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Orthopaedics

Modular Taper Junction and Neck Strength in Total Hip Arthroplasty

Re s tora ti on® Modular Revi s i on Hip Sys temTechnical Mon ogra ph

I. Established History of Modular Tapers used in THA Implants

Taper junctions have been an effective means of independentlysecuring distal and proximal components together in modularhip stem implants.

The strength of the taper junction has been a prominent designconsideration due to the high load concentration that resultsthrough this region. In addition to having adequate fatiguestrength capabilities, taper junctions must be designed towithstand fretting failure and corrosion, as well as maintaintorsional stability. To enhance the performance of a modulartaper junction, additional treatments following the machining of the taper may be required. This monograph will describe thetesting and key issues surrounding the manufacture andperformance of the Restoration® Modular Hip System taperjunction design.

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II. Surface Tre a t m e n t s

Early high failure rates in femoral hip prostheses may beattributed to a range of material properties, including fatiguestrength, compressive strength, and other mechanicalproperties. The performance of modular taper junctions infemoral stems is further susceptible to corrosion and frettingoccurring from relative movement at the taper junction, andincreased stresses caused by the loss of proximal bone support.

What can be done to improve the materialp ro p e r t i e s ?Treatments to improve the taper strength of titanium-basedfemoral prostheses can be either mechanical or non-mechanical.Non-mechanical treatments harden a material’s surface byaltering the surface’s structure. Examples of non-mechanicaltreatments are heat treatments, nitride impregnation, andanodizing. Non-mechanical treatments affect the surface to adepth less than 0.1mm.1 , 2

Heat treatment is a material-hardening process in which thetitanium material is heated to a specific temperature, held at thattemperature for the appropriate amount of time, and cooledaccording to prescribed methods. This process may improve theoverall strength of the taper, but it does not produce a hardenedsurface which will significantly resist fretting failure.

Modular Taper Junctionand Neck Strength in Total Hip Arthroplasty

Re s tora ti on® Modu l a rRevi s i on Hip Sys temTechnical Mon ogra ph

Orthopaedics

Figure 1SEM image of a non-shot peened modular taper junction at 500µm .

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Figure 2SEM images of modular taper junction at 500µm. The dimpled effect is aresult of the shot peening process which hardens the taper junction throughcompression of surface molecules.

II. Surface Treatments (continued)

Nitride impregnation is a surface-hardening process thatintroduces nitrogen into the taper surface. The nitriding processgenerates a very thin (typically no more than 4 microns)titanium nitride layer on the titanium.2

A n o d i z i n g is a surface-hardening process which pulls oxygeninto the surface of the material, generating an oxide layer. Thistype of surface treatment is mainly effective on aluminum.

Mechanical treatments change the structural arrangement (i.e.,microstructure) of the material’s surface molecules.3 E x a m p l e sof mechanical treatments are swaging, burnishing, and shotpeening. Mechanical treatments affect the surface to a depth ofapproximately 0.25mm – 0.50mm.3 , 4

S w a g i n g is a surface-hardening process in which the taper isplastically deformed to its final shape using high pressures.Swaging is similar to forging except in swaging the material iscold worked.

B u r n i s h i n g is another form of a mechanical surface-hardeningprocess. In this process, the taper starts out with a machinedfinish, which looks like a fine thread form, or a series of hills andvalleys, when magnified. The action of roller burnishing pushesthe hills into the valleys through a rotation of hardened rollersover the machined surface. The result is a work-hardened surface.

Shot Peening is a surface-hardening process in which smallspheres of ceramic, steel or other materials (less than 0.5mm indiameter) are used to bombard the taper surface.5 Each sphere(shot) acts as a tiny peening hammer imparting a small indentionor dimple on the surface (Figures 1 & 2).4 This process packsthe surface molecules tighter and places them in compression,resulting in a layer of residual compressive stress, or work-hardened surface.

Since nearly all cracks caused by fretting originate on or near thesurface of a part, compressive surface stresses induced by shotpeening slow crack growth.4 In order for fretting forces toadversely affect the taper’s yield strength, the magnitude of stresscreated by fretting must exceed the magnitude of the residualcompressive stresses. Hence the compressive forces ‘pre-stress’the surface area of the taper in the opposite direction than thatof the fretting forces.

Mechanical treatments provide an advantage over non-mechanical treatments in affecting the surface of thetitanium-based tapers to a greater depth, and ultimatelyenhancing the fatigue strength of the taper.1 , 2 , 3 , 4

To improve resistance to fretting failure and increase the fatiguestrength of the taper junction, Stryker® Orthopaedics utilizes aproprietary shot peening process on its Restoration® ModularHip System taper junction.6

R e s t o r a t i o n®M o d u l a r

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R e s t o r a t i o n® M o d u l a rTest Results6

ISO Guideline7 , 8

Figure 4Results of ISO 7206-4 fatigue test configuration for the Restoration® Modulartaper junction.7

Modular taper junction @ 10 million cycles.

III. Te s t i n g

Test Methodology for this Next Generation SystemThe fatigue limits of the Restoration® Modular hip taperjunction and neck were tested in a manner consistent withcurrently accepted ISO (International Standards Organization)r e c o m m e n d a t i o n s .

The Restoration® Modular implants are dynamically tested to 10 million cycles, simulating the average load on an implant overa ten-year period.

Modular Taper Junction Te s t i n gOne of the design objectives for the development of theRestoration® Modular taper junction was the ability of the maletrunnion (taper) to endure a 4450N (1000 lbf)6 cyclic loadwhen used in conjunction with the largest offset proximal body (44mm base offset) and extended offset femoral head (V-40™ X-Long +12mm) (Figures 3 & 4). The total lateral offsetfrom hip center to femoral axis, achieved using this combinationof proximal body and femoral head is approximately 53mm.

The Restoration® Modular taper junction exceeds the guidelinesestablished by ISO (2300N or 517 lbf)7 , 8 and meets the designobjective of 4450N (1000 lbf) head load.6

To compare the effects of shot peening the taper, identicaltesting was performed utilizing distal stems generated throughthe same process minus the shot peening operation. Test resultsshow the shot peened components to have a minimum increasein fatigue strength of 33%.9

Figure 3ISO 7206-4 fatigue test configuration for the Restoration® Modular taperj u n c t i o n .

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R e s t o r a t i o n®M o d u l a r

R e s t o r a t i o n® M o d u l a rTest Results1 0

S e m l i t s c hG u i d e l i n e1 1

Figure 5ISO 7206-6 fatigue test configuration for the Restoration® Modular ConeBody neck.1 2

Modular Neck Fatigue @ 10 million cycles.

Neck Te s t i n gIn addition to testing the taper junction, neck testing wasperformed on the Restoration® Modular proximal bodies. Allof the proximal bodies were found to withstand fatigue loadingof 5340N (1200 lbf) .1 0 Semlitsch et al.1 1 suggests testing fatiguestrength at 4300N (996 lbf) for the neck region of a hip stem(Figure 5). The Restoration® Modular Proximal Body neckexceeds this suggested fatigue strength.1 0

Shot peening has also been used to improve the fatigue strengthin the neck of the proximal bodies. Results have shown that shotpeening increases the fatigue strength of Ti-6Al-4V alloy byapproximately 10% - 15%.1 3

Figure 6Results of torsional stability testing of the taper junction under loading.1 4

* This system is contraindicated for high activity.

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Torsion Te s t i n gWhy is torsion testing needed?

The torsional stability of the taper junction is required toprevent slippage between the components, helping to ensurethat proper implant anteversion is maintained during and afterthe surgical procedure. For hip implants, this is especiallyimportant during periods of extreme loading, which may occurduring stair climbing or rising from a seated position.

The torsional stability of the Restoration® Modular Hip Systemwas tested in a specially designed apparatus. The componentswere assembled using surgical technique recommendations, a n d

the torque required to cause rotation of the proximal body withrespect to the distal stem was measured. A comparison of thetorsional stability of dry tapers vs. tapers coated with soft tissue,fat and marrow, was performed, and the results were expressedin Newton-meters (Nm) (Figure 6) .1 4

What are the results for the Restoration®

Modular implants?

The torsion testing showed comparable results for both drytapers and tapers coated with the contaminates of soft tissue,fat, and marrow. This suggests that torsional stability isunaffected by contaminates.14

I V. Summary

Stryker® Orthopaedics has designed the Restoration® Modularstem implants as the next generation modular hip system forrevision surgery. This system meets and exceeds currentindustry testing requirements. Utilizing a proprietary shotpeening manufacturing process, we are confident that themodular taper junctions and proximal body necks have beendesigned to the highest orthopaedic manufacturing standards.

Ascending Stairs1 5 , 1 6 High Activity*1 6 R e s t o r a t i o n® M o d u l a rShot Peened Dry Taper1 4

R e s t o r a t i o n® M o d u l a rShot Peened

PLUS Contaminant1 4

1. Watanabe et al., “An Effect of Peening on Fretting Fatigue” The 7th International Conference on Shot Peening.2. Titanium Information Group, “Titanium and Titanium Alloys – Titanium Nitride Coatings, Production andApplications,” h t t p : / / w w w . a z o m . c o m / D e t a i l s . a s p ? A r t i c l e I D = 2 6 2 0. 3. Dreschler, Kiese, J. and Wagner L., “Effects of Shot Peening and Roller-Burnishing on Fatigue Performance of VariousTitanium Alloys,” The 7th International Conference on Shot Peening.4. Shot Peening Applications, Eighth Edition – Metal Improvement Company, Inc.5. Military Specification, “Shot Peening of Metal Parts,” June 7, 1989.6. Stryker H.O. Allendale, ATG Group, “Technical Report No. MT03051,” 2002.7. ISO 7206-4 Implants for Surgery - Partial and Total Hip Joint Prostheses - Part 4: Determination of Endurance Propertiesof Stemmed Femoral Components.8. ISO 7206-8 Implants for Surgery - Partial and Total Hip Joint Prostheses - Part 8: Endurance Performance of StemmedFemoral Components with Application of Torsion.9. Stryker H.O. Allendale, ATG Group, “Technical Report No. MT05049,” 2005.10. Stryker H.O. Allendale, ATG Group, “Technical Report No. MT03065,” 2002.1 1 . Semlitsch M., Panic B., “15 Years of Experience with Test Criteria for Fracture-Proof Anchorage Stems of ArtificialJoints.” Technical Principals, Design and Safety of Joint Implants (1994): 22-36.1 2 . ISO 7206-6 Implants for Surgery - Partial and Total Hip Joint Prostheses - Part 6: Determination of Endurance Propertiesof Head and Neck Region of Stemmed Femoral Components.1 3 . “Shot Peening Applications” Eighth Edition, Metal Improvement Company, Inc.1 4 . Stryker H.O. Allendale, ATG Group, “Technical Report No. MT02094,” 2002.1 5 . Davy et al., “Telemetric Force Measurements Across the Hip After Total Hip Arthroplasty.” J Bone Jt Surg, Vol. 70-A(1):45-50, 1988.1 6 . Bergmann et al., “Is Staircase Walking a Risk for the Fixation of Hip Implants?” J Biomech, Vol. 28:535-553, 1995.

The information presented in this material is intended to demonstrate the breadth of Stryker product offerings. Always refer tothe package insert, product label and/or user instructions before using any Stryker product. Surgeons must always rely on t h e i rown clinical judgment when deciding which treatments and procedures to use with patients. Products may not be available inall markets. Product availability is subject to the regulatory or medical practices that govern individual markets. Please contactyour Stryker representative if you have questions about the availability of Stryker products in your area.

Stryker Corporation or its divisions or other corporate affiliated entities own the registered trademarks: Restoration, Stryker.Stryker Corporation or its divisions or other corporate affiliated entities use or have applied for the following trademark: V40.

Literature Number: L R M H - T MTG/GS 2.5M 06/06 8028

Copyright © 2006 Stryker.Printed in USA.

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