Measurement of rotational laxity of the knee: in vitro comparison of accuracy between the tibia, overlying skin, and foot.

BACKGROUND

Posterolateral corner (PLC) injuries are difficult to diagnose and cause significant morbidity. The ideal method for the dial test and its accuracy remain unclear.

PURPOSE

This study compares the accuracy of measuring tibial external rotation at the skeletal level to measuring the patella-tubercle angle (PTA) and the thigh-foot angle (TFA) in the supine position to assess the most accurate method to measure rotation during the dial test.

STUDY DESIGN

Controlled laboratory study.

METHODS

Measurements were compared simultaneously using rotational goniometers at a cutaneous splint over the tibia, at a foot splint, and directly from the tibial skeleton. Six lower limbs were used. The femur was held rigidly and the knee tested at 90° and 30° of flexion. External rotation torque up to 8 N·m was applied through the foot splint, and the rotations were measured by 2 testers.

RESULTS

Measurements at the tibial splint and directly on the tibia showed significant correlation at both knee flexion angles. The mean tibial external rotation was 24° at 90° of flexion and 26° at 30° of flexion (P < .05). The soft tissue effect caused the tibial splint to overestimate rotations by a mean of 6° and 9° at 90° and 30° of flexion, respectively. Foot splint measurements did not correlate significantly with tibial rotation, overestimating rotations by a mean of 103%. Intratester and intertester intraclass correlations were significant for the skin-mounted tibial splint measurements at both flexion angles but not for foot splint measurements at either flexion angles.

CONCLUSION

Rotation of the foot did not accurately represent the tibial external rotation at the knee, which could be measured more accurately by an instrument resting on the skin via a molded tibial splint. These results suggest that the PTA, and not the TFA, should be used in the dial test. This would support the use of the supine position during the dial test.

CLINICAL RELEVANCE

The dial test is a commonly used method for diagnosing PLC injuries. This study helps to identify the ideal position and measuring points to use for this test; measurements based on the tibia were more accurate than those that used rotation of the foot.