EFFECT OF LOW-INTENSITY PULSED ULTRASOUND STIMULATION ON CALLUS REMODELING IN A GAP HEALING MODEL – EVALUATION BY BONE MORPHOMETRY USING 3-DIMENSIONAL QUANTITATIVE MICRO COMPUTED TOMOGRAPHY

Abstract

Introduction: Low-intensity pulsed ultrasound stimulation (LIPUS) can enhance bone regeneration and callus healing during fracture repair. However, whether a certain phase of the healing process in fracture repair in particular is infiuenced by LIPUS treatment remains unclear. In this investigation, the effect of LIPUS on callus remodeling in a gap healing model was evaluated by bone morphometric analyses using 3-dimensional (3D) quantitative micro computed tomography (μCT) at the healing site, providing information on the temporal sequence of mineralized remodeling events that characterize the gap healing.

Materials and Methods: The rabbit osteotomy model with 2-mm gap for the right tibia was immobilized with four pins fixed to an external fixator with double side bars. 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 exactly the same condition as the LIPUS group. After the harvested tibia was scanned by μCT, region of interest was set at the callus healing area. It defined as a center of the osteotomy gap with a width of 1 mm. Morphometric parameters used for evaluation were mineralized callus volume (BV, cm³) and volumetric bone mineral density of mineralized tissue comprising the callus (mBMD, mBMD = BMC/ BV, mgHA/cm³). The whole ROI was measured and was subdivided into three zones. The periosteal callus zone (External), the medullary callus zone (Endosteal) and the remaining zone was the cortical gap zone (Intercortical). For each zone, BV and mBMD were measured. Data of the μCT evaluations were analyzed using a one-way ANOVA test. Statistically significant difference was set at p < 0.05.

Results: In the LIPUS groups, BV for the Endosteal zone was significantly lower for the 8-week group than for the 4-week group. Comparing results at the same time point, the LIPUS group at 8 weeks was significantly higher than that of the control group in the Intercortical zone. As for mBMD, in the LIPUS group, the 8-week group was significantly higher than the 4-week group for Total, External, Internal, and Endosteal zones, respectively. Comparing results at the same time point, mBMD was significantly higher for the LIPUS group at 8 weeks than for the control group in both External and Intercortical zones.

Discussion: The most striking finding in our study was that LIPUS accelerated bone formation in the Intercortical zone and callus resorption in the Endosteal zone. This suggests that LIPUS could shorten the time required for remodeling. However, the results of this study do not clarify whether an early phase in callus formation in particular is infiuenced by LIPUS.