Aro H T, Markel M D, Chao E Y
Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minnesota.
J Trauma. 1993 Nov;35(5):776-85. doi: 10.1097/00005373-199311000-00022.
The pin-bone interface is the weakest link in the mechanical stability of external skeletal fixation. In this investigation, a canine model was used to characterize the nature of cortical bone reactions at the pin-bone interface. Unilateral external fixators were applied to the tibiae of 61 dogs using six tapered cortical half-pins. The pins were inserted after predrilling both cortices, and pin insertion torque was measured. A transverse or oblique osteotomy was performed in each tibia and stabilized under different gap conditions. Unrestricted weight bearing was allowed immediately after surgery. Sequential functional evaluations of the animals and biweekly roentgenograms of the pin tracks were done. Pin removal torques were recorded at the time the animals were killed at different time periods. Pin tracks were analyzed using quantitative tetracycline histomorphometry and microradiography. The results showed that cortical bone undergoes extensive creeping substitution around external fixation half-pins. New bone accounted for approximately 43% of the intracortical space along the pin track, and cortical bone porosity showed a fourfold increase compared with intact bone value. This cortical bone remodeling resulted in a time-related decrease of pin removal torque (p < 0.001). In inherently unstable oblique osteotomies, and less in stable rigidly fixed transverse osteotomies, immediate postoperative weight bearing caused bone thread resorption and adverse cortical bone remodeling at the entry cortex of external fixation half-pins. The unicortical loosening of half-pins that became evident during the first month of fixation obviously represents a consequence of micromotion and local bone yielding failure caused by high dynamic stresses of the pin-bone interface. Effective precautions should be taken to reduce such stresses.
针-骨界面是外骨骼固定机械稳定性中最薄弱的环节。在本研究中，使用犬模型来表征针-骨界面处皮质骨反应的性质。使用六根锥形皮质半针，对61只犬的胫骨应用单侧外固定器。在预先钻透两侧皮质后插入针，并测量针的插入扭矩。在每只胫骨上进行横向或斜向截骨，并在不同间隙条件下进行固定。术后立即允许无限制负重。对动物进行连续功能评估，并每两周拍摄针道的X线片。在不同时间段处死动物时记录针的拔除扭矩。使用定量四环素组织形态计量学和显微放射摄影术分析针道。结果表明，皮质骨在外固定半针周围经历广泛的爬行替代。新骨占沿针道皮质内空间的约43%，与完整骨值相比，皮质骨孔隙率增加了四倍。这种皮质骨重塑导致针拔除扭矩随时间下降（p<0.001）。在固有不稳定的斜向截骨中，以及在稳定的刚性固定横向截骨中较少见，术后立即负重会导致外固定半针进入皮质处的骨螺纹吸收和不良的皮质骨重塑。在固定的第一个月内明显出现的半针单皮质松动显然是针-骨界面高动态应力引起的微动和局部骨屈服失败的结果。应采取有效的预防措施来降低此类应力。
