Force measurement in circular external fixation frames reflects construct stability: A laboratory proof-of-concept study.

BACKGROUND

Circular external fixation frames are widely used in limb reconstruction, however determining the optimal time for frame removal remains challenging. Current approaches rely on clinical and radiological assessment, both of which have recognised limitations. Mechanical assessment of construct stability offers a potential alternative, but existing approaches have not translated widely into clinical practice, in part due to reliance on absolute measurement thresholds.

OBJECTIVE

To determine whether force measurement within a circular external fixation frame varies systematically with fracture site stability, and to assess whether this measurement may support monitoring of healing progression.

METHODS

Three similar four-ring Ilizarov frame constructs were assembled using a Perspex bone analogue. A load cell was integrated into the frame and used to measure force during axial loading to 700 N. Models representing different stages of fracture healing were simulated using varying fracture gaps and materials of differing stiffness. Measurements were obtained across multiple sensor positions and configurations. Force measurements were compared between models using non-parametric statistical methods. Receiver operating characteristic (ROC) analysis was performed to assess separation between conditions.

RESULTS

Force measured within the frame decreased progressively with increasing construct stiffness. A significant relationship between model stability and measured force was observed (Kruskal-Wallis p < 0.0001). Clear separation was demonstrated between grossly unstable and intact constructs; however, overlap between higher stability models and intact bone was evident. ROC analysis showed strong discrimination when considering all unstable conditions (AUC ≥ 0.91), but performance was reduced when restricted to higher stability models (AUC 0.699-0.838). Absolute force measurements varied with sensor configuration, although the rank order of conditions was preserved.

CONCLUSION

Force measurement within a circular external fixation frame reflects construct stability in an in vitro model. Whilst absolute values are dependent on construct configuration and show reduced discrimination near union, consistent directional changes suggest that serial within-subject measurement may provide a useful approach to monitoring healing progression. These findings support further evaluation of longitudinal measurement strategies in clinical studies.