Mid-Term Results of Metal-on-Metal Hip Resurfacing for Treatment of Osteoarthritis Secondary to Developmental Dysplasia of the Hip: A Minimum of 8-Years of Follow-Up

Background

Developmental dysplasia of the hip (DDH) is an important cause of childhood disability, and the most common cause of secondary osteoarthritis (OA) in young adults. It is reported that DDH underlies up to 9% of all primary hip replacements and up to 29% of those in people aged 60 years and younger [1]. Conventional total hip arthroplasty (THA) for DDHs has success in pain relief and activity improvement. However, total hip arthroplasty requires complex reconstruction and has a less reliable outcome when compared with the results in patients with primary degenerative osteoarthritis [2].

Hip resurfacing offers an attractive alternative to conventional total hip arthroplasty in patients with DDH [3,4]. Hip resurfacing preserves the size of the femoral head close to normal and near normal biomechanical function, enables an excellent range of motion, minimizes the risk of postoperative dislocation, results in less blood loss, and makes rehabilitation easier [5–7].

However, there are few studies in the literature evaluating the functional outcome and longevity of the articular surface replacement (ASR™, DePuy Orthopedics, Inc., Warsaw, IN; Corin Medical Ltd, Cirencester, United Kingdom) for treatment of osteoarthritis secondary to Developmental Dysplasia. Our aim was to prospectively assess mid-term clinical outcomes of our experience using metal-on-metal hip resurfacing for treatment of osteoarthritis secondary to developmental dysplasia and identify the potential causes of early failures.

Material and Methods

EVALUATION METHOD:

All patients were followed up at the 1st, 2nd, 3rd,6th, and 12th months after the operation and annually thereafter. The Harris hip score was used to evaluate the surgical results [10], and were determined preoperatively and postoperatively at each follow-up examination. The clinical result was excellent if Harris score was higher than 90, good if it was 80–89, fair if it was 70–79, and poor if was less than 70. The hip score at latest follow-up was used to grade the functional result. Clinical examinations to final follow-up were all available. Evaluation of pain was carried out by patients with the use of a 10-point visual analog scale. Complications were recorded at the same time.

The radiological assessment during each follow-up session and pre-operation included a standing anteroposterior radiograph of the pelvis with the X-ray beam centered at the pubic symphysis and a lateral radiograph of the operated hip joint. All the plain films were evaluated by 1 orthopedic surgeon. The Immediate postoperative radiograph was considered as the original plain film. The abduction angle of the acetabular component and the stem shaft angle of the femoral prosthesis were measured. The femoral component was considered malpositioned if it was 5° more horizontal (varus) than the medial trabecular system of the proximal femur [11]. The acetabular component was considered malpositioned if the abduction angle was greater than 65° or less than 30°.

Failure was defined by removal or revision of the prosthesis or consideration for revision based on reduction in function of the hip with radiographic evidence of loosening of the components, such as change in position of either the femoral or acetabular component or extensive radiolucent lines around the acetabular component and resorption of bone.

STATISTICAL ANALYSIS:

The changes in preoperative and postoperative hip scores were compared for statistical significance using the t test. Kaplan-Meier survival analysis was used at the deadline of the follow-up corresponding to 95% confidence limits. All calculations were performed with SPSS version 13.0 software and p<0.05 was considered significant.

Results

CLINICAL OUTCOMES:

The overall survival was 88.2% at a minimum follow-up of 8 years (Figure 1), but the survival was 91.2% after excluding the infections as the cause of component loosening and failure. The mean Harris hip score improved from 48.27±3.13 (range, 14–71) to 89.63±3.42 (range, 65–100) at latest follow-up. Flexion improved from a mean of 75±8.05° (range, 20°–98°) to a mean of 107±9.34° (range, 85°–128°) between preoperative and postoperative evaluations (Table 1). Most patients experienced no pain and 3 hips (8.8%) had occasional pain in the first 6 months postoperatively, but the pain relieved at 1 year after the operation. The Figures 2 and 3 showed a patient with no pain and an excellent range of movement.

Until the deadline of the follow-up, a total of 4 failures (11.8%) occurred in these patients. One (2.9%) of the failures was due to an infection, 1 (2.9%) was due to a femoral neck fracture, and 2 (5.9%) were due to aseptic femoral component loosening. All 4 patients required revision to a conventional total hip arthroplasty.

Of the 29 patients, 17 maintained an equal limb length postoperatively. Eleven patients had less than1.5 cm (0.6–1.5 cm) shorter limb length, and only 1 case had 2.2 cm change in the length of operated legs compared to the preoperative length. However, all these limb length discrepancies disappeared after their surgery by implanting the socket prosthesis in the true acetabular location, restoring the hip rotation center and appropriately increasing the neck-shaft angle.

RADIOGRAPHY OUTCOMES:

The evaluation methods mentioned previously yielded the following radiographic evaluations: postoperative radiographs revealed technical errors in 2 patients, 1 of which had a malpositioned femoral component with greater than 5° more varus postoperatively measured versus the medial trabecular system and the other malpositioned acetabular components with the larger abduction angle (Table 2); radiography showed narrowing of the femoral neck after the operation in 6 patients (Figure 4).

COMPLICATIONS:

The most common complications seen at any time during the follow-up included deep infection, periprosthetic fracture, and dislocation. Of these patients, 1 patient had a deep infection with Staphylococcus aureus at 15 months after surgery, and 1 had femoral neck fracture from playing basketball (Table 3). There was no nerve palsy or deep vein thrombosis in our series.

Discussion

Metal-on-metal total hip resurfacing appears to be an effective alternative to conventional total hip arthroplasty in patients with osteoarthritis secondary to developmental dysplasia of the hip. Hip resurfacing offers several functional benefits over THA: the size of the femoral head and neck remains close to normal, the resurfaced hip is stable and capable of an excellent range of motion, and the joint retains a greater degree of normal biomechanical function. Resurfacing provides adequate pain relief and improved level of function and activity in younger patients who warrant a hip replacement. The patients in the current study showed a marked improvement in Harris hip score and flexion. The overall survival was 88.2% at a minim follow-up of 8 years, but the survival was 91.2% after excluding the infections as the cause of component loosening and failure. We found no significant complications due to high ion levels in blood or metallosis. The implant survival and the functional outcomes score are comparable to those observed in other patients managed with metal-on-metal surface arthroplasty [12,13].

In this study, the traditional posterolateral approach was used to perform the hip resurfacing arthroplasty, and no secondary avascular necrosis after resurfacing of the femoral head occurred. The low incidence of secondary avascular necrosis in this series supports the use of the posterior approach despite historical concerns [14]. With recent dramatic improvement of hip arthroplasty technology, the trochanteric osteotomy for wider exposure has been avoided in consideration of its common complications. One study compared the posterolateral and direct lateral approaches for complications, and reported that both approaches offer excellent pain reduction and return to function with no difference in survival or in the incidence of complications [15].

Although it has advantages, several complications can arise, such as femoral neck fractures, aseptic loosening, infection, and metal hypersensitivity. Risk factors resulting in aseptic loosening of the femoral component have been identified, such as large femoral head cysts, female sex, younger age, higher body mass index (BMI), and smaller component size in males [16,17]. A total of 4 hips underwent a revision (11.8%), and the reasons for failures were infection in 1, femoral neck fracture in 1, and aseptic femoral component loosening in 2. Femoral neck fracture is a rare complication after hip resurfacing, occurring at reported rates of 0% to 7%. Femoral components placed in 5° valgus have a factor of 6.1 reduction in the relative risk of an adverse outcome [18,19]. These findings emphasize that sufficient and careful preoperative planning is essential to improve the outcomes and longevity of surface arthroplasty. The main issues were the location of the true acetabulum with its ideal coverage and fixation, the exact match of the acetabulum and femoral prosthesis, and the length discrepancy of both legs [20]. In our experience, the aseptic femoral component loosening in 2 patients occurred at the early stage of performing surface arthroplasty, which was similar to those in previous reports, indicating that a higher rate of complications occurs at the beginning of the learning curve when first performing surface arthroplasty.

Our study has certain limitations. Due to a relatively small sample size, our study was an initial retrospective analysis of patients. Pathologic specimens of failed cases were not available to show the reasons for failure. Finally, because this was a single patient series, there were no patients or groups available for direct comparison.

Conclusions

Although greater technical challenges exist in osteoarthritis secondary to developmental dysplasia of the hip and there is a long learning curve, metal-on-metal resurfacing arthroplasty showed perfect results at a minimum of 8 years of follow-up in our study, and we expect to have results from longer follow-up in the future.