Influence of Ankle Position and Radiographic Projection Angle on Measurement of Supramalleolar Alignment on the Anteroposterior and Hindfoot Alignment Views.

BACKGROUND: Using digitally reconstructed radiographs (DRRs), we determined how changes in the x-ray beam projection angle from the horizon, tibiotalar joint angle, and axial rotation of the foot influenced measurements of the medial distal tibial angle (MDTA) on the anteroposterior (AP) and hindfoot alignment views (HAV).
METHODS: Seven cadaver foot-ankle specimens were scanned by computed tomography (CT) at fixed tibiotalar joint positions, ranging from 15 degrees of dorsiflexion to 25 degrees of plantarflexion. DRRs were created from each CT scan to simulate alterations in the horizontal projection angle (0 to 25 degrees) and foot axial rotation (-30 to 30 degrees). The MDTA was measured on each DRR and compared with that quantified on the baseline HAV and AP view.
RESULTS: Altering the horizontal projection angle by ≥5 degrees and >10 degrees significantly altered the MDTA for the AP view and the HAV, respectively. Shifting dorsiflexion and plantarflexion caused minor changes in the MDTA that were only statistically significant for the HAV. Axial rotation significantly changed the MDTA on both views, but deviations were more pronounced for the HAV.
CONCLUSIONS: Compared with the HAV, the MDTA on the AP view was less sensitive to changes in foot-ankle position. However, increasing the tilt of the x-ray beam from the horizon altered the MDTA on the AP view substantially. CLINICAL RELEVANCE: To avoid misinterpretation of the MDTA, we recommend using the AP view to quantify supramalleolar alignment as it is less sensitive to changes in positioning of the foot-ankle. When acquiring an AP film, the x-ray beam should be directed along the horizon to ensure consistent assessment of the MDTA across patients.