Reverse shoulder prosthesis and related methods

Abstract

A prosthetic joint assembly for joining the humerus to the scapula is disclosed, the prosthetic joint assembly comprising a humeral component adapted to engage the humerus with a concave dish and a scapula component adapted to engage the scapula with a convex surface adapted to engage the concave dish, the prosthetic center of rotation being displaced in an inferior and medial direction relative to the natural center of rotation and the humerus being displaced in an inferior direction relative to the natural center of rotation when the components are implanted and engaged in a rest position.

Description

[Technical Field]

[0001] The present invention relates generally to prosthetic implants, and more particularly to prosthetic implants for use in total shoulder replacement procedures. [Background technology]

[0002] Figures 1 and 2 illustrate the characteristics of a normal human shoulder in both rest and abduction positions. The human shoulder joint is formed where the head (2) of the humerus (4) engages with the glenoid cavity (5) of the scapula (6). During normal shoulder function, the articular surface of the humeral head (2) fits into the glenoid cavity (5) like a ball and socket, allowing the humerus (4) to freely pivot relative to the scapula (6) while remaining held within the shoulder joint. This pivoting occurs around a center of rotation (8) between the humerus (4) and scapula (6) (hereafter referred to as the "natural center of rotation" or "nCOR"). This center of rotation (8) is usually located at or near the center of the humeral head (2). In a healthy shoulder, upward movement of the humerus (4) (in the frontal plane) is limited by a process of the scapula (6) called the acromion (10), and specifically by a portion of the scapula called the coracoid process (12). Furthermore, in a healthy shoulder, the humeral head (2) is held within the glenoid cavity (5) by a complex of muscles and tendons commonly referred to as the rotator cuff (not shown for clarity), which surround and stabilize the shoulder joint.

[0003] Due to injury, trauma, degenerative changes, disease (such as arthritis), or other conditions, some people experience pain, discomfort, or difficulty moving their shoulder through its range of motion, or may be unable to move their shoulder at all. In some situations, shoulder joint conditions are treated by a partial joint replacement. In a partial replacement, the head (2) of the humerus (4) is replaced with a prosthetic implant, while the glenoid cavity (5) is left relatively intact. However, there are many cases where the glenoid cavity (5) is so altered or damaged that it is no longer able to engage and retain the head (2) of the humerus (4), making partial replacement either inadvisable or impossible.

[0004] When partial replacement is not possible, one viable treatment is to replace the head (2) of the humerus (4) and the glenoid cavity (5) with a prosthetic shoulder joint in a procedure commonly referred to as a total shoulder arthroplasty. Furthermore, in the majority of conditions requiring a total shoulder arthroplasty, the rotator cuff is also damaged, preventing the head (2) of the humerus (4) from stabilizing within the glenoid cavity (5). Therefore, the components of a total shoulder arthroplasty are reverse-type. In a reverse-type total shoulder arthroplasty, the components implanted into the scapula (corresponding to the glenoid cavity (5)) are convex or ball-shaped, while the components implanted into the head (2) of the humerus (4) are concave or socket-shaped. This reverse configuration has been shown to enhance stability in the absence of a fully healthy rotator cuff.

[0005] While efforts have been made to develop reverse shoulder prostheses, they have often not produced the desired results. Drawbacks of currently available reverse shoulder prostheses include, for example, a very limited range of motion, a tendency to dislocation, excessive stress on the bone resulting in failure and / or fracture of the prosthesis, complications such as infection, and premature wear resulting in the need for additional surgeries over the patient's lifetime. Furthermore, current methods for implanting such prosthetic shoulder joints often result in poor joint alignment, resulting in inadequate joint function and range of motion.

[0006] Therefore, there is a need in the art for a reverse shoulder prosthesis and related methods for implanting the same that provides the patient with a range of motion and alignment approximating that of a healthy shoulder, is durable, provides adequate support for the remaining humerus and sternum, and avoids some or all of the drawbacks of existing shoulder prostheses. Summary of the Invention [Problem to be solved by the invention]

[0007] The inventors have determined that the deficiencies of existing reverse total shoulder prostheses result from (a) an improper location of the center of rotation between the scapular and humeral components of the prosthesis (hereinafter "prosthesis center of rotation" or "pCOR") and (b) an improper absolute position of the humerus relative to the scapula after the prosthesis is placed.

[0008] The present invention provides a novel reverse total shoulder prosthesis that, when implanted, properly positions the pCOR and humerus to provide an optimally functioning prosthesis. More specifically, the pCOR is positioned medially and inferiorly to the position of the nCOR. Furthermore, for optimal positioning, the humerus is translated lower than the position of the nCOR. [Means for solving the problem]

[0009] Referring to FIG. 3, which shows a close-up of the contact surface between the scapula (6) and humerus (4), the inventors have determined through experiments and simulations that the translation vector (20) of the natural center of rotation (8) relative to the optimal pCOR position (22) has a range of ratios between the inferior (24) and medial (26) components of the vector (20). The ratio between the inferior (24) and medial (26) components of the center of rotation translation vector (20) ranges from 0.6 to 1.2 (resulting in an angle range of 30 to 50 degrees below horizontal), with a preferred ratio being 0.85 to 1.15 (resulting in an angle range of 40 to 49 degrees below horizontal). In many cases, the optimal solution is one in which the medial (26) and inferior (24) components are equal, i.e., a ratio of 1 (resulting in an angle of 45 degrees below horizontal). Similarly, the optimal magnitude of the center of rotation translation vector (20) also ranges from 60% to 80% of the radius (28) of the patient's humeral head (2). In many cases, the optimal magnitude of the center of rotation translation vector (20) is approximately 70% of the radius (28) of the patient's humeral head (2).

[0010] Similarly, the inventors have determined that the orientation angle (32) of the humeral translation vector (30) relative to the nCOR (8) is between 75 and 105 degrees below horizontal. In many cases, an optimal solution is achieved when the humeral translation vector (30) is 90 degrees below horizontal. Similarly, the optimal magnitude of the humeral translation vector (30) ranges from 80% to 120% of the radius (28) of the patient's humeral head (2). In many cases, the optimal magnitude of the humeral translation vector (30) is approximately 100% of the radius (28) of the patient's humeral head (2).

[0011] To this end, a prosthetic joint assembly for joining the humerus to the scapula is disclosed. The humerus and scapula have a natural center of rotation relative to one another, the humerus has a humeral head diameter, and the humerus is positionable relative to the scapula between a rest position and an abducted position. The prosthetic joint assembly includes a humeral component having two opposite ends, a first end having a humeral stem adapted for fixed engagement with the humerus and a second end having a concave dish; and a scapular component having two opposite sides, the first side having a scapular base adapted for fixed engagement with the scapula and the second side having a convex surface adapted to engage the concave dish. When the concave dish and convex surfaces are engaged, the humeral component is free to pivot relative to the scapular component about the prosthetic center of rotation. When the humeral stem engages the humerus, the scapular stem engages the scapula, and the concave and convex surfaces are engaged, the prosthesis center of rotation is displaced downward and medially relative to the natural center of rotation. When the humeral stem engages the humerus, the scapular base engages the scapula, the concave and convex surfaces are engaged, and the humerus is in a resting position, the humerus is displaced downward relative to the natural center of rotation. The direction of humeral displacement is 75 to 105 degrees below horizontal. The ratio of the downward displacement of the prosthesis center of rotation to the medial displacement of the prosthesis center of rotation is within the range of 0.6 to 1.2 (30 to 50 degrees below horizontal), preferably within the range of 0.85 to 1.15 (40 to 49 degrees below horizontal), and optimally equal to 1 (45 degrees below horizontal). The distance of the prosthetic center of rotation displacement relative to the natural center of rotation is 60% to 80% of the radius of the humeral head, optimally equal to 70%. The distance of the humerus displacement relative to the natural center of rotation is 80% to 120% of the radius of the humeral head, optimally equal to 100%.

[0012] Also disclosed is a prosthetic joint assembly for joining a humerus to a scapula, wherein the humerus and scapula have a natural center of rotation relative to one another, the humerus has a humeral head diameter, and the humerus is positionable relative to the scapula between a rest position and an abducted position. The prosthetic joint assembly includes a humeral component having two opposite ends, the first end adapted for fixed engagement with the humerus and the second end having a concave surface; a scapula baseplate having a longitudinal dimension and two opposite sides, the first side adapted for fixed engagement with the scapula and the second side having a trunnion offset downwardly from the center of the longitudinal dimension; and a glenosphere component having two opposite sides, the first side having an opening adapted for fixed engagement with the trunnion and the second side having a convex surface adapted to engage the concave surface. When the concave and convex surfaces are engaged, the humeral component is free to pivot relative to the glenosphere about the prosthesis center of rotation. In this embodiment, the humeral component optionally includes: a stem component having a longitudinal axis and two opposite ends, a first end having a humeral stem and a second end having a coupler contact surface; a coupler component having two opposite ends, the first end having a stem contact surface adapted to fixedly engage the coupler contact surface of the stem component and the second end having a cup contact surface; and a cup component having two opposite sides, the first side having a coupler contact surface adapted to fixedly engage the cup contact surface of the coupler component and the second side having a concave surface.

[0013] A method for joining the humerus to the scapula with a prosthesis is also disclosed. The humerus and scapula have a natural center of rotation relative to each other, the humerus has a humeral head diameter, and the humerus is positionable relative to the scapula between a rest position and an abducted position. The method comprises the steps of (1) fixedly engaging a scapular component with the scapula, and (2) fixedly engaging a humeral component with the humerus, the humeral component adapted to engage the scapular component and freely pivot relative to the scapular component about the prosthesis center of rotation. (3) When the humeral component and the scapular component are engaged, the prosthesis center of rotation is displaced inferiorly and medially relative to the natural center of rotation. (4) When the humeral component is engaged with the scapular component in the rest position, the humerus is displaced inferiorly relative to the natural center of rotation. (5) The direction of humeral displacement is 75 to 105 degrees below horizontal, optimally 90 degrees below horizontal. (6) The ratio of the inferior displacement of the prosthesis center of rotation to the medial displacement of the prosthesis center of rotation is in the range of 0.6 to 1.2 (30 to 50 degrees below horizontal), preferably in the range of 0.85 to 1.15 (40 to 49 degrees below horizontal), and optimally equal to 1 (45 degrees below horizontal). (7) The displacement distance of the prosthesis center of rotation from the natural center of rotation is 60% to 80% of the radius of the humeral head, and optimally 70%. (8) The displacement distance of the humerus from the natural center of rotation is 80% to 120% of the radius of the humeral head, and optimally equal to 100%.

[0014] While the present invention is illustrated and described herein as embodied in a shoulder prosthesis, it is not intended to be limited to the details shown, as various modifications and structural changes can be made therein without departing from the spirit of the invention and within the scope of the appended claims and equivalents thereof. Moreover, many of the principles and techniques described in the following description are applicable to prostheses used in other joints of the human anatomy.

[0015] The inventive features, together with further objects and advantages thereof, will be best understood from the following description of specific disclosed embodiments taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0016] [Figure 1] 1 shows a human shoulder bone in a resting position as a reference for explaining the operating principles of the present invention; [Figure 2] FIG. 1 shows the human shoulder bones in an abducted position as a reference for explaining the operating principles of the present invention. [Figure 3] FIG. 1 is a close-up view of the contact surface between a human scapula and humerus, illustrating the relocation of the center of rotation and humerus position in accordance with the present invention. [Figure 4] FIG. 1 is an isometric view of the scapular component of a shoulder prosthesis in accordance with the present invention. [Figure 5] FIG. 5 is an exploded orthographic view of the scapular component of the shoulder prosthesis shown in FIG. 4. [Figure 6] FIG. 5 is a lateral view of the scapular component of the shoulder prosthesis shown in FIG. 4 . [Figure 7] 1A and 1B show various variations of the scapular component of the shoulder prosthesis of the present invention, each having different sizes. [Figure 8] 1 is an isometric view of the humeral component of a shoulder prosthesis in accordance with the present invention; [Figure 9] FIG. 9 is an exploded orthographic view of the humeral component of the shoulder prosthesis shown in FIG. 8 . [Figure 10] FIG. 9 is a side view of the humeral component of the shoulder prosthesis shown in FIG. 8 . [Figure 11] 1A and 1B illustrate several variations of different sizes of the humeral component of a shoulder prosthesis according to the present invention. [Figure 12] 1 shows a shoulder prosthesis according to the present invention implanted in a human scapula and humerus in a rest position. [Figure 13] FIG. 13 shows the shoulder prosthesis of FIG. 12 with the humerus and scapula omitted for clarity. [Figure 14] 1 shows a shoulder prosthesis according to the present invention implanted in a human scapula and humerus in an abducted position. [Figure 15] FIG. 15 shows the shoulder prosthesis of FIG. 14 with the humerus and scapula omitted for clarity. [Figure 16]10A-10C show an optional variation of the humeral component according to the invention with a large convex bearing element. [Figure 17] 10A-10C show an optional variant of the humeral component according to the invention with a two-part concave bearing element. DETAILED DESCRIPTION OF THE INVENTION

[0017] The goal of the prosthesis disclosed herein is to optimize the final alignment of the pCOR and humerus, as detailed above and shown in Figure 3, which is an enlarged view of the interface between the humerus and scapula, showing the translation vectors and final alignment of the pCOR and humerus. This optimal positioning is achieved through the use of the scapular component (100) and humeral component (200), as described in more detail in the following paragraphs.

[0018] Figures 4, 5, and 6 are isometric, exploded, and side views, respectively, fully illustrating the structure of the scapular component (100). The scapular component (100) includes a base portion (102) adapted for fixed engagement with the glenoid cavity (5) of the scapula (6) via one or more threaded holes (104, 106, 108, 110). The threaded holes are adapted to receive one or more polyaxial locking screws (112) or uniaxial locking screws (116) with corresponding locking caps (114) to secure the base portion (102) to the glenoid cavity (5). The posterior surface (118) of the base portion (102) optionally includes a stem (120) adapted for implantation in the glenoid cavity (5) to provide additional torsional support to the scapular component (100).

[0019] The front surface 122 of the base 102 includes a generally cylindrical trunnion 124. The trunnion 124 optionally includes a bore and has internal threads 148 adapted to receive a monoaxial locking screw (not shown) or a polyaxial locking screw 126 and a corresponding locking cap 128. The outer surface of the trunnion 124 is adapted to receive a glenosphere core 130, which in turn is adapted to receive a hollow glenosphere cover 132. The glenosphere core 130 has one or more protrusions 134 that closely match corresponding openings in the glenosphere cover 132, thereby preventing the glenosphere cover 132 from rotating relative to the glenosphere core 130 after assembly. Once assembled, the glenosphere core (130) and glenosphere cover (132) comprise a spherical glenosphere assembly (144) having an outer surface (152) adapted to contact the humeral component (200).

[0020] The glenosphere core 130 and glenosphere cover 132 have threaded holes 136, 138 aligned with the trunnion 124 and locking cap 128, and are adapted to receive a locking screw 140 for securing the glenosphere assembly 144 to the base 102. To accomplish this, the locking cap 128 has internal threads 142 recessed therein that correspond to the external threads of the locking screw 140. If the trunnion 124 is not bored and does not receive the polyaxial locking screw 126 and locking cap 128, an internally threaded hole can be provided at the tip of the trunnion 124 to engage the locking screw 140. While a two-piece glenosphere assembly 144 is shown in the described embodiment, a single-piece glenosphere (not shown) can also be used to achieve the same effect.

[0021] With the exception of the glenosphere cover 132, all components of the scapular component 100 are preferably metallic in composition, such as, but not limited to, biocompatible surgical alloys, such as cobalt-chromium-molybdenum (CoCrMo) or titanium alloys suitable for biomedical applications such as joint replacement. The glenosphere cover 132 is formed from a durable yet resilient plastic material, such as, for example, ultra-high molecular weight polyethylene (UHMWPE). If a single-piece glenosphere is used, it may be of metal or plastic construction.

[0022] As can be clearly seen, the trunnion 124 is located significantly below the center 150 of the base 102. This ensures that the center 146 of the glenosphere assembly 144, which serves as the prosthesis center of rotation 22, is located sufficiently below. Furthermore, because the center 146 of the glenosphere assembly 144 is located very close to the base 102, it is also located sufficiently medially. As previously mentioned, locating the prosthesis center of rotation 22 below and medially relative to the natural center of rotation 8 is one of the primary objectives achieved by the arrangement of the components described.

[0023] Next, FIG. 7 illustrates several scapular component (100) variations in size that can be used depending on the patient's anatomy and the physician's intended magnitude and angle of translation of the pCOR (22). As can be seen, some sizes have a single mounting screw and corresponding base hole, while other sizes have up to four holes and screws. Furthermore, the base (102) shapes vary from circular to oval. However, the base (102) can have other shapes without departing from the principles of the disclosed invention. The described modular configuration allows glenosphere assemblies (144) of various sizes and shapes to be used with bases (102) of various sizes and shapes to assemble the optimal scapular component (100) for the patient's anatomy and the desired location of the prosthesis' center of rotation (22).

[0024] Figures 8, 9, and 10 are isometric, exploded, and side views, respectively, fully illustrating the structure of the humeral component 200. The humeral component 200 includes a stem 202, a coupler 204, a cup 206, and an assembly screw 208. The stem 202 is a substantially elongated member with an intermedullary stem 210 at one end and a conical portion 212 at the other end. The stem 202 is bored along its longitudinal axis so that the assembly screw 208 can enter the intermedullary stem 210 end and engage the coupler 204 at the conical portion 212 end. The intermedullary stem 210 comprises a typical bone stem adapted to penetrate the medullary bone of the humerus 4 and securely engage the bone. The intermedullary stem 210 can be adapted for use with or without cement. The conical portion (212) has a gradually increasing diameter and terminates in one or more fins (214) adapted to engage trabecular bone within the humerus (4) to transfer torsional loads and prevent rotation of the stem (202) after implantation. The upper portion of the conical portion (212) has a stem shaft opening (218) adapted to receive the coupler (204).

[0025] The coupler 204 includes a stem-engaging shaft 220 adapted to engage with the stem shaft opening 218 to form a secure interference or press fit between the stem 202 and the coupler 204. Additionally, the bottom surface of the stem-engaging shaft 220 includes an opening 222 with internal threads 238. The internal threads 238 receive the threads 224 of an assembly screw 208 inserted through the bottom of the medullary stem 210 side of the stem 202. In one embodiment, the stem-engaging shaft 220 and the shaft opening 218 form a Morse taper that provides a secure friction fit. To further secure the engagement between the stem 202 and the coupler 204 against torsional forces, the stem-engaging shaft 220 may be offset from the centerline of the stem 202. The off-center location of the stem engagement shaft (220) combined with the Morse taper provides a very strong, torsion-resistant fit between the stem (202) and coupler (204) after the assembly screws (208) are tightened.

[0026] The coupler 204's upper surface 226 includes a sloped receiving area with an opening for receiving the cup 206. The slope of the upper surface 226 is angled to properly displace the humerus 4 laterally and inferiorly relative to the pCOR 22 after the prosthesis is assembled. The intermediate coupler portion 227 is configured to further lower the humerus 4 relative to the pCOR 22, if needed to optimally position the humerus. The cup shaft opening 228 in the upper surface 226 is adapted to receive a cup shaft 230 to secure the cup 206 to the coupler 204. The cup shaft opening 228 and cup shaft 230 may include additional Morse tapers to ensure a secure engagement between the coupler 204 and the cup 206. Additionally, coupler (204) has one or more inner (232) and outer (234) suture attachment points.

[0027] One end of the cup 206 includes a concave or dished surface 236 adapted to closely and correspondingly engage the outer surface 152 (see FIGS. 4-6) of the glenosphere assembly 144. The other end of the cup 206 includes a cup shaft 230. The cup shaft 230 engages the cup shaft opening 228 to form an interference fit, as previously described, to securely engage the cup 206 with the coupler 204. Additionally, the cup shaft 230 may optionally include a Morse taper or other type of shallow, self-retaining taper.

[0028] All parts of the humeral component (200) are preferably metallic compositions, such as, but not limited to, biocompatible surgical alloys, such as cobalt chromium molybdenum (CoCrMo) or titanium alloys suitable for biomedical applications such as joint replacement.

[0029] 11 illustrates a variation of several differently sized humeral components (200) that can be used depending on the patient's anatomy and the physician's intended amount and angle of further translation of the humerus (4). As can be seen, the various sizes include stems (202) of different diameters, conical sections (212) of different tapers, intermediate coupler sections (227) of different lengths, and concave or dished sections (236) of different diameters and curvatures that fit into corresponding glenosphere assemblies (144). The described modular configuration allows for the use of differently sized stems (202) with differently sized couplers (204) and cups (206) to assemble the optimal humeral component (200) for the patient's anatomy and the optimal displacement of the humerus (4) relative to the prosthesis' center of rotation (22).

[0030] The procedure for implanting the disclosed shoulder prosthesis into a patient involves the following general steps: First, the size and relative positions of the patient's anatomy—humerus (4), humeral head (2), scapula (6), glenoid cavity (5), and natural center of rotation (8)—are measured. Next, based on the measurements, the scapular component (100) and humeral component (200) are assembled using various modular components, including appropriately sized base (102), glenosphere assembly (144), stem (202), coupler (204), and cup (206) components. The glenoid cavity (5) is then prepared to receive the scapular component (100), which is implanted in the appropriate position to achieve the desired level of reduction in pCOR (22). Next, the humeral head is removed from the humerus (4), and the humeral component (200) is implanted in its place. Finally, the scapular component (100) and humeral component (200) are mated, and the shoulder joint is moved from a rest position to an abducted position and vice versa for testing. If any impingement between the humerus and scapula is detected, one or more modular elements of the scapular component (100) or humeral component (200) can be replaced to optimize the alignment of the shoulder joint.

[0031] Figures 12 and 13 show a fully assembled and implanted shoulder prosthesis of the present disclosure in its "rest" position. Figure 12 shows an assembled shoulder prosthesis according to the present invention implanted into a human scapula (6) and humerus (4). Figure 13 shows the same assembled prosthesis, including the scapular component (100) and humeral component (200), but without the bones for clarity. As can be seen, the prosthesis center of rotation (22) is significantly offset medially and inferiorly relative to the natural center of rotation (8), and similarly, the humerus (4) is also significantly offset inferiorly relative to its natural center of rotation (8). This arrangement optimizes the humerus (4) for full rotation into an abducted position, minimizing the risk of impingement on any part of the scapula (6).

[0032] Next, Figures 14 and 15 show a fully assembled and implanted shoulder prosthesis of the present disclosure after it has been rotated into an "abduction" position. Figure 14 shows an assembled shoulder prosthesis according to the present invention implanted into a human scapula (6) and humerus (4). Figure 15 shows the same assembled prosthesis, including the scapular component (100) and humeral component (200), but without the bones for clarity. As can be seen, the prosthesis center of rotation (22) remains significantly offset medially and inferiorly relative to the natural center of rotation (8). In the abduction position, the humerus (4) is in approximately the same position as in a healthy shoulder (see Figure 2), and the shoulder's range of motion has been nearly fully restored.

[0033] The disclosed prosthesis can be modified in various ways to accommodate specific situations that arise from time to time. One example of such a situation is when, after a reverse total shoulder arthroplasty using the disclosed prosthesis, it is determined that the patient is no longer suitable for continued use of the reverse shoulder prosthesis. This situation may arise, for example, if a reinjury or degenerative changes in the patient cause the scapula to no longer support the scapular component (100). In such a situation, the scapular component can be removed and the humeral component modified to place a glenosphere in place of the cup to contact the natural glenoid cavity. This avoids a complete replacement of the humeral component, which can be difficult and / or traumatic for the patient. Figure 16 illustrates an optional modification of the humeral component (200') to address such a situation. As shown, the cup component (206) of the humeral component is removed and a large glenosphere (240) is attached to the coupler (204) in its place.

[0034] FIG. 17 shows an alternative embodiment of the humeral component (200") in which the concave bearing surface is formed of a plastic material and the remainder is metal. This is achieved by replacing the cup (206) with a two-piece component consisting of a metal tray (242) and a cooperating plastic cup insert (244) having a concave surface.

[0035] While numerous embodiments of the present invention have been described, these embodiments are illustrative and not limiting, and numerous variations will be apparent to those skilled in the art. For example, each element described herein can be configured to any desired size (e.g., each element described herein can be configured to any desired custom size, or each element described herein can be configured to any desired size selected from a "group" of sizes, such as small, medium, or large). Furthermore, one or more of the components can be formed from (a) any biocompatible material (which biocompatible material may be applied to allow or prevent bone formation on the surface, depending on the physician's intent), (b) plastic, (c) fiber, (d) polymer, (e) metal (pure metal and / or alloy), or (f) any combination thereof. Furthermore, any prosthesis can employ any number of projections (e.g., for initial fixation by bonding with cement and / or for secondary fixation by bonding with cement). Furthermore, any prosthesis can employ any number of female features to increase the bonding area. Additionally, any prosthesis may employ any number of bone-engaging male features to enhance primary / secondary fixation. Additionally, any prosthesis may employ any number of bone screws (e.g., for primary fixation and / or secondary fixation). Additionally, any of the steps described herein may be performed in any desired order (and any additional steps may be freely added and / or any steps may be freely omitted).

[0036] Furthermore, various modifications and additions can be made to the described exemplary embodiments without departing from the scope of the present invention. For example, while the above-described embodiments refer to particular features, the scope of the present invention also includes embodiments having different combinations of features or embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to include all alternatives, modifications, variations, and equivalents that fall within the scope of the claims.

Claims

1. 1. An artificial joint assembly for joining a humerus to a scapula, wherein the humerus and scapula have a natural center of rotation relative to one another, the humerus has a humeral head radius, and the humerus is positionable between a rest position and an abducted position relative to the scapula, the artificial joint assembly comprising: a humeral component having two opposed humeral epiphysis, a first end of the humeral epiphysis having a humeral stem adapted for fixed engagement with the humerus, and a second end of the humeral epiphysis having a concave dish; b. A scapular component having a scapular baseplate and a glenosphere assembly, the scapula base plate having a surface adapted to fixedly engage the scapula and a trunnion having a first threaded hole configured to receive a threaded fastener; a scapular component, the glenosphere assembly having an opening configured to contact the trunnion, a second screw hole coaxial with the first screw hole and opening into the opening, and a convex surface adapted to engage the concave dish; c) the threaded fastener extending from the second threaded hole, passing through the first threaded hole, and extending beyond the surface of the scapular baseplate to engage the scapular component to the scapula; Equipped with d. when the concave dish and the convex surface are engaged, the humeral component is free to pivot relative to the scapular component about a prosthesis center of rotation; e. When the humeral stem engages the humerus, the scapular component engages the scapula, and the concave dish and convex surface engage, the prosthesis center of rotation is displaced a distance in an inferior and medial direction relative to the natural center of rotation. Artificial joint assembly.

2. The ratio of the downward displacement of the prosthesis center of rotation to the inward displacement of the prosthesis center of rotation is within a range of 0.6 to 1.

2. The prosthetic joint assembly of claim 1.

3. The ratio of the downward displacement of the prosthesis center of rotation to the inward displacement of the prosthesis center of rotation is within a range of 0.85 to 1.

15. The prosthetic joint assembly of claim 1.

4. a ratio of the inferior displacement of the prosthesis center of rotation to the inward displacement of the prosthesis center of rotation is about 1; The prosthetic joint assembly of claim 1.

5. The distance of displacement of the prosthetic center of rotation relative to the natural center of rotation is 60% to 80% of the radius of the humeral head. The prosthetic joint assembly of claim 1.

6. The distance of the displacement of the humerus relative to the natural center of rotation is 80% to 120% of the radius of the humeral head. The prosthetic joint assembly of claim 1.

7. When the humeral stem engages with the humerus, the scapular component engages with the scapula, and the concave dish portion and the convex surface engage, and the humerus is in the rest position, the humerus is moved a distance in a direction downward at an angle between 75 and 105 degrees from the horizontal relative to the natural center of rotation. The prosthetic joint assembly of claim 1.

8. The screw fastener is a. a first screw configured to contact the first threaded hole; b. a second screw configured to contact the second screw hole; c. a locking cap configured to securely engage the first thread and the second thread; Including, The prosthetic joint assembly of claim 1.

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