Metal-on-Metal Hybrid Surface Arthroplasty:
Two to Six Year Follow-up
Harlan C. Amstutz, M.D.; Paul E. Beaule, M.D., FRCSC; Frederick J. Dorey Ph.D.; Michel J. Le Duff, MA.; Pat A. Campbell, Ph.D.; Thomas A. Gruen, MS
Introduction
Methods & Materials
Results
Radiographic Results
Discussion
References
Abstracted from manuscript accepted for publication and now in press for The Journal of Bone & Joint Surgery which analyzes the results of Dr. Amstutz's initial cohort of 355 patients (400 hips) who had the Conserve Plus surface arthroplasty. This report represents the first comprehensive analysis of patients undergoing M/M surface arthroplasty anywhere in the world.
Introduction
Although total hip arthroplasty is quite predictable and durable in older patients, young and active patients have higher revision rates 1,2,3,4,5 and these rates are increased when the etiology is osteonecrosis 6,7. Because there was no satisfactory biological or prosthetic solution for advanced arthritis in the young and active patient, we began to investigate metal-on-metal surface arthroplasty in the early 1990's. Our concept of applying metal-on-metal bearings to surface arthroplasty followed the re-introduction of metal-on-metal bearings to total hip arthroplasty in Europe in 1988 8,9. The measured wear of first generation metal-on-metal retrievals of these implants has been reported to be only a few microns per year 10,11. Unlike the adverse effects of increased volumetric wear of polyethylene as a function of increased head size, the wear is minimally affected by increasing the head size in metal-on-metal components 10,12. For these reasons, we, as well as others, began to implant metal-on-metal surface arthroplasties. Although the Conserve Plus design began in 1992, initially we used the fully cemented McMinn 13 and a small number of a complex cementless Wagner 14 design. However, we observed short-term loosening of the acetabular component and inconsistent manufactured tolerances in the McMinn Design and the overall results were disappointing and 6-9 year results are in press15. The purpose of this study was to evaluate the clinical and radiographic performance of this implant in the first consecutive 400 hips of the Conserve Plus™ design.
Methods and Materials
Patient Cohort
Between November 1996 and November 2000, the first 400 Conserve Plus™ (Wright Medical Technology, Arlington, Tennessee) surface arthroplasties were performed in 355 patients. The study was approved by the hospital Institutional Review Board. The most common indications for the procedure included patients of a young age, and or high activity level. In some older patients with abnormal proximal femoral morphology, we also chose to perform surface arthroplasty. Many of our young patients had been advised to defer surgery and had extremely severe degenerative changes but we did not exclude any who met the above criteria because of severe femoral head cysts or osteopenia. The demographics and etiology of the arthritis of the patients who underwent hip resurfacing (average age 48.2 years, range fifteen to seventy-seven) are shown in Table 1.
Table 1. Patient demographics (mean, standard deviation and range)
Thirty-two patients had bilateral arthroplasty at the same operation, and thirteen patients had sequential bilateral procedures between 2.5 and 34 months after the first side.
The Implant
The Conserve Plus acetabular shell is nearly hemispherical (170o). Its exterior surface has sintered beads ranging from 50 µm to 150 µm in diameter for cementless fixation. The one-piece acetabular shell is five millimeters in thickness. Insertion is press-fit by under reaming one millimeter16. The femoral component has the same design as the Conserve® hemi-resurfacing (Wright Medical Technology, Arlington, Tennessee) which was approved by the FDA in 1995. This component has a short metaphyseal stem to facilitate accurate component alignment and permits a cement mantle that averages 1.25mm around the femoral head. The femoral component is greater than a hemisphere (208o degrees), which in most instances, enables coverage of all of the reamed bone by the component, and maintains length of the femoral head and neck. The surface finish is approximately 0.008 µm (0.3 microinches). The specifications for roundness are strict to permit adequate diametrical clearances for lubrication and to minimize wear. There are ten femoral and acetabular component sizes are in two-millimeter increments: the acetabulum sizes are 46 to 64 mm and the femoral head sizes are 36 to 54 mm. All components are made of cast F-75 cobalt chromium molybdenum alloy that is heat-treated and solution annealed. Although the device is classified by the Food and Drug Administration as investigational, the initial multi-center investigational device exemption trial has been completed and the manufacturer has submitted an application for a PMA (pre-market approval). Implantations are now being performed in ten centers under "continued access" pending final approval.
Surgical Technique and Hospital Course
A detailed description of the technique and instrumentation has been published 15,17. Most of the modifications of the surgical technique for this cohort were made during the first 100 hips operated but the technique continues to be refined with new instrumentation to facilitate the procedure. Initially we used a standard posterior approach but changed to a hockey stick incision over time because of the musculature of our many active patients. The incision length has been minimized and depends on the size of the patient. The acetabulum is reamed to one millimeter under size and checked carefully for depth and roundness with gauges and placing the acetabular component at 25o to 30o of anteversion and 45o of abduction 17. The femoral component is aligned with the anatomic main axis of the femoral neck to avoid notching the neck, especially laterally, and to cover all of the reamed bone with the femoral prosthesis. 17 The target angle for the femoral component is now 140o with the femoral shaft17,18. Once the head is cylindrically reamed to size, the dome is removed with an oscillating saw, a tower alignment guide is applied to ream for the tapered stem, and the head is chamfered. The bone is meticulously prepared by removing all soft tissue from cysts and additional holes are placed in the dome and chamfered area to improve fixation. After jet lavage to clean the head, a femoral suction tip of the same dimensions as the metaphyseal stem is inserted into the head to suction out the blood prior to fixation with acrylic cement. All femoral components were cemented but only a small number of metaphyseal stems were cemented. Early in the series, the stem was cemented in fifteen hips because of severe neck osteopenia or large defects. In thirty-nine of the last forty-four cases of this series, the stem was routinely cemented to better evaluate the effects of cementation to improve initial fixation, and to evaluate any possible negative consequences such as stress shielding. Prior to closure a range of motion is performed and impinging bone is removed from the acetabular walls and occasionally the posterior trochanteric ridge which enables patients to regain and in occasional instances exceed pre-existing range of motion of the hip.
Post-operative Management
All patients have prophylactic antibiotics, adjusted low dose Warfarin, Indomethacin or 700 rads of radiation pre operatively to prevent sepsis thromboembolic phenomona and heteroptic bone formation. Ambulation begins on the first postoperative day, allowing weight bearing as tolerated, using crutches for 4-5 weeks. A cane is occasionally used for an additional 2-3 weeks. Sports are generally permitted at 3-6 months post-operatively17
Outcome Evaluation
The average follow-up is 3.5 years (range 2.8 - 6.8). Follow-up visits including hip range of motion and radiographic examination were scheduled preoperatively and postoperatively at three to four months, one year and at yearly intervals in which all patients were followed prospectively to evaluate pain, walking, function, and activity according to the UCLA hip scores 19, SF-12 20, and Harris hip score 21. Leg length discrepancy was assessed using blocks of different thickness placed under the patient's foot until the pelvis leveled. This measurement was performed preoperatively and at each visit after surgery. The majority (91%) of the patients were examined by the senior author in the Los Angeles clinic or in one of fifteen special clinics held annually in other cities in the United States. Online self-evaluation forms were submitted at the prescribed follow-up intervals and 8% of patients were clinically evaluated by local orthopedists who sent us radiographs for review. The patients were then contacted by phone to discuss their progress. Two patients (three hips) died at twenty-one and twenty-three months postoperatively of causes unrelated to the surgery. Only three patients (0.75%) have been lost to clinical follow-up. All of the patients had antero-posterior, modified table-down lateral, and Johnson cross-table lateral radiographs23 of the pelvis taken preoperatively and, where possible, during each follow-up. An independent reviewer (T.G.) evaluated the radiographic series on an annual basis.
Results
Clinical Results
The average duration of follow-up was three and a half years (range, 2.2 to 6.2 years). Clinical results (UCLA hip score, Harris hip score and SF-12 scores) are summarized below.
Table 2. Summary of the clinical results from UCLA hip scores, SF-12, and Harris hip scores (HHS). Average scores and (ranges)
The average Harris hip score was 93.5. Charnley class and Harris hip scores were associated with 95.2 average (range 61 to 100) for Charnley class A and 93.3 (range 66 to 100) for Charnley class B (p = 0.008). The average Harris hip score was 80.7 (range 41 to 100) for Charnley class C, which was inferior to both Charnley A and B (p=0.001). Postoperative SF-12 scores did not differ significantly from the average score of the general United States population matched for age (Physical = 50.01 9.69 and Mental = 53.10 9.40 20). The range of motion improved from a mean 85.5o (range 5o to 150o ) in flexion, 30.5o (range 0o to 90o) in abduction-adduction measured in extension, and 18.5o (range 0o to 85o) rotation arc measured in extension, to 122.0o (range 55o to 170o), 69.8o (range 25o to 130o) and 73.7o (range 10o to 125o), respectively.
Leg length discrepancy
Seventy-eight patients had leg length discrepancies preoperatively. Fifty-three were <1cm, sixteen were 1 to 2 cm, eight were 2 to 3 cm, and one was > 3cm. After surgery, only twenty-five patients had a leg length discrepancy: twenty-two had leg length discrepancies that were <1cm and three patients had a discrepancy of 1 to 2 cm but all were less than their preoperative levels.
Radiographic Results
Heterotopic Ossification
One hundred and six hips had some heterotopic bone (36% of the males and 12% of the females). The average pain score for this group was 9.4 (range 2 to 10), which did not differ statistically from the rest of the patient group. Brooker Grade III and IV bone was observed in twenty-eight hips, all in male patients (7% of the whole group, 9.5% of male patients). As a group these cases showed a decreased range of motion in flexion arc (mean 109.5 degrees (range 55 to140) vs. 121.9 degrees (range 85 to155) for the rest of the male patients, p = 0.001). All of the patients had a functional arc of rotation and abduction-adduction arc. After implementation of our radiation protocol for one stage bilateral patients, our overall rate of heterotopic ossification Brooker grade III or IV declined to 5.3% (three Brooker grade III cases in 56 hips).
Hip Biomechanics
The stem shaft angle increased (more valgus) significantly (p=0.001) between the first 100 (average 131.1 degrees, range 110 to 150) and the subsequent operations (average 137.8 degrees, range 111 to 153). The stem-shaft angle was significantly (p=0.0255) lower (more varus) for the patients revised for femoral loosening (average129 degrees, range 110 to 148) compared to the rest of the cohort (average 136 degrees, range 111 to 163). The stem-shaft angle was negatively correlated (r=-0.374, p<0.001) to the abductor moment arm. In Charnley class A patients, normal hip abductor mechanics were restored as denoted by a hip ratio that was similar between the operated side (average 0.584, range 0.40 to 0.83) and the contralateral, unoperated side (average 0.571, range 0.40 to 0.78) (p=0.193).
Acetabular Radiolucencies
Two hundred and sixty-one hips (67%) had no radiolucencies and 122 (32%) had radiolucencies in one or two zones. There have been no progressive radiolucencies in Delee and Charnley zones I or II.
Metaphyseal Stem Radiolucencies
According to our rating scheme, sixteen hips (4.2%) that have not been revised have femoral metaphyseal stem radiolucencies score of >=7. The average pain and activity scores of the group with a radiolucency score of >=7 were not statistically different from the rest of the cohort. Several factors correlated with femoral fixation scores and the relationship of the main clinical factors associated with the presence of radiolucencies is summarized below.
Table 3. Clinical variables in relationship with femoral radiolucencies greater or equal to 7 based on Cox multivariate proportional hazard model (n=384). Hazard ratio represents the increase in risk of apparition of a femoral radiolucency between the mentioned group and its counterpart for comparison between groups, and between one level and the next increment for continous variables. For example, females are 3.1 times more at risk of early development of a radiolucencythan males and, in males, patients 10kg lighter than others are 1.56 times more at risk.
N = 400
|
Mean
|
SD
|
Range
|
Age at surgery (years)
|
48.2
|
10.9
|
15 -77
|
Weight (Kg) Females/Males
|
68.6/88.6
|
12.7/16.4
|
45-107/57-164
|
Height (cm) Females/Males
|
165.2/178.4
|
6.8/7.2
|
148-183/157-198
|
BMI Females/Males
|
25.1/27.8
|
4.3/4.5
|
17.5-42.3/19.2-46.4
|
Male Patients
|
73%
|
-
|
-
|
Female Patients
|
27%
|
-
|
-
|
Charnley class A
|
49%
|
-
|
-
|
Charnley class B
|
44%
|
-
|
-
|
Charnley class C
|
7%
|
-
|
-
|
Etiologies
|
Percent
|
Osteoarthritis
|
65.6%
|
Osteonecrosis (14% ON Ficat III and 86% ON Ficat IV)
|
9.0%
|
Developmental dysplasia (77% Crowe class I 23% Crowe class II)
|
10.8%
|
Post-traumatic arthritis
|
7.8%
|
Legg Calve Perthes
|
2.5%
|
Slipped capital femoral epiphysis
|
1.8%
|
Ankylosing spondylitis
|
1.0%
|
Juvenile rheumatoid arthritis
|
0.8%
|
Rheumatoid arthritis
|
0.8%
|
Melorheostosis
|
0.3%
|
Previous surgeries
|
6.3%
|
Failed osteotomy
|
6
|
Failed coring
|
10
|
Failed hemisurface arthroplasty
|
2
|
Failed pinning
|
5
|
Failed Judet graft
|
1
|
Failed acetabular reconstruction
|
1
|
Copyright 2005 Joint Replacement Institute Los Angeles, CA · (213) 484-7600 · JRIinfo@dochs.org
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