Measurement of Bone Mineral Density in the Tunnel Regions for Anterior Cruciate Ligament Reconstruction by Dual-Energy X-Ray Absorptiometry, Computed Tomography Scan, and the Immersion Technique Based on Archimedes' Principle

Purpose: To determine, for anterior cruciate ligament (ACL) reconstruction, whether the bone mineral density (BMD) of the femoral tunnel was higher than that of the tibial tunnel, to provide objective evidence for choosing the appropriate diameter of interference screws. Methods: Two groups were enrolled. One group comprised 30 normal volunteers, and the other comprised 9 patients with ACL rupture. Dual-energy X-ray absorptiometry was used to measure the BMD of the femoral and tibial tunnel regions of the volunteers' right knees by choosing a circular area covering the screw fixation region. The knees were also scanned by spiral computed tomography (CT), and the 3-dimensional reconstruction technique was used to determine the circular sections passing through the longitudinal axis of the femoral and tibial tunnels. Grayscale CT values of the cross-sectional area were measured. Cylindrical cancellous bone blocks were removed from the femoral and tibial tunnels during the ACL reconstruction for the patients. The volumetric BMD of the bone blocks was measured using a standardized immersion technique according to Archimedes' principle. Results: As measured by dual-energy X-ray absorptiometry, the BMD of the femoral and tibial tunnel regions was 1.162 +/- 0.034 g/cm(2) and 0.814 +/- 0.038 g/cm(2), respectively (P < .01). The CT value of the femoral tunnel region was 211.7 +/- 11.5 Hounsfield units, and the value of the tibial tunnel region was 104.9 +/- 7.4 Hounsfield units (P < .01). The volumetric BMD of the bone block from the femoral tunnel (2.80 +/- 0.88 g/cm(3)) was higher than the value from the tibial tunnel (1.88 +/- 0.59 g/cm(3)) (P < .01). Comparing the data between male and female patients, we found no significant difference in both femoral and tibial tunnel regions. Conclusions: For ACL reconstruction, the BMD of the femoral tunnel is higher than that of the tibial tunnel. This implies that a proportionally larger-diameter interference screw should be used for fixation in the proximal tibia than that used for fixation in the distal femur.