EFFECT OF LOW-INTENSITY PULSED ULTRASOUND STIMULATION ON GAP HEALING IN A RABBIT OSTEOTOMY MODEL EVALUATED BY QUANTITATIVE MICRO COMPUTED TOMOGRAPHY-BASED 3-DIMENSIONAL CROSS-SECTIONAL MOMENT AND CROSS-SECTIONAL MOMENT OF INERTIA

Abstract

Introduction: Low-intensity pulsed ultrasound stimulation (LIPUS) reportedly enhances restoration of strength at fracture healing sites. However, evaluation of strength by mechanical testing was limited to only one direction, with either bending or torsion. Quantitative micro computed tomography (μCT) scans allow us to calculate strength-related parameters such as cross-sectional moment (CSM) and cross-sectional moment of inertia (CSMI). Previous studies have performed 2-dimensional (2D) analyses, and 3-dimensional (3D) evaluations have not been described. The purpose of this study was thus to investigate the effects of LIPUS on osteotomy healing using 3D analyses of CSM and CSMI.

Materials and Methods: Bilateral, transverse, mid-tibial osteotomies with a 2-mm gap were performed in 42 rabbits. LIPUS was continued for both the treatment group (n=7/group/time point) and the control group (n=7/ group/time point), for 20 min, six times/week, for 4, 6, or 8 weeks. The control group also received a sham inactive transducer under the same condition as the LIPUS group. After the tibia was scanned by μCT, region of interest (ROI) was set at the center of the osteotomy gap with a width of 1 mm. Center of gravity for the ROI and the XYZ coordinate was calculated. An optional line (I) can be drawn in this coordinate. The angle of the Z axis (𝛉) was measured, and also the degree of angle of the X axis (φ) was measured. The 3D CSM [I (φ, 𝛉)] around this line was calculated using the following equation: I (φ, 𝛉) = ∫ r²dV (mm5), where r is the distance of a voxel to the center of gravity (mm) and dV is the area of a voxel (mm3). The axial CSM was defined as CSMx: I (0, 90), CSMy: I (90, 90), whereas the polar CSM was also defined as CSMp: I (any, 0). 3D CSMI weighted by density distribution was calculated using the following equation: I’ (φ, 𝛉) = ∫ r²dm = ∫ ρr²dV (mg.mm²), ρ is the measured volumetric callus mineral density. Likewise CSMIx, CSMIy and CSMIp were calculated. These data of the μCT evaluations were analyzed using a one-way ANOVA test (p< 0.05).

Results: When 3D CSMs at the same time point were compared, values for the LIPUS groups were significantly higher than those for control groups for CSMx at 6 weeks and CSMp at 8 weeks. As for comparison of 3D CSMIs at the same time point, values for the LIPUS groups were significantly higher than those of the control groups for CSMIx, CSMIy, and CSMIp at 6 and 8 weeks.

Discussion: Bone healing by 3D CSM and CSMI has not been described before. Our results demonstrate that these bone strength parameters improved with LIPUS during the early phases. However, whether the late phase of callus formation is infiuenced remains unclear.