Introduction: Osteolysis of the pelvis secondary to polyethylene wear of uncemented acetabular implants has emerged as the most serious and challenging consequence of THR. A very large number of patients have and will continue to receive implants at the risk of being associated with osteolysis. The early detection of osteolysis allows the initiation of treatment programs that preserve bone stock. Because osteolysis occurs and progresses in the absence of clinical symptoms, appropriate follow-up surveillance must be instituted. Our initial study of the usefulness of CT scans in detecting clinically silent and radiographically unobservable osteolysis indicated that x-rays greatly understated the incidence and location of osteolysis. The purpose of this study was to determine the incidence of CT scan identifiable osteolysis in young, active patients with a single cup design and a minimum follow-up of 7 years.
Methods: Between 1990–1995, 117 hips (105 patients) underwent an uncemented total hip replacement with a patient-matched femoral component and a titanium plasma sprayed, multi-holed acetabular shell with a compression molded, polyethylene, irradiated in air. 57 patients underwent a CT scan using a metal subtraction software technique. All patients were classified based on their CT scans: Group I: no osteolysis; Group II: cavitary osteolysis; Group III: segmental osteolysis. All patients had standard AP, Frog – lateral and shoot-through lateral radiographs, performed at the time the CT scan was obtained.
Results: 37.2% of hips were in Group I, 53.5% in Group II and 9.3% in Group III. No patients in Group I had x-ray evidence of osteolysis (i.e. there were no false negative CT scans). 12% of patients in Group II had x-ray evidence of osteolysis. 22% of patients in Group III had x-ray evidence of osteolysis. There was no correlation between the incidence of osteolysis seen on CT scans with: 1) activity level; 2) age; 3) sex; 4) weight, and 5) size of acetabular component. There was no correlation between polyethylene wear measured using the Martel method and pelvic osteolysis. There was a correlation between the length of implantation and pelvic osteolysis. The average follow-up for patients in Group III was 105.5 months (range 85 – 115) vs. 89.4 months (57 – 117) for Group II and 81.5 months (51 – 112) for Group I. Of the patients with follow-up greater than eight years, 25% had Group III osteolysis. No patients have required revision or polyethylene liner exchange thus far.
Discussion: This study indicates that: 1) x-rays are an unreliable method for determining the presence. Location or extent of osteolysis, 2) the incidence of osteolysis based upon CT scans (Group II – III) is 63%; 3) Osteolysis, even if extensive (Group 3) is NOT associated with symptoms; 4) the pattern of osteolysis seen on CT scan strongly suggests that the presence of screws plays an important factor in the process; 5) CT scans are helpful in the planning process for acetabular revision. They allow the precise determination of the location and extent of osteolysis; 6) CT scans are also potentially useful for determining the impact of medical (e.g. alidronate) or surgical (e.g. bone grafting) treatment of osteolysis; 7) CT scans may be very helpful in assessing whether new polyethylenes are associated with reduced osteolysis.
The authors strongly recommend that: 1) a surveillance program be established for careful, regular follow-up of patients with THR in place more than 7 years; 2) CT scans be considered as part of that surveillance program, and 3) cups with screws NOT be used routinely in primary THR surgery.
