The Accuracy And Reproducibility Of The KT1000 Knee Ligament Arthrometer

The accuracy and reproducibility of the KT1000 has been reported by   a number of investigators.

The correlation between measurements of A-P displacement recorded by a skeletal pin motion and the KT1000 has been reported by Daniel (10) (Fig. 1) and Shoemaker (42).

The change in anterior displacement after ACL section has been reported by Burks (6) and Daniel (17) (Fig. 2).

Satisfactory reproducibility testing with the KT1000 has been reported by Malcom (31) (Fig. 3), Highenboten (28), Steiner (45), Hanten (25), and Daniel (13). Forster (20) is the only reported study were the reproducibility was not satisfactory.

Arthrometer Measurements vs  Actual Skeletal Displacements  in Cadavers Fig.1

Effect of Anterior Cruciate Ligament Section on Anterior Displacement (Frequency of Ant Displacement Increase) Fig. 2

Anterior Drawer: Magnitude and Side to Side Difference Variation Fig.3

The precision of A-P displacement measurements are dependent on a standardized method of placing the leg and securely stabilizing the patella in the femoral trochlea. With adequate patellar stabilization, tibial tubercle motion relative to the patella accurately reflects motion of the tibia relative to the femur. It is necessary to flex the knee 20 to 30 degrees in order to engage the patella in the femoral trochlea. In patients with patella alta or lateral tracking patella, the knee may need to be flexed to 40 degrees.

The patella is stabilized in the femoral trochlea by direct pressure which should be oriented to seat the patella (Fig. 4).

The patellar stabilization pressure must remain constant. Altering the patellar pressure may result in motion of the patellar sensor, especially in subjects with a joint effusion or patellar chondromalacia. The hand stabilizing the patella in the femoral trochlea should rest on the thigh and prevent the instrument from rotating during the test.

To facilitate patella stabilization, the femur should be positioned so the patella is facing up, or in slight external rotation. If there is excessive rotation, the thigh should be supported with a thigh strap (Fig. 5). The foot support should not be used to internally rotate the leg, but to simply support the feet.
The test variability between the KT1000 and the clinical examination or different testing systems has been reported (15, 23, 28, 40, 44, 45). Though the absolute displacement differences between devices vary, there is good correlation on side-to-side difference measurements with instruments that have a tibial tubercle and a patellar sensor pad.

It is important to establish the precision of a testing system prior to utilizing it in decision making. The manufacturer recommends monthly monitoring of the KT1000 instrument to confirm accuracy of the load-sensing handle and the displacement sensors. The important indicator of pathology is side-to-side difference. The same machine must be used on both sides to minimize the significance of small calibration errors in the machine itself. The crucial element in the testing process is to duplicate the testing technique on the second knee that was used on the first knee. Important points are:

1. Muscle relaxation.
2. Similar limb orientation.
3. Similar Arthrometer placement on the leg in respect to the instrument marker at the joint line and instrument rotation in relation to the patella.
4. Consistent patella pad pressure technique and establishing the testing reference position.
5. Establishing the testing reference position.
6. Similar speed and vector of force application.

Fig. 4 — The knee is supported in a flexed position to engage the patella in the femoral trochlea. In some patients the thigh support must be raised 3 to 6 more centimeters to provide sufficient knee flexion to engage the patella in the femoral trochlea. This may be done by placing a board under the thigh support. The thigh should be supported so the patella is facing up. Occasionally thigh strap is used to accomplish this task (Fig. 5). The examiner stabilizes the patella sensor with manual pressure. The stabilizing hand should rest against the lateral thigh and apply 2 to 5 pounds of pressure on the patella sensor pad. The hand position, patella sensor position and patella sensor pressure must remain constant throughout the test. Varying the pressure on the patella sensor pad and rotation of the pad is a common cause of measurement error.
 
 
 
 
 
 
 
 
 
 

Measurements in the PCL-injured patient have been reported (11, 13, 14, 21, 36, 37, 48). The key to measurement of posterior tibial subluxation is to reference the measurements to the quadriceps active position at the quadriceps neutral angle (Fig. 6).
Fig.6