The Journal of Arthroplasty Vol. 23 No. 8 2008

Femoral Neck Narrowing After Metal-on-MetalHip Resurfacing

Simon Spencer, MBChB, MRCS (Glasg), Robert Carter, MBChB, FRCS (Tr & Orth),Helen Murray, RGN, and R.M. Dominic Meek, MD, FRCS (Tr & Orth)

Abstract: We reviewed 40 Corin Cormet 2000 (Corin, Cirencester, UK) metal-on-metal resurfacing hips, in 36 patients, for the presence of femoral neck narrowing.A neck-to-prosthesis ratio was calculated by dividing the diameter of the femoralneck with that of the implant. This ratio was measured on plain anteroposteriorpelvis radiographs taken immediately and 2 years postoperation. Subsequentradiographs were measured up to a maximum 7 years (mean, 5.3 years)postoperation. Femoral neck narrowing was observed in 90% of hips at 2 years,with the average neck narrowing ranging from a ratio of 0.865 to 0.811. Importantly,no further narrowing occurred beyond this point up to 7 years postoperation. Wedescribed a simple reproducible method of measuring neck narrowing on plainradiographs and discuss possible causal factors for neck narrowing after hipresurfacing. Key words: hip resurfacing, femoral neck narrowing, stress shielding,metal on metal, Corin Cormet 2000.© 2008 Elsevier Inc. All rights reserved.

Hip resurfacing is becoming a popular alternative tototal hip arthroplasty in the younger patient.Fracture of the neck of the femur is a recognizedcomplication of hip resurfacing, with an incidence of1.46% in a recent large study [1]. Known intrao-perative risk factors are neck notching and signifi-cant varus placement of the prosthesis [1]. Thepresence of femoral neck narrowing below theprosthesis has been noted on plain radiographswith different resurfacing implant designs [2,3]. Atpresent, there is no consistent evidence correlatingneck narrowing with fracture. Although the sig-

From the Department of Orthopaedic Surgery, Southern GeneralHospital, Glasgow, United Kingdom.

Submitted January 27, 2007; accepted October 14, 2007.No benefits or funds were received in support of this study.Reprint requests: Simon Spencer, MBChB, MRCS, Specialist

Registrar in Orthopaedic Surgery, Department of Orthopaedicsand Trauma Surgery, Southern General Hospital, 1345 GovanRoad, G51 4TF Glasgow, UK.

© 2008 Elsevier Inc. All rights reserved.0883-5403/08/2308-0002$34.00/0doi:10.1016/j.arth.2007.10.014

1105

nificance of neck narrowing is not yet known, ifexcessive narrowing were to occur, this couldincrease the risk of neck fracture, particularly inpatients with osteopenic bone. As far as we areaware, only a few studies have reported earlypostoperative narrowing but none with long-termfollow-up of contemporary resurfacing arthroplasty[2,3]. The aim of this study was to assess whetherfemoral neck narrowing occurs in one particulartype of hip resurfacing postoperatively and quantifythe degree of neck narrowing.

Patients and Methods

From August 1998 to September 2001, 48consecutive surface replacements arthroplasties(44 patients) were carried out by a single surgeon,using the Cormet 2000 metal-on-metal prosthesis(Corin, Cirencester, UK). The posterior approachwas performed for all operations. All componentswere fixed using uncemented acetabular andcemented femoral components. One patient died7 months postoperatively from unrelated causes and

Fig. 1. Calculation of neck-prosthesis ratio.

1106 The Journal of Arthroplasty Vol. 23 No. 8 December 2008

1 patient sustained a high-energy traumatic femoralneck fracture at 4 months. Care was taken using setpositions to standardize lower limb rotation whileobtaining radiographs by placing the limb in 10° ofinternal rotation and if necessary stabilizing thepatient with foam blocks. The radiographs wereassessed to ensure they were of adequate technicalquality, and only good-quality anteroposterior (AP)radiographs were accepted. Symmetry of the tro-chanters was then evaluated qualitatively on eachindividual pelvic film and between subsequentfollow-up films to ensure identical femoral neckversion. Patients with either poor-quality or missingradiographs from the immediate postoperative or2 year follow-up stage could not be used in thestudy; therefore, 6 patients were excluded. Of note,the 6 excluded patients were followed up clinicallyand encountered no known complications. Therewere 36 patients (40 hips) for follow-up: 15 werefemale and 21 were male; average age at operationwas 49.9 years. Diagnosis at operation was 35osteoarthritis, 3 avascular necrosis, 1 septic arthritis,and 1 ankylosing spondylitis. Two hips were revised,1 at 2.5 years and 1 at 3.5 years, for acetabularloosening with the femoral stem found to be wellfixed on both occasions.All radiographs were measured by 2 independent

observers, both orthopedic surgeons, using thetechnique outlined below. The neck-prosthesisratio on AP pelvis radiographs was measured inmillimeters as illustrated in (Fig. 1). Diameter A isthe diameter of the femoral neck exactly at theprosthesis. Diameter B is the diameter of the implantexactly at the level of its opening edge. The neck-to-prosthesis ratio was calculated by dividing A by B.Diameter A was found to represent the narrowestpoint of the neck in all cases.All radiographs were measured, and the bone-to-

implant ratio was calculated. The benefit of using aratio over a simple measurement in millimeters isthat it allows direct comparison between subsequentradiographs to be made without having to standar-dize film magnification. Neck narrowing was indi-cated by the presence of a reduced ratio betweensubsequent radiographs as illustrated in (Fig. 2),with the ratio dropping from 0.88 postoperation to0.77 at 2 years in this case.Statistical analyses were performed using SPSS

12.0 (SPSS for Windows, Rel. 12.0.0, 2003; SPSSInc, Chicago, Ill). Analysis of the reliability of this asa method of measuring neck ratios was carried out.For each comparison, both scatterplot and correla-tion analyses were performed. The objective Pear-son's correlation coefficient was 0.953 (P b .001) forintraobserver error and 0.985 (P b .001) for

interobserver error therefore demonstrating highlystatistically significant degrees of correlationbetween observations thus validating its use. Com-parison of mean ratios at the different postoperativeintervals was performed using the 1-way analysisof variance.

Results

A degree of femoral neck narrowing was observedin 90% of prosthesis at 2 years postoperation, withthe average neck narrowing from a ratio of 0.86(range, 0.96-0.75) to 0.81 (range, 0.93-0.68). Six(15%) of the 40 had narrowing of greater than 10%at 2 years. Average neck ratios were calculated up to7 years postresurfacing (Table 1) demonstrating nosignificant further neck narrowing beyond the2-year level (Fig. 3). Neck-to-prosthesis ratios at2 years were significantly reduced as comparedto neck-to-prosthesis ratios at immediate postopera-tive (F = 11.781; df = 1; P = .001). No significantdifference was found to exist between ratios atfollow-up beyond 2 years (F = 0.473; df = 5; P =.795). With a standard radiographic magnificationfactor of 115%, this would represent in real terms amean neck narrowing of 2.1 mm at 2 years (mean,39.0-36.9 mm). A range of neck ratios wereobserved throughout follow-up at any given time,from the widest 0.96 in one patient immediatelypostoperative to the narrowest 0.68 in a differentpatient at 5 years. An example of above-average

Fig. 2. Illustration of neck narrowing with reduction in ratio from 0.88 to 0.77 at 2 years.

Femoral Neck Narrowing Following Hip Resurfacing � Spencer et al 1107

narrowing is illustrated in Fig. 4. Despite this range,there appeared to be no correlation between degreeof narrowing observed at 2 years and stabilization ofnarrowing after this.

Discussion

This study demonstrates that significant femoralneck narrowing occurs below the Corin Cormet2000 hip resurfacing prosthesis in the first 2 yearsafter surgery, with a mean bone-to-prosthesis ratiodrop of 0.865 to 0.811. Importantly, narrowingstabilizes with no significant progression observed toa maximum of 7 years.Other studies have commented on the presence of

femoral neck narrowing during early follow-up ofresurfacing systems. To date, no articles havemeasured the progress of the neck resorptionbeyond this early follow-up. McMinn [2] describedthe presence of neck thinning measured on APradiographs of 235 McMinn hips, with a mixedgroup of cemented and uncemented femoral com-ponents. Forty percent had less than 1-mm thin-ning, 16% had 1 to 2 mm, and 44% had greater

Table 1. Mean Neck-Prosthesis

Time Postoperation Immediate 2 y 3 y

n 40 35 26Mean neck-prosthesisratio (SD)

0.865 (0.053) 0.811 (0.059) 0.812 (0.052)

than 2 mm at 3 years with an overall mean thinningof 1.47 mm [2]. Our results gave a mean necknarrowing of 2.1 mm at 2 years. Unfortunately, wefeel there is insufficient detail in the methodologydescribing how and where in the neck narrowingwas calculated in theMcMinn study to allow reliabledirect comparisons to be made. Lilikakis [3] calcu-lated a prosthesis-to–femoral neck ratio on plainradiographs postoperatively and at mean 2.4 yearsfollow-up with the Corin Cormet 2000 prosthesisused with a hydroxyapatite-coated femoral compo-nent. An inverse ratio was used in this study (ie,implant diameter divided by neck diameter), andtherefore, the calculated ratio increased with time asthe neck narrowed. They observed a neck narrow-ing ratio drop from 1.05 (range, 1.0-1.3) post-operation to 1.1 (range, 1.0-1.4) at 2.4 years, with17 (27%) of 63 having greater than 10% necknarrowing [3]. Of interest, our results showed only 6(15%) of 40 cases having greater than 10% necknarrowing. This difference could be associated withthe type of femoral stem fixation used. Uncementedimplants are known to have inconsistent ingrowth,and spot-weld fixation may not transmit forces tothe femoral neck as well as cemented implants, with

Ratio After Hip Resurfacing

4 y 5 y 6 y 7 y

21 30 16 40.809 (0.053) 0.797 (0.054) 0.803 (0.049) 0.813 (0.048)

ig. 4. Illustration of greater than average neck narrowingatio, 0.68 at 5 years postoperation.

Fig. 3.Neck-to-prosthesis ratio in patients at 0, 2, 3, 4, 5, 6,and 7 years postoperatively. Bars represent 95% con-fidence intervals of the mean, and circles represent means.

1108 The Journal of Arthroplasty Vol. 23 No. 8 December 2008

resulting increased stress shielding and narrowingof the neck.One criticism of our study is that neck narrowing

in the sagittal plane has not beenmeasured. It wouldbe difficult to standardize a lateral hip radiograph toallow accurate and reproducible measurements ofneck narrowing, and therefore, only the AP wasmeasured in our study. Although not formallymeasured, we did not observe any cases of excessneck narrowing on the lateral views.The authors accept that, by using more invasive

methods such as roentgen stereophotogrammetricanalysis (RSA), rotation of the hip between sub-sequent films may be assessed with a more certaindegree of accuracy. However, this study aimed touse and to validate a simple and reproduciblemethod of measuring neck narrowing on plainradiographs. By aiming to control the position of thefemur and assessing symmetry of the trochantersbetween subsequent radiographs, it was felt thatgood-quality AP radiographs could be achieved.Only good-quality AP films were accepted and used.The previous studies that commented on necknarrowing also performed measurements on plainAP radiographs [2,3]. We have shown that thismethod of measuring is statistically reliable, and ourresults showed stabilization of narrowing over timewould lead us to believe that accurate results can beachieved with this method.At present, the cause of neck narrowing after hip

resurfacing is not known. Possible contributingfactors may be (i) stress shielding, (ii) bone necrosisas a result of femoral preparation and cementing atsurgery, (iii) wear debris, or (iv) fluid pressureaffecting cancellous neck bone.

Hip resurfacing aims to provide a more physiolo-gic loading of the proximal femur, avoiding stressshielding and associated bone resorption. It isunclear whether stress shielding occurs in thefemoral neck below a resurfacing prosthesis inactual clinical practice. Current literature givesconflicting results. Previous finite element analysishas indicated stress concentration at the peg andstress shielding at the anterosuperior femoral neck[4] or the stem unloading the inferomedial cortex[5], both of which may cause bone resorption in thefemoral neck below the prosthesis. However, recentdual energy x-ray absorptiometry (DEXA) studies ofthe proximal femur after hip resurfacing indicatethat femoral bone stock is preserved with noevidence of stress shielding [6,7].

It has been suggested that femoral head bloodsupply is compromised during hip resurfacing bothmeasured intraoperatively [8] or observed by thepresence of osteonecrosis in retrieved femoral headsobtained at revision [9]. In contrast to this, otherstudies showed that the retrieved heads maintainedgood blood supply [10,11]. It is not known whethercementing of the femoral component has anyimpact on the degree of neck narrowing that occurs.McMinn et al [2] noted no difference in necknarrowing when comparing cemented and unce-mented femoral components.

Although wear debris–induced bone loss was aproblem with early designs of hip resurfacing, it isnow significantly reduced with newer metal-on-metal implants [12]. Wear debris may however still

Fr

Femoral Neck Narrowing Following Hip Resurfacing � Spencer et al 1109

have some impact on neck narrowing, particularlywhen combined with increased fluid pressure.Fluid pressure has been shown to cause osteolysis

via different mechanisms. One theory is that highfluid pressure interferes with the oxygenation ofbone resulting in bone resorption [13]. Other studiesdemonstrate that cyclic high hydrostatic pressure,particularly in the presence of wear particles,increases tumor necrosis factor α production byprimary human macrophages and thereforeincrease the macrophage osteolytic potential [14].There is no consistent evidence in the literature

reporting contemporary resurfacing implants thatneck narrowing leads to fracture. Although areview of failure mechanisms in 61 Indianaconservative hip prostheses, implanted between1977 and 1981, showed all 8 femoral failuresoccurring beyond 10 years with some degree ofnarrowing (4 loosening and 4 neck fractures) [15],this is not an evidence that neck narrowing leads tofracture. In fact, perhaps we should expect to seesome narrowing in almost all implants, with 90%of implants in our study showing some degree ofneck narrowing.The early neck narrowing observed may be

because of one, or a combination, of the abovecausal factors. However, this study shows thatnarrowing occurs early and is not usually progres-sive for 7 years. This pattern of response maytherefore be more in keeping with a process of boneadaption possibly as a result of initial stress shieldingof the femoral neck below the implant.

References

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2. McMinn D, Treacy R, Lin K, et al. Metal on metalsurface replacement of the hip: experience of theMcMinn prosthesis. Clin Orthop Relat Res 1996;329S:S89.

3. Lilikakis A, Vowler S, Villar R. Hydroxyapatite-coatedfemoral implant in metal-on-metal resurfacing hiparthroplasty: minimum of two years follow-up.Orthop Clin North Am 2005;36:215.

4. Watanabe Y, Shiba N, Matsuo S, et al. Biomechanicalstudy of the resurfacing hip arthroplasty, finite elementanalysis of the femoral component. J Arthroplasty2000;15:505.

5. De Waal M, Malefijt C, Huiskes R. A clinical,radiological and biomechanical study of the TARAhip prosthesis. Arch Orthop Trauma Surg 1993;112:220.

6. Harty J, Devitt B, Harty L, et al. Dual energy x-rayabsorptiometry analysis of peri-prosthetic stressshielding in the Birmingham resurfacing hip replace-ment. Arch Orthop Trauma Surg 2005:1.

7. Kishida Y, Sugano N, Nishii T, et al. Preservation ofthe bone mineral density of the femur after surfacereplacement of the hip. J Bone Joint Surg Br 2004;86:185.

8. Steffen RT, Smith SR, Urban JP, et al. The effect of hipresurfacing on oxygen concentration in the femoralhead. J Bone Joint Surg Br 2005;87:1468.

9. Little CP, Ruiz AL, Harding IJ, et al. Osteonecrosis inretrieved femoral heads after failed resurfacingarthroplasty of the hip. J Bone Joint Surg Br 2005;87:320.

10. Campbell P, Mirra J, Amstutz HC. Viability offemoral heads treated with resurfacing arthroplasty.J Arthroplasty 2000;15:120.

11. Howie DW, Cornish BL, Vernon-Roberts B. Theviability of femoral heads after resurfacing hiparthroplasty in humans. Clin Orthop Relat Res 1993;291:171.

12. Grigoris P, Roberts P, Panousis K, et al. The evolutionof hip resurfacing arthroplasty. Orthop Clin North Am2005;36:125.

13. Schmalzried T, Callaghan J. Wear in total hip andknee replacements. J Bone Joint Surg Am 1999;81-A:115.

14. Ingham E, Fisher J. The role of macrophages inosteolysis of total joint replacement. Biomaterials2005;26:1271.

15. Ritter MA, Lutgring JD, Berend ME, et al. Failuremechanisms of total hip resurfacing: implication forthe present. Clin Orthop Relat Res 2006;453:110.