Virtual Mechanical Testing from Low-Dose CT Scans Predicts Tibial Fracture Time to Union and Outperforms Subjective Outcomes Scoring

Abstract

Background: Quantitative outcomes assessment remains a persistent challenge in orthopaedic trauma. Although patient-reported outcomes measures (PROMs) and radiographic assessments such as RUST scores are frequently used, very little evidence has been presented to support their validity for measuring structural bone formation or biomechanical integrity. Methods: A sequential cohort of tibial shaft fracture patients was prospectively recruited for observation following standard reamed intramedullary nailing in a Level I trauma center. Follow-ups at 6, 12, 18, and 24 weeks included X-rays and completion of PROMs (EQ-5D and pain scores). Low-dose computed-tomography (CT) scans were also completed at 12 weeks. Scans were reconstructed in 3D and subjected to virtual mechanical testing via the finite element method to assess fracture limb torsional rigidity relative to intact bone. Results: Patients reported progressive longitudinal improvement in mobility, self-care, activity, and health over time, but the PROMs were not correlated with structural bone healing. RUST scoring showed moderate intra-rater agreement (ICC = 0.727), but the scores at 12 weeks were not correlated with time to union (R2 = 0.103, p = 0.193) and were only moderately correlated with callus structural integrity (R2 = 0.346, p = 0.010). In contrast, patient-specific virtual torsional rigidity (VTR) was significantly correlated with time to union (R2 = 0.383, p = 0.005) and clearly differentiated one case of delayed union (VTR = 10%, union at 8 months) from the rest of the normally healing cohort (VTR > 60%, median union time 19 weeks) using CT data alone. Conclusions: PROMs provide insight into the natural history of the patient experience after tibial fracture, but have limited utility as a measure of structural bone healing. RUST scoring, although repeatable, is not a valid longitudinal predictor of time to union. In contrast, virtual mechanical testing from low-dose CT scans provides a quantitative and objective structural callus assessment that reliably predicts time to union and may enable early diagnosis of compromised healing. Level of Evidence: Diagnostic Level II