Using a Knee Arthrometer to Evaluate Tissue-specific Contributions to Knee Flexion Contracture in the Rat

This method can help answer key questions in the musculoskeletal field, such as the effect of an intervention on knee joint range of motion using a rat model. The main advantage of this technique is that it provides a precise, reproducible, user independent method for measuring rat knee joint extension. Start by using scissors and forceps to deglove the lower extremity of the euthanized rat and remove skin from underlying facia.

Then, position the body on its side with the experimental leg facing upwards. Place the femur in the grooved metal clamp that is integrated into the mounting stage of the arthrometer. Secure the femur by punching holes through through muscle with the precision screwdriver to place the clamp distal to the greater trochanter.

Then, adjust the lateral femoral condyle over the center of rotation of the arthrometer. Position the movable arm with two upright posts behind the leg, just proximal to the calcaneus to push the knee into passive extension once the electric motor is activated. Use a hex key to tighten the femur clamp at its base until it is secured.

Ensure that the camera is correctly mounted on the arthometer using a screwdriver. Check that the camera is on manual focus, and focus the camera on the femora condyle. Set the direction of the arthrometer clockwise or counterclockwise depending on the direction of the knee range of movement being tested and the position of the rat.

Activate the arthrometer motor by simultaneously pushing the power and start buttons. The arthrometer motor will move at a speed of 6.6 rpm and then stop for 2.1 seconds upon reaching the first preset torque. When the first torque is reached, the corresponding LED will light up and the digital camera will take a picture of the knee automatically.

Once the picture is taken, the arthrometer will continue to the next higher preset torque. Once the four torques have been applied, the arthrometer will stop. Once the rat is positioned on the arthrometer and testing is initiated, the total time for testing one knee is approximately 18.8 seconds.

To isolate the arthrogenic or non-muscular component of a contracture, reset the torque sequence to release the force applied to the leg, then perform a myotomy the posterior transarticular muscles. Dissect the muscle proximal enough to the knee joint to ensure that the capsule is not cut. After testing both legs before and after myotomy, and dissecting out any required tissues, dispose of the animal carcass and all biohazardous materials following institutional protocol and clean the arthrometer.

Open the file containing the digital image taken by the camera mounted on the arthrometer in Image J.The person performing the analysis should be blinded to the experimental grouping of the animal. Select the angle tool from the main toolbar and trace the femoral-tibial angle by drawing a femoral line from the middle of the femur clamp to the lateral condyle, and the tibial line from the lateral femoral condyle to the lateral malleolus. Use the measuring tool by clicking Analyze then Measure to show the calculated angle produced by the two lines drawn above.

Use the convention of zero degrees to mean full extension. This figure shows maximum knee extension following 16 weeks of immobilization for seven rats compared to the non-immobilized contralateral limb. For the immobilized knee, the ability for maximum extension was reduced compared to the contralateral.

Following myotomy, the maximal extension capability for the experimental and contralateral knee increased, however, the experimental knee continue to demonstrate a flexion contracture. While attempting this procedure, it's important to ensure that the femoral clamp is well-secured in order to avoid movement of the knee during testing. Following this procedure, other methods like histologic analysis of the knee, articular structure can be performed to answer additional questions pertaining to the tissue and cellular changes that occur in the knee joint to affect its range in motion.