Quantitative assessment of mechanical laxity in the functionally unstable ankle

Delineating between functional and mechanical instability in those with chronic ankle dysfunction is a challenging task. Current methods of assessing ankle ligamentous laxity are subjective in nature and limit our ability to identify the site and extent of instability; therefore, a need exists for objective laxity measurements. The purpose of this study was to determine whether subjects with self-reported, functional ankle instability (FAI) demonstrated increased mechanical laxity when tested with instrumented arthrometry and stress radiography.

Both ankles were tested in 51 subjects with self-reported unilateral FAI. An instrumented ankle arthrometer measured ankle-subtalar joint motion for total anteroposterior (AP) displacement (mm) during loading at 125 N and total inversion-eversion (I-E) rotation (degrees of ROM) during loading at 4 N x m. The Telos GA-II/E device provided either anterior or lateral stress (15 kp) while fluoroscopic radiographs were recorded for anterior displacement (mm) and talar tilt (degrees).

The arthrometry measurements of anterior and total AP displacement and the radiographic measurements of anterior displacement were greater (P < 0.05) in the FAI ankles when compared with the uninjured ankles. There were no differences in total I-E rotation, inversion rotation, or talar tilt between ankles when analyzed with either measurement technique.

The ability to objectively measure mechanical instability in the functionally unstable ankle is important to understanding the nature and cause of the instability. Ankle arthrometry and stress radiographic measurements are objective assessment tools for mechanical laxity. Despite finding greater laxity in the functionally unstable ankle, the clinical significance of the observed displacement remains unanswered. Further research is needed to determine the amount of laxity that constitutes mechanical instability and how this relates to FAI.