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Wear-Through of a Modular Polyethylene Liner: Three Case Reports
C. Anderson Engh, Jr., MD;
Robert H. Hopper, PhD;
Charles A. Engh, MD; and James P. McAuley, MD
These three cases involving the S-ROM Total Hip System Polydial polyethylene liner illustrate the importance of and difficulties in detecting polyethylene wear-through before a complete acetabular revision is necessary. The patients in these cases had full-thickness polyethylene wear-through 11 or more years after total hip arthroplasties. Dorr and Livermore wear measurements from radiographs of the patients underestimated the actual wear and did not indicate wear-through. Because excessive head penetration was not evident radiographically with the S-ROM components, the cases showed the necessity of using additional measures to diagnose wear-through, particularly with these components. Repeat radiographs at a higher kV were used to better visualize the outline of the femoral head, and hip aspiration was used to diagnose metallosis. These cases also showed the importance of yearly followup evaluations – especially 10 or more years after surgery for patients with the S-ROM polydial locking mechanism.
The most important aspect of these case reports was the pattern of polyethylene wear. In each case, wear-through occurred at the dome of the polyethylene; thus, the locking mechanism was not disrupted, and the polyethylene did not fragment. Because the locking mechanism of the polyethylene liner and the metal shell was intact, it was possible to treat three of the four patients with a liner and ball exchange, rather than perform a complete cup revision. Reports of wear-through on other liners have involved fragmentation of the polyethylene and most of the cases in those reports have required exchange of the acetabular metal shell.
In these cases, the wear measurement techniques of neither Dorr and Wan (1) nor Livermore et al. (2) accurately or consistently reflected polyethylene wear-through (Table 1); consequently, radiographs were not helpful in determining when to perform a revision. The technique of Dorr and Wan calculates wear that occurs parallel to the face of the acetabular component. However, because wear in these cases was directed toward the acetabular dome, rather than parallel to the face of the cup, the technique of Dorr and Wan underestimated head penetration. Polyethylene wear also was measured in this study with the technique of Livermore et al. Because this technique measures the shortest distance between the femoral head and the metal shell, it provides the ability to identify wear that is directed toward the acetabular dome. Although the values obtained with the technique of Livermore et al. were closer to actual wear, they did not predict wear-through in two of the four cases.
In ascertaining wear-through, it is important to visualize the entire femoral head. Unfortunately, in these cases the thick metal backing of the S-ROM acetabular shells obscured a portion of the femoral head, making it impossible to determine when the femoral head wore through the polyethylene. Currently, to address this problem, the surgeons in this study take two radiographs of patients who have more than 10 years of follow-up and in whom the acetabular shell obscures the radiographic contour of the femoral head. One radiograph is taken using a bone technique to identify radiolucencies and osteolysis, and a second radiograph is taken at a higher kV (15 to 20 kV more) to improve visibility of the ball within the acetabular shell. This metal radiographic technique could help other surgeons recognize polyethylene wear in situations where liner wear-through is a possible diagnosis.
Because the radiographs were not helpful in predicting polyethylene wear-through, patient symptoms and other radiographic findings were evaluated. In two cases, the decision to reoperate was based on the presence of hip pain, polyethylene wear, and progressively enlarging osteolytic lesions. The surgeons were surprised when they encountered a worn-through polyethylene liner at surgery in these cases. Unfortunately, the other two patients did not have hip pain, excessive polyethylene wear, or osteolysis. One patient had no symptoms until the femoral head wore through the polyethylene and the metal shell. The remaining patient had no hip symptoms; instead, he had leg swelling and was diagnosed when a computed tomogram of the pelvis revealed a cyst connected to his hip.
Based on the findings in these four cases, yearly followup evaluations should be done, especially 10 or more years after surgery, for patients with the S-ROM polydial locking mechanism. Orthopaedists need to be familiar with the thickness of S-ROM polyethylene liners, particularly the thin liners associated with a 32-mm head size. When patients present with a change in hip function, this should raise the level of suspicion for polyethylene wear-through, even if excessive head penetration is not evident radiographically. Additional measures, such as repeat radiographs at a higher kV to better visualize the outline of the femoral head or a hip aspiration to diagnose metallosis, should be considered.
1. Dorr LD, Wan Z: Comparative results of a distal modular sleeve, circumferential coating, and stiffness relief using the Anatomic Porous Replacement II. J Arthroplasty 11: 419-428, 1996.
2. Livermore J, Ilstrup D, Morrey B: Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg 72A: 518-528, 1990.
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