主动负重和被动加载时骨折局部力学环境的比较研究

目的比较主动和被动加载时长骨骨折局部的生物力学环境，为选择理想的加载模式提供参考。方法建立胫骨中段横行骨折外固定系统的三维模型，通过改变骨痂弹性模量模拟骨折愈合早期（4—8周）愈合骨的刚度，利用有限元法计算主动负重与被动加载时骨折局部的力学环境及钉孔处的应力分布。结果主动负重时骨痂内部始终为压应力；被动加载在骨折愈合早期也为压应力，但随着骨痂刚度的提高，靠近外固定支架侧仍为压应力，对侧逐渐转为张应力。骨折断端的位移随着骨痂刚度的提高而降低，到术后8周位移接近为零。两种载荷方式钉孔处存在较高的压应力，但主动载荷时应力更高。结论在骨折愈合早期，宜采用体外的机械装置对愈合骨施加载荷，其力学影响因素为位移；随着骨痂的生长与钙化，应逐渐改为由患者主动部分和完全负重，其力学影响因素为应力。

A comparison of biomechanical environments of fracture healing: active weight-bearing vs. passive loading

Objective To find an optimal loading method for long bone fracture healing after comparison of the local biomechanical environments under active and passive loadings. Methods A 3-D model of the external fixation system for transverse tibial mid-shaft fracture was constructed. The rigidities of the healing bone during the early stage (4 to 8 weeks) were simulated by varying elastic moduli to the callus. Then the mechanical environments of the fracture site and the stress distributions at the pin tract under active and passive loadings were evaluated using the finite element model. Results Active loading always produced the compressive stress in the callus; passive loading did also during the early stage of healing. With the stiffening of callus under passive loading, the compressive stress remained at the adjacent side of external fixator but changed into tensile stress at the contralateral side. Displacement of the fragment ends decreased with stiffening of callus, and nearly stopped at 8 weeks. High compressive stress was observed near the inlet of pin tract in both groups, but even higher one under the active loading condition. Conclusions Passive loading resulting from mechanical apparatus is appropriate during the early stages of healing when the chief biomechanical factor is displacement of bone fragments. Active part or full weight-bearing should be done gradually during increased calcification of callus when the chief biomechanical factor is bone stress.