Ankle alignment on lateral radiographs. Part 1: sensitivity of measures to perturbations of ankle positioning.

BACKGROUND

In ankles with end-stage osteoarthritis or with total ankle replacement (TAR), radiographic landmarks based on joint surface morphology usually are obscured and inadequate for radiographic measurement. Furthermore, because of difficulty in reproducibly positioning the ankle for a standing radiograph, any radiographic measure to accurately describe ankle alignment must tolerate perturbations of ankle positioning on clinical radiographs. To identify a radiographic measure of anteroposterior tibial-talar alignment that meets those requirements, three methods were compared to determine their sensitivity to perturbations in ankle positioning.

METHODS

Ten cadaver ankles had lateral radiographs taken in varying ankle positions in nine prespecified positions in the transverse plane and in seven positions in the sagittal plane. The anteroposterior tibial-talar alignment was quantified by three methods. Sensitivities to changes of ankle position in each plane were then compared.

RESULTS

With the tibial-axis-to-talus ratio (T-T ratio: the ratio into which the midlongitudinal axis of the tibial shaft divides the longitudinal talar length), sensitivity to ankle positional changes in either plane was lowest, with errors associated with 10 degrees of ankle malpositioning being 2.2%. The posterior-tibial-line-to-talus ratio (P-T ratio: a similar ratio, but using the posterior longitudinal line of the tibial shaft) showed higher sensitivity in the transverse plane than the T-T ratio, though the associated errors in either plane were nearly comparable. The tibial-axis-to-lateral-process distance (T-L distance: the perpendicular distance from the tibial axis to the tip of the lateral talar process) showed highest sensitivity in both planes.

CONCLUSIONS

The T-T ratio tolerated perturbations of ankle positioning best among the tested measures. This measure is potentially applicable to clinical radiographic measurement when determining the anteroposterior tibial-talar alignment in ankles with articular degeneration or TAR. The P-T ratio also appears to have reasonable tolerance.