Fracture healing of the sheep tibia treated using a unilateral external fixator. Comparison of static and dynamic fixation

It is generally accepted that when a fracture is treated with external fixa tion, dynamization of the fixation accelerates formation of the bony callus by transferring part of the functional loads. The aim of the research pres ented here was to validate this principle using in vivo measurements of cal lus stiffness. We created a transverse fracture in the mid third of the tib ial shaft in 12 sheep and maintained a 3 mm gay between the fragments for 3 weeks. Two types of unilateral external fixators were applied. Axial loadi ng was permitted (dynamization) from the fourth week onwards in 6 animals. In the other 6 animals, fixation remained static for both types of fixation . Weekly measurements of callus stiffness were obtained using a goniometer and load cell to assess bending stiffness. Two slightly different fixators were used. Callus formed in all 12 animals. Callus stiffness increased expo nentially to reach the degree of stiffness measured on the contralateral si de. There was no clear difference in healing between the two types of fixat ions nor between dynamic and static fixation. If a unilateral fixator was a pplied which did not maintain absolutely rigid fixation, the fracture gener ally healed well even without contact between the fragment ends. If the pro cess of callus formation had begun normally, dynamic fixation offered no fu rther benefit. Measuring the stiffness of the callus with an adequate measu rement apparatus in vivo indicated that the fixator could be removed earlie r than would have been authorized on the basis of radiological evidence alo ne.