A seventy-two-year-old woman was seen at our clinic in November 1996 because of pain in the right thigh. She had been managed with a total hip arthroplasty with a Zweymüller metal-on-metal prosthesis at another institution in November 1995. The acetabular component was a Weber Metasul cup (Sulzer Medica) made of cobalt-chromium-nickel-molybdenum alloy and polyethylene, and it was fixed with cement. The femoral component was a Zweymüller stem (AlloPro AG) made of Ti-6Al-4V, and it was fixed without cement. The radiograph showed that the inclination angle of the socket was 47 degrees and the stem was inserted in a neutral position. The socket was in minimum anteversion or retroversion (Fig. 1). There was a demarcation line that was more than one millimeter wide in zone 1 of DeLee and Charnley3. The apparent radiographic density of the proximal aspect of the femur was relatively low, which was consistent with stress-shielding. Osteolysis was suspected in the calcar and the greater trochanter. The right lower limb was about two centimeters longer than the left lower limb, and the right hip had a 10-degree abduction contracture.
The patient had had pain in the right thigh since the time of the total hip arthroplasty. It was not relieved by nonoperative treatments, such as administration of nonsteroidal anti-inflammatory medications. We performed a revision total hip arthroplasty with cement on March 5, 1997. During the operation, we found that the joint space was stained black and the acetabular component was loose. After removal of the component, we discovered that the acetabular bone-cement interface was also stained black. The stem was rigidly fixed in the femur distally, and the black staining was found only in the region of the calcar, where osteolysis was observed. The black-stained tissue and all of the components were removed. The hip joint was replaced with a total hip prosthesis consisting of a twenty-two-millimeter alumina ceramic head and a polyethylene socket that was fixed with cement. At four months after the operation, the patient no longer had pain in the right thigh.
Retrieval Study
A groove that was 11.7 millimeters long, 2.5 millimeters wide, and 1.0 millimeter deep was observed on the anteroinferior aspect of the neck of the femoral component (Fig. 2). The groove corresponded to the position of the impingement between the socket and the femoral neck during maximum flexion of the hip. Although there was no apparent damage to the metal cup, a thin scratch was identified in the polyethylene just lateral to the metal bearing; the scratch corresponded to the position of the impingement as well.
Histological Study
Histological examination of the pseudocapsular tissue around the hip prosthesis revealed numerous multinucleated giant cells and metallic particles (Fig. 3). Analysis of the black pseudocapsule by scanning electron microscopy with electron-probe microanalysis revealed particles of titanium (Fig. 4), but cobalt and chromium were not detected. Therefore, we concluded that the main source of the metallic debris was the femoral stem made of titanium alloy.
Despite some reports of short-term failures1, many surgeons have been surprised to find excellent results fifteen to twenty years or more after use of the McKee-Farrar prosthesis (Howmedica International, Limerick, Ireland)5,8. Ray8 reported loosening of only one femoral component in a study of seventeen patients who had been followed for more than twenty years. In a comparative survivorship analysis of 177 consecutive total hip arthroplasties (in 169 patients who were followed for an average of twenty years [range, nineteen to twenty-one years]), Jacobsson et al.5 found a 77 percent rate of survival without aseptic loosening for the McKee-Farrar prosthesis and a 73 percent rate for the Charnley prosthesis (Chas. F. Thackray, Leeds, United Kingdom). Those authors5,8 did not report any progressive granulomatous lesions with metallic particles (metallosis) or wear of the cup. The wear of a well functioning metal-on-metal prosthesis is very low compared with that of a prosthesis with a metal-on-polyethylene bearing6. Weber10 stated that observations such as low wear rates and some excellent long-term results were the main motivation for the development of the Metasul metal-on-metal bearing.
However, some studies of retrieved metal-on-metal prostheses have revealed evidence of impingement of the neck on the socket. McKellop et al.6 reported such impingement in six (29 percent) of twenty-one hips that had been managed with one of three types of metal-on-metal prostheses: the McKee-Farrar, Müller, and Ring prostheses, which are all made of cast cobalt-chromium-molybdenum alloy. Willert et al.12 reported impingement between the neck and the socket in two of nineteen hips in which one of these three types of metal-on-metal prostheses had been used. Amstutz and Grigoris1 noted that the tissue discoloration that was observed during revisions of metal-on-metal implants could have been the result of impingement or motion at the implant-cement surface, or both. However, there was usually little damage to the implants, as long as twenty-five years after implantation in many instances. It was thought that the damage was not serious because both components were made of the same cast cobalt-chromium-molybdenum alloy and, often, the impingement was believed to have occurred after loosening of the socket6.
In the patient described in the present study, the cobalt-chromium-nickel-molybdenum alloy of the bearing surface of the cup was much harder than the Ti-6Al-4V alloy of the femoral component. Furthermore, the edge of the bearing had a thin, sharp configuration. These factors are considered to be the main causes of the notch in the neck and the severe titanium metallosis that developed within a short period after implantation. The study of the retrieved implants showed that the notch in the neck was deep enough to scrape the polyethylene outside of the metal cup. We believe that, if the impingement had continued for a longer period, the notch would have become larger and would have resulted in a larger amount not only of metallic debris but also of polyethylene debris.
Dorr et al.4 reported one case of metallosis after recurrent dislocation of a Metasul metal-on-metal prosthesis. That patient elected to have a revision operation three years after the initial implantation for the treatment of the recurrent dislocation. At the time of the operation, impingement was evidenced by a notch in the femoral neck and the tissues were stained black with metallic debris. The notch was clearly evident on the lateral radiograph of the neck, as shown in Figure 4 of their article.
Our patient had no history of dislocation or any other major complication. The pain in the thigh was considered similar to the pain in the thigh that occurs in association with other stems that are fixed without cement. If the patient had not had this pain, we would not have revised the hip until we found radiographic evidence of metallosis or other notable wear due to impingement. Furthermore, metallosis itself usually does not cause pain7. However, the titanium alloy contains aluminum and vanadium and, although these are considered to be essential elements present in the body tissue and fluid, an excessive amount can lead to a clinical abnormality. Aluminum has been implicated in osteomalacia, microcytic anemia, and neurological disorders, including encephalopathy, as has been well established in patients who have renal failure and are managed with hemodialysis9,11,13. Vanadium, which is in the most toxic oxidation state (+5), is more toxic than nickel and chromium, and it accumulates in the kidney, liver, bone, and spleen2,14.
The complication that we described can occur anytime, without symptoms or associated complications, after a hip arthroplasty with a metal-on-metal prosthesis. The anteroposterior and lateral radiographs of the hip should be examined carefully during the follow-up evaluation to determine whether a notch is present in the neck and to find radiographic evidence of metallosis. The range of motion between this twenty-eight-millimeter-head and neck combination and the Weber Metasul cemented cup is approximately 102 degrees. Although we can make every effort to prevent impingement by positioning the components properly during the operation, it is impossible to avoid this complication in every patient, especially in one who had a good preoperative range of motion. We concluded that the presence of metallosis should be suspected even in asymptomatic patients who have a prosthesis with a Metasul bearing, and we are concerned regarding the use of titanium in the manufacture of this implant.