Patent Document

TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to the field of orthopedics, and, more particularly, to glenoid component apparatuses for shoulder arthroplasty and methods for making them. 
     BACKGROUND 
     Arthroplasty is the surgical replacement of one or more bone structures of a joint with one or more prostheses. Shoulder arthroplasty often involves replacement of the glenoid fossa of the scapula with a prosthetic glenoid component. The conventional glenoid component typically provides a generally laterally or outwardly facing generally concave bearing surface against which a prosthetic humeral head (or, alternatively, the spared natural humeral head in the case of a glenoid hemi-arthroplasty) may bear during operation of the joint. The conventional glenoid component typically also includes a generally medially or inwardly projecting stem for fixing the glenoid component in a cavity constructed by suitably resecting the glenoid fossa and suitably resecting cancellous bone from the glenoid vault. 
     Various stem designs have been proposed for ensuring proper alignment and secure and lasting fixation of the glenoid component within the glenoid vault. However, the glenoid vault has a complex morphology. While three-dimensionally shaping a stem for compatibility with the endosteal walls of the glenoid vault can potentially significantly enhance fixation of the glenoid component, historical designs have not taken full advantage of this opportunity. 
     One advantageous approach is described in co-pending application Ser. No. 10/259,045, published on Apr. 1, 2004 as US2004/0064189 A1 and entitled “Concave Resurfacing Prosthesis”, the disclosure of which is incorporated herein by reference. In this approach, a glenoid component is fitted to at least partially fill a cavity formed in the glenoid vault. The component has a generally oval inverted dome shape to generally conform to the shape of the vault. However, it is recognized in the &#39;045 Application that exact sizing of the glenoid component to the vault cavity is made difficult by the unique anatomy of each patient. To address this difficulty, the &#39;045 Application discloses providing a series of differently sized glenoid components. 
     There remains a need for a glenoid component that is more nearly sized and shaped in three-dimensions to fill the cavity in the glenoid vault. There is a further need for a technique that facilitates preparation of such a component, and especially a component that has more universal applicability to the anatomy of most patients. 
     SUMMARY OF THE INVENTION 
     The present invention provides a glenoid component apparatus for shoulder arthroplasty. The apparatus includes a bearing portion and further includes a stem portion extending from the bearing portion. The stem portion models a normalized or pathologic glenoid vault morphology. 
     In an alternative embodiment, the present invention provides a method for making a glenoid component for shoulder arthroplasty. The method includes obtaining a model of a normal or pathologic glenoid vault morphology and further includes producing a portion of the glenoid component based on the model. 
     In another alternative embodiment, the present invention provides a glenoid component apparatus for a shoulder joint including at least one of a natural humeral component and a prosthetic humeral component. The apparatus includes a means for bearing against at least one of the natural humeral component and the prosthetic humeral component. The apparatus further includes a means, extending from the bearing means, for modeling a normal glenoid vault morphology. 
     The above-noted features and advantages of the present invention, as well as additional features and advantages, will be readily apparent to those skilled in the art upon reference to the following detailed description and the accompanying drawings, which include a disclosure of the best mode of making and using the invention presently contemplated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded perspective view of an exemplary shoulder prosthesis including an exemplary glenoid component according to the present invention; 
         FIG. 2   a ,  FIG. 2   b , and  FIG. 2   c  show a flow diagram of an exemplary method for configuring the stem of the prosthesis of  FIG. 1  to model a normal or pathologic glenoid vault morphology; 
         FIG. 3  shows a rectangular (“Cartesian”) coordinates reference system relative to the plane body of a typical scapula as defined by three surface points of the scapula; 
         FIG. 4  shows the superior-inferior (“SI”) dimension and the anterior-posterior (“AP”) dimension of the typical glenoid fossa; 
         FIG. 5  shows a table listing exemplary range and exemplary average SI dimension for six exemplary sub-groups of scapulae from a scapulae sample based on their SI dimensions; 
         FIG. 6  shows an exemplary substantially complete tracing (toward the inferior end of the typical glenoid fossa) of the endosteal walls of the typical glenoid fossa; 
         FIG. 7  shows an exemplary partial tracing (toward the inferior end of the typical glenoid fossa) of the endosteal walls of the typical glenoid fossa as a result of fossa occlusion in the region of the typical scapular spine; 
         FIG. 8  shows views of a volumetric rendering of a relatively complex model of the normal glenoid vault morphology of the scapulae sample; 
         FIG. 9  shows views of a volumetric rendering of an intermediate 3-D model of the normal glenoid vault morphology of the scapulae sample based on the relatively complex 3-D model of  FIG. 8 ; 
         FIG. 10  shows a perspective view of a simplified 3-D model of the average normal glenoid vault morphology of the scapulae sample based on the intermediate 3-D model of  FIG. 9 ; 
         FIG. 11  shows a superior view of each of the triangular cross sections of the simplified 3-D model of  FIG. 10 ; 
         FIG. 12  shows a table listing the respective width dimension, depth dimension, and resulting area of the triangular cross sections of the simplified 3-D model of  FIG.10 ; and 
         FIG. 13  shows a table listing the coordinates for the respective vertexes of the triangular cross sections of the simplified 3-D model of  FIG. 10  relative to the rectangular (“Cartesian”) coordinates reference system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Like reference numerals refer to like parts throughout the following description and the accompanying drawings. 
       FIG. 1  shows an exploded perspective view of an exemplary shoulder prosthesis  100  including an exemplary glenoid component  120  according to the present invention. Prosthesis  100  also includes an exemplary humeral component  140 . Humeral component  140  is configured in a known manner for implantation in a humerus  160  and replacement of a natural humeral head (not shown) and, accordingly, includes a prosthetic humeral head  180 . 
     Glenoid component  120  is configured for implantation in a scapula  200  and replacement of a natural glenoid fossa (not shown in  FIG. 1 ). Glenoid component  120  includes a bearing  220 . Bearing  220  is made from a durable biocompatible plastic or any other suitable durable biocompatible material. For example, bearing  220  may be made from a polyethylene. One particular polyethylene that is well suited for bearing  220  is a high molecular weight polyethylene, for example ultra-high molecular weight polyethylene (“UHMWPE”). One such UHMWPE is sold as by Johnson &amp; Johnson of New Brunswick, N.J. as MARATHON™ UHMWPE and is more fully described in U.S. Pat. Nos. 6,228,900 and 6,281,264 to McKellop, which are incorporated herein by reference. Bearing  220  includes a generally concave surface  240  that is configured as known for bearing against prosthetic humeral head  180  or, in cases where the natural humeral head is spared, for bearing against the natural humeral head. Bearing  220  further includes a post  260 , or some other feature or mechanism capable of mating the bearing to a stem element of the glenoid component, such as stem  280  discussed below. 
     Glenoid component  120  also includes a stem  280 . As discussed further below, stem  280  is configured to model a normal or pathologic glenoid vault morphology such that stem  280  fits within a cavity  300  that may be defined, at least partially, by endosteal walls  320  of scapula  200 . To this end, it is noted that the present invention may provide a series of rigidly scaled or sized versions of stem  280  for accommodating various glenoid vault sizes that may be presented among different patients. It should also be appreciated that the glenoid vault of scapula  200  may include some cancellous bone  340 . 
     Stem  280  is made from a suitable biocompatible metal such as, for example, a cobalt chromium alloy, a stainless steel alloy, a titanium alloy, or any other suitable durable material. In alternative embodiments, stem  280  may include a porous coating to facilitate bone in-growth into glenoid component  120 . The porous coating may be any suitable porous coating and may for example be POROCOAT®, a product of Johnson &amp; Johnson of New Brunswick, N.J. and more fully described in U.S. Pat. No. 3,855,638 to Pilliar, which is incorporated herein by reference. Stem  280  can be solid or a thin shell of suitable durable material. 
     Stem  280  includes a generally superior surface  360 , a generally inferior surface  380 , a generally anterior-medial surface  400 , a generally posterior-medial surface  420 , and a generally lateral surface  440 . Stem  280  defines a socket  460  that extends inwardly from surface  440 . Socket  460  receives post  260  (of bearing  220 ). Stem  280  may also define a through-channel  480  that extends, coaxially with socket  460 , through stem  280 . 
     Glenoid component  120  further includes a fastener  500  in the form of, for example, a screw. The screw, or screws, may be any screw capable of additionally securing glenoid component  120  within scapula  200 . For example, the screw may be a cortical screw such as DePuy Ace catalog number 8150-36-030 available from DePuy Orthopaedics, Inc. of Warsaw, Ind. The screw has a diameter sufficient to properly secure glenoid component  120  within scapula  200  and may, for example, have a diameter of about two to five millimeters. The screw may have any suitable length capable of properly securing glenoid component  120  within scapula  200 . For example, the screw may have a length of from 10 to 60 millimeters. The screw may be secured to stem  280  in any suitable manner. In the exemplary embodiment, fastener  500  extends through through-channel  480  (of stem  280 ). However, it is noted that fastener  500  is not indispensable and may be omitted from alternative embodiments. 
     Bearing  220  is secured to stem  280  in any suitable manner. For example, bearing  220  may be bonded to stem  280 , or bearing  220  could be made from polyethylene and compression molded to stem  280 . Alternately, the bearing  220  may be glued to stem  280  by, for example, an adhesive. Alternatively, bearing  220  may be mechanically interlocked to stem  280  by taper locking or otherwise press-fitting post  260  in socket  460 , or post  260  and socket  460  may include any other suitable interlocking features, for example, rib(s), lip(s), detent(s), and/or other protrusion(s) and mating groove(s), channel(s), or indent(s) (not shown). Additionally, it is noted that in alternative embodiments, bearing  220  and stem  280  may be integrated into a single part made from UHMWPE or any other suitable material—with or without an omission of fastener  500 . 
     The present invention contemplates a method for preparing a glenoid component that will satisfy a majority of patient anatomies. Thus, in accordance with one method, the steps described in flow diagrams of  FIGS. 2   a - 2   c  correspond to one exemplary method used to model the normal or pathologic glenoid vault morphology, and ultimately to prepare an optimally sized and configured implant. 
     In a first step  1020  ( FIG. 2   a ), a suitable sample of human scapulae (“scapulae sample”) is selected to represent a reasonable demographic cross section of an anticipated patient population. In the exemplary embodiment, the scapulae sample included sixty-one human scapulae selected from different sources, thirty-two left-sided and twenty-eight right-sided. Various criteria were applied to the selection process so that the sample was as representative of the patient population as possible, including height, sex, gender and ethnicity. 
     At step  1040  ( FIG. 2   a ), volumetric scan of each scapula in the sample was performed using a Siemens Volume Zoom Scanner (a CT scanner available from Siemens Medical Systems of Malvern, Pa.). It is noted that the initial orientation of the scapulae in the CT images is dependent on the physical placement and orientation of the scapulae within the CT scanner, which is inherently difficult to reproduce. Nevertheless, the scapulae were placed in a supine anatomic position and axial images were obtained in one mm increments (with 0.27 to 0.35 mm in-plane resolution). The images were acquired at 120 kV, 100 mA, using a 180 mm field-of-view, large focal spot, and rotation speed of 0.5 sec/rev. A medium-smooth reconstruction algorithm was used for reconstruction of the images. 
       FIG. 3  shows a rectangular (“Cartesian”) coordinates reference system  1060  relative to the plane body of a typical scapula  1080  as defined by three surface points  1100 ,  1120 , and  1140  of the scapula  1080 . As at least partially discernable from  FIG. 3 , point  1100  represents an inferior tip of the scapula  1080 , point  1120  represents a medial pole of the scapula  1080  where the spine intersects the scapula  1080 , and point  1140  represents the center of the typical glenoid fossa  1160 . Further, it should be appreciated that coordinates reference system  1060  defines, among other things, an XZ-plane  1180 , an XY-plane  1190 , a vector  1200  extending from the medial pole of the scapula to the center of the glenoid fossa  1160 , and an X-axis  1220 . 
     At step  1230  ( FIG. 2   a ), the three-dimensional (“3-D”) images of the scapulae were re-sampled to align them on coordinates reference system  1060  (see  FIG. 3 ) for subsequent analysis. In the exemplary embodiment, points  1100 ,  1120 , and  1140  were interactively chosen on the 3-D image of each scapula and the scapulae were again re-sampled such that the plane of the body of each scapula was aligned parallel to the XZ-plane  1180  of the coordinates reference system  1060  (see  FIG. 3 ), and such that the vector  1200  extending from the medial pole of the scapula to the center of the glenoid fossa  1160  was parallel to the X-axis  1220  (see  FIG. 3 ). 
       FIG. 4  shows the superior-inferior (“SI”) dimension  1240  and the anterior-posterior (“AP”) dimension  1260  of the glenoid fossa  1160 . At step  1280  ( FIG. 2   a ), the SI dimension  1240  and the AP dimension  1260  (see  FIG. 4 ) of each scapula was determined by interactively placing points on the 3-D images using a suitable software program. 
     At step  1300  ( FIG. 2   a ), the scapulae sample were arbitrarily divided into six sub-groups based on their SI dimensions  1240  (see  FIG. 4 ) to reduce the initial number of morphological comparisons and to facilitate determination of the relationship between the global or overall typical glenoid vault size and the typical glenoid vault morphology.  FIG. 5  shows a table listing exemplary range and exemplary average SI dimension for the six sub-groups of scapulae based on their SI dimensions. 
     At step  1420  ( FIG. 2   a ), the endosteal walls  320  of the glenoid vaults of the scapulae were manually traced and digitized.  FIG. 6  shows a substantially complete tracing  1320  (toward the inferior end of the typical glenoid fossa  1160 ) of the endosteal walls  320  of the typical glenoid fossa  1160 .  FIG. 7  shows a partial tracing  1360  (toward the inferior end of the typical glenoid fossa  1160 ) of the endosteal walls  320  of the typical glenoid fossa  1160  as a result of fossa occlusion in the region of the typical scapular spine  1380 . Reference line  1400  ( FIG. 7 ) is discussed further below. Each endosteal boundary was traced on each of the two-dimensional (“2-D”) XY-slices of the respective re-sampled image (see  FIG. 6 ), starting at the respective glenoid fossa and extending medially to the scapular spine  1380  (see  FIG. 7 ), but not into the interior of the spine. Both the anterior and posterior wall tracings in the region of the spine are terminated at reference line  1400  (see  FIG. 7 ), which was defined to be simultaneously perpendicular to the plane of the respective glenoid fossa and tangential to the surface of the respective endosteal notch. 
     At step  1440  ( FIG. 2   a ), each endosteal tracing defining the respective glenoid vault was normalized by its extent in the SI dimension. This measurement was made from the inferior limit of the endosteal walls of the glenoid fossa to the superior limit in the Z-dimension (see  FIG. 3 ) of the image. The vaults were rigidly scaled in all three dimensions (i.e., X, Y, and Z) to normalize the SI dimension of the vault tracing to the average within its corresponding sub-grouping. This approach substantially eliminated size differences between the different vaults, facilitating an appropriate shape determination. An assumption was made that right-sided and left-sided scapulae are approximately anatomically symmetrical. Under this assumption, right-sided vaults were mirrored about the XZ-plane (see  FIG. 3 ) to allow morphological determinations to be made within the entire sample. In the exemplary embodiment, the normalized vaults within each of the six scapular sub-groupings were spatially aligned (i.e., “registered”) using an iterative closet point (“ICP”) algorithm such as discussed in Besl P. J. and McKay N. D., “A method for registration of 3-D shapes,” IEEE Trans. Pattern Analysis and Machine Intelligence 1992, volume 14, pages 239-256, which is incorporated herein by reference. 
     At step  1460  ( FIG. 2   b ), a 3-D model of the normalized glenoid vault morphology was then constructed for each sub-group of the scapulae of the scapulae sample based on the morphological constraints imposed by each of the vaults in the sub-group. For each sub-group, the set of registered glenoid vaults were overlaid and the approximate average endosteal walls  320  (see  FIG. 6  and  FIG. 7 ) of the sub-group were manually digitized. Each endosteal boundary was traced on each of the two-dimensional (“2-D”) XY-slices of the respective re-sampled image (see  FIG. 6 ), starting at the respective glenoid fossa and extending medially to the scapular spine  1380  (see  FIG. 7 ), but not into the interior of the spine. Both the anterior and posterior wall tracings in the region of the spine were terminated at reference line  1400  (see  FIG. 7 ), which was defined to be simultaneously perpendicular to the plane of the respective glenoid fossa and tangential to the surface of the respective endosteal notch. The resulting 3-D model satisfied the endosteal wall boundaries for each vault within the group. 
     At step  1480  ( FIG. 2   b ), a relatively complex 3-D model  1500  (see  FIG. 8 ) approximating the average normalized glenoid vault morphology of the entire scapulae sample was constructed based on the morphological constraints imposed by the models for each sub-group. The registered glenoid vaults for the sub-groups were overlaid and the approximate average endosteal walls  320  (see  FIG. 6  and  FIG. 7 ) of the sub-group models were manually digitized. Each endosteal boundary was again traced on each of the two-dimensional (“2-D”) XY-slices of the respective re-sampled image (see  FIG. 6 ), starting at the respective glenoid fossa and extending medially to the scapular spine  1380  (see  FIG. 7 ), but not into the interior of the spine. Both the anterior and posterior wall tracings in the region of the spine were terminated at reference line  1400  (see  FIG. 7 ), which was defined to be simultaneously perpendicular to the plane of the respective glenoid fossa and tangential to the surface of the respective endosteal notch. The resulting 3-D model  1500  satisfies the endosteal wall boundaries for each vault within the scapulae sample. 
       FIG. 8  shows views of a volumetric rendering of the relatively complex 3-D model  1500  generated in the previous steps. As at least partially discernable in  FIG. 8 , model  1500  includes a generally superior surface  1520 , a generally inferior surface  1540 , a generally anterior-medial surface  1560 , a generally posterior-medial surface  1580 , and a generally lateral surface  1600 . It should be appreciated that generally superior surface  360  (of stem  280 ) corresponds roughly to generally superior surface  1520 , generally inferior surface  380  (of stem  280 ) corresponds roughly to generally inferior surface  1540 , generally anterior-medial surface  400  (of stem  280 ) corresponds roughly to generally anterior-medial surface  1560 , generally posterior-medial surface  420  (of stem  280 )corresponds roughly to generally posterior-medial surface  1580 , and generally lateral surface  440  (of stem  280 ) corresponds roughly to generally lateral surface  1600 . 
     At step  1720  ( FIG. 2   b ), intermediate 3-D model  1700  was constructed by inscribing a plurality of mutually parallel triangular cross sections within the boundaries defined by the model walls on a plurality of XY-plane (see  FIG. 3 ) cross-sections of relatively complex 3-D model  1500  (see  FIG. 8 ).  FIG. 9  shows views of a volumetric rendering of this intermediate 3-D model  1700  of the normalized glenoid vault morphology of the scapulae sample based on relatively complex 3-D model  1500  (see  FIG. 8 ). 
     At step  1800  ( FIG. 2   b ), a simplified 3-D model  1820  (see  FIG. 10 ) of the average normalized glenoid vault morphology of the scapulae sample was constructed by selecting five equidistantly inferior-superior spaced-apart mutually parallel triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) (see  FIGS. 10 and 11 ) from intermediate 3-D model  1700  (see  FIG. 9 ). These triangular cross-sections were selected to account for more than 90% of the volume of intermediate 3-D model  1700  with almost negligible spatial deviation of the anterior and posterior walls. It should be appreciated that simplified 3-D model  1820  thus provides a concise geometrical model of the normalized glenoid vault morphology while substantially preserving the morphological nuances inherent to the endosteal walls  320  (see  FIG. 1 ). 
     A perspective view of this simplified 3-D model  1820  of the average normalized glenoid vault morphology of the scapulae sample is shown in  FIG. 10 .  FIG. 11  shows a superior view of each of the triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) obtained from the simplified 3-D model  1820 . As at least partially discernable from  FIGS. 10 and 11 , cross section  1840  includes a generally medially positioned vertex  2000 , a generally anteriorly and generally laterally positioned vertex  2020 , and a generally posteriorly and generally laterally positioned vertex  2040 . Similarly, cross section  1860  includes a generally medially positioned vertex  2060 , a generally anteriorly and generally laterally positioned vertex  2080 , and a generally posteriorly and generally laterally positioned vertex  2100 . Cross section  1880  includes a generally medially positioned vertex  2120 , a generally anteriorly and generally laterally positioned vertex  2140 , and a generally posteriorly and generally laterally positioned vertex  2160 . The next cross section  1900  includes a generally medially positioned vertex  2180 , a generally anteriorly and generally laterally positioned vertex  2200 , and a generally posteriorly and generally laterally positioned vertex  2220 . Finally, cross section  1920  includes a generally medially positioned vertex  2240 , a generally anteriorly and generally laterally positioned vertex  2260 , and a generally posteriorly and generally laterally positioned vertex  2280 . 
     Further, cross section  1840  includes a “base” edge  2400  extending between vertex  2020  and vertex  2040 , cross section  1860  includes a “base” edge  2420  extending between vertex  2080  and vertex  2100 , cross section  1880  includes a “base” edge  2440  extending between vertex  2140  and vertex  2160 , cross section  1900  includes a “base” edge  2460  extending between vertex  2200  and vertex  2220 , and cross section  1920  includes a “base” edge  2680  extending between vertex  2240  and vertex  2280 . 
     In addition, cross section  1840  includes a “left” edge  2500  extending between vertex  2000  and vertex  2020 , cross section  1860  includes a “left” edge  2520  extending between vertex  2060  and vertex  2080 , cross section  1880  includes a “left” edge  2540  extending between vertex  2120  and vertex  2140 , cross section  1900  includes a “left” edge  2560  extending between vertex  2180  and vertex  2200 , and cross section  1920  includes a “left” edge  2580  extending between vertex  2240  and vertex  2260 . 
     Finally, cross section  1840  includes a “right” edge  2600  extending between vertex  2000  and vertex  2040 , cross section  1860  includes a “right” edge  2620  extending between vertex  2060  and vertex  2100 , cross section  1880  includes a “right” edge  2640  extending between vertex  2120  and vertex  2160 , cross section  1900  includes a “right” edge  2660  extending between vertex  2180  and vertex  2220 , and cross section  1920  includes a “right” edge  2480  extending between vertex  2260  and vertex  2280 . 
     The respective base edges ( 2400 ,  2420 ,  2440 ,  2460 ,  2680 ) of the triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) define lateral boundaries of simplified 3-D model  1820 , corresponding to the region of the typical glenoid fossa  1160  (see  FIG. 3 ). Further, the respective left edges ( 2500 ,  2520 ,  2540 ,  2560 ,  2580 ) of triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) define anterior boundaries of simplified 3-D model  1820 , while the respective “right” edges ( 2600 ,  2620 ,  2640 ,  2660 ,  2480 ) of triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) define posterior boundaries of simplified 3-D model  1820 . The respective generally medially positioned vertexes ( 2000 ,  2060 ,  2120 ,  2180 ,  2260 ) of triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) sweep from a more posterior orientation at the inferior end of simplified 3-D model  1820  to a more anterior orientation at the superior end of simplified 3-D model  1820 . 
     Each of the triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) has a respective width dimension (“w”) and a depth dimension (“d”). The table in  FIG. 12  summarizes the respective width dimension (“w”) (see  FIG. 11 ), depth dimension (“d”) (see  FIG. 11 ), and resulting area of triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ). The table in  FIG. 13  lists the coordinates for the respective vertexes of triangular cross sections ( 1840 ,  1860 ,  1880 ,  1900 ,  1920 ) relative to rectangular (“Cartesian”) coordinates reference system  1060  (see  FIG. 3 ). 
     It is contemplated that simplified 3-D model  1820  may be rigidly scaled according to SI size (see  FIG. 4 ) to accommodate larger or smaller glenoid vaults while maintaining the integrity of the basic morphological model. 
     At step  3000  ( FIG. 2   c ), stem  280  is initially fashioned in the shape of the simplified 3-D model  1820 . In one embodiment, this step  3000  contemplates loading the coordinates of each of the vertexes defining the simplified 3-D geometrical model  1820  into a suitable stereo lithography system. The stereo lithography system may be operated to produce a corresponding 3-D form made of a plastic, wax, or any other suitable material as is known in the art. A mold is then prepared from the 3-D form and a stem  280  is fashioned, such as by injection molding using this mold. In alternative embodiments stem  280  may be otherwise suitably produced in accordance with simplified 3-D model  1820  via stereo lithography, by hand, or by any other suitable method (with or without an intervening form or mold) as known. 
     In subsequent steps, the stem  280  is machined to provide the features necessary to prepare the stem for implantation. Thus, at step  3020  ( FIG. 2   c ), socket  460  is bored into stem  280 . At step  3040  ( FIG. 2   c ), through-channel  480  is bored (coaxially with socket  460 ) through stem  280 . It should be understood that the rough stem produced from the 3-D model may be machined according to other protocols depending upon the interface between the stem  280  and the bearing  220 . It is further contemplated that the stem  280  may be formed as a solid or a hollow body and may further be provided with certain surface features to facilitate fixation of the stem within the glenoid vault. 
     The improved stem may then be implanted in accordance with known surgical procedures. For instance, cancellous bone  340  may first be removed from the glenoid vault of scapula  200  to construct cavity  300 , which extends to endosteal walls  320  (see  FIG. 1 ). Stem  280  is then inserted into cavity  300  into intimate contact with endosteal walls  320  to facilitate alignment and reliable fixation of glenoid component  120  within scapula  200 . Bone cement may be used to enhance fixation of the stem within the bone. Fastener  480  is inserted through through-channel  480  into engagement with scapula  200 . After fastener  480  is fully inserted into scapula  200 , post  260  is inserted into socket  460  and bearing  220  is secured to stem  280 . 
     The foregoing description of the invention is illustrative only, and is not intended to limit the scope of the invention to the precise terms set forth. Further, although the invention has been described in detail with reference to certain illustrative embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 
     For example, the glenoid components may be solid or hollow bodies. In particular, the stem  280  may be formed as a solid implant, but may be preferably at least partially hollow to reduce the weight and material requirements for the component. If the implant component is hollow, it must have sufficient wall thickness to maintain its strength and integrity under maximum expected physiological loads. 
     The present invention contemplates a glenoid stem component that is formed to closely approximate a normalized glenoid vault morphology. In the embodiments discussed above, this normalized morphology is generated from a relatively large sample size of human scapulae from which relevant measurements were obtained. It was found that the normalized component dimensions obtained in accordance with the invention well approximated the actual dimensions of the sample population. In particular, it was found that at least 85% of the surface points of the sampled glenoid vaults varied by less than 2.0 mm, which represents a minimal variation given the overall dimensions of the endosteal walls of the vault. 
     In generating the vault models for the different groups noted above, it was discovered that for the entire set of vault geometries, 98.5% of the surface points comprising the interior surface models varied by less than 2.0 mm. This finding refuted the a prior assumption that vault morphology was dependent upon the global vault size. As a result, a single vault model was derived from the group models using the same steps described above. This final model is depicted in  FIG. 9 . From that model of the actual glenoid vault morphology for the entire sample population, the simplified geometric model was developed as described above. This simplified model was found to account for over 80% of the volume of the model of the actual sample population, while also preserving the morphological nuances inherent to the endosteal surfaces of the glenoid vault. 
     In one aspect of the invention, a morphological model is developed for several discrete groups of glenoid sizes. The groups may be preferably grouped by SI (superior-inferior) dimension, as summarized in the table of  FIG. 5 . The simplified model used to create the component mold in the illustrated embodiment corresponded to Group  4 , but it is understood that the simplified model for the other groups may be obtained by directly scaling the dimensions as a function of the ratio of SI values.

Technology Category: 7