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© 1999 The Journal of Bone and Joint Surgery, Inc. Catastrophic Failure of a Cemented, Collarless, Polished, Tapered Cobalt-Chromium Femoral Stem Used with Impaction Bone-Grafting. A Report of Two Cases* , NEW YORK, N.Y.Investigation performed at the Musculoskeletal Research Center, Department of Orthopaedic Surgery, New York University Medical Center-Hospital for Joint Diseases, New York City
Revision total hip arthroplasty performed with use of impaction bone-grafting and insertion of a tapered femoral component with cement in patients who have severe bone loss in the proximal part of the femur has yielded encouraging short-term results4,9,10,13,16,17,20. The collarless, polished, tapered (CPT) femoral stem (Zimmer, Warsaw, Indiana), which was designed for this purpose, is manufactured from a high-strength material (forged cobalt-chromium) in order to avoid the substantial rate of stem fracture that has been observed in association with a component of similar geometry, the original Exeter femoral prosthesis (Howmedica, Rutherford, New Jersey)14. Femoral stems made of forged cobalt-chromium-molybdenum alloy rarely fracture; the only such failures of which we are aware were attributed to defects that had been introduced during the manufacturing process18,22. In the original series of 433 polished, stainless-steel Exeter stems, which were implanted between 1970 and 1975, the rate of fracture was 3 percent (thirteen stems) and the fractures were attributed to the use of inferior materials and the thin distal aspect of the stem14. We are not aware of any reported fractures of the CPT femoral stem, despite its original highly tapered design. The purpose of this report is to describe the catastrophic failure of two cemented CPT femoral stems that had been used in conjunction with the impaction bone-grafting technique. Both failures apparently were related to implant design and operative technique rather than to defects introduced during the manufacturing process. The results presented here are important because more orthopaedic surgeons are utilizing this technique to address deficiency in the proximal part of the femur in patients managed with revision total hip arthroplasty.
CASE 1. A fifty-nine-year-old woman who had a history of degenerative arthritis of the hip secondary to slipped capital femoral epiphysis was managed with a bipolar arthroplasty with cement in 1981 and conversion to a total hip replacement with cement in 1985. The patient was 170 centimeters tall and weighed 100 kilograms at the time of the original procedure. In 1989, the femoral component was revised, because of loosening, with a long-stem component that was inserted with cement; the acetabular component was well fixed and was not revised. A third revision was performed, in 1992, because of aseptic loosening of both components. The femoral stem was revised with a proximally porous-coated, press-fit, bowed, long-stem component that had a distal diameter of eighteen millimeters, and the acetabular component was revised with a porous-coated, press-fit, hemispherical component. In February 1995, a fourth revision was performed because of pain in the thigh and groin due to aseptic loosening of the femoral component; the acetabular component was found to be well fixed, with minimum polyethylene wear. The proximal femoral bone stock was deficient medial and anterior to the subtrochanteric region. The proximal part of the femur was reconstructed with wire mesh, a 3.5-millimeter reconstruction plate, and a lateral onlay femoral strut allograft that was secured with cerclage wires. The femoral component was revised, without trochanteric osteotomy, with use of impaction bone-grafting (that is, with the graft extending beyond the tip of the implant) and insertion of a CPT femoral prosthesis (size C) with CPT instruments. Three years after the fourth revision, in 1998, the patient was pain-free, had a persistent Trendelenburg gait, and walked with a cane. Radiographs of the hip revealed five millimeters of subsidence of the femoral stem, which was essentially unchanged from that observed one year postoperatively, and proximal bone resorption about the lesser trochanter. One month later, the patient had an acute onset of pain in the thigh and buttock; she recalled no history of trauma. The pain was unrelieved by medication and rest. Physical examination revealed tenderness and swelling in the thigh and hip and a painful range of motion of the hip. Radiographs of the hip showed no changes (Fig. 1-A) compared with previous radiographs, and the symptoms were attributed to overexertion. The patient was managed with rest and acetaminophen.
Two weeks later, the symptoms increased and the patient needed crutches in order to walk. Physical examination revealed increased tenderness and swelling in the thigh. Repeat radiographs of the hip showed catastrophic failure of the femoral component (Fig. 1-B). Intraoperative findings revealed that the prosthesis and the femur were fractured at the same level, that the lateral femoral strut allograft was intact and had united to the distal part but not to the proximal part of the femur, and that the distal portion of the broken stem was well fixed in the cement mantle and the impacted bone graft. There was no incorporation of the impacted allograft proximally. The hip was revised with a proximal femoral allograft and a long-stem femoral component that was inserted with cement. At the time of the four-month follow-up, the patient was walking with crutches. Radiographs did not show loosening of the femoral component.
The retrieved component was cleaned of loose debris and was visually inspected under a dissecting microscope. The appearance of a rough fracture surface near the medial side of the stem as well as the appearance of a series of increasingly larger concentric ridges emanating from the site of fatigue failure indicated that the fracture had initiated at the lateral aspect of the stem. Scanning electron microscopy (model JSM T300; JEOL Technics, Tokyo, Japan) and metallographic analysis confirmed the forged structure of the stem and revealed no evidence of structural defects or abnormalities (fatigue-crack initiators such as notches, inclusions, or microporosity) introduced during the manufacturing process that could otherwise explain the fracture of the stem. CASE 2. A fifty-one-year-old woman who was 165 centimeters tall and weighed fifty-nine kilograms sustained a fracture of the left femoral neck in 1973. The fracture initially was treated with open reduction and internal fixation. Subsequently, avascular necrosis developed. A proximal femoral osteotomy was performed in 1975, and a primary total hip arthroplasty was done in 1986. Two subsequent revisions were performed because of aseptic loosening; the most recent of these, done in 1990, involved the insertion of a porous-coated anatomic (PCA) femoral component (Howmedica) without cement. This femoral component subsequently loosened aseptically and was revised, in 1994, with use of the impaction bone-grafting technique and insertion of a CPT prosthesis with cement. Two years later, the patient was pain-free but had persistent weakness of the hip in flexion and abduction. Radiographs of the hip revealed an adequate position of the stem, a uniform cement mantle distally, and incorporation of the impacted allograft at the distal aspect of the stem only (Fig. 2-A). Three months later (two years and three months after the insertion of the CPT stem), the left hip buckled and the patient fell while walking in her home. Radiographs of the hip revealed a periprosthetic fracture with bending of the CPT femoral component (Fig. 2-B). At the time of the revision, the femoral stem was well fixed distally and there was no evidence of incorporation of the impacted allograft proximally. The hip was revised with use of a proximal femoral allograft and a long-stem femoral component that was inserted with cement. At the time of the sixteen-month follow-up, the patient was walking without assistance and radiographs showed no signs of loosening of the femoral component.
A thin, tapered femoral component made of polished stainless steel (Exeter hip system; Howmedica) was developed in 1970 for use in primary total hip arthroplasties performed with cement14,21. In the initial series, 3 percent (thirteen) of 433 such stems fractured after a minimum duration of follow-up of twenty years14. These fractures were attributed to both the stem design and the low yield strength of stainless steel. A matte surface was added in 1976 in an attempt to increase the strength of the stem. Five thousand Exeter stems with such a surface were inserted between 1976 and 1984, with a resultant decrease in the fracture rate to 0.22 percent (eleven stems)14. However, the matte finish was associated with an increased rate of endosteal osteolysis, which was believed to be due to particulate debris generated by the surface. In 1984, the implant material was changed to a stronger forged stainless-steel alloy (Orthinox; Howmedica), and to our knowledge no stem fractures have been reported since this change. The CPT femoral stem, which is made of a high-strength forged cobalt-chromium alloy, was designed in an effort to avoid the substantial rate of stem fracture seen with the original tapered Exeter stems. The mechanism of failure in our first patient (Case 1) is believed to have been cantilever bending secondary to good distal fixation in the presence of poor proximal bone support resulting from poor proximal incorporation of the impacted allograft. Cantilever bending has been well documented as a cause of failure of cast cobalt-chromium and stainless-steel stems1-3,6-8,11,12,15,19 as well as of forged cobalt-chromium stems with obvious manufacturing defects secondary to laser etching22 or large variations in grain size18. In the present report, we describe a fatigue fracture that occurred in a forged cobalt-chromium stem that did not have an obvious manufacturing defect at the site of failure. Fractures of the femoral stem have been well documented and may be attributed to one factor or a combination of factors7,12, including (1) high stresses in the stem due to increased patient weight, a high level of activity, or a relatively undersized prosthesis; (2) poor proximal bone support or fixation, which may be due to the absence of the calcar; (3) varus orientation of the stem; (4) cantilever bending resulting from good distal fixation in the presence of an inadequate proximal cement mantle; and (5) material defects in the stem itself. In one of the cases described in the present study (Case 1), the weight of the patient in combination with the poor proximal support and good distal fixation of the implant are postulated to have resulted in catastrophic failure of the stem. In the other patient (Case 2), the poor proximal support and good distal fixation of the implant contributed to bending of the stem after the proximal part of the femur fractured. In both patients, the failure of the combination of wire mesh, a reconstruction plate, and cerclage wires to adequately contain impacted bone graft proximally also resulted in poor bone support. Thus, poor proximal bone support and good distal fixation contributed to both of these catastrophic failures. Bone-remodeling and allograft incorporation have been reported in association with the impaction bone-grafting technique17,20. However, we are not aware of any reports that have described allograft incorporation following the use of proximal wire mesh or a reconstruction plate. The CPT system requires that calcar defects be reconstructed with wire mesh or a reconstruction plate, or both, in order to achieve proximal stability. In both of our patients, excessive soft-tissue stripping of the proximal part of the femur was necessary in order to place the wire mesh, the reconstruction plate, and the cerclage wires that secured the allograft. The proximal part of the femur receives its blood supply from both endosteal and periosteal vessels; however, when the soft tissues are removed from the outside of the bone, the proximal part of the femur is devascularized. As a result, poor proximal incorporation of the impacted allograft, as well as increased susceptibility to fracture in the proximal part of the femur, especially near the tip of the femoral stem, should be expected. In patients with poor calcar support who are amenable to the impaction bone-grafting technique, the use of a calcar-replacing femoral prosthesis in conjunction with the impaction bone-grafting technique may be preferable to the placement of circumferential wire mesh5. In patients with substantial uncontained bone loss in the proximal part of the femur, the use of the impaction bone-grafting technique in conjunction with proximal femoral reconstruction with wire mesh and insertion of a CPT stem should be viewed with caution as this stem design, when combined with loss of proximal bone support, may predispose to catastrophic failure.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
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