Monitoring In Vivo Load Transmission Through an External Fixator

Abstract

This work presents a portable non-invasive external fixator to assess and monitor fracture healing in real time. To evaluate the potential of this fixator, a transverse osteotomy was performed in the tibia of six adult sheep (mean age 3+/-0.5 years and weight 63+/-5 kg). The fractures were stabilized by a specially designed unilateral external fixator, which was instrumented by means of a set of strain gauges. Strains in the external surface of the fixator were monitored during all the healing process. A wireless, remote monitoring of the implant was developed through a specially designed external telemetric device. The strain gauges were arranged in two different half-bridge Wheatstone configurations, allowing easy post-processing of the signal. Thus, bending loads were measured in two planes of the external fixator acting as a load cell. The load through the fixator was evaluated for the gait cycle during all the healing process. Full weight bearing of the injured leg was observed from the beginning. The load transmission mechanism in the fixator was quite similar in all operated tibias and radiographic images showed a successful healing in all animals. Although the fixator has only been tested in an animal model, after further testing this system may have clinical potential.

Influence of high-frequency cyclical stimulation on the bone fracture-healing process: mathematical and experimental models

“…An appropriate mechanical environment was used at each time point. In this way, we designed a fixator that ensured a maximum interfragmentary strain of 10 per cent within the fracture gap [1]. Design criteria also included low weight of the fixator, ease of implantation, biocompatibility and comfort for the animal during the surgery and recovery.…”

“…The gap size was 2 mm and the monolateral external fixator was attached medially to the transversally osteotomized right tibiae [1]. Sheep were divided into two groups of two sheep in each group: a stimulated group and a static or control group.…”

“…First, we designed an external static fixator from a biomechanical perspective [1]. The mechanical environment in the fracture gap was previously analysed at different time points in the healing process by means of static finite-element analyses (figure 2).…”

“…A monolateral external fixator, described previously [1], was developed in order to study experimentally the process of fracture healing. This fixator allowed us to monitor telemetrically the repair process in osteotomized long bones, correlating the load transmission through the main body of the system to the increasing stiffness of the regenerated bone.…”

“…As this process is mechanically driven, both experimental and computational studies have been performed in an attempt to improve the understanding of this process. In fact, external fixators have usually been designed from biomechanical [1] and clinical [2] considerations; similarly, treatments for bone healing are based on either mechanical or biological processes [3].…”