Early Clinical Experience With the Use of the Birmingham Hip Resurfacing System

Abstract

This study reviews the early clinical experience with the Birmingham Hip resurfacing system (Smith & Nephew, Memphis, Tenn) in the United States since its approval by the FDA. A total of 230 patients were followed for a mean of 16 months (range, 6 months). There was a significant improvement in the mean preoperative Oxford hip score at the latest follow-up (44 points [range, 30-58 points] to 17 points [range, 12-28 points]). The most commonly used femoral component was 50 mm, and the mean duration of the procedures was 95 minutes (range, 65-180 min). There were 3 revisions (1.3%). Early results are favorable. As with any device, there is a continuing need for long-term monitoring and large-scale epidemiologic surveillance.

Total hip resurfacing has been controversial for the past 5 decades. From the device proposed by Charnley in 1958 (Teflon-on-Teflon; Imperial Chemical Industries, Cheshire, UK) to the first FDA-approved modern resurfacing system (Birmingham Hip; Smith & Nephew, Memphis, Tenn) many designs have been studied worldwide.^1-3 The majority of these femoral bone-conserving devices failed at an early stage, and total hip arthroplasty (THA) became the gold standard for the treatment of painful degenerative joint disease. Several theories about the cause of failure of the early resurfacing devices have been proposed. Two widely accepted hypotheses include the “stress-shielding” effect of the femoral component in the normal biology of bone mineralization and the deleterious consequences of osteonecrosis.^4-6 It is now known that earlier polyethylene-containing resurfacing systems (Teflon-on-Teflon, metal-on-polyethylene, ceramic-on-polyethylene) failed due to accelerated component wear,^7-9 primarily because the materials used in those resurfacing procedures were suboptimal.

The critical requirements of an ideal bearing surface in hip arthroplasty include adequate biocompatibility and structural strength, low friction and wear, and high resistance to corrosion (decay, oxidation). Over the past decades, hip arthroplasty techniques and materials have evolved, and a favorable impact on the quality of life of patients has been observed.^10-13 As a result, indications for the procedure have been expanded to include many hip pathologies and the procedure offered to younger, more active patients. Unfortunately, these young, active patients treated with conventional bearings showed higher and earlier failures than did older subjects. As indicated previously, the limiting factor in the long-term survival of THA was also polyethylene wear debris-induced osteolysis.

Metal-on-metal total hip resurfacing has evolved over the past 10 years. The orthopedic surgeon now has more than 10 different designs to choose from, whereas only one device was available worldwide in 1997. Several changes in metallurgy and surgical technique have been reported in the literature,^3,9,14 but long-term clinical studies are not currently available. The objective of this study is to report on early clinical experience with the use of the Birmingham Hip resurfacing system in the United States since its approval by the FDA.

Materials and Methods

Since 2006, the senior investigator has performed more than 300 procedures using the Birmingham Hip resurfacing system. All cases have been included in a single surgeon database. Patients with less than 6 months of follow-up were excluded from the study. A total of 230 patients were followed both clinically and radiographically for a mean of 16 months (range, 6-24 months). There were 168 men, and mean patient age was 55 years (range, 25-74 years). Mean body mass index was 29.6 (range, 20.3-52.1). Institutional review board consent was obtained to review the hospital records for all patients, including data from preoperative studies, operative reports, and postoperative office visits. The most common diagnosis (200 patients) leading to total hip resurfacing was severe osteoarthritis unresponsive to conventional nonoperative techniques (such as lifestyle modifications, pharmacologic interventions, and physical therapy).

Device, Surgical Technique, and Postoperative Management

Conventional instrumentation for the Birmingham Hip system was used for all patients. The acetabular component is nearly hemispherical, with a beaded surface coated with hydroxiapatite for cementless fixation. Notably, the beads are cast in during the initial manufacturing process, therefore avoiding excessive heat to adhere them to the cup, preventing loosening, and third body wear. The femoral component preserves bone stock by cemented fixation to the proximal metaphysis using a short, thin stem. Since 1997, the Birmingham Hip system has been produced as “as cast” cobalt-chrome alloy containing microscopic carbide residues.¹⁵ By eliminating postcasting heat treatments (used to reduce microporosity), the device has superior wear resistance compared with previous resurfacing models.¹⁶

All surgeries were performed in a laminar airflow environment through a standard posterolateral approach. One gram of cefazolin was given with the administration of anesthesia, and three additional doses were given over the next 24 hours. The patient was then placed in the lateral position on the unaffected hip. The proximal curve of the greater trochanter and its posterior margin were outlined with a marking pen. The line of incision was started 7 cm distal to the tip of the trochanter on its posterior margin and extended for approximately 7 cm obliquely backward. Skin and subcutaneous tissue were incised, and hemostasis was obtained. Blunt dissection of the fibers of the gluteus maximus was performed along the line of the skin incision and from the underlying trochanteric bursa. Undermining of the skin incision was performed distally for approximately 2 cm. A conventional Charnley retractor was then positioned, ensuring that the sciatic nerve was not caught in the posterior wing. The insertion of the gluteus maximus to the femur was divided as needed in all patients. The bursa over the trochanter was incised along the posterior margin of the greater trochanter, exposing the external rotators. The posterior edge of the gluteus medius was retracted to expose the piriformis tendon. The gluteus minimus was then separated from the piriformis tendon and released from the underlying capsule with electrocautery. A pin retractor was inserted into the ischium. The piriformis and remaining external rotators along with the capsule were divided close to their insertion. A thin cuff of the quadratus femoris was left attached to the femur for reattachment. Care was taken to preserve the soft-tissue envelope of the femoral neck during this dissection. The superior capsule was divided and the hip dislocated. The acetabular component placement was performed using offset acetabular reamers and an offset cup introducer. The target position was 35· to 45· of abduction and 20· of anteversion. Once the cup was secured in the floor of the acetabulum, the femur was rotated to so that the neck and intertrochanteric region could be viewed. For varus-valgus alignment of the femoral jig, a guidewire was drilled to measure the templated distance on the intertrochanteric crest and to attach the femoral jig. The femoral head was sequentially reamed with care to avoid notching the femoral neck. After pulse lavage of the reamed head, the femoral component was cemented in place. As indicated by McMinn and colleagues, the key points of the procedure are (1) to ensure a neutral or mild valgus position of the component; (2) to place a suction vent through the lesser trochanter to reduce risk of systemic embolization and to ensure femoral neck viability by reducing small vessel intraosseous embolization; (3) to avoid notching of the femoral neck; (4) to preserve soft-tissue cover over the neck; (5) to provide adequate peripheral support on the femoral head to ensure line-to-line contact between the inside of the femoral component and the periphery of the prepared femoral head for good cement microinterlock; and (6) to use low-viscosity cement to facilitate good microinterlock.¹⁷

All patients began transfers and ambulation on the day of surgery. On the first postoperative day, all patients were included in a structured physical therapy program that included propiosception and strengthening exercises. Patients were allowed to fully weight bear as tolerated and were typically discharged home on the second postoperative day. Criteria for discharge include adequate pain control with oral medication, ability to walk 250 feet, and ability to get in and out of bed without assistance. Patients were encouraged to continue hip-strengthening exercises 3 times per week. The first 30 patients received a strict regimen of prophylactic anticoagulation with low-molecular-weight heparin. Due to increased complications in superficial wound healing, the protocol was modified to aspirin daily for 1 month after the procedure for the remaining patients.

Patients underwent postoperative evaluation at 2 and 6 weeks; 3 and 6 months; 1 year; and annually thereafter. A patient-based evaluation was performed using the Oxford Hip score. This score is based on answers to 12 questions administered pre- and postoperatively to each patient. It is a disease-specific, self-administered health status measure that addresses clinically important symptoms about pain and physical function. Each question is assigned a score of 1 (none) to 5 (extreme). These scores are then totaled for a minimum raw score of 12 (best) and a maximum raw score of 60 (worst) points (Table).¹⁸ A subanalysis was performed dividing the hips based on the patient age into young (<50 years, n = 68) and older (>50 years, n = 162) age groups.

Table: Summary of Oxford Hip Questionnaire

Radiographic Analysis

Preoperative radiographic templates were obtained for all patients. Standard anteroposterior and lateral radiographs were obtained during the immediate postoperative period and at each clinical visit for all patients. To assess radiolucencies at the bone interface of the femoral and acetabular components, the investigators followed several methodologies described in detail elsewhere.^19,20 In addition, the stem-shaft angle, the cup-inclination angle, and the femoral neck-shaft angle were recorded during each radiographic evaluation.

Results

There was a marked improvement in the mean pre- and postoperative Oxford hip score at the latest follow-up. The mean preoperative score was 44 points (range, 30-58 points) compared with a mean postoperative score of 17 points (range, 12-28 points).

The most commonly used femoral components had a diameter of 50 mm in 86 hips (37%), 46 mm in 71 hips (30%), and 54 mm in 29 hips (18%), with a range of 38 to 54 mm. The overall mean head size was 48 mm. The mean duration of the procedure was 95 minutes (range, 65-180 min).

In the subanalysis comparing the young and the older patients, scores were obtained for 65 (96%) of the young and 158 (98%) of the older patients. Both groups showed statistically significant improvements in pain and function. Of note, the mean Oxford Hip score was significantly higher (P = .01) for the young patients (19.3 points; SD, 8.7) than for the older patients (16.2 points; SD, 6.2).

There were 3 revisions (1.3%). One patient was a 45-year-old man with a diagnosis of osteonecrosis. During the immediate postoperative period, a large, loose foreign body was identified. The patient was returned to the operating room where the fragment was removed from the acetabular cup. Both the femoral and acetabular components were left in place, and the patient was doing well with no radiographic or clinical evidence of failure at the latest follow-up. The second patient was a 47-year-old man with a diagnosis of osteoarthritis. Shortly after the index procedure, the patient presented with a dislocation of the affected hip that was effectively reduced with nonoperative maneuvers. However, almost 1 year later, the patient underwent revision of the acetabular component to a dysplasia cup. Six months after the revision, the patient’s Oxford hip score was 16 points, and there was no radiographic evidence of loosening, radiolucencies, or migration of the cup. The third patient was a 56-year-old woman with a diagnosis of osteoarthritis. She presented with a femoral neck fracture 11 months after the index procedure. Her femoral component was revised, and her current radiographic and clinic outcomes are also satisfactory (Oxford Hip score, 19 points).

There has been no evidence of migration of the cup in the most recent radiographic analysis for all patients. One patient presented with acetabular radiolucencies. An additional radiolucency was also reported around the femoral component in a different patient. Both patients presented with a thin sclerotic line in zone 2 that has remained unchanged for the past 12 months. Of note, there was no radiographic evidence of bone density loss in the proximal aspect of the affected femur.

One patient had a major medical complication after the procedure. During the immediate postoperative period, a myocardial infarction was diagnosed and effectively treated in an intensive-care-unit setting. This patient was a 61-year-old woman with known coronary artery disease and hypertension. She recovered from the coronary event and has adequate function and activity level with the hip resurfacing. There has been no evidence of deep venous thrombosis or pulmonary embolus in any patient with the current method of postoperative anticoagulation.

Discussion

In 1991, clinical trials were initiated in the United Kingdom to study a new system of metal-on-metal total hip resurfacing. The results of these initial series, published in 1996, showed that a hybrid fixation with a cemented femoral component and a cementless acetabular cup provided an optimal combination.²¹ One year later, important changes and improvements in metallurgy of the same device allowed the introduction of the current Birmingham Hip resurfacing system. In 2006, the USFDA approved the marketing of this device for total hip resurfacing. This article reviews the early experience with the use of the device in the United States in a large community medical center.

As noted previously, the concept of a femoral resurfacing articulating with an acetabular cup is not new. Several failures in previous devices and the overall good results with modern THA motivate the ongoing debate regarding the indications and limitations of the current resurfacing systems. The investigators acknowledge that there is a high rate of success with circumferential bead-coated uncemented stemmed femoral components at 10 to 20 years.^22-24 There have been no long-term adverse consequences of femoral stress shielding with a diaphyseal component.²⁵ The investigators also agree that patient selection is important and that many demographic variables may encourage a significant number of orthopedic surgeons to avoid use of resurfacing devices. Also, resurfacing is technically more difficult than THA, and it is associated with a different learning curve.²⁶ Blood and urine metal ion levels, capsular lymphocytic aggregation, and hypersensitivity are valid concerns with metal-on-metal resurfacings.

The investigators believe that the current Birmingham resurfacing system addresses most of the previous limitations in design and metallurgy. As with THA, the resurfacing system removes only enough acetabular bone stock to achieve optimal component position and fixation. In contrast to THA, in resurfacing, the femoral neck and most of the head are retained. In addition, the proximal femur is optimally loaded.²⁷ Back and colleagues studied 210 sets of radiographs from patients who received the Birmingham Hip system and found no decrease in bone density in the femoral neck.²⁸ Moreover, Kishida and colleagues reported that in addition, patients with total hip resurfacing showed increased bone mineral density in the proximal femur, compared with what had been observed in patients who underwent THA.²⁹ In a dual energy x-ray absorptiometry scan study of proximal femoral bone mineral density, they found a median increase of 11% in patients with modern Birmingham Hip resurfacings compared with a 17% bone mineral density decrease in patients with an uncemented metal-on polyethylene hip replacement at 2 years in the critical region of the calcar femorale.²⁹ As indicated in this study, the femoral component is 6 mm smaller than the maximum head diameter. After the resurfacing, the head:neck ratio is reduced with implications for increased range of motion (ROM) and decreased neck-cup impingement.³⁰ The difficult learning curve associated with Birmingham Hip resurfacing has been a matter of controversy.^3,31 A study involving 140 surgeons performing more than 3000 resurfacing procedures with the same device showed an implant survival rate of 97% at 5 years.^2,32 Considering the large number of surgeons (each with a different learning curve), these results are reassuring about the short- to midterm reliability of the device. We believe that, as with every other surgical device, certain contraindications should be observed with hip resurfacing. Severely osteopenic femoral heads and necks and the presence of large cysts in the femoral head are contraindications to hip resurfacing. Patients with a relatively low offset, a significantly varus neck-shaft angle, or limb length discrepancy could also obtain improved outcomes with THA. Hip resurfacing has been shown in several studies to have significantly lower rates of material wear compared with procedures involving other bearing surfaces.^33,34 In 15 years of use of metal-on-metal bearings, Dowson and colleagues showed a production volume of metal debris equivalent to one pin head.^35,36 Of note, the Birmingham Hip system provides a device in an “as cast” conformation of cobalt chrome in both the cup and the femoral component. Sacrificing microporosity, this device guarantees a high carbide concentration reducing metallic wear and debri.¹⁶

Currently, we note several absolute contraindications for the procedure. Patients with severe renal failure or a diagnosis of severe osteoporosis (bone mineral density 2.5 standard deviations or more below the young adult mean in the presence of one or more fragility fractures) are instructed against having a Birmingham Hip Resurfacing system. Also, patients with significant limb length inequality are not candidates for the procedure. Several intraoperative findings (such as massive subchondral cysts) may also contraindicate the resurfacing of the hip.

We acknowledge several limitations in this study. This series represents the experience of a single surgeon at a community hospital. The relatively short follow-up of our series precludes several statistical analyses and comparisons with other treatment methods. In addition, the lack of long-term survival data with the use of the modern resurfacing devices should be considered. There is no validation for the optimal postoperative treatment algorithm for patients with hip resurfacing, and our favorable results with the approach presented here cannot be confirmed statistically with our current data. The senior investigator trained with the surgeon who developed the Birmingham Hip device in the United Kingdom before implanting the system in the United States, stressing the importance of a structured learning curve with dedicated additional training. However, the investigators believe that the early results with the device, the low complication rate, the high patient satisfaction, and the overall rationale for the technique support the trend to offer the resurfacing procedure to selected candidates for THA.

Conclusion

The excellent low-friction, low-wear, high-biocompatibility properties of modern metal-on-metal devices make them optimal bearing couples for hip resurfacings. Young and active patients are able to return to daily activities with superior ROM and a relatively decreased risk of dislocation after resurfacing compared with THA. As with any new device, there is a continuing need for long-term monitoring and large-scale epidemiologic surveillance.

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Authors

Drs Marulanda, Wilson, Edwards, and Raterman are from the Department of Orthopaedics and Sports Medicine, University of South Florida, Tampa, Fla.

Drs Marulanda, Wilson, Edwards, and Raterman have no relevant financial relationship to disclose.

Correspondence should be addressed to German A. Marulanda, MD, Department of Orthopaedics and Sports Medicine, University of South Florida, 3500 East Fletcher Avenue, Suite 511, MDC106, Tampa, FL 33613.