Hip Resurfacing for the Treatment of Developmental Dysplasia of the Hip

Abstract

Total hip arthroplasty (THA) with ceramic-on-ceramic bearings and an uncemented design is considered an effective treatment of developmental dysplasia of the hip, especially for young, active patients. The new generation of hip resurfacing with large femoral heads offers more stability, better range of motion (ROM), and more bone preservation than conventional THA. Twenty-one consecutive patients (26 hips) with osteoarthritis secondary to developmental dysplasia of the hip underwent metal-on-metal hip resurfacing. Average patient age at the time of surgery was 46.5 years (range, 37-59 years). Six patients (28.6%) were men and 15 (71.4%) were women. During the same period, another 21 patients (26 hips) with developmental dysplasia of the hip secondary to osteoarthritis were treated with ceramic-on-ceramic THA. Average patient age at the time of surgery was 48.2 years (range, 38-64 years). At follow-up, no complications (eg, dislocation, infection, or symptomatic deep venous thrombosis) occurred in the 2 groups. No significant difference was noted in Harris Hip Score between the 2 groups, but the average ROM of the hip resurfacing group was significantly better than the THA group (P<.05). All patients reported significant pain relief on their operated hips, with the postoperative visual analog scale scores <2. No signs of early loosening were observed on radiographs. The short-term results of the metal-on-metal hip resurfacing have been encouraging in the treatment of developmental dysplasia of the hip, with better ROM recovery than conventional THA.

Historically, artificial arthroplasty for developmental dysplasia of the hip has been challenging to orthopedic surgeons. The dysplastic hip has a deformed acetabulum and femur, which makes it difficult to reconstruct. On the acetabulum side, deficiencies in the anterior and superior wall make it difficult to obtain sufficient bony coverage of the cup.¹ On the femoral side, the narrow medullary canal and increased torsion result in excessive anteversion and a posterior location of the greater trochanter.² These distorted anatomies also affect most of the soft tissues around the hip, making the muscles around the hip joint shortened and the capsule thickened. In severe cases, because of the long period of high dislocation, the sciatic nerve is shortened and the femoral nerve and profunda femoris artery distorted.³

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Despite these issues, many orthopedic surgeons still treat developmental dysplasia of the hip with total hip arthroplasty (THA). The literature reports that THA with cementless acetabular designs has encouraging follow-up results in young, active patients.^4-7 As the cementless acetabular prosthesis was used more extensively in young, active patients, its aseptic loosening rate decreased and its survival rate increased. In mild cases of developmental dysplasia of the hip (Crowe type I and II¹), the operating procedure is no different from primary THA, and the follow-up result can be just as successful. In severe cases of developmental dysplasia of the hip (Crowe type III and IV¹), because of anterior and posterior deficiencies of the acetabulum and high dislocation of the hip, THA becomes challenging. Coverage of the acetabular prosthesis and recovery of the rotation center are the main considerations. However, with the technique of bulk bone grafting and the use of the S-ROM femoral prosthesis (DePuy, Warsaw, Indiana), THA has shown satisfactory short- and long-term results in this patient population.^8,9 Total hip arthroplasty is the first choice for the treatment of severe coxitis from developmental dysplasia of the hip.

Compared with the average THA patient population, patients with developmental dysplasia of the hip are typically younger and more active, and often need a large range of motion (ROM) of the hip. Most patients are women who pay attention to their stance in walking and sitting, so it has generally been understood that increased ROM of the hip is advantageous in this patient population. The optimal aim of THA for developmental dysplasia of the hip is increased ROM of the hip, complete pain relief, and an unrestricted return to activity. The new generation of hip resurfacing can minimize the risk of postoperative dislocation, increase the ROM of the hip with the use of large femoral heads, preserve the proximal femoral bone stock to maintain better stability and near normal biomechanical loading, and enable early exercise. There is also low wear of metal-on-metal articulation for longer prosthesis survival. This study compared the early results of hip resurfacing in a group of patients with developmental dysplasia of the hip with those of a matched group of patients undergoing THA for the same diagnosis.

Materials and Methods

Study Design and Patient Cohort

The study group consisted of a consecutive series of 21 patients (26 hips) who had undergone metal-on-metal hip resurfacing for developmental dysplasia of the hip from September 2005 to May 2007. No patient was excluded from the study. All patients were followed clinically and radiographically for a minimum of 17 months, with a mean follow-up of 27 months (range, 17-37 months). Six patients (28.6%) were men and 15 (71.4%) were women, and mean patient age at time of surgery was 46.5 years (range, 37-59 years). Hospital records for all patients were reviewed, including data from preoperative studies, operative reports, and postoperative follow-up information.

The 26 hips with developmental dysplasia were matched with the same number of hips in a cohort of patients who underwent total hip resurfacing for developmental dysplasia of the hip during the same period. Matching was done for surgeon, degree of developmental dysplasia of the hip, surgical approach, and patient sex. Average follow-up was 26 months (range, 16-37 months). Six patients (28.6%) were men and 15 (71.4%) were women, and mean patient age at time of surgery was 48.2 years (range, 38-64 years). If the patient had a previous pelvic or femoral osteotomy, this was recorded. The indication for hip resurfacing and THA was the same for both groups, which was painful end-stage joint degeneration. All the patients were informed of the advantages and disadvantages of the 2 surgical techniques preoperatively, and all operations were confirmed by the patients themselves. Patients in the hip resurfacing group were counseled on the paucity of information about the risk of long-term exposure to raised levels of metal ions, while patients in the THA group were informed of the risk of less ROM of the hip than with hip resurfacing.

The severity of dysplasia was classified according to Crowe et al.¹ In the 2 matched groups, 38 hips were classified as Crowe type I (19 hips in each group) and 14 hips as Crowe type II (7 hips in each group). Thirty-two patients were classified as Charnley category A (unilateral hip disease) and 10 as Charnley category B (bilateral hip disease, no other functional disabilities).¹⁰ Two hips (2 patients) in the resurfacing group had had structural bone-grafting surgery to support the deficiency of acetabular coverage, but the pain relief of the hips was not obvious.

Surgical Technique and Postoperative Management

All operations were performed by a single surgeon (W.X.) through a lateral position via the posterolateral approach without trochanteric osteotomy. Standard instruments for the Durom hip resurfacing system (Zimmer, Swindon, Wiltshire, United Kingdom) were used for all patients in the hip resurfacing group. The system is composed of an acetabular component with circumferential fins and a plasma-sprayed titanium coating for cementless fixation, and a femoral component with a cobalt-chromium stem for cemented fixation. After the femoral head was dislocated, the femoral head and acetabulum were prepared following the routine procedure.¹¹ The acetabulum was reamed to accommodate the acetabular component, and the cup was implanted in a press-fit manner. The pin-shaft angle was measured with a goniometer to achieve a target of 140° (±5°). All femoral components were cemented. The excised extortors were anatomically sutured after the prosthesis was implanted in place, and no drainage was used in the incision.

In the THA group, standard instruments for the Secur-Fit HA ceramic-on-ceramic total hip system (Stryker, Mahwah, New Jersey) were used, along with the same body position and surgical approach as was used in the resurfacing group. Both the acetabular and femoral components of the hip system have a commercially pure titanium coating for cementless fixation, and the acetabular cup has assistant screws for augmentation. The femoral head and liner of the cup are ceramic with high wear resistance and low fracture risk, which has the potential to extend the lifetime of the THA.

Prophylactic antibiotics were routinely given in both groups, with one dose intraoperatively and continued for 3 days postoperatively. Low molecular heparin (3800 IU, started 12 hours preoperatively and continued for 10 days postoperatively) was also used for prophylaxis of deep venous thrombosis.¹² A small dose of indomethacin was routinely prescribed to prevent heterotopic ossification unless patients could not tolerate the medication; beginning on the night of the operation, one 100-mg indomethacin suppository was given for 3 days, and then an oral dose of 100 mg was given as 50 mg twice a day with mucoprotection, up to the 14th postoperative day.¹³

All patients had no restrictions for on-bed activities postoperatively. They were encouraged to move their hips at will without using triangle pillows or antirotation shoes. In unilateral cases in the hip resurfacing group, patients were restricted to 20% weight bearing (with the aid of a walker) until the fifth postoperative day, then weight bearing was advanced to 50% (with cane or crutch in opposite hand). At 2 weeks postoperatively, all patients were advanced to full weight bearing as tolerated. For bilateral cases, on-bed functional exercises on the quadriceps femoris were the main course until the fifth postoperative day, then patients were restricted to 20% weight bearing (with the aid of a walker) for the next 5 days and 50% (with the aid of 2 crutches) for another 10 days. Approximately 3 weeks later, they were permitted to bear full weight. In the THA group, weight-bearing exercises were the same as in the resurfacing group, and the danger of early dislocation was made known. Most patients were discharged home within 1 week postoperatively. The average hospital stay was 5 days for patients with unilateral or simultaneous bilateral THA. Follow-up was performed at 6 weeks; 3, 6, and 9 months; and then yearly.

Clinical and Radiographic Analysis

Clinical assessment of pain, function, deformities, and ROM was based on the evaluation system of Harris.¹⁴ A Harris Hip Score of ≥90 points was defined as an excellent outcome; 80 to 89 points, a good outcome; 70 to 79 points, a fair outcome; and <70 points, a poor outcome. The visual analog scale was used to evaluate hip pain during the follow-up period: 0~2 as no pain, 2~4 as mild pain, 4~6 as moderate pain, 6~8 as severe pain, and 8~10 as worst possible pain. Radiologic assessment during each follow-up session included a standing anteroposterior radiograph of the pelvis with the radiograph centered at the pubic symphysis and a lateral radiograph of the operated hip joint. According to the zone classification of DeLee and Charnley,¹⁵ the radiolucent lines >1 mm thick around the acetabular shell were recorded. In resurfacing group, radiolucency and osteolysis on the femoral side were evaluated using the zone classification of Amstutz et al,¹⁶ while in the THA group, the radiographic analysis of the femoral prosthesis was performed according to Gruen et al.¹⁷ Migration of the acetabular and femoral component center was evaluated by comparing the horizontal and vertical distance from the inferior points of the teardrops and the center of the lesser trochanter on the immediate postoperative and final radiographs.¹⁸ Ectopic ossification was classified according to the system described by Brooker et al.¹⁹

Data Analysis

Postoperative complications included dislocation, infection, fracture, nerve injury, and symptomatic deep venous embolism. All complications during the follow-up period were recorded. Failures of hip resurfacing included conversion to conventional THA for any reason.

Paired Student t tests were used to compare variables between the 2 study groups. All P values <.05 were considered significant.

Results

Clinical Results

At final follow-up, the average Harris Hip Scores of the resurfacing and THA groups were similar: 93 and 91 points, respectively. This difference was not significant by analysis of variance (P>.05). Outcomes in the resurfacing group were considered excellent in 20 patients and good in 6. Outcomes in the THA group were considered excellent in 18 and good in 8. All patients had significant pain relief. All postoperative visual analog scores were <2.

The average postoperative ROM in all planes was significantly better in the resurfacing group (298.1°; range, 195°-390°) than in the THA group (221.3°; range, 135°-290º) (P<.05; Table).

In the resurfacing group, 12 patients maintained equal limb length pre- and postoperatively. Four patients had a <1-cm (0.5-0.8 cm) change in limb length and 5 patients a >1-cm (1.0-1.2 cm) change in limb length of their operated limbs pre- and postoperatively. However, all limb length discrepancies disappeared postoperatively by implanting the socket prosthesis in the true acetabular location, restoring the hip rotation center, and appropriately increasing the neck-shaft angle. In the THA group, 15 patients maintained equal limb length pre- and postoperatively. Six patients had a discrepancy of 0.5~1.0 cm in limb length. Postoperatively, equal limb lengths were achieved by implanting the socket prosthesis in the true acetabular location, restoring the hip rotation center, and selecting the suitable length of the femoral head (-5, 0, or +5).

Figure 1: Preoperative (A) and postoperative (B) radiographs at 12-month follow-up. Figure 2: Twelve months postoperatively, the patient can squat (A), and she reports no pain in her right hip (B).

Radiographic Results

In the resurfacing group, the outer diameter of the cup averaged 52 mm (range, 48-58 mm) with a mean abduction angle of 46.9° (range, 33°-55°). In the THA group, the outer diameter of cup also averaged 52 mm (range, 46-58 mm) with a mean abduction angle of 45.2° (range, 37°-52°). The coverage of the acetabular prosthesis in both groups was >80%. At most recent follow-up, all prostheses were fixed in place and no radiographic lucencies were detected. There was no evidence of migration of the acetabular and femoral components. No patients in our study group developed ectopic ossification during the latest follow-up period.

Case Report

A 58-year-old woman was admitted to our institution September 11, 2006, diagnosed with right hip osteoarthritis secondary to developmental dysplasia of Crowe type II with 25 years of pain in the right hip and 5 years of motional restriction. Physical examination revealed that her right hip had a discrepancy of 1.2 cm compared to the left hip, and ROM was restricted. Preoperative Harris Hip Score on the right was 31.5. One year postoperatively, it had increased to 98. She gained equal limb length and has complete pain relief of the right hip. Most encouraging, she can squat as before (Figures 1, 2).

Discussion

Today, with the modern prosthesis design and the development of surgical technique, cementless THA to treat mild developmental dysplasia of the hip (Crowe types I and II) has been approved.^4-7 In Crowe type I dysplastic hips, with <50% of proximal migration of the measured femoral head, the acetabulum is normal or ovoid in the vertical plane, and the femur is near normal. The bone quality is good and a standard prosthesis may be used. In Crowe type II dysplastic hips, with 50% to 75% of proximal migration of the measured femoral head, the anatomy has changed. The acetabulum is shallow and oval, and the femur is deformed with a straight and narrow medullary canal. The main problem is the dysplastic acetabulum, which often shows a superior acetabular defect and various degrees of anterior and posterior deficiencies. It is difficult to obtain total coverage of the acetabular cup in these cases, which is the main reason for long-term aseptic loosening of the acetabular cup. The literature reports that reaming up to the medial wall and superolateral structural grafting can increase the coverage and stability of the acetabular component.^6,7

The bearing surface of a conventional THA is designed for metal-on-polyethylene, and the wear debris can limit the implant’s longevity. Modern ceramic material with high wear resistance and low fracture risk has the potential to extend the lifetime of THA, which makes the procedure potentially more suitable for young, active patients. Murphy et al²⁰ reported the results of 194 ceramic-on-ceramic THAs for an average follow-up of 4.3 years (range, 2-9 years). The implant survivorship for all hips with aseptic revision of any component was 96% (confidence interval, 91-100) at 9-year follow-up; survivorship for hips without prior surgery was 99.3% (confidence interval, 98-100). There was a 1.7% incidence of implant-related complications. Colwell et al²¹ have followed 1635 ceramic-on-ceramic THAs for 9 years and reported encouraging results. Their dislocation rate is 1.1% (3 anterior and 15 posterior dislocations), with only 1 case of revision.

In our matched ceramic-on-ceramic THA groups, 19 hips were diagnosed as Crowe type I and 7 hips were diagnosed as Crowe type II. Reaming up to the medial wall was performed in all Crowe type II cases, and the coverage of the acetabular cup can reach 80%. Superolateral structural grafting was not used in our cases. At the latest follow-up, no dislocation occurred, and all prostheses were fixed well with no aseptic loosening.

Ceramic-on-ceramic cementless THA, with or without screw augmentation, has had great success in the treatment of young, active patients. The follow-up results are encouraging and the long-term survival rate is improving. As the design of the femoral head is smaller than the normal anatomic structure, the ROM of the hip may be less than that of the normal hip. In patients with developmental dysplasia of the hip, the soft tissues are also deformities: the hamstrings, adductors, quadriceps, and iliopsoas are short, which may increase the rate of postoperative dislocation. Hip resurfacing is well known for its large diameter femoral head design, which increases the ROM of the hip and decreases the rate of postoperative dislocation, thus enabling early functional exercises. This technique is especially suitable for young, active patients due to the protection of bone mass.^16,22

The process of hip resurfacing to treat mild cases of developmental dysplasia of the hip is similar to THA. On the acetabular side, both cups are cementless. The only difference is that the Secur-Fit HA cup (Stryker) has screw augmentation, while the Durom cup (Zimmer) does not. To obtain better coverage of the cup, reaming up to the medial wall was performed in Crowe type II cases in the resurfacing group, but none with structural grafting. All cups were reconstructed in the true acetabular location with coverage >80%. On the femoral side, the difference is obvious. For Crowe type I cases, the femoral head is normal and the neck is in valgus. The preparation of the femoral side is the same as in hip resurfacing. But in Crowe type II cases, the head is small and the neck is short and anteverted. Reaming is based on the center of the femoral neck, not the center of the femoral head, which, in such dysplasia cases, is often eccentrically located on the femoral neck. In our hip resurfacing series, the average pin-shaft angle was 143°, which was close to the target of 140° (±5°).

In our study, both THA and resurfacing showed encouraging results in the treatment of mild cases of developmental dysplasia of the hip at a mean follow-up of 26 and 27 months, respectively. All patients were pain free and gained good Harris Hip Scores. Postoperative radiographic analyses showed that all prostheses were fixed in place with no radiolucencies and no ectopic ossification. The only difference in patient recovery was in ROM, with the resurfacing group achieving better ROM than the THA group. Because our follow-up time was short (26 to 27 months), the main influencing factor of ROM was the prosthesis itself. Medial and long-term influencing factors may include functional exercise, muscle contracture, and ectopic ossification. In our matched groups, we excluded interference factors such as surgeon, degree of developmental dysplasia of the hip, surgical approach, and gender. Our study has confirmed that hip resurfacing can improve ROM in young, active patients with developmental dysplasia of the hip.

Although hip resurfacing greatly increased ROM of the hip, this technique is new and long-term risk factors are still unknown, such as the effect of the high level of serum metal ion.^23,24 Most patients with developmental dysplasia of the hip are young women, and it occurs in some before birth. Few reports have discussed whether the high level of serum metal ion will be hazardous to infants.

Another limitation of resurfacing is that it cannot increase the offset of the hip. The recovery of limb discrepancy is achieved by implanting the socket prosthesis in the true acetabular location, restoring the hip rotation center, and appropriately increasing the neck-shaft angle. For severe cases with high dislocation of the hip (Crowe type III and IV), in addition to the above problems, the true acetabulum is so small that resurfacing is not appropriate and cannot achieve equal limb length.

Ceramic-on-ceramic THA with a ceramic interface has no such complications, and the wear rate is as low as a metal-on-metal interface. The design of the acetabular cup is press-fit and cementless with screw augmentation, so the rate of aseptic loosening of the acetabular is low. The femoral head has three stands (-5, 0, or +5) for different levels of limb discrepancy.

Conclusion

Hip resurfacing was used to treat 21 patients (26 hips) for mild developmental dysplasia of the hip. The early results, at an average follow-up of 27 months, are excellent compared with a matched group of patients treated with ceramic-on-ceramic THA. Both were pain free at last follow-up and had gained an encouraging Harris Hip Score postoperatively, with no complications. The ROM of the resurfacing group is significantly better than that of the THA group. The long-term survival rate of the implants is still unknown. As there are only 52 hips (26 hips in each group) in our series, the complication rates also have limitations. However, in our experience, hip resurfacing may be a reasonable option for young patients with mild developmental dysplasia of the hip.

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Authors

Drs Li, Xu (Weidong), Xu (Ling), and Liang are from the Department of Orthopedics, Changhai Hospital, Shanghai, China.

Drs Li, Xu (Weidong), Xu (Ling), and Liang have no relevant financial relationships to disclose.

Correspondence should be addressed to: Weidong Xu, MD, Department of Orthopedics, Changhai Hospital, 168 Changhai Rd, Shanghai 200433, People’s Republic of China.