Evaluation of callus stiffness by axial load-share ratio to judge safe removal of external skeletal fixator

Abstract

Objective To create a credible quantitative method to evaluate callus’s stiffness which can be used to judge appropriate time to remove external skeletal fixator safely. Methods Firstly, a model of tibial transverse fracture was built to determine the relationship between callus stiffness and loading on the external skeletal fixator on which pressure sensors and data collecting system were installed. The space between bone ends was imbedded respectively with materials with various degrees of stiffness to simulate the process of callus developing from hematoma to sclerosis. The whole unit was tested on the biomechanical test system BOSE 3510, with an axial load of 70 kg. Secondly, medical records of 21 patients were collected who had been treated by external skeletal fixation from September 2013 to March 2016, involving the tibia and fibula in 19 cases and the femur in 2. Twenty cases underwent treatment of fracture and one bone lengthening. Open fractures were treated in 16 cases, fracture and infection in 4 and bone defects in 2. In the different process of bone healing, changes in load-share ratio of external fixation were measured with different axial loading. Before removal of the external skeletal fixation, the load-share ratio of external fixation with full loading was measured to determine the appropriate time to remove the external skeletal fixator on the basis of the patients’ clinical situation and X-ray plain films. Results A linear equation resulted from the sim-ulative experiment: y=0.243x-564.882, with x representing the reading of pressure sensor and y representing loading value. The varying bone callus’s stiffness during the fracture healing process could be simulated by different materials. The axial load-share ratios were relevant to the callus stiffness. No such complications as refracture happened after removal of the external fixation in 18 patients who had a load-share ratio 10%, one had refracture after removal of external fixation and 2 had to sustain continuous treatment and follow-up without removal of the external fixation. Conclusions The axial load-share ratio may be an appropriate indicator in evaluation of the bone callus stiffness. It can be used to judge appropriate time to remove external skeletal fixator safely so as to effectively reduce the complications related to callus’s healing. It may be safe to remove the external skeletal fixation when the load-share ratio is < 10%. Key words: External fixators; Bony callus; Fracture healing; Load-share ratio; Safe removal time