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

Background: 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. Methods: 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. Results: 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. Conclusion: 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. Clinical Relevance: Caution should be used when applying the score to fractures stabilized with rigid fixation methods without residual fracture gaps.