Significance of changes in the reference position for measurements of tibial translation and diagnosis of cruciate ligament deficiency

Measurements of tibial translation in response to an external load an used in clinical and laboratory settings to diagnose and characterize knee-ligam ent injuries. Before these measurements can be quantified, a reference posi tion of the knee must be established (defined as the position of the knee w ith no external forces or moments applied). The objective of this study was to determine the effects of cruciate ligament deficiency on this reference position and on subsequent measurements of tibial translation and, in so d oing, to establish a standard of kinematic measurement for future biomechan ical studies. Thirty-six human cadaveric knees were studied with a robotic/ universal force-moment sensor testing system. The reference positions of th e intact and posterior cruciate ligament-deficient knees of Is specimens we re determined at full extension and at 30, 60, 90, and 120 degrees of flexi on, and the remaining five-degree-of-freedom knee motion was unrestricted. Subsequently, under a 134-N anterior-posterior load, the resulting knee kin ematics were measured with respect to the reference positions of the intact and posterior cruciate ligament-deficient knees. With posterior cruciate l igament deficiency, the reference position of the knee moved significantly in the posterior direction, reaching a maximal shift of 9.3 +/- 3.8 mm at 9 0 degrees of flexion. For the posterior cruciate ligament-deficient knee, p osterior tibial translation ranged from 13.0 +/- 3.4 to 17.7 +/- 3.6 mm at 30 and 90 degrees, respectively; when measured with respect to the referenc e positions of the intact knee. When measured with respect to the reference positions of the posterior cruciate ligament-deficient knee, these values were significantly lower, ranging from 11.7 +/- 4.3 mm at 30 degrees of kne e flexion to 8.4 +/- 4.8 mm at 90 degrees. A similar protocol was performed to study the effects of anterior cruciate ligament deficiency on 18 additi onal knees. With anterior cruciate ligament deficiency, only a very small a nterior shift in the reference position was observed. Overall, this shift d id not significantly affect measurements of tibial translation in the anter ior cruciate ligament-deficient knee. Thus, when the tibial translation in the posterior cruciate ligament-injured knee is measured when the reference position of the intact knee is not available, errors can occur and the mea surement may not completely reflect the significance of posterior cruciate ligament deficiency. However, there should be less corresponding error when measuring the tibial translation of the anterior cruciate ligament injured knee because the shift in reference position with anterior cruciate ligame nt deficiency is too small to be significant. We therefore recommend that i n the clinical setting, where the reference position of the knee changes wi th injury, comparison of total anterior-posterior translation with that of the uninjured knee can be a more reproducible and accurate measurement for assessing cruciate-ligament injury, especially in posterior cruciate ligame nt-injured knees. Similarly, in biomechanical testing where tibial translat ions are often reported for the ligament-deficient and reconstructed knees, a fixed reference position should be chosen when measuring knee kinematics . If such a standard is set, measurements of knee kinematics will more accu rately reflect the altered condition of the knee and allow valid comparison s between studies.