The need for a standardized whole leg radiograph guideline: The effects of knee flexion, leg rotation, and X-ray beam height

Introduction: Lower limb malalignment is a major risk factor for knee osteoarthritis (OA) and is mainly diagnosed using the Hip Knee Ankle Angle (HKA). Therefore, accurate HKA measurements are indispensable. Objectives: This study aimed to research the effects of knee flexion, leg rotation, and X-ray beam height on the accuracy of the HKA measurement. To convert our findings into a guideline for obtaining whole leg radiographs (WLR) in favor of accuracy and reproducibility. Methods: An in vitro experiment was designed using sawbones (in 5° varus) of the whole lower limb, fixated in different leg rotation angles, knee flexion angles, and three different X-ray beam heights. Results: The HKA measurement error was 1° per 20° of leg rotation without flexion (P < .01). When 5° of flexion was added, the HKA measurement error was 0.8° per 20° rotation (P < .01). With 15° knee flexion, the HKA measurement error became 4° per 20° rotation (P < .01). Varying X-ray beam heights of 5cm (P = .959) and 10 cm (P = .967) did not cause any significant measurement errors. Conclusion: This study showed that leg rotation alone (without knee flexion) can lead to clinically relevant measurement errors when exceeding 9°. When there is 15° of knee flexion and 10° leg rotation the error becomes approximately 2°. Varying X-ray beam heights within a range of 10 cm does not affect the accuracy. Based on these findings, we propose guidelines for system setup and patient positioning during a WLR that is easy to apply and aims at minimizing errors when measuring the HKA.