Effect of callus development on the deformation of external fixation frames

Abstract

Purpose We designed a sensor that measures the bending moments at the articulations and the torque of the rod of a Hoffmann II® external fixation. We considered the effect of the callus formation in the stabilisation of a "fracture-fixation system." Methods Four Hoffmann II® frame configurations were mechanically tested. Two carbon fibre tubes represent the bone fragments (length 180 mm, outer diameter 25 mm, inner diameter 19 mm). The callus is represented by the interposition of springs of different rigidity (10-405 N/mm) in the fracture gap between the tubes. Results The deformation of the frame is in inverse proportion to the stiffness of the callus; the slope of the curve drops rapidly during early development of the callus, to reach a plateau after some 50 % of recovery of the normal mechanical characteristics of the bone. This simulation supports the theoretical approach, i.e. the external frame resists larger stresses at the start of the fracture healing. Over a callus stiffness of some 200 N/mm the pattern of the curves remains similar, regardless of the frame configuration. Conclusion An optimisation of the frame is possible, adapted to the actual mechanical situation of the callus. A monitoring system is deemed reliable after making sure that the elementary components behave the same way in the clinical condition as in the laboratory. In an experimental set up we confirmed its reliability in a clinical-like situation.

The Effect of External Fixator Configurations on the Dynamic Compression Load: An Experimental and Numerical Study

“…This behaviour could be justified by the global stiffness variation of the external fixator. A smaller free pin span makes the whole structure stiffer than a larger one [29,30], and for any given regenerate stiffness of callus, a more flexible fixation tends to increase the ratio of the total load supported by the callus [31,32]. Nevertheless, the results of Figure 5, and Tables 3 and 4 show also that increasing the axial distance from 70 mm to 90 mm does not change load share ratios very much.…”

A Biofeedback System for Continuous Monitoring of Bone Healing

“…To better understand such information and to reach an evidence level, the consequent next step is to build up a database. Distinct healing patterns for normal and aberrant courses of healing could be extracted and interpreted (Burny et al, 2012;Claes et al, 2002). Ideally, data collection should concentrate on human patients to increase the significance of results.…”

A Novel Capacitive Measurement Device for Longitudinal Monitoring of Bone Fracture Healing

“…A 20 mm gap was left at the fracture site and the bone callus was represented by springs of different rigidity: k 1 = 0 N/mm, k 2 = 0.153 N/mm and k 3 = 0.460 N/mm. We used springs with increasing rigidity to simulate increasing bone callus stiffness, while the values of springs were chosen to be feasible with benchtop tests and in accordance with previous works [ 13 ]. The behaviour of each component of the external fixator was assumed to be isotropic, linear, and homogenous, based on data provided by the datasheet [ 23 ], and according to similar studies observed in the literature [ 21 , 24 ].…”

“…In indirect measurement approaches, the strain of the implant is evaluated against the healing time. Since the deformation of the frame is inversely proportional to the callus stiffness, information on the state of the bone can be obtained by monitoring the level of deformation on the external fixator [ 13 ]. Only a few studies in the past have evaluated the displacement of the external fixator components as a mean for monitoring fracture healing.…”

“…However, as assessed by the authors, the results should be evaluated with caution, considering the high variability among the animals. Burny et al [ 13 ] highlighted the importance of considering the presence of bone callus as it can cause variations in load transmission. Authors covered the rods of a Hoffmann ® external fixator (Striker Corp., Kalamazoo, MI, USA) with strain gauges in order to monitor changes in the torque applied to the rods and the moments at the articulations.…”