What is an arthrometer?

Physicians or physical therapists generally use an arthrometer, also known as a goniometer, as an orthopedic diagnostic tool to measure range of motion in joints. Manufacturers create each device to measure the flexibility and stiffness of the ligaments in a particular joint. Arthrometers can be made for shoulder, spinal, or ankle joints, but most are used to assess knee ligament problems. Sensors in the device record the movement of the joints and typically transmit the information to a connected chart, providing a visual and paper-based display of the joint's articulation during the assessment.

Health care providers may use an arthrometer for initial and postoperative evaluations of an ACL knee injury. The patient usually lies on a table with padded support under the legs. The brace keeps the leg in proper alignment and provides the necessary amount of knee flexion, which is typically 20 to 35 degrees. The arthrometer is placed along the upper leg, from the knee to the tibia, and is fastened with Velcro® straps.

A knee arthrometer usually has two sensors. One is in contact with the kneecap or patella, and the other is located on the tibia or shinbone. A force handle allows 15, 20, or 30 pounds of constant pressure to be applied below the knee during the stability testing process. Using varying amounts of pressure, the lower leg can be moved, or raised by the patient himself, while supporting the knee. Different range of motion tests, often referred to as the drawer test, Lachman test, and pivot shift, allow the doctor or therapist to determine abnormal motion between the tibia and the patella.

The amount of movement the tibia experiences to or from the patella determines the amount of ligament damage. An injured knee often exhibits a greater range of motion than an uninjured knee. Practitioners can use the device on both knees, comparing the amount of movement in the injured leg to that in the uninjured leg. Arthrometer measurements on injured knees usually show 3 millimeters or more of displacement than the healthy joint.

Clinicians may choose to use an arthrometer for initial diagnostic purposes, as patients typically experience little or no knee pain with the device compared to the discomfort caused by conventional manual manipulation. An arthrometer may also be used during physical therapy as a means of monitoring progress. The accuracy of the instrument is highly dependent on the training and experience of the individual using it. Incorrect placement can affect sensor contact and may provide erroneous measurements.

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