In patients who are managed with resurfacing arthroplasty of the hip, the femoral head and neck are conserved and continue to be loaded by the prosthesis, thereby preventing stress-shielding and preserving the quality of bone in the proximal part of the femur1. Should failure of the femoral component occur, conversion to a total hip arthroplasty should thus be a straightforward procedure.
Since its inception, “revisability” has been claimed as one of the major advantages of resurfacing arthroplasty of the hip. However, to our knowledge, this theoretical advantage has never been clearly demonstrated. Earlier-generation resurfacing devices with a metal-polyethylene bearing frequently proved to be difficult to revise on the acetabular side, primarily because of the use of large, cemented acetabular components and osteolysis, which was frequently extensive secondary to polyethylene wear. To our knowledge, only three published reports have specifically addressed the revision of failed hip resurfacing procedures2-4. Unfortunately, they were published in the early 1980s and they lacked adequate operative details, outcomes measures, and comparative groups to permit meaningful analyses.
The objective of the present study was to test the hypothesis that a failed femoral component in a hip with a modern-generation metal-on-metal resurfacing arthroplasty can be easily and successfully converted to total hip arthroplasty. To date, the vast majority of resurfacing failures have been caused by either femoral-sided aseptic loosening or femoral neck fracture. Therefore, the present report addresses conversions performed for femoral-side failures.
Materials and Methods
Between May 1997 and October 2005, the senior author (H.C.A.) performed 844 primary metal-on-metal hip resurfacing arthroplasties and 157 primary total hip arthroplasties. All of the resurfacing procedures were performed with Conserve Plus devices (Wright Medical Technology, Arlington, Tennessee). The femoral head was cemented in all hips, but the metaphyseal stem was cemented in only 328 hips (38.9%). The total hip arthroplasty devices included 115 cementless stems (73%) (ATH; Kinamed, Camarillo, California) and forty-two cemented stems (27%) (CTN; Kinamed, or Perfecta; Wright Medical Technology). All femoral components articulated with cementless acetabular components. Twenty-three total hip arthroplasties (15%) (all of which were performed prior to the availability of cross-linked polyethylene) involved conventional polyethylene bearings (Interseal; Wright Medical Technology), sixty-four (41%) involved cross-linked polyethylene bearings (Trilogy/Longevity; Zimmer, Warsaw, Indiana, or Duraloc/Marathon; DePuy, Warsaw, Indiana), and seventy (45%) involved metal-on-metal bearings (Transcend or Conserve; Wright Medical Technology).
Twenty-one hips in twenty patients with an average age of 50.2 years (range, twenty-three to seventy-two years) underwent conversion from the metal-on-metal resurfacing device to a total hip arthroplasty. The indication for conversion surgery was a femoral neck fracture in five hips and femoral component loosening in sixteen (Figs. 1-A, 1-B, and 1-C). There were no failures of the acetabular component in these patients. In the group of patients who required conversion to a total hip arthroplasty, one patient with developmental dysplasia of the hip had had a periacetabular and proximal femoral osteotomy prior to the original resurfacing procedure. None of the other patients in the conversion group had had previous hip surgery. The average time between resurfacing and conversion surgery was 40.9 months (range, 1.4 to 99.8 months). In eighteen hips, the acetabular component was retained and the femoral component was revised to a stemmed femoral component with a large femoral head that matched the inner diameter of the cup. The three remaining hips required acetabular revision because a unipolar head (Big Femoral Head (BFH); Wright Medical Technology) matching the cup size was not available at the time of surgery.
During the same period, sixty-three patients (sixty-nine hips) who were younger than sixty-five years of age (average age, 50.8 years; range, twenty-seven to sixty-four years) underwent a primary total hip arthroplasty that was performed by the same surgeon at the same institution. The indications for primary total hip arthroplasty were either a denial from the patient's insurance carrier for resurfacing (because of the investigational device status of the implant during this period) or because the bone quality of the femoral head was thought to be compromised by either extensive osteonecrosis or cystic degeneration. In addition, some patients chose to have a traditional total hip arthroplasty instead of a resurfacing arthroplasty. To allow for a fair comparison between the groups with regard to operative time and blood loss, five patients (five hips) were excluded from the total hip arthroplasty group because they had either severe (Crowe5 type-III or IV) dysplasia or because they required structural bone-grafting or extensive hardware removal at the time of the primary total hip arthroplasty. This left sixty-four hips in fifty-eight patients. The comparative demographic data between the study and control groups are presented in Table I.
A posterior approach to the hip was used for all patients in the study and control groups. In the five patients in whom resurfacing arthroplasty failed because of a femoral neck fracture, the fracture was well proximal to the lesser trochanter and therefore did not adversely affect the optimal level for a neck osteotomy at the time of conversion. Stability of the acetabular component was tested at the time of conversion by applying a number of blows through a tamp onto the rim of the cup.
Postoperatively, patients in both groups were instructed to follow posterior hip dislocation precautions, which included no hip flexion past 120° and avoidance of a position of combined hip flexion, adduction, and internal rotation. Hip abduction pillows were not used routinely in either group. Patients were restricted to partial (25% to 50%) weight-bearing for one month and were told to use crutches during this time, but compliance with these instructions could not be verified.
The clinical outcome of the procedures was evaluated with use of the University of California at Los Angeles (UCLA) hip-scoring system6, the Harris hip score7, and the Short Form-12 (SF-12)8, which were administered at the time of the last follow-up visit.
The operative time, the estimated blood loss, all perioperative complications, and the length of the hospital stay were recorded. The reported operative time in both groups included the time for patient positioning (the time between the completion of induction of anesthesia and the application of the surgical dressing) and the time required to make intraoperative radiographs (to ensure the accuracy of limb-length restoration).
Radiographic assessment included low anteroposterior pelvic, modified table-down lateral, and frog-leg lateral views of the hip, made both immediately postoperatively and at the time of the final follow-up. We measured the femoral offset and the horizontal hip center of rotation as described by Silva et al.9 and measured limb length as described by Austin et al.10. The positioning of the stem in the frontal plane was recorded and classified into one of three categories: neutral, valgus, or varus. A stem was considered to be in neutral orientation whenever its main axis deviated from the femoral shaft axis by <5°. The fixation of the femoral component was assessed for each Gruen zone11 with use of the fixation score devised by Engh et al.12 for cementless stems. A similar evaluation was made for each DeLee and Charnley zone13 on the acetabular side. The radiographic data collection was performed by a single observer (S.T.B.) who was blinded to patient and group identity.
Student t tests were performed to establish the significance of the differences in average values observed between groups. The level of significance was set at p < 0.05.
The study was approved by the research committee/institutional review board of our institution.
The average duration of follow-up was forty-six months (range, twelve to 113 months) for the study group and fifty-seven months (range, twenty-four to 105 months) for the control group.
There were no significant differences between the study group and the control group with regard to the UCLA pain, walking, function, and activity scores; the SF-12 physical and mental component scores; or the Harris hip score. The average values of these tests for the overall study group and the control group are presented in Table II.
The average operative time was 178 minutes (range, 140 to 255 minutes) for the study group and 169 minutes (range, 110 to 265 minutes) for the control group. This difference was not significant (p = 0.263). The average intraoperative estimated blood loss was 509 mL (range, 100 to 1200 mL) for the study group and 578 mL (range, 250 to 1600 mL) for the control group (p = 0.314).
Acetabular revision was performed during only three of the twenty-one conversion procedures. These three stable cups had to be removed with use of curved osteotomes because, at the time of conversion surgery, sufficiently large prosthetic femoral heads were not available. In the remaining eighteen hips, the cup was well fixed and only the femoral side was revised. The operative time and blood loss for the three conversion procedures that involved cup revision were similar to those for the rest of the conversion procedures. The average acetabular bone loss in these three hips, as measured by the change in outer diameter from the original cup to the revision cup, was 4 mm (range, 2 to 6 mm). The average duration of hospitalization was 4.0 days (range, three to six days) for the study group and 4.2 days (range, three to eight days) for the control group (p = 0.479). A weak negative correlation (r = –0.223, p < 0.05) was found between the duration of hospitalization and how recent the surgery was, illustrating a decrease in hospital stay over time at our institution.
At the time of conversion surgery, visual inspection of the femoral and acetabular components revealed no appreciable metallic wear in any of the hips. Furthermore, osteolysis was not observed in any hip. Wear analysis, performed for fourteen of the twenty-one retrieved femoral components, demonstrated an average of 7 μm (range, 0 to 45 μm) of wear. Wear analysis was performed for only one retrieved acetabular component, which demonstrated 0 μm of wear. However, this component was revised just 2.1 months after implantation.
All of the revision cementless stems were integrated into bone, and the fixation scores showed no difference between the conversion group and the control group (p = 0.365). No radiolucent lines were recorded for any of the cemented stems in either of the two groups. There was no difference in limb-length discrepancy between the two groups (p = 0.705). Femoral offset and the horizontal position of the center of rotation of the hip were also similar between the two groups (p = 0.712 and p = 0.224, respectively). Eighty-one percent of the stems were in neutral position in the conversion group, compared with 84% in the control group. Two cementless stems in each group subsided (by 2 and 5 mm in the conversion group and by 2 mm each in the control group), but all had stabilized and had good fixation scores at the time of the latest follow-up. The acetabular fixation scores were also comparable between the conversion group and the control group (p = 0.279). All eighteen cups that were retained at the time of conversion surgery remained stable, with no migration and no progressive radiolucent lines, for an average of eighty-five months (range, twenty to 116 months) from the date of the primary resurfacing arthroplasty until the time of the latest follow-up. The three hips that underwent revision of both the acetabular and the femoral component also had stable implants with no signs of loosening.
Three patients in the conversion group experienced a complication. One patient had a femoral nerve palsy that completely resolved. One patient had an intraoperative, nondisplaced, proximal femoral fracture, which was treated with cerclage wires. One patient had a perioperative myocardial infarction.
There were six complications in the control group. There were three femoral nerve palsies, each of which also completely resolved. There were two periprosthetic femoral shaft fractures; one occurred intraoperatively and was treated with cerclage wiring, and one presented one month after surgery and was treated with open plate fixation and cerclage wiring. Last, there was one case of deep infection, which required a two-stage revision.
Current-generation metal-on-metal resurfacing arthroplasty of the hip is most commonly performed for younger, active adults, in whom conventional total hip arthroplasty has historically been associated with relatively low prosthetic survival rates14-18. In this high-demand patient population, the reported revision rates for contemporary resurfacing designs have been comparatively low, ranging from 0.02% to 3% at approximately three to five years of follow-up19-21. The most common mechanisms of failure have been aseptic loosening of the femoral component and femoral neck fracture. Acetabular-side failure is rare.
The theoretical advantage of easy “revisability” of a resurfacing arthroplasty is supported by our results. With the numbers studied, the procedure was similar to primary total hip arthroplasty in terms of operative time and blood loss. The radiographic results after conversion were also similar to those in the primary total hip arthroplasty group in terms of osseointegration, the position of the center of rotation, femoral offset, stem position, and limb-length equality.
The postoperative clinical outcomes also were similar for the conversion group and the primary total hip arthroplasty group. At an average duration of follow-up of approximately three and one-half years, all patients in the conversion group had a good or excellent outcome as measured with the Harris hip score and SF-12 scores. Furthermore, these patients maintained a high level of activity, with an average UCLA activity score of 6.8 and with ten of the twenty patients continuing to participate in sports regularly.
Historically, the most common complication of conversion hip surgery or revision total hip arthroplasty has been dislocation. The rate of dislocation has been reported to be approximately 10% among patients who have been managed with conversion of a hemiarthroplasty to a total hip arthroplasty22, and it has been reported to be 2% among those who have been managed with total hip arthroplasty after a previous periacetabular osteotomy23. In a recent large series, the dislocation rate following revision total hip replacement was 7% to 11%24. In contrast, in the present series of conversions of failed femoral resurfacing arthroplasties, there were no dislocations and no infections. In our patients in the conversion group, the absence of postoperative instability was likely related to the large femoral head size (average, 43.5 mm), as the increased stability of large femoral heads in the revision setting has been clearly established by a number of previous studies25-28. The results of the present study were very encouraging; nonetheless, they will need to be confirmed in a larger series of patients with a longer follow-up.
All four of the femoral nerve palsies in our series were related to the use of a lateral hip decubitus positioning system with anterior pads that were positioned laterally against the anterior superior iliac spine and medially against the pubis. The lateral pad often slipped during the procedure and pressed against the femoral triangle. The device was initially attractive because it was translucent to x-rays, but, because of the neurologic complications, its use was discontinued. We now stabilize the pelvis with a padded support on the pubis, the sacrum, and the anterior and posterior aspects of the thorax.
After resurfacing arthroplasty, normal stresses and bone density are maintained in the proximal part of the femur1,29. For this reason, conversion to a total hip arthroplasty is similar to primary total hip arthroplasty with regard to achieving osseointegration of a cementless stemmed prosthesis, and all twenty-one conversion stems in the present series were integrated into bone. In contrast, femoral-side revision of failed conventional total hip arthroplasty can be technically demanding and the results are less predictable, with increased rates of dislocation, infection, and aseptic loosening30-33.
With the current generation of resurfacing implants, the metal acetabular shell is thin, and, therefore, minimal amounts of acetabular bone are removed at the time of the index procedure, similar to a traditional total hip arthroplasty. The fixation of this porous-coated acetabular component has performed as well as similar porous-coated components that have been used for decades for total hip arthroplasty34. Furthermore, osteolysis is rare in association with metal-on-metal bearings and was not an issue in this short-term series of hip resurfacing. Three acetabular components required revision not because of loosening, but because the conversion surgery predated the availability of matching large femoral heads to mate with the existing shell. It is important to note that in those hips, an average of 4 mm of bone was lost.
The present study had two primary limitations. First, the duration of follow-up was relatively short. This was unavoidable because the first implantation of current-generation resurfacing devices was performed only nine years ago. A study of the conversion of earlier-generation resurfacing devices would allow a longer follow-up period but would not be relevant to today's designs. The second shortcoming was the relatively small number of patients. Fortunately, this was because the current-generation devices are performing well, with only a limited number of revisions to date. Despite these limitations, the current study supports the premise of the easy and safe revisability of metal-on-metal resurfacing arthroplasties of the hip. At the time of early follow-up, the clinical outcomes were excellent after conversion. In order to determine the long-term efficacy, additional follow-up is still required.
In conclusion, conversion of a femoral-side failure of this current-generation metal-on-metal resurfacing arthroplasty design to a total hip replacement can be performed with a technical effort similar to that for a primary total hip replacement. The short-term clinical outcomes of these conversions were similar to those of conventional primary total hip arthroplasty. The resurfacing acetabular component has performed well and is unlikely to require revision during conversion for a femoral-side failure. Our early experience supports the concept of “revisability” as an advantage of the current generation of hip resurfacing. ▪
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Wright Medical Technology. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Wright Medical Technology). Also, a commercial entity (Wright Medical Technology) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Joint Replacement Institute at Orthopaedic Hospital, Los Angeles, California
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