Smartphone versus knee ligament arthrometer when size does not matter.

Purpose: The use of available mechanical methods to measure anterior tibial translation (ATT) in anterior cruciate ligament (ACL)-deficient knees are limited by size and costs. This study evaluated the performance of a portable device based on a downloadable electronic smartphone application to measure ATT in ACL-deficient knees.

Methods: A specific smartphone application (SmartJoint) was developed for this purpose. Two independent observers nonsequentially measured the amount of ATT during execution of a maximum manual Lachman test in 35 patients with an ACL-deficient knee using KT 1000 and SmartJoint on both involved and uninvolved knees. As each examiner performed the test three times on each knee, a total of 840 measurements were collected. Statistical analysis compared intertest, interobserver and intra-observer reliability using the interclass correlation coefficient (ICC). An ICC > 0.75 indicates excellent reproducibility among measurements.

Results: Mean amount of ATT on uninvolved knees was 6.1 mm [standard deviation (SD = 2)] with the KT 1000 and 6.4 mm (SD = 2) with SmartJoint. Mean side-to-side difference was 8.1 mm. (SD = 4) with KT 1000 and 8.3 mm (SD = 3) with SmartJoint. Intertest reliability between the two methods yielded an ICC 0.797 [95 % confidence interval (CI) 0.717-0.857] for the uninvolved knee and of 0.987 (CI 0.981-0.991) for the involved knee. Interobserver ICC for SmartJoint and KT 1000 was 0.957 (CI 0.927-0.976) for the uninvolved knee and 0.992 (CI 0.986-0.996) for the involved knee and 0.973 (CI 0.954-0.985) for the uninvolved knee and 0.989 (CI 0.981-0.994) for involved knee, respectively.

Conclusion: The performance of SmartJoint is comparable and highly correlated with measurements obtained from KT 1000. SmartJoint may provide a truly portable, noninvasive, accurate, reliable, inexpensive and widely accessible method to characterize ATT in ACL-deficient knee.