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1 MacLeod, A., Sullivan, N., Witeouse, M. R., & Gill, H. S. (016). Large Diameter Total Hip Artroplasty Modular Heads Require Greater Assembly Forces for Initial Stability. Bone and Joint Researc, 5(8), DOI: / BJR R1 Publiser's PDF, also known as Version of record License (if available): CC BY-NC Link to publised version (if available): / BJR R1 Link to publication record in Explore Bristol Researc PDF-document Tis is te final publised version of te article (version of record). It first appeared online via BJR at ttp:// Please refer to any applicable terms of use of te publiser. University of Bristol - Explore Bristol Researc General rigts Tis document is made available in accordance wit publiser policies. Please cite only te publised version using te reference above. Full terms of use are available: ttp://

2 57.BJRBJR / BJR R1 researc-article016 Freely available online open Access BJR Hip Large-diameter total ip artroplasty modular eads require greater assembly forces for initial stability A. R. MacLeod, N. P. T. Sullivan, M. R. Witeouse, H. S. Gill University of Bat, Bat, United Kingdom Objectives Modular junctions are ubiquitous in contemporary ip artroplasty. Te ead-trunnion junction is implicated in te failure of large diameter metal-on-metal (MoM) ips wic are te currently te topic of one te largest legal actions in te istory of ortopaedics (estimated costs are stated to exceed $4 billion). Several factors are known to influence te strengt of tese press-fit modular connections. However, te influence of different ead sizes as not previously been investigated. Te aim of te study was to establis weter te coice of ead size influences te initial strengt of te trunnion-ead connection. Materials and Metods Ti-6Al-4V trunnions (n = 60) and two different sizes of cobalt-cromium (Co-Cr) eads (8 mm and 36 mm; 30 of eac size) were used in te study. Tree different levels of assembly force were considered: 4 kn; 5 kn; and 6 kn (n = 10 eac). Te strengt of te press-fit connection was subsequently evaluated by measuring te pull-off force required to break te connection. Te statistical differences in pull-off force were examined using a Kruskal Wallis test and two-sample Mann Witney U test. Finite element and analytical models were developed to understand te reasons for te experimentally observed differences. A. R. MacLeod, MEng, PD, Researc Associate, University of Bat, Claverton Down Rd, Bat, Nort East Somerset BA 7AY, UK. N. P. T. Sullivan, BM BS, B Med Sci (Hons), M Ed & DIC, MRCS, Ortopaedic Registrar, Soutmead Hospital, Department of Trauma and Ortopaedics, Nort Bristol NHS Trust, Soutmead Way, Bristol, BS10 5NB, UK. M. R. Witeouse, BSc, MSc, PD, Consultant Senior Lecturer in Ortopaedic Surgery, University of Bristol, Musculoskeletal Researc Unit, Soutmead Hospital, Bristol, UK, BS10 5NB, UK. H. S. Gill, BEng, PD, Professor of Healtcare Engineering, University of Bat, Claverton Down Rd, Bat, Nort East Somerset BA 7AY, UK. Correspondence sould be sent to Prof. H. S. Gill; r.gill@bat.ac.uk doi: / BJR R1 Bone Joint Res 016;5: Received: 1 Marc 016; Accepted: 6 May 016 Results 36 mm diameter eads ad significantly lower pull-off forces tan 8 mm eads wen impacted at 4 kn and 5 kn (p < 0.001; p < 0.001), but not at 6 kn (p = 0.1). Mean pull-off forces at 4 kn and 5 kn impaction forces were approximately 0% larger for 8 mm eads compared wit 36 mm eads. Finite element and analytical models demonstrate tat te differences in pull-off strengt can be explained by differences in structural rigidity and te resulting interface pressures. Conclusion Tis is te first study to sow tat 36 mm Co-Cr eads ave up to 0% lower pull-off connection strengt compared wit 8 mm eads for equivalent assembly forces. Tis effect is likely to play a role in te ig failure rates of large diameter MoM ips. Cite tis article: Bone Joint Res 016;5: Keywords: Head Size; Trunnion; Taper; Connection; Pull-off; Strengt Article focus To evaluate weter te initial connection strengt of modular total ip artroplasty eads is influenced by te size of te ead used; To investigate wy te observed differences occur. Key messages Te larger ead size used in our study (36 mm) produced lower connection strengts tan te smaller ead size (8 mm) for te same assembly forces; Te increased rigidity of te larger diameter eads is te reason for te reduction in initial connection strengt. Strengts and limitations Experimental study wit a large number of samples (n = 60); Experimental results supported by finite element and analytical modelling; Only two ead sizes were examined (8 mm and 36 mm). vol. 5, NO. 8, August

3 339 Large-diameter total ip artroplasty modular eads require greater assembly forces for initial stability Introduction Modular junctions are ubiquitous in contemporary ip artroplasty. Te ead-trunnion junction is implicated in te failure of large diameter metal-on-metal (MoM) ips, 1, wic are te currently te topic of one te largest legal actions in te istory of ortopaedics (te estimated cost to Jonson & Jonson is stated to exceed $4 billion 3 ). Modularity as te advantage of allowing intra-operative correction of anatomy (lengt and offset) and stability, decreasing implant inventories 4-6 and permitting future revision of te femoral ead wile retaining te femoral stem. 5-7 Assembly of tese components is via a press-fit connection comprising a male taper (trunnion) and a female ball (ead). 4,5 Relative motion at te trunnionead interface, owever, can produce significant amounts of metallic debris 4,8 and contribute to fretting and corrosion. 5,9,10 Te metal debris associated wit trunnion fretting and corrosion can induce biological activity responsible for adverse local tissue reactions 1,9-1 and secondary inflammatory pseudotumour formation. 13,14 MoM resurfacing and THA popularised te use of larger-diameter eads, going up to 60 mm. 15 In THA, diameters larger tan 8 mm are gaining popularity due to te reduced risk of dislocation, owever, it as been suggested tat 36 mm is te largest ead size tat sould be used 17 as larger ead sizes ave been associated wit increased fretting and corrosion at te trunnion-ead interface. 6,8,17,19-1 Following te unexpected problems wit MoM articulations, a report by SCENIHR (Scientific Committee on Emerging and Newly Identified Healt Risks) stated: Tese larger eads put larger loads on te taper junction and are suspected to be responsible for te problems. Te importance of ead size regarding implant failures is clear: failure rates of MoM articulations in THA are strongly predicted by ead size. 15,3 Tis is also supported by retrieval studies wic sow tat corrosion and fretting scores correlate strongly wit increasing ead offset (lever arm between te centre of te ead and te centre of pressure of te trunnion) and ead size. 8,9,4 Even wit bearing types oter tan MoM, studies ave reported corrosion and wear at te taper junction,,9,13,5,6 wit te majority relating to larger ead sizes. For example, evidence of significantly increased fretting and corrosion as been seen in 36 mm, compared wit 8 mm, eads in metal-on-polyetylene (MoP) bearings. 7 Te assembly conditions are known to influence te integrity of modular junctions. 6,18,5,8-30 For example, greater assembly forces ave been sown to increase te contact area at te ead-trunnion interface 31 and reduce te rate of corrosion. 3 No studies, owever, ave considered te effect of assembly force in relation to ead size. It is not clear, terefore, if taper connections using larger-diameter eads are compromised at te point of assembly, or if te differences between ead sizes are entirely due to te increased lever arm accelerating te rate of damage accumulation under pysiological loading. Additionally, understanding te mecanical environment around te ead-trunnion interface may elp to explain differences in performance between ead sizes and drive design improvements. Te aim of te current study was to investigate weter ead size influences initial pull-off strengt at different levels of impaction force. In order to acieve tis aim, we used a combination of experimental, numerical modelling and analytical metods. Materials and Metods Experimental Testing. A total of 60 cobalt-cromium (Co-Cr) eads (diameter 36 mm: n = 30 and diameter 8 mm: n = 30; medium offset (+0 mm)), along wit 60 1/14 titanium alloy (Ti-6Al-4V) trunnions, were used in te study. 3 All specimens were manufactured by an ortopaedic device manufacturer (JRI Ortopaedics, Seffield, United Kingdom) wit te angle of te male and female tapers equal to 5.67, standard deviation (sd) 0.08, and an engagement lengt (along te trunnion axis) of mm. Te trunnions were manufactured witout te femoral stem for ease of testing. A custom-made impaction rig, similar to tat previously described by Remer, Bisop and Morlock, 33 was designed using a swinging ammer (Fig. 1a). Te material of te impactor tip is known to influence te assembly, 33 terefore a 6.3 mm tick plastic cap (Nylon-66) was used to reduce te impulse transmitted to te prostesis compared wit a direct MoM blow. 34 Te impaction load vector was aligned wit te taper axis using a digital inclinometer (STM SmartTool Digital Inclinometer 360 ; M-D Building Products, Inc., Oklaoma City, Oklaoma). Eac trunnion specimen was secured in a steel specimen older using two 3 mm pins. Te specimen older was tigtened to te rig base wit a torque of 40 Nm. Prior to impaction, a settling force equal to te weigt of te impactor (1.5 kg) was used to ensure consistency between specimens. Te relationsip between impaction rig inclination angle and measured peak impaction force is sown in Figure 1b. Peak impaction forces produced by our rig were found to be igly repeatable wit linear regression of peak force versus drop eigt producing an R value of (95% confidence interval (CI) to 0.996). Te ambient temperature in te room during testing was between 0.5 C and 1.0 C. All specimens ad been stored in te room for at least 4 ours prior to testing. Te mean impaction force applied by surgeons as been reported to be approximately 4.4 kn. 8 Te test specimens were divided into six groups (n = 10 per group) consisting of tree impaction levels (4 kn; 5 kn and 6 kn) for eac ead diameter. Te assembled specimens were ten disassembled using a materials testing macine (Series 5965; Instron, Norwood, Massacusetts). Te eads were removed from te stems by applying BONE & JOINT RESEARCH

4 A. R. MacLeod, N. P. T. Sullivan, M. R. Witeouse, H. S. Gill mm Head Trunnion 6.3 mm θ Impact force (kn) Specimen Holder Fig. 1a Drop angle ( ) An illustration sowing, a) experimental set-up sowing drop rig and test specimen and a grap sowing b) te influence of drop angle and impaction force sowing ten repeats for eac drop angle. Fig. 1b tensile displacement at a rate of mm/s (sd ), maintaining te alignment tolerances specified in te ISO : 003 standard for axisymmetric experiments. 35 Te surface rougness of every trunnion was assessed before and after testing using a non-contact profiler (Proscan 000; sensor model: S11/03; resolution: 1 nm; Scantron Industrial Products Ltd, Taunton, United Kingdom). A linear profile traversing te entire engagement lengt was taken at te same location for bot scans. Te trunnions were assessed using te maximum eigt of te asperities witin te sampling lengt (5 mm) compared wit te mean surface eigt, R p. Te statistical differences in pull-off force and R p measurements between te ead sizes were examined using a Kruskal Wallis test and two-sample Mann Witney U test using SPSS (IBM Corp. Released 013. IBM SPSS Statistics for Windows, Version.0. IBM Corp., Armonk, New York). Spearman s rank correlation coefficient was also used to identify any relationsip between te rougness measures and pull-off force. Finite element analysis. To investigate differences in internal stresses between te ead sizes, an axisymmetric finite element (FE) model of te ead and trunnion was created (Abaqus 6.1; Simulia Corp., Providence, Rode Island) using dimensions provided by te manufacturer. A mes convergence study was performed wit convergence occurring at 068 linear tetraedral elements, wit a mean element edge lengt at te contacting surfaces of 0.5 mm. Increasing te mes density by a factor of 0 altered stress predictions by 0.5% and did not cange pull-off force predictions. Te contact interface between te trunnion and te ead was represented using a surface-to-surface contact wit a standard coulomb friction coefficient of In order to introduce variability into te model, two additional frictional coefficients were evaluated (0. and 0.3) 7,36 and used to provide a range of results. Te analysis was conducted in two steps: quasistatic loading representing te peak impaction forces used in te experimental tests (4, 5 and 6 kn) followed by pull-off by displacement. Te maximum tensile force during disassembly (equivalent to pull-off strengt) was recorded for eac ead size. Analytical modelling. In order to understand te mecanics of te problem, a simple analytical model was developed, assuming te entire surface of te trunnion and ead to be in contact and te interface pressure to be evenly distributed. 38,39 Using equations from MacDonald et al 38 and Fessler and Fricker, 39 te force arising from frictional resistance at te trunnion-ead interface, τ, depends upon te interface friction, µ, te pressure at te interface, p f, te slope of te trunnion, α, and te total contact area, A: τ = Aµ cos α p f Eqn. 1 Te net assembly force is equal to te impaction force minus te total frictional resistance, τ: F = F Aµ cos αp Eqn. in impact f Te resistance of te ead to press-fit assembly, F res, is dependent upon te interface pressure, p f : F = Asinα p Eqn. 3 res f Balancing te net assembly force, F in, wit te resistance of te ead, F res, and rearranging for, p f : Asinα p = F Aµ cosα p Eqn. 4 f impact f vol. 5, No. 8, August 016

5 341 Large-diameter total ip artroplasty modular eads require greater assembly forces for initial stability r f = 6.5 r f = 6.5 (mm) r r E ν r = 1.3 E ν r = E t ν t E t ν t Fig. a 14.1 Fig. b Key dimensions for (a) 8 mm a (b) 36 mm diameter eads. p f Fimpact = A( sinα + µ cosα ) Eqn. 5 During pull-off, extraction forces are resisted by frictional forces, t, but a component of te interface pressure, p f, contributes: Assuming te ead to act as a tick-walled cylinder surrounding te trunnion, te interface pressure, p f, can be described if te interference between te components, u t, is known: 40 p f = rf E r r + rf rf ut rf + ν E + t ( 1 ν ) t Eqn. 6 Were r f is te distance to te interface, r is te radius of te surrounding material (tickness of te ead at te location of interest) and E, E t, v, v t, are te Young s moduli and Poisson s ratio of te ead and trunnion, respectively (Fig. ). Re-arranging Equation 6 for u f and substituting Equation 5 for p f : u t = F impact rf E r + r f rf r r + ν f E + t A sinα + µ cosα ( ) ( ) 1 νt Eqn. 7 Te radial stress, σ r, and circumferential stress, σ c, at a distance, r, from te interface can be predicted for a given level of interference, u t : 40 σ r σ c Eur t f r = 1 r r Eur t f r = 1+ r r Eqn. 8 Eqn. 9 F = Aµ cosαp Asin α p Eqn. 10 F extract f f extract = p A µ cosα sin α Eqn. 11 f ( ) As te radial stress at te interface, σ r, is equal to p f, te pull-off force is given by: F extract Eur = A µ cosα sinα t f 1 r ( ) r r Eqn. 1 f As te frictional coefficient and contact area are equal for te two ead sizes, te pull-off force is entirely dependent upon te interference generated and is linear wit force, wit te slope influenced by ead size. In order to assess te relative rigidity of te two ead sizes, te force per unit interference was calculated using Equation 7, and termed interference rigidity. Results Experimental testing. Te larger-diameter eads were found to produce significantly lower pull-off forces at te 4 kn and 5 kn levels of impaction force (p < 0.001; p < 0.001) but not at 6 kn (p = 0.1) (Table I). Mean (sd) pull-off forces for te tree load levels (4 kn, 5 kn, 6 kn) were: N (sd 101.3); N (sd 150.4); and N (sd 16.3), respectively, for te 8 mm eads and N (sd 77.8); N (sd 79.9); N (sd 7.9), respectively, for te 36 mm eads (Fig. 3). For te 4 and 5 kn impaction forces, te pull-off force was, on average, 37% of te assembly force for te 8 mm eads and 31% for te 36 mm eads. Pull-off forces were 0.8% and 19.5% larger for te 8 mm eads at 4 kn and 5 kn, respectively. BONE & JOINT RESEARCH

6 A. R. MacLeod, N. P. T. Sullivan, M. R. Witeouse, H. S. Gill 34 Table I. Pull-off force for te different ead size groups and impaction forces Head size = 8 mm Head size = 36 mm Mann Witney U test Pull-off force (N) Pull-off force (N) Kruskal Wallis test Mean sd Mean sd Impaction force (kn) p < p < p < p = 0.1 sd, standard deviation; kn, kilonewtons Table II. Values of rougness, R p, pre-testing, post-testing and te cange in rougness for te different ead size groups and impaction forces Head size = 8 mm Head size = 36 mm Mann Witney U test Rougness pre-test, R p, (μm) Rougness pre-test, R p, (μm) Mean sd Mean sd Kruskal Wallis test Impaction force (kn) p = 0.16 p = p = p = 0.47 Rougness post-test, R p, (μm) Rougness post-test, R p, (μm) Mean sd Mean sd Impaction force (kn) p = 0.85 p = p = p = 0.57 Cange in rougness (post- pre), R p, (μm) Cange in rougness (post- pre), R p, (μm) Mean sd Mean sd Impaction force (kn) p = 0.1 p = p = p = 0.79 sd, standard deviation; R p, surface eigt Te trunnions for bot ead size groups were found to ave similar rougness, R p, before (p > 0.40) and after (p > 0.4) testing at all load levels (Table II). Tere was also no difference between ead sizes considering te cange in rougness before and after te tests (p > 0.81). Additionally, tere was no relationsip between pull-off force and initial, post-testing or cange in rougness. Te Spearman s rank correlation coefficient for eac measure was (p = 0.73); (p = 0.47); and (p = 0.6), respectively. Finite element analysis and analytical modelling. Altoug te absolute values of pull-off forces predicted by te finite element (FE) simulation were larger tan tose in te experimental testing, (Fig. 4), te relative difference in pull-off force between ead sizes matced te experiments closely at te 4 kn and 5 kn levels of impaction force. Te FE predictions were 0.0%, 18.4% and 19.1% larger for 8 mm eads at te 4 kn, 5 kn and 6 kn levels of impaction force, respectively. Te predicted stresses arising due to assembly forces were compared for te FE and analytical models (Fig. 5). Pull-off force (kn) Impaction force 4 kn 5 kn 6 kn 8 mm 36 mm Fig. 3 Grap sowing pull-off forces for different ead sizes and impaction forces used in te study. Te difference in agreement in absolute radial stress predictions between te FE and analytical models at te trunnion-ead interface was 6.% and 5.% for 8 mm and 36 mm eads, respectively (Fig. 5c). Circumferential vol. 5, No. 8, August 016

7 343 Large-diameter total ip artroplasty modular eads require greater assembly forces for initial stability Pull-off force (N) Experimental Finite element Analytical 8 mm 36 mm Fig. 4 Experimental and finite element pull-off forces for different ead sizes at an impaction force of 4 kn. Error bars sown for te finite element and analytical model are generated by altering te coefficients of friction used. stress differences were 8.% and.3%, respectively. Despite te similarity in stresses, te differences in pull-off strengt between ead sizes predicted by te analytical model were muc smaller tan tose predicted by te FE analysis or by te experimental mecanical testing, being 4.9% larger for te 8 mm eads at all load levels (Fig. 4). Te interference rigidity calculated analytically was kn/mm for te 8 mm eads and kn/mm for te 36 mm eads; 18.6% larger for te 36 mm eads. Discussion Modular components are assembled intra-operatively by te surgeon; as a result, differences in connection strengt obtained can be considerable. 8 Pull-off force is generally used as a surrogate measure of in-service connection strengt, and factors tat are known to influence tis variable include impaction force; 8,41 te number of impaction strikes; 8 taper design 41,4 and surface rougness. 9,4 It is not clear, owever, weter tere is any influence of te size of te ead on te initial strengt of press-fit acieved. Te current study evaluated weter ead size influences initial pull-off strengt for different levels of impaction force. Assumptions. Taper designs vary between manufacturers, and design differences can produce variation in performance. 41 All design variables in te current study oter tan ead size were kept constant; te trunnions used for eac ead size were identical. As 57% of all THA operations reported by te National Joint Registry (NJR) in 015 used eiter 8 mm or 36 mm diameter eads, 3 tese two ead sizes were selected for tis study. Te Co-Cr-titanium material combination was also selected due to its popularity (61% of THA procedures reported by te NJR). 3 Additionally, a previous study found tat tis material combination produced te largest pull-off resistance. 33 Our relatively simple, quasi-static finite element model was able to predict te increase in pull-off strengt accurately for te 8 mm eads compared wit te 36 mm eads, as sown by te experimental results. Tis indicates tat te penomenon is not dynamic, affected by te speed of impaction, nor influenced by inertial effects due to te increased mass of te largerdiameter eads. Additionally, quasi-static assembly of components (pus-on) as been previously found to produce similar pull-off forces to dynamic assembly. 8 Neverteless, it is possible tat dynamic effects could alter te relative difference between ead sizes. Fretting and corrosion are tougt to occur due to a combination of rocking and pistoning motions taking place during pysiological loading. 7 Tese involve a combination of axial, bending and torsional forces. Previous studies ave used pull-off connection strengt as an indicator of resistance to loosening and interface micromotion, 33,34 owever, it as been suggested tat te torsional resistance of modular connections may be more important to te life of te connection. 38 Te two measures are closely related, altoug te relationsip between te torsional connection strengt and assembly force is known to be more variable. 38 Reports of post-operative complications relating to wear at modular taper junctions are becoming increasingly common. 1,8,15 Taper wear appears to be unaffected by bearing surface wear, 8 and is present even in ceramic bearings.,5,1 Tere is increasing evidence tat largerdiameter eads perform poorly wit regard to fretting and corrosion at te ead-trunnion interface. 8,43 Te assumption as been tat larger-diameter eads, due to teir increased lever arm to te centre of rotation, are exposed to larger torques, exacerbating te corrosionfretting process. Two previous studies ave developed non-linear finite element models to understand te influence of various parameters on ead-trunnion fretting, using contact stress and micromotion to predict wear rates. 7,43 Te studies predict tat ead-centre offset (te distance from te centre of te ead to te centre of te stress witin te trunnion) is te main factor governing trunnion wear. Interestingly, bot studies also found tat increased impaction force did not significantly reduce te predicted wear. Tis was because altoug contact stresses increased, micromotion between te ead and trunnion decreased proportionally, meaning tat te combined effect was almost negligible. If tis is te case in reality, ten larger impaction forces will not be able to compensate for te differences between ead sizes in terms of wear rates in te long-term. Unfortunately, neiter of tese FE studies reported te initial connection strengt, and terefore cannot be directly compared wit te current study. Additionally, it is not clear weter te wear predictions of tese FE studies (using a product of contact stress and micro-motion) are realistic as tey do not take into account corrosion-assisted wear wic may be more dependent upon connection strengt. BONE & JOINT RESEARCH

8 A. R. MacLeod, N. P. T. Sullivan, M. R. Witeouse, H. S. Gill mm 4 kn 36 mm 4 kn 30 5 Hoop 0 15 r r Stress (N/mm ) Analytical FE 8 mm 36 mm von Mises stress (N/mm ) Fig Radial Distance from contacting surface, r (mm) Von Mises (equivalent) stress predictions of te finite element analysis: a) at an assembly load of 4 kn; b) built-in stresses after assembly; and c) comparison of radial and circumferential stress against te analytical predictions of a cylinder. Te mean pull-off strengts measured in te present study are similar in magnitude to pull-off strengts in previous studies, 8,34,41 approximately 37% and 31% of te impaction force for 8 mm and 36 mm eads, respectively. To our knowledge, tis study is te first to demonstrate tat given a constant impaction force, te pull-off connection strengt of larger-diameter eads is inferior to smaller eads, even prior to pysiological loading. We found tat te taper connection strengt of 36 mm eads was around 0% lower tan tat of 8 mm eads at 4 kn and 5 kn impaction forces. Tis study also evaluated weter tere were any differences between te ead sizes wit regard to canges in surface rougness tat may ave occurred during testing. We found no significant differences in rougness between te ead size groups at any load level. Our experimental, numerical and analytical models were all in agreement tat, for 4 kn and 5 kn assembly loads, smaller diameter eads ave larger pull-off connection strengts. Te extent of te difference sown by our experimental tests was well predicted by our FE simulation wit a similar increase in pull-off strengt predicted for te 8 mm compared wit te 36 mm eads (witin 1.1% at 4 kn and 5 kn assembly loads). Te stress predictions of te FE model closely matced te analytical model wit a difference of 6.% for radial stresses at te connection interface. Te largest difference between te ead sizes was te circumferential stress (oop stress). Te FE and analytical models bot predicted a substantially larger oop stress for te 8 mm eads compared wit te 36 mm eads (50% and 41% larger, respectively). Te radial stresses for te 8 mm eads were 16% and 15% larger tan tose of te 36 mm eads for te FE and analytical models, respectively. It is te radial stress tat is responsible for te increase in pulloff strengt (Equation 11). Using te analytical model, we evaluated te interference rigidity of te two ead sizes (force per unit interference at te ead-trunnion interface). Te interference rigidity of 8 mm eads was 18.6% iger for 36 mm eads, corresponding well wit te experimentally measured pull-off force tat was 0.7% lower tan tat of te 36 mm eads. Terefore, muc of te difference between pull-off forces for te two ead sizes can be explained by interference rigidity. Te remainder may be as a result of localised stress concentrations caused by deformation of te trunnion and/ or ead. Tese local increases in contact pressure were observed in te FE results and could explain wy te FE simulation was able to capture te true extent of te experimentally measured difference, wile te analytical model was not. Some of te difference in predictions between te experimental tests and FE simulations may be due to our assumption of perfectly matcing taper angles. Sligt mismatces could reduce te contact area between te ead and trunnion, tereby reducing te connection strengt (Equation 11). Anoter process tat can increase te strengt of te taper connection is cold welding due to plastic deformation. 5 Witt et al 31 demonstrated tat plastic deformation can occur at assembly forces as low as 500 N. As our FE vol. 5, No. 8, August 016

9 345 Large-diameter total ip artroplasty modular eads require greater assembly forces for initial stability model did not include te ridged profile of te trunnion surface, local stress concentrations at te tips of te ridges were not predictable. Neverteless, our findings suggest tat te larger interface pressures generated by 8 mm eads make plastic deformation more likely to occur. Tis localised effect could alter te tigtness of fit or frictional resistance at te contact interface, tereby influencing pull-off strengt. 5,39 We found no significant differences in rougness between te ead size groups at any load level, indicating tat te differences in pull-off strengt are not due to measurable damage occurring at te trunnion-ead interface. Reduced taper diameters and lengts were introduced to expand te range of motion 5 and to create universal tapers witin implant ranges tat would allow te use of ceramic eads, 5 owever, elastic deformation of te trunnion as been blamed for increasing micromotion and increasing susceptibility to fretting and corrosion. 9,6 Morlock 5 ypotesised tat tree variables could be largely responsible for te dramatic failures of MoM articulations: trunnion diameter, trunnion lengt and ead size. Using Equation 1, a 1 mm diameter trunnion would reduce contact area by 14.3% and pull-off strengt by 3.8% compared wit an equivalent 14 mm diameter trunnion. Similarly, trunnion lengt would ave a 1:1 influence on contact area and tus influence pull-off strengt. Terefore, we ave sown tat tese tree variables suspected to influence te long-term survivability - trunnion diameter, trunnion lengt and ead size - also influence te initial connection strengt. Te clinical importance of tese variables is similar, wit an approximate % cange in connection strengt per millimetre cange in dimension. It sould be noted tat our study assumed no angular mismatc between te male and female tapers; varying degrees of angular mismatc could terefore produce different results, particularly in te case of trunnion lengt. Previous studies ave examined te minimum force 3,44 or torque 34 required to produce a drop in galvanic potential at te ead-trunnion interface, tereby exposing it to corrosion. Tere is agreement tat larger connection strengts (resulting from larger assembly forces) require larger forces to initiate corrosion. Terefore, as 8 mm eads ave larger connection strengts tan 36 mm eads for te same assembly force (up to 5 kn), we infer tat tey will also ave greater corrosion resistance. Furter work, owever, needs to be undertaken to confirm tis. A possible reason tat previous studies ave overlooked te influence of assembly effects wen using larger-diameter eads is because offset as been used to simulate te effect of larger-diameter eads. 7,1 Tis may not capture te differences in connection strengt tat occur during assembly. Limitations. A previous study by Kinbrum, Traynor and Collins 37 was also unable to detect canges in rougness before and after testing using impaction forces of 4 kn. Altoug our assembly forces were larger (up to 6 kn), it is possible tat our scanning resolution was not ig enoug to detect te surface canges. It is not clear wy te pull-off force for 8 mm eads at 6 kn impaction force did not follow te linearly increasing trend seen for 36 mm eads. Bot FE and analytical models suggest tat te trend sould continue for muc larger impaction forces, owever, tese models were not equipped to capture any material non-linearity. Altoug tere was no significant cange in measured surface rougness of te trunnion, indicating tat no damage ad occurred, it is possible tat damage occurred on te female taper, despite aving a larger Young s modulus, due to te confinement of te male taper. Neverteless, te typical force applied by surgeons as been sown to be 4409 N (sd 660) 8 in laboratory conditions. Te lack of constraint of te eadtrunnion construct in vivo makes it even less likely tat an impaction force of 6 kn is regularly and reliably acieved clinically. It is also important to note tat our findings relate to solid THA eads and may not apply to ollowed sell eads suc as te Birmingam Hip Modular Head (Smit & Nepew plc, London, United Kingdom), te ASR XL Head (DePuy International, Leeds, United Kingdom), or modular ead designs wit a titanium sleeve insert. Despite studies demonstrating tat initial connection strengt influences corrosion resistance in vitro, 3,44 it is still unclear weter tis effect influences te long-term survivorsip of a ip prostesis. Furter studies sould consider measuring connection strengt immediately after impaction and after cyclic loading in order to determine weter connection strengt is reduced over time due to pysiological loading. In conclusion, te autors note tat as far as tey are aware, tis is te first study to demonstrate tat te eadtrunnion connection strengt of larger-diameter eads is inferior to tat of smaller ead sizes at te point of assembly. Heads of 36 mm diameter require approximately 0% greater assembly forces (6 kn vs 5 kn) to provide a similar initial pull-off strengt to tat acieved by 8 mm eads. Any reduction in te initial taper connection strengt is likely to compound oter effects suc as increased centre offset. Altoug initial stability may not necessarily translate into long-term in vivo stability, te lower connection strengt may elp to explain te greater extent of fretting and corrosion associated wit large diameter eads. If a link between initial and longterm stability is definitively found, tese differences would need to be accounted for clinically during THA. It sould be noted tat tis effect is likely to be even more pronounced for ead diameters larger tan 36 mm. References 1. Esposito CI, Wrigt TM, Goodman SB, Berry DJ, Clinical, Biological and Bioengineering Study Groups from Carl T. Brigton Worksop. Wat is te trouble wit trunnions? Clin Ortop Relat Res 014;47: Carli A, Politis A, Zukor D, Huk O, Antoniou J. Clinically significant corrosion at te ead-neck taper interface in total ip artroplasty: a systematic review and case series. Hip Int 015;5:7-14. BONE & JOINT RESEARCH

10 A. R. MacLeod, N. P. T. Sullivan, M. R. Witeouse, H. S. Gill Feeley J. J&J to Pay as Muc as $40 Million More in ASR Hip Accord. www. bloomberg.com/news/articles/ /j-j-to-pay-as-muc-as-40-million-morein-asr-ip-accord (date last accessed 1 July 016). 4. Lieberman JR, Rimnac CM, Garvin KL, Klein RW, Salvati EA. An analysis of te ead-neck taper interface in retrieved ip prosteses. Clin Ortop Relat Res 1994;300: Hussenbocus S, Kosuge D, Solomon LB, Howie DW, Oskouei RH. Head-neck taper corrosion in ip artroplasty. Biomed Res Int 015;015: Wassef AJ, Scmalzried TP. Femoral taperosis: an accident waiting to appen? Bone Joint J 013;95-B(11 Suppl A): Donaldson FE, Coburn JC, Siegel KL. Total ip artroplasty ead-neck contact mecanics: a stocastic investigation of key parameters. J Biomec 014;47: Langton DJ, Sidaginamale R, Lord JK, Nargol AV, Joyce TJ. Taper junction failure in large-diameter metal-on-metal bearings. Bone Joint Res 01;1: Goldberg JR, Gilbert JL, Jacobs JJ, et al. A multicenter retrieval study of te taper interfaces of modular ip prosteses. Clin Ortop Relat Res 00;401: McKellop HA, Sarmiento A, Brien W, Park SH. Interface corrosion of a modular ead total ip prostesis. J Artroplasty 199;7: Purdue PE, Koulouvaris P, Nestor BJ, Sculco TP. Te central role of wear debris in periprostetic osteolysis. HSS J 006;: Del Balso C, Teeter MG, Tan SC, Lanting BAHJ, Howard JL. Taperosis: does ead lengt affect fretting and corrosion in total ip artroplasty? Bone Joint J 015;97-B: Witeouse MR, Endo M, Zacara S, et al. Adverse local tissue reactions in metal-on-polyetylene total ip artroplasty due to trunnion corrosion. Bone Joint J 015;97-B: Langton DJ, Jameson SS, Joyce TJ, et al. Early failure of metal-on-metal bearings in ip resurfacing and large-diameter total ip replacement: A consequence of excess wear. J Bone Joint Surg [Br] 010;9-B: Smit AJ, Dieppe P, Vernon K, et al. 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Scientific Committee on Emerging and Newly Identified Healt Risks (SCENIHR). Opinion on: Te safety of Metal-on-Metal joint replacements wit a particular focus on ip implants. European Commission. ttp://ec.europa.eu/ealt/scientific_ committees/emerging/docs/scenir_o_04.pdf (date last accessed 17 May 016). 3. No autors listed. National Joint Registry for England, Wales, Nortern Ireland and te Isle of Man. 1t Annual Report. ttp:// Portals/0/Documents/England/Reports/1t%0annual%0report/NJR%0 Online%0Annual%0Report%0015.pdf (date last accessed 17 May 016). 4. Hexter A, Panagiotidou A, Sing J, Skinner J, Hart A. Corrosion at te eadtrunnion taper interface in large diameter ead metal-on-metal total ip artroplasty: a comparison of five manufacturers. Bone Joint J 013;95-B(Supp 1):3. 5. Morlock MM. Te taper disaster ow could it appen? Hip Int 015;5: Tan SC, Teeter MG, Del Balso C, Howard JL, Lanting BA. Effect of taper design on trunnionosis in metal on polyetylene total ip artroplasty. 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In vitro corrosion testing of modular ip tapers. J Biomed Mater Res B Appl Biomater 003;64: Funding Statement JRI donated materials used in te performance of te tests described in te manuscript. H.S. Gill reports funding received from Smit & Nepew wic is related to tis article. Professor Gill is a member of te Editorial Board of Te Bone & Joint Journal, and a member of Executive Committees of te Britis and European Ortopaedic Researc Societies. M. Witeouse reports several fees and grants received from DePuy, Heraeus, Stryker and HQIP, none of wic are related to tis article. Autor Contribution A. R. MacLeod: Substantial contributions to researc design, Acquisition, analysis and interpretation of data, Drafting and critically revising paper. N. P. T. Sullivan: Substantial contributions to data acquisition and interpretation, Critically revising paper. M. R. Witeouse: Substantial contributions to researc design and interpretation of data, Critically revising paper. H. S. Gill: Substantial contributions to researc design and interpretation of data, Drafting and critically revising paper. ICMJE conflict of interest None declared. 016 MacLeod et al. Tis is an open-access article distributed under te terms of te Creative Commons Attributions licence (CC-BY-NC), wic permits unrestricted use, distribution, and reproduction in any medium, but not for commercial gain, provided te original autor and source are credited. vol. 5, No. 8, August 016