The effects of external mechanical stimulation on the healing of diaphyseal osteotomies fixed by flexible external fixation.

OBJECTIVE: The purpose of this study was to investigate the effect of an externally applied mechanical stimulus on fracture healing under flexible fixation. DESIGN: Stimulation of fracture healing under various conditions of interfragmentary movement in an in vivo fracture model on 41 sheep. BACKGROUND: It is generally accepted that small interfragmentary movements (IFMs) yield better bone healing results than larger IFMs (> 1 mm). However, the optimal size of IFM within the 1-mm range remains undetermined. METHODS: Standardized transverse osteotomy of 3 mm gap size in the left ovine tibia was fixed with an unilateral external fixator. The sheep were divided into four IFM groups of 0.0, 0.2, 0.4 and 0.8 mm and stimulated with this amplitude for 1200 cycles per day at 1 Hz. After a healing period of 6 weeks, bone mineral density and biomechanical stability were evaluated to determine the quality of healing. RESULTS: The amount of callus formation increased significantly with increasing IFM (P <0.05). However, highest biomechanical stability of the healed bone and mineral density of the gap tissue was achieved with an IFM of 0.4 mm, although the differences were not significant. CONCLUSIONS: These results suggest that the optimal interfragmentary movement for acceleration of delayed fracture healing is in the range of 0.5 mm. However, the enhancement of the healing of flexibly-fixed fractures by external application of interfragmentary movement is limited. RELEVANCE: In this model the external application of a mechanical stimulus in addition to the stimulation caused by normal loading and the flexibility of the fixation did not enhance the healing process significantly. It appears that the external application of interfragmentary motion is promising perhaps only for patients unable to stimulate their fracture healing by weight-bearing.