Mechanical analysis of the healing of different osteotomies fixed externally

Under standard conditions the left tibia was osteotomised in nine sheep. The osteotomy was transverse in six, and oblique in three. The osteotomies were fixated using a strain-gauge-monitored unilateral external fixator. The fixation was rigid in three sheep with a transverse osteotomy (group 1) and also in three sheep with an oblique osteotomy (group 2). The fixation was dynamic in three sheep with a transverse osteotomy (group 3) using a roller-bearing mechanism allowing 1 mm of axial displacement. The bone callus stiffness increased progressively with time. One month after osteotomy the load force through the fixator in groups 1 and 2 was more than 20%, whereas the equivalent load force in group 3 was only about 5%.