Patent Publication Number: US-2018049758-A1

Title: Patient-specific surgical guide

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to patient-specific surgical guides. In particular, but not exclusively, the invention relates to such guides for use in minimally invasive surgical procedures within joints, whereby the guide is located in a pre-operatively defined position and orientation within the joint relative to both articular bone surfaces thereof. 
     BACKGROUND TO THE INVENTION 
     Surgical guides are used to assist surgeons during surgical procedures, for example to align bones during joint replacement procedures, and to guide tools for preparing bone surfaces to receive prosthetic components. 
     Patient-specific guides having a contoured surface to mate with a corresponding articular bone surface are known. The contoured surface may be determined pre-operatively, through techniques including the use of computer-assisted image methods based on three-dimensional images of the patient&#39;s anatomy reconstructed from MRI, CT, ultrasound, X-ray, or other three- or two-dimensional medical scans of the patient&#39;s anatomy, to ensure that the guide fits closely to the bone surface once located in position during a surgical procedure. Opposing joint surfaces each require their own single-surfaced guide. Such guides having a single surface require the surgeon to manually compress them against the articular surface in order to be kept in place (or alternatively to use screws, or other means, to keep it in place once they are happy with its location). 
     Moreover, in order to place such known guides in position, and then to use the guides, for example for placement of guide pins in the opposing bone surfaces of the joint, it is typically required for the surgeon to make a relatively large incision, or series of incisions. This is so as to be able to dislocate the joint to provide the necessary clear access and passage both for placement of the guide(s) into position and for insertion of the guide pin(s) using the guides—usually perpendicular to the associated articular surface—at each of the two bones. A perpendicular view of the articular surfaces is needed in order to insert the guide pins in position. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, defined by the accompanying claim  1 , there is provided a patient-specific surgical guide for use in a minimally invasive procedure in a joint, the guide comprising: a first contoured surface; and a second contoured surface; wherein the first and second contoured surfaces are configured for engagement with respective opposing articular bone surfaces in said joint such that the guide is located in a pre-operatively determined position and orientation within the joint relative to both bones. 
     Such a patient-specific guide streamlines guide placement, because it is effectively auto-aligned and oriented within the joint, once inserted between the opposing bone surfaces, by virtue of the two contoured surfaces and a clamping effect produced by soft tissue tension across the joint, which is maintained because of the minimally-invasive nature of the procedure. Because both articular surfaces are mapped onto the guide, the joint can apply its natural compressive force which, together with some manipulation of the joint by the surgeon, will urge the guide to self-locate because it wants to find its lowest energy state (i.e. the joint orientation with the least force). 
     The required joint exposure is also minimised. The guide can be used in a minimally invasive setting where the joint is not dislocated. 
     In addition, the guide enables new means of bone alignment and preparation, as described below in the context of optional further features as set out in dependent claims. In particular, by having both articular surfaces on the guide, the bones can be locked relative to each other, which in turn allows guide pins to be driven into place without having to have a perpendicular view of the articular surface (because they can be driven through the adjacent bone from outside the joint). This again minimizes invasiveness and is not possible with currently available single surface guides. 
     In one embodiment, the guide comprises a monoblock component. 
     In an alternative embodiment, the guide is modular, comprising: a first modular component on which the first contoured surface is formed; a second modular component on which the second contoured surface is formed; and means for securing the first modular component to the second modular component. Alternatively, the two modular parts may be split in such a way that each has a part of both articular bone contours, and the two modular parts may be assembled to the bone ends from a transverse direction. According to such modular embodiments, the means for securing the first modular component to the second modular component may comprise respective mating features on said first and second modular components. Those respective mating features may comprise a groove on one of the first and second modular components and a corresponding tongue on the other of the first and second modular components. Alternatively, any other suitable mating, e.g. male and female, components could be used instead. The individual components may themselves be split into sub-components that can be secured together by suitable fixation means. One advantage of a modular construction is that it allows for greater intra-operative flexibility to the surgeon, because the different components or sub-components can conceivably be swapped-out for others, or removed to allow for improved access within and around the joint during surgery. Also, it is conceivable that the components and/or sub-components can be inserted through the minimally-invasive incision(s), for assembly together within the confines of the joint. Smaller components and sub-components of course require smaller incisions. 
     The modular guide may further comprise a spacer for insertion between the first and second modular components, wherein the spacer comprises part of said means for securing the first modular component to the second modular component. The provision of a spacer allows for the thickness of the assembled modular guide to be adjusted, which can be useful to allow for the surgeon to take into account different joint laxities, for example. 
     The first contoured surface may comprise a three dimensional surface closely mateable in only a single position with one of said articular surfaces. Likewise, the second contoured surface may comprise a three dimensional surface closely mateable in only a single position with the other of said articular surfaces. In conjunction, these mating surfaces help to ensure that the guide can be fitted into a unique position, which properly-located position can easily be determined by the surgeon through feel when inserting the guide in the joint. 
     The first and second contoured surfaces may further be configured to lock the articular bone surfaces in a pre-operatively determined configuration relative to the guide and hence relative to one another. As such, the bones may be set in a particular clinically useful pose. 
     The guide may further comprise at least one datum, so as to provide a fixed point of reference for the surgeon. 
     The guide may further comprise means for attachment of and/or guiding of at least one surgical instrument or intra-operative tracking marker or sensor. The guide according to this embodiment may further comprise an additional modular component for attachment to said guide in a pre-operatively determined position and orientation relative to the guide, for guiding at least a selected one of the following exemplary surgical procedures: guide wire insertion, drilling, cutting, reaming, resecting, augmenting, injecting, imaging and screwing. When for use in guide wire insertion, the guide may further comprise a clearance hole and an associated slot such that a guide wire can be inserted through the hole for attachment to at least one of the bones and such that the guide is removable from the joint with the wire in place by the passage of the wire through the slot. 
     The guide may further comprise at least one fixation hole for receiving a fixing screw or other attachment means to secure the guide to the joint, in use. The guide may also or instead further comprise at least one hole for use in making a reference mark on the underlying bone surface, which mark can be used to help guide subsequent surgical steps. In certain embodiments, the fixation hole may be used for making that reference marking, for example by virtue of the hole left by the screw acting as marker. 
     The guide may be manufactured from a medical-grade polymer or metallic material including but not limited to: polyethylene, 316 steel, nylon 6, acrylic, cobalt chrome, titanium and PEEK. These materials are especially conducive to the guide being manufactured using additive manufacturing, which is a particularly good way to make guides specific to each patient and procedure with minimal wastage of stock material. 
     The guide may be for a shoulder joint, in which case the first contoured surface is a humerus-contacting surface, sized and shaped to substantially match the geometry of at least a portion of the patient&#39;s humerus, and in which the second contoured surface is a glenoid-contacting surface, sized and shaped to substantially match the geometry of at least a portion of the patient&#39;s glenoid cavity. In alternative embodiments, the guide may instead be for other articular joints, such as the knee, hip or elbow. In further alternative embodiments, the guide may be adapted for use in association with other opposing bones or bone portions, such as for use in realignment of bone fractures, or in sarcoma surgery. 
     The guide may further comprise chamfered surfaces positioned to aid in separation of the two bone surfaces, against the passive joint tension, during insertion of the guide. 
     The guide may be at least partially coated with a low friction coating, so as to assist in insertion of the guide into between the opposing bone surfaces. 
     Where the guide is modular, the first and second modular components may include respective means for connection to associated first and second arms of a pivoting surgical instrument and the means for securing the first modular component to the second modular component may comprise a mechanism for locking the first and second arms relative to one another. By way of example, the first and second modular components may each include a hole for securely receiving a prong on the end of the respective arms of the surgical instrument, and the locking mechanism may be a ratchet, as known in self-retaining soft tissue retractor instruments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows an posterior-inferior isometric view of an assembled modular guide according to one embodiment, located in situ within a shoulder joint, with portions of the humerus and scapula shown in wire-frame; 
         FIG. 2  is a close-up view of the guide of  FIG. 1 ; 
         FIG. 3  corresponds to  FIG. 2 , but from a posterior viewpoint; 
         FIG. 4 a    corresponds to  FIGS. 2 and 3 , but from an anterior-inferior isometric viewpoint; 
         FIG. 4 b    corresponds to  FIG. 2 , but with the guide removed to show the underlying bone surfaces; 
         FIGS. 5 a -5 c    depict the assembled modular guide of the preceding figures, in respective inferior, anterior-inferior isometric and posterior views, but with the bones removed for clarity—note that  FIG. 5 b    corresponds to  FIG. 4  and that  FIG. 5 c    corresponds to  FIG. 3 ; 
         FIGS. 6 a  and 6 b    depict a humeral-side component of the guide in respective anterior and posterior views; 
         FIGS. 7 a  and 7 b    depict the humeral-side component of  FIGS. 6 a  and 6 b   , but with a modular tab sub-component removed; 
         FIGS. 8 a -8 d    depict the modular tab sub-component of the humeral-side component in respective anterior, posterior, anterior-superior and superior views; 
         FIGS. 9 a -9 d    depict a scapular-side component of the guide in respective anterior, anterior-inferior, anterior-inferior-lateral and posterior-superior isometric views; 
         FIG. 10  is a posterior view of an optional spacer for insertion between the humeral- and scapula-side components of the guide; and 
         FIGS. 11   a - d  depict an embodiment in which the humeral-side component includes a slot feature for the passage of a guide wire therethrough, as well as drilling steps for its insertion. 
     
    
    
     DETAILED DESCRIPTION 
     The following description will be made in the context of a patient-specific surgical guide for use in a minimally invasive surgical procedure on a patient&#39;s shoulder joint. It should be understood, however, that the principles and teachings can be applied mutatis mutandis to produce guides useable in other articular joints, such as the knee, hip or elbow. Suitable applications include, but are not limited to, total or partial joint replacement procedures and soft-tissue repairs such as ligament repair or replacement. 
     The term ‘minimally invasive surgical procedure’ is considered to have an established meaning within the art and is intended to encompass, inter alia, arthroscopic and mini-open surgical approaches, including those using multiple minimally-invasive incisions. 
     A patient-specific surgical guide assembly (‘PSG’)  10  is shown located in situ between a patient&#39;s humerus  12  and scapula  14  in  FIGS. 1 to 4 . The PSG  10  is modular, comprising a first modular component  20  for location at the humeral side of the joint (the humeral-side component  20 ) and a second modular component  30  for location at the scapula side of the joint (the scapula-side component  30 ). 
     The humeral-side component  20  and the scapula-side component  30  are secured to one another via a mating connection  40 , perhaps best seen in  FIGS. 3 and 5   c . The mating connection  40  here comprises a tongue  42  protruding from an end of the humeral-side component  20  closest to the scapula-side component  30 , said tongue  42  slidingly received in a mating groove  44  formed on an opposing end of the scapula-side component  30 . It will be understood that numerous alternative mating connections are envisaged and that the skilled person would be able to conceive of suitable such connections. By way of example only, the tongue  42  could project from the scapula-side component  30  with the corresponding groove  44  being in the humeral-side component  20  or, rather than a sliding tongue and groove arrangement, the secure connection could be made via a series of interconnecting fasteners. 
     As best seen in  FIG. 5 b   , the humeral-side component  20  has a contoured surface  60 , customised to match corresponding contours on the surface of the patient&#39;s humerus  12 . This first contoured surface  60  will be described in greater detail below. Likewise, the scapula-side component  30  has a contoured surface  80 , customised to match corresponding contours on the surface of the patient&#39;s scapula  14 , and more particularly the glenoid and surrounding surfaces. This second contoured surface  80  will also be described in greater detail below. 
     The first and second contoured surfaces are determined pre-operatively, through known techniques including the use of computer-assisted image methods based on three-dimensional images of the patient&#39;s joint anatomy reconstructed from MRI, CT, ultrasound, X-ray, or other three- or two-dimensional medical scans of the patient&#39;s anatomy. 
     In the pre-operative planning stage, imaging data of the relevant anatomy of a patient can be obtained using one of medical imaging methods described above, as needed for joint modeling, mechanical/alignment axis determination or for other alignment purposes. The imaging data obtained and other associated information can be used to construct a three-dimensional computer image of the joint or other portion of the anatomy of the patient. An initial pre-operative plan can be prepared for the patient in image space and can include bone or joint preparation, planning for resections, milling, reaming, broaching, implant selection and fitting, as well as designing patient-specific guides, templates, tools and alignment methods for the surgical procedure. 
     The imaging data can thus be used to design the first and second contoured surfaces  60 ,  80  of the PSG  10  to ensure that when the PSG  10  is located between the opposing articular bone surfaces  12 ,  14  in the patient&#39;s joint during the surgical procedure, several objectives are achieved. 
     A first objective is to ensure that the PSG is located in a pre-operatively determined position and orientation within the joint relative to both bones. This is known as registration of the guide to the joint, and assists the surgeon in knowing that the guide is properly located before continuing further with the procedure so that the PSG  10  can be used as a platform for further surgical steps requiring knowledge of a bone&#39;s position and orientation. A PSG  10  according to the invention is capable of achieving automatic self-location to the desired position within the joint through a combination of said first and second contoured surfaces  60 ,  80  and the passive tension of surrounding trans-articular soft tissues (e.g. muscles, tendons, ligaments, and/or joint capsule) which will tend to result in the bones  12 ,  14  of the joint positioning and orienting themselves in the desired configuration, which is that with the lowest passive tension allowable when the PSG  10  is present within the joint. 
     To this end, it is preferable for at least one (and more preferably both) of the first and second contoured surfaces  60 ,  80  to comprise a three-dimensional surface that is closely mateable with the associated bone surface in only a single position. 
     A second objective is to ensure that the bones are ‘locked’—i.e. located in a fixed, pre-operatively determined configuration—relative to the PSG and hence to each other. This is known as alignment. The PSG  10  according to this embodiment would thus allow new bone preparation techniques to be performed which simultaneously prepare both bones  12 ,  14  given that the relative configuration of the two bones is known and can be pre-operatively chosen to correspond to a clinically useful pose. 
     By way of example, a relatively shallow or small contoured surface might contain enough ‘landmarks’ i.e. unique points and regions matching those of an associated bone surface such that the surface can only be fitted neatly to the bone in a single, unique position. However, the bone could be susceptible to being shifted out of that position, for example by relative translational movement of the bone away from the surface, or through a relative twisting action. 
     To mitigate against that, the contoured surface may be extended, for example to ‘hook’ over a portion of the bone. In the exemplary example described herein, the contoured surface  60  of the humeral-side component  20  comprises a relatively shallow, generally concave portion  61  contoured to match the convex contours of at least part of the end of the patient&#39;s humeral head  12   a . The contoured surface  60  further comprises a sidewall extension portion  62  that enshrouds at least part of the side of the patient&#39;s humeral head  12   a , and an overhanging lip  63  that fits under at least a part of the patient&#39;s humeral head  12   a , within the humeral neck  12   b . In this way, the contoured surface  60  not only accommodates the humerus  12  in a single position relative to the PSG  10 , but also resists relative movement (translation and/or rotations) between the humerus  12  and the PSG  10  once located in position.  FIGS. 4 a , 5 b  and 6 a   , as well as  FIGS. 8 a -8 d    perhaps best show these features. 
     The contoured surface  80  of the scapula-side component  30  comprises a relatively shallow, generally convex portion  81  contoured to match the concave contours of at least part of the end of the patient&#39;s glenoid cavity  14   a . The contoured surface  80  further comprises an inwardly-angled sidewall extension portion  82  that enshrouds and accommodates at least part of the patient&#39;s posterior glenoid rim  14   b , and posterior glenoid vault  14   c . In this way, the contoured surface  80  not only accommodates the scapula  14  in a single position relative to the PSG  10 —particularly by accommodating the patient&#39;s posterior glenoid rim  14   b  within a region  83  formed between the convex portion  81  and the sidewall extension portion  82 , but also resists relative movement (translation and/or rotations) between the scapula  14  and the PSG  10  once located in position.  FIGS. 4 a , 4 b , 5 b  and 9 a -9 c    perhaps best show these features. 
     In embodiments in which the surface contours are extended to lock the PSG  10  in position relative to the bones  12 ,  14  and vice versa, the very features that resist the relative movement (e.g. the sidewall  62  and lip  63  of the contoured surface  60 , or the sidewall  82  of the contoured surface  80 ) also make it more difficult to fit the PSG in place. So, one or both of the modular components  20 ,  30  can be split into sub-components to help in fitting their respective contoured surfaces  60 ,  80  to and around the associated bone surfaces. 
     In the exemplary embodiment described herein, the humeral-side component  20  comprises two sub-components: a base portion  20   a  (see  FIGS. 7 a , 7 b   ), which includes the tongue  42  that secures the humeral-side component  20  to the scapula-side component  30 , and which includes a first part  60   a  of the first contoured surface  60 ; and a fin or tab portion  20   b  (see  FIGS. 8 a  to 8 d   ), which includes a second part  60   b  of the first contoured surface  60 . Broadly speaking, the first part  60   a  of the first contoured surface  60  comprises the relatively shallow, generally concave portion  61 , and the second part  60   b  of the first contoured surface  60  comprises the sidewall extension portion  62  and overhanging lip  63 . However, it will be appreciated that the division between the concave portion  61  and the sidewall extension portion  62  may not be a well-defined boundary, such that the second part  60   b  may include part of the concave portion  61  and/or the first part  60   a  may include part of the sidewall extension portion  62 . 
     The base portion  20   a  includes an L-shaped recess  22  on its posterior external surface  24  adjacent the end that is closest to the scapula-side component  30 . A hole  26  is formed through a widest part of the recess  22 . The tab portion  20   b  includes an L-shaped protrusion  23  on its anterior external surface  25  adjacent the end that is closest to the scapula-side component  30 , the protrusion  23  matching the recess  22 . A through-hole  27  extends through the widest part of the protrusion  23  and through the rest of the adjacent part of the tab portion  20   b  to the posterior external surface  29 . The base portion  20   a  and the tab portion  20   b  are assembled together by fitting the protrusion  23  into the recess  22 , and secured together in that position by means of a fastener, such as a bolt (not shown) threaded into the hole  27 . In this assembled configuration, the first and second parts  60   a ,  60   b  of the first contoured surface  60  are contiguous (see  FIG. 6 a   ). 
     The humeral-side component  20  may be divided into more than two sub-components if necessary and according to the patient&#39;s specific anatomy and needs, as well as the procedure being carried out with the use of the PSG. It may be beneficial to remove a sub-component during the surgical procedure, for example to provide better access to a part of the joint to allow surgical steps to be performed. An alternative embodiment may allow both the base and tab portions to be made as one piece, joined by a thin hinge line, which can be bent into the desired shape when placed within the joint, and secured in that configuration, for example by a snap-together mechanism. 
     As illustrated, the scapula-side component  30  is a unitary part, and it can be fitted in place by a combination of a translation and a rotation, to accommodate the patient&#39;s posterior glenoid rim  14   b  within the region  83  formed between the convex portion  81  and the sidewall extension portion  82 . It will be understood, however, that in certain circumstances, such as those described with reference to the humeral-side component  20 , it may be better to split the component into two or more sub-components, which could be secured together in the manner described above with reference to the humeral-side component  20 . 
     The PSG  10  may include at least one datum (not shown) for use as a reference point or reference plane, for example. The PSG  10  may also or instead include means (not shown) for attachment of and/or guiding of at least one surgical instrument or intra-operative tracking marker or sensor. 
     Additional modular components (not shown) may be included for attachment to the PSG  10  in a pre-operatively determined position and orientation relative to the guide, for guiding at least a selected one of the following exemplary surgical procedures: guide wire insertion, drilling, cutting, reaming, resecting, augmenting, injecting, imaging and screwing. To this end, the, or each, additional modular component would either be of a pre-operatively planned patient-specific design, or would be of a standard design but with patient-specific attachment locations on the PSG  10  so as to ensure that the surgical steps are carried out in the appropriate location and orientation for that particular patient. 
     The PSG  10  includes a hole  90  for use in marking the underlying glenoid surface with a reference mark for subsequent steps of the procedure once the PSG has been removed. The PSG  10  may further include one or more fixation holes (not shown) for receiving a fixing screw to secure the guide to the joint, in use. In one embodiment, the hole  90  may be used as such a fixation hole, with the screw-hole made in the bone surface during placement of the screw therein acting as the reference mark. As illustrated in the exemplary embodiment, the hole  90  is provided through the scapula-side component  30 , for fixation of a screw into the scapula  14 —in particular the posterior glenoid rim  14   b  thereof, but it will be appreciated that additional or alternative hole locations could be used instead, depending for example on the patient&#39;s joint geometry and operational needs. 
     The PSG  10  is typically disposable and made of lightweight materials, including certain metals and polymers including, but not limited to: polyethylene, nylon 6, acrylic, PEEK, 316 steel, cobalt chrome, and titanium, as well as alloys thereof. Where made of modular components, the components could be manufactured from different materials, to take advantage of different material properties, if desirable. The PSG  10  may conveniently be manufactured using additive manufacturing techniques 
     In use, a surgeon would insert the components and sub-components of the PSG  10  through one or more minimally-invasive incisions for assembly within the area of the joint. Insertion of the PSG  10  into place between the bone surfaces during such a minimally-invasive procedure may require quite some force, by virtue of the fact that the soft tissues in and surrounding the joint (e.g. the joint capsule, and rotator cuffs comprised of the infraspinatus, teres minor, subscapularis, and supraspinatus) are not greatly affected by the minimally-invasive incisions, and thus continue to produce their natural and ever present tonic forces—which regularly compress the two articular surfaces together and prevent the joint from dislocating—throughout the procedure. Just as it helps to hold the natural components of the joint firmly in place, the passive joint stiffness provided by the soft tissue tension helps to retain the PSG  10  firmly in place once inserted, thereby ensuring a stable guide location that the surgeon can rely on being accurately located. 
     To assist in the insertion of the PSG  10  into location between the opposing articular bone surfaces and against the latent soft tissue tension, the PSG  10  may include one or more tapered or chamfered surfaces so that it has a general wedge shape, which would help to separate the bone surfaces. In the illustrated embodiment, such tapered surfaces are provided on an anterior side of the PSG: the humeral-side component  20  including a first tapered surface  130 ; and the scapula-side component  30  including a second tapered surface  132 , a portion of which in fact continues across on to the humeral-side component. This is best seen in  FIGS. 5 a    and  11 . The location of the tapered surfaces  130 ,  132  on the anterior side of the PSG  10  will assist in its insertion from the posterior. 
     Specifically, humeral-side base portion sub-component  20   a  and scapula-side component  30  will typically be assembled together outside of the patient using the above described fixation method, while the humeral-side tab sub-component  20   b  will not be attached until after the above components with their tapered shape are inserted into between the bone surfaces. Once assembled together, these combined components  20   a ,  30  are inserted typically through a posterior incision, while sub-component  20   b  will be inserted within the soft tissue envelope of the joint, slotted onto component  20   a  by mating surfaces  22  and  23  and connecting them using a fastener. Once connected, the scapula-side component  30  can be confirmed to be properly located as a result of surface  80  mating with the scapula  14 . Subsequently, the humerus  12  can be passively adjusted by the surgeon until it can be felt to be properly located relative to surface  60  (e.g. the humerus feels physically locked in place due to the rigid connection the guide creates between the scapula and humerus). 
     To further assist in insertion, one or more of the surfaces of the PSG  10  may be at least partially coated with a low-friction coating. 
     Because it can be difficult to determine pre-operatively how much tension the joint will be under from the soft tissues, it is desirable to allow for the ‘thickness’ of the PSG  10  to be altered intra-operatively so that the resulting soft tissue tension with the PSG  10  in place can be altered accordingly. To this end, a spacer  100 , as shown in  FIG. 10 , may be provided to be interposed between the humeral-side component  20  and the scapula-side component  30 . The spacer  100  includes a groove  102  that corresponds to the groove  44  in the scapula-side component  30  and which is therefore able to slidingly receive the tongue  42  of the humeral-side component  20  for secure connection thereto. The spacer  100  further comprises, on an opposing surface, a tongue  104  that corresponds to the tongue  42  of the humeral-side component  20  and which is therefore able to be slidingly received within the groove  44  on the scapula-side component  30  for secure connection thereto. In embodiments including such a spacer  100 , the spacer could be fitted between the humeral-side sub-component  20   a  and scapula-side component  30  outside of the patient, and the assembly  20   a ,  100 ,  30  would be inserted typically through a posterior incision as described above, with the tab sub-component  20   b  being secured to the assembly once the assembly is located between the humerus  12  and the scapula  14 . Alternatively, the humeral facing and scapular facing components may be inserted into the joint, and the spacer part then inserted between them, so as to tense the surrounding joint soft tissues. 
     A range of spacers  100  of different thicknesses may be provided to allow even greater flexibility to the surgeon in selecting the ‘thickness’ of the PSG  10  intra-operatively, so that an optimum fit and optimum passive joint stiffness can be selected. More than a single spacer  100  could be used; a number of spacers being stacked for insertion between the humeral-side sub-component  20   a  and the scapula-side component  30 . 
     Note that the use of zero or more spacers  100  will depend on the laxity of the patient&#39;s joint and the surgeon&#39;s experience/preference, whereby spacers can be added to increase the passive tension in a lax joint, and thus produce a greater compressive force on the inserted components to help in properly maintaining their position. 
     In one embodiment, rather than comprising an assembly of component parts  20 ,  30 , the PSG  10  may instead comprise a single, monoblock component (not shown), typically formed from a single piece of material. An advantage of a monoblock unitary construction of the PSG is that it likely to be stiffer and stronger than a similar PSG of modular construction. It would also not require assembly within the confines of the joint during the surgical procedure, so could be less fiddly to insert. However, the larger overall dimensions of the monoblock PSG as compared to the modular components of an equivalent modular PSG would likely require a larger surgical incision and might therefore not be suitable for minimally-invasive procedures. It is also not possible to insert a spacer component to adjust the ‘thickness’ of the PSG to find the tightest possible fit between the bones. To mitigate for this fact, a range of monoblock PSGs of different ‘thicknesses’ could be provided, so that the surgeon can select the optimum size intra-operatively. 
     In an alternative embodiment, the two bone-facing components may be adapted for attachment to a surgical instrument configured in the same way as a self-retaining soft tissue retractor instrument with a central pivot between two parts such that it works like a scissors mechanism. For example, the bone-facing components may have holes in them which mate with prongs on the ends of the arms of the retractor instrument and which also have means to ensure that they lock into a predetermined position. Thus, in use, the two bone-facing components may be inserted into the joint while close together (back to back), and then the surgeon may squeeze the handles of the retractor together, thereby tensing the surrounding soft tissues of the joint as the bone-facing components move apart and are retained in the spaced-apart position by means of a ratchet mechanism in the retractor instrument. 
     As known in the art, certain procedures require the insertion of a guide wire  200 , and an associated drilling step. This is illustrated in  FIGS. 11   a - d . FIGS.  11   a  and  b  show the passage of a driver  210  through the patient&#39;s humerus  12  and beyond, through the PSG  10 ′, into the scapula  14 , the bones being held secure in position by the PSG  10 ′, and the driver  210  being aligned by reference to the PSG  10 ′, optionally through use of an additional guide component (not shown) to aid in accurately inserting the guide wire  200  extra-articularly. In  FIGS. 11 c  and  d   , the humerus is removed for clarity and the passage of the driver  210  and guide wire  200  can be seen through the generally concave portion  61  of the contoured surface  60 , which includes a clearance hole  218  positioned in a pre-operatively planned location and orientation for proper positioning of the guide wire during the procedure. In particular, the relative configuration of the two primary contoured surfaces  60 ,  80  as dictated by the pre-operative plan ensures that the desired axes of the guide wire  200  for each bone  12 ,  14  are collinear, thus allowing a guide wire  200  to be driven along this axis (typically using said attached secondary guide) to concurrently produce the required guide for both bones. 
     In particular, the guide wire  200  according to this embodiment is relatively short, for insertion into just the glenoid, not extending back into the humerus. This short guide wire portion  200  is attached to a mating driver  210  to drive the wire through the humerus  12  and into the glenoid. As known in the art of guide wire placement, the distal end of the guide wire includes features (not shown) that enable it, as driven by the attached bit  210 , to ‘self-drill’ through bone. The short guide wire  200  is thus able to be driven through the humerus and into the glenoid without the need for a pilot hole. Once located in position within the glenoid, the guide wire  200  can be detached from the driver  210 , acting in effect as a pin in the glenoid, aligned to the pre-determined, properly-oriented axis along which it was inserted, for use to guide further steps in the procedure. The driver  210  would then be retracted back through the humerus, leaving a bone tunnel. The bone tunnel produced through the humerus  12  by the passage of the short guide wire  200  and associated driver  210  can itself act as a humeral guide. In traditional surgery, an independent guide wire would be inserted into the humerus, being inserted from the articular surface and oriented as required, but in this embodiment, the humeral guide wire can be replaced by the bone tunnel because the bone tunnel and the guide wire  200  are necessarily aligned along a common axis as a result of the insertion technique. This can be advantageous because the shorter length of the guide wire  200  means that the technique can be undertaken in a more confined space while still providing a means of guiding surgical steps (guide pin for the scapula, and bone tunnel for the humerus). 
     The driver  210  may be mated to the guide wire  200  by any suitable connection that allows the guide wire  200  to be driven with sufficient force for insertion through the humerus  12  and into the glenoid. Suitable examples would include, but are not limited to, screwdriver- or allen-type bits and associated mating slots. 
     In the illustrated embodiment, the driver comprises a drill bit  210  with a connection feature (not shown) incorporated into its distal end. The use of a drill bit  210  as the driver allows for a relatively wide diameter bone tunnel to be formed through the humerus  12  simultaneously with the driving of a relatively narrow diameter short guide wire  200  into the glenoid. Alternatively, the bone tunnel could be enlarged using a conventional drill bit in a separate and subsequent step to the insertion of the guide wire  200 . 
     A through slot  220  is provided from the clearance hole  218  to the anterior side of the PSG  10 ′ so that the PSG  10 ′ can be removed from the joint once the guide wire  200  has been fixed in place, the guide wire  200  ‘passing through’ the slot  220  as the PSG  10 ′ is removed to the posterior. 
     In certain embodiments, a further degree of freedom could be provided in the PSG, for example by including features on the mating surfaces of adjacent components or sub-components that allow them to rotate relative to one another before they are locked or fixed in position. This would provide the surgeon with additional flexibility with regards to limb orientation. In other words, one could pre-plan a range of valid relative bone configurations, then design and manufacture a PSG incorporating features that allow the surgeon the scope to select any one of those configurations once the patient is prepped and on the operating table. 
     In addition to the above-described applications in articular joint structures, similar principles can be used to produce patient-specific guides for use in realignment of bone fractures, or in sarcoma surgery. In this context, rather than the guide being made to fit between two opposing articular joint surfaces, it would instead fit in between two bone fragments where part of the original bone has been lost, for example either due to the compressive nature of the fracture or where it has been removed deliberately in a surgical procedure. The two bone fragments can be properly spaced and aligned by the guide, then plated. 
     It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. Accordingly, individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure as defined by the accompanying claims.