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Objectives and questions: Sagittal alignment changes can significantly impact knee kinematics, soft tissue tensioning, and implant longevity in total knee arthroplasty (TKA). Traditional instrumentation in non-robotic TKA use the intramedullary axis as a reference for femoral component positioning. Recently, short-segmented knee radiographs (SSR) have been increasingly utilized to reduce patient radiation exposure. Give the potential impact of femoral component flexion-extension positioning on TKA balance and kinematics, this study aimed to analyze the difference in femoral component position when referencing the femoral intramedullary canal in SSR versus conventional lateral knee radiographs (CLR).

Material and methods: The intramedullary axis was determined in 153 lateral knee radiographs, both at full length as well as at a 12.5 cm segment. The angle between the axes was measured, and the distal position of the SSR axis relatively to the CLR axis was reported. The arc length formula L= θ × r was employed to calculate the distance between both axes at the femoral entry point of the intramedullary (IM) guiding rod. Cases with a difference greater than 2° were classified as outliers. A one-sample t-test was conducted to assess whether the mean angle difference was significant.

Results: The mean femoral length in CLR was 19.91 cm (SD; 3.22 cm). On average, the intramedullary axis determined in SSR was 2.23° (SD; 1.14°) more posterior compared to CLR. This angular difference corresponds to a 48.63 mm more posterior femoral entry point of the IM guiding rod. The angle difference was found to be statistically significant (p < 0.01). There was no statistically significant difference between females and males (females [n = 85; 55.56%]: 2.33° [SD; 1.83] vs. males [n = 68; 44.44%]: 2.12° [SD; 1.10°]; p = 0.285). In 83 (54.25%) cases, the angle difference exceeded 2°.

Discussion and conclusions: The intramedullary axis measured on SSR was consistently more posterior compared to CLR, leading to an average of 2.23° increased femoral component flexion and a 48.63 mm more posterior femoral entry point of the IM guiding rod. Surgeons should consider these differences when using SSR, as they may ultimately influence knee kinematics and TKA outcomes.