Semirigid Rod Fixation for Long-Bone Fracture

Abstract

Experiments on 119 New Zealand rabbits were performed to measure healing strength of femurs. Four pairs of normal rabbit femurs were also studied in vitro by use of a four-point bending test to measure bending rigidity. Rabbit-sized octagonal cross-sectioned intramedullary rods with 12%, 20%, 50%, 75%, and 100% of femoral bending rigidity were specially prepared. For each animal, one intramedullary rod was implanted in an osteotomized femur through a standard lateral approach, with the other femur serving as a control. The results from the 12% rigidity rods were widely variable. In the 20% and 50% rigidity groups, the callus diameter was increased over the 75% rigidity group through all phases of healing. The torque ratio in this group increased up to eight weeks and then converged to the ratios of the other rigidities. The 75% rigidity group had shown increase of angular displacement up to eight weeks, while the 20% and 50% rigidity groups continued to improve and reached control strength at 12 weeks of treatment. Furthermore, the 20% and 50% groups absorbed more energy to failure in the later stage of healing than the 75% and 100% rigidity groups. Rods of 20% and 50% rigidity stimulate external callus formation and improve the stability of healing bone.