Subjects were enrolled as part of a randomized, placebo-controlled, double-blind study of Enbrel^® (Etanercept, Immunex Corp., Seattle, Washington, USA) for prosthetic hip loosening. (This study will be unblinded in March 2002). Patients with X-ray evidence of acetabular loosening on plain X-ray films were recruited from the clinical practice of the Department of Orthopedics at the University of Rochester (Rochester, NY, USA). Subjects that had an uncemented acetabular component, were ambulatory, and were at least five years post implant were eligible for this study. Exclusion criteria included inflammatory arthritis, history of prosthetic joint infection, current active infection, severe medical illness, chronic use of systemic glucocorticoids, use of calcitonin, and metabolic bone disease (except osteoporosis). Informed consent was obtained from all subjects. The Human Subjects Review Board of the University of Rochester approved this study. Plain X-rays and CT studies were performed on all patients.

Two-dimensional analysis, based on anterior-posterior pelvis radiographs was performed as we have previously described [14,15]. With this technique, the mean volumetric wear accuracy is 4.88% ± 10.9% for two-dimensional volume.

CT images were acquired using GE CTi with 1 mm collimation, a pitch of 1.5, and a 14–22 cm field of view. The raw data was reconstructed for 1 mm slices. The area within 5 cm from the prosthesis-bone interface in all directions was evaluated. The volume of osteolysis was calculated by VirtualScopics (Rochester, NY, USA). This volumetric CT measurement of osteolysis is accurate to 0.1 mm³ and it has a reproducibility of ± 4%.

A beam artifact elimination algorithm is used to correct degradation of the CT images caused by the hip prosthe-sis. The algorithm suppresses streak artifacts and corrects the tissue density information for every voxel in the CT data. Once corrected, the CT data are segmented using a region-based algorithm. This algorithm automatically analyzes the image, finding cortical bone and metal based on density information and defining the boundaries between different tissues based on tissue contrast. The algorithm then proceeds to extract regions of uniform density, and labels each region according to its relative size. Regions are then combined using the segmentation algorithm that extends the largest regions of uniform density by merging them with contiguous, smaller regions of similar density. An expert radiologist then reclassifies the segmented regions. A region is labeled as lytic if no trabecular structures or cortical bone is seen in any of the orthogonal original CT images. The reclassified regions are then used for 3D rendering and volume measurements.

Function was evaluated using the Harris Hip Scale, the Western Ontario and McMaster University Osteoarthritis Index (WOMAC), and the short form 36 questionnaire (SF-36). The Harris Hip Scale is widely used in the orthopedic literature to quantify the effects of hip surgery [16]. It is a disease-specific index with validated reliability and responsiveness [17]. WOMAC is a self-administered, disease-specific index widely used to evaluate both surgical and medical interventions in osteoarthritis of the knee and hip [18,19]. SF-36 is a self-administered questionnaire that assesses the overall quality of life. It appears to be generally applicable and has been used in a wide variety of clinical trials [20,21].

SPSS 10.0 for Windows was used to calculate the correlation between parameters. P < 0.05 was considered to be significant.

Twenty patients, whose plain films indicated periacetabular osteolysis, were recruited from the outpatient practice of the Orthopedic Department at the University of Rochester (Rochester, NY, USA) (Table 1). The average age was 63.8 years and the average time post surgery was 10.8 years. All of the patients had THA with an uncemented acetabular component. Fifteen of the patients had THA for osteoarthritis and five had THA for idiopathic or post-traumatic avascular necrosis. None of the patients had inflammatory arthritis.

Three-dimensional reconstruction and 3D-segmented CT measurement of osteolytic volume were obtained in all 20 patients. Figure 1 shows a comparison of plain film and 3D-CT images. In this series, where periacetabular osteolysis on plain films was a criteria for entry into the study, the volume of osteolysis averaged 30.0 mm³ and ranged from 2.9 to 92.7 mm³. We found that several subjects had lytic lesions that were readily identified by 3D-segmented CT scan, but were undetectable by plain X-rays. Interestingly, we also had one patient with an apparent large lytic lesion on plain films, but the 3D-segmented CT scan revealed a lesion that was only 2.9 cm³. In general, we thus found that plain films are a poor measure of periacetabular osteolysis.

Polyethylene wear was determined using digitalized plain radiographs. In 15 subjects, films were obtained at least six months post-operation. For five additional subjects, wear volume was estimated assuming that the prosthesis was originally centered in the acetabulum. One subject had a Biomet prosthesis in which, by design, the femoral head is not centered in the cup. The average volume of wear was 1010 mm³ with a range of 72–2400 mm³.

When CT measurements of osteolysis were correlated with the polyethylene wear for all 20 patients, osteolysis correlated directly with wear (RR = 0.494, P = 0.027). In two patients, however, the measurement of linear wear probably underestimated the amount of wear debris (see the two points indicated by the solid arrow in Fig. 2). One of these patients had an acetabular revision and thus may have had wear debris left over from the prior prothesis. The second patient had recurrent dislocation of the prosthesis and thus may have had wear debris that was not measured by our technique. If these two patients and the patient with the Biomet prosthesis (see clear arrow, Fig. 2) are excluded, the correlation between wear and osteolysis improves (RR = 0.685, P = 0.002). These data suggest that most patients have a fairly tight relationship between wear and osteolysis, but that there are outliers who have a relatively high amount of wear compared to the amount of osteolysis seen. These outliers, are enclosed by the rectangle in Fig. 2.

The Harris Hip score tended to correlate inversely with volumetric CT measurements of osteolysis, but this did not reach significance (RR=-0.418, P = 0.066). The WOMAC and SF-36 values tended to correlate well with each other and with the Harris Hip score, but their correlations with osteolysis were no better than that shown by the Harris Hip score.