Range of motion (ROM) refers to the extent a joint can move from its fully extended to its fully flexed position. For the knee, this measurement is a fundamental metric used to assess joint health, track recovery following an injury or surgery, and evaluate treatment effectiveness. Accurately documenting the knee’s available motion provides a consistent record for healthcare providers. This data helps establish a baseline, identify functional limitations, and monitor progress as the patient works toward regaining mobility.
Defining Normal Knee Movement
The knee joint primarily moves in two directions: flexion and extension. Flexion is the action of bending the knee, which decreases the angle between the thigh and the lower leg. Extension is the opposite movement, where the knee straightens until the leg is fully aligned. Full knee extension is the accepted zero-degree point for measurement.
The range of motion typical for a healthy adult knee is zero degrees of extension to approximately 135 to 150 degrees of flexion. Some individuals may possess a slight degree of hyperextension, extending a few degrees beyond the zero point. Understanding these normative values provides context for the recorded numbers. These baseline figures help determine if a patient’s mobility is restricted or progressing toward a functional range.
Preparing the Goniometer and Patient
Measuring knee ROM requires a specialized tool known as a goniometer, which measures joint angles in degrees. The goniometer consists of a stationary arm, a moving arm, and a central axis (fulcrum). Before measuring, the patient is positioned lying flat on their back (supine) on a firm surface. This position stabilizes the body and minimizes compensatory movements from the hip or pelvis.
Proper alignment of the goniometer is essential for accurate readings. The central axis must be placed directly over the lateral epicondyle of the femur, the bony prominence on the outside of the knee. The stationary arm is aligned with the lateral midline of the femur, pointing toward the greater trochanter of the hip. This preparation ensures the measurement accurately reflects the rotation occurring at the knee joint.
Measuring Knee Flexion and Extension
The process begins by establishing the zero point, which is the degree of full extension. The patient straightens their leg completely, pressing the back of the knee toward the table. The moving arm of the goniometer is aligned with the lateral midline of the fibula, pointing toward the lateral malleolus at the ankle. If the leg is perfectly straight, the measurement is zero degrees; any inability to straighten fully is recorded as an extension deficit.
Measuring knee flexion involves instructing the patient to bend their knee, bringing their heel toward their buttocks as far as they comfortably can. The goniometer alignment remains the same, with the fulcrum on the lateral epicondyle and the stationary arm fixed along the femur. The examiner follows the movement of the lower leg, reading the final degree value from the scale where the moving arm rests. This measurement is typically performed by the patient independently, which is referred to as active range of motion (Active ROM).
Active ROM shows the motion achieved through muscle contraction. Passive range of motion (Passive ROM) is taken when the examiner gently assists the limb to move beyond the active point. Passive ROM often yields a slightly greater degree of motion because external force overcomes muscular or soft tissue resistance. Measuring both Active and Passive ROM provides a complete picture of the joint’s mechanical limitations versus the patient’s functional capacity. Maintaining stabilization of the thigh throughout measurements is important to prevent movement at the hip joint, which would skew the knee reading.
Recording and Interpreting Results
The standardized method for documenting knee range of motion uses a three-number notation system to communicate the full arc of movement. This system uses the zero degree of full extension as the reference point between any extension deficit or hyperextension and the maximum flexion achieved. For example, a healthy knee might be recorded as 0-0-140, where the middle zero represents full extension, the first zero indicates no extension loss, and 140 is the end of flexion.
An extension deficit is documented when the knee cannot fully straighten to zero degrees. If the patient’s knee straightens only to 10 degrees, the measurement is recorded as 10-0-140, indicating a 10-degree lack of extension. If the knee can hyperextend five degrees past the straight position, the notation becomes -5-0-140 or 5-0-140, with the first number representing the hyperextension. Some documentation uses a simplified method, such as 0-10-140 to indicate a 10-degree contracture.
Documenting the measurement must also include whether it was active or passive, the date, and patient-reported pain levels at the end range of motion. Interpreting these numbers involves comparing them to the normal range and to prior measurements of the same knee. A progressive increase in the flexion number or a decrease in the extension deficit indicates successful rehabilitation and improved mobility. A lack of progress suggests the need for an adjustment in the treatment plan.