Range of Motion (ROM) refers to the distance a joint can move in a specific direction. The knee primarily moves through flexion (bending) and extension (straightening). Accurate documentation of knee ROM is necessary for tracking a patient’s progress during rehabilitation following injury or surgery. This measurement provides healthcare professionals with a universal language to communicate changes in mobility and establish functional treatment goals. A standardized approach ensures measurements are comparable across clinical settings and over time, allowing for effective patient management.
Preparing for Accurate Knee Measurement
The primary tool used to measure knee mobility is the goniometer, a specialized protractor designed to measure joint angles in degrees. Correct patient positioning is essential to begin the measurement process. For measuring extension, the patient typically lies on their back (supine). If checking full extension, a small towel roll or bolster may be placed under the ankle to prevent the examination table from limiting the knee, allowing for the detection of hyperextension.
Measuring knee flexion often requires the patient to be positioned supine, sliding the heel toward the buttocks, or prone (lying on the stomach) to bend the knee toward the hip. Regardless of the movement, the goniometer must be aligned with specific anatomical landmarks for reliability. The instrument’s center point, or fulcrum, is placed over the lateral epicondyle of the femur, the bony prominence on the outside of the knee joint.
The stationary arm of the goniometer is aligned with the longitudinal axis of the thigh, pointing toward the greater trochanter of the femur. The moving arm is aligned with the lower leg, pointing toward the lateral malleolus of the ankle. Before recording the final angle, a distinction must be made between Active ROM (AROM) and Passive ROM (PROM). AROM is the range achieved by the patient using their own muscles, while PROM is the range achieved when a clinician moves the limb further. Both measurements offer distinct information about muscle strength and joint mobility, and both should be recorded.
Standardized Recording of Range of Motion
The universally accepted method for recording joint mobility is the Neutral-Zero Method, which establishes full extension (a completely straight knee) as the zero-degree starting point. This system uses a three-number notation to concisely communicate the full range of movement. The format is written as Extension Deficit or Hyperextension – Neutral Point – Flexion. For example, \(5^\circ – 0^\circ – 135^\circ\) documents the complete range of the joint.
The first number indicates any loss of extension or the presence of hyperextension, representing the knee’s starting position relative to zero. If the knee cannot fully straighten, a number like \(5^\circ\) signifies an extension deficit, meaning the knee is resting \(5^\circ\) away from the straight position. If the knee straightens past the zero point, this hyperextension is recorded as the first number, which the three-number format distinguishes from a deficit.
The middle number is always zero, representing the standard reference point of a fully straight knee. This neutral position is the baseline from which both extension and flexion are measured. Maintaining this standardized zero point allows for the universal understanding of documented ROM values.
The third number records the maximum angle of flexion achieved when the joint is bent as far as possible. A flexion measurement of \(135^\circ\) indicates the knee can bend to that angle from the zero-degree point. If the knee achieves full extension with no hyperextension, the first and middle numbers are the same, leading to a notation like \(0^\circ – 0^\circ – 135^\circ\), which is commonly shortened to \(0^\circ – 135^\circ\).
The documentation must clearly state the type of measurement taken, using abbreviations such as AROM (Active Range of Motion) or PROM (Passive Range of Motion). Other common abbreviations include L. Knee or R. Knee to specify the limb, and the patient’s position (e.g., supine or prone). Consistency in this method allows for immediate understanding of the knee’s status, indicating whether the limitation is due to soft tissue restriction or muscle weakness.
Understanding Normal and Restricted Knee Motion
A healthy knee joint typically possesses a range of motion spanning from \(0^\circ\) of extension to about \(135^\circ\) to \(150^\circ\) of flexion. Some healthy individuals, particularly younger people, may exhibit a few degrees of hyperextension, which is considered a normal variant. For most functional daily activities, however, the knee only requires mobility between \(0^\circ\) and about \(120^\circ\) of flexion.
Deviations from this normal range provide important clinical indicators regarding the patient’s condition. An inability to achieve full extension, known as an extension deficit, can be detrimental to function. Even a small loss of \(5^\circ\) of extension can lead to an abnormal gait pattern, increased joint loading, and is associated with quadriceps weakness and the risk of developing osteoarthritis.
Limited flexion often suggests restrictions within the joint capsule or surrounding soft tissues, such as swelling or pain. This limitation may prevent the individual from performing activities that require a deeper bend, like squatting or rising from a low chair. Without standardized ROM documentation, clinicians cannot effectively track whether intervention strategies are improving joint mobility or justifying continued treatment. The documented angles serve as a quantifiable measure of recovery and functional capacity.