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Reverse shoulder glenoid loosening: an evaluation of the initial fixation associated with six different reverse shoulder designs.

A septic glenoid loosening is one of the primary historical failure modes of reverse total shoulder arthroplasty (rTSA). Over the past decade, numerous noncemented rTSA prostheses have been developed to maximize initial fixation and prevent aseptic glenoid loosening. Design variations include baseplate profile, baseplate size, backside geometry, center of rotation (COR), surface finish and coatings, fixation screw diameters, number of fixation screw options, and type of screw fixation. Investigators have previously outlined some factors that influence initial fixation in glenoid bone: bone quality, COR position, screw position, and screw diameter appear to play an important role. (1-10) However, little comparative biomechanical data exist to substantiate one design consideration over another.

Comparative biomechanical studies of different rTSA glenoid designs in clinically-relevant loading scenarios could aid the shoulder surgeon in deciding between the multiple implant offerings available on the market, particularly when confronted with high-demand or compromised bone situations. This study quantifies the initial glenoid fixation of six different rTSA designs in a low and high density polyurethane bone-substitute model by measuring glenoid baseplate/glenosphere displacement before and after cyclic loading of simulated abduction. It is estimated that these six designs represent over 75% of the rTSA US market share.

Materials and Methods

A displacement test was conducted according to a previously presented test method (9,11-12) and used to quantify glenoid fixation of six different rTSA designs: 38 mm Equinoxe standard offset (EQ), 38 mm Equinoxe lateral offset (EQL), 36 mm Depuy Delta III (DRS), 36 mm Zimmer, (ZRS), 32 mm neutral DJO RSP (DJO), and a 36 mm DePuy Delta III with 29 x 10 mm of bone-substitute "graft" behind the baseplate (BIO-RSA). A quantity of seven of each reverse shoulder prosthesis was secured to a low and high density polyurethane bone-substitute block as a shear (357 N), and a compressive (50 N) load was applied before and after cyclic loading. The two different densities (0.24 and 0.48 g/[cm.sup.3]) of polyurethane bone-substitute blocks (76 mm x 57 mm x 48 mm; Pacific Research, Inc.; Vashon, WA), each conforming to ASTM F 1839, (13) are intended to mimic the modulus, density, and strength range typical of patients receiving reverse shoulder arthroplasty, where the low and high density blocks simulate poor and good quality glenoid cancellous bone, respectively. (13-17) As described in Table 1, each reverse shoulder glenoid prosthesis was secured with 30 mm bone screws except for the BIO-RSA components, which used 45 mm bone screws to secure the 29 x 10 mm "bone graft" cylinders; these bone graft cylinders were manufactured from two different densities (0.24 and 0.48 g/(cm.sup.3)) to simulate the BIO-RSA surgical technique. (18-22)

The reverse shoulder glenoid loosening method consisted of two tests: a displacement test and a cyclic test and was conducted in three phases: phase 1) pre-cyclic displacement test, phase 2) cyclic test, and phase 3) post-cyclic displacement test. In the displacement test, the axial test machine (Model 8872; Instron Corp; Norwood, MA.; axial resolution of 0.001 mm and an accuracy of 0.017 mm) and 3 digital indicators (Model ID-C112EXB; Mitutoyo, Japan; resolution of 0.001 mm and an accuracy of 0.003 mm) measured displacement as a 50 N compressive axial load was applied perpendicular to the glenoid and a 357 N shear load was applied parallel to the face of the glenoid baseplate along its superior/inferior (S/I) axis and then performed a second time turning the component 90[degrees] and loading it along its anterior/ posterior (A/P) axis (Fig. 1). Two dial indicators were used to subtract out any compliance of the test construct; displacement was measured in the direction of the applied shear and compression loads to the nearest micron and applied along the S/I and then the A/P axes of each prosthesis.

In the cyclic test, a 750 N axial load was constantly applied through the center of the humeral liner as the glenosphere/glenoid baseplate/bone-substitute block were rotated about the humeral component with a stepper motor to create a sinusoidal angular displacement profile encompassing an arc of 55[degrees] at 0.5 Hz for 10,000 cycles (Fig. 2). This loading profile (over the 55[degrees] arc) would induce a maximum calculated shear load of 456 N (with a corresponding compressive load of 595 N) at the lower extreme of rotation and a maximum compressive load of 750 N (with no corresponding shear load) when applied perpendicular to the baseplate. (9,11) The components were cooled with a continuous jet of air with no lubrication during the cyclic test. It should be noted that the appropriate diameter of humeral liner (e.g., 32, 36, 38 mm) was manufactured with a 145[degrees] neck angle to test each device; a 145[degrees] humeral liner was utilized to ensure each device was subjected to the same combination of shear and compression loads during the cyclic test and isolate the effect of glenoid design on initial fixation in this model. A two-tailed unpaired Student's t-test was used to compare S/I and A/P glenoid prosthesis displacements relative to each density block in the direction of the applied shear load before and after cyclic loading, where p < 0.05 denotes significance.

Results

The average pre- and post-cyclic displacement associated with each reverse shoulder design in the low and high density polyurethane substrates are described in Tables 2 and 3, respectively. The average displacement of the EQ, EQL, ZRS, DJO, DRS, and BIO-RSA devices in the low density substrate was 182, 137, 431, 321, 190, and 256 microns, respectively. The average displacement of the EQ, EQL, ZRS, DRS, and BIO-RSA devices was 102, 95, 244, 138, and 173 microns, respectively. Pre- and post-cyclic displacement was significantly less in the high density bone substitutes than in the low density bone substitutes for the majority of implant comparisons. The EQ, EQL, and DRS devices were associated with significantly lower (p < 0.05) pre- and post-cyclic displacement than the ZRS, DJO, and BIO-RSA devices in each substrate, where the EQL device was associated with the least displacement in either substrate.

All seven of the EQ, EQL, and DRS devices remained well fixed throughout cyclic loading in both the low and high density blocks. During the cyclic test, six of seven ZRS devices failed from catastrophic loosening in the low density polyurethane blocks at an average of 2,603 [+ or -] 981 cycles (range: 1,144 to 3,810) (Fig. 3). Four of seven BIO-RSA devices failed from catastrophic loosening in the low density polyurethane blocks at an average of 2,926 [+ or -] 978 cycles (range: 1,600 to 3,913) (Fig. 4). Finally one of the seven DJO failed in the low density blocks at 7,342 cycles due to fatigue fracture of the central screw (Fig. 5). As a result of this implant failure, the DJO device was unable to be tested in the high density blocks. For the DJO device, the mean A/P and S/I displacement prior to cyclic loading was significantly lower than mean A/P (p = 0.005) and S/I (p = 0.001) displacement after cyclic loading.

Discussion

The results of this study demonstrate significant differences in fixation (p < 0.05), both before and after cyclic loading, for six different reverse shoulder designs; these results suggest that glenoid baseplate/glenosphere design does impact initial fixation with reverse shoulder arthroplasty in this model. Of the six designs evaluated, the EQ, EQL, and DRS had no catastrophic failures when tested in either the low and high density substrates and were associated with significantly less displacement (p < 0.05) than the ZRS, DJO, and BIO-RSA devices. The ZRS, DJO, and BIO-RSA devices each had at least one catastrophic failure when tested in the low-density substrate.

It is unclear which design parameters were most responsible for the observed differences in fixation. However, some general statements about the designs can be made. Catastrophic failures were witnessed in various implants with COR lateralization ranging from 2.5 mm to 10 mm from the glenoid surface. Certainly, the more lateral the COR from the fixation surface, the greater the torque on that fixation surface, biomechanically explaining the observed fatigue failure of the DJO device. When lateralization was created through the BIO-RSA technique, there did appear to be an adverse effect on fixation as the BIO-RSA results were significantly poorer in both the low and high density models than the DRS design when used without "graft," despite the use of four, 15 mm longer screws with the BIO-RSA device. However, a lateralized COR alone is insufficient to describe the observed differences in fixation described in this study, as there were implants (EQL) with as much as 6.5 mm of COR lateralization that exhibited no failure and excellent maintenance of fixation. In fact, the EQL design was associated with the lowest overall displacement before and after cyclic loading in both the low and high density substitute. Interestingly, the EQL design had superior fixation than the EQ design in both the low and high density bone substitutes (as the primary differences between these designs is that EQL has a COR 4 mm more lateral and 1 mm less inferior shifted than the EQ design). These results suggest that superior/inferior changes in COR may play a more significant role on fixation in this model than medial/lateral changes in COR.

In this model, implants with less-rough surfaces (e.g., HA and grit blasted) performed significantly better than implants with rougher coatings (e.g., porous trabecular metal and Ti plasma spray). However, the significance of this finding is unclear. Given that rougher coatings should better potentiate bone in-growth, (23) it is possible that (in this model) rougher coatings could inadvertently be less favorable to fixation due to these coatings creating more abrasion at the implant-substrate surface. As the low and high density bone substitute used in this study cannot possibly simulate bone in-growth, this testing methodology only evaluates initial fixation and makes no attempt to simulate biologic fixation. Therefore, we are unable to make any conclusions regarding the contribution of surface roughness.

Four of the tested devices utilized four screws (EQ, EQL, DRS, and BIO-RSA), and in the other two, five screws (DJO) and two screws (ZRS) were used. Most devices utilized screws that were 4.5 mm in diameter; the DJO implant utilized the largest screws (one 6.5 mm central compression screw and four 5 mm compression screws). Some implants used locking screws, some used compression screws, some used compression screws with locking caps, and some used a combination of types. The ZRS implant with the greatest number of catastrophic failures in the low density bone substitute only utilized two screws, the least number of the implants tested; it may be that two screws is too few to provide sufficient initial fixation in certain clinical scenarios.

With the exception of the DJO device, each implant had a unique, though similar method of press-fit central fixation. All but the DJO device utilized a press-fit peg to supplement peripheral screw fixation be it a porous peg, cage peg, or monolithic peg. The DJO device utilized a 6.5 mm central compression screw. Interestingly, one fatigue failure was observed with this 6.5 mm central screw at the minor diameter of the first (most lateral) thread (e.g., at the minimum cross sectional area) (Fig. 5). It should be noted that the fixation surface utilized by the DJO device had the smallest cross-sectional area of all the other devices tested. Given that there are several published reports of 6.5 mm central screw fracture with the DJO device, (24-28) it may be that the cross sectional area of this central screw is insufficient to withstand long-term expected loading in all clinical scenarios.

Regarding glenoid baseplate shape, there was also a combination of curved-back (EQ, EQL, DJO) and flat backed (ZRS, DRS, BIO-RSA) glenoid baseplate designs, without any distinct pattern noted in their performance. Additionally, it is difficult to say whether the size of the baseplate played any role in the performance. However, it is worth noting that the devices with the biggest glenoid baseplates/largest surface area (EQ and EQL) were associated with the least displacement before and after cyclic loading in both the low and high density polyurethane bone-substitute substrates.

This study has some limitations. It is purely biomechanical and may not represent the actual clinical condition perfectly. However, we believe that the method utilized in this study does simulate the primary function of reverse shoulders (e.g., abduction) better than that of other studies for reasons presented previously. (9) This study was performed in polyurethane blocks and not cadaveric scapulae. While foam blocks are ideal for their consistency, they differ in character and shape from actual scapula bone. It is unclear how deformity or anatomical morphological variations impact fixation and if it may be seen clinically.

Conclusions

This comparative biomechanical study quantified the glenoid fixation of six different rTSA designs before and after cyclic loading of simulated abduction when loaded at 750 N for 10,000 cycles in both a low and high density polyurethane bone-substitute model. Significant differences (p < 0.05) in glenoid fixation were observed in nearly every testing configuration, with the EQ, EQL, and DRS devices performing best (having significantly lower pre- and post-cyclic displacement than the ZRS, DJO, and BIO-RSA devices). While some general trends were

identified, future work using this cyclic-abduction test method should seek to isolate implant design differences in order to gain a greater understanding of which design parameters contribute most to achieve initial glenoid fixation with rTSA.

Caption: Figure 1 Depiction of the displacement test in which shear and compressive loads were applied while the shear and compressive loads were applied before and after cyclic loading.

Caption: Figure 2 Depiction of the cyclic test in which a 750 N load is applied through the humeral liner as the glenoid component is cycled about an arc of 55[degrees] at 0.5 Hz for 10,000 cycles; note that the 10[degrees] angular bias was performed but is not depicted in the image above.

Caption: Figure 3 Representative image of the ZRS device in the low density substitute after disassociation during the cyclic test.

Caption: Figure 4 Representative image of the BIO-RSA device in the low density substitute after disassociation during the cyclic test.

Caption: Figure 5 Representative image of the DJO device after fatigue fracture of the central screw during the cyclic test.

Disclosure Statement

Funding for this study was provided by Exactech, Inc., Gainesville, Florida. Nick Stroud and Christopher P. Roche are employed by Exactech, Inc. Pierre-Henri Flurin, M.D., is a consultant for Exactech, Inc., and receives royalties on products related to this article. Matthew J. DiPaola, M.D., has no financial or proprietary interest in the subject matter or materials discussed in this article, including but not limited to, employment, consultancy, stock ownership, honoraria, and paid expert testimony.

References

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(13.) ASTM F 1839-08 (2012) Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments. Available at: www. astm.org/Standards/F1839.htm. Accessed August 1, 2013.

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(24.) Frankle M, Siegal S, Pupello D, et al. The Reverse Shoulder Prosthesis for glenohumeral arthritis associated with severe rotator cuff deficiency. A minimum two-year follow-up study of sixty patients. J Bone Joint Surg Am. 2005 Aug; 87(8): 1697-705.

(25.) Frankle M, Levy JC, Pupello D, et al. The reverse shoulder prosthesis for glenohumeral arthritis associated with severe rotator cuff deficiency. A minimum two-year follow-up study of sixty patients surgical technique. J Bone Joint Surg Am. 2006 Sep; 88 Suppl 1 Pt 2:178-90.

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(27.) Holcomb JO, Cuff D, Petersen SA, et al. Revision reverse shoulder arthroplasty for glenoid baseplate failure after primary reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2009 Sep-Oct; 18(5):717-23. doi: 10.1016/j.jse.2008.11.017.

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Nick Stroud, M.S., Matthew J. DiPaola, M.D., Pierre-Henri Flurin, M.D., and Christopher P. Roche, M.S., M.B.A.

Nick Stroud, M.S., and Christopher P. Roche, M.S., M.B.A., are employed by Exactech, Gainesville, Florida. Matthew J. DiPaola, M.D., is in the Department of Orthopaedics, Wright State University, Boonshoft School of Medicine, Dayton, Ohio. Pierre-Henri Flurin, M.D., is at the Bordeaux-Merignac Clinique du Sport, Merignac, France.

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

Table 1  Comparison of Reverse Shoulder Designs Utilized in this
Glenoid Loosening Study

Reverse Shoulder            EQ                      EQL

Surface Finish              Grit-blasted            Grit-blasted
Center of Rotation          2.5 mm lateral to       6.5 mm lateral
                              bone interface          to bone interface

Glenosphere Diameter        38 mm                   38 mm
Baseplate Profile           Oval: 34 mm long,       Oval: 34 mm long,
                              25 mm wide              25 mm wide
Backside Geometry           Curved back             Curved back
Central Fixation Method     Press-fit tapered       Press-fit tapered
                              cage peg: 8x16 mm       cage peg:
                                                      8x16 mm
Screws (Number, Diameter/   4, 4.5x30 mm self-      4, 4.5x30 mm self-
  Length, and Screw Type)     tapping bone screws     tapping screws

Reverse Shoulder            ZRS

Surface Finish              Porous
Center of Rotation          2.5 mm lateral to
                              bone interface

Glenosphere Diameter        36 mm
Baseplate Profile           Circle: 28 mm
                              diameter
Backside Geometry           Flat-back
Central Fixation Method     Press-fit porous
                              peg: 8x15 mm
Screws (Number, Diameter/   2, 4.5x30 mm self-
  Length, and Screw Type)     tapping bone screws

Reverse Shoulder            DJO

Surface Finish              Plasma Spray
Center of Rotation          10 mm lateral to
                              the bone interface

Glenosphere Diameter        32 mm
Baseplate Profile           Circle: 26 mm in
                              diameter
Backside Geometry           Curved back
Central Fixation Method     6.5 mm integral
                              compression screw
Screws (Number, Diameter/   4, 5.0x30 mm self-
  Length, and Screw Type)     taping bone screws

Reverse Shoulder            DRS

Surface Finish              HA Coated
Center of Rotation          0 mm lateral to
                              the bone interface

Glenosphere Diameter        36 mm
Baseplate Profile           Circle: 29 mm in
                              diameter
Backside Geometry           Flat-back
Central Fixation Method     Press-fit peg:
                              8x16 mm
Screws (Number, Diameter/   2, 4.5x30 mm self-
  Length, and Screw Type)     tapping bone screws
                              & 2, 4.5x30 mm
                              locking screws

Reverse Shoulder            BIO-RSA

Surface Finish              HA Coated
Center of Rotation          10 mm lateral to the
                              bone substitute
                              graft interface
Glenosphere Diameter        36mm
Baseplate Profile           Circle: 29 mm
                              in diameter
Backside Geometry           Flat-back
Central Fixation Method     Press-fit peg:
                              8x16 mm
Screws (Number, Diameter/   4, 4.5x45 mm
  Length, and Screw Type)     self-tapping
                              bone screws

Table 2 Pre- and Post-Cyclic Glenoid Displacement in the Low
Density Polyurethane Substrate

Displacement    EQ                EQL               ZRS
(microns)

SI Shear Pre    181 [+ or -] 30   137 [+ or -] 25   381 [+ or -] 59
SI Shear Post   186 [+ or -] 34   129 [+ or -] 11   NA
AP Shear Pre    180 [+ or -] 58   136 [+ or -] 15   481 [+ or -] 73
AP Shear Post   181 [+ or -] 70   146 [+ or -] 21   NA

Displacement    DJO               DRS               BIO-RSA
(microns)

SI Shear Pre    238 [+ or -] 13   186 [+ or -] 15   232 [+ or -] 14
SI Shear Post   368 [+ or -] 60   189 [+ or -] 19   249 [+ or -] 15
AP Shear Pre    266 [+ or -] 27   190 [+ or -] 25   269 [+ or -] 19
AP Shear Post   414 [+ or -] 88   196 [+ or -] 27   272 [+ or -] 11

Table 3 Pre- and Post-Cyclic Glenoid Displacement in the High
Density Polyurethane Substrate

Displacement    EQ                EQL               ZRS
(microns)

SI Shear Pre    102 [+ or -] 11   87 [+ or -] 11    247 [+ or -] 66
SI Shear Post   112 [+ or -] 28   80 [+ or -] 10    207 [+ or -] 65
AP Shear Pre    98 [+ or -] 31    104 [+ or -] 43   254 [+ or -] 73
AP Shear Post   96 [+ or -] 25    108 [+ or -] 52   269 [+ or -] 161

Displacement    DJO            DRS               BIO-RSA
(microns)

SI Shear Pre    did not test   130 [+ or -] 13   160 [+ or -] 10
SI Shear Post   did not test   134 [+ or -] 11   164 [+ or -] 27
AP Shear Pre    did not test   144 [+ or -] 29   182 [+ or -] 19
AP Shear Post   did not test   144 [+ or -] 11   185 [+ or -] 32


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Article Details
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Author:Stroud, Nick; DiPaola, Matthew J.; Flurin, Pierre-Henri; Roche, Christopher P.
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Report
Geographic Code:1USA
Date:Apr 15, 2013
Words:4209
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