Authors: Andrew G Tsai [1]; Volker Musahl (corresponding author) [1]; Hanno Steckel [1]; Kevin M Bell [1]; Thore Zantop [1]; James J Irrgang [1]; Freddie H Fu [1]
Background
The anterior cruciate ligament (ACL) is the primary restraint to anterior tibial translation, secondary restraint to valgus rotation, and tertiary restraint to internal tibial rotation [1]. Rotational knee laxity may predict later osteoarthritis [2, 3] and is closely correlated with patient discomfort [4]. Though reduced after single-bundle ACL reconstruction [5], rotational laxity may still exist, even though the anterior translation has been adequately restored [6, 7]. Anatomic double-bundle reconstruction, in contrast, replaces the anteromedial (AM) bundle as well as posterolateral bundle (PL) [8, 9]. Anatomic ACL reconstruction may better restore normal knee kinematics in six degrees of freedom (6-DOF) [10, 11, 12, 13]. Diagnosis of rotational knee laxity after ACL tears in the office is based on patient history and subjective un-instrumented physical examination. Concomitant injury to other ligamentous structures can lead to false positives, and patient guarding can reduce the sensitivity of tests and lead to false negatives.
Un-Instrumented Physical Exams
Un-instrumented physical examination is the gold standard for assessing knee ligamentous injuries, although these exams are subjective and dependent on examiners skill and experience. The Lachman test, which is performed with the knee at 30[degrees] of flexion, is the most sensitive test [14]. At this flexion angle, the PL bundle is starting to become tight and is the primary restraint to anterior tibial translation. A difference of >3 mm in anterior tibial translation as compared to the uninjured, contralateral knee as well as a soft endpoint indicate a positive Lachman test, which is indicative of injury to the ACL, both in the AM and PL bundles [15]. The less-sensitive anterior drawer test is tested with the knee at 90[degrees] of flexion. When performing the anterior drawer test, the examiner draws the proximal tibia forward and uses his thumbs to palpate the tibiofemoral step off; the test is repeated with the foot in neutral, internal, and external rotation. The quality of the end point as well as the difference in translation between the patient's injured and uninjured knees indicates damage to the ACL, particularly the AM bundle, as well as secondary supports [16]. The Slocum test is similar to the anterior drawer test except it tests for rotational laxity and is performed with the foot and tibia internally rotated 30[degrees] and with the tibia externally rotated 15[degrees] [16, 17]. The internal and external rotation tightens up the lateral and medial ligamentous structures respectively. A positive Slocum test is indicative of anterolateral or anteromedial laxity. The pivot-shift test is the most specific test for ACL injury and is oftentimes only testable during examination under anesthesia (EUA). To administer the pivot-shift test, the tester rotates the patient's tibia inward while the knee is flexed at 30[degrees] [18, 19, 20]. The tester then extends and subsequently flexes the knee. If a pivot shift is present, the examiner should feel an anterior subluxation of the knee during extension and a glide, clunk, or gross reduction during flexion, corresponding to grades I, II, or III, respectively [21, 22].
Instrumented Physical Exams
There are several commercially-available arthrometers used clinically to quantify anterior-posterior knee laxity, including the KT-1000 (MEDmetric Corporation, San Diego, CA), Rolimeter (Aircast, Summit, NJ), Acufex Knee Signature System (Acufex Inc., Mansfield, MA), and Stryker Knee Laxity Tester (Stryker Corporation, Kalamazoo, MI) [16, 23, 24], though the KT-1000 is the most widely used [16]. The Rottometer [25] and Vermont Knee Laxity Device [26] may also be used to determine rotational knee laxity. The Rottometer is a modified chair design with the foot strapped down to a plate; knee rotation is measured with a goniometer. The Rottometer is a simple, easy-to-use device; however, it is not portable and is not capable of measuring off-axis loading. The Vermont Knee Laxity Device is a large, complex device that is capable of accurately measuring knee kinematics and simulating various loading situations. Its size, however, prohibits it from being portable and being used in the office setting. A simple device that measures rotation in a non-invasive manner is the Lars Rotational Laxiometer [27], which is strapped externally to the subjects' tibia. The Lars Rotational Laxiometer, however, is not able to measure the moment applied by the observer during testing and is unable to cancel out coupled motion of the femur; both were suggested as deficiencies by Bleday, et al. Additionally, computer assisted surgery (CAS) devices make it possible for a surgeon to accurately measure kinematics of the knee, but they are costly, complex, and require the patient to undergo surgery and thus can not be used for clinical follow-up [28, 29].
There is no simple, commercially available device to measure knee rotation [30, 31]. In a previous study, a simple device for non-invasive measurement of rotational laxity was described [32]. This device has been shown to have acceptable levels of test re-test reliability to measure knee rotation in a best case scenario in cadaveric knees. Therefore, the objective of this study was to determine the reliability of the new device to measure knee rotation in human subjects with normal knees. Specifically, the intra-tester reliability within a single testing session, the test-retest reliability between two testing sessions, and the inter-tester reliability were evaluated. It was hypothesized that knee rotation measurements obtained with the new device will have acceptable limits of reliability for clinical use and interpretation.
Methods
The rotational knee laxity measurement device consists of an …
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