Metal-on-metal hip resurfacing has recently been introduced into the U.S. market, and increasing numbers of these procedures have been performed each year. In Australia1, hip surface replacement has been reported to account for 7.5% of all primary total hip arthroplasties in patients of all ages and 17.7% of all total hip procedures in patients under the age of sixty-five years. Prior experience with hip resurfacing suggests that caution should be exercised when introducing newer versions of this procedure into the U.S. market2. Careful assessment of this technology with comparison with accepted contemporary standard total hip arthroplasties appears to be prudent.
Currently, two hip-resurfacing devices have been approved for the U.S. market. The Birmingham Hip Resurfacing System (Smith and Nephew, Warwick, United Kingdom) was approved in May 20063 on the basis of the results of a single-center UK study. The Cormet 2000 Hip Resurfacing System (Corin, Cirencester, United Kingdom) was approved in July 20074 on the basis of the results of a multicenter U.S. Investigational Device Exemption study, which began in 2001. A ceramic-on-ceramic total hip replacement system (Osteonics ABC System; Stryker, Mahwah, New Jersey), approved in February 20035, served as the historical control in that study. The Cormet Hip Resurfacing device was introduced at a time when patient selection criteria and technical features of successful implantation were incompletely understood. Thus, we thought that the results of this Investigational Device Exemption study might provide helpful insights for surgeons regarding the criteria for successful introduction of this procedure into the United States.
The purpose of the Cormet Investigational Device Exemption study was to establish the safety and efficacy of this hip resurfacing prosthesis by demonstrating non-inferiority in comparison with an approved ceramic-on-ceramic total hip arthroplasty system. A secondary purpose was to evaluate the types and prevalences of adverse events that might be expected with hip resurfacing and ceramic-on-ceramic total hip arthroplasty devices that were both intended for the treatment of younger, more active patients with degenerative hip disease.
Study Design and Subject Population
A premarket, prospective, multicenter Investigational Device Exemption study was conducted in the United States in order to demonstrate the safety and efficacy of the Cormet 2000 Hip Resurfacing System. Enrollment of the pivotal study group was initiated in May 2001 and completed in August 2003. Of 337 patients enrolled, sixteen had a revision less than twenty-four months postoperatively, one died less than twenty-four months postoperatively, and twenty-eight were lost to follow-up before twenty-four months. This left 292 patients (86.6%) with the original resurfacing device in place to be evaluated at a minimum twenty-four months postoperatively.
A premarket, prospective, randomized, multicenter Investigational Device Exemption study of the ceramic-on-ceramic Osteonics ABC System was performed, with the enrollment phase beginning in October 1996 and ending in October 1998. The Osteonics ABC System I or System II was used in 266 patients with unilateral hip disease. Three patients had a revision less than twenty-four months postoperatively, two patients died less than twenty-four months postoperatively, and five were lost to follow-up. This left 256 patients (96.2%) with the original prosthesis in place available for evaluation at a minimum twenty-four months postoperatively, and these patients served as the control group for the study of the Cormet Hip Resurfacing System.
Table I shows a comparison of the elements of the two study designs. The core data collected from these studies were the same. In addition, the follow-up time points were similar.
A patient could be included in the resurfacing study if he or she had end-stage hip disease and was skeletally mature, mentally capable of participating in the postoperative follow-up evaluation, available for two-year postoperative follow-up, and deemed a candidate by the investigator. A patient was excluded if the patient was a prisoner, was pregnant, had a known allergy to metals, had had a previous ipsilateral hip operation, had had an above-the-knee amputation of either extremity, had a diagnosis of developmental dysplasia of the hip, or had a preoperative Harris hip score6 of =70 points. The criteria for inclusion in the standard total hip arthroplasty cohort were very similar to those for inclusion in the resurfacing cohort with the exception that the patient age was limited to between twenty-one and seventy-five years, there was no maximum preoperative Harris hip score, a diagnosis of developmental dysplasia of the hip was permitted, and above-the-knee amputation and neoplastic disease were not exclusion criteria.
Implants
The Cormet prosthesis is a third-generation resurfacing device for which hybrid fixation is used. It comprises two components—a resurfacing head and a monoblock acetabular component. The acetabular component is manufactured from cast cobalt-chromium-molybdenum alloy and is finished with a bicoat consisting of hydroxyapatite on plasma-sprayed unalloyed titanium, which provides the bone-ongrowth surface on the back of the component. The acetabular component is intended for use without bone cement and is available in nine sizes (diameters) in 2-mm increments: 46 through 62 mm. The femoral component, intended for use with bone cement, is manufactured from cast cobalt-chromium-molybdenum alloy. At the time of this study, this component was available in five sizes (diameters) in 4-mm increments: 40 through 56 mm. Currently, it is available in 2-mm increments: 44 through 56 mm.
The Osteonics ABC implants consisted of two cup designs (Systems I and II), both of which had ceramic-on-ceramic bearings. System I comprised a porous-coated titanium shell and an alumina bearing couple (ABC) insert. System II had an arc-deposited titanium shell with hydroxyapatite coating and an ABC insert. All patients received the same femoral stem (Omnifit HA; Stryker, Mahwah, New Jersey).
Propensity analysis was used to assess the comparability of the patient populations of the two cohorts. The model included age in decades, weight in 40-lb (18.1-kg) increments, male sex, the baseline total Harris hip score in 10-point increments, and having a baseline Harris hip score pain category of "marked pain" or "totally disabled." The analysis demonstrated that any between-group difference in patient populations would not affect conclusion of non-inferiority in comparison with the standard total hip arthroplasty system.
Data Collection
Baseline demographics included patient age, sex, weight, and diagnosis, which are summarized in Table II. The preoperative clinical evaluation was conducted by the surgeon, and the Harris hip score form was completed for each patient. The two cohorts were similar except for a slightly increased mean age in the total hip arthroplasty cohort. The surgical information that was collected included the surgical approach, sizes of the implant components, operative time, and method of anesthesia.
Postoperative evaluations were completed at six weeks, six months, one year, two years, and annually thereafter. Evaluations were based on the Harris hip score and anteroposterior and lateral radiographs. Radiographic evaluation, performed by an independent reviewer, consisted of assessment of acetabular7 and femoral8 radiolucencies by zone, acetabular migration, femoral migration, femoral subsidence and tilt, and osteolysis.
Adverse events were collected according to the definitions of the International Conference on Harmonization Good Clinical Practice Program9. This analysis included a summary of baseline and patient demographics, Harris hip scores, radiographic findings, hip and device-related adverse events, and revisions in the resurfacing and control patient groups. In order to demonstrate non-inferiority of the hip resurfacing prosthesis, relative to the total hip prosthesis in terms of clinical success at a minimum of twenty-four months, a newly proposed performance standard, the composite clinical success (CCS) score, was used. The four components of the CCS score are a Harris hip score of =80 points, radiographic evidence of success, absence of device-related complications, and absence of revision. Each procedure was rated as either a success or a failure on the basis of the four components of the CCS score. The proportion of successful procedures was compared between the resurfacing group and the matching cohort. For the resurfacing study, the non-inferiority delta was equal to 0.08, with the aim of demonstrating that the proportion of CCS scores in the resurfacing group was no more than 8% worse than that in the comparison group. The null hypothesis, that the investigational device is inferior to the control device, is rejected if the lower bound of the confidence interval is larger than -0.08.
Table III provides an overview of the Harris hip scores at a minimum of twenty-four months. Both the resurfacing and the control group had an excellent mean total score (=90 points), and the two groups had similar percentages of Harris hip scores indicating success (=80 points). The functional outcomes, as measured with the Harris hip score, were statistically similar at the two-year time point10.
Radiographs of 281 subjects in the resurfacing group were available for review at a minimum of twenty-four months, and the findings have been reported in a previous publication11. To summarize, ten patients had radiographic evidence of instability of the femoral component at a minimum of twenty-four months. Femoral subsidence of =5 mm was identified in these patients, with one of them also exhibiting radiolucency in all three zones around the femoral component. No progressive zonal radiolucencies were found, and no radiograph demonstrated failure of the acetabular component.
Operative-Site-Related Complications
Operative-site-related complications were reported in 115 patients in the resurfacing group; there were eighty-three hip-related complications and thirty-two device-related complications. The control group had a similar number of operative-site-related complications: 102, with eighty-one hip-related complications and twenty-one device-related complications. At the time of the latest follow-up, twenty-four revisions had been done in the resurfacing group (sixteen prior to the twenty-four-month time point and eight in the follow-up time period) and five revisions had been done in the control group (three prior to the twenty-four-month time point and two in the follow-up time period). The results confirm that the resurfacing device showed "non-inferiority" in comparison with the control device (Tables IV and V).
To our knowledge, this study is the first in the literature to compare the results of a multicenter U.S. study of a total hip resurfacing system with the results of a ceramic-on-ceramic standard total hip replacement in a matching group. Both groups had a hard-on-hard bearing surface without the risk of issues associated with polyethylene debris. In addition, excellent results have been reported following use of the total hip arthroplasty device5,12,13, making the patients treated with that implant an appropriate group for comparison with the resurfacing cohort.
As both studies were prospective, multicenter, monitored, premarket evaluations of new devices, the number and frequency of adverse events might be expected to be higher than those typically reported in postmarket studies. Hip-related complications were the most frequently reported postoperative adverse events concerning the hip or operative site. Eighty-three (24.6%) of the patients in the resurfacing group had a hip-related adverse event, a rate similar to that in the patients treated with the total hip arthroplasty (30.5%) (p = 0.118).
Femoral neck notching is associated only with the investigational resurfacing device since the femoral neck is removed as an early step in total hip arthroplasty. Neck notching has been reported in the literature8; however, very few rates of this event have been published. Notching was identified in sixteen (25%) of sixty-three patients in one series14, and it was reported in twenty-six (52%) of fifty cases in another15. Femoral neck notching is known to be dependent on the surgical technique, and the rate might be expected to be higher in a multicenter trial such as this one. Neck notching is thought to be a problem as it may be a risk factor for future femoral neck fracture. However, none of the four patients in this series who reportedly had intraoperative femoral neck notching went on to have a femoral neck fracture later in their treatment course.
A weakness of this study is that the two Investigational Device Exemption studies were not conducted by the same sponsor. The comparison group's higher rates of specific adverse events common to both procedures (wound-related and soft-tissue-trauma perioperative events) may have been due to study-specific reporting requirements and not to the surgical procedures themselves. Nevertheless, there were comparable rates of typically reported operative-site-related events such as bursitis, hip pain, heterotopic bone formation, and deep joint infection. Events more common to the total hip arthroplasty group included intraoperative chipping of the ceramic insert (p = 0.007), postoperative femoral fracture (p = 0.007), and dislocation (p = 0.025). Chipping and breakage of the ceramic insert during surgical impaction is a known risk related to devices with this bearing couple. The devices are usually replaced intraoperatively with no known clinical sequelae. Six cases of such chipping and breakage were reported in this study.
The rate of dislocation in the resurfacing group was significantly lower than that in the control group. This difference highlights the benefits of using a large-diameter head, which is one feature of resurfacing devices. Ceramic heads that ranged in size from 28 to 32 mm were used in the comparison group. In contrast, the head sizes ranged from 40 to 56 mm in diameter in the resurfacing group. It has been well established that an increase in head size decreases the dislocation rate by increasing the range of motion available prior to impingement, and increasing jump height16-18. In two other publications, the dislocation rate following resurfacing was reported to be 0.75%8,19, which was similar to the results in the current study.
Periprosthetic femoral fracture is more common following total hip arthroplasty than it is following resurfacing arthroplasty, and that difference was found in the comparison of these two groups. The group treated with the standard total hip arthroplasty had a rate of femoral fracture (intraoperatively and postoperatively) of 2.3%, whereas no periprosthetic femoral fracture occurred in the resurfacing group. This difference can be attributed to the nature of the implant design. Total hip arthroplasty relies on a large metal femoral component that cannulates the femur and extends into the diaphysis. Resurfacing, however, is done with a small femoral stem that extends only into the femoral neck. Femoral fracture is a serious device-related adverse event that must be corrected surgically with additional fixation and/or revision of the component to a longer stem that bypasses the fracture. This risk is avoided by the nature of resurfacing femoral component designs.
Device-related events that were more common in the hip resurfacing group included femoral neck fracture (p = 0.002) and femoral component loosening (p = 0.007). Femoral neck fracture is a known mechanism of failure of hip resurfacing devices. In this study, the rate of postoperative femoral neck fracture was 3.3%. While this rate is higher than that in a recently published review of fifty cases from the Australian registry1, the registry-study conclusions are relevant to our study results. The Australian registry study indicated that older, overweight women had a higher risk of fracture. The Australian registry study also linked femoral neck fracture to superior neck notching at the time of implantation and placement of the femoral component in >5° of varus. Thus, surgical technique and patient selection are important factors influencing the rate of this complication. The surgeon's learning curve has also been reported to be a factor in the rate of femoral neck fracture20.
Femoral component loosening is also more prevalent after resurfacing than it is after total hip arthroplasty, although it may be observed in only a small percentage of patients. The small and minimally penetrating femoral stem is seated on a resurfaced native femoral head and secured with bone cement. In total hip arthroplasty, the larger femoral component is wedged into the cortical bone of the metaphysis and extends down the diaphysis. In this study, the resurfacing group showed a 3.9% rate of femoral component loosening while none of the femoral components used in the total hip arthroplasties loosened in the minimum two-year time frame.
Overall, the rates of device-related adverse events were similar between the investigational (9.5%) and control (7.9%) groups in this study. The types of events differed between the groups, however, as the investigational group exhibited higher rates of femoral neck fracture and component loosening, while the control group had higher rates of periprosthetic femoral fracture and dislocation. The notably higher revision rate in the investigational group may be explained by a variety of factors. The ceramic-on-ceramic control devices were implanted by a group of orthopaedic surgeons who were highly trained in the performance of total hip arthroplasty. However, the hip resurfacing procedures as well as the investigational devices were new to each of the investigators in the resurfacing study, since no hip resurfacing system had been approved for the U.S. market at the time of enrollment into this Investigational Device Exemption study. Rigorous patient-selection criteria and extensive technical training were not features of this investigation.
Female sex, component size, and a lower preoperative Harris hip score were found to be significant risk factors for revision. Thirteen (12%) of the 109 female patients in the investigational group had a revision compared with eleven (4.8%) of the 228 male patients (p = 0.03). Similarly, thirteen (17%) of the seventy-eight patients with a 40 or 44-mm femoral component had a failure compared with eleven (4.2%) of 259 patients with a 48, 52, or 56-mm component (p = 0.001). A decrease of 5 points in the preoperative Harris hip score increased the risk of fracture by a factor of 1.32 (p < 0.0001).
In conclusion, a new hip resurfacing device implanted with a new surgical technique was compared with proven hard-bearing total hip arthroplasties in this Investigational Device Exemption trial. While the types of hip-related and device-related complications differed between the two procedures, the overall frequency of complications did not. Hip resurfacing appears to be an appropriate alternative to the ceramic-on-ceramic control device for properly selected younger, more active patients. Patient selection and technical consistency must be improved to achieve even higher success rates.