To perform a posterior stress X-ray utilizing the Telos device, the patient is positioned in the lateral decubitus position, index knee dependent on the radiolucent table. The knee is positioned at 90° of flexion inside the Telos device (Fig. 5.3). The knee must be in neutral rotation. A 15-kPa force is exerted on the anterior tibial tubercle, and a lateral X-ray is performed. The knee must be positioned in a true lateral position, which should be confirmed by superimposition of the lateral and medial femoral condyles on the radiograph.

Measurement of displacement is performed by using the Telos template, aligning the inferior horizontal line parallel to and overlying the tibial plateau. The perpendicular “zero” line is then lined up with the posterior border of the tibial plateau. The measurement of posterior displacement is then made in millimeters between the posterior border of the tibial plateau and the posterior border of femoral condyles.

The degree of posterior displacement is measured with a template on the lateral stress X-ray (Fig. 5.4). In this example, the posterior displacement is 17 mm.

The difficulties with this method are:

One of the most significant challenges with the Telos system is ensuring standardized measurement. Following a standardized protocol when performing the radiographs produces reliable and reproducible measurements [10].

A recently published study conducted over 12 years using the Telos device for the evaluation of knee instability in more than 1000 patients found it to be reliable and effective at diagnosing posterior laxity [11]. They found that a measurement of greater than 8 mm of posterior displacement was diagnostic for complete PCL rupture, while a measurement of greater than 12 mm was indicative of injury to secondary supporting structures as well (PLC and/or PMC).

The Telos system has not been as helpful in assessing the magnitude of anterior laxity of the knee. Rijk et al. found that an anterior displacement of more than 7 mm was abnormal, with a false-negative rate of 12 % [12]. The patient and device positioning for anterior stress testing is essentially identical to the posterior stress exam, with the position reversed.

Recently, Dejour et al. demonstrated that the Telos device in conjunction with clinical examination (pivot shift test) was helpful in differentiating partial from complete ACL ruptures [13].

The KT-1000 knee ligament arthrometer (MEDMetric Corp., San Diego, CA), developed by Dale Daniel and Larry Malcolm [14], has become the standard for the measurement of ACL laxity. Starting from its introduction in the early 1980s, it has continued to be found to be accurate and reliable in the measurement of anterior translation of the tibia on the femur [15]. It has proven to have strong reliability, with good inter- and intra-rater performance [16]. It has recently performed equally compared with intra-operative computer-assisted surgery/navigation [17]. The device is used with the patient supine and a support platform placed under both thighs to maintain approximately 25–35° flexion of both knees. The feet are supported on the lateral aspects by a second platform to ensure the same relative rotation of both lower legs. This position is ideal for the performance of an instrumented Lachman test on both knees. The arthrometer is placed secured with Velcro straps on the knee and lower leg such that the force pad is located over the tibial tubercle, and the patellar pad is resting on the anterior surface of the patella. The patella pad is gently stabilized while the force handle is pushed and pulled to achieve tibiofemoral translation readings (Fig. 5.5). The maximum manual test has been found to have the highest diagnostic value for the determination of ACL laxity and is performed by using a hand behind the calf to produce a maximal anterior translation force [18].

The best results with the KT-1000 are obtained when comparing side-to-side difference within the same patient, and when the same examiner performs the repetitive exams. Though the KT-1000 arthrometer is simple to use, there is still an association of increased accuracy and reproducibility with the experienced user.

The KT-1000 has not, however, achieved the same level of acceptance for the quantitative measurement of posterior instability. Daniel [19] described the method of measuring posterior laxity by first determining the quadriceps neutral point.

The principle of the measurement as described by Daniel is to determine the four levels of anterior-to-posterior motion:

Initially, the patient contracts the quadriceps muscle sufficiently to bring the tibial forward to the “quadriceps neutral” position.

The posterior motion from this point is then recorded as the posterior sag, and then the posterior displacement with 20 pounds of posterior force is measured and noted as the posterior displacement (Fig. 5.6). The total amount of posterior motion is determined when these two later values are added.

In our experience, it is often difficult to get the patient to contract their quadriceps sufficiently to bring the tibia fully forward to the neutral position. This amount of forward displacement is often underestimated. Johnson presented a study to the PCL study group in 1995, comparing the KT value against the stress X-ray [20]. The results were:

When the millimeters of displacement of the KT is expressed as a percentage of the Telos: