Abstract
Medial Displacement Osteotomy (MDO) of the os calcis is used to correct the hind foot valgus in a flat foot deformity. Screw fixation is commonly used although contemporary locking plate systems are now available. This study tested the hypothesis that a 10mm MDO would support a higher load to failure with a locked step plate than with a single cannulated screw.
Materials and Methods
Eight pairs of embalmed cadaveric limbs harvested 10cm below the knee joint were axially loaded using a mechanical testing rig. Two pairs served as non-operated controls loaded to 4500N. The remaining limbs in pairs underwent a 10mm MDO of the os calcis and were stabilised with a locked step plate or a 7mm cannulated compression screw. One pair was loaded to 1600N (twice body weight) as a pilot study and the remaining 5 pairs were loaded to failure up to 4500N. The force-displacement curve and maximum force were correlated with observations of the mechanism of failure.
Results
In one pair of control limbs, failure occurred with fractures through both os calcis bones, whilst the other pair did not undergo mechanical failure to 4500N. In the pilot osteotomy, the plate did not fail whilst loss of fixation with the screw was observed below 1600N. For the remaining five pairs, the median (with 95% Confidence Intervals) of the maximum force under load to failure were 1778.81N (1099.39 – 2311.66) and 826.13N (287.52 – 1606.67) for the plate and screw respectively (Wilcoxon Signed Rank test p=0.043). In those with screw fixation loaded to 4500N, the tuberosity fragment consistently failed by rotation and angulation into varus.
Conclusion
In this model of load to failure with a medial displacement os calcis osteotomy, a locked step plate supported a significantly higher maximum force than a single large cannulated screw.
