​Written by Peter Loubert, PT, PH.D., and Caroline Gwaltney, PT, DPT, CWS

Students in the CMU DPT program are taught a variety of techniques to measure joint range of motion, including goniometric measurement using both the landmark method and parallel lines method. The parallel lines method is a lesser-known technique, but a valuable tool for accurate ROM assessment. Dr. Peter Loubert provided some helpful background to understand the rationale for choosing the parallel lines method of measurement.

There are similarities between the landmark and parallel lines methods. Both methods measure the arc of movement between 2 lines in the same plane of motion using concepts of geometry. Some joints move in a single plane only (e.g., the elbow and knee) while others move in multiple planes (e.g., the shoulder and hip). The ROM measurement is always taken within the relevant plane of movement. Measuring an angle requires that 2 lines be defined. In ROM measurement, one line is defined as the “Reference” line, and the other line is defined as the “Indicator” line.

In geometry, every line must be defined by at least 2 points. In ROM measurement, the lines of the body are often represented by 2 anatomical landmarks that serve as the minimum of 2 points necessary to define a line. The validity of this procedure is dependent upon how well the anatomical landmarks form a line that is parallel to the long axis of the body segment being represented. However, too often the respective anatomical landmarks are not an accurate representation of the long axis of a body segment. The result, then, may be a measurement that is reliable, but not accurate (valid).

Let’s use measurement of elbow flexion as an example. In application of the landmark method, the lateral epicondyle of the humerus and the radial styloid process are often cited as the appropriate anatomical landmarks for alignment of a measuring instrument to the forearm segment, and they generally are a good representation of the long axis of the forearm as long as the forearm is fully supinated. If the forearm is not in full normal supination, the distal point (radial styloid process) is moved anterior to the long axis of the forearm and is therefore no longer an accurate representation of the forearm segment of the body. The parallel lines method instead aligns the indicator arm of the goniometer so it is parallel to the posterior border of the ulna in the sagittal plane. This eliminates error from out-of-plane supination/pronation because the ulna doesn’t rotate with forearm supination and pronation, and it provides much more than just 2 points to represent the line of the forearm in measuring an elbow angle.

Continuing the example, the long axis of the humerus is the appropriate line for representing the arm segment in elbow flexion/extension ROM measurements. The corresponding anatomical landmarks most often cited to represent the arm for landmark-based measurements are the lateral epicondyle of the humerus and the acromion process. This is almost always problematic because the acromion is several centimeters wide in the sagittal plane. To be consistent (reliable), the clinician must choose a specific point on the acromion for their instrument alignment, which would otherwise vary by more than 6 degrees (a clear threat to the validity of the measure) for a person with a 35 cm long arm and an acromion 4 cm wide (the size of Dr. Loubert’s arm!). The validity of elbow flexion measurements by the landmark method are further jeopardized if they need to be measured with the shoulder in any amount of flexion or extension. This is because the acromion changes its relationship to the long axis of the humerus with shoulder flexion and extension.  If the shoulder is flexed during elbow ROM measurement, the measured elbow angle will be systematically greater than the true elbow angle, and if the shoulder is extended, the measured elbow angle will be systematically less than the true elbow angle. In the parallel lines method, the reference arm of the goniometer is aligned parallel to the long axis of the humerus whether or not the acromion falls on that line. This assures a more accurate and consistent measurement of elbow flexion and extension.

According to well established geometric theory, to accurately measure the angle of any joint, the following criteria are necessary and sufficient in the alignment of a goniometer:

1. The goniometer must be aligned so the plane of the goniometer is parallel to the plane of the joint motion to be measured.
2. One arm of the goniometer must be aligned parallel to the long axis of the reference segment of the joint being measured
3. The other arm of the goniometer must be aligned parallel to the indicator segment of the joint being measured.

If the above criteria are met, the joint angle will be reproduced accurately on the goniometer.  It also doesn’t matter where the axis of the goniometer is in relation to the body or any anatomical landmarks. It only matters that the plane of the goniometer is in the same plane as the movement being measured and the arms of the goniometer are aligned parallel to the relevant body segments. This “parallel lines method” of goniometry is often simpler to apply than the landmark method, and is not subject to the kinds of errors created by anatomical variation and changes in relationships of landmarks when body segments change position relative to one another.

Though the concept of parallel lines goniometry may seem foreign when first contemplated, it is likely to seem more familiar when analyzing the use of a typical finger goniometer. Finger goniometers are aligned with the arms of the goniometer applied to the dorsum of adjacent phalanges, with the arms of the goniometer made parallel to the dorsal surfaces. When the goniometer is aligned correctly, the axis of the goniometer is nowhere near the axis of the joint being measured. But the measure is accurate as long as the plane of the goniometer is parallel to the plane of the IP joint, and the arms of the goniometer are parallel to the long axis of the phalanges involved. In fact, a finger goniometer inherently measures by the parallel lines method, not the landmark method.