A six-degrees-of-freedom instrumented linkage for measuring the flexibility characteristics of the ankle joint complex.

Diagnosis of ligament injuries to the ankle joint complex is a difficult clinical problem which relies primarily on manual physical examination and on radiographic evaluations. In an attempt to develop a reliable, quantitative diagnostic tool for such injuries we developed a six-degrees-of-freedom instrumented linkage (Ankle Flexibility Tester-AFT) capable of measuring the flexibility characteristics of the ankle joint complex in vivo. The unique non-serial structure of this linkage was such that these characteristics were recorded directly in an ]anatomical coordinate system which enhanced clinical interpretation. The goal of the present study was to develop this linkage and to test its accuracy and its test-retest reliability. The positional accuracy of the AFT was measured and was found to be better than 0.5 mm for translation and 1.2 degrees for rotations. The results obtained from a study conducted on two cadaveric specimens indicate negligible effect of loading rate on the flexibility characteristics within the range of possible manually applied loads. Finally, the reliability of the AFT was examined from test-retest studies conducted on a total of thirteen young healthy volunteers. The intraclass correlation coefficient (ICC), calculated from the test-retest data, indicated a reliability higher than 0.85. It was concluded that the high reliability and accuracy of the AFT, its simplicity of operation, the easy alignment procedure, the on-line load-displacement results, and the elimination of complex data processing render this device suitable for use in the clinic as well as in the research laboratory.