CORTICAL BONE REACTIONS AT THE INTERFACE OF EXTERNAL FIXATION HALF-PINS UNDER DIFFERENT LOADING CONDITIONS

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 t ibiae of 61 dogs using six tapered cortical half-pins. The pins were i nserted after predrilling both cortices, and pin insertion torque was measured. A transverse or oblique osteotomy was performed in each tibi a and stabilized under different gap conditions. Unrestricted weight b earing was allowed immediately after surgery. Sequential functional ev aluations 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 r esults showed that cortical bone undergoes extensive creeping substitu tion around external fixation half-pins. New bone accounted for approx imately 43% of the intracortical space along the pin track, and cortic al bone porosity showed a fourfold increase compared with intact bone value. This cortical bone remodeling resulted in a time-related decrea se of pin removal torque (p < 0.001). In inherently unstable oblique o steotomies, and less in stable rigidly fixed transverse osteotomies, i mmediate postoperative weight bearing caused bone thread resorption an d adverse cortical bone remodeling at the entry cortex of external fix ation half-pins. The unicortical loosening of half-pins that became ev ident during the first month of fixation obviously represents a conseq uence of micromotion and local bone yielding failure caused by high dy namic stresses of the pin-bone interface. Effective precautions should be taken to reduce such stresses.