KT1000 ARTHROMETER : TEST KNEE LAXITY

The KT1000 arthrometer was designed to measure the anterior translation of the tibia while maintaing the femur in position. The results obtained provide an objective diagnostic of the state of the ACL to the medical practitioner. 

While running a test, the patient is placed in the supine position on an examination table. A bolster (provided with the KT1000) is placed under the thighs so that the knees remain at approximately 30° of flexion. While placing the patient, the heels are positioned symmetrically on a positioning cup (also provided with the KT1000) which places the tibia a 15° rotation. Once correct positioning is achieved, the examiner can start placing the device on the knee of the patient.

Located at the top of the device, two rectangular shaped plastic pads serve as indicators to place the arthrometer on the knee. The first one (sensor pad) is positioned on the knee patella and the second one (TTA pad) on the anterior tibial tuberosity. The device is then attached to the leg by means of two straps. Further to this, the displacement is measured through the sensor pad which is the only moveable part of the KT1000. The displacement is consequently measured by calculating the relative motion between the sensor pad and the TTA pad that is given on a gauge located on top of the device. To run the test correctly, the examiner must always recalibrate the KT1000 arthrometer and determine the zero point. This is done by performing several anterior translations on the tibia by means of pulling the handle located on the arthrometer.

Once calibration is achieved, tests may be carried out to objectively assess knee laxity. While applying an anterior tibial translation by pulling on the handle, three distinct tones can be heard. These guide the examiner in knowing the load he is applying on the tibia through the device:

• Tone 1 : 15 lb (67 Newtons).
• Tone 2 : 20 lb (89 Newtons).
• Tone 3 : 30 lb (134 Newtons).

The healthy leg is always to be tested first followed by the injured leg. The side-to-side differences are then evaluated at each force, which allows the diagnosis of the state of the anterior cruciate ligament (ACL).

Over the past decades, the KT1000 was quickly referenced as the number one device for evaluating knee laxity. Results provided using this medical device might be read the following way.

(1) For a side-to-side difference residing between 3 and 5 mm, it is considered that anterior posterior laxity is unusually high. The ACL might therefore be partially or completely torn.

(2) For a side-to-side difference higher than 5 mm, it is considered that anterior posterior laxity is very high which inidicates a complete ACL tear. 

Nowadays, it is considered that a KT1000 test is clinically more accurate and useful when diagnosing ACL instability compared to an MRI scan. This arthrometer can also be used to follow-up post-operative results, but the fact that the forces are applied manually represents a risk (risk not present with the GNRB arthrometer).

After surgery, the side-to-side displacement differential should indeed decrease to a normal range (less than 3 mm). If the ACL reconstruction surgery and the rehabilitation following it were successful, there should indeed be no increased side-to-side difference left.

As the KT1000 has been around since the 1980s, a newer and more advanced arthrometer was of course developed since then : the GNRB & the DYNEELAX. Technology having drastically evolved over the past decades, the method of knee laxity assessment used by this device was of course based on a more computerized approach which consequently lead the GNRB to being a more reliable arthrometer (click here to find studies on the GNRB).

The method used by the GNRB & DYNEELAX mainly holds its position on the podium thanks to automation. This attribute, present at the core of the GNRB & DYNEELAX indeed places these as the leaders regarding knee laxity assessment as it has a direct impact of accuracy and more importantly reproducibility.

DYNEELAX offers the advanatge of also measuring knee rotation instability which is extremely useful to plan surgeries adequately.

Various sensors and parameters dispatched on the device help guide the medical practitioner during the knee laxity tests. Accuracy is thus more advanced as the displacements are measured at every force applied between 0 and 200 N (there is consequently much more data to analyse in comparison to the three displacements measured with the KT1000, see graph "GNRB stiffness curves" on the left). 

In addition, the GNRB & DYNEELAX are also safer to use after surgery as the forces applied on the knee are manually chosen. It therefore becomes possible to run GNRB & DYNEELAX tests at the very beginning of the rehabilitation program in order to control the healing process and maximise the chances of gaining knee stability.