Materials and Methods

From a series of more than 950 hips treated with metal-on-metal hybrid resurfacing (Conserve Plus; Wright Medical Technology, Arlington, TN, USA), 208 patients (238 hips) underwent the procedure between November 1996 and June 2005 for a diagnosis other than primary OA.

The degeneration of the articular cartilage was secondary to DDH in 82 hips (34.5%), ON in 70 (29.4%), PT in 35 (14.7%), LCP disease in 20 (8.4%), SCFE in 13 (5.5%), inflammatory joint disease in 15 (6.3%), pigmented villonodular synovitis in 2 (0.8%), and melorheostosis in 1 (0.4%). There were 129 males (62%) and 79 females (38%). The average age of the patients at the time of surgery was 41.4 years (range, 14-63). Forty-six hips (19.3%) had undergone a prior operation before resurfacing, including 13 osteotomies, 12 core decompressions, 14 pinnings of the femoral head, 2 hemiresurfacings, and 5 other procedures.

All the procedures reported here were performed by the senior author. The surgical technique employed in this series has been described in detail in previous publications [28-30], and the effects of the modifications made from the initial surgical technique have been evaluated [31].

The patients were evaluated preoperatively, immediately after surgery, at 3 to 4 months, at 1 year, and then at yearly intervals. Radiographic data consisting of a low anteroposterior pelvis view, a modified table down-lateral, and a Johnson lateral view [32] were collected at each visit. The radiographic analysis was similar to that reported in our previous publications [21]. Two patients were lost to follow-up, leaving 236 hips for review.

The clinical outcome of the surgeries was evaluated pre- and postoperatively using the University of California at Los Angeles (UCLA) hip scoring system [33] and the Short-Form 12 questionnaire (SF-12) [34]. The Harris hip score [35] was calculated postoperatively as an overall assessment of success comparable to other studies. The Surface Arthroplasty Risk Index (SARI) [22] was calculated for each hip to evaluate the suitability of the group to be treated with a resurfacing procedure.

A statistical analysis was performed using Kaplan-Maier survivorship curves and log-rank tests for comparison of survivorship data. Paired Student's t tests were used for comparison of preoperative to postoperative clinical scores, and two-sample equal-variance t tests were used for comparisons of clinical scores with other groups of patients.

Results Clinical Results

At a mean follow-up of 5.6 years (range, 1.0-9.5), all clinical scores improved significantly, although they did not quite reach the average scores of primary OA patients, except for the physical component of the SF-12 survey (Table 1). SARI scores were high on average for the study group (3.2 vs 2.3 for the primary OA patients, P = 0.001), and this difference was explained by a greater percentage of previous surgeries (19.3% vs 0.2%), a lower body weight (78.2 kg vs 85.1 kg, P = 0.001), and a greater percentage of hips with cystic defects larger than 1 cm in the study group (55.0% vs 30.9%).

Radiographic Results

Seven hips (2.9%) from the study group presented substantial metaphyseal stem radiolucencies [21] at the last radiographic follow-up. Only one of these was associated with clinical symptoms of loosening in a patient who was lost to follow-up. The others were all pain free despite an average follow-up time of 4.6 years (range, 2.0-7.0) since the appearance of the radiolucency (Fig. 1).

A narrowing of the femoral neck of 10% or more at the junction with the femoral component was observed in ten hips, but no definite association could be made with femoral component failure.

Table 1. Clinical scores of the study group (pre- and postoperative) and in comparison with patients operated for primary osteoarthritis (OA)

Study group, preoperative

P

Study group, postoperative

P

Primary OA, postoperative

UCLA hip scores

Pain

3.3

0.001

9.3

0.008

9.5

Walking

5.9

0.001

9.5

0.002

9.7

Function

5.4

0.001

9.3

0.014

9.6

Activity

4.4

0.001

7.2

0.001

7.7

SF-12

Physical

31.6

0.001

50.6

0.718

50.9

Mental

46.1

0.001

51.2

0.001

54.2

HHS

91.8

0.023

93.5

UCLA, University of California at Los Angeles; SF-12, Short-Form 12 questionnaire; HHS, Harris hip score

UCLA, University of California at Los Angeles; SF-12, Short-Form 12 questionnaire; HHS, Harris hip score

Fig. 1. Seven-year-postoperative radiograph of a 40 year-old woman who underwent metal-on-metal resurfacing for developmental dys-plasia of the hip (DDH). The region of interest highlights a radiolucency, which has been visible around the metaphyseal stem for more than 6 years, indicating imperfect initial fixation with first-generation cementing technique (cyst size was 2 cm). The patient has no clinical symptoms, indicating a degree of stability commensurate at this time with her activity level of 7 and her weight of 67kg

Complications

There were a total of 14 complications (overall rate, 5.9%) that did not require conversion to a total hip replacement (THR) in this series. Four were dislocations (1.7%), from which 3 resolved with closed reduction and 1 necessitated acetabular component reorientation. There were 4 femoral nerve palsies (1.7%), which all fully recovered without any specific treatment. There was also 1 femoral vein clot (0.4%) followed by extracapsular bleeding secondary to the use of heparin. One hematogenous sepsis happened 10 days after surgery and was treated with soft tissue debridement and antibiotics. One of 5 patients operated through a lateral transtrochanteric approach developed a trochanteric bursitis, which resolved with the removal of wires used in the reattachment of the greater trochanter.

A component size mismatch that occurred early in the series before prepackaging of the components was resolved with replacement of the acetabular shell with a 2-mm-thicker custom component of the appropriate inner diameter. One hip required a reexploration to remove residual bone cement trapped in the joint after hip reduction. Finally, one hip needed acetabular reconstruction after the acetabular shell protruded through the acetabular wall. The patient was heavy, had poor bone quality, and had undergone simultaneous bilateral resurfacing (the event occurred on the first hip operated). In addition, the wall had presumably been further weakened by overreaming.

Conversions to THR

Thirteen hips were converted to a THR in this series. The reasons for revision included 2 for fracture of the femoral neck, 9 (in 8 patients) for femoral component loosening, 1 for late hematogenous sepsis, and 1 for recurrent subluxation secondary to ischial-trochanteric impingement. The femoral neck fractures occurred at 2 and 5 months after surgery (both with a diagnosis of DDH in patients with poor bone quality) [36], and the loosening of the femoral component occurred at an average of 53.4 months (range, 23-100) after resurfacing.

Taking any revision as endpoint, the Kaplan-Maier survivorship of the study group at 4 years was 95.0% (95% confidence interval, 90.1-97.5). In comparison, the hips operated for primary OA had a slightly superior 4-year survivorship with 96.6% (95% confidence interval, 93.4-98.3; log-rank test, P = 0.056). However utilizing second-generation technique [31], there has been only 1 loosening and 2 radiolucencies in the most recent 138 hips, and none when the stem was cemented in despite the presence of large cystic defects.

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