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HIP

The relationship between the angle of version and rate of wear of retrieved metal-on-metal resurfacingsA PROSPECTIVE, CT-BASED STUDY

A. J. Hart, K. Ilo, R. Underwood, P. Cann, J. Henckel, A. Lewis, J. Cobb, J. Skinner

From Imperial College, London, United Kingdom

A. J. Hart, MD FRCSG (Orth), Consultant Orthopaedic Surgeon and Senior LecturerDepartment of Musculoskeletal Surgery

K. Ilo, BSc, Medical StudentR. Underwood, Research

AssociateP. Cann, PhD, Research

FellowJ. Henckel, MRCS, Specialist

Registrar in Orthopaedic Surgery

A. Lewis, FRCS (Orth), Consultant Orthopaedic Surgeon

J. Cobb, FRCS, Professor of Orthopaedic SurgeryImperial College London (Charing Cross Hospital), Fulham Palace Road, London W6 8RF, United Kingdom.

J. Skinner, FRCS (Orth), Consultant Orthopaedic SurgeonRoyal National Orthopaedic Hospital, Brockley Hill, Stanmore HA7 4LP, United Kingdom.

Correspondence should be sent to Mr A. J. Hart; e-mail: a.hart@imperial.ac.uk

©2011 British Editorial Society of Bone and Joint Surgerydoi:10.1302/0301-620X.93B3. 25545 $2.00

J Bone Joint Surg [Br] 2011;93-B:315-20.Received 19 July 2010; Accepted after revision 10 November 2010

We measured the orientation of the acetabular and femoral components in 45 patients (33 men, 12 women) with a mean age of 53.4 years (30 to 74) who had undergone revision of metal-on-metal hip resurfacings. Three-dimensional CT was used to measure the inclination and version of the acetabular component, femoral version and the horizontal femoral offset, and the linear wear of the removed acetabular components was measured using a roundness machine.

We found that acetabular version and combined version of the acetabular and femoral components were weakly positively correlated with the rate of wear. The acetabular inclination angle was strongly positively correlated with the rate of wear. Femoral version was weakly negatively correlated with the rate of wear. Application of a threshold of > 5 μm/year for the rate of wear in order to separate the revisions into low or high wearing groups showed that more high wearing components were implanted outside Lewinnek’s safe zone, but that this was mainly due to the inclination of the acetabular component, which was the only parameter that significantly differed between the groups.

We were unable to show that excess version of the acetabular component alone or combined with femoral version was associated with an increase in the rate of wear based on our assessment of version using CT.

The inclination angle of the acetabular compo-nent of metal-on-metal (MoM) hip replace-ments has been statistically significantlyassociated with the rate of wear identified fromexplanted components.1 The mechanisminvolves edge loading but version of the acetab-ular component may also affect edge loadingby impingement,2 subluxation/dislocation,3 andreduced cover of the head of the femoral com-ponent. De Haan et al4 attributed the failure of19 MoM hips to insufficient or excessive ver-sion of the acetabular component.

The effect of version of the acetabular compo-nent on the rate of wear has not been the focus ofany published study. However, Langton et al5

reported that 16 patients with inflammatorysoft-tissue reactions had a median version of 7°greater than that of asymptomatic patients (p <0.001). Of these, eight patients had a rate ofwear of the femoral head greater than that ofseven patients in whom failure was due to frac-ture of the femoral neck.5 One study found noeffect of version on the rate of wear of retrievedceramic-on-ceramic (CoC) hips and anotherfound a negative correlation.6,7 In the latter astripe pattern of wear across the femoral head

was observed which was attributed to impinge-ment of the neck of the femoral component onthe rim of the acetabular component.8

Robust analysis of the effect of version ofthe acetabular component in vivo on any out-come variable needs to overcome the difficul-ties of obtaining valid three-dimensionalmeasurements. CT measurements allow fem-oral version to be added to version of theacetabular component in order to obtain thecombined version of both components.9

The recent alert from the Medicines andHealthcare products Regulatory Agency of theUnited Kingdom concerned all MoM hips,10

encompassing resurfacing and total hip replace-ment (THR) using a large femoral head(> 36 mm diameter) together with more con-ventional MoM bearings with heads of 28 mmin diameter. However, because conventionalMoM bearings seem to have a long clinical her-itage with minimal problems,11 we have focusedon designs with head sizes > 36 mm diameter.

Our aim was to determine the effect of isolatedacetabular version and combined acetabular andfemoral version on the rate of wear of failedMoM hips of the current generation. We wished

RENNIKS.J ,BBOC.J ,SIWEL.A ,LEKCNEH.J ,NNAC.P ,DOOWREDNU.R ,OLI.K ,TRAH.J.A613

THE JOURNAL OF BONE AND JOINT SURGERY

to achieve this prospectively so that pre-revision CT could beused for the measurement of the version of components andcould be related to the wear of the hip after revision.

Patients and MethodsThe approval of the Institutional Review Board wasobtained. A consecutive series of 45 patients (45 hips and90 available components) underwent revision between Feb-ruary 2008 and February 2010. All the patients had CTscans performed before revision. We excluded eightpatients who did not have adequate CT scans or MoMcomponents of the current generation. In all, there were33 men and 12 women with a mean age of 53.4 years (30 to74) at primary hip replacement.

There were 25 Birmingham Hip Resurfacings (BHR)(Smith & Nephew, Warwick, United Kingdom); eight ASR(DePuy, Leeds, United Kingdom); four Cormet 2000(Corin, Cirencester, United Kingdom); four Durom (Zim-mer, Winterthur, Switzerland) and four Magnum (Biomet,Bridgend, United Kingdom) resurfacing devices.

We recorded gender, the age at implantation, the dura-tion of implantation, the device used and the head size.From the pre-revision CT scans we measured the inclina-tion and version of the acetabular component, the femoralversion and the horizontal femoral offset.Three-dimensional (3D) CT measurement. This was per-formed using 0.75 mm collimation (high resolution) and anartefact minimisation sequence (16-bit data processed onan extended scale), both of which allowed visualisation ofthe detail required to separate the face of the metal

acetabular component from the metal femoral head. Theradiation dose was only 1.7 mSv,12 compared with that of atraditional pelvic CT scan of 10 mSv. The anatomical incli-nation and version angles of the acetabular componentwere measured using a 3D-CT reconstruction softwarepackage (Robin 3D software).13,14 In order to quantify theproportion of patients inside and outside Lewinnek’s safezone, we transformed the radiological values used to out-line the perimeter into anatomical angles using the formulaprovided by Murray.15

The retrieved acetabular components were measured forthe position and depth of the wear scar using a Talyrond 365roundness instrument (Taylor Hobson, Leicester, UnitedKingdom).16 This precision measuring machine uses a stylusprobe to measure form, roundness and cylindricity to submi-cron accuracy (spindle accuracy of 0.02 µm, minimum gaugeaccuracy 12 nm). Each measurement was repeated threetimes and the mean value taken as the final reading. Circum-ferential and polar measurements of the explanted acetabu-lar components were made. For the former, the componentwas mounted horizontally on the Talyrond rotating stage,centred and levelled to submicron accuracy using a functionwithin the software. A total of 12 circular measurement pro-files were made in increments of 1 mm along lines of latitudeon the bearing surface of the component parallel to its rim(Fig. 1). The profiles were analysed using Ultra Software(Taylor Hobson Ltd, Leicester, United Kingdom). A maxi-mum inscribed circle was fitted to each profile to representthe unworn surface of the component (Fig. 1). From theanalysed profiles, the position and extent on any wear scarcould be located in three dimensions (Fig. 2).

For the polar measurements of the acetabular component12 profiles, at increments of 15° around the rim, were mea-

Fig. 1

Diagram showing the roundness profile of an acetabular componentfrom a failed MoM hip resurfacing (MI, maximum inscribed; RONv,roundness valley; RONt, roundness total).

Fig. 2

3-D map of a failed MoM acetabular component created from 12 circularmeasurement profiles, showing the position and depth of the wear scaron the acetabular component. The wear scar was 295 µm deep(RONt roundness total).

THE RELATIONSHIP BETWEEN THE ANGLE OF VERSION AND RATE OF WEAR OF RETRIEVED METAL-ON-METAL RESURFACINGS 317

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sured from its rim through the pole and back to the rim alongthe lines of longitude on the bearing surface. The polar mea-surement encompasses the region near the pole not covered bythe circumferential measurement and allows wear and form tobe distinguished from the circumferential measurement.

The results of the measurements are presented as themedian value with the IQR.Femoral head wear measurement. The heads were mea-sured by mounting the head vertically on the Talyrondrotating stage. Partial roundness profiles were taken fromthe base of the head, through the pole, back to the base. Inall 12 partial roundness profiles were taken by rotatingthe head at 15° increments. For analysis, a Minimum Cir-cumscribed arc was fitted through each profile. This arc isthe minimum diameter arc that completely enclosed themeasured profile and provides an approximation of themanufactured shape of the head. However, head compo-nents are subject to form error of up to 20 μm, making itimpossible to identify wear scars from a single profile. Inorder to separate the wear and form, all 12 profiles weresuperimposed using a Matlab program. This allows theworn and unworn regions of each profile to be clearlyidentified. The maximum linear wear depth can be calcu-

lated from the maximum deviation between worn andunworn profiles, and thus separates the form error fromthe wear calculation.Measurement of femoral anteversion. The angle betweenthe stem of the femoral component and the posterior con-dylar axis of the knee along the anatomical axis of thefemur was recorded. The horizontal femoral offset wasdefined as the shortest distance between the centre of rota-tion of the femoral head and the anatomical axis.Statistical analysis. The Pearson correlation coefficientwas used to test the strength of possible associationsbetween the wear rate and the following variables: incli-nation and version of the acetabular component, the hor-izontal femoral offset and the combined version of the twocomponents.

The data were then categorised into low and high wear-ing groups after calculation of the mean linear wear ratefrom published MoM hip retrieval studies (Table I). Highwear was therefore defined as a wear rate > 5 μm per year.We used the non-parametric Mann-Whitney U test to testfor significant differences in the variables between the highand low wearing groups. A p-value ≤ 0.05 was consideredto be significant.

Table I. Reports of the wear analysis of retrieved metal-on-metal hip articulations

Author/s Number of hips Prosthesis*Mean (range) time before explanted

Mean (range) in vivo linear wear rate (μm/yr)

Langton et al5 15 ASR hip resurfacings and 1 ASR THR

18.0 mths (0.0 to 46.0) Femoral components (n = 15) 1.3 Acetabular components (n = 5) 1.6

Kwon et al18 8 pseudotumours 22 other causes

Hip resurfacings Pseudotumour group3.6 years (1.1 to 6.6)Non-pseudotumour group2.3 years (1.0 to 5.8)

Femoral components 8.1 for pseudotumours and 1.8 for other causes

Witzleb et al38 10 BHR 13.0 mths Volumetric wear rates up to 27.0 mm3/year

Tuke et al19 5 2 McKee-Farrar; 2 Ring, 1 Müller

18.4 yrs (Femoral and acetabular components) 2.2 (0.8 to 14.0)

Morlock et al27 32 Hip resurfacings 99.0 to 1200.0 days Volumetric 1.1 to 26.0 mm3/yearRieker et al20 337 Metasul 38 mths (1.0 to 12.0 yrs) 6.2 (no range reported)Reinisch et al21 22 28 mm diameter plus

Endiprothetik AG32 mths (1.0 to 6.0 yrs) 7.6 (2.9 to 12.8)

McKellop et al22 11 acetabular components McKee-Farrar 16.0 yrs 2.1 (0 to 195)Schmidt et al23 13 acetabular components McKee-Farrar 16.3 yrs 4.9 (0.1 to 300)Mean 5.0

* THR, total hip replacement; BHR, Birmingham hip resurfacing

Table II. Details of the results. Correlation coefficients were calculated for each variable against the wear rate

Median 25th centile 75th centile Pearson correlation

Acetabular inclination (°) 50.1 43.3 58.6 0.55Acetabular version (°) 24.5 10.0 36.0 0.17Femoral version (°) 17.0 7.5 21.0 -0.13Combined version (°) 41.5 25.3 48.5 0.18Maximum acetabular wear depth (μm) 11.8 1.9 27.2 0.92Duration in situ (mths) 32.5 22.0 45.0 0.04Acetabular rate of wear (μm/yr) 5.6 3.2 27.2 1.00Horizontal femoral offset (mm) 41.5 38.0 46.0 0.01Femoral rate of wear (μm/yr) 8.7 0.0 8.4 0.7

318 A. J. HART, K. ILO, R. UNDERWOOD, P. CANN, J. HENCKEL, A. LEWIS, J. COBB, J. SKINNER

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ResultsThe measured parameters are summarised in Table II. Therange of version of the acetabular component was greaterthan that of either its inclination or that of femoral ver-sion. The rate of wear of the acetabular component wasvery strongly positively correlated with its maximumacetabular wear depth (microns). Therefore all furtheranalysis used the wear rate of the acetabular componentfor comparison, since this is the most frequently quotedparameter in this field.

The angle of inclination of the acetabular componentwas strongly positively correlated with the rate of wear(Fig. 3). Version of the acetabular component and com-bined version were weakly positively correlated with therate of wear of the acetabular component (Figs 4 and 5).Femoral version was weakly negatively correlated with therate of wear of the acetabular component.

The analysis comparing low and high wearing groups isshown in Table III. Inclination of the acetabular componentwas the only parameter studied which showed statistical sig-nificance (Mann-Whitney U test, p = 0.004). This is in agree-ment with the results of our correlation coefficient data.Plotting inclination, version and rate of wear by category(low wear ≤ 5 μm/year, high wear > 5 μm/year) showed thatthe proportion of low wearing hips inside Lewinnek’s safezone3 was double that of high wearing hips: seven of 19(36.8%) and five of 26 (19.2%), respectively (Fig. 6).

Horizontal femoral offset was not correlated with therate of wear of the acetabular component. Furthermore,there was no correlation between the difference in horizon-tal femoral offset, that is the difference between the oper-ated and the contralateral hip and the rate of wear of theacetabular component. The horizontal femoral offset wasdefined as the shortest distance between the centre of rota-

300

20

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100

Wea

r ra

te (

µm

/yr)

120

140

160

180

35 40 45 50

Inclination (º)

55 60 65 70

y = 2.5294x - 104.09R2 = 0.43969

75

Fig. 3

Scatter plot of the rate of wear of the acetabular component versus itsangle of inclination as measured by CT.

Version (°)

y = 0.2808 + 17.113R2 = 0.02844

180

160

140

120100

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60

4020

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-60 -40 -20 0 20 40 60

Wea

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te (

µm

/yr)

Fig. 4

Scatter plot of the rate of wear of the acetabular component versus itsangle of version as measured by CT.

Combined version (°)

y = 0.3111x + 13.765R2 = 0.03051

180

160

140

120

100

8060

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-40 -20 00

20 40 60 80

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µm

/yr)

Fig. 5

Scatter plot of the combined acetabular and femoral version versus therate of wear of the acetabular component as measured by CT.

Table III. Comparison between the groups with low and high wear ratesfor the most relevant CT measured variables. The Mann-Whitney U testwas used to assess statistical significance

Median 25th centile 75th centile p-value

Acetabular version (°)Low wear group 25.0 15.0 36.0 0.70High wear group 22.5 6.2 36.0

Combined version (°)Low wear group 42.0 29.3 47.8 0.79High wear group 37.0 20.8 51.8

Acetabular inclination (°)Low wear group 49.0 41.0 53.8 0.004High wear group 55.0 48.3 60.0

Horizontal femoral offset (mm)Low wear group 44.6 39.0 46.0 0.4High wear group 40.0 38.0 43.0

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tion of the femoral head and the anatomical axis. Analysisof correlations between the rate of wear of the acetabularcomponent and either inclination, acetabular version orcombined version for the 25 BHR hips in isolation showedsimilar Pearson correlation coefficients of 0.58, 0.1 and0.18, respectively, to those obtained for all the hips in ourseries (Table II).

The rate of wear of the femoral component was stronglycorrelated with the rate of acetabular wear (Table II) andproduced the similar correlations against acetabular ver-sion and combined acetabular and femoral version of 0.21and 0.16, respectively.

DiscussionWe have analysed the effect of positioning of the compo-nents of MoM resurfacings on their performance in vivo, inparticular, on the rate of wear. This is relevant because ofthe high proportion of unexplained failures of MoM hipresurfacings in contrast to standard replacements,17 and theassociation between the rate of wear and the formation ofpseudotumour in MoM hips.18 Our study is the first toinvestigate this relationship, in any type of replacementusing CT. Importantly, our CT measurements includedacetabular and combined component version.9

Our results showed that the wear rate of failed MoMresurfacings was strongly associated with the angle of incli-nation of the acetabular component, but only weaklyassociated with version, whether isolated acetabular or com-bined component version. A positive correlation between theacetabular inclination angle and the rate of wear wasreported by Morlock et al1 using plain radiologicalmeasurement, but our use of 3D CT quantified this associa-tion more precisely. We showed a visible demarcation (Fig. 3)between low and high wear rates at 57° of acetabular incli-nation which is similar to the cut-off value of 55° proposedby De Haan et al4 when correlating blood metal ion levelswith acetabular component inclination angles.

Most of the previous MoM hip retrieval studies1,5,19-27

concern either McKee-Farrar,19,22-24,26 other older-generation MoM hips,25 and articulations of small diame-ter (< 28 mm).20,23 A few have analysed the current MoMhips of large diameter,1,5,27 but only one series of eight hipsincluded the pre-revision radiological assessment of acetab-ular version.5 It is difficult to determine a cut-off betweenthe expected and excessive wear of MoM hips in vivo. It ispossible that there is a variable response to levels of metalions, suggesting that the definition of excessive wear maydepend upon the individual patient. Nevertheless, thisshould not detract from our investigation into the relation-ship between the position of the acetabular component andthe rate of wear.

Other data are available from two reports of retrievedceramic-on-ceramic (CoC) hips,6,7 although we recognise thatthis type of bearing surface behaves differently from the MoMhip. One showed no association between the rate of wear andeither acetabular inclination or version, and the other showedno association with acetabular inclination and a negative cor-relation with version. There was a statistically significantincrease in the rate of wear for components with an acetabularversion < 15°.7 The lack of an association between the angle ofinclination and the rate of wear in CoC hips may be becausethe rate of wear is so low that it is close to the detection limitbetween form error and actual wear. Alternatively, CoC hipsmay be more tolerant to edge loading compared with MoMhips, as shown by hip simulator studies.28,29

An important strength of our study was the use of 3D CTto measure both components. It is known that 3D CT is moreprecise and accurate than plain radiography and that this ismore important for acetabular version than inclination.30,31

The difficulty in obtaining valid radiological measurements ofversion is a possible reason for the limited number of reportsinvestigating version as a factor. The effect of acetabular ver-sion should ideally be considered in combination with fem-oral version to give the combined version.9 This requiresmeasurement of the axis of femoral rotation and thereforeinformation about the knee by CT as in our protocol.

There have been only two studies which have reportedon the relationship between version and levels of bloodmetal ions.5,32 The first of these found a positive correlationfor components with a bearing surface diameter of < 51 mmand a negative correlation for those > 51 mm but com-mented that the lowest levels of metal ions were found withversion between 10° and 20°. The second found a positivecorrelation with version for components with a bearing sur-face diameter of < 51 mm and that the lowest levels of metalions were found with version of between 10° and 20°.

Our study has several limitations. First, as with allretrieval studies, it was only possible to measure total wearand therefore did not allow separation of the bedding-inperiod, considered to occur within the first year of use, fromsteady-state wear.33 However, only four components failedwithin one year, all after nine months. Furthermore, therewas a very strong positive correlation between total wear

Inclination angle (º)

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Lewinnek’s box

Fig. 6

Scatter plot of the version of the acetabular component versus its angleof inclination (converted into anatomical values). The rate of wear isquantified and Lewinnek’s radiological safe zone is plotted. There was agreater proportion of hips with a low rate of wear inside the safe zonecompared with those with a high rate of wear

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and the rate of wear. It must be acknowledged that there arelimited data on the situation in vivo so that the quantifica-tion of the separation of bedding-in and steady-state wearcan only be identified from hip simulator investigations.

Secondly, when measuring the position of the acetabularcomponents in the pelvis we have assumed that the anteriorpelvic plane is vertical.15 However, the pelvis tilts during sit-ting, walking and lying supine by a mean of 49° (-22° to27°).34-36

Thirdly, not all the implants studied were produced bythe same manufacturer, which influences the effective angleof version because of variation in the articular arc angle ofthe acetabular component.37 However, with the exceptionof the eight ASR resurfacings, the remaining 37 hips had amean articular arc angle of 161 ± 4°.37 The effect on versionwill be half the difference in the articular arc angle. How-ever, this will be much less than that introduced by theuncertainty on plain radiological analysis of version com-pared with CT. Further reassurance of our results may begained from our analysis of BHRs in isolation. The rela-tionship between version and the rate of wear was very sim-ilar to that of the whole group. Lastly, it is possible that ourstudy was underpowered. However, it is the largest study ofits kind with the inclusion of 45 hips (90 components), andthe first to correlate the rate of wear of failed MoM hipresurfacings with pre-revision CT measurements and alsothe first to study the relationship between acetabular ver-sion and the rate of wear in a large series of hips.

The authors are extremely grateful for the help provided by G. Lloyd (ImperialCollege) and P. Coward (Royal National Orthopaedic Hospital).

We wish to acknowledge the help of M. Borroff from the Association of BritishHealthcare Industries and S. Muirhead-Allwood and M. Porter for steering com-mittee advice. We also wish to acknowledge all the patients who contributedtheir data and hip implants, and the surgeons who sent them to us.

This work was funded by the British Orthopaedic Association (BOA) throughan industry consortium of nine manufacturers: DePuy, Zimmer, Smith &Nephew, Biomet, JRI, Finsbury, Corin, Mathys and Stryker. The contract allowsfor freedom to publish all results.

No benefits in any form have been received or will be received from a com-mercial party related directly or indirectly to the subject of this article.

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