Measurement of the dynamic axial load-share ratio in vivo could indicate sufficient callus healing in external fixators.

Background: Fracture healing is commonly evaluated through physical examination and radiographic results. However, these methods rely on the surgeons' subjective experience, without including the objective biomechanical properties of the bony callus. This paper presents an innovative method for measuring the callus stiffness in vivo to evaluate fracture healing, further instructing surgeons to remove external fixator safely.

Methods: A novel dynamic axial load-share ratio (D-LS) index and its associated measuring system was introduced, including the system's composition (hexapod and insole modules), theoretical model, and method for D-LS measurement. From Jan 2022 to May 2024, 36 patients with tibial shaft fracture treated by Taylor Spatial Frame were evaluated in this prospective study. Once the patient had reached clinical bone healing conditions, the in vivo D-LS measurement was conducted. The patients' demographic data, clinical outcomes, particularly D-LS value and refracture rate were recorded.

Results: At a mean follow-up of 16.50 ± 5.79 months, a total of 36 patients completed the final follow-up. Fixators were removed with an average of 24.81 ± 4.51 weeks. The result of hexapod module's precision examination were maximum errors of 3.72 N, 3.31 N and 2.68 N in x-, y- and z-axis, respectively. The measuring process took an average system installation time of 15.42 ± 4.88 min. Two patients (5.56%) reported fracture site pain. Each patient's D-LS was determined after three rounds of measurement. The average D-LS value was 15.58 ± 2.77% (range, 9.60-20.52%). None of the 36 patients reported refracture at the last follow-up.

Conclusions: The novel D-LS measurement system can measure the dynamic forces of lower limb for patients with external fixator in vivo. An objective biomechanical indicator of the regenerate callus was provided by the D-LS. The D-LS measurement is a complement to standard radiological assessment only after radiologically confirmed bone union. Measuring the D-LS in vivo could indicate whether the callus healing is sufficient in external fixation, and 15.6% (average) was recommended as a reference D-LS value for safe fixator removal.