Instrumented Measurement of Ankle Laxity

Abstract & Commentary

Synopsis: Clinically objective and reliable measures of ankle displacement and rotation were demonstrated with the use of a portable ankle arthrometer in subjects without a history of injury to the ankle.

Source: Kovaleski JE, et al. Instrumented measurement of anteroposterior and inversion-eversion laxity of the normal ankle joint complex. Foot Ankle Int 1999;20(12):808-814.

Kovaleski and colleagues used a portable ankle arthrometer to measure dominant and non-dominant ankle anteroposterior (AP) translation and inversion-eversion (I-E) rotation, and determined the test-retest reliability and precision of the device. The arthrometer consisted of an adjustable plate fixed to the foot, a load-measuring handle attached to the foot plate, and a pad attached to the tibia. Subjects were tested while supine on a table with the knee in extension and the heel securely fixed on the foot plate. A computer with an analog-to-digital converter recorded six-degrees-of-freedom motion. AP displacement in millimeters was measured at 75 N, 100 N, and 125 N force loads. I-E rotation in degrees was recorded at 2000 N-mm, 3000 N-mm, and 4000 N-mm loads. The measurements reflected the motion of the calcaneus with respect to the tibia. The ankles of 41 male and female volunteers without a history of previous injury were tested at 0° of flexion.

To determine intratester reliability for both AP and I-E measurements, the device was removed and then repositioned for a retest procedure, and intraclass correlation coefficients were calculated. The standard error of measurement was used to estimate precision of measurement, and t-tests were used to compare differences between dominant and nondominant sides. An ICC of 0.75 or better was considered high reliability of measurement.

The reliability coefficients ranged from 0.82 to 0.89 for AP displacement, and 0.86 to 0.97 for I-E rotation. The range of AP displacement was from 10.18 mm (SEM = 1.23) at 75 N load to 18.47 mm (SEM = 2.06) at the 125 N load. For I-E rotation, the range was from 23.14° (SEM = 1.74) at 2000 N-mm to 47.41° (SEM = 2.47) at 4000 N-mm. No significant differences were found between dominant and nondominant sides for AP displacement and I-E rotation.

COMMENT BY DAVID H. PERRIN, PhD, ATC

Instrumented measurement of knee joint displacement has received a great deal of attention in the scientific literature and in orthopedic practice. The advantages of instrumented arthrometry over clinical measurements are standardization of applied force and quantification of observed displacement. This study demonstrated high reliability and precision of measurement using a portable ankle arthrometer. As with the KT1000 knee arthrometer, the advantage of a portable ankle arthrometer is the ability to measure displacement without the need for stress radiography.

As with any laboratory or clinical instrument, it is important to establish the reliability and precision of joint arthrometers. While the study of healthy populations is a logical starting point, the primary limitation of this paper was that subjects with a history of ankle ligamentous injury were not included in the study. The reliability of ankle arthrometry in healthy subjects cannot be generalized to patients who have partial or complete ligament tears. As such, the next phase of establishing the reliability and validity of this device should include assessment of the injured population for which the device has been designed.

Additional limitations of the device would appear to be related to test position and isolation of the joints contributing to motion of the ankle. The sensitivity of the device in assessing the structures most commonly injured with inversion ankle sprain might be enhanced if measurement of I-E rotation occurred with foot placement in plantar flexion, rather than the position of 0° of flexion described in the methods of this study. Others have discussed the importance of determining the individual contribution of talocrural and subtalar joint laxity to ankle joint instability, in order to effectively treat patients with recurrent ankle sprain.1 The arthrometer used in this study measures the sum of motions at the talocrural and talocalcaneal joints, and thus does not quantify the individual contribution of these joints to instability of the ankle complex. Separating the contribution of each component of instability diagnostically would help direct therapeutic intervention.

Joint arthrometers are not without their limitations, yet they do provide some advantages for the quantification of laxity and the accompanying functional instability that frequently occurs with joint injury. Kovaleski et al have made a useful contribution to our understanding of instrumented assessment of laxity of the normal ankle joint complex.

Reference

1. Hertel J, et al. Talocrural and subtalar joint instability after lateral ankle sprain. Med Sci Sports Exerc 1999;31(11):1501-1508.

The ankle arthrometer used in the study by Kovaleski et al determined laxity by measuring:

a. talar tilt.

b. sum of motions at the talocrural and talocalcaneal joints.

c. talocrural motion.

d. subtalar motion.