Patent Publication Number: US-2010125336-A1

Title: Reverse shoulder prosthesis

Description:
The present invention relates to a shoulder prosthesis, and in particular to a scapula component and a humerus component which together form a reverse shoulder prosthesis. 
     Reverse (or inverse) shoulder prostheses are known. Such prostheses are termed reverse as they reproduce the scapula-humerus interaction in reverse. In a normal shoulder, a spherical element located on the proximal end of the humerus interacts with the glenoid cavity on the scapula. In a reverse shoulder prosthesis the spherical element is located on the scapula and the cavity, which interacts with the spherical element, is located on the proximal end of the humerus. Reverse shoulder prostheses are typically used in the event of serious muscular degeneration of the shoulder, particularly of the rotator cuff muscles. The degeneration of such muscles causes a prevalent action by the deltoid which tends to draw the humerus upwards and raises the risk of it contacting the acromion on the scapula. To avoid this contact the shoulder joint is reversed. The reverse shoulder prosthesis provides stability to the joint and maximises mobility of the shoulder. Reverse shoulder prostheses are the most common form of prosthesis used to treat severe rotator cuff tears. 
     A significant problem with existing reverse shoulder prostheses is impingement of the humerus component of the prosthesis on the inferior border of the scapula. Contact with the scapula caused by the impingement may create notches in the bone. These notches may result in bone loss and loosening of the scapula component of the prosthesis. Studies have shown that in 62% of patients with reverse shoulder prostheses, notching is observed within 2 years of inserting the prosthesis. Modelling studies have shown that especially in low-level tasks, requiring only a low degree of humeral elevation, impingement can occur. There is therefore a need for a reverse shoulder prosthesis which reduces or eliminates impingement. Such a device may extend the life of the prosthetic device, reduce the risk of potential dislocation and reduce the risk of damage to the scapula. 
     According to a first aspect, the invention provides a scapula component for a reverse shoulder prosthesis comprising a part spherical, convex, surface which is less than a hemisphere and wherein the part spherical surface does not have a rotational axis of symmetry. 
     The part spherical surface may have one or more planes of symmetry. Preferably the part spherical surface only has one plane of symmetry. 
     Preferably the part spherical, convex, surface comprises a main circular surface portion and one or more subsidiary surface portions depending from the edge of the main surface portion. 
     According to a second aspect, the invention provides a scapula component for a reverse shoulder prosthesis comprising a part spherical, convex, surface which is less than a hemisphere, wherein the part spherical, convex, surface comprises a main circular surface portion and one or more subsidiary surface portions depending from edge of the main surface portion. 
     Preferably the edge of the main circular surface portion is defined by a plane which intersects the part spherical surface. 
     Preferably the main circular surface portion is the area bounded or defined by the largest circle that will fit onto the part spherical surface. 
     The one or more subsidiary surface portions may depend from only a part, or from all, of the edge of the main circular surface portion. 
     Preferably the scapula component comprises two subsidiary surfaces on opposite sides of the main surface portion. Preferably the two subsidiary surfaces are crescent shaped. 
     Preferably the scapula component comprises a third subsidiary surface which extends from the circular edge at the top of the main surface portion. 
     The main surface portion and the one or more subsidiary surfaces together form the part spherical surface. All the surfaces which form the part spherical surface have the same radius of curvature. The overall shape of the part spherical surface is preferably determined by where load forces will be placed on the surface of the scapular component when it is in use. An aim is to have a part spherical surface which includes only the load bearing those surfaces which would be present if the surface was hemispherical. 
     Preferably the transition between the main surface portion and the subsidiary surface portions is smooth, and may form one continuous surface with a constant radius of curvature. 
     Preferably the part spherical surface does not have a rotational axis of symmetry. The part spherical surface may have one or more planes of symmetry. 
     Preferably the main surface portion of the part spherical surface has a rotational axis of symmetry. 
     Preferably the surface area of the main surface portion of the part spherical surface is less than the surface area of a hemisphere. Preferably the surface area of the part spherical surface is less than the surface area of a hemisphere. 
     Preferably the surface area of the part spherical surface is less than the surface area of a hemisphere with the same radius of curvature as the part spherical surface. 
     Preferably the two most distant points on the edge of the part spherical surface are spaced apart by less than two times the radius of curvature of the part spherical surface. 
     Preferably the surface area of the part spherical surface is at least about 5% less than the surface area of a hemisphere which has the same radius of curvature as the part spherical surface. Preferably the surface area of the part spherical surface is between about 5% and about 30% less, more preferably between about 20% and about 30% less, than the surface area of a hemisphere which has the same radius of curvature as the part spherical surface. Preferably the surface area of the part spherical surface is at least about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30%, less than the surface area of a hemisphere which has the same radius of curvature as the part spherical surface. 
     Preferably the surface area of the main surface portion of the part spherical surface is at least about 5% less than the surface area of a hemisphere with the same radius of curvature as the part spherical surface and the main surface portion. Preferably the surface area of the main surface portion is between about 5% and about 35% less, more preferably between about 20% and about 35% less, than the surface area of a hemisphere which has the same radius of curvature as the part spherical surface and the main surface portion. Preferably the surface area of the main surface portion is at least about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 or 35%, less than the surface area of a hemisphere which has the same radius of curvature as the part spherical surface and the main surface portion. 
     Preferably the volume occupied by the portion of the scapula component with a part spherical surface is between about 5% and about 50% less than the volume that would be occupied by a hemisphere with the same radius of curvature, preferably the volume occupied is between about 10% and about 30% less. 
     Preferably the volume occupied by the scapula component of the invention is less than the volume a hemisphere comprising the same radius of curvature as the part spherical portion of the scapula component would occupy. 
     Preferably the part spherical surface defines a bearing surface which in use contacts the humerus component of a reverse shoulder prosthesis. 
     Preferably the part spherical surface forms the front face of the scapula component. 
     The part spherical surface may have a radius of curvature of between about 10 mm and about 30 mm, preferably between about 15 mm and about 25 mm, more preferably between about 18 mm and about 22 mm. 
     In known reverse shoulder prostheses the scapula component comprises a substantially hemispherical surface which forms the bearing surface. The requirement that the component is hemispherical has dictated the size of the scapula component that can be used in a reverse shoulder prosthesis. Insertion of the scapula component during shoulder replacement surgery is difficult, and there is little room for the surgeon to manoeuvre. Surgeons have therefore preferred to use smaller scapula components, typically with a hemispherical bearing surface having a diameter of between 36 and 38 mm (a radius of curvature of 18 or 19 mm). Although components with larger diameters, say 40 or 42 mm (radius of curvature of 20 or 21 mm) are known, the increase in size makes them difficult to position in the scapula and therefore unpopular with surgeons and much less commonly used. 
     However, a scapula component according to the invention in which the part spherical surface is less than a hemisphere, and is shaped to retain only those surfaces which are needed to take the bearing/forces put on the component in use, takes up less volume than a hemispherical component with the same radius of curvature. A scapula component according to the invention with a part spherical surface which has a radius of curvature of between about 18 and about 22 mm, may take up a volume which is same, or less than a conventional substantially hemispherical prostheses with a diameter of 36 or 38 mm (a radius of curvature of 18 or 19 mm). By way of example, a scapula component according to the invention with a radius of curvature of 21 mm may be configured to occupy the same, or less, volume than a Delta® III scapula component from DePuy™ with a diameter of 36 mm (a radius of curvature of 18 mm). 
     The advantage of providing a surface which is part spherical, but less than a hemisphere, is that the advantages of having a surface with a larger radius of curvature can be obtained without having to increase the overall size and/or volume of the scapula component. Having a bearing surface with a larger radius of curvature minimises the risk of impingement of the underlying bone, as the larger radius of the bearing surface of the scapula component drives the cup on the humeral component away and therefore reduces the risk of impingement. 
     Scapula components according to the invention may therefore be smaller, than known hemispherical components with the same radius of curvature, and may therefore be easier to insert during surgery. 
     Preferably the scapula component comprises a mounting means to mount the scapula component either directly or indirectly to the scapula. 
     The mounting means may be a hole or a screw or any other suitable fixation means which extends from the front of the scapula component to the back of the scapula component. Preferably the mounting means has a longitudinal axis. 
     Preferably the main surface portion of the part spherical surface of the scapula component has a rotational axis of symmetry that is offset at an angle from the longitudinal axis of the mounting means. Preferably the two axes are offset by an angle of between about 5 degrees and about 45 degrees, preferably the two axes are offset by an angle of between about 10 and about 30 degrees. Preferably the two axes are not laterally displaced. 
     The benefit of offsetting the rotational axis of symmetry of the main surface portion of the part spherical surface, relative to the longitudinal axis of the mounting means, is that it allows the part spherical surface of the scapula component to be more easily positioned for contact with a humeral component when in use. 
     Preferably the surface area of the part spherical surface is less than the surface area of a hemisphere with the same radius of curvature due to the removal of material at the base of the hemisphere. Preferably the part of the hemisphere that is missing in the part spherical surface would not have been load bearing if it had been present in the scapula component of the invention. 
     In addition to the part spherical surface the scapular component may also comprise a part cylindrical surface. Preferably the part spherical surface and the part cylindrical surface are on the front of the scapular component. 
     Preferably the part cylindrical surface extends from the back of the part spherical surface. Preferably, the part cylindrical surface does not form part of the bearing surface of the scapula component. The part cylindrical surface may be included on the scapula component to assist in fixation of the scapula component to the scapula, either directly or indirectly. 
     The scapula component may be attached directly or indirectly to the scapula. If the component is attached indirectly a mounting member may be used. The mounting member may attach on one side to the scapula and on the other side to the scapula component. Preferably the mounting member has a convex back surface which, in use, contacts the scapula. 
     According to a further aspect the invention provides a humerus component for a reverse shoulder prosthesis comprising a cup member having a part spherical, concave, bearing surface surrounded by an oval rim. 
     Preferably the part spherical bearing surface of the cup member is configured to match, and in use contact, the part spherical bearing surface of a scapula component. 
     The part spherical surface of the cup member may have a radius of curvature of between about 10 mm and about 30 mm, preferably between about 20 mm and about 25 mm, more preferably between about 18 mm and about 22 mm. 
     Preferably the rim surrounding the part spherical bearing surface is not of an even width. Preferably the rim includes a wider portion which extends around between about 10% and about 50% of the edge of the part spherical bearing surface. Preferably, in use, the wide rim portion is located inferior to the narrow rim portion. Preferably the transition between the wide rim portion and the narrow rim portion is gradual. In use the wide rim portion is intended to be positioned where the cup portion will bear most of the load from the scapula component. This is typically in an inferior position relative to the narrow rim portion. Preferably the rim is between about 0.2 mm and about 10 mm wide. 
     The wide and narrow profile of the rim defines the oval mounting in which the spherical bearing surface is located. 
     Preferably the rim is chamfered. Preferably it is chamfered away from the edge of the part spherical bearing surface. Preferably the rim is chamfered to reduce the risk of impingement of the scapula by the humerus component, without compromising joint stability. 
     The humerus component may be arranged to be mounted directly to the humerus of a patient, or it may be arranged to be mounted via a cup support and/or an anatomical stem. 
     Preferably the humerus component is arranged to be mounted at an angle between about 55 and about 70 degrees, preferably about 60 degrees, relative to the longitudinal axis the mounting stem and/or to the longitudinal axis of the humerus. At an angle of about 60 degrees the risk of impingement on the scapula is reduced. 
     According to a further aspect, the present invention provides a reverse shoulder prosthesis comprising a scapula component according to the first or second aspect of the invention and a humerus component according to the third aspect of the invention. 
     The part spherical (bearing) surface of a scapula component according to the invention may be made of stainless steel, titanium, cobalt chrome, a combination thereof or any other suitable material. 
     The bearing surface of a humerus component according to the invention may be made of polyethylene, such as ultra high molecular weight polyethylene which may have been cross-linked, or any other suitable material. 
    
    
     
       Embodiments of the present invention will now be described in more detail, by way of example only, with reference to the following figures. 
         FIG. 1A  shows a schematic perspective view of a reverse shoulder prosthesis according to an embodiment of the invention; 
         FIG. 1B  shows a schematic perspective view of a reverse shoulder prosthesis according to an embodiment of the invention located in a shoulder; 
         FIG. 2A  shows a sketch drawing of a prior art reverse shoulder prosthesis, the arrow indicates where impingement has occurred and the bone can be seen to be notched; 
         FIG. 2B  is a photomicrograph of a section through a shoulder with a reverse prosthesis according to the prior art; 
         FIG. 2C  is a photograph of a shoulder with a reverse prosthesis according to the prior art; 
         FIGS. 3A to 5  show perspective side and plan views of a scapula component according to a first embodiment of the invention; 
         FIGS. 6A to 8A  show perspective side and plan views of a scapula component according to a second embodiment of the invention; 
         FIG. 8B  shows a perspective side view of a scapula component according to a third embodiment of the invention; 
         FIGS. 9A to 9C  show perspective views of a scapula mounting component according to an embodiment of the invention; 
         FIG. 10  shows a perspective view of a scapula mounting component and a scapular component according to the second embodiment of the invention; and 
         FIGS. 11A to 11D  show perspective views of a humerus component according to an embodiment of the invention. 
     
    
    
     Referring to the drawings,  FIG. 1A  shows a reverse shoulder prosthesis according to the invention comprising a scapula component  1  and a humerus component  200 . The bearing surface  3  of the scapula component  1  is shown in contact with the bearing surface (not visible) of the humerus component  200 . In use ( FIG. 1B ), the scapula component  1  is mounted in the glenoid cavity on a patient&#39;s scapula  8  and the humerus component  200  is mounted to the proximal end of the patient&#39;s humerus  2 . In use, the bearing surface  3  on the scapula component  1  interacts with the bearing surface of the humerus component  200  to allow the humerus  2  to move relative to the scapula  8 . 
       FIGS. 2A and 2B  illustrate a prior art reverse shoulder prosthesis.  FIG. 2A  schematically illustrates notching (indicated by the arrow) on a shoulder fitted with a reverse shoulder prosthesis. It is believed that this notching may be caused by impingement of the scapula by the humerus component of the prosthesis.  FIG. 2B  is a photomicrograph of a section in the frontal plane through the glenoid and the scapula component of a shoulder prosthesis, illustrating the erosion of the inferior pole of the glenoid, leaving the prosthesis unsupported by bone.  FIG. 2C  is a photograph showing the erosion around the inferior pole of the glenoid and the scapula neck. The inferior screw and the inferior part of the glenoid base plate (mounting portion) of the scapular component are denuded of bone. Wear of the polyethylene cup portion of the humerus component is also visible (Nyffeler et al (2004) J Bone Joint Surg [Br] 86-B:1187-91). 
       FIGS. 3A to 5  illustrate, in more detail, a scapula component  1  according to a first embodiment of the invention. 
       FIGS. 3A to 3D  show a scapula component  1  according to the invention with a part spherical, convex, surface which is less than a hemisphere. The part spherical, convex, surface comprises a main surface portion  3  and three subsidiary surface portions  5 ,  6 ,  7  depending from the edge of the main surface portion  3 . These portions are more clearly depicted in  FIGS. 4 and 5 . In  FIG. 4 , a plan view of the scapula component  1 , a circle  4  has been drawn onto the surface of the part spherical surface  1  to clearly illustrate the main surface portion  3 . The circle  4  drawn is the largest circle that can be placed on the part spherical surface, and defines the main surface portion  3 . In  FIG. 5 , a side elevation of the scapula component  1 , plane  8  defines the circular edge of the main surface portion  3 . 
     The main surface portion  3  is circular, and the three subsidiary surface portions  5 ,  6 ,  7  which depend from the edge of the main surface portion  3  are crescent shaped. The subsidiary portions  5  and  6  are mirror images of each other and are located on opposite sides of the main surface portion  3 . The crescents of subsidiary portions  5  and  6  are elongated. The third subsidiary portion  7  is located at the top, superior, edge of the main surface portion  3  and is also crescent shaped, but is less elongated than portions  5  and  6 . 
     Preferably the subsidiary surface portions have a total surface area which is less than the surface area of the main surface portion. Preferably the total surface area of the subsidiary surface portions is less than about 90%, 80%, 70%, 60%, 50%, 40% or 30% of the surface area of the main surface portion. Preferably the width of a subsidiary portion, at its widest part, is less than about 10 mm, more preferably it is less than about 5 mm. 
     The main  3  and subsidiary  5 ,  6 ,  7  surface portions together form the part spherical surface of the scapular component  1 . In use this surface forms the bearing surface that contacts the humerus component of the reverse shoulder prosthesis. 
     Modelling studies have shown that the subsidiary surface portions  5 ,  6 ,  7  are needed, in addition to the main surface portion  3 , in order to bear the load when a reverse shoulder is in use. 
       FIGS. 3D and 5  illustrate the difference between the scapula component  1  of the invention and a hemisphere. It is clear that the scapula component of the invention is significantly less than a hemisphere. The scapula component  1  is shown located in a hemisphere defined by lines  11  and  12 . The figures illustrate that it is the base of the hemisphere, which would have reached line  12 , that has been removed/is missing from the scapula component  1  when compared to a hemisphere. 
     The scapula component  1  also has a hole  14  though it, which extends from the main surface portion  3  of the part spherical surface of the scapular component  1  into a cavity  16  defined by the scapular component  1 . The hole  14  defines a longitudinal axis indicated by the dashed line  15  in  FIG. 5 . 
     The main portion  3  of the part spherical surface has a rotational axis of symmetry, illustrated by the dashed line  17  in  FIG. 5 . The rotational axis of symmetry  17  of the main surface portion  3  of the part spherical surface is offset relative to the longitudinal axis of hole  14 . 
     Hole  14  acts as a mounting means and allows a screw, or any other suitable fixing means to pass through the scapula component  1  to attach the component directly to the scapula or to a mounting means, in the embodiment illustrated the scapula component  1  would usually be attached to a mounting means and then to the scapula. 
       FIG. 6A  to  FIG. 8A  show a second embodiment of the scapular component  100  to that depicted in  FIGS. 3A to 5 . In this embodiment the scapular component comprises a part spherical surface and a part cylindrical surface. 
     The part spherical surface is the same shape and configuration as illustrated in  FIGS. 3A to 5  and is indicated by dotted line  119  in  FIGS. 7 and 8A . 
     More specifically, the scapula component  100  comprises a part spherical, convex, surface  110  and a part cylindrical surface  120 . The part spherical surface  110  is front facing and comprises a main surface portion  103  and three subsidiary surface portions  105 ,  106 ,  107  depending from the edge of the main surface portion  103 . The circle  104  in  FIG. 7  illustrates the boundary of the main surface portion  103 . The circle  104  is the largest circle that can be placed on the part spherical surface.  FIG. 8A  shows a side elevation of the scapula component  100 , in which plane  108  defines the circular edge of the main surface portion  103 . 
     As in the first embodiment illustrated in  FIGS. 3A to 6 , the main surface portion  103  is circular, and the three subsidiary surface portions  105 ,  106 ,  107  which depend from the edge of the main surface portion  3  are crescent shaped. 
     In use the part spherical surface  110  forms the bearing surface that contacts the humerus component of the reverse shoulder prosthesis. 
     As in  FIGS. 3A to 5  the scapula component  100  also includes a hole  114 , which extends from the main surface portion  103  of the part spherical surface of the scapular component  100  into a cavity  116  defined by the scapular component  100 . The hole  114  defines a longitudinal axis which is offset from the rotational axis of symmetry on the main surface portion  103 . 
     The skilled man will appreciate that the cavity  116  may be of any suitable shape, including domed (as illustrated), cylindrical and cuboid, provided that it can be mounted to a mounting member or directly to a scapula. 
     In addition to the part spherical surface  110 , the scapula component  100  includes a part cylindrical surface  120 . The part cylindrical surface  120  extends backwards from at least a part of the back edge of the part spherical surface  110 . The depth of the part cylindrical surface  120  varies along its length. The part cylindrical surface  120  extends from most of the back edge of the part spherical surface  110 . The part cylindrical surface  120  may extend from between about 50% and about 100% of the back edge of the part spherical surface  110 . 
     The part cylindrical surface  120  has a back edge  127  which is substantially flat. In use, the back edge  127  of the part cylindrical surface  120  typically contacts the scapula. 
     In the embodiment illustrated the back edge  127  of the part cylindrical surface  120  is a few millimetres, about 2 mm, short of reaching the plane where the back surface of a hemisphere with the same radius of curvature as the part spherical surface  110  would have been. This leaves a lip  113  at the inferior edge of the part spherical surface  110 . The skilled man will appreciate that this distance could be greater, or that the back surface  127  could extend to where the back surface of the hemisphere would have been as depicted in  FIG. 8B . 
       FIGS. 9A to 9C  depict a mounting member  130  which may be used to mount the scapula component of  FIGS. 6A to 8A  to a scapula. The mounting member is arranged to be mounted onto the prepared surface of a patient&#39;s scapula (not shown) and then to engage with the cavity  116  in the scapula component  100 . 
     Mounting member  130  is essentially domed shape. The top of the dome has been removed to leave a concave top surface  128 . Sidewalls  121  slope from the top  128  to meet a base  123 . The base  123  of the mounting member  130  has a convex surface  127 . Preferably the convex surface  127  allows the base  123  to fit to the contours of the scapula, providing improved contact and reducing stresses. A mounting projection  129  extends from the base  23 . The mounting projection  129  has a longitudinal axis  122  as shown in  FIGS. 9A and 9C . Preferably the longitudinal axis  122  of the mounting projection  129  is on the same axis as the longitudinal axis  115  of the scapular component  100  when the two components are fixed together ( FIG. 10C ). The mounting projection  129  is tubular, and the free end  131  has a chamfered edge  133 . Four supporting struts  135  are equally spaced around the mounting projection  129 . The mounting projection  129  may define a hole which is used to screw the mounting member  130  to a scapula. Preferably a corresponding hole is made in the patient&#39;s scapula to accommodate the mounting projection  129 . 
     Four screw recesses  137  are equally spaced around the circumference of the mounting member  130  in the sidewalls  121  of the mounting member  130 . A screw hole  139  extends from each of the screw recesses  137  through to the base  123 . Each screw recess  137  is large enough to accommodate a head of a screw  141 .  FIG. 9C  shows a screw  141 , as it would be positioned in a screw recess  137  and screw hole  139  when the mounting member is attached to the scapula (not shown). 
     A screw hole  143  is located in the centre of the flattened top  128  of the mounting member  130 . The screw hole  143  has a longitudinal axis  122  in  FIGS. 9B and 9C . This screw hole  143  extends through the centre of the mounting member  130  and through the tubular mounting projection  129 . The screw hole  143  is capable of accommodating a central screw (not shown) which can secure the mounting member  130  to a patient&#39;s scapula (not shown). 
     The mounting member  143  may be attached to the scapula using four circumferential screws, or by using a central screw located in hole  143 , or by using both means. 
     As an alternative to locating a screw in the central screw hole  143 , the outer surface of the mounting projection  129  may be threaded and may screw directly into the scapula. 
     The fixing means discussed in the specific example are just one example of how the scapular component may be mounted, and the skilled man will appreciate that the device may be mounted in any suitable way. For example more or less fixing means than illustrated may be used. The fixing means may include screws, pins, nail, staples and any other suitable means. 
     Referring to  FIG. 10 , screw hole  143  in the mounting member  130  is arranged to align with hole  114  in the scapula component  100 , and a central screw (not shown) can be used to secure the scapula component  100  to the mounting member  130 . 
       FIGS. 11A to 11D  show a humerus component  200  according to the invention. The humerus component  200  comprises an upper cup portion  202  ( FIG. 11D ) and a lower mounting portion  204  ( FIG. 11C ). 
     The cup portion  202  comprises a front facing convex part spherical surface  206  surrounded by an oval shaped rim  208  with a chamfered surface. The side of the cup portion forms a cylindrical surface  209 . In use, the convex surface  206  forms a bearing surface which contacts the scapula component. The underside (not shown) of the cup portion  202  is substantially flat. 
     The width of the rim  208  varies around the part spherical surface  206 . At the lower/inferior end of the part spherical surface  206 , the rim  211  is wider than at the upper/superior end of the part spherical surface  206 , where the rim  212  is narrower. This difference in width of the rim is determined by how much load a particular part of the humerus component  200  will take when in use. For example, in use a lot of the load forces will be concentrated at the lower inferior end of the part spherical surface  206 , where the rim  212  is wider. The wider rim  212  allows these forces to be absorbed and reduces the risk of damage to the prosthesis. Where the rim is narrower, for example at the upper end of the part spherical surface  206 , the forces on the corresponding part spherical surface  206  are less and thus a narrower rim  212  can be used. 
     The chamfering on the rim  208  is designed to minimise impingement on the scapula which may be caused by the humerus component. As the bearing/load forces do not act directly on the surface of the rim  208 , the rim can be chamfered without affecting performance of the prosthesis. 
     Preferably the top surface on the cup portion  202  does not all lie in the same plane. As depicted in  FIG. 11B , the lower half of the top surface of the cup portion  202  is about 2 degrees (illustrated as angle y) from the plane  240  define by the central part of the top surface of the cup portion  202 , and the upper half of the top surface of the cup portion  202  is 5 degrees (illustrated as angle x) from plane  240 . The skilled man will appreciate that this is merely an example and that these angles can be varied somewhat and that the top surface may all lie in the same planar. The angling of the upper surface is intended to further minimise the risk of the humerus component impinging on the scapula. 
     The cup portion  202  also comprises a stem  215  which projects from the lower surface of the cup portion  202 . The stem  215  is tubular and is arranged to mount the cup portion on the lower mounting portion  204 . 
     The side  209  and the underside (not shown) of the cup portion  202  has a notch  218  at the lower most edge. This notch  218  engages with a corresponding raised portion  227  on the mounting portion  204 . The notch  218  and raised portion  227  are designed to impart more strength to the lower end of the humerus component  200  where most of the load is concentrated when the component  200  is in use. The reason for this arrangement is that the lower mounting portion  204  is made of a stronger material, typically metal, which will provide the strength needed to support the load and reduce the risk of damage to the prosthesis. 
     The lower mounting portion  204  comprises a support portion  221 , configured as a disc, with an upper surface  223  having a small spherical curvature and a lower flat surface (not shown). The small spherical curvature on the upper surface  223  reduces torsional stresses on the portion. The upper surface  223  carries a raised portion  227  which corresponds to the notch  218  on the underside of the cup portion  202 . 
     The upper surface  223  of the humerus mounting portion  204  has a cup-portion mounting hole  229  which extends through the humerus mounting member  204  and partially into a stem  224  projecting from the underside of the support portion  221 . The cup-portion mounting hole  229  engages with the cup mounting projection  215  on the cup portion  202 . The hole  229  and the projection  215  can be any suitable complementary shape. They may be oval to prevent rotation. 
     The stem  224  projects at an angle of 60 degrees from a plane defined by the lower surface of the mounting portion  204 . This angle may be varied, however the intention is that the part spherical cup of the cup portion is presented to the scapula component at an angle of about 60 degrees to the longitudinal axis of the humerus. 
     The stem  224  is tubular and is capable of fitting into a standard or custom anatomical stem (not shown). The stem may have any suitable cross sectional shape. The stem may be oval to prevent any rotation when in use. 
     The transition between the stem  224  and the lower surface of the mounting portion  204  is a smooth gradual slope. A strut  233  extends from the lower surface of the mounting portion  204  to half way along the stem  224  and serves to prevent rotation of the stem once in situ in a standard or custom anatomical stem.