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iomechanical comparison of component position andardware failure in the reverse shoulder prosthesis

ergio Gutiérrez, MS,a,b R. Michael Greiwe, MD,a,c Mark A. Frankle, MD,a Steven Siegal, MD,a and

illiam E. Lee III, PhD,b Temple Terrace and Tampa, FL

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here has been renewed interest in reverse shoulderrthroplasty for the treatment of glenohumeral arthritisith concomitant rotator cuff deficiency. Failure of therosthesis at the glenoid attachment site remains aoncern. The purpose of this study was to examinelenoid component stability with regard to the anglef implantation. This investigation entailed a biome-hanical analysis to evaluate forces and micromotionn glenoid components attached to 12 polyurethanelocks at �15°, 0°, and �15° of superior and inferior

ilt. The 15° inferior tilt had the most uniform compres-ive forces and the least amount of tensile forces andicromotion when compared with the 0° and 15° su-eriorly tilted baseplate. Our results suggest that im-lantation with an inferior tilt will reduce the incidencef mechanical failure of the glenoid component in aeverse shoulder prosthesis. (J Shoulder Elbow Surg007;16:9S-12S.)

otator cuff deficiency with glenohumeral arthritisresents a unique challenge to the reconstructive sur-eon. The complex motions of the shoulder joint re-uire stability throughout an extended range of mo-

ion. When the rotator cuff is deficient oronfunctional, total shoulder replacement has yieldedoor results as a result of eccentric loading of thelenoid leading to loosening and early failure.12 In

he modern era, multiple procedures have been rec-mmended to resolve this problem. These includeemiconstrained7,22,27 and constrained total shoulderrthroplasty,4,26 shoulder arthrodesis,6,20,31,35 andemiarthroplasty.1,13,23,32,35 Hemiarthroplasty, theurrent standard of care for this condition, offers only

rom the aMusculoskeletal Research Foundation, Florida Orthopae-dic Institute, Temple Terrace, and bBiomechanics Laboratory,Biomedical Engineering, and cCollege of Medicine, University ofSouth Florida, Tampa.

eprint requests: Mark A. Frankle, MD, Florida OrthopaedicInstitute, 13020 N Telecom Pkwy, Temple Terrace, FL33637(E-mail: frankle@pol.net).opyright © 2007 by Journal of Shoulder and Elbow SurgeryBoard of Trustees.

058-2746/2007/$32.00

toi:10.1016/j.jse.2005.11.008

imited goals for functional improvement21 and only aodest improvement in pain.2,11

Recently, there has been renewed interest in semi-onstrained reverse shoulder arthroplasty. Currently,owever, there are minimal basic science data avail-ble on which to base rational clinical decisions.everal authors have reported promising results in thehort and medium term using a reversed or invertedhoulder implant.3,14,29,30,33 The most recent studynvolving the Delta III prosthesis (DePuy Orthopae-ics, Warsaw, IN) in the treatment of glenohumeralsteoarthritis with massive cuff rupture, a multicentertudy of 80 shoulders in 77 patients, reported signif-cant improvements in all 4 areas of the Constantcore. However, 49 cases (63.6%) were noted toave medial component encroachment and scapularotching without evidence of loosening.30

The Reverse shoulder prosthesis (Encore Medical,ustin, TX) attempts to address the issue of scapularotching by providing the option for a more lateralenter of rotation. However, this lateral placementields a greater moment arm and hence generatesreater torque at the glenoid baseplate-bone inter-ace, creating concerns regarding early looseningnd failure. In an effort to address this concern, theeverse shoulder prosthesis uses enhanced baseplatexation by use of a fixed-angle central screw with 4eripheral locking screws. This configuration hasemonstrated stability to cyclic loading equivalent to

hat of the Delta III design in the laboratory.16

To understand the mechanical factors involved inhese early failures better, we examined the effect ofaseplate orientation on the distribution of forces andicromotion at the bone-prosthesis interface. Threengles of implantation were examined: �15°, 0°,nd �15° of scapular plane tilt.

ATERIALS AND METHODS

An apparatus was developed to simulate abduction ofhe humerus through 60° of abduction (Figure 1). A mov-ble sled with a 500-lb load cell (model LCH-500; Omegangineering, Stamford, CT) was connected via a cablehrough a series of pulleys to the distal portion of a steelipe used to simulate the humerus. The angle of abduction

� 0.01°) was measured by use of an electronic goniome-

er (Greenleaf Medical, Palo Alto, CA) attached via a ring

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hat moved with the steel pipe. At approximately half theistance between the glenohumeral joint and the cablettachment, a spring was attached (spring constant �8.67 pound force/in) that gradually increased the forcest the glenoid, simulating the forces present at the glenohu-eral joint during humeral abduction. Silicone spray wassed in the joint to simulate synovial fluid.

The reverse baseplate (Standard 25-mm Central Screwaseplate; Encore Medical) was attached to a solid rigidolyurethane block (30 pounds per cubic foot; Pacific Re-earch Laboratories, Vashon, WA) via a central attachmentcrew and peripheral captured screws. The baseplate wasmplanted with a custom-made torque screwdriver (Encore

edical) to approximately 60 in/lb. The peripheral screwsere all torqued to 20 in/lb. FlexiForce force transducers

Tekscan, Boston, MA) were attached to the underside ofhe baseplate with cyanoacrylate at the superior and infe-ior positions. A linear voltage displacement transducerRDP Electrosense, Pottstown, PA) was placed with its tip athe base of the glenosphere and measured microdisplace-ent (� 0.003 mm) in the superior and inferior directions.Eight different blocks were used for each different base-

late angle (15° superior inclination, 15° inferior inclination,nd 0° [or normal] inclination), and ten runs were performeder block. Data were collected by use of a custom-madeabVIEW graphic interface (National Instruments Corpora-ion, Austin, TX), and the following information was gathered:uperior and inferior forces between the baseplate and theoam, superior and inferior displacement of the glenosphere,ngle of humeral abduction, and force at the origin of theable. Data were exported into a Microsoft Excel spreadsheetMicrosoft, Redmond, WA), and means and SDs were calcu-ated. Statistical analysis was performed by use of a 1-waynalysis of variance and a Student t test.

ESULTS

Table I summarizes the biomechanical data.oth superior and inferior forces under the base-late increased when going from an inferior incli-ation to a superior inclination (Figure 2). The type

igure 1 Experimental apparatus shown with its basic components.

f force, though, changed when going from an h

nferior inclination to a superior inclination. Thenferior transducer in the inferior inclinationhowed a progression from a lesser compressiveorce to a greater compressive force. The same heldrue for the normal inclination, although the magni-ude of the compressive force was less when 60°as reached. Superior inclination had no compres-

ive force present in the inferior force transducer.orces under the superior force transducer, on thether hand, were compressive forces. The magni-

ude of this force increased when going from annferior inclination to a superior inclination.

The displacement data showed that the majority ofovement was in the superior direction (Figure 3). Itas not until 50° was reached in the inferior inclina-

ion and 60° in the normal inclination that movementn the inferior direction was noted. The magnitude ofll displacement remained under 60 �m, well under

he crucial displacement of 150 �m, when osteocytesannot rebuild bone.10

ISCUSSION

Laboratory testing provides a biomechanical basisor rational clinical decision making. We can infer,y looking at results obtained by use of high-densityolyethylene blocks, that glenoid component position-

ng may affect the stability of the baseplate-bonenterface. Implants with 15° of inferior tilt had theost uniform compressive forces and the least micro-otion when compared with the 0° and 15° superi-rly tilted baseplate. These results indicate that annferior tilt of approximately 15° will maximize im-lant stability and minimize mechanical failure for thelenosphere and baseplate component of the Reversehoulder prosthesis.

Stable fixation that minimizes resultant micromotionas been demonstrated to be a critical factor for pro-oting durable implant fixation via bony ingrowth.5,9

he baseplate used in this study has a porous titaniumurface. In our biomechanical model, the magnitude ofisplacement remained under 60 �m. Whereas a max-

mum micromotion of 100 to 150 �m has been reportedo be a threshold value to allow bony ingrowth,25 recenttudies have suggested that the value may be as low as0 to 40 �m.17,18 Although the exact threshold value isnclear, what is certain is that a lack of stable fixationesults in the formation of a fibrous membrane, predis-osing shoulders to early loosening and poor clinicalutcomes.19,24,34 In addition, even distribution of com-ressive forces and minimization of sheer strain at theone-prosthetic interface also promote ingrowth anday likewise play a critical role in the implant-boneicroenvironment.28

Reverse total shoulder arthroplasty has emerged aspromising surgical solution for patients with gleno-

umeral arthritis and rotator cuff deficiency.8,13,15

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arly results have been encouraging, but failure at thelenoid baseplate–host bone interface remains a con-ern. The moment arm of the glenoid componentroduces torque at the bone-prosthetic interface. Al-

eration of the angle of this lever will alter the magni-ude of force at the interface. Furthermore, the anglef the interface relative to the applied force (move-ent of the arm) will affect the types of stress occur-

ing at the interface. In addition, the distribution of theypes of stress (compression or shear) is likewisessociated with the tilt of the component. The benefitsf implanting a baseplate in an inferior inclination are1) decreased overall magnitude of force, (2) a de-rease in the total micromotion over the full range ofbduction, and (3) more even distribution of compres-ive forces beneath the baseplate.

able I Results from Baseplate Inclination

Abduction angle(degrees)

Superior forcemean (N)

Superior forStDev (N)

5° Inferior 10 13.82 10.5520 29.40 19.1530 39.72 24.9640 43.36 26.9650 42.75 26.5460 36.91 22.28

° Normal 10 35.47 17.5220 65.65 31.3030 86.88 39.6540 98.49 45.6850 102.51 49.8860 98.46 50.83

5° Superior 10 46.45 29.7520 78.04 28.3030 108.43 35.9640 126.03 41.6750 136.26 43.8060 137.59 44.19

igure 2 Difference in force between superior and inferior forceransducers (bars below 0 N indicate a decrease in compressiveorce from initial pre-compression). The graph shows an increase inhe magnitude of forces, as well as a decrease in compressiveorces, when going from an inferior inclination to a superiornclination.

Maximizing stability by closely approximating the S

deal angle of implantation theoretically provideshort- and long-term benefits. In the short term, the riskf mechanical failure is minimized while simulta-eously promoting osseous ingrowth necessary fortable long-term implant incorporation. The percent-ge of osseous ingrowth necessary and the clinicalignificance of radiolucent lines under the baseplateave yet to be determined for this implant type.

No published studies have evaluated component po-itioning of the Reverse shoulder prosthesis. In a multi-enter trial of the Delta prosthesis, Sirveaux et al30

ention that it is better to position the glenoid compo-ent with a slight tilt. However, there is no further dis-ussion of this finding nor are any clinical or biome-hanical data presented in support of this statement.

The limitations of our study are as follows: (1) The

nferior forcemean (N)

Inferior forceStDev (N)

Displacementmean (�m)

DisplacementStDev (�m)

�11.58 18.55 16.93 6.29�9.28 18.41 27.44 10.61�1.03 16.69 28.44 6.3918.73 15.31 17.26 7.8645.43 17.84 �4.14 10.3975.94 23.99 �32.59 14.23

�12.16 7.75 20.37 3.81�16.77 9.80 36.05 5.23�14.82 8.82 41.64 8.57

�6.71 7.08 34.75 14.7411.27 10.93 17.04 22.6337.50 20.66 �8.95 28.34

�32.48 20.56 26.61 7.97�41.55 23.26 43.98 6.21�47.08 26.08 52.03 7.00�47.25 26.06 49.05 7.11�42.71 25.43 35.48 8.52�31.85 20.30 11.93 13.74

igure 3 Difference in displacement between different inclinationngles (bars below 0 � show displacement in the inferior direc-

ion). The inferior inclination shows less superior displacement andore inferior displacement when compared with the other inclina-

ions. The superior displacement is greater in magnitude and islways in a superior direction.

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ratories) have a mechanical stiffness, yield, andltimate strength similar to those of the human gle-oid, but conditions differ from cadaveric glenoidsnd, therefore, do not simulate a cadaveric study. (2)he active muscle forces were not simulated, and notabilizing forces from the ligaments and joint capsuleere present—the absolute magnitudes of measured

orces and displacements cannot be correlated tohose occurring in vivo.

In conclusion, our results indicate that an inferior tiltf approximately 15° will maximize implant stabilitynd minimize early mechanical failure for the gleno-phere and baseplate component of the Reversehoulder prosthesis. The magnitude of displacementemained under 60 �m, well below the critical thresh-ld of 100 to 150 �m necessary to promote bonyngrowth and implant incorporation. The relationshipetween the amount of osseous versus fibrous in-rowth and long-term implant survivorship remains toe determined by cadaveric retrieval studies.

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