Summary

The addition of a synthetic insert limited the tensile strains at the plate and lateral cortex which may protect them against failure following and osteotomy

Abstract

Introduction

There are a number of proposed reasons to fill the opening wedge defect following Medial Opening Wedge High Tibial Osteotomy (MOWHTO), including improvements in bone healing and haemostasis. Little data is available on the mechanical benefits to the osteotomy and hardware when combined with a locking plate fixator.

Purpose

To determine the effectiveness of a synthetic bone insert for improving MOWHTO mechanical integrity in response to cyclical loading.

Methods

Twelve fresh-frozen cadaveric knee specimens (mean [SD] age of 73.5 [9.9] years) were sectioned at tibial and femur lengths of 16cm from the joint centre. A MOWHTO was performed on all specimens to a standardized correction of 9mm and stabilized with a compression fixation plate (SBM Orthopaedics, France). The specimens were randomized to either: i) a synthetic insert condition (n=6), in which a 9mm bio-absorbable beta tri-calcium phosphate NEOTIS wedge (SBM Orthopaedics, France) was inserted into the gap space prior to plate fixation; or ii) a plate-only condition (n=6), wherein the wedge was used only to ensure the proper opening width and removed once the plate was secured.
Uniaxial strain gauges were attached to the lateral cortex and fixation plate, and the specimens were secured within a materials testing system. The specimens were then subjected to a staircase cyclical loading protocol where the load was initially applied as a sinusoidal waveform between 100N and 800N, and increased by increments of 200N every 5000 cycles until failure or a maximum load of 2400N was reached. Peak strains at failure and cycles to failure were compared between the insert and plate-only conditions using a one-tailed independent samples t-test (a=0.05).

Results

There was no significant difference in the mean [SD] cycles to failure between the insert (1197 [ 650] cycles) and plate-only (598 [591] cycles). With respect to the fixation plate, the strains were significantly different between the insert and plate only conditions (p=0.02), transitioning from a compressive strain with the wedge (mean [SD] = -8.6 [-3.6] µe) to a tensile strain without the wedge (mean [SD] =12.9 [23] µe). The strains measured at the lateral cortex were also significantly affected by the inclusion of a synthetic bone insert (p=0.016), increasing from -55.58 (-54.30) µe when the insert was utilized to 23.66 (55.72) µe when only the plate was used for fixation.

Conclusion

The results presented suggest that the addition of a synthetic insert to a MOWHTO did not significantly increase its survivability under cyclical loading. However, the addition of a synthetic insert limited the tensile strains at the plate and lateral cortex. This suggests that the addition of a synthetic insert may protect the plate and lateral cortex from fracture during prolonged cyclic loading.