Physicians or physical therapists generally use an arthrometer, also referred to as a goniometer, as an orthopedic diagnostic tool for measuring range of motion in joints. Manufacturers create each device to gauge the flexibility and stiffness of ligaments in a particular joint. Arthrometers can be manufactured for shoulder, vertebral, or ankle joints but most are used to assess problems in knee ligaments. Sensors on the device record joint movement and usually relay the information to a connected graph, providing a visual and hard copy display of joint articulation during the assessment.
Health care providers might use an arthrometer for initial and postoperative assessments of an anterior cruciate ligament knee injury. The patient usually lies on a table with a cushioned support under his legs. The support maintains 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 top of the leg, from the knee down to the tibia, and is held in place with Velcro® straps.
A knee arthrometer usually has two sensors. One is in contact with the patella, or kneecap, and the other lies on the tibia, or shin bone. A force handle allows 15, 20, or 30 pounds of steady pressure to be applied below the knee during the process of assessing stability. Using varying amounts of pressure, the lower portion of the leg can be moved, or the patient himself can lift it, while the knee is supported. Different range of motion tests, often referred to as the drawer test, Lachman’s test, and the pivot shift, allow the physician or therapist to determine abnormal movement between the tibia and patella.
The amount of movement the tibia experiences away from or toward the kneecap determines the amount of ligament damage. An injured knee often exhibits a greater degree of movement than an uninjured knee. Professionals may use the device on both knees, comparing the amount of movement in the injured leg to that of the uninjured leg. Arthrometer measurements on injured knees typically show 3 millimeters or more of displacement than the healthy joint.
Physicians might opt to use an arthrometer for initial diagnostic purposes, as patients generally experience little or no knee pain with the device compared to the discomfort caused by conventional manual manipulation. An arthrometer might also be used during physical therapy as a means of monitoring progress. The accuracy of the instrument largely depends on the training and experience of the individual using it. Improper placement can affect sensor contact and may provide erroneous measurements.