Patent Publication Number: US-8123753-B2

Title: Cutting guide assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a surgical instrument. In particular, the present invention relates to a surgical instrument used during an implantation procedure for a reverse shoulder prosthesis. More particularly, the present invention relates to a cutting guide assembly for resecting a head of a bone. The present invention also relates to a method of resecting a head of a bone using the cutting guide assembly. 
     BACKGROUND OF THE INVENTION 
     A humerus-scapular joint (referred to herein as a shoulder joint) prosthesis comprises a humeral component having a stem part which can be fitted into a reamed cavity within the medullary canal of the humerus, and a glenoid component for attachment to the glenoid. The humeral component and the glenoid component comprise corresponding bearing surfaces which articulate together as the joint moves. In a natural shoulder joint the humeral component comprises a convex head, which articulates against a concave bearing surface on the glenoid. This structure is reproduced in an “anatomic” shoulder joint prosthesis, in which the humeral component includes a stem part and a head part with a convex bearing surface and the glenoid component provides a concave bearing surface. The stem part is implanted within the humerus. The head part is fitted to the stem part, or is formed integrally with the stem part, so that it sits above a resection surface of the humerus. Anatomic prostheses are suitable for implantation in patients where joint tissue has degraded (for example, due to arthritis). 
     The structure of the anatomic joint is reversed in a “reverse” shoulder joint prosthesis. The glenoid component includes a convex head, and the humeral component has a concave recess in the epiphysis, in which the head on the glenoid component can be received and articulate. 
     The humeral component of a reverse joint prosthesis, including the epiphysis part which provides the bearing surface, may be implanted almost entirely within the humerus. 
     The biomechanical properties of the patient&#39;s joint are altered when a reverse shoulder joint prosthesis is implanted because the center of rotation of the joint is shifted medially. A reverse shoulder joint prosthesis is suitable for implantation in a patient with damaged cuff muscle tissue. The shift of the center of rotation allows manipulation of the arm using the deltoid muscle because of the increased mechanical advantage. 
     A reverse shoulder prosthesis is described in WO-2007/039820 (DePuy (Ireland) Ltd). Such a joint prosthesis is available commercially and sold by DePuy Products Inc. under the trade name Delta Xtend. 
     The process of implanting a reverse shoulder prosthesis involves the use of a range of surgical instruments. Embodiments of the present invention relate to improved surgical instruments for use in such an implantation procedure. 
     As a preliminary step during the process of implanting a reverse shoulder prosthesis the head of the humerus must be resected. It is important that the precise location and orientation of the required resection surface is accurately determined. Furthermore, once the location of the required resection surface is determined it is important that the resection is performed accurately to avoid causing misalignment of the implanted prosthesis. 
     It is an objection of embodiments of the prior art to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a cutting guide assembly for resecting a head of a bone, comprising: an elongate support rod configured to be partially disposed in and rotatable within a cavity extending into the head of the bone, the support rod defining a longitudinal axis and including a reference formation; and a first cutting plate and a second cutting plate, each of the first cutting plate and the second cutting plate separately couplable to the support rod, each cutting plate defining a cutting surface arranged such that when either the first cutting plate or the second cutting plate is coupled to the support rod, the bone can be cut to form a resection surface by aligning a cutting tool with the cutting surface; wherein the first cutting plate when coupled to the support rod is positioned laterally of the head of the bone and the second cutting plate when coupled to the support rod is positioned anteriorly of the head of the bone, and in either case, the first cutting plate and the second cutting plate are oriented such that the bone can be cut to form the same resection surface, without rotating the support rod. 
     The cutting guide assembly may further comprise a plurality of alignment holes extending through the support rod in a plurality of radial directions, the cutting guide assembly further comprising an alignment rod arranged to be slidably received in an alignment hole and to extend transversely from the support rod for alignment with a point remote from the head of the bone to adjust the rotational position of the support rod within the cavity. 
     The support rod may further comprise a handle that is manipulable to rotate the support rod within the cavity such that the orientation of an axis perpendicular to the formed resection surface about a longitudinal axis of the cavity is determined by the rotational position of the support rod within the cavity. 
     The cutting surface may lie in a plane that is inclined to the longitudinal axis of the support rod at an angle of between 50° and 80°. 
     The support rod may be partially disposed within the cavity, the cutting surface of the first cutting plate or the second cutting plate when coupled to the support rod lies in a plane that is inclined to the longitudinal axis of the cavity and faces medially and superiorly. The first cutting plate may comprise a concave medial side arranged to face the head of the bone and the second cutting plate may comprise a straight anterior side arranged to face the head of the bone. 
     According to a second aspect of the present invention there is provided a method of resecting a head of a bone, comprising the steps of: inserting an elongate support rod into a cavity extending into the head of the bone such that the support rod is partially disposed in and rotatable within the cavity, the support rod defining a longitudinal axis and including a reference formation; selecting a first cutting plate or a second cutting plate, each cutting plate defining a cutting surface, according to whether a greater proportion of the lateral or anterior face of the exposed head of the bone is visible respectively; coupling the selected cutting plate to the support rod such that the orientation of the selected cutting plate about the longitudinal axis of the support rod is defined by the reference formation; aligning a cutting tool with the cutting surface of the selected cutting plate; and cutting the head of the bone can be cut to form a resection surface; wherein, without rotation of the support rod, if the first cutting plate is selected, the first cutting plate couples to the support rod such that the first cutting plate is positioned laterally of the head of the bone, and, if the second cutting plate is selected, the second cutting plate couples to the support rod such that the second cutting plate is positioned anteriorly of the head of the bone, and said step of cutting the head of the bone forms the same resection surface with either the first cutting plate or the second cutting plate. 
     The method may further comprise the step of rotating the support rod within the cavity such that the orientation of an axis perpendicular to the formed resection surface about a longitudinal axis of the cavity is determined by the rotational position of the support rod within the cavity. 
     The method may further comprise the steps of: providing a support rod that has a plurality of alignment holes that extend through the support rod in a plurality of radial directions, and the step of rotating the support rod within the cavity further comprises: inserting an alignment rod into an alignment hole such that the alignment rod is slidably received in the alignment hole and extends transversely from the support rod; and aligning the alignment rod with a point remote from the head of the bone to adjust the rotational position of the support rod within the cavity. The step of rotating the support rod within the cavity may comprise rotating the support rod within the cavity until the cutting surface of the selected cutting plate lies in a plane that is inclined to the longitudinal axis of the cavity and faces medially and superiorly. 
     According to a third aspect of the present invention there is provided a cutting guide assembly for resecting a head of a bone, comprising: an elongate support rod configured to be partially disposed and rotatable within a cavity extending into the head of the bone, the support rod defining a longitudinal axis and including a reference formation; a cutting plate defining a cutting surface configured such that the head of the bone can be cut to form a resection surface by aligning a cutting tool with the cutting surface; and a cutting plate mount arranged to couple the cutting plate to the support rod, the orientation of the cutting plate mount about the support rod being defined by the reference formation such that the orientation of an axis perpendicular to the formed resection surface about a longitudinal axis of the cavity is determined by the rotational position of the support rod within the cavity, the cutting plate mount comprising a first clamp; wherein the cutting plate assembly further comprises a post extending superiorly from the cutting plate parallel to the longitudinal axis of the support rod, the post being receivable within the first clamp such that the cutting plate is slidable parallel to the longitudinal axis of the support rod. 
     The post may comprise at least one indicator marking configured such that the position of the cutting plate along an axis parallel to the longitudinal axis of the support rod is indicated by the position of the at least one indicator marking relative to the first clamp. 
     The cutting surface may lie in a plane which is inclined to the longitudinal axis of the support rod at an angle of between 50° and 80°. 
     The cutting plate mount may further comprise a second clamp configured to engage the support rod reference formation and a shaft extending from the second clamp parallel to the plane of the cutting surface, the first clamp being slidably mounted on the shaft such that the cutting plate can be moved parallel to the plane of the cutting surface towards and away from the head of the bone. 
     The cutting plate may further have first and second guide holes configured to act as a drilling guide for drilling holes into the head of the bone, the cutting guide assembly further comprising fixation pins configured to be inserted through the guide holes into the head of the bone to support the cutting plate on the head of the bone such that the cutting plate can be decoupled from the cutting plate mount and the support rod by sliding the first clamp over the post extending superiorly from the cutting plate. The first and second guide holes may define parallel axes such that the cutting plate can be removed from the pins and inverted such that the post extends inferiorly from the cutting plate. The cutting plate may define a second cutting surface parallel to the first cutting surface on the opposite side of the cutting plate from the first cutting surface. 
     The cutting plate may further have a third guide hole extending transverse to the axis of the first and second guide holes to act as a drilling guide for drilling a third hole into the head of the bone such that a third pin can be inserted through the third guide hole into the head of the bone securing the cutting plate to the head of the bone. 
     According to a fourth aspect of the present invention there is provided a method of resecting a head of a bone, comprising: inserting an elongate support rod into a cavity extending into the head of the bone such that the support rod is at least partially disposed in and rotatable within the cavity, the support rod defining a longitudinal axis and including a reference formation; coupling a cutting plate defining a cutting surface to the support rod via a cutting plate mount that couples to the reference formation, the cutting plate comprising a post extending superiorly from the cutting plate parallel to the longitudinal axis of the support rod, the post being receivable within a first clamp forming part of the cutting plate mount such that the cutting plate is slidable parallel to the longitudinal axis of the support rod; rotating the support rod within the cavity such that the cutting plate is rotated about the support rod and the orientation of the cutting plate about the longitudinal axis of support rod is defined by the reference formation; sliding the post through the cutting plate mount clamp such that the cutting plate mount slides relative to the support rod longitudinal axis; aligning a cutting tool with the cutting surface; and cutting the head of the bone to form a resection surface; wherein the orientation of an axis perpendicular to the formed resection surface about a longitudinal axis of the cavity is determined by the rotational position of the support rod within the cavity. 
     Rotating the support rod within the cavity may comprise rotating the support rod within the cavity until the cutting surface lies in a plane which is inclined to the longitudinal axis of the cavity and faces medially and superiorly. 
     The cutting plate mount may further comprise a second clamp configured to engage the support rod reference formation and a shaft extending from the second clamp parallel to the plane of the cutting surface, the first clamp being slidably mounted on the shaft, the method further comprising sliding the first clamp along the shaft such that the cutting plate moves parallel to the plane of the cutting surface towards and away from the head of the bone. 
     The cutting plate may further have first and second guide holes. The method may further comprise: drilling holes through the first and second guide holes into the head of the bone; inserting fixation pins through the guide holes and into the head of the bone such that the cutting plate is supported on the head of the bone; and decoupling the cutting plate from the cutting plate mount. 
     The first and second guide holes may define parallel axes. The method may further comprise: removing the cutting plate from the pins; and inverting the cutting plate such that the post extends inferiorly from the cutting plate; wherein the cutting plate defines a second cutting surface parallel to the first cutting surface on the opposite side of the cutting plate from the first cutting surface. 
     The cutting plate may further have a third guide hole extending transverse to the axis of the first and second guide holes. The method may further comprise: drilling a hole through the third guide hole into the head of the bone; and inserting a third fixation pin through the third guide hole into the head of the bone to secure the cutting plate to the head of the bone. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
       The present invention will now be described, by way of example only, with reference to the following figures, in which: 
         FIG. 1  illustrates in an exploded view a cutting guide for guiding a cutting tool used to resect the head of a humerus, the cutting guide being arranged to be used when a superior-lateral surgical approach exposes the humerus; 
         FIG. 2  illustrates in an exploded view a cutting guide for guiding a cutting tool used to resect the head of a humerus, the cutting guide being arranged to be used when a deltoid-pectoral surgical approach exposes the humerus; 
         FIG. 3  illustrates the cutting guide of  FIG. 1  assembled and in position on the head of a humerus, and illustrating the use of an alignment pin for determining the orientation of the cutting guide; 
         FIG. 4  illustrates the cutting guide of  FIG. 1  assembled and in position on the head of a humerus; 
         FIG. 5  illustrates the cutting guide of  FIG. 2  assembled and in position on the head of a humerus; 
         FIGS. 6 and 8  illustrate a cutting plate forming part of the cutting guide of  FIG. 1  attached to the head of a humerus in first and second configurations; 
         FIGS. 7 and 9  illustrate a cutting plate forming part of the cutting guide of  FIG. 2  attached to the head of a humerus in first and second configurations; 
         FIG. 10  illustrates a first reamer being used to ream portions of a glenoid such that a glenoid component of a reverse shoulder prosthesis can be attached to the glenoid; 
         FIG. 11  illustrates a second reamer being used to ream portions of a glenoid such that a glenoid component of a reverse shoulder prosthesis can be attached to the glenoid; 
         FIG. 12  illustrates the reamer of  FIG. 11  in a side view positioned against the reamed glenoid; 
         FIG. 13  illustrates an intramedullary reaming guide and an alignment instrument for aligning the intramedullary reaming guide relative to a resected humeral head when the intramedullary reaming guide is inserted into a cavity reamed in the medullary canal of the humerus; 
         FIG. 14  illustrates the intramedullary reaming guide and the alignment instrument of  FIG. 13  coupled together; 
         FIGS. 15 and 16  illustrate the intramedullary reaming guide and the alignment instrument of  FIG. 13  during implantation of the intramedullary reaming guide into a cavity reamed in the medullary canal of the humerus; 
         FIG. 17  illustrates the intramedullary reaming guide of  FIG. 13  implanted into a cavity reamed in the medullary canal of the humerus; 
         FIG. 18  illustrates a centered reaming adapter coupled to a intramedullary reaming guide implanted into a cavity reamed in the medullary canal of the humerus as illustrated in  FIG. 17 ; 
         FIGS. 19 and 20  illustrate first and second sizing guides respectively coupled to the centered reaming adapter illustrated in  FIG. 18 ; 
         FIG. 21  illustrates a first eccentric reaming adapter coupled to a intramedullary reaming guide implanted into a cavity reamed in the medullary canal of the humerus as illustrated in  FIG. 17 ; 
         FIG. 22  illustrates a first sizing guide coupled to the eccentric reaming adapter illustrated in  FIG. 21 ; 
         FIG. 23  illustrates a second sizing guide coupled to a second eccentric reaming adapter illustrated, which in turn is coupled to a intramedullary reaming guide implanted into a cavity reamed in the medullary canal of the humerus as illustrated in  FIG. 17 ; 
         FIG. 24  illustrates a reaming head being used to ream an epiphysis cavity in a resected humeral head; 
         FIG. 25  illustrates a broach and a broach insertion instrument being used to enlarge a reamed cavity within the medullary canal of a resected humeral head; 
         FIG. 26  illustrates portions of the broach and broach insertion instrument of  FIG. 25  being used to measure a rotational offset between the rotational position of the broach and a line which extends normal to a resection surface and intersects a longitudinal axis of the humerus defined by the reamed cavity; 
         FIG. 27  illustrates part of a modular humeral component of a reverse shoulder prosthesis; 
         FIG. 28  illustrates part of a convex bearing head forming part of a reverse shoulder prosthesis and part of a convex bearing head orientation guide for correctly aligning the eccentricity of the convex bearing head; 
         FIG. 29  illustrates the convex bearing head and convex bearing head orientation guide of  FIG. 28  during attachment of the convex bearing head to a glenoid; 
         FIG. 30  illustrates the orientation guide of  FIG. 29  in cross section along its longitudinal axis during attachment of the convex bearing head to a glenoid, including a screwdriver passing through a central bore of the orientation guide; 
         FIG. 31  illustrates a front view of the convex bearing head of  FIG. 28 ; 
         FIG. 32  illustrates a reaming guide coupled to the epiphysis of the humeral component of a reverse shoulder prosthesis for use within a revision procedure to remove cortical bone around the epiphysis; 
         FIG. 33  illustrates a reaming head coupled to the reaming guide of  FIG. 32 ; and 
         FIG. 34  illustrates a humeral head implant couple to a humeral head after reaming using the reaming head of  FIG. 33 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     A reverse shoulder prosthesis may be of the form available commercially and sold by DePuy Products Inc. under the trade name Delta Xtend Reverse Shoulder System. Such a reverse shoulder system particularly suitable for treating shoulder cuff tear arthropathy. The normal biomechanics of a patient&#39;s scapula and humeral components are reversed. Advantageously, the gleno-humeral joint center of rotation is moved medially and inferiorly increasing the deltoid lever arm and the deltoid tension thus allowing the muscles of the deltoid group to compensate for rotator cuff deficiency. 
     A reverse shoulder prosthesis comprises two primary components: a humeral component implanted into a reamed cavity within the medullary canal of a resected humeral head and a glenoid component attached to a reamed portion of the glenoid part of the scapula. The humeral component may either comprise a modular humeral stem part and an epiphysis part or a single integral component comprising both a humeral stem and an epiphysis. The modular humeral component is preferably designed to form a press fit in a reamed humeral cavity. The integral humeral component is preferably designed to be cemented in position. For press fit humeral components the surface of the implant may be coated with a material which encourages bone in growth thereby securing the implant in position, for instance hydroxyapatite (HA) coated titanium alloy. The glenoid component may be secured primarily by screws into the glenoid with a HA coating for secondary fixation. 
     The stem part of the humeral component may be similar in form to the stem part of an anatomic shoulder prosthesis. For a modular humeral component it is known for the epiphysis part to be either centered upon the humeral component or offset in a posterior direction to allow for adjustable retroversion, thereby allowing for increased internal rotation of the joint. 
     The plane of the upper face of the epiphysis part is typically at 155° to the axis of the stem part, which increases the stability of the implanted prosthesis. 
     The glenoid component comprises a mounting plate (alternatively referred to as a metaglene) arranged to be attached to a reamed portion of the glenoid and a convex bearing head (alternatively referred to as a glenosphere) comprising a convex bearing surface mountable upon the mounting plate. The convex bearing head comprises part of a sphere. The convex bearing head may be eccentric (that is, having a fixation hole that is not positioned at the centre of the bearing surface of the convex bearing head) in order to increase the range of motion of the shoulder prosthesis and reduce the risk of scapular erosion. 
     Between the humeral component and the glenoid component there is provided a humeral cup formed from a material having a low friction surface, such as polyethylene, in order to maximize the range of motion of the shoulder prosthesis and reduce the risk of scapular erosion. The humeral cup is typically coupled to the epiphysis. 
     In the event of problems arising within implanted reverse shoulder prostheses a reverse shoulder prosthesis can be converted to an anatomical prosthesis. To achieve this, the convex bearing head and the mounting plate are removed from the glenoid and the humeral cup is removed from the epiphysis. A convex bearing head may then be attached to the epiphysis, arranged to articulate against the glenoid and the acromion. 
     A surgical procedure for implanting a reverse shoulder prosthesis and optionally converting the prosthesis to an anatomic prosthesis, and particular the surgical instruments used in such a procedure will now be described. 
     Prior to surgery an initial assessment is made of the humerus and the glenoid using radiographic and CT imaging to determine whether there is sufficient bone stock for implantation of the humeral component and the glenoid component. If the patient is suitable for treatment, then the imaging may be measured in order to determine the appropriate size of implants, though the final decision is typically left to the surgeon&#39;s discretion. 
     A reverse shoulder prosthesis may be implanted using a surgical approach involving either a superior-lateral incision or a deltoid-pectoral incision. The decision is subject to the surgeon&#39;s preference and clinical parameters. The chosen approach affects the surgical instruments and techniques used, in particular the instruments used for resecting the humeral head, as will be described in greater detail below. 
     A superior-lateral approach comprises forming an incision either anterior-posterior along the lateral edge of the acromion or in a lateral direction starting from a superior position on the shoulder. The shoulder is dissected until the humeral head is visible at the anterior edge of the acromion. The arm may then be externally rotated and the head dislocated antero-superiorly to facilitate positioning of a cutting guide. The superior-lateral approach allows for a clear view of the glenoid and therefore facilitates the implantation of the glenoid implant components, in particular when the glenoid is retroverted. 
     A deltoid-pectoral approach comprises forming an incision from the midpoint of the clavicle to the midpoint of the arm. The shoulder is dissected until the humeral head is visible and can be dislocated. The deltoid-pectoral approach has the advantage of offering an enhanced view of the inferior part of the glenoid. If revision surgery is required in order to convert the humerus-scapula joint to an anatomical configuration the deltoid-pectoral approach is preferred as it allows for a longer humeral incision. 
     Regardless of the surgical approach, once the humeral head is visible and has been dislocated the first step is to form an intramedullary cavity. The cavity runs from the humeral head parallel to the longitudinal axis of the humerus. The cavity defines a longitudinal axis extending along the cavity into the humerus. A pilot hole must first be drilled into the humeral head, passing directly down into the medullary canal along the bone. A series of hand reamers having progressively larger diameters are then used to enlarge the cavity until there is contact with cortical bone of the intramedullary canal of the humerus. The diameter of the final reamer used determines the size of the cutting guide assembly support rod, intramedullary reaming guide and the final humeral component, as will be described herein. For example, if a 12 mm reamer begins to gain purchase in the intramedullary cortical bone (and so is the largest reamer used) then a 12 mm stem for the humeral component will be required. 
     Once the intramedullary cavity has been formed, then the humeral head can be resected. Referring to  FIGS. 1 and 2  these respectively illustrate in exploded views alternative cutting guide assemblies that may be used to guide a cutting tool for resecting the humeral head. The cutting guide assembly illustrated in  FIG. 1 , generally referred to as reference numeral  1 , is arranged to be used when the surgical approach is superior-lateral, whereas the cutting guide assembly illustrated in  FIG. 2 , generally referred to as reference numeral  1   a , is arranged to be used when the surgical approach is deltoid-pectoral. 
     The required resection of the humeral head is the same regardless of the surgical approach. The resection surface is typically required to be at an angle of 155° to the longitudinal axis of the humerus defined by the intramedullary cavity. The cutting guide assemblies  1 ,  1   a  illustrated in  FIGS. 1 and 2  present a cutting surface on a cutting plate that is automatically orientated at 155° to the longitudinal axis of the humerus. The resection surface faces medially and superiorly; that is, it faces towards the glenoid. For a superior-lateral approach the visible portion of the humeral head is predominantly the superior and lateral portions of the humeral head, whereas for a deltoid-pectoral approach it is predominantly the anterior portion of the humeral head that is visible. Consequently, a separate cutting guide assembly is required for each surgical approach. Each cutting assembly is suitable for performing surgical procedures on either a patient&#39;s left or right arm. The superior-lateral cutting guide assembly of  FIG. 1  may be used for either the right arm (as illustrated) or the left arm by simply rotating the whole assembly. The deltoid-pectoral cutting guide of  FIG. 2  may be used for either the right arm (as illustrated) or the left arm by rotating the cutting plate  4 . As shown in  FIG. 2 , the cutting plate  4  is engraved with “RIGHT” on a first side indicating that it is orientated for used on a right shoulder and “LEFT” on a second opposite side (not visible) indicating that it is orientated for use on a left shoulder. It will be apparent from the description below that the cutting guide assemblies  1 ,  1   a  comprise a selection of modular components, some of which are common to each surgical approach.  FIGS. 1 and 2  illustrate cutting guide assemblies  1 ,  1   a  suitable for resecting the head of a right humerus of a patient. 
     Each cutting guide assembly  1 ,  1   a  comprises a cutting plate  2 ,  4  illustrated in  FIGS. 1 and 2  respectively. Each cutting plate comprises a cutting surface that defines the resection surface, and may be maneuvered until it is in the optimal position for performing the resection. The resection is then achieved by the surgeon aligning a cutting tool with the cutting surface such that the resection surface is parallel to the cutting surface of the cutting plate. 
     The cutting guide assemblies  1 ,  1   a  illustrated in  FIGS. 1 and 2  each comprise the same support rod  6 . The support rod  6  comprises an elongate rod including a first portion  8  for insertion into the reamed intramedullary cavity. Each cutting guide assembly is provided with a range of support rods  6 , with each support rod  6  being provided with a different diameter first portion  8  corresponding to the differing diameters of the reamers used to form the intramedullary cavity. For instance, if a 12 mm intramedullary cavity has been formed then a 12 mm diameter support rod  6  must be used to ensure that the support rod  6  forms a close fit in the intramedullary cavity. 
     Each support rod  6  comprises a flange  10  which forms a depth stop preventing over insertion of the support rod  6  into the intramedullary cavity by coming to rest against the top surface of the humeral head. Adjacent to the flange  10  the support rod  6  further comprises a reference formation  12 . The reference formation  12  is formed as a rib. The orientation of the reference formation  12  relative to the longitudinal axis of the humerus determines the orientation of the resection surface about the longitudinal axis of the humerus. The support rod  6  further comprises a T shaped handle  14  which may be manipulated by a surgeon in order to rotate the support rod  6  within the intramedullary cavity to adjust the orientation of the resection surface, as will be described in greater detail below. 
     Once the support rod  6  has been fully inserted into the intramedullary cavity the remainder of the cutting guide assembly may be assembled. The cutting guide assemblies  1 ,  1   a  illustrated in  FIGS. 1 and 2  each comprise a separate cutting plate mount  16 ,  18  respectively for coupling the cutting plate  2 ,  4  to the support rod  6 . Each cutting plate mount  16 ,  18  comprises a clamp formed as a collar  20  arranged to surround the shaft of the support rod  6  and engage the reference formation  12 . The collar  20  incorporates a groove  22  arranged such that when coupled to rib  12 , collar  20  is prevented from rotating about the support rod  6 . A locking screw  24  is provided, which passes through a corresponding hole in the collar  20  and engages the shaft of the support rod  6  holding the collar in place. 
     The purpose of cutting plate mounts  16 ,  18  is to position the cutting plates  2 ,  4  in an appropriate position to define the plane of the resection surface. Consequently, each cutting plate mount  16 ,  18  further comprises a shaft  26 ,  28  that extends from the collar  20 . Shafts  26 ,  28  extend from collar  20  along an axis parallel to the plane of the cutting surface of the cutting plate  2 ,  4  (and hence parallel to the resulting resection surface). For the cutting guide assembly illustrated in  FIG. 1  when assembled for a superior-lateral approach of a right shoulder the shaft  26  extends from the collar  20  laterally so as to protrude from the patient&#39;s shoulder through the incision. Consequently, in order to lie parallel to the resection surface, shaft  26  extends superiorly and laterally. For the cutting guide illustrated in  FIG. 2  when assembled for a deltoid-pectoral approach of a right shoulder the shaft  28  extends from the collar  20  anterially so as to protrude from the patient&#39;s shoulder through the incision. Consequently, in order to lie parallel to the resection surface, shaft  28  extends perpendicularly from the support rod  6 . 
     Cutting plate  2  illustrated in  FIG. 1  is provided with a concave curved edge  30  on the side that in use will be adjacent to the humeral head. Curved edge  30  is intended to reflect the profile of the humeral head so as to allow the cutting plate  2  to be positioned close to the lateral portion of the humeral head. Conversely, cutting plate  4  illustrated in  FIG. 2  has a straight edge  32  on the side that in use will be adjacent to the anterior portion of the humeral head. Extending superiorly from each cutting plate  2 ,  4  and parallel to the axis of the support rod  6  is a post  34 ,  36 . Posts  34 ,  36  are slidably received within a respective clamp  38 ,  40 , which in turn is slidably mounted upon shafts  26 ,  28  respectively. The position of the clamps  38 ,  40  with respect to posts  34 ,  36  can be locked by tightening screws  42 ,  44  in order to preserve the height adjustment of the cutting plate  2 ,  4  selected by the surgeon. Clamps  38 ,  40  remain free to slide along shafts  26 ,  28  so that the cutting plate  2 ,  4  can be slid close to the humerus before being secured in position (as described below). 
     The arrangement of the cutting plate mounts  16 ,  18  is such that for each cutting guide assembly the cutting plates  2 ,  4  may be raised or lowered parallel to the longitudinal axis of support rod  6  by sliding posts  34 ,  36  through clamps  38 ,  40 . This allows the surgeon to select the appropriate position of the resection surface along the longitudinal axis of the humerus. Posts  34 ,  36  are provided with color coded markings, comprising a central red marking  46  and outer green markings  48 . Normally, the post  34 ,  36  will be locked in position by the respective clamp  38 ,  40  such that only the green markings  48  are visible either side of the clamp  38 ,  40 . This ensures that the resection surface is located along the longitudinal axis of the humerus at the correct position for most patients (if the support rod  6  is inserted into the medullary canal sufficiently far for the flange  10  to contact the upper surface of the humeral head). However, on a patient specific basis, the sliding adjustment of posts  34 ,  36  allows the position of the resection surface along the longitudinal axis of the humerus to be adjusted according to clinical parameters. 
     Furthermore, the arrangement of the cutting plate mounts  16 ,  18  is such that for each cutting guide assembly the cutting plates  2 ,  4  may be brought closer to, or in contact with, the humeral head by sliding clamps  38 ,  40  along shafts  26 ,  28 . Given that shafts  26 ,  28  extend parallel to the required resection surface, once the surgeon has selected the appropriate level of the resection surface along the longitudinal axis of the humerus, the cutting plates  2 ,  4  can be slid towards the humeral head parallel to the resection surface, such that the position of the resection surface is not affected. Bringing the cutting plates into contact with the humeral head advantageously allows the surgeon to cut the humeral head by running the cutting tool along the cutting surface with less risk of inaccuracy caused by stray motion of the cutting tool. 
     As illustrated in  FIGS. 1 and 2 , the cutting surface is defined by the superior side  50 ,  52  of the cutting plates  2 ,  4 . However, as will be appreciated, the arrangement is such that a surgeon would be compelled to cut around posts  34 ,  36  and also the support rod  6  where it extends into the humeral head. As will be described below, once the cutting plate  2 ,  4  has been appropriately positioned, the cutting plate  2 ,  4  can be locked in position on the humeral head and the support rod  6  and the cutting plate mount  16 ,  18  removed in order to ease a surgeon&#39;s work in resecting the humeral head. 
     As has been described above, the provision of separate cutting guide assemblies  1 ,  1   a  optimized for use with either a superior-lateral or a deltoid-pectoral surgical approach allows a surgeon to accurately position a cutting plate  2 ,  4  (and hence the resection surface) at a desired level along the longitudinal axis of the humerus by adjustment of clamp  38 ,  40 . The desired orientation about the longitudinal axis of the humerus is set by rotation of the support rod  6 . The correct angle of the resection surface with respect to the longitudinal axis of the humerus is set automatically by the angle subtended between the cutting plate  2 ,  4  and post  34 ,  36 , which in use is parallel to the longitudinal axis of cavity and hence parallel to the longitudinal axis of the bone. The cutting guide assemblies  1 ,  1   a  allow these parameters of the resection surface to be set in a controlled fashion which is not solely dependent upon the surgeon&#39;s skill and judgment in order to correctly position the resection surface. The cutting guide assemblies  1 ,  1   a  allow the position of the resection surface to be finely adjusted before any cutting step is required. 
     As noted above, the orientation of the resection surface about the longitudinal axis of the humerus can be adjusted by rotating the support rod  6  within the intramedullary cavity. In order to assist the alignment of the resection surface, an upper portion  54  of the support rod  6  further comprises a series of alignment holes  56  which pass through the handle  6 . The alignment holes  56  include a primary alignment hole  58  indicated by a flared entrance hole. As can be seen, the axis of the primary alignment hole is parallel to the axis of the reference formation  12  defined by the long axis of the rib. The remaining alignment holes  56  form a series of alignment holes extending though the support rod  6  at differing radial directions. 
     When the support rod  6  is inserted into the intramedullary cavity, an alignment rod  60  can be inserted into one of the alignment holes  56  and use to rotationally align the cutting guide handle  6 , as is shown in  FIG. 3 .  FIG. 3  shows a cutting guide in accordance with  FIG. 1  being used to locate cutting plate  2  relative to a humeral head  62  when the humeral head  62  has been exposed using a superior-lateral surgical approach. Soft tissue of the patients shoulder is shown held back by retractors  64 . It will be appreciated that an alignment rod  60  may be used to align the cutting guide of  FIG. 2  in the same way, given that the support rod is the same for each cutting guide. 
     Adjusting the rotational position of the resection surface varies the degree of retroversion or anteversion (that is the rotational position about the longitudinal axis of the humerus of a line which is normal to the resection surface and intersects the longitudinal axis of the humerus) applied to the implanted reverse shoulder prosthesis. The retroversion or anteversion of the final implant position can be assessed by comparing the axis  68  of the alignment rod  60  with the patient&#39;s forearm axis  66 . Rotating the support rod  6  within the intramedullary cavity until the alignment rod axis  68  is parallel to the patient&#39;s forearm axis  66  ensures that required degree of retroversion or anteversion is set for the resection surface. If the alignment pin  60  is inserted into the primary alignment hole  58  then 0° retroversion is set for the resection surface. If alternatively one of other alignment holes  56  are used then a predetermined degree of retroversion or anteversion can be provided to the resection surface. Typically 0-10° retroversion is applied since excessive retroversion can restrict joint mobility, especially internal rotation. However, care must be taken not to damage the subscapularis insertion by resecting the humeral head  62  with excessive anteversion. 
     Once the desired degree of retroversion or anteversion has been set, the level of the resection surface can be adjusted as discussed above, typically such that only the green markers  48  are visible on post  34 . Usually 1-2 mm of the proximal area of the greater tuberosity is resected (at the level of the supraspinatus insertion on an intact shoulder). The cutting plate  2 ,  4  can then be slid into contact with the humeral head  62  by adjusting the position of the clamp  38 ,  40  along shaft  26 ,  28 .  FIGS. 4 and 5  show the cutting guide assemblies  1 ,  1   a  illustrated in  FIGS. 1 and 2  respectively assembled and positioned upon a humeral head  62  such that the cutting surface  50 ,  52  is positioned defining the plane of the chosen resection surface. For clarity, soft tissue surrounding the humeral head  62  is not shown. 
     As noted above, the cutting plate  2 ,  4  may be secured to the humeral head  62  such that the remainder of the cutting guide assembly can be removed, assisting the surgeon in resecting the humeral head  62  by passing a cutting tool over the cutting surface  50 ,  52 . As shown in  FIGS. 4 and 5 , each cutting plate  2 ,  4  further comprises a pair of guide holes  70 ,  72  respectively at each end of the cutting plate  2 ,  4 . Once the cutting plate  2 ,  4  is correctly positioned, holes may be drilled through the guide holes  70 ,  72  into the cortical bone with a 3.2 mm drill bit, using the cutting plate  2 ,  4  as a drill guide. Fixation pins  74  may then be passed through the guide holes  70 ,  72  preserving the alignment of the cutting plate  2 ,  4  relative to the humeral head  62 . 
     Once fixation pins  74  are in position, the cutting plate mount  16 ,  18  can be removed from the cutting plate  2 ,  4  by slackening off locking screw  42 ,  44  thereby freeing post  34 ,  36  which extends from the cutting plate  2 ,  4 . Slackening off locking screw  24  allows the clamp  20  to be lifted parallel to the longitudinal axis of the support rod  6  such that cutting plate mount  16 ,  18  is decoupled from the cutting plate  2 ,  4  without disturbing its position relative to the humeral head  62 . The support rod  6  can then be released from the intramedullary cavity. The cutting plate  2 ,  4  is supported on the humeral head  62  by the fixation pins  74  as shown in  FIGS. 6 and 7  in the relative position determined through the above alignment steps. The cutting plate  2 ,  4  can be further secured to the humeral head  62  by passing a third fixation pin  74  through a third guide hole  76 ,  78 . The outer pair of guide holes  70 ,  72  are arranged to be parallel to one another such that the cutting plate  2 ,  4  can slide along fixation pins  74  towards and away from the humeral head parallel to the resection surface  50 ,  52 . The third, central guide hole  76 ,  78  defines an axis which is divergent from the axes of the first two guide holes  70 ,  72 . Consequently, when the third fixation pin  74  is inserted the cutting plate  2 ,  4  is held firmly in position. 
     As will be appreciated from  FIGS. 6 and 7  the surgeon is able to resect the humeral head  62  by passing a cutting tool parallel to and next to the cutting surface  50 ,  52 . Removal of the other components of the cutting guide assembly greatly reduces the complexity of the cutting step that must be performed by the surgeon. However, post  34 ,  36  extending from the cutting plate is still in the way of the resection and must be cut around, possibly resulting in a less accurate resection. In order to avoid this obstacle, before the third fixation pin  74  is inserted, the cutting plate  2 ,  4  can be removed from the initial pair of fixation pins  74  by sliding along the axes of the parallel pair of fixation pin  74 . The cutting plate  2 ,  4  can then be inverted and replaced over the fixation pins  74  as shown in  FIGS. 8 and 9 . The cutting plate  2 ,  4  additionally defines a second cutting surface  80 ,  82 , which now faces superiorly and in the plane previously occupied by the first cutting surface  50 ,  52 . Second cutting surface  80 ,  82  is parallel to the first cutting surface  50 ,  52  and equidistant from the fixation pins  74 . Consequently, the second surface  80 ,  82  is parallel to the same chosen resection surface. Advantageously, there is no post protruding from the second cutting surface  80 ,  82  allowing for the resection to be performed as a single cutting action resulting in a more even resection. Again, a third fixation pin  74  can be provided through divergent guide hole  76 ,  78  securing the cutting plate  2 ,  4  in position. 
     It will be appreciated that in alternative embodiments the cutting plate assembly may be varied. In particular, the coupling mechanism to the support rod may be modified to provide alternative mechanisms for coupling the cutting plates such that cutting plates designed for different surgical approaches are aligned to the same desired resection plane. Similarly, the height adjustment of the cutting plate may be modified, for instance by coupling to the cutting plate mount via an alternative connection. It will be desirable that any alternative cutting guide assembly retains the ability for the cutting plate assembly to be disassembled while the cutting plate remains attached to the head of the bone. 
     Once the resection has been performed, the fixation pins  74  and the cutting plate  2 ,  4  can be removed from the humeral head  62 . A humeral resection protecting plate can be placed over the resected surface in order to protect the bone from damage during the following surgical steps preparing the glenoid. 
     A forked retractor can be passed under the scapula in order to lever the humeral head  62  out of the way in order to allow unimpeded access to the glenoid. If the glenoid is not fully visible then a further resection of the humeral head  62  may be required. The forked retractor is placed under the inferior glenoid labrum to move the humerus distally or posteriorly according to the chosen surgical approach (superior-lateral or deltoid-pectoral respectively). 
     Once the glenoid is fully visible, preparation of the glenoid can begin. Firstly, any remnants of the labrum must be removed from the glenoid face. Additionally, any osteophytes present may also have to be removed to prevent later interference when attaching the mounting plate and the convex bearing surface to the glenoid. 
     Particular care is needed when determining the attachment point of the mounting plate as this affects the resultant center of rotation of the reverse shoulder prosthesis. The correct mounting plate position achieves optimal glenoid fixation (that is, the mounting plate is fully in contact with the glenoid), good range of motion of the shoulder joint and minimal potential for bone impingement (the humeral component contacting the scapula around the convex bearing head). Ideally, the mounting plate should be positioned on the inferior circular portion of the glenoid. A mounting plate positioning tool may be used to determine the optimal mounting plate position. This comprises a generally circular sizing plate including cut outs such that the glenoid surface is visible through the sizing plate mounted upon a positioning handle which can be manipulated by the surgeon at a point remote from the glenoid. The positioning handle couples to the sizing plate at a point eccentric of the center of the sizing plate such that the center of the plate is visible, and couples to the sizing plate along an axis which diverges from an axis normal to the plate (for instance 20°) in order to allow for maximum visibility of the glenoid. 
     Once the sizing plate is positioned correctly (for instance, such that its border follows the inferior edge of the glenoid and the sizing plate is parallel to the glenoid face, or with a slight superior tilt) a guide pin is inserted through a guide hole in the center of the sizing plate into the glenoid. The guide pin is inserted either perpendicularly to the glenoid or with a slight superior tilt as determined by the position of the sizing plate. This ensures that an axis defined by the convex bearing head will be either perpendicular to the glenoid or with a slight inferior tilt, thus reducing the risk of scapular notching due to contact between the humeral epiphysis component and the scapula. The position of the guide pin determines the resulting position of the mounting plate as further steps preparing the surface of the glenoid are performed using the guide pin to locate the surgical instruments, as will be described below. The guide pin comprises a 2.5 mm diameter rod and is inserted 3-4 cm into the glenoid using a power tool. The sizing plate and positioning handle may then be removed by sliding over the guide pin. 
     The mounting plate comprises a circular disc having a slightly convex rear side to be mounted within a corresponding concave depression reamed on the glenoid surface. In order to prepare the glenoid surface a two step reaming process is required. In a first reaming step the glenoid is prepared using a powered circular reamer that is arranged to prepare a reamed portion of bone that is the same size as the mounting plate. As shown in  FIG. 10  the powered reamer  100  comprises a circular reaming shell driven by a power tool  102 . The reaming shell  100  and the power tool  102  are passed over the guide pin indicated by dashed line  104  by sliding a cannulated shaft over the guide pin such that the reaming position is fixed. The reaming shell  100  may be 27 mm in diameter to ream a concave depression on the glenoid surface corresponding to a typical mounting plate. 
     Although the mounting plate will be seated correctly after the initial reaming step, the convex bearing head to be mounted upon the mounting plate extends outside of the reamed area. In order to avoid conflict between the convex bearing head and the superior area of the glenoid it is necessary to ream the superior area of the glenoid outside of the first reamed area. As shown in  FIGS. 11 and 12 , a manually driven reamer  108  is used to ream the superior area  110  of the glenoid  106 . The reamer  108  comprises a cannulated shaft  112  which slides over the guide pin indicated by dashed line  104 . This ensures alignment of the second reaming step with the first reamed area. The reamer  108  comprises a guide portion  114  which is shaped to be received within the first reamed area. The guide portion  114  has a non-reaming lower surface which is arranged to slidably engage the first reamed portion of the glenoid as the cannulated shaft  112  is rotated about the guide pin. The guide portion  114  includes cut away portions  116  such that its position relative to the first reamed portion of the glenoid  106  can be observed. 
     Reamer  108  further comprises an eccentric reaming lobe  118  which extends from the guide portion  114  about a portion of the periphery of the guide portion  114 . Eccentric reaming lobe  118  has a reaming lower surface positioned to engage the superior area of the glenoid  106 . The reaming surface may comprise reaming formations, such as teeth, as is known in the art. By rotating cannulated shaft  112  about the guide pin while applying pressure towards the glenoid  106  the reaming surface of the eccentric reaming lobe  118  is arranged to remove surface portions of the glenoid, until the guide portion  114  is fully seated within the previously reamed portion of the bone. Once this is achieved, as shown in side view in  FIG. 12 , the glenoid surface is fully prepared to receive the mounting plate and the convex bearing head. 
     Advantageously, by providing the second reamer as an eccentric reaming lobe, the second reamer is reduced in size compared to a conventional circular reamer such as is used in the first reaming step. This allows the second reamer to be inserted through a smaller incision that would otherwise be the case. For instance, the maximum dimension (that is, the length) of the eccentric reaming lobe  118  may be approximately the same as the diameter of the guide portion  114 . The radial extent of the eccentric reaming lobe (from the edge of the guide portion  114  to the edge of the eccentric lobe) may be approximately 8 mm, which is approximately 0.3 times the diameter of the guide portion. Preferably the maximum length and the maximum radial extent from the guide pin of the eccentric reaming lobe is less than the diameter of the guide portion. 
     As the second reamer is eccentric, it is necessary to manually drive the second reamer such that the eccentric reaming lobe  118  can be rotated back and forth over the superior area of the glenoid in order reduce the impact on the remainder of the glenoid and surrounding tissue. However, if necessary, the second reamer can be used to remove other portions of the glenoid face anteriorly, posteriorly and inferiorly about the circular reamed mounting plate portion. 
     Optionally, after the second reaming step has been completed, the preparation of the glenoid can be checked by passing a glenoid level checker over the guide pin. The glenoid level checker comprises a disc of the same shape as the mounting plate and an eccentric lobe corresponding to the same amount of bone that is required to be removed from the superior area of the glenoid. The glenoid level checker includes cut outs so that the surface of the glenoid may be viewed while checking the reaming. No space should be visible between the glenoid level checker and the glenoid surface if the reaming has been completed correctly. If space is visible between the glenoid surface and the glenoid level checker then further reaming with either the first and/or the second reamer may be required. 
     It will be appreciated that in alternative embodiments the eccentric reamer may be varied. For instance, it could be modified to be driven by a motor with a reciprocating action such that the eccentric reaming lobe is repeatedly passed over the same portion of the glenoid surface. 
     After reaming of the glenoid is complete the guide pin is left in place and used as a drilling guide for drilling a central hole into the glenoid to receive a central pin of the mounting plate. A cannulated stop drill includes a central cavity to receive the guide pin is used. The cannulated stop drill includes a flange ensuring that the central hole is not over drilled. 
     The mounting plate comprises a disc sized and shaped to be received in the first reamed portion of the glenoid. The mounting plate further includes a central pin corresponding to the central hole drilled in the glenoid. The central pin incorporates a threaded bore for later attachment of the convex bearing head (as will be described in greater detail below). The exterior surface of the central pin is ribbed so as to form a push fit in the central hole. The mounting plate further comprises four fixation holes to receive fixing pins passing into the glenoid to secure the implant. Once the central pin is fully received in the central hole in the glenoid, if necessary the mounting plate may be rotated such that the inferior fixation hole is aligned with the inferior pillar of the glenoid. The surface of the mounting plate further comprises a vertical alignment mark to ensure correct orientation by aligning the vertical alignment mark with the scapular pillar inferiorly and the base of the coracoid process superiorly (that is, the vertical alignment mark is aligned with the long axis of the glenoid). The mounting plate may be gently impacted to ensure that the mounting plate pin is fully seated. Screws may then be implanted through the fixation holes to complete the implantation. The screws may be locking screws, as are known in the art, and may be such that the angle of implantation can be varied to ensure implantation into good bone stock. Alternative, any other suitable form of screw may be used. The mounting plate implantation is then secure and further humeral head preparation can be carried out. 
     To ream the resected humeral head so as to create a cavity to receive the epiphysis component of the humeral implant, it is necessary to insert an intramedullary reaming guide into the cavity in the reamed medullary canal. Referring to  FIG. 13 , the intramedullary reaming guide  200  comprises an elongate stem portion  202  which defines a longitudinal axis, and a neck portion  204 , which defines a neck axis inclined to the longitudinal axis. The intramedullary reaming guide  200  is provided in a range of sizes determined by the diameter of the stem portion  202 . The size of intramedullary reaming guide chosen is determined by the diameter of the intramedullary cavity reamed into the humeral head  62 , as described above. The intramedullary cavity defines a longitudinal axis, which is parallel to or aligned with the longitudinal axis of the humerus. Consequently, when the intramedullary reaming guide  200  is inserted into the intramedullary cavity, rotating the stem portion  202  rotates the neck portion  204  about the longitudinal axis of the humerus. 
     The neck portion  204  further comprises a flange  206 , such that when the intramedullary reaming guide  200  is fully inserted into the intramedullary cavity, further insertion is prevented by the flange  206 . Adjacent to the flange  206  is a reference formation  208 , comprising a rib. The reference formation  208  serves to ensure that any posterior offset when reaming the epiphysis cavity is precisely orientated relative to the neck portion  204 , as will be described in greater detail below. The reference formation  208  also allows the intramedullary reaming guide to be coupled to an alignment instrument, as will be described below. 
     The stem portion  202  further comprises at least one and preferably two ribs  210  arranged to cut into the cancellous bone around the intramedullary cavity as the intramedullary reaming guide  200  is driven into the intramedullary cavity. The ribs  210  prevent the fully inserted intramedullary reaming guide from being rotated about the axis of the stem once the intramedullary reaming guide  200  is fully inserted. Therefore, it is essential that the intramedullary reaming guide  200  is correctly orientated before being driven into the humerus. 
     As noted above, the neck portion  204  defines the reaming axis for reaming the humeral head  62  in order to create a cavity for the epiphysis portion of the humeral component. It is important to ensure the epiphysis cavity is correctly reamed, such that once implanted the rim of the epiphysis portion is exactly parallel to the resection surface. The rim of the epiphysis portion may be required to be congruent with the resection surface. Normally, this requires that the axis of the neck portion  204  is perpendicular to the resection surface, however the axis of the neck portion  204  may lie anywhere within a plane defined by the axis of the cavity and a line extending from the axis of the cavity perpendicular to the resection surface. To ensure that the axis of the neck portion  204  lies within this plane, the intramedullary reaming guide  200  must be correctly orientated before being driven into the bone and locked in position by the ribs  210 . 
     In order correctly orientate the intramedullary reaming guide  200  an alignment instrument  212  is provided as illustrated in  FIG. 13 . Alignment instrument  212  comprises a handle  214  and coupler  216  for coupling to the intramedullary reaming guide  200 . The coupler  216  comprises a clamp arranged to engage rib  208  on the intramedullary reaming guide  200 . Neck portion  204  is passed through coupler  216  until flange  206  comes to rest against the coupler  216 . The clamp may then be tightened onto rib  208  by turning knob  218 , which is coupled to internal rod  220  which in turn couples to the clamp. Knob  218  is turned until internal rod  220  is no longer visible. Knob  218  further comprises an impaction surface  218 A. Once the intramedullary reaming guide  200  is fully received in alignment instrument  212 , the longitudinal axis of handle  214  is aligned with the longitudinal axis of the stem component  202  while the stem portion  202  is partially received in the intramedullary cavity. Once correctly aligned, an impaction force can be applied to impaction surface  218 A drives the intramedullary reaming guide  200  fully into the cavity. 
     The alignment instrument  212  further comprises a plane finder  222 . Plane finder  222  comprises a plate having a surface which defines a plane forming an angle with respect to the longitudinal axis of the handle  214  which is the same as that at which the resection surface intersects the longitudinal axis of the humerus. Typically, this is 155°. If the neck portion  204  is arranged to be perpendicular to the resection surface, then this is the same angle at which the axis of the neck portion  204  intersects the axis of the stem portion  202  of the intramedullary reaming guide  200 . 
     Referring now to  FIG. 14 , this illustrates the intramedullary reaming guide  200  coupled to the alignment instrument  212 . As can be seen, the plane finder  212  is slidably mounted with respect to the handle  214  such that it can be raised and lowered parallel to the longitudinal axis of handle  214 . The plane finder  222  is formed as a horse shoe such that it can slide over the intramedullary reaming guide  200 . The plane finder  222  is further provided with parallel support bars  224 ,  226  which are arranged to be parallel to the longitudinal axis of the handle. Support bars  224 ,  226  are slidably received in holes passing through coupler  216 , which comprises a mounting bracket. Cross bar  228  prevents the plane finder  222  from being fully removed from the alignment instrument  212 . 
     The process of inserting the intramedullary reaming guide  200  into the intramedullary cavity begins with sliding plane finder  222  parallel to the axis of handle  214  until it is fully extended over the intramedullary reaming guide  200 . The stem portion  202  can then be progressively inserted into the intramedullary cavity until the plane finder  222  contacts the resection surface. The handle  214  (and thus the plane finder  222  and the intramedullary reaming guide  200 ) can then be rotated about the longitudinal bone axis until the plane defined by the surface of the plane finder  222  is parallel to the resection surface, as shown in  FIG. 15 . As can be seen in  FIG. 15 , the ribs  210  have not yet made contact with the cancellous bone of the humeral head  62  and so the alignment instrument  212  together with the intramedullary reaming guide  200  can freely rotate and slide within the intramedullary cavity. Once correctly positioned, the intramedullary reaming guide  200  can be driven home by applying an impaction force to impaction surface  218 A until the plane finder is brought up against mounting bracket  216  as shown in  FIG. 16 . At this point, ribs  210  engage the cancellous bone surrounding the intramedullary cavity and prevent further rotation of the intramedullary reaming guide  200 . 
     The flange  206  of intramedullary reaming guide  200  is received within a recess on the underside of mounting bracket  216 , and the plane finder  222  is similar received within a peripheral recess around the underside of mounting bracket  216 , such that when the plane finder  222  is in contact with both the resection surface and the mounting bracket  216  the intramedullary reaming guide  200  is fully inserted into the intramedullary cavity. The underside of flange  206  is in contact with the resection surface. The alignment instrument  214  can then be decoupled from the intramedullary reaming guide  200  by unscrewing knob  218  leaving the intramedullary reaming guide  200  in position with neck portion  204  protruding from the resection surface of the humeral head  62  as shown in  FIG. 17 . 
     As will be appreciated, the alignment of the neck portion  204  is directly related to the alignment of the support rod about the axis of the cavity during the initial resection step described above. That is, after the initial rotational alignment of the cutting guide assembly relative to the patient&#39;s forearm, each surgical step performed upon the humeral head  62  is intended to preserve that original orientation. 
     It will be appreciated that in alternative embodiments the plane finder may differ. For instance it need not be formed as a horse shoe, and may instead be any other shape such as an elongate bar. The only limitation to the shape of the plane finder is that it must be arranged to move relative to the longitudinal axis of the alignment instrument and arranged to contact the resection surface, such that rotation of the alignment instrument causes the plane finder to rotate until it is parallel to the plane of the resection surface. 
     After the intramedullary reaming guide  200  has been implanted, the humeral head is ready for reaming to create a cavity for the epiphysis component. As discussed above, the humeral component may either be a single integral implant incorporating both the stem component and the humeral component, or it may be modular in which different size stem components and epiphysis components can be coupled together. Advantageously this allows the epiphysis component to be offset from the position of the neck portion  204  of the intramedullary reaming guide  200  in a posterior direction, which can increase joint mobility. Furthermore, in order to achieve a more secure implantation, it is preferable to insert the stem component in an anatomic orientation referenced to the bicipetal groove (as discussed in greater detail below). However, the orientation of the epiphysis component may differ from the anatomic position according to the orientation chosen by the surgeon when resecting the humeral head, as discussed above. Consequently, the modular humeral implant allows for this variation (i.e., distal offset) between the stem component and the epiphysis component. 
     As will now be described, an instrument kit for reaming an epiphysis cavity allows for an optional posterior offset of the epiphysis component. Additionally, the diameter of the reamed epiphysis cavity may be varied. Advantageously, the center of the reamed epiphysis cavity and the size of the reamed epiphysis cavity may be chosen in order to ensure the best possible coverage of the resection surface (that is, the largest epiphysis cavity).  FIG. 18  illustrates the resected humeral head  62  with the neck portion  204  extending perpendicularly from resection surface (only the tip of neck portion  204 , flange  206  and reference formation  208  are visible). Positioned over the neck portion  204  is a centered adapter sleeve  300  which is arranged to ensure that reaming for the epiphysis cavity is centered about the neck portion  204  of the intramedullary reaming guide  200 . The centered adapter sleeve  300  comprises a generally cylindrical component, the outer surface of which comprises a reaming guide such that a reaming head having a cylindrical bore can be positioned over the adapter sleeve  300 , thereby ensuring that the reaming head is correctly aligned with the humeral head  62 . 
     Adapter sleeve  300  further comprises a bore  302  corresponding to and configured to accept the diameter of the neck portion  204 . The bore  302  extends to a proximal part of the adapter sleeve  300  such that the tip of the neck portion  204  is visible, thus confirming that the adapter sleeve  300  is fully seated on the intramedullary reaming guide  200 . At a distal end of adapter sleeve  300  is a collar  304 , comprising a groove shaped to accept the reference formation  208 . 
     Consequently, when the adapter sleeve  300  is fully seated on neck portion  204  it is prevented from rotating about the neck portion  204 . 
     The adapter sleeve  300  shown in  FIG. 18  is a centered adapter sleeve, that is the bore  302  is coincident with the outer surface of the adapter sleeve  300 , and thus is suitable for reaming where no posterior offset of the epiphysis cavity is required. As will be described below, adapter sleeves  300  with varying degrees of posterior offset (that is, non-coaxial bores  302 ) may be used to achieve a posterior offset of the epiphysis component. 
     In addition to allowing for variable posterior offset, the instrument kit further allows for different diameter epiphysis cavities to be reamed using reamers with different size reaming heads. However, before reaming begins, reaming sizing guides can be used to determine the correct size of reaming head. Referring to  FIGS. 19 and 20 , these respectively show the same centered adapter sleeve  300  illustrated in  FIG. 18  in combination with first and second sizing guides  306 ,  308 . The sizing guides  306 ,  308  comprise a sleeve  310 ,  312  arranged to fit over the outside of the centered reaming adapter  300  and a disc  314 ,  316 . Discs  314 ,  316  come to rest against the resection surface so that the surgeon may view what the diameter of the reamed epiphysis would be if a reamer having a reaming head of the same size as the sizing guide disc is used. Discs  314 ,  316  include cut outs so that the resection surface can be viewed through, as well as around, the disc. The sizing guide  308  shown in  FIG. 19  is a smaller, size 1, guide, and the sizing guide shown  308  in  FIG. 20  is a larger size 2 guide. The disc  314  of the size 1 guide  306  may be approximately 38 mm in diameter and the disc  316  of the size 2 guide may be approximately 42 mm in diameter. The bore of each sizing guide  306 ,  308  fitting over the adapter sleeve  300  is the same, thus allowing the adapter sleeves  300  and sizing guides  306 ,  308  to be interchanged. The sizing guides  306  may be color coded so that they can be matched to the same color (and same size) reaming head when reaming is conducted. 
     As can be seen in  FIGS. 19 and 20 , both sizing guide  306 ,  308  extend outside of the resection surface anteriorly. Consequently, it is apparent that the centered adapter sleeve  300  is not appropriate for this particular humeral head  62 . By providing a posterior offset using a posteriorly eccentric adapter sleeve, improved coverage of the resection surface can be obtained, as will now be described. 
     Referring now to  FIG. 21 , the centered adapter sleeve  300  shown in  FIGS. 19 and 20  has been replaced by a new adapter sleeve  318  which presents a posterior offset. The offset adapter sleeve  318  presents generally the same outer cylindrical shape such that it can receive the same sizing guides  306 ,  308  shown in  FIGS. 19 and 20 . However, for offset adapter sleeve  318 , the bore  320  which receives neck portion  204  is offset from the axis of the adapter sleeve defined by the outer cylindrical surface. Markings on the outside of the offset adapter sleeve  318  ensure that the offset is positioned in a posterior direction as shown in  FIG. 21  (which illustrates the arrangement for a right humerus). 
     As with the centered adapter sleeve  300  shown in  FIG. 18 , the bore  320  extends to a proximal part of the reaming adapter  318  such that the tip of the neck portion  204  is visible, thus confirming that the adapter sleeve  300  is fully seated on the intramedullary reaming guide  200 . At a distal end of adapter sleeve  318  is a collar  322 , comprising a groove shaped to accept the reference formation  208 . Consequently, when the adapter sleeve  300  is fully seated on neck portion  204  it is prevented from rotating about the neck portion  204 . As will be apparent, the groove  322  for offset adapter sleeve  318  is exactly the same as for centered adapter sleeve  300  and in the same relationship with bore  320  in order to ensure a correct fit over neck portion  204 . However, the groove  322  is offset from the central axis of the offset adapter sleeve  318  such that the groove is defined by two fingers of differing thicknesses. 
     As noted above, both centered adapter sleeve  300  and offset adapter sleeve  318  are generally cylindrical and have the same exterior diameter to ensure compatibility with the sizing guides  306 ,  308 . The exterior diameter of the adapter sleeves  300 ,  318  is larger than the diameter of flange  206  to ensure that even for the offset adapter sleeve  318  the exterior surface of each adapter sleeve extends further outwards than the flange  206  and the reference formation  208  in all radial directions about the neck portion  204 . This ensures that when a reaming head is passed over the adapter sleeves  300 ,  318  (or any other adapter sleeve with a different degree of posterior offset), there is no contact between the reaming head and the intramedullary reaming guide. 
     Referring to  FIG. 22 , the smaller size 1 sizing guide  306  is positioned over offset adapter sleeve  318  to check for coverage of the resection surface. Offset reaming guide  318  is color coded the same color as the size 1 sizing guide  306 . Six different epiphyses are available for the final implant: first and second size centered epiphyses (corresponding to the size 1 and size 2 sizing guides shown in  FIGS. 19 and 20 ), an epiphysis with a first degree of offset having the diameter of the size 1 sizing guide shown in  FIG. 22  (in left and right shoulder options) and an epiphysis with a second, larger, degree of offset having the diameter of the size 2 sizing guide shown in  FIG. 23  discussed below (also in left and right shoulder options). Each offset epiphysis has left and right options, which are mirror images of one another. 
     While a reamer matched to the size of the larger sizing guide could be used on the adapter sleeve shown in  FIGS. 21 and 22 , this would not match a final implant epiphysis. Consequently, while either size sizing guide may be used in conjunction with the centered adapter sleeve, the correctly color matched sizing guide must be used with each offset adapter sleeve. If the bone coverage is not sufficient then a new adapter sleeve  324  with an increased posterior offset as shown in  FIG. 23  can be used in combination with the size 2 sizing guide  308 . Increased offset adapter sleeve  324  is color coded the same color as the size 2 sizing guide  308 . 
     In alternative embodiments of the present invention there may be any number of adapter sleeves with differing degrees of offset. Similarly, there may be any number of sizing guides, which may be used with any adapter sleeve (offset or centered). However, it will be appreciated that such flexibility would necessarily be at the expense of having to provide a larger number of different sized and shaped epiphyses for the final implant to account for all possible combinations of size of offset and size of reaming head (corresponding to the sizing guide). 
     The color (and hence size) of the chosen sizing guide  306 ,  308  must be matched to the same color reaming head. Careful note must be taken of whether a centered or which posterior offset adapter sleeve is used, and the size of the reaming head used as this determines which epiphysis component to use during final implantation of the humeral component. 
     Once the optimal adapter sleeve and sizing guide have been selected the sizing guide is removed and the matching reaming head  326  is passed over the adapter sleeve such that powered reaming of the epiphysis cavity can begin as shown in  FIG. 24 . Reaming head  326  comprises a reaming shell  328  including spaced apart reaming formations  330  and an exterior smooth reamer flange  332 . Reaming is complete when the exterior reamer flange  332  is fully in contact with the resection surface around the whole of the reamer shell  328 . 
     Once reaming of the humeral head  62  is complete the reaming head  326  and the adapter sleeve can be removed from neck portion  204 . The intramedullary reaming guide  200  can then be extracted from the intramedullary cavity by connecting the intramedullary reaming guide  200  to the alignment instrument  212  shown in  FIG. 13  and pulling axially out of the intramedullary cavity. If any cancellous bone remains unreamed within the epiphysis cavity around the previous position of the intramedullary reaming guide  200  then this can be manually removed. 
     After reaming of the epiphysis cavity is complete, the intramedullary cavity must be enlarged in order to accommodate the stem portion of the humeral component. As described above, the intramedullary cavity is initially formed as a continuous diameter reamed bore. At a distal portion, the stem portion comprises a corresponding diameter shaft (a range of diameter stem portions being available corresponding to the largest size reamer used to create the intramedullary cavity). However, proximally, the stem portion comprises anterior and posterior ribs and, optionally, a pronounced medial rib, all of which serve to prevent rotation within the intramedullary cavity and also to increase the engagement of the stem portion with cancellous bone. Therefore, the intramedullary cavity must be enlarged in these areas. 
     As discussed above, the resection surface, and hence the position of the epiphysis component, can be orientated about the longitudinal axis of the bone defined by the intramedullary cavity in order to provide a desired degree of retroversion or anteversion to the reverse shoulder prosthesis. Consequently, the resection surface may be rotationally offset from the anatomical position (that is, the rotational position of the natural humerus neck axis about the longitudinal axis of the humerus). However, it is advantageous to insert the stem portion in an anatomical position in order to increase the strength of the joint. Additionally, this provides the maximum amount of cancellous bone for the stem portion to engage. Therefore, it is necessary to measure the rotational offset between the rotational position of the stem portion cavity (that is, the anatomical position of the natural humerus if the stem portion is exactly aligned with the anatomical position) and a line extending normal to the resection surface and intersecting the longitudinal axis of the intramedullary cavity. This measurement may either be performed at the same time as enlarging the intramedullary cavity or as a separate processing step. The measured rotational offset may then be used during assembly of the humeral component to rotationally offset the stem portion and the epiphysis portion. It is important to correctly measure this offset in order to ensure that the rim of the epiphysis portion is parallel to the resection surface. Typically, the rim of the epiphysis component is required to be congruent with the resection surface. 
     Referring now to  FIG. 25 , this illustrates a broach  400  and a broach insertion instrument  402 . The broach insertion instrument  402  incorporates means for measuring the rotational offset. The broach  400  comprises at a distal portion a smooth shaft of a corresponding diameter to that of the intramedullary cavity reamed before.  FIG. 25  illustrates the broach  400  partially inserted into the humeral head  62 . At a proximal end the broach  400  comprises cutting teeth  404  adapted to engage and cut into cancellous bone as the broach  400  is driven into the humeral head  62 . Cutting teeth  404  form an anterior cutting fin  406 , a medial cutting fin  408  and a posterior cutting fin (not visible in  FIG. 25 ). 
     The enlarged portion of  FIG. 25  illustrates from a superior angle the anterior cutting fin  406 . To ensure that the broach  400  (and hence, the implanted stem portion) are in the anatomic position, the anterior cutting fin  406  should be aligned with the anterior aspect of the bicipital groove  410 . 
     The broach insertion instrument  402  includes an engagement mechanism  412  for engaging the distal end of broach  400  that may comprise a clamp which is engaged by manipulating lever  414 . The instrument  402  also comprises a handle portion  416 , which terminates at an impaction surface (not shown in  FIG. 25 ) to which an impaction force may be applied to drive the broach  400  into the intramedullary cavity. 
     The instrument further comprises a depth stop  418 , which comprises a rocker bar extending through a portion of the instrument proximal to the broach engagement mechanism  412 . The rocker bar  418  pivots within the instrument  402  and extends from the instrument  402  on the anterior and posterior sides. The rocker bar comprises a plane finder. As broach  400  is driven into the intramedullary cavity the rocker bar contacts the resection surface at the cortical shell of the humeral head  62  and aligns itself with the plane of the resection surface. In the event that the resection surface is oriented in the anatomical position (that is, there is no rotational offset) both arms of the rocker bar  418  will contact the resection surface at the same time. However, if the resection surface is retroverted or anteverted one or the other arm of the rocker bar  418  will contact the resection surface first, causing the rocker bar to pivot about its mid point. Insertion of the broach  400  into the intramedullary cavity continues until both arms are in contact with the resection surface. The rocker bar  418  therefore ensures the correct extent of insertion of the broach  400  into the intramedullary cavity, and therefore ensures the cavity is correctly sized to receive the stem portion. Increased rotational offset results in an increased pivot angle of the rocker bar  418  relative to the broach insertion instrument  402 . 
     Referring to  FIG. 26 , this illustrates the broach insertion instrument  402  once the broach  400  has been fully inserted into the intramedullary cavity. As can be seen, both arms of rocker bar  418  are in contact with the resection surface. The rocker bar  418  is pivoted with respect to the instrument handle  416  due to the resection surface being retroverted relative to the anatomical position of the broach  400 . The instrument  402  further comprises a yoke  420  which is slidably mounted on the instrument  402  such that it can slide in the plane in which the rocking bar  418  pivots. Yoke  420  comprises legs  422  that are configured to contact the rocking bar  418 . When the rocking bar  418  pivots, it causes the yoke  420  to rise up away from the rocker bar  418 . It will be appreciated that the plane in which yoke  420  slides may differ from the pivot plane of rocker bar  418  and that the yoke  420  will still move so long as its plane is not perpendicular to the rocker bar pivot plane. 
     If there is no rotational offset (zero retroversion) both legs  422  will be in contact with the rocker bar  418  and the yoke will not rise up from its rest position. However, once the rocker bar  418  begins to pivot, only one leg  422  will be in contact with the rocker bar  418 . Yoke  420  slides within parallel grooves formed in the sides of insertion instrument  402 . The yoke  420  is constrained by these grooves such that it cannot pivot, the degree to which the yoke  420  rises up is the same regardless of which arm of rocking bar  418  is rising up. 
     It will be appreciated that an increased rotational offset will cause the rocker bar  418  to pivot by an increased amount. The direction in which rocker bar  418  pivots (that is, which arm is uppermost) is dependent upon whether the resection surface is retroverted or anteverted. The amount by which yoke  420  rises up when rocker bar  418  pivots it directly proportional to the magnitude of the of the rocker bar pivot, and hence is indicative of the rotational offset between the stem portion and the epiphysis portion. The enlarged portion of  FIG. 26  illustrates a scale  424  applied to either side of the yoke  420 . Scale  424  is read at a reference mark  426  on the body of the broach insertion tool. When there is no rotational offset (rocker bar  418  is level and the yoke  420  is at its lowest position) the reference mark  426  indicates 0° retroversion. Scale  424  is calibrated to directly indicate the rotational offset. Therefore, if the resection surface is not in the anatomical position reading scale  424  will indicate the degree of rotational offset (in  FIG. 26  the scale  424  indicates up to a 30° rotational offset). Whether the rotational offset is in respect of retroversion or anteversion is determined by noting which arm is uppermost (and in contact with yoke  420 ). If the anterior arm of rocker bar  418  is uppermost, the resection surface is retroverted and if the posterior arm is uppermost, the resection surface is anteverted. 
     It will be appreciated that in alternative embodiments of the present invention the broach insertion instrument may comprise alternative means for measuring movement of the yoke (or other plane finder component) such as electronic detection means. 
     As will be appreciated, it is advantageous for the rocker bar  418  to be as long as possible, and for the yoke legs  422  to contact the rocker bar  418  as far apart as possible as this amplifies the degree to which the yoke  420  rises up for a given rotational offset. Typically, the rocker bar is 54 mm, though it will be appreciated that the length of the rocker bar must be greater than the diameter of the cavity reamed in the resection surface of the epiphysis. The proximal place of the implanted epiphysis has a diameter which typically ranges between 38 mm and 41 mm according to the required size of the implant. Therefore, the rocker bar may vary between 40 mm and 70 mm. The yoke legs  422  are arranged to contact the rocker bar  418  towards either end of the rocker bar  418 , for instance spaced apart by between 40 mm and 70 mm. 
     It will be appreciated that other mechanisms for measuring the rotational offset could be provided. The rocker bar  418  constitutes a plane finder adapted to alter its position relative to the broach insertion tool to conform to the plane of the resection surface. Specifically, the movement is a pivot motion about an axis which is perpendicular to the axis of the broach insertion instrument handle  416 . The movement relative to the handle  416  could take other forms. For instance, the plane finder may be formed as a plate having a plane which intersects the axis of the handle  416  at the same angle as that at which the resection surface intersects the longitudinal axis of the intramedullary canal. The plane finder may be arranged to be rotatable about the handle  416  such that as the broach  400  is driven into the bone the plane finder slides round until its plane is congruent with the resection surface. The rotation of the plane finder about the handle  416  may then be measured and is equivalent to the rotational offset. As noted above, the reaming and measurement steps may be separated such that a separate measurement tool could be used have a first component for insertion into the intramedullary cavity and a plane finder as discussed above. 
     Referring now to  FIG. 27 , this illustrates the humeral component for insertion into the reamed humeral head  62 . Alternatively, the component illustrated in  FIG. 27  may be a trial component for insertion and testing prior to insertion of the final component.  FIG. 27  illustrates a proximal part of the stem portion  428  and the epiphysis portion  430 . The stem portion  428  further comprises a reference formation  432  and the underside of the epiphysis  430  comprises a series of notches  434  and a scale  436 . An upper surface of the stem  428  further comprises a single protrusion (not visible in  FIG. 27 ) arranged to engage one of notches  434 . The measured rotational offset of the enlarged intramedullary cavity and the epiphysis cavity (measured using the yoke system of the broach insertion instrument  402  of  FIGS. 25 and 26 ) according to whether the offset is retroverted or anteverted, is used to select the appropriate notch  434  into which to engage the stem protrusion, as selected by aligning reference formation  432  with the required position on scale  436 . A locking screw (not visible in  FIG. 27 ) passes through the epiphysis portion into the stem portion, thereby locking the humeral component and preserving the selected degree of retroversion or anteversion such that the humeral component conforms to the shape of the cavity reamed in the humeral head  62 . The stem portion  428  is selected according to the diameter of the reamed intramedullary cavity. The epiphysis portion  430  is selected from an available range according to whether the reamed epiphysis cavity was centered or posteriorly offset, and according to the size of the epiphysis reaming head used. 
     Once assembled, the humeral implant can be manipulated using a humeral component driver which comprises means for releasably engaging the inside part of the epiphysis component. This allows the humeral component to be inserted into the intramedullary cavity without contacting the exterior surface of the implant (thereby preserving the hydroxyapetite coating which serves to encourage bone in growth securing the implant in position). The humeral component driver incorporates alignment holes to receive an alignment pin similar to the alignment pin shown in  FIG. 3  allowing the alignment of the humeral component to be referenced to the patient&#39;s forearm, thereby ensuring that the humeral component is aligned with the resection surface. Furthermore, during insertion of the humeral component, the anterior rib of the stem component is aligned with the anterior aspect of the bicipetal groove similar to the alignment of the broach shown in  FIG. 25 . 
     As noted above, in place of the modular humeral component an integral humeral implant comprising both a stem and an epiphysis may be used. The integral humeral component is particularly suited to applications in which the humeral component is secured using bone cement. The surgical steps for preparing the intramedullary cavity to receive the integral implant are generally the same as described above for the modular implant. However, it is not possible to provide a posterior offset for the epiphysis. Consequently, only a single, centered reaming adapter is provided for reaming the epiphysis cavity, although a choice of size of reaming head is available and reaming sizing guides may be used to determine the reaming head to be used, as described above. It is not necessary to enlarge the intramedullary cavity using a broach to receive an integral component as fixation is achieved using bone cement and therefore the humeral component stem does not incorporate fins. During insertion of the humeral component into the intramedullary cavity rotational alignment of the component and the resection surface is achieved by using an alignment pin orientated to be parallel to the patient&#39;s forearm axis, as described above for the modular humeral implant. 
     Once the humeral implant is in position, the convex bearing head can be attached to the mounting plate. As with the humeral component, a trial convex bearing head may first be attached so that the optimal positioning and size of the convex bearing head can be determined. The convex bearing head comprises a convex dome including a recessed cavity on the reverse side corresponding to the size and shape of the mounting plate. As is shown generally in  FIG. 30  and described in more detail below, a hole passes through a central portion of the convex bearing head such that the convex bearing head can be secured to the mounting plate by passing a screw into a threaded socket extending into the central pin of the mounting plate. At its broadest point the convex bearing head is preferably either approximately 38 mm or 42 mm in diameter according to the chosen size, but the size may range from 35 mm to 45 mm. Additionally, the convex bearing head may be circular or eccentric about the screw hole. The convex bearing head preferably overlaps the glenoid inferior limit by about 3 mm to 5 mm. The overlap reduces contact between the humeral epiphysis and the scapular pillar during rotation of the shoulder. 
     When securing the convex bearing head to the mounting plate a 1.5 mm diameter guide pin may be inserted into the central hole in the mounting plate in order to ensure correct alignment of the convex bearing head. A fixation hole within the convex bearing head is passed over the guide pin until the recessed cavity on the reverse side of the convex bearing head is in contact with the mounting plate. A fixing screw includes an axial bore configured so as to permit the fixing screw to pass over the guide pin. The fixing screw can be tightened using a cannulated hexagonal screw driver. Once the fixing screw is engaged in the threaded bore within the mounting plate central pin the guide pin can be removed before fully tightening the screw. The screw is preferably tightened until the scapula begins to rotate in response to motion of the screw driver. 
     For an eccentric convex bearing head, it is important that the eccentricity is in the correct radial position. The maximum eccentricity should be directed towards the base of the glenoid. Referring to  FIG. 28 , in order to rotate the convex bearing head upon the mounting plate the convex surface of the convex bearing head  500  incorporates a reference formation  502 . The reference formation  502  can be manipulated by a convex bearing head orientation guide  504 , which is arranged to slide over the screw driver. As shown in  FIG. 28 , the reference formation  502  comprises an eccentric slot enlarging part of the fixation hole  506 . The fixation hole  506  including slot  502  is shown more clearly in a front view of the convex bearing head  500  in  FIG. 31 . The convex bearing head orientation guide  504  includes a pin  508  which protrudes from the body of the guide  504  and is configured to be received within the slot  502 . The orientation guide  504  may further comprise a circular guide arranged to be received within the fixation hole  506 . The screw driver is passed through the guide  504  and engages the fixing screw within fixation hole  506 . The convex bearing head  500  can be rotated about the fixation hole  506  by manipulating the body of the convex bearing head orientation guide  504  while tightening the fixing screw. 
     Referring to  FIG. 29 , the outside surface of the convex bearing head orientation guide  504  preferably includes an arrow  510 . The correct rotation position of the convex bearing head  500  can be achieved by rotating the convex bearing head orientation guide  504  (and hence the convex bearing head  500 ) until the arrow  510  is aligned with a predetermined part of the scapula  512 . For instance, if the required radial position of maximum eccentricity is towards the inferior portion of the glenoid, the reference formation  502  (which is aligned with the maximum eccentricity) should also point to the inferior portion of the glenoid. The arrow  510  is on the opposite side of the orientation guide  504  from the pin  508  and should be aligned with the superior portion of the glenoid. In particular, the convex bearing head  500  and the orientation guide  504  should be rotated until the arrow  510  points to the base of the coracoid process in order to correctly align the convex bearing head before tightening the fixing screw while maintaining the guide  504  in position. The convex bearing head may be further secured to the mounting plate by applying an impaction force to the convex bearing head and then further tightening the fixing screw. 
     It will be apparent to the skilled person that in alternative embodiments the reference formation may differ from that illustrated in  FIG. 28 , for instance it could be a different shape or may not be formed together with the fixation hole. 
     Referring to  FIG. 30 , this illustrates a cross section view of the orientation guide  504  engaged with the convex bearing head  500 . Guide pin  508  is received within slot  502  within the fixation hole  506 . A screw driver  514  is shown extending through an axial lumen within the orientation guide  504  to engage fixing screw  516  which secures the convex bearing head  500  to the mounting plate  518 . In alternative embodiments in which the reference formation is not combined with the fixation hole, the alignment guide and the screwdriver may be provided separately. 
     The reverse shoulder prosthesis is completed by positioning a cup in a recess in the upper surface of the epiphysis. The cup presents a concave bearing surface in which the convex bearing head is received. The size of the cup (for example, 38 mm or 42 mm in diameter) is chosen to match the size of the convex bearing head. Additionally, the cup is available in a range of thicknesses. The cup thickness chosen is dependent upon the precise positioning of the resection surface and the mounting plate. If the implanted prosthesis results in an insufficiently tensioned shoulder joint (in which the joint tends to dislocate during motion) then a thicker cup may be used to increase the tension by increasing the distance between the scapula and the humerus. 
     It can be necessary to change the humeral component to the anatomic configuration (and also to change the glenoid component to the anatomic configuration). This may either be during a revision procedure due to glenoid loosening or during the initial surgical procedure to implant the prosthesis if it becomes apparent that there is insufficient glenoid bone stock to attach the mounting plate after the point at which the humerus has been resected. 
     In order to change the humeral component to the anatomic configuration it is necessary to remove cortical bone in the medial and lateral regions around the humeral head. This is because the anatomic head to be fitted to the implanted humeral implant overlaps the cortical bone in these regions. It is important to minimize any disturbance the humeral stem during this bone preparation stage. 
     The first step is to remove the humeral cup from the epiphysis. A reaming guide  600  can then be inserted into the cavity within the epiphysis  602  as shown in  FIG. 32  and connected to the existing epiphysis  602  by a taper junction. That is, the epiphysis  602  comprises a shallow cavity formed generally as a portion of a cone. The sides of the open cavity formed in the epiphysis  602  diverge towards the open end. Similarly, the reaming guide  600  generally comprises a disc having tapering edges arranged to match the taper of the epiphysis open cavity. The taper junction forms a firm connection between the epiphysis  602  and the reaming guide  600  during reaming of the bone, while allowing the reaming guide  600  to removed later. The reaming guide  600  comprises two sockets  604  that define reaming axes. The reaming axes diverge as they extend from the epiphysis  602 . The reaming guide  600  is positioned in the epiphysis  602  such that anterior and posterior slots  606  are aligned with corresponding slots  608  in the rim of the epiphysis  602 . The reaming guide  600  forms a press fit with the epiphysis component cup temporarily securing guide  600  in position. The reaming axes are directed medially and laterally. 
     As shown in  FIG. 33 , an appropriate reaming head  610  is driven about an axial guide  615  that is arranged to be inserted into socket  604 . The reaming head  610  is driven by a means (not shown) to remove cortical bone in the lateral (or medial, depending on the chosen axis) direction without contacting the implanted stem component. The reaming head  610  comprises a reaming ring  612  that is configured to pass around the epiphysis  602  contacting the cortical bone immediately surrounding the epiphysis  602  predominantly in the medial (or lateral) direction. Reaming ring  612  is driven radially about axial guide  615  to remove bone on the proximal portion of the humerus. 
     As shown in  FIG. 34 , once bone is removed on the medial and/or lateral portions of the humerus, an appropriately sized humeral head implant  614  is secured to the epiphysis forming a press fit in the epiphysis cavity overlapping the epiphysis and the cortical bone in the medial and lateral directions. 
     Although surgical instruments and techniques described above are primarily related to a reverse shoulder prosthesis implantation procedure it will be appreciated that some or all of the surgical instruments and surgical techniques described may be equally applicable elsewhere. For instance, they may find utility in the implantation of other prostheses, such as a hip prosthesis. Additionally, some or all of the surgical instruments and techniques described may be equally applicable to the implantation of anatomic prostheses as opposed to reversed anatomy prostheses. 
     Other modifications and applications of the present invention will be readily apparent from the description herein without departing from the scope of the appended claims.