The original 5-mm thick acetabular component was approved by the FDA for use in hip resurfacing in 2009, and at our center we implanted the 3.5-mm shell under off-label use. None of the components was hydroxyapatite-coated.
One surgeon (HCA) completed all of the surgeries using a previously described posterior approach . A 1-mm press-fit preparation and identical insertion techniques were used for both acetabular cup designs.
Preoperative and postoperative UCLA hip scores  were recorded by the senior author (HCA) for all hips at each patient visit. Complications were noted from chart review and revisions of the resurfacing device. AP radiographs were taken at each followup. Particular attention was directed to the interface between bone and the cementless acetabular component. At the time of this writing, five patients (1.8%) in the 5-mm shell group and 16 patients (7.2%) in the 3.5-mm shell group were missing radiographs beyond the 1-year mark. These patients were not included in the subsequent comparative analysis of radiographic lucencies around the cup. Radiolucencies in the DeLee and Charnley zones  and the presence of gaps on the immediate postoperative film were recorded. All radiographs were examined from digital films with a high level of magnification. Three of the authors (MA-H, KMT, MJL) performed the initial radiographic review. Intraobserver and interobserver reliabilities were assessed using a subset of 43 patients evaluated twice by all three experimenters. The two measurements were made with a lapse of 24 hours between the two readings while blinded to the previous reading and those of the other experimenters. Kappa coefficients were computed and yielded excellent agreement for intraexperimenter reliability on gaps (kappa = 0.84), substantial agreement for intraexperimenter reliability on cup interface radiolucencies (kappa = 0.64) and interexperimenter reliability on gaps (kappa = 0.69), and moderate agreement for interexperimenter reliability on cup interface radiolucencies (kappa = 0.47). The senior author (HCA) reviewed post hoc the radiographs flagged by the three experimenters as having radiolucent lines at the bone-cup interface and minor differences were resolved by consensus. Einzel-Bild-Roentgen-Analyse software (EBRA-CUP - Release 2003; University of Innsbruck, Innsbruck, Austria) was used to calculate acetabular component abduction and anteversion angles [8, 18]. A subgroup of 121 patients, 53 with 3.5-mm shells and 68 with 5-mm shells, had previous serum metal ion studies with a minimum followup of 1 year after surgery However, we restricted this subgroup to 87 patients (46 with 5-mm shells and 41 with 3.5-mm shells) who had optimal acetabular component positioning defined as a contact patch to rim (CPR) distance of 10 mm or more  and a radiographically secure (absence of three-zone radiolucency) bone-cup interface to control for possible surgical technique bias and ensure a fair comparison of serum ion concentrations between the two designs. For these patients, the mean time of followup to the blood drawing was 78 months (range, 12-158 months).
UCLA hip scores and serum cobalt and chromium concentrations were compared using the Mann-Whitney U-test. Chi-square tests were used to compare complication rates and the prevalence of radiographic features. To focus the study on the comparative performance of the two cup designs, Kaplan-Meier survivorship estimates were computed using time to revision for aseptic acetabular component failure as the end point and the survival curves were compared between groups using the log-rank test. The Cox proportional hazard ratio was used to evaluate the predictive value of postoperative radiographic gaps on the loosening rate of the cups, adjusting for component positioning (CPR distance) and size.
Institutional review board approval was granted to perform this study.
Postoperative Functional Outcomes
UCLA hip scores were similar between patients with 5-mm and 3.5-mm shells (pain, p = 0.0976; walking, p = 0.9571; function, p = 0.9316; activity, p = 0.2085) (Table 2).
Patients with the 5-mm thick shell had a higher complication rate (p = 0.0431). Eighteen patients in the 5-mm thick shell group (6.4%) experienced complications (six dislocations, five femoral nerve palsies, eight blood-related complications; one patient had femoral nerve palsy and thrombophlebitis). Six patients (1.8%) in the 3.5-mm group experienced complications (two dislocations, one peroneal nerve palsy, three blood-related complications). All nerve palsies resolved without treatment and all blood-related complications resolved with conventional treatments. Two of the dislocations required reoperations, whereas the others stabilized after closed reduction.
Both groups had similar survivorship (Fig. 2; log-rank test, p = 0.318). There were eight revision surgeries for cup failure performed in the 5-mm shell group (three in men and five in women). The mean time to revision was 115 months (range, 56-169 months) and mean femoral component size was 44.7 mm (range, 42-48 mm). In the 3.5-mm shell group, there were two such revisions. One was performed 46 months after surgery in a man with a 52-mm femoral component and the other at 44 months in a woman with a 44-mm femoral component. The 7-year Kaplan-Meier survivorship for the 5-mm thick group was 99.6% (95% CI, 97.1%-99.9%) and for the 3.5-mm group was 98.7% (95% CI, 94.7%-99.7%). The 10-year Kaplan-Meier survivorship for the 5-mm group was 97.7% (95% CI, 0.93.8%-99.2%).
Both groups had similar rates of gap in Zone 2 (p = 0.0813). There were 34 hips (12%) showing a gap in Zone 2 on the postoperative radiograph in the 5-mm cup group while 17 hips (7%) from the 3.5-mm cup group had a similar feature (Fig. 3).
In addition, from the total 51 hips with postoperative gaps, only eight showed cup radiolucencies on the last followup films, and we found no association between the presence of a postoperative gap and the rate of revision for acetabular component loosening (p = 0.111) (Table 3).
Although there appeared to be a difference in the rates of radiolucency in at least one of the DeLee and Charnley zones at last followup (p = 0.0001), we found similar results when accounting for followup time (p = 0.1450). Among the hips reconstructed with a 5-mm cup, 37 (13%) had a radiolucency in at least one of the DeLee and Charnley zones at the time of last followup (Fig. 4) versus 10 (5%) in the group treated with the 3.5-mm shell. However, the average followup time for patients who received the 5-mm cup was 108 months compared with 54 months for the patients who received the 3.5-mm cup (p = 0.0001). Therefore, to account for the difference in followup time, we reviewed the radiographic data of the 5-mm group within 5 years of surgery, excluding radiographs taken beyond, and noted 23 cups with radiolucencies (8%). In these conditions, the lucency rate in the 3.5-mm group was similar to that of the 5-mm group.
Metal Ion Concentrations
Both groups had similar serum cobalt and chromium ion concentrations among patients with a CPR distance of 10 mm or greater (n = 46 in the 5-mm group and n = 41 in the 3.5-mm group). The median cobalt concentration was 1.1 μg/L (range, 0.3-5.6 μg/L) for the 5-mm group and 1.0 μg/L (range, 0.4-6.0 μg/L) for the 3.5-mm group (p = 0.404). The median chromium concentration was 1.4 μg/L (range, 0.1-6.3 μg/L) for the 5-mm group and 1.3 μg/L (range, 0.3-6.4 μg/L) for the 3.5-mm group (p = 0.250).
The advantage of using a thinner acetabular shell in hip resurfacing is the ability to conserve bone stock on the acetabular or the femoral side, or both . However, it is necessary to verify that mid- to long-term safety and efficacy of such devices are at least as good as those of the original components before their use can be generalized. In this study, we compared UCLA hip scores, complication rates, acetabular component survivorship, radiographic features relevant to socket fixation, and serum metal ion concentrations between groups of patients treated with the same femoral hip resurfacing device but acetabular components of different thicknesses.
The limitations of our study come from the consecutiveness of the two series that were compared. It is possible that the surgeon's surgical technique for implantation of the acetabular component kept improving, although no intentional surgical change was made between the two implantation periods. However, these possible improvements would have been small considering that the senior author (HCA) had been performing hip resurfacing arthroplasties for more than 20 years when this series was initiated. In addition, there was a difference in length of followup between the two groups and this could have affected our comparison of clinical scores and metal ion levels. However, there is evidence in the literature that clinical scores after THA are maintained beyond 7 years , and metal ion levels do not increase in well-functioning hips once stable state wear rate has been reached .
We found no difference in UCLA hip scores between the two groups. This result was expected because the modifications made to the acetabular shell aimed at conserving bone with minimal alteration of the device geometry and maintaining coverage of the femoral head by the acetabular shell. The magnitude of the scores recorded was well within the range of UCLA hip scores reported for this procedure and the device used in this study .
The reduction in complication rates between the 5-mm and the 3.5-mm shell groups can hardly be attributed to the use of a different acetabular component, but rather to progress in instrumentation and technique. Approximately 1/3 of the reported complications were femoral nerve palsies associated with the use of an anterior pelvic stabilizer, which may have pressed on the patient's femoral triangle. The use of this stabilizer was discontinued before the series of 3.5-mm shells was initiated . However, it is possible that the slight reduction in dislocation rate may be a consequence of a more favorable head-to-cup-diameter ratio as suggested by Kelley et al. .
The survivorship of the two cup designs was similar. This is most likely the result of using the same surgical technique and the same porous coating on both acetabular components. The 7-year survivorship of both cups was high and the 10-year survivorship of the 5-mm cup confirms the potential of this technology for long-term durability. These results match those of the most successful current designs [13, 27].
We did not find any association between the presence of postoperative radiographic gaps in De Lee and Charnley Zone 2 and the occurrence of aseptic loosening of the cup. This might suggest that initial bone-cup apposition at the periphery of the component may be sufficient to ensure durable osseointegration. This result is in agreement with those of previous reports of filling of the postoperative gap within a few years after surgery and no incidence of component loosening [11, 25, 30]. The prevalence of radiolucencies around the cup at 5 years for both groups was comparable to the rate reported by Ollivere et al.  with the Birmingham prosthesis with the same followup time.
Serum cobalt and chromium ion concentrations did not differ between the two subgroups of patients with metal ion data and good socket positioning. This was expected because the bearing characteristics of the two acetabular components were identical and the manufacturing tolerances for roundness and clearance were the same. In addition, the median values for serum cobalt and chromium were as low as any previously reported for any type of hip resurfacing design [1, 16, 21, 22, 28].
Based on the results of our study, the clinical performance of the 5-mm and the 3.5-mm thick cups was equally good considering that the only difference observed was a lower rate of postoperative complications explained by a change in instrumentation. With similar clinical outcomes at 5-years minimum followup, comparable metal ion concentrations, and 7-year survivorship, there is no reason to abstain from using the 3.5-mm thick acetabular component. Excellent results of ultraporous trabecular-like acetabular component backing (made of titanium or tantalum) have been reported [7, 20] for conventional THAs and open the way for additional improvement of cementless fixation of hip resurfacing devices.
1. Allan DG, Trammell R, Dyrstad B, Barnhart B, Milbrandt JC. Serum cobalt and chromium elevations following hip resurfacing with the Cormet 2000 device. J Surg Orthop Adv.
2. Amstutz HC, Beaulé PE, Dorey FJ, Duff MJ, Campbell PA, Gruen TA. Metal-on-metal hybrid surface arthroplasty: surgical technique. J Bone Joint Surg Am.
3. Amstutz HC, Campbell PA, Dorey FJ, Johnson AJ, Skipor A, Jacobs JJ. Do ion concentrations after metal-on-metal hip resurfacing increase over time? A prospective study. J Arthroplasty.
4. Amstutz HC, Duff MJ. Eleven years of experience with metal-on-metal hybrid hip resurfacing: a review of 1000 conserve plus. J Arthroplasty.
2008;23:6 suppl 136-43 10.1016/j.arth.2008.04.017.
5. Amstutz HC, Duff MJ, Campbell PA, Dorey FJ. The effects of technique changes on aseptic loosening of the femoral component in hip resurfacing: results of 600 Conserve Plus with a 3 to 9 year follow-up. J Arthroplasty.
6. Amstutz HC, Thomas BJ, Jinnah R, Kim W, Grogan T, Yale C. Treatment of primary osteoarthritis of the hip: a comparison of total joint and surface replacement arthroplasty. J Bone Joint Surg Am.
7. Baad-Hansen T, Kold S, Nielsen PT, Laursen MB, Christensen PH, Soballe K. Comparison of trabecular metal cups and titanium fiber-mesh cups in primary hip arthroplasty: a randomized RSA and bone mineral densitometry study of 50 hips. Acta Orthop.
8. Biedermann R, Tonin A, Krismer M, Rachbauer F, Eibl G, Stockl B. Reducing the risk of dislocation after total hip arthroplasty: the effect of orientation of the acetabular component. J Bone Joint Surg Br.
9. Bruyère O, Ethgen O, Neuprez A, Zégels B, Gillet P, Huskin JP, Reginster JY. Health-related quality of life after total knee or hip replacement for osteoarthritis: a 7-year prospective study. Arch Orthop Trauma Surg.
10. Carrothers AD, Gilbert RE, Jaiswal A, Richardson JB. Birmingham hip resurfacing: the prevalence of failure. J Bone Joint Surg Br.
11. Delaunay C. Kapandji AI [10-year survival of Zweymüller total prostheses in primary uncemented arthroplasty of the hip][in French]. Rev Chir Orthop Reparatrice Appar Mot.
12. DeLee JG, Charnley J. Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop Relat Res.
13. Gross TP, Liu F. Hip resurfacing with the Biomet Hybrid ReCap-Magnum system: 7-year results. J Arthroplasty.
14. Herman KA, Highcock AJ, Moorehead JD, Scott SJ. A comparison of leg length and femoral offset discrepancies in hip resurfacing, large head metal-on-metal and conventional total hip replacement: a case series. J Orthop Surg Res.
15. Kelley SS, Lachiewicz PF, Hickman JM, Paterno SM. Relationship of femoral head and acetabular size to the prevalence of dislocation. Clin Orthop Relat Res.
16. Kim PR, Beaulé PE, Dunbar M, Lee JK, Birkett N, Turner MC, Yenugadhati N, Armstrong V, Krewski D. Cobalt and chromium levels in blood and urine following hip resurfacing arthroplasty with the conserve plus implant. J Bone Joint Surg Am.
17. Kostensalo I, Junnila M, Virolainen P, Remes V, Matilainen M, Vahlberg T, Pulkkinen P, Eskelinen A, Mäkelä KT. Effect of femoral head size on risk of revision for dislocation after total hip arthroplasty: a population-based analysis of 42,379 primary procedures from the Finnish Arthroplasty Register. Acta Orthop.
18. Langton DJ, Sprowson AP, Mahadeva D, Bhatnagar S, Holland JP, Nargol AV. Cup anteversion in hip resurfacing: validation of EBRA and the presentation of a simple clinical grading system. J Arthroplasty.
19. Duff MJ, Wang CT, Wisk LE, Takamura KB, Amstutz HC. Benefits of thin-shelled acetabular components for metal-on-metal hip resurfacing arthroplasty. J Orthop Res.
20. Macheras G, Kateros K, Kostakos A, Koutsostathis S, Danomaras D, Papagelopoulos PJ. Eight- to ten-year clinical and radiographic outcome of a porous tantalum monoblock acetabular component. J Arthroplasty.
21. Moroni A, Savarino L, Cadossi M, Baldini N, Giannini S. Does ion release differ between hip resurfacing and metal-on-metal THA? Clin Orthop Relat Res.
22. Moroni A, Savarino L, Hoque M, Cadossi M, Baldini N. Do ion levels in hip resurfacing differ from metal-on-metal THA at midterm? Clin Orthop Relat Res.
23. Ollivere B, Duckett S, August A, Porteous M. The Birmingham Hip Resurfacing: 5-year clinical and radiographic results from a District General Hospital. Int Orthop.
24. Robb C, Harris R, O'Dwyer K, Aslam N. Radiographic assessment of biomechanical parameters following hip resurfacing and cemented total hip arthroplasty. Hip Int.
25. Roth A, Winzer T, Sander K, Anders JO, Venbrocks RA. Press fit fixation of cementless cups: how much stability do we need indeed? Arch Orthop Trauma Surg.
26. Su EP, Sheehan M, Su SL. Comparison of bone removed during total hip arthroplasty with a resurfacing or conventional femoral component: a cadaveric study. J Arthroplasty.
27. Treacy RB, McBryde CW, Shears E, Pynsent PB. Birmingham hip resurfacing: a minimum follow-up of ten years. J Bone Joint Surg Br.
28. Witzleb WC, Ziegler J, Krummenauer F, Neumeister V, Guenther KP. Exposure to chromium, cobalt and molybdenum from metal-on-metal total hip replacement and hip resurfacing arthroplasty. Acta Orthop.
29. Yoon JP, Duff MJ, Johnson AJ, Takamura KM, Ebramzadeh E, Amstutz HC. Contact patch to rim distance predicts metal ion levels in hip resurfacing. Clin Orthop Relat Res.
© 2014 Lippincott Williams & Wilkins, Inc.
30. Zwartelé RE, Olsthoorn PG, Pöll RG, Brand R, Doets HC. Primary total hip arthroplasty with a flattened press-fit acetabular component in osteoarthritis and inflammatory arthritis: a prospective study on 416 hips with 6-10 years follow-up. Arch Orthop Trauma Surg.