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

Abstract

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.