Determination of the Precision of Dual Energy X-ray Absorptiometry (dexa) When Applied at the Knee

Introduction: Dual energy x-ray absorptiometry (DEXA) is one of the most widely used methods measuring periprosthetic bone mineral density (BMD). Several reports have presented the precision of DEXA when used at the hip, whereas similar reports on factors influencing the precision at the knee are sparse. There are two noticeable aspects when measuring BMD at the knee. First, no software specific for the knee has been developed. In stead, the BMD measurements have to be performed by using software developed for the hip. Second, the limited amount of soft tissue around knee may cause problems for the densitometer to determine the baseline for the BMD measurement. Therefore different soft tissue equivalents (rice bags, Plexiglas rod) have been developed to aid the measurements. The effect of these on the precision has, however, not been presented. When using DEXA in analyzing the longitudinal bone remodeling around total knee arthroplasty (TKA) occurring after operation, knowledge about factors influencing the precision is required. The aim of this study, therefore, was to analyze which factors influence the precision of DEXA when applied to the knee. Methods: Subjects: 12 knees in 10 patients (6 women and 4 men, mean age 70 years) operated with TKA 1 to 3 years previously were investigated. In 3 of the patients the contra-lateral non-operated knee was also investigated. All prosthetic knees were operated with the Miller-Galante II TKA (Zimmer, Warsaw, Indiana, U.S.A.). The tibial component had 4 pegs but no stem. In 6 knees the implants were uncemented, and 6 were cemented. Bone densitometer: A dual energy x-ray absorptiometer (DPX-L, Lunar Corp., Wisconsin, U.S.A.) with a fast 3000 μA scan model was used. To evaluate the effect of different soft tissue equivalents, scans were performed either with a 150-mm long Plexiglas rod positioned in the detector opening, or with rice bags placed around the knee.