Is the use of retrospectively collected radiographic scores an accurate method to determine time to union? A retrospective clinical cohort.

Abstract

To evaluate the accuracy of time to union (TTU) in a cohort of patients after intramedullary nailing (IMN) of tibial shaft fractures where radiographic union was measured retrospectively with uncontrolled intervals between radiographs. Design: Retrospective cohort. A single Level I academic Trauma Center. Included were adult patients (18 years and older) treated with IMN for an acute tibial shaft fracture (OTA/AO 42A, 42B, 42C) between January 2020 to January 2025 with at least 6 week and 12-week post-operative radiographic evaluations. Patients who required a reoperation to promote union were excluded. The modified radiographic union scores for tibial fractures (mRUST) were determined at each post-operative radiographic follow-up. The observed TTU was defined as the days from IMN to radiographic union (defined as mRUST ≥ 12). Cohort median time intervals were determined for: the time from IMN to the radiograph immediately prior to union; the time between the radiograph prior to union and union, and the TTU. The inter-quartile range to median ratio (IQR:median) was calculated for these intervals to normalize the time interval ranges and was used to define accuracy. Accuracy was defined as follows: low accuracy = IQR:median ≥0.5; moderate accuracy = IQR:median 0.2-0.5; and high accuracy = IQR:median ≤0.2. A predicted TTU for the cohort was calculated by considering all mRUST scores from the entire cohort over time to estimate the time at which 50% of the population would be expected to achieve radiographic union. This was a way to account for the changing nature of mRUST scores with time in between clinical visits. To determine if the observed TTU over- or underestimated the predicted TTU, the predicted TTU was compared to the observed TTU by taking the difference between the two. 89 patients with a mean age of 43.1 years (SD = 13.7 years) and 65.9% male were included. The median observed TTU was 96 days (mean 208.6, SD 363.8 days) with an IQR:median of 0.9, indicating low accuracy. The median time from surgery to the radiograph immediately prior to union was 47 days (mean 58.7, SD 37.3 days) with an IQR:median of 0.4 (moderate accuracy). The median time between the radiograph immediately prior to union and union was 49 days (mean 150, SD 358 days) with an IQR:median of 1.1 (low accuracy). The predicted TTU was 61.8 days. The observed TTU over-estimated TTU by 34.2 days. In patients treated with tibial IMN, retrospectively determining radiographic time to union (TTU) with uncontrolled interval radiographic evaluations was an inaccurate method to determine TTU. The observed TTU over-estimated a predicted TTU by 34.2 days. These findings highlight the inaccuracy of retrospectively determined TTU in orthopaedic clinical cohorts. Level III.