Hip

The femoral head/neck offset and hip resurfacing

P. E. Beaulé, N. Harvey, E. Zaragoza, M. J. Le Duff, F. J. Dorey From the University of Ottawa, Ottawa, Canada


P. E. Beaulé, MD, FRCSC, Associate Professor Department of Orthopaedic Surgery University of Ottawa, The Ottawa Hospital, 501 Smyth Road, Suite 5004, Ottawa K1H 8L6, Ontario, Canada.
N. Harvey, MD, Orthopedic Resident
M. J. Le Duff, MA, Clinical Research Co-ordinator Joint Replacement Institute University of Southern California, 2400 South Flower Street, Los Angeles, California 90007, USA.
E. Zaragoza, MD, Associate Clinical Professor Department of Radiology Santa-Monica UCLA Medical Center, 1245 16th Street, Santa Monica, California 90404, USA.
F. J. Dorey, PhD, Professor and Statistitician Department of Pediatrics Norris University of Southern California Cancer Center, 657OPT Children’s Hospital, 4650 Sunset Boulevard, Los Angeles, California 90027, USA. Correspondence should be sent to Dr P. E. Beaulé; e-mail: [email protected] ©2007 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.89B1. 18011 $2.00 J Bone Joint Surg [Br] 2007;89-B:9-15. Received 11 April 2006; Accepted after revision 9 August 2006

Because the femoral head/neck junction is preserved in hip resurfacing, patients may be at greater risk of impingement, leading to abnormal wear patterns and pain. We assessed femoral head/neck offset in 63 hips undergoing metal-on-metal hip resurfacing and in 56 hips presenting with non-arthritic pain secondary to femoroacetabular impingement. Most hips undergoing resurfacing (57%; 36) had an offset ratio ≤ 0.15 pre-operatively and required greater correction of offset at operation than the rest of the group. In the nonarthritic hips the mean offset ratio was 0.137 (0.04 to 0.23), with the offset ratio correlating negatively to an increasing α angle. An offset ratio ≤ 0.15 had a 9.5-fold increased relative risk of having an α angle ≥ 50.5˚. Most hips undergoing resurfacing have an abnormal femoral head/neck offset, which is best assessed in the sagittal plane.

The main advantages of resurfacing over a standard total hip replacement are preservation of the proximal femoral bone stock and enhanced stability owing to the larger size of the femoral head.1 With the introduction of metal-on-metal bearings, hip resurfacing has shown excellent short- and medium-term results, with patients returning to high levels of activity.2-5 However, appropriate patient selection and proper positioning of the implant have been shown to influence the outcome.4,6,7 Orientation of the femoral component in the coronal plane has been the focus of particular attention, as a valgus orientation favours survival of the implant but is associated with a decrease in femoral offset.8,9 Both Silva et al10 and Loughead et al11 have shown that femoral offset is reduced with hip resurfacing compared with a stemmed replacement. Although a decreased femoral offset with a stemmed device is associated with increased polyethylene wear12 and instability of the hip,13 the clinical implications of a reduced femoral offset in hip resurfacing have yet to be defined.14 The function of the hip is also influenced by the femoral head/neck offset ratio,15-17 which when insufficient leads to femoroacetabular impingement, causing hip pain and labral tears.15,18-22 Although an insufficient femoral head/neck offset ratio has long been recognised under various descriptions, such as tilt or a pistol grip deformity,23,24 there were no radiological criteria to define this deformity until recently. With the advent of specialised imag-

VOL. 89-B, No. 1, JANUARY 2007

ing techniques, it is apparent that it is best characterised in the sagittal plane.15,17,21 More importantly, as was postulated by both Murray23 and Stulberg et al,24 and reemphasised by Harris,25 this deformity is the underlying cause of arthritis of the hip in over 40% of cases. Therefore, because the femoral head/neck junction is preserved with hip resurfacing, patients undergoing this type of procedure might be at risk of impingement, either from malposition of the implant or from lack of correction of the underlying deformity. We have sought to determine what represents an abnormal femoral head/neck offset ratio, and hypothesise that a majority of patients undergoing hip resurfacing have an abnormal offset ratio consistent with femoroacetabular impingement.

Patients and Methods The femoral head/neck offset ratio of a consecutive series of patients undergoing hip resurfacing was measured and compared with that of a group of patients with non-arthritic hip pain secondary to femoroacetabular impingement. A total of 65 hips (60 patients) were treated for osteoarthritis with a hybrid metal-on-metal hip resurfacing procedure (Conserve Plus, Wright Medical Technology, Arlington, Tennessee) as part of an investigational device exemption (IDE) clinical trial approved by our Institutional Review Board. The aetiology was idiopathic osteoarthritis in 48 hips (74%), 9

10

P. E. BEAULÉ, N. HARVEY, E. ZARAGOZA, M. J. LE DUFF, F. J. DOREY

Fig. 1 a) A femoral head spherometer is used to re-establish sphericity of the head. b) With the prominent head/neck osteophytes removed, the true axis of the neck is identified, permitting the alignment of the cylindrical reamer to place the guide wire. c) Preparation of the head is completed, demonstrating the restored femoral head/neck offset.

osteonecrosis in six (9.2%), post-traumatic arthritis in four (6.1%), dysplasia in five (7.7%), slipped capital femoral epiphysis in one (1.5%) and Perthes’ disease in one (1.5%).

Of the 60 patients, 39 were Charnley class A and 21 Charnley class B.26 All the operations were performed by the senior author (PEB), using an approach for surgical dislocation originally described by Ganz et al27 and adapted for hip resurfacing.28 A trochanteric slide osteotomy was used, leaving the short external rotators intact and the hip dislocated anteriorly after a Z-shaped capsulotomy. Prior to placing the guide wire into the femoral neck and preparing the femoral head, prominent anterolateral osteophytes were removed using head spherometer gauges (Wright Medical Technology) (Fig. 1a) to optimise the head/neck offset. Care was taken not to damage the retinacular vessels, which are located mainly superolaterally.29 With the neck axis defined, a cylindrical reamer matching the size of the femoral head was used as a guide to introduce the Kwire (Fig. 1b). Further optimisation of the offset could be obtained by translating the femoral component anteriorly. Once the components had been placed, with proper restoration of the femoral head/neck offset (Fig. 1c), the trochanteric fragment was fixed with two or three 4.5 mm cortical screws. The comparison group consisted of 51 consecutive patients (56 hips) with hip pain, who had a positive labral tear on magnetic resonance imaging with gadolinium arthrography. The mean duration of their symptoms was 14.5 months (3 to 36). All had signs and symptoms consistent with impingement, with a positive impingement sign as well as pain in the hip on prolonged sitting. Hips with evidence of acetabular dysplasia with a centre edge angle < 25˚ and/or arthritis of a Tonnis grade30 >1 were excluded. Institutional Review Board approval was obtained for this study. Impingement secondary to an insufficient femoral head/neck offset ratio was diagnosed by three-dimensional computed tomography (CT), as reported by Beaulé et al.17 With the contour of the femoral head overlaid with a perfect circle centred over the head, the α angle21 is measured by the angle from the centre-line of the femoral neck to the centre-point of the femoral head, and the anterior edge of the femoral neck where this crosses the radius of the femoral head. A larger angle corresponds to diminished concavity or a lack of offset at that head/neck junction. All measurements were performed by the same observer (EZ). All patients had a standard anteroposterior (AP) pelvic radiograph as well as a cross-table lateral view. In the patients who underwent hip resurfacing, a radiograph was obtained after the operation. Two hips in the resurfacing group had inadequate radiographs which did not allow measurements, leaving 63 hips (58 patients) available for study. One observer (NH) retrospectively reviewed the radiographs for all 119 hips. All were taken using either traditional techniques or digital imaging printed at 100% magnification. On the AP pelvic radiographs of the non-arthritic hips the shape of the femoral head was subdivided into two categories. Those with an aspherical and/or a pistol-grip appearance24 had a femoral head with flattening on the THE JOURNAL OF BONE AND JOINT SURGERY

THE FEMORAL HEAD/NECK OFFSET AND HIP RESURFACING

11

Fig. 2 Anteroposterior radiograph of the pelvis of a 45-year-old man with pain in the left hip associated with a pistol-grip deformity. The insets show the associated anterior labral tear as well as the cross-table lateral radiograph with an offset ratio of 0.15. The α angle was measured at 90.0˚.

normally concave surface of the lateral femoral neck, or a head with lack of sphericity (Fig. 2). Spherical heads had a concave lateral surface with a round appearance (Fig. 3). In the resurfacing group, the femoral stem-shaft angle and the angle of abduction of the acetabular component were measured on the post-operative radiograph.7 On the cross-table lateral radiograph, the femoral head/ neck offset was measured by the offset ratio.16 A line bisecting the longitudinal axis of the femoral neck, not necessarily through the centre of the head, was drawn, followed by a second line parallel to this and a tangent to the anterior part of the neck, and finally a third line parallel to the first two and tangential to the anterior aspect of the head. The offset ratio was then calculated by dividing the distance between the second two lines by the diameter of the head. In the resurfacing group, the offset ratio was also measured after operation (Fig. 4), as was the version of the femoral component defined as the angle between the stem and the femoral shaft. All measurements were made by one observer (NH). A parametric Student’s t-test was used to compare the individual measurements on both the plain radiographs and the three-dimensional CT scans. The standard deviations (SD) were also calculated. The association between variables was tested using Pearson’s correlation coefficient after a normal distribution was confirmed. Cox regression and multivariate linear regression analysis was performed VOL. 89-B, No. 1, JANUARY 2007

to determine any association between offset ratio, the appearance of the head, the α angle, the diagnosis and patient-related factors. A two-tailed p-value < 0.05 was considered to be statistically significant.

Results The patients undergoing hip resurfacing and the comparison group were similar in age and gender, with a mean age of 45.7 years (22 to 60) and 43 years (22 to 55), respectively (p = 0.3). Of the resurfacing group, 42 of 58 were male and in the control group 31 of 51 were male. The mean pre-operative offset ratio for the resurfacing group was 0.15 (0.07 to 0.30) and not significantly different between the different diagnostic groups (p = 0.11) (Table I). The mean offset ratio for the resurfacing group was not significantly greater than in the comparison group: 0.150 (0.07 to 0.30) vs 0.137 (0.04 to 0.23) (p = 0.326). Of the resurfaced hips 36 of 63 (57%) had an offset ratio ≤ 0.15. The mean post-operative offset ratio of 0.20 (0.13 to 0.34) was significantly greater than in both the preoperative and the comparison groups (p < 0.0001). The mean correction of the offset ratio was significantly greater for the hips that presented with an offset ratio ≤ 0.15 preoperatively (36 hips) compared with the remaining group (27 hips), 0.066 (0.07 to 0.14) vs 0.027 (0.15 to 0.30), respectively (p = 0.003). The mean size of the femoral component was 47.6 mm (40 to 54). The mean angle of

12

P. E. BEAULÉ, N. HARVEY, E. ZARAGOZA, M. J. LE DUFF, F. J. DOREY

Fig. 3 Anteroposterior radiograph of the pelvis of a 30-year-old man with pain in the left hip associated with a spherical femoral head. The inset is of the cross-table lateral radiograph with an offset ratio measured at 0.08. The α angle was measured at 100˚.

Table I. Offset ratio before and after operation subdivided by the aetiology Average offset ratio Aetiology (n)

Pre-operative (range)

Post-operative (range)

Dysplasia (5) Osteoarthritis (47) Osteonecrosis (5) Post-traumatic arthritis (4)

0.17 0.14 0.14 0.16

0.19 0.19 0.19 0.22

(0.14 to 0.21) (0.07 to 0.30) (0.10 to 0.19) (0.13 to 0.20)

(0.17 to 0.21) (0.13 to 0.31) (0.14 to 0.25) (0.16 to 0.34)

abduction of the acetabular component was 44.4˚ (35˚ to 60˚). The mean femoral stem-shaft angle on the AP and cross-table lateral views was 139.8˚ (125˚ to 150˚) and 21.0˚ (10˚ to 35˚), respectively. There were no correlations between correction of offset and the size of the femoral (r = -0.012) or the stem-shaft angle on the cross-table lateral view (r = -0.127).

In the comparison group, the mean α angle as measured by three-dimensional CT was 69.2˚ (41.5˚ to 100˚). A total of 11 hips had an α angle < 50.5˚. Of the 56 hips, 30 had an aspherical and/or pistol-grip appearance, the remainder being spherical. The former had a significantly larger mean α angle than the spherical (75.6˚ (42.1˚ to 100˚) vs 61.9˚ (41.8˚ to 100˚), respectively; p = 0.003). The mean femoral head/neck offset ratio was 0.137 (0.04 to 0.23, SD 0.05). The offset ratio was significantly negatively correlated with the α angle (r = -0.40, p = 0.002). The mean offset ratio was not significantly smaller for the abnormally shaped femoral heads than for the spherical (0.125 (0.04 to 0.224) vs 0.149 (0.05 to 0.229); p = 0.118). The offset ratio did not correlate with age (p = 0.32) or gender (p = 0.20). To determine the value of an offset ratio which was consistent with femoroacetabular impingement, we chose an α angle ≥ 50.5˚ as diagnostic.7,21 In 45 of 56 hips with an α angle ≥ 50.5˚, the mean offset ratio was significantly smaller than those with an α angle < 50.5˚ (0.13 (0.04 to 0.22) vs 0.18 THE JOURNAL OF BONE AND JOINT SURGERY

THE FEMORAL HEAD/NECK OFFSET AND HIP RESURFACING

13

Fig. 4 Cross-table lateral radiograph of a 46-year-old man before and after resurfacing of the hip. The offset ratios were 0.07 and 0.16, respectively.

(0.10 to 0.23); p = 0.03). Based on logistic regression, an offset ratio of ≤ 0.15 has a 9.5-fold increase in relative risk VOL. 89-B, No. 1, JANUARY 2007

of having an α angle ≥ 50.5˚ (p = 0.007). Consequently, the mean α angle was significantly larger with an offset ≤ 0.15

14

P. E. BEAULÉ, N. HARVEY, E. ZARAGOZA, M. J. LE DUFF, F. J. DOREY

(76.8˚ (46.5˚ to 100˚) vs 62.8˚ (41.8˚ to 94.3˚)), if the offset ratio is ≥ 0.15 (p = 0.0004). Multivariate regression analysis showed that lack of sphericity of the head on an AP view (p = 0.01) and a decreasing offset ratio (p = 0.05) were significantly correlated to an α angle ≥ 50.5˚.

Discussion Hip resurfacing presents a number of technical challenges in terms of the position and size of the femoral implant.6 Although significant emphasis has been placed on the orientation of the femoral component in the coronal plane and its impact on biomechanical reconstruction,8,10,11 little is known about its placement in the sagittal plane and how this relates to the femoral head/neck offset.28,31,32 However, we do know that with stemmed hip replacements an appropriate head/neck offset is important to minimise the risk of impingement,33,34 as this relates to the functional range of movement,35,36 the stability of the hip37 and wear of the bearing surface.38,39 In our study, 57% of hips treated by resurfacing had an abnormal head/neck offset ratio preoperatively. Correction of this deformity becomes important, as the experience with metal-on-metal total hip replacements showed early failures due to poor design of the femoral stem, with an inappropriate head/neck offset leading to component loosening and abnormal patterns of wear.38,40-42 A recent paper by Howie et al38 noted that a third of retrieved McKee-Farrar prostheses had evidence of impingement between the acetabular and the femoral components. This was associated with evidence of fatigue wear in the retrieved components, leading to increased production of cobalt-chrome particles and fragments, increasing the potential for third-body wear. This is in contrast to the Ring prosthesis, which had a better stem design.43 Although there is little information on retrieved metal-onmetal hip resurfacings,44 prosthetic impingement after hip resurfacing has been reported.31,45 In 109 retrieved Wagner metal-on-polyethylene resurfacing components, Wiadrowski et al45 found evidence of eccentric wear at the rim of the acetabular component secondary to impingement of the femoral neck or residual cement at the edge of the femoral component in 84% of cases. In our comparison group, which we used to determine what offset ratio leads to impingement, the results must be viewed in the context of certain limitations, including the number of patients, one radiological observer, and the technical limitations of obtaining the correct projection of the impinging femoral head/neck area with the cross-table lateral radiograph. Despite this, and because of its simplicity in the clinical setting, the cross-table lateral radiograph should be part of the surgical planning, with an offset ratio ≤ 0.15 representing a significant risk for femoroacetabular impingement. The main limitation of this study in terms of determining the risk of impingement is that only the bony aspect was studied, whereas impingement after hip replacement is multifactorial and includes factors such as the soft tissue and the orientation of the acetabular com-

ponent.33,37 D’Lima et al33 demonstrated that there is a complex interaction between abduction of the acetabular component, its anteversion and the anteversion of the femoral component in determining the maximum range of movement available. With hip resurfacing, the lack of modularity on the femoral side requires the use of other means to optimise the femoral head/neck offset. Optimum positioning of the femoral component and correction of any underlying abnormality to maximise the femoral head/neck offset ratio will minimise the risk of impingement and maximise the functional range of movement.35,37 Because the femoral head overlaps the cortex of the neck and projects most prominently posteriorly,46 it is necessary to identify the neck axis or else there will be a tendency to place the femoral component more posteriorly on the neck. This will add to the deficient anterior offset already present, which, if left uncorrected, can result in persistent pain secondary to impingement.47 Although the larger diameter of the femoral head with hip resurfacing may provide a greater functional range of movement,48 Chandler et al35 compared the range of movement of normal and prosthetic hips and demonstrated that bony contact eventually becomes the limiting parameter, so that an increase in the diameter of the head no longer contributes to increased movement. The importance of an appropriate head/neck offset ratio has long been recognised in total hip replacement.34,35,49 We believe that this may influence the development of arthritis in the hip23-25 and its treatment by resurfacing. Recognition of the lack of femoral head/neck offset permits its correction, which will affect the functional end-point of a stable range of movement. We have shown that in osteoarthritic hips removal of the anterior osteophytes enables correction of the deficient femoral head/neck offset ratio without putting the neck at increased risk of fracture after hip resurfacing.50

Supplementary Material A further opinion by Mr Richard Field is available with the electronic version of this article on our website at www.jbjs.org.uk The authors or one or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article.

References 1. Beaulé PE. Surface arthroplasty of the hip: a review and current indications. Semin Arthroplasty 2005;16:70-6. 2. Amstutz HC, Beaulé PE, Dorey FJ, et al. Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study. J Bone Joint Surg [Am] 2004;86A:28-39. 3. Treacy R, McBryde CW, Pynsent P. Birmingham Hip Resurfacing arthroplasty: a minimum follow-up of five years. J Bone Joint Surg [Br] 2005;87-B:167-70. 4. Daniel J, Pynsent PB, McMinn DJW. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg [Br] 2004;86-B:177-84. 5. Back DL, Dalziel R, Young D, Shimmin A. Early results of primary Birmingham Hip Resurfacings: an independent prospective study of the first 230 hips. J Bone Joint Surg [Br] 2005;87-B:324-9. THE JOURNAL OF BONE AND JOINT SURGERY

THE FEMORAL HEAD/NECK OFFSET AND HIP RESURFACING

6. Beaulé PE, Antoniades J. Patient selection and surgical technique for surface arthroplasty of the hip. Orthop Clin North Am 2005;36:177-85. 7. Beaulé PE, Dorey FJ, LeDuff MJ, Gruen T, Amstutz HC. Risk factors affecting outcome of metal on metal surface arthroplasty of the hip. Clin Orthop 2004;418:87-93. 8. Beaulé PE, Lee J, LeDuff M, et al. Orientation of femoral component in surface arthroplasty of the hip: a biomechanical and clinical analysis. J Bone Joint Surg [Am] 2004;86-A:2015-21. 9. Shimmin A, Back D. Femoral neck fractures following Birmingham hip resurfacing: a national review of 50 cases. J Bone Joint Surg [Br] 2005;87-B:463-4. 10. Silva M, Lee KH, Heisel C, dela Rosa M, Schmalzried TP. Biomechanical results of total hip resurfacing arthroplasty. J Bone Joint Surg [Am] 2004;86-A:40-1. 11. Loughead JM, Chesney D, Holland JP, McCaskie AW. Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty. J Bone Joint Surg [Br] 2005;87-B:163-6. 12. Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop 2001;388:125-34. 13. McGrory BJ, Morrey BF, Cahalan TD, An K-N, Cabanela ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg [Br] 1995;77-B:865-9. 14. Girard J, Lavigne M, Vendittoli PA, Roy A. Biomechanical reconstruction of the hip: a randomised study comparing total hip resurfacing and total hip arthroplasty. J Bone Joint Surg [Br] 2006;86-B:721-6. 15. Ito K, Minka MA 2nd, Leunig S, Werlen S, Ganz R. Femoroacetabular impingement and the cam-effect: a metal-based quantitative anatomical study of the femoral head-neck offset. J Bone Joint Surg [Br] 2001;83-B:171-6. 16. Eijer H, Leunig M, Mahomed MN, Ganz R. Cross-table lateral radiographs for screening of anterior femoral head-neck offset in patients with femoro-acetabular impingement. Hip Int 2001;11:37-41. 17. Beaulé PE, Zaragoza EJ, Motamedic J, Copelan N, Dorey FJ. Three-dimensional computed tomography of the hip in the assessment of femoro-acetabular impingement. J Orthop Res 2005;23:1286-92. 18. Myers SR, Eijer H, Ganz R. Anterior femoroacetabular impingement after periacetabular osteotomy. Clin Orthop 1999;363:93-9. 19. Reynolds D, Lucas J, Klaue K. Retroversion of the acetabulum: a cause of hip pain. J Bone Joint Surg [Br] 1999;81-B:281-8. 20. Siebenrock KA, Schoeniger R, Ganz R. Anterior femoro-acetabular impingement due to acetabular retroversion: treatment with periacetabular osteotomy. J Bone Joint Surg [Am] 2003;85-A:278-86. 21. Notzli HP, Wyss TF, Stoecklin CH, et al. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg [Br] 2002;84-B:556-60. 22. Ganz R, Parvizi J, Leunig M, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop 2003;417:112-20. 23. Murray RO. The aetiology of primary osteoarthritis of the hip. Br J Radiol 1965;38:810-24. 24. Stulberg SD, Cordell LD, Harris WH, Ramsey PL, MacEwen GD. Unrecognized childhood hip disease: a major cause of idiopathic osteoarthritis of the hip. In: Amstutz HC ed. The Hip. Procs Third Open Scientific Meeting of the Hip Society. St. Louis: C. V. Mosby, 1975:212-28. 25. Harris WH. Etiology of osteoarthritis of the hip. Clin Orthop 1986;213:20-33. 26. Charnley J. Low friction arthroplasty of the hip: theory and practice. New York: Springer-Verlag, 1979. 27. Ganz R, Gill TJ, Gautier E, et al. Surgical dislocation of the adult hip: a new technique with full access to the femoral head and acetabulum without the risk of avascular necrosis. J Bone Joint Surg [Br] 2001;83-B:1119-24.

VOL. 89-B, No. 1, JANUARY 2007

15

28. Beaulé PE. A soft tissue-sparing approach to surface arthroplasty of the hip. Oper Tech Ortho 2004;14:75-84. 29. Lavigne M, Kalhor M, Beck M, Ganz R, Leunig M. Distribution of vascular foramina around the femoral head and neck junction: relevance for conservative intracapsular procedures of the hip. Orthop Clin North Am 2005;36:171-6. 30. Tonnis D, Heinecke A. Acetabular and femoral anteversion: relationship with osteoarthritis of the hip. J Bone Joint Surg [Am] 1999;81-A:1747-70. 31. Clarke IC. Biomechanics: multifactorial design choices: an essential compromise? Orthop Clin North Am 1982;13:681-707. 32. Freeman MAR, Bradley GW. ICLH double cup arthroplasty. Orthop Clin North Am 1982;13:799-811. 33. D’Lima DD, Urquhart AG, Buehler KO, Walker RH, Colwell CWJ. The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios. J Bone Joint Surg [Am] 2000;82-A:315-21. 34. Amstutz HC, Markolf KL. Design features in total hip replacement. In: Harris WH, ed. The Hip Society. St. Louis, etc 1974:111-24. 35. Chandler DR, Glousman R, Hull D, et al. Prosthetic hip range of motion and impingement: the effects of head and neck geometry. Clin Orthop 1982;166:284-91. 36. Amstutz HC, Lodwig RM, Schurman DJ, Hodgson AG. Range of motion studies for total hip replacements: a comparative study with a new experimental apparatus. Clin Orthop 1975;111:124-30. 37. Scifert CF, Brown TD, Pedersen DR, Callaghan JJ. A finite element analysis of factors influencing total hip dislocation. Clin Orthop 1998;355:152-62. 38. Howie DW, McCalden R, Nawana RW, et al. The long-term wear of retrieved McKee-Farrar metal-on-metal total hip prostheses. J Arthroplasty 2005;20:350-7. 39. Barrack RL, Schmalzried TP. Impingement and rim wear associated with early osteolysis after a total hip replacement: a case report. J Bone Joint Surg [Am] 2002;84-A:1218-20. 40. Zahiri CA, Schmalzried TP, Ebramzadeh E, et al. Lessons learned from loosening of the McKee-Farrar metal-on-metal total hip replacement. J Arthroplasty 1999;14:326-32. 41. Klapperich C, Graham J, Pruitt L, Ries MD. Failure of a metal-on-metal total hip arthroplasty from progressive osteolysis. J Arthroplasty 1999;14:877-81. 42. Howie D, Cornish B, Vernon-Roberts B. Resurfacing hip arthroplasty: classification of loosening and the role of prosthetic wear particles. Clin Orthop 1990;255:14459. 43. Ring PA. Total replacement of the hip joint: a review of a thousand operations. J Bone Joint Surg [Br] 1974;56-B:44-58. 44. Beaulé PE, LeDuff M, Campbell P, et al. Metal-on-metal surface arthroplasty with a cemented femoral component: a 7-10 year follow-up study. J Arthroplasty 2004;19(Supp 13):17-22. 45. Wiadrowski TP, McGee M, Cornish BL, Howie DW. Peripheral wear of Wagner resurfacing hip arthroplasty acetabular components. J Arthroplasty 1991;6:103-7. 46. Harty M. Surface replacement arthroplasty of the hip: anatomic considerations. Orthop Clin North Am 1982;13:667-79. 47. Schmalzried T, Silva M, de la Rosa MA, Choi E-S, Fowble VA. Optimizing patient selection and outcomes with total hip resurfacing. Clin Orthop 2005;441:2004. 48. Amstutz HC, Grigoris P, Dorey FJ. Evolution and future of surface replacement of the hip. J Orthop Sci 1998;3:169-86. 49. Bartz RL, Noble PC, Kadakia NR, Tullos HS. The effect of femoral component head size on posterior dislocation of the artificial hip joint. J Bone Joint Surg [Am] 2000;82-A:1300-7. 50. Mardones RM, Gonzalez C, Chen Q, et al. Surgical treatment of femoroacetabular impingement: evaluation of the effect of the size of the resection. J Bone Joint Surg [Am] 2005;87-A:273-9.