During the 1980s several ligament testing devices were developed in an attempt to quantitate antero-posterior (AP) displacement of the knee joint. The subjective clinical grades given to the Lachmann, pivot shift and anterior draw test often vary from examiner to examiner. Objective quantitative ligament testing devices provide the opportunity to compare populations of patients more accurately. There are a number of different ligament testing devices currently available. They are not commonly used in the United Kingdom and I am aware of only 2 or 3 surgeons besides myself in the country that routinely use these devices. The device that I use is the KT1000 arthrometer which has most often been reported in the literature.
The KT1000 is a device that was developed to measure anterior and posterior translation of the tibia relative to the femur in a clinical setting. The patient is placed into the supine position on the examination table, with the thighs resting on the bolster. This keeps the knees in approximately 30° of flexion. The heels are position symmetrically on a positioning cup, which maintains the tibia in a symmetric position of rotation (15°). The examiner must make certain that the patient’s thighs remain relaxed throughout the examination. The arthrometer has 2 sensing paddles: one is positioned on the patella, the other is placed on the tibial tubercle. The arthrometer is then secured to the lower leg using 2 Velcro straps. The sensor pads are freely moveable so that the difference in the AP displacement is determined by the distance or relative motion between the 2 pads. The calibrate gauge allows the examiner to determine the amount of displacement to the nearest 0.5mm. Next the examiner must determine the zero point. This is done by performing or forming several anterior and posterior translations through a handle that is located 10cm distal to the joint line and adjusting the calibrator gauge to the zero position. This should be performed several times to ensure that the true zero point has been established. As the examiner applies an anterior force through the handle a tone is heard at 15lb (67 newtons) of force. A second tone is heard as the force reaches 20lb (89 newtons) and a third tone is heard at 30lb (133 newtons). Finally a manual maximum anterior displacement is performed with the force being applied anteriorily through the proximal calf rather than the handle. The readings at 15, 20, 30lb and manual maximum are all recorded. Tests should be repeated on the uninvolved extremity to determine normal values. The side to side differences are then calculated at each force application including the manual maximum test. The arthrometer should be zeroed and recalibrated after each test. A quads active test can be performed by asking the patient to gently lift the heels while the knee is resting at approximately 30° of flexion thus causing contraction of the quadriceps muscles. This will cause disruption of the KT1000 needle. The amount of displacement is then recorded. A quads neutral angle can be determined. This is the angle of knee flexion at which the quadriceps contraction results in neither anterior nor posterior tibial translation. This angle is approximately 70°. At this position, the patellar tendon is perpendicular to the joint surface of the tibia. This angle is more important in determination during posterior cruciate ligament laxity testing than anterior cruciate ligament laxity assessment.
Accurate and reproducible results using a KT1000 require an experienced examiner as inter and intra examiner variability exists.
The side to side differences comparing the injured and normal extremity allow determination of diagnosis of increased anterior posterior laxity. A side to side difference of less than 3mm at 30lb and manual maximum is considered normal. A side to side difference between 3 and 5mm is considered a grey area. A side to side difference of more than 5mm is considered diagnostic of an ACL tear.
A KT1000 assessment is clinically more accurate and useful at diagnosing ACL instability than an MRI scan.
The KT1000 can also be used to follow-up post-operative results. The increased side to side difference pre-operatively should decrease to within normal range post-operatively. There should be no increased side to side difference over time post-operatively in a well performed ACL reconstruction. There is some data to suggest that hamstring ACL grafts stretch out over time which will show a progressively increasing side to side difference post-operatively. This may be due to the graft physically stretching or the fixation slipping, both of which potentially are issues with hamstring ACL grafts. The other reason the side to side difference may increase post-operatively, is if there is a re-tear of the ACL graft or of the normal knee following a further injury.