The accuracy and reproducibility of radiographic assessment of stress-shielding. A postmortem analysis

BACKGROUND: Although periprosthetic bone loss remains a major concern in total hip arthroplasty, radiographic assessment of such loss is both difficult and subjective. In the present study, we assessed the ability of orthopaedic surgeons to reproducibly recognize changes in periprosthetic bone density on radiographs. We hypothesized that assessment of periprosthetic bone loss on plain radiographs is not reliable enough to justify its use in outcomes research. METHODS: Twenty-nine unilateral total hip replacements and the surrounding bone were retrieved at autopsy, and radiographs were made; radiographs of the contralateral, normal femur were also made after implantation of an identical prosthesis and used as a control. Three orthopaedic surgeons independently examined the specimen radiographs and classified bone loss in each of sixteen femoral zones. Bone loss was recorded as present if the bone of the femur that had had in vivo implantation showed evidence of cortical thinning, increased porosity, or decreased density (either cortical or trabecular) when compared with the control femur. The kappa coefficient was used to quantify interobserver and intraobserver reproducibility in determining bone loss for the 464 zones examined and in determining the Engh and Bobyn stress-shielding classification of each femur. In fourteen femoral pairs, bone loss was also quantified with dual-energy x-ray absorptiometry, and the resulting value was then compared with the bone-loss classification that had been determined radiographically. RESULTS: First, the surgeons agreed on the presence or absence of bone loss in 73 percent (337) of the 464 zones. The interobserver kappa value of 0.58 denoted only good reproducibility. The intraobserver reproducibility was better; the surgeon's initial evaluation of bone loss agreed with his second evaluation for 90 percent of the zones (kappa = 0.74). Second, the three surgeons agreed on the degree of stress-shielding, according to the Engh and Bobyn classification, in 66 percent (nineteen) of the twenty-nine femora. The kappa value for this comparison was only 0.27, indicating marginal reproducibility. Third, although there was some agreement among reviewers when there was 20 to 60 percent reduction in bone-mineral content as determined with dual-energy x-ray absorptiometry, excellent agreement among the examiners (kappa = 0.85) was not achieved until bone loss averaged 70 percent. CONCLUSIONS: On the basis of these results, we suggest caution in interpreting results from studies of femoral bone loss that have used plain radiographic analysis if the authors have not provided interobserver reliability data. We question the utility of evaluating periprosthetic bone loss on radiographs, since the loss is not reproducibly recognized until 70 percent of the bone is gone.