Does Fixation Method Affect the Correlation of mRUST and Healing Strength?

Previous biomechanical studies correlating strength of healing with mRUST are limited to a single mode of intramedullary fixation. This study evaluated the correlation between mRUST and biomechanical strength in a fracture healing model using fixation methods that generated different modes of healing.

Biomechanical data were sourced from previous ovine osteotomy studies and included 24 sheep, 12 fixed with rigid constructs and 12 fixed with a single relatively stable construct. The sheep were sacrificed at 9 weeks and the tibiae were loaded to failure in torsion. Load to failure was recorded as a percentage of the contralateral intact tibia. Standardized 9 week radiographs were reviewed and the mRUST score was recorded.

A fracture was considered biomechanically healed if it retained 72% of the strength of the contralateral side. In the rigid group, the mRUST score correctly determined the biomechanical healing state in 6/12 fractures. Specifically, it correctly labeled 6 fractures ununited and incorrectly labeled 6 ununited fractures as healed. In the relative stability group, the mRUST correctly determined the biomechanical healing state in 9/12 fractures. Specifically, it correctly labeled 1 fracture ununited and 8 fractures united. The mRUST correctly predicted healing in 9/12 fractures stabilized with a residual fracture gap, but only 4/12 stabilized without a residual fracture gap.

This is the first study to evaluate the biomechanical accuracy of the mRUST score in fracture models using both rigid and relatively stable fractures. The results suggest a disparity in the accuracy of the mRUST to predict biomechanical healing in rigid fixation versus relative fixation constructs and in fractures stabilized with and without residual fracture gaps.

Caution should be used when applying the score to fractures stabilized with rigid fixation methods without residual fracture gaps.