A novel liner locking mechanism enhances retention stability

Medical Engineering & Physics 25 (2003) 747–754www.elsevier.com/locate/medengphy

A novel liner locking mechanism enhances retention stability

Warren Macdonalda,∗, Anders Aspenberga, C. Magnus Jacobssona, Lars V. Carlssona,b

a Department of Biomaterials Research, Institute for Surgical Sciences, University of Gothenburg, S-413 90, Gothenburg, Swedenb Department of Orthopaedics, Sahlgrens University Hospital, University of Gothenburg, S-413 45, Gothenburg, Sweden

Received 14 October 2002; received in revised form 22 May 2003; accepted 9 June 2003

Abstract

Acetabular liner retention of a novel design of liner locking was evaluated in static and cyclic endurance modes. The lockingmechanism combines geometric form and accurate machining to give high conformity to the acetabular shell and minimise relativemotion against the metal shell, minimising debris generation and escape or ingress. Using amended test liners with integral coupling,mean static pullout strength was determined to be 399± 53 N and lever-out strength 28.03± 2.8 N m. Cyclic loading of 5 N mfor up to 10 million cycles caused no significant reduction in strength, no detectable fretting wear, and the sealing mechanismprevented particle access between the cup interior and the “effective joint space”. The stability measured ensures secure and reliablein vivo retention of the liner, comparable with extant component designs using other liner locking mechanisms. 2003 IPEM. Published by Elsevier Ltd. All rights reserved.

Keywords: Acetabular implant; Polyethylene; Modular component; Mechanical testing; Disassembly

1. Introduction

Most uncemented acetabular systems consist of ametal shell or cup fitted with an ultra-high-molecular-weight polyethylene (UHMWPE) liner. Despite its longhistory in orthopaedics, concerns about UHMWPEinclude the debris generated when it wears, its relativelylow mechanical strength, and its oxidation/degradationbehaviour. More recently a further concern has arisen:fracture or failure of the liner retention leading tocomponent dislocation or failure[1–3].

Also, heightened concern about the role of wear debrisin prosthetic loosening[4–7], particularly fine polyethyl-ene debris[8,9], and the possibility that insufficient con-nection of modular components may further generatedebris [10,11] or allow its dissemination[12] haveturned greater attention to the design and retainingstrength of polymeric liners.

Failure of UHMWPE liners is related to design andmanufacture as well as choice of materials[13,14]. Wearresistance of UHMWPE is strongly dependent on base

∗ Corresponding author. Address: 5 St James’ Square, Boscombe,Dorset BH5 2BX, UK. Tel.:+44-1202-422-101; fax:+44-1202-422-131.

E-mail address: [email protected] (W. Macdonald).

1350-4533/$30.00 2003 IPEM. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S1350-4533(03)00112-7

resin properties (molecular weight, extent of consoli-dation [15,16], additives [17]), articulating conditions(femoral head finish and material[18,19], lubricationand temperature[20]) and manufacturing conditions(oxidation due to sterilization and storage[21], degreeof crosslinking). In addition, accelerated wear or grosscomponent failure has been linked to thin sectioned orincongruent PE components[22–24], design choicesaffected by cup design and fixation (screwed or screw-less) [25,26], and the liner retention mechanism[27].Some liner retention mechanisms have also been foundto be inadequate in vivo[28].

Excessive wear at the cup/liner or liner/femoral headinterfaces can contribute to severe osteolysis in the acet-abular bone if wear debris can gain access to the bone[29,30] or is generated adjacent to the fixation surfaces.Sealing of the acetabular cavity, integration and fixationbetween bone and metal cup, and between metal cupand articulating liner, can prevent particle ingress andinhibit osteolysis.

Modular liner stability has been laboratory tested inpullout and lever-out modes monotonically[31], inrepetitive cyclic endurance[10,32], and to examine sea-ling against particle or fluid flow[12].

The aim of this investigation was to determinewhether a liner retention based on highly congruent sur-

748 W. Macdonald et al. / Medical Engineering & Physics 25 (2003) 747–754

faces and peripheral locking could offer secure resist-ance to disrupting forces initially and after cyclic load-ing.

2. Materials and methods

2.1. Liner design

An acetabular cup/articulating liner assembly wasdesigned utilising dual geometry: cylindrical form at theopening combined with spherical form over the remain-der of the liner (Fig. 1). A combined textured band andcylindrical congruency at the (near cylindrical) cupmouth provided stability to rocking or lever-out. Spheri-cal form at the “dome” with reduced metal thicknessallowed maximised polyethylene thickness in the load-transfer zone and increased liner/shell congruence.Around the cylindrical opening zone a flexible texturedlocking band ensured good sealing and initial retentionupon assembly of the components in surgery.

2.2. Test methods

Test liners were designed and produced with integralcouplings to enable application of the repetitive loads(torsional and rocking) and single cycle tests in pulloutand lever-out (Fig. 2); the inner 28 mm bore was partmachined to allow realistic liner deformation, but a cen-tral column was retained to serve as the loading stem.Test cups were manufactured in c.p. titanium withinternal form, finish and dimensions to nominal implantdimensions, but with simplified outside form. Withdimensions of the metal cups and the correspondingliners controlled at 20 °C, the nominal fit at 20 °C and

Fig. 1. Improved liner design showing spherical dome section, cylin-drical periphery with lamellar locking band circumferentially.

Fig. 2. Test liner after 10 000 cycles vertical loading with sealingtesting. The integral stem is shown connected to the test coupling, andmethylene blue can be seen to have intruded (the grey discolourationof the link stem and cylindrical band) only down to the textured sea-ling band.

the anticipated fit at body temperature could be variedto investigate the effect of dimensional variation onliner stability.

Liners were stored at 4 °C until selected for testing.They were then assembled into the test cup, the cup/linerassembly was bolted into the test chamber and circu-lation of water at 37 °C was commenced. After a mini-mum of 30 min, to allow the polyethylene to equilibrateto 37 °C, mechanical testing or loading was commenced.

A cyclic test rig was developed which could applyrepetitive forces, torsion or lateral force at the liner axis,whilst allowing the application of a continuous verticalload to mimic body weight and with the test componentsimmersed in the circulating fluid at 37 °C. Mechanicalactuation of the loading allowed testing at 1–2 Hz, andinitial tests were performed in 0.9% Ringer’s solution.

For non-cyclic testing, pullout was performedimmediately using an Instron Universal Materials Test-ing Machine (Instron 4301; Instron Ltd, HighWycombe), with a crosshead speed of 5 mm/min (asused by Tradonsky et al. [31]); load and displacementwere monitored continuously (Fig. 3). Specimens wereprepared over a range of diametral interference fits (as

749W. Macdonald et al. / Medical Engineering & Physics 25 (2003) 747–754

Fig. 3. Liner testing in pullout mode (the water bath has beenremoved to reveal the test cup A with the liner B assembled in placeand bolted to the extraction link).

described at room temperature) so that testing couldexamine the influence of interference fit on retention invarious modes. Lever-out was achieved with an attachedlever arm of 500 mm actuated by the Instron UTS actingat a crosshead speed of 100 mm/min (Fig. 4). Torsionaltesting was also performed on the Instron machine, withthe crosshead driving a quadrant of 250 mm radius.

Tests were undertaken to investigate the dependenceof pullout strength on interference, initially with widervariation in dimensions than would normally be accept-able in production. Subsequently limiting the fit to spe-cific values of interference within the design tolerances,the dependence was again investigated.

For cycled testing, the PE liner link of 500 mm wasreversibly loaded by 10 N, at a frequency of 1 Hz anda triangular waveform. Thus, repetitive forces of ±5 Nm torque were applied in either rocking mode (appliedabout an axis in the plane of the cup opening) or tor-sional mode (applied about the vertical axis of the linerand load stem). The test components were also immersedin a bath of Ringer’s solution maintained at 37 °C duringtesting. After cycling for the requisite number of cycles,the cup/liner specimen was removed from the repetitiveloading rig (with its water bath) and transferred to theInstron UTS machine, where pullout was performed.

To verify the cyclic testing against in vivo perform-

Fig. 4. Liner testing in lever-out mode (the water bath C obscuresthe test cup and liner); the lever link and quadrant D are connectedby braided wire F to the crosshead, which is drawn around the pulleyby upward motion of the crosshead.

ance, tests were also undertaken in a hip joint simulator.Using four stations of a Los Angeles-style hip jointsimulator (MTS 8 station hip wear simulator) [33] work-ing at 1 Hz with a “Paul” -type loading curve [34] from250 to 2500 N, liner retention loading was induced byarticulating the acetabular components against deliber-ately roughened cobalt–chromium femoral heads. Thistechnique has been shown to result in increased wear atthe metal/PE articulation of the liner bore, and to pro-duce torques of up to 1 N m in the plane of the cup,and as high as 4 N m in the lever-out sense [35].

Lubrication was provided by 25% bovine calf serumin Ringer’s solution, with 0.1% sodium azide additionsto inhibit bacterial breakdown. No external heating wasapplied, but the heat generated during articulationensured that the bulk serum temperatures stayed between30.5 and 35 °C. Component dimensions were adjustedto ensure the same dimensional fit at these temperaturesas in vivo. On completion of 5 million cycles, thecomponents were removed from the hip simulator,chilled to allow easy extraction of the liners withoutinducing artefactual damage, and inspected under opticalstereomicroscopy at magnifications up to 40×.

Particle sealing was investigated with an improvedcyclic testing rig. Similar liner and cup specimens and


specimen supports were employed, but pneumatic actu-ation enabled torsional and vertical loadings to besynchronised and programmed cyclically. A jacketedspecimen support chamber enabled isolation of the testfluid from the circulating heating fluid, allowing methyl-ene blue to be used as the test fluid. Specimens weremounted in the test cup in the usual way, and thechamber was filled with a 50% solution of methyleneblue. After 30 min at 37 °C, to equilibrate the UHMWPEand secure the assembly, the test assembly was subjectedto the cyclic loading regime (1000, 10 000, 50 000 and100 000 cycles).

On completion of the requisite number of cycles, thecirculating fluid was stopped and the test chamber wasdrained. The chamber was further washed in warm water(37 °C) until all traces of methylene blue around thespecimens had been removed, and the assembly wasplaced inverted in a refrigerator at 7 °C for 30 min. TheUHMWPE liner could then be easily extracted from thetest cup, revealing any traces of methylene blue carriedinto the interfaces.

3. Results

3.1. Static tests

Pullout retention strengths measured initially (after nocycling) demonstrate a linear dependence on the inter-ference measured at 20 °C (Fig. 5, Table 1). Multipleregression analysis shows that this reliance on inter-ference is highly significant (p � 0.00012), whilst goodcorrelation (R 2 = 0.77) and the 95% confidence limitsshow that 250 N is the minimum possible retention atthe design limit of 30 µm interference. After the pilotseries of specimens showed such strong dependence oninterference, the tests were re-worked for design limitsof interference (30–105 µm interference) and the resultsdemonstrated similar predictable behaviour (Fig. 6).

Liners were also tested in lever-out, using a similar

Fig. 5. Pullout strength vs. liner interference fit (regression line and95% confidence limits are plotted).

experimental arrangement to that for pullout. Lever-outstrength was found to be independent of interference,with a mean value of 28 N m (Table 1) (p = 0.679).

The torque strength of the liner retention was alsodetermined, and found to be 16.8 N m (±2.6).

The pullout strengths of the liner system after cyclicloading are presented in Fig. 7. It is apparent that cyclicloading to 5 million cycles did not significantly reducethe retention of this design. The distribution of the reten-tion strengths thus measured indicated that the strengthof retention remains greater than 250 N. Microscopicinspection of the liner exterior surfaces after cyclingrevealed no evidence of fretting.

3.2. Endurance tests

After 5 million and 10 million cycles, specimensshowed no evidence of fretting wear between liner andcup; retained machining marks were present over all ofthe dome and the side regions. In no region was pol-ishing of the machining marks found. Some grossscratches were observed, due to the disruption duringpullout testing. Otherwise, the liner appeared unchangedfrom its as-manufactured state.

Likewise, after 5 million cycles in the hip simulator,no evidence of fretting wear could be observed. Retainedmachining marks, unchanged from the as-machined con-dition, were observed on all surfaces.

3.3. Interfacial sealing

Methylene blue was carried down the interface fromthe cup opening only as far as the peripheral lockingband, but was prevented from further intrusion by thetextured interlock (Fig. 2), after all of the cyclic test life-times.

4. Discussion

Bone/implant stability and surgical accuracy had beenaddressed by an osseointegrated implant and instrumen-tation [36]. The acetabulum imposes a generally hemi-spherical form on any design solutions: a strong formenabling metal thickness to be minimised, and polymerthickness to be maximised, especially if 32 mm diameterheads are avoided. Dual geometry assembly of the linerin the metal cup as proposed provided stability to lever-out and increased liner/shell congruence (Fig. 1). Thetextured peripheral band ensured good sealing and initialretention at room temperature, and effective sealing tothe shell wall resisted fluid flow (Fig. 6).

Congruence between liner and shell is dependent upondesign and manufacturing tolerances. With maximalcongruence from design, manufacture needs to takeaccount of thermal variation in polyethylene. In fact,


Table 1Liner lever-out strengths initially (without cycling) and pullout strengths without cycling [31]

Cup design Lever-out strength, mean (N m) Standard deviation Pullout strength, mean (N) Standard deviation Reference

Duraloc 79 13 2949 291 [31]S-ROM 64 1.7 2144 21 [31]APR 51.5 3.8 1446 48 [31]HGP II 37.5 1.5 529 28 [31]Goteborg 28.0 2.1 399 53 Present studyOmnifit 25.8 3.3 458 88 [31]PCA 16.4 2.9 378 132 [31]Optifix 8.2 0.3 271 12 [31]Triloc 4.9 0.2 129 6 [31]

Fig. 6. Pullout strength vs. liner interference fit. Restricted size vari-ation (regression line and 95% confidence limits).

since the thermal coefficient of expansion for titaniumis 7.6 × 10�6 K�1 and for PE is 1.8 × 10�4 K�1, thethermal expansion can be used to compensate for manu-facturing variations rather than contributing to them, afeature incorporated in this system design. In that way,fluid sealing and mechanical retention are maintainedand enhanced under the operating conditions in vivo.

The combination of a modular liner and cup design

Fig. 7. Residual liner strength as a function of endurance testing lifetime.

(as provided by a large number of systems presently)offers many advantages, such as:

1. the use of bone contacting metals found to be unsuit-able as articulating components (e.g. titanium),

2. the ability to use adjunctive fixation such as screwswithout affecting the articulation,

3. modular and hence adjustable liner orientation afterimplantation of the bone fixation element (throughoffset bore or long posterior wall liners),

4. the articulation element is replaceable without disrup-tion of bone fixation.

Initial fixation at both interfaces will prevent ingressof wear debris to developing or stable fixation zones.But lost stability later in the service life of the implantsystem will permit particle ingress, and result in delayedosteolysis rather than enduring fixation. Therefore thestability of the interfaces must endure throughout thecyclically loaded service of in vivo conditions.

Tradonsky et al. [31] tested the monotonic retainingstrength of a range of modular acetabular components,using two different modes. Direct extraction of the linerfrom the metal cup was tested by pushing out from the


bottom of the cup, whilst “ lever-out” required the drill-ing of an anchorage hole in one side of the liner to givegrip to the end of the lever. The forces required for thesemodes of disruption varied widely, pushout strengthfrom 29 to 663 pounds-force (129–2950 N) and lever-out from 43 to 684 inch-pounds (4.9–77 N m) The extentof damage to the polymeric liner and the metal retainingfeatures also varied widely.

Design of a modular cup system to avoid liner failureor osteolysis requires the following system attributes[37]:

1. Stable metal implant fixation into a close-fittingbone cavity.

2. Cup stability without the need for adjunct screw fix-ation.

3. Liner stability against disruptive forces (especiallylever-out and pullout).

4. Enduring stability throughout the expected service life(10 years minimum; 25 years preferred).

5. Congruence between metal cup interior and PE linerexterior surfaces, and adequate thickness of polymer,to avoid increased articulating wear.

With a solely press-fit cup, the absence of screw-headsin the metal bore means that there is no mechanism forpushing the liner out of the cup; extraction of the linerfrom the cup will necessitate the imposition of tractiveforces through the liner bore. Any forces generated inthis way will be of low magnitude (relying on “suction”of the femoral head in the hemispherical bore). Never-theless, an adequate retention strength is still advisableto prevent disassembly through whatever mechanismsmay occur in vivo.

The pullout strengths and lever-out strengths meas-ured in the system presented in this study comparefavourably with the range of values reported by Tradon-sky et al. [31]. By virtue of the simple design, withoutscrews or any components interfering with the hemi-spherical bore of the metal cup, the tendency to produce“pushout” of the liner is strongly reduced and thestrengths measured can be considered adequate for invivo service.

Testing of acetabular cup fixation stability has shownstrengths ranging from 500 to 2000 N in pullout mode,20 to 75 N m in rotation, and 5 to 40 N m in lever-out[36]. Lever-out testing of liners has been reported [31],and failures of modular acetabular systems in lever-outmodes have been reported [1,2]. However, it wouldappear that these were due to unacceptably weak PEretention systems due to design inadequacy [3] com-bined with the increased susceptibility to lever-out fail-ure of elevated rim PE liners. Elevated or angled rim PEliners are used in cases of recurrent dislocation, oftenbecause the implant has been wrongly positioned at sur-gery.

The relationship between cup/liner interference andretention strengths in the two modes sheds interestinglight on the design choices. In pullout, there is a signifi-cant dependence on interference, even at the level of 30µm difference. This obviously has important impli-cations for the manufacturing control of such devices. Itshould be noted that the test components were manufac-tured with dimensional control of better than 5 µm, andprocess control to the required dimensions is feasible.This is in strong contrast to the statement of Ritter et al.[38], who undertook a study of dimensional variation infemoral heads between manufacturers, but did notrequest dimensions of the polyethylene componentbecause “ the specifications cannot be as closely moni-tored” . Many manufacturers appear to experience widevariation in the dimensions of their polyethylene compo-nents, probably with severe consequences for the fit andperformance of their assemblies.

The lever-out strength is independent of interferenceover the range of values investigated, indicating theadvantage of using dual geometry in the liner retention.The same advantage applies to the performance of thisretention in vivo under lever-out modes of loading.

Specimens were tested in the MTS hip joint simulatorbecause the combination of motions, forces and environ-ment is closest to the physiological. But the torques thatcan be applied to the liner retention (those developed byfriction at the articulating interface between head andliner) are of necessity limited; hip simulators aredesigned to model normal conditions and not the abnor-mal transients which may cause liner disruption. Thus,it was considered that a test machine using less complexmotions but more direct coupling to the test specimensmight enable more rigorous testing.

Testing in saline or water instead of serum is of ques-tionable validity in wear testing of articulating bearingssuch as the femoral head/liner bore interface. However,in fretting testing the tendency to form a transfer filmand hence suffer accelerated wear could actuallyincrease the sensitivity of the testing for liner retention.Furthermore, the formation of a transfer film occurs withwear over large sliding distances, and the expectationin liner/cup retentions is that no motion should occur,especially with the present design. On that basis it wasfelt that testing in saline (in the first instance) wasacceptable, as long as validation was undertaken in morerepresentative lubrication conditions.

Likewise, it is not thought that exact reproduction ofthe loading pattern occurring during normal gait (the“Paul cycle” ) is essential for valid testing of liner reten-tion. The importance of the “Paul cycle” is the combi-nation of force magnitudes and sliding velocities occur-ring during articulation of the hip, which are nottransmitted directly to the liner/cup interface but are fil-tered by the head/liner articulation and then affect theliner/cup interface. Notwithstanding, validation on a hip


simulator of the results from the simpler test rig confirmsboth the safety of the implant design and the efficacy ofthe simplified test mode.

Methylene blue is a large molecular weight colouringagent, which nevertheless is significantly smaller in par-ticle size than the 0.1–1 µm wear particles typically pro-duced from wear of polyethylene, and even than the nan-ometre sized particles being found with metal–metal orceramic–ceramic bearings. The peripheral sealing bear-ing band and congruent spherical surfaces inherent inthe present design demonstrate adequate resistance to theintrusion of particles under low fluid pressure and cyclicloading. Khalily et al. tested for particle intrusion atpressures up to 300 kPa, with a dedicated test rig ableto develop such pressures [12]. This is a useful directionin which to pursue such intrusion tests.

Testing for fretting wear of polyethylene componentsremains an inexact science, whilst the preferred meansof detection is still microscopic inspection of the relevantsurfaces of the polyethylene components. Any wear willresult in removal of material preferentially from the highpoints of the machining striations, “burnishing” the sur-faces and removing the machined finish. This is con-sidered to be quite a sensitive indicator of this mode ofwear [22], although an objective and clearer techniqueis desirable.

5. Conclusions

1. Use of congruent textured surfaces and peripherallocking for liner retention provides adequate stabilityfor modular acetabular systems.

2. Representative pre-clinical tests can be performedwith simplified test equipment, avoiding the need forjoint simulators.

3. Liner retention through congruent textured surfacesand peripheral locking also serves to prevent lowpressure ingress of particles and fluid.

Acknowledgements

The support and collaboration of Stig Wennberg inthe design and development of the retention system isgratefully acknowledged. We would also like to thankProf. John Fisher and Mr. Devon Derby, of the Univer-sity of Leeds, for scratching and characterising the fem-oral heads used in the simulator study, and Dr. TubaYamac and Dr. Julia Shelton of Queen Mary, Universityof London, for undertaking the hip simulation.

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A novel liner locking mechanism enhances retention stability