Precision of Dual Energy X-ray Absorptiometry in Determining Periprosthetic Bone Mineral Density of the Hydroxyapatite Coated Hip Prosthesis

The objectives of this study were to demonstrate the potential usefulness of dual-energy X-ray absorptiometry (DEXA) in the assessment of dynamic changes in bone mineral density (BMD) around the femoral stem of non cemented hydroxyapatite-coated hip prostheses and to establish our procedures by determining the variabilities associated with error introduced by the machine, operator, and/or subject. Methods Regional BMD were measured twice by DEXA for 27 patients. All scans were analysed twice by applying large size regions of interest (ROI's), 7 in total, defined by ‘modified’ Gruen zones. The data of 27 patients were analysed for the second time using smaller size ROI's, 14 in total, 7 small ROI's on each of the medial and lateral sides of the prosthesis. Three patients, all of whom exhibited intense ectopic ossification, were scanned twice. The variation associated with rotation of the femur was assessed by scanning six patients first in neutral rotation, thereafter by ± 15° in internal and external rotation position. Results: The overall coefficient of variation (CV) using ‘modified’ Gruen zones was 2.40% and varied according to the zone assessed. When smaller zones were used, the overall CV was 3.42%. The overall CV in the patients with ectopic ossification was 7.56%. When the leg was rotated by ± 15° externally or internally, the overall CV was approximately 10%. Conclusions Patient positioning is probably the most variable condition in the clinical setting. Using large ROI's such as ‘modified’ Gruen zones yields significantly better overall precision than using smaller ROI's. Patients with intense ectopic ossification are not suitable candidates for longitudinal study.