[Serial strain gauge measurement of bone healing in Hoffmann external fixation]

Since 1978, the author has applied Hoffmann external fixation to the treatment of open fractures and infected pseudoarthrosis of long bones in the lower limbs, but has some difficulties in determining when weight bearing should be started after operation, how much weight bearing should be and when the pin should be removed. As new method to mechanically analyze the callus strength, I tried to estimate the amount of strain at intervals of 2 to 3 weeks, beginning from the second week after operation, by bending or compressing the fracture site through the strain gauge glued to the middle of the external fixator's connecting rod. From a strength test by means of a model of fracture using a vinyl chloride pipe and also from a study of computer calculation using the model of plane beam structure for architectural design, it was found that the amount of the strain on the connecting rod decreased hyperbolically when the mechanical properties of the callus increased, and that it became constant when the mechanical properties of the callus reached 50% of the intact bone. The strength test using an cadaveric skin bone demonstrated that the callus volume was one of the most important and affecting factor. Twenty-three cases were treated by Hoffmann external fixation, and the bone healing was achieved in 20 of them. On the basis of the bone healing curve obtained by the serial strain gauge measurement in those cases, the bone healing process could be classified into 5 types: normal healing, slow healing, non-union, arrest in evolution and breakage of callus; and were employed as indexes in the post-operative rehabilitation program.