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Comparison of bone removed with reverse total shoulder arthroplasty.

Recent successes with reverse total shoulder arthroplasty (rTSA) have led to an expansion of its indications and an increase in the number of prosthetic designs available in the global marketplace. These rTSA designs have different design parameters (e.g., humeral neck angles, humeral offsets, and glenoid offsets) and are implanted using a variety of preparation techniques (e.g., different angles and amounts of the humeral osteotomy, different amounts of humeral retroversion, sit-on-top versus sit inside of humerus implants, inferior shifted placement of the glenoid, inferiorly tilted orientation of the glenoid, etc.). The impact of these different design parameters and implantation techniques on the amount of humeral and glenoid bone removed and how this amount of bone removal impacts implant fixation is not well understood. To that end, a computer analysis is conducted to quantify the amount of humeral and glenoid bone removed by three different rTSA designs when implanted using various implantation techniques. Specifically, the purpose of this comparative study is three-fold:

1. Quantify the amount of cortical and cancellous humeral and glenoid bone removed to implant three different rTSA prostheses,

2. Quantify the amount of cortical and cancellous glenoid bone removed by inferiorly tilting the glenoid baseplate with two different rTSA prostheses, and

3. Quantify the amount of cortical and cancellous glenoid baseplate surface contact with three different rTSA prostheses when implanted with and without inferior tilt and when implanted in a 10 mm medially eroded glenoid.

Methods

A 3-D computer model was developed in Unigraphics (Siemens PLM; Plano, TX, USA) and used to quantify the cortical and cancellous humeral and glenoid bone removed to implant three different rTSA designs. The 36 mm x 18 mm Grammont Delta III (Depuy Orthopaedics, Inc; Warsaw, IN, USA) has a 29 mm diameter glenoid baseplate with a flat-back, a humeral neck angle of 155[degrees], a center of rotation 0 mm lateral to the glenoid fossa, and a humeral stem/liner medial offset of 9.8 mm. The 32 mm x 26 mm RSP (DJO Surgical; Austin, TX, USA) has a 26 mm diameter glenoid baseplate with a curved-back, a humeral neck angle of 135[degrees], a center of rotation 10 mm lateral to the fossa, and a humeral stem/liner medial offset of 10.9 mm. The 38 mm x 21 mm Equinoxe (Exactech, Inc.; Gainesville, FL, USA) has a 34 mm long and 25 mm wide oval baseplate with a curved-back, a humeral neck angle of 145[degrees], a center of rotation 2 mm lateral to the fossa, and a humeral stem/liner medial offset of 20.8 mm.

Each reverse shoulder design was geometrically modeled and implanted in a 3-D digitized scapula and humerus (Pacific Research Laboratories, Inc.; Vashon Island, WA). The digital scapula and humerus had a uniform thick 1 mm shell to simulate cortical bone, where the volume of each serves as the control in this analysis. The bone removed to implant each prosthesis was calculated as the volume difference between the test condition and the anatomic humerus and glenoid bone models. To isolate the volumetric differences in cortical and cancellous humeral and glenoid bone removed by the three rTSA devices, each prosthesis was implanted according to the manufacturer's recommendations, so that each glenoid component aligns with the inferior glenoid rim with 0[degrees] glenosphere tilt and each humeral component was oriented at 20[degrees] retroversion. To quantify the amount of cortical and cancellous glenoid bone removed by inferiorly orienting the glenoid component, the two devices in which this is sometimes recommended (Depuy Delta III and the DJO RSP) were implanted in 15[degrees] of inferior glenosphere tilt. In each of these conditions, the glenoid baseplate surface contact area with the cortical and cancellous glenoid bone was also quantified. To simulate the usage of all three rTSA devices in a medially eroded glenoid, each scapula was medially eroded by 10 mm and each glenoid baseplate was aligned with the inferior glenoid rim with 0[degrees] glenosphere tilt; glenoid baseplate surface contact area with the cortical and cancellous glenoid bone was then quantified. It should be noted that glenoid baseplate contact area with the cortical and cancellous glenoid bone did not consider the central fixation member or the area contributed by any additional locking or compressive screws; only the back-side geometry of each rTSA baseplate contacting the fossa was considered.

Results

All three rTSA design utilized a different humeral osteotomy that was reflective of each device's humeral implant geometry (Fig. 1). As described in Table 1, the Delta III design with the 155[degrees] humeral neck angle and the largest proximal body that resides within the proximal humerus resects the least cortical bone but the most cancellous bone and the most overall humeral bone (47.4 [cm.sup.3]). The RSP design with the 135[degrees] humeral neck angle and the smallest proximal body that resides within the proximal humerus resects the most cortical bone, the second most cancellous bone, and the second most overall humeral bone (38.0 [cm.sup.3]). The Equinoxe design with the 145[degrees] humeral neck angle and second largest proximal body whose humeral liner sits on-top of the osteotomy resects the second most cortical bone, the least cancellous bone, and the least overall humeral bone (31.7 [cm.sup.3]).

As described in Table 2, when each of the three rTSA designs were implanted along the inferior glenoid rim with 0[degrees] glenosphere tilt (e.g., perpendicular to the glenoid), each device removed approximately the same amount of cortical glenoid bone. The 26 mm diameter curved-back RSP baseplate removed the most cancellous glenoid bone and most overall glenoid bone (3.7 [cm.sup.3]). The 29 mm diameter flat-back Delta III baseplate removed the second most cancellous bone and the second most overall glenoid bone (3.6 [cm.sup.3]). The 34 mm long and 25 mm wide oval curvedback Equinoxe baseplate removed the least cancellous bone and least overall glenoid bone (3.3 [cm.sup.3]). Frequently, it is recommended that the Delta III baseplate and the RSP baseplates be placed in inferior tilt, whereas the Equinoxe baseplate is recommended to be placed in neutral. Therefore, when the Delta III baseplate was implanted along the inferior glenoid rim with 15[degrees] inferior tilt, 20.9% more cortical bone, 45.7% more cancellous bone, and 38.0% more overall glenoid bone was removed relative to its 0[degrees] inferior tilt configuration. When the RSP baseplate was implanted along the inferior glenoid rim with 15[degrees] inferior tilt, 21.0% more cortical bone, 80.5% more cancellous bone, and 62.7% more overall glenoid bone was removed relative to its 0[degrees] inferior tilt configuration.

As described in Table 3, when each of the three rTSA designs were implanted along the inferior glenoid rim with 0[degrees] glenosphere tilt, the Equinoxe baseplate had the most cortical bone surface contact, the most cancellous bone surface contact, and the most overall glenoid bone surface contact (501.3 [mm.sup.2]). The RSP baseplate had the least cortical bone surface contact, second most cancellous bone surface contact, and the second most overall glenoid bone surface contact (386.0 [mm.sup.2]). The Delta III baseplate had the second most cortical bone surface contact, least cancellous bone surface contact, and the least overall glenoid bone surface contact (360.6 [mm.sup.2]).

When the Delta III baseplate was implanted along the inferior glenoid rim with 15[degrees] inferior tilt, 21.8% less cortical bone surface contact, 6.3% more cancellous bone surface contact, and 2.1% more overall glenoid bone surface contact occurred relative to its 0[degrees] inferior tilt configuration. When the RSP baseplate was implanted along the inferior glenoid rim with 15[degrees] inferior tilt, 4.1% more cortical bone surface contact, 6.5% less cancellous bone surface contact, and 5.9% less overall glenoid bone surface contact occurred relative to its 0[degrees] inferior tilt configuration.

When each of the three rTSA designs were implanted along the inferior glenoid rim of a 10 mm medially eroded glenoid with 0[degrees] glenosphere tilt, the same trends applied. Specifically, the Equinoxe baseplate had the most cortical bone surface contact, the most cancellous bone surface contact, and the most overall glenoid bone surface contact (383.1 [mm.sup.2]), 23.6% less surface contact than when implanted in a non-worn glenoid. The RSP baseplate had the least cortical bone surface contact, second most cancellous bone surface contact, and the second most overall glenoid bone surface contact (296.9 [mm.sup.2]), 23.1% less surface contact than when implanted in a non-worn glenoid. The Delta III baseplate had the second most cortical bone surface contact, least cancellous bone surface contact, and the least overall glenoid bone surface contact (374.1 [mm.sup.2]), 24.0% less surface contact than when implanted in a non-worn glenoid.

Discussion

The results of this study demonstrate that different rTSA designs and different preparation techniques impact the amount of humeral and glenoid bone removed to implant each device. Differences in the amount of humeral and glenoid bone removed by each rTSA design has implications on implant fixation and may influence the viability of using each device during revision arthroplasty. Both the Delta III and RSP designs had humeral preparation techniques that removed a substantial amount of the proximal humerus in order to fit the humeral liner within the proximal humerus. Conversely, the Equinoxe design utilized a platform humeral stem (e.g., the same humeral stem utilized to perform an anatomic total shoulder arthroplasty) that had a humeral tray and humeral liner residing on-top of the humeral osteotomy--this "on-top" design feature was the primary reason that the Equinoxe removed the least amount of proximal humeral bone.

These results also demonstrate that different rTSA designs and different scapula morphologies impact the amount of baseplate surface contact with the glenoid fossa. Differences in the amount of cortical and cancellous glenoid surface contact area achieved by each rTSA design has implications on both the magnitude of stress and the associated stress profile, potentially impacting the fixation of each device. Both the Equinoxe and RSP designs utilize baseplates with a curved-back and had the largest surface contact area with the glenoid fossa; conversely, the Delta III utilized a baseplate with a flat-back and had the least surface contact area with the glenoid. The RSP baseplate was the smallest in diameter, whereas the Equinoxe baseplate was the largest and most anatomically shaped given its oval profile--this difference in baseplate size was the primary reason the Equinoxe had the largest surface contact area with the glenoid. The larger baseplate was also demonstrated to be more forgiving when utilized in a medially eroded glenoid, as the Equinoxe baseplate in a medially eroded glenoid had more surface contact than the Delta III when used in a non-worn glenoid (383.1 vs. 360.6 [mm.sup.2]) and nearly the same surface contact area as the RSP when used in a non-worn glenoid (383.1 vs. 386.0 [mm.sup.2]). Future work should seek to optimize implant geometry and glenoid preparation techniques to minimize bone removed, maximize cortical glenoid bone contact, and maximize overall glenoid bone contact with the baseplate to reduce contact stress and improve fixation.

Nigo and coworkers quantified the surface contact area of several commercially available rTSA devices when each was implanted perpendicular to a "block" substrate (e.g., without inferior tilt) and then quantified the impact on that surface contact area when each device was counter-sunk into the glenoid by varying amounts to contribute the surface contact of the glenosphere with the glenoid. (1) In that study, Nigo and coworkers reported that the baseplate surface contact area of the Depuy Delta III and Tornier Aequalis (i.e., Delta III equivalent) were both 488.3 [mm.sup.2], the baseplate surface contact area of the DJO RSP was 432.8 [mm.sup.2], and the baseplate surface contact area of the Exactech Equinoxe was 585.9 [mm.sup.2]. These values are slightly larger than those reported in our study due to Nigo and coworkers including the surface contact area contribution of each device's central fixation feature in their calculation. We reported the baseplate surface contact with the glenoid fossa only, as we believe it to be more clinically relevant. Like Nigo and coworkers, we found that the Equinoxe baseplate had the largest surface contact area (501.3 [mm.sup.2]); however, unlike Nigo and coworkers we found that the RSP baseplate had the second most surface contact area (386.0 [mm.sup.2]) and the Delta III baseplate had the least surface contact area (360.6 [mm.sup.2]). Also, we did not simulate the test scenario of Nigo and coworkers in which each device was counter-suck into the glenoid to facilitate glenosphere contact with the glenoid bone as we do not recommend this technique. Counter-sinking the glenoid baseplate eliminates cortical bone support, which as demonstrated by Walch and colleagues (2) increases the probability of glenoid subsidence. Counter-sinking the baseplate may also prevent full taper engagement between the glenosphere and glenoid baseplate, thereby increasing the incidence of glenosphere disassociation as well.

This study has some limitations. The computer model simulated one medium-sized glenohumeral anatomy with and without medial wear; therefore, this study does not account for all possible anatomical and morphological variations in which these devices are implanted. Future work should consider the impact of other glenoid wear patterns on this bone removal and surface contact area analysis. Similarly, this computer model simulated the most commonly used size of three different rTSA designs when each was implanted along the inferior glenoid rim (with and without inferior tilt) with the same humeral retroversion to facilitate a direct comparison; therefore, this study does not account for all possible techniques in which these devices could be implanted. Future work should consider the impact of inferiorly shifting each device (as this is commonly recommended) to achieve inferior glenosphere overhang on this bone removal and surface contact analysis. Additionally, the computer model simulated a 1 mm cortical shell; as the cortical thickness of the scapula and humerus is of non-uniform thickness, the actual values may not reflect the clinical reality. However, given the direct comparison between designs, the observed trends are still applicable. Finally, the volume of bone removed was based upon the normal shoulder; it is unknown if the normal shoulder is the best reference as the collapsed condition of the glenohumeral joint resulting from the pathologies in which the reverse shoulder is indicated.

Conclusions

Different rTSA design parameters and preparation techniques impact the amount of humeral and glenoid bone removed when implanting each device. The Equinoxe platform humeral stem utilizing a humeral liner "on-top" of the humeral osteotomy preserved the most humeral bone, whereas the Delta III humeral stem with the large proximal body and humeral liner residing with-in the humerus removed the most humeral bone. The Equinoxe glenoid baseplate was the largest but, because of its curved-back and more-anatomic oval profile, removed the least glenoid bone and was associated with the largest surface contact area with the glenoid fossa. The RSP baseplate was the smallest but removed the most glenoid bone due to its small backside curvature. The Delta III baseplate was flat-backed and was associated with the least surface contact area when used with and without tilt and when used in a normal and medially eroded glenoid.

Caption: Figure 1 Comparison of humeral osteotomy according to manufactures' specified techniques (anatomic shoulder in gray): Delta III (left, blue), RSP (middle, orange), Equinoxe (right, green).

Disclosure Statement

Funding for this study was provided by Exactech, Inc., Gainesville, Florida. Christopher P. Roche, Phong Diep, and Matthew A. Hamilton are employed by Exactech, Inc. Pierre-Henri Flurin, M.D., and Howard D. Routman, D.O., are consultants for Exactech, Inc., and receive royalties on products related to this article.

References

(1.) Nigro PT, Gutierrez S, Frankle MA. Improving glenoid-side load sharing in a virtual reverse shoulder arthroplasty model. J Shoulder Elbow Surg. 2013 Jul;22(7):954-62. doi: 10.1016/j. jse.2012.10.025.

(2.) Walch G, Young AA, Boileau P, et al. Patterns of loosening of polyethylene keeled glenoid components after shoulder arthroplasty for primary osteoarthritis: results of a multicenter study with more than five years of follow-up. J Bone Joint Surg Am. 2012 Jan 18;94(2):145-50. doi: 10.2106/JBJS.J.00699.

Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., Matthew A. Hamilton, Ph.D., Pierre-Henri Flurin, M.D., and Howard D. Routman, D.O.

Christopher P. Roche, M.S., M.B.A., Phong Diep, B.S., and Matthew A. Hamilton, Ph.D., are employed by Exactech, Inc., Gainesville, Florida. Pierre-Henri Flurin, M.D., is at the Bordeaux-Merignac Clinique du Sport, Merignac, France. Howard D. Routman, D.O., is with Atlantis Orthopaedics, Palm Beach Gardens, Florida.

Correspondence: Christopher P. Roche, M.S., M.B.A., Exactech, Inc., 2320 NW 66th Court, Gainesville, Florida 32653; chris. roche@exac.com.

Table 1 Humeral Bone Volume Removed by Osteotomy in
20[degrees] Humeral Retroversion

Volume           Cortical Humeral   Cancellous Humeral   Total Humeral
([cm.sup.3])     Bone Removed       Bone Removed         Bone Removed

36 mm Delta III   5.6                41.8                 47.4
32 mm RSP         6.4                31.6                 38.0
38 mm Equinoxe    6.0                25.7                 31.7

Table 2 Glenoid Bone Volume Removed by Reaming with
0 and 15[degrees] of Inferior Tilt

Volume ([cm.sup.3])                  Cortical Glenoid
                                     Bone Removed

36 mm Delta III (0[degrees] tilt)    1.1
36 mm Delta III (15[degrees] tilt)   1.4
32 mm RSP (0[degrees] tilt)          1.1
32 mm RSP (15[degrees] tilt)         1.3
38 mm Equinoxe (0[degrees] tilt)     1.1

Volume ([cm.sup.3])                 Cancellous Glenoid   Total Glenoid
                                    Bone Removed         Bone Removed

36 mm Delta III (0[degrees] tilt)    2.5                  3.6
36 mm Delta III (15[degrees] tilt)   3.6                  5.0
32 mm RSP (0[degrees] tilt)          2.6                  3.7
32 mm RSP (15[degrees] tilt)         4.7                  6.0
38 mm Equinoxe (0[degrees] tilt)     2.2                  3.3

Table 3 Glenoid Baseplate Surface Contact Area

Volume ([mm.sub.2])                  Cortical          Cancellous
                                     Surface Contact   Surface Contact

36 mm Delta III (0[degrees] tilt)    53.1              307.5
36 mm Delta III (15[degrees] tilt)   41.5              326.8
36 mm Delta III (0[degrees] tilt,    40.6              233.5
  10 mm medial wear)
32 mm RSP (0[degrees] tilt)          21.2              364.8
32 mm RSP (15[degrees] tilt)         22.1              341.2
32 mm RSP (0[degrees] tilt, 10 mm    36.7              260.2
  medial wear)
38 mm Equinoxe (0[degrees] tilt)     68.4              432.9
38 mm Equinoxe (0[degrees] tilt,     50.8              332.3

Volume ([mm.sub.2])                  Total Glenoid
                                     Surface Contact

36 mm Delta III (0[degrees] tilt)    360.6
36 mm Delta III (15[degrees] tilt)   368.3
36 mm Delta III (0[degrees] tilt,    274.1
  10 mm medial wear)
32 mm RSP (0[degrees] tilt)          386.0
32 mm RSP (15[degrees] tilt)         363.3
32 mm RSP (0[degrees] tilt, 10 mm    296.9
  medial wear)
38 mm Equinoxe (0[degrees] tilt)     501.3
38 mm Equinoxe (0[degrees] tilt,     383.1
  10 mm medial wear)

* Note that surface area of central fixation feature and any
supplemental locking screws is not considered in this analysis,
only the backside surface of each devices baseplate is considered.


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Article Details
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Author:Roche, Christopher P.; Diep, Phong; Hamilton, Matthew A.; Flurin, Pierre-Henri; Routman, Howard D.
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Report
Geographic Code:1USA
Date:Apr 15, 2013
Words:3256
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