July 2016 No. 41
Identifying and Lessening the Dreaded EDC Lag
How does one identify the cause of this posture when finger extension is attempted? Either adherence or weakness of the extrinsic finger extensors or the presence of severe interosseous muscle tightness can create this posture.
In the case of severe interosseous muscle tightness, both metacarpophalangeal (MP) joint extension and interphalangeal (IP) joint flexion is limited. Testing for interosseous muscle tightness is described in detail in Clinical Pearls No. 19-21.
Determining the cause of extensor lag is more complex. The lag can result from weakened muscles which is most often seen when muscle innervation is returning following a nerve injury. In the injured hand, the active lag is usually caused by adherence of the extrinsic extensor tendons somewhere along their path.
Consider the extrinsic extensors (herein referred to as EDC) in an uninjured hand. Since IP joint extension is accomplished primarily by the intrinsic finger muscles, testing the EDC focuses on MP joint extension. Since the EDC also crosses the wrist, the ability of the EDC to extend the MP joints is made more difficult with greater wrist extension because simultaneous wrist and MP joint extension requires maximum muscle contraction to produce maximal proximal tendon glide. Conversely, wrist flexion creates passive tension on the EDC, thereby passively extending the MP joints without requiring active EDC muscle contraction. When the wrist is positioned close to neutral, the EDC excursion (and muscle contraction) required to accomplish MP joint extension is less than when the wrist is in extension.
To test the maximum proximal excursion of the EDC, the patient must actively hold both the wrist and the MP joints in extension simultaneously, without IP joint extension. In this photo of a patient without a nerve injury, one sees that with the wrist extended about 30 degrees there is a 45 degree extension lag at the MP joints. In this case, the first question is whether the MP extension lag changes when the wrist position changes. If MP joint extension decreases as the wrist moves into extension and increases as the wrist moves to neutral, the adherence is proximal to the wrist. If, however, the MP extension lag does not change when the wrist position is changed, the adherence is distal to the wrist; (over the metacarpals) assuming passive MP joint motion is present.
To reduce EDC lag/adherence regardless of the location of the adherence, start by determining the wrist position where the patient demonstrates the best MP joint extension, even though less than full. If the adherence is over the metacarpals, less wrist extension may not alter the MP joint extension at all, but a wrist position closer to neutral maximizes the EDC pull across the metacarpals because the muscle does not have to work so hard.
The patient repeats active MP joint extension with the wrist in the chosen position and the IP joints in relaxed flexion until almost full MP joint extension is regained. When MP joint extension is gained in the initial position, the wrist is then serially re-positioned in greater extension until the patient is able to simultaneously maximally extend both the wrist and the MP joints. Only then is it appropriate to ask the patient to add active IP joint extension as part of the exercise.
One should keep in mind that an injured stiff hand may have both interosseous muscle tightness as well as an extensor lag from adherence, requiring more than one exercise to remediate the problems.
Thanks to Patricia Rappaport MPT, CHT for editing and comments.
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Disclaimer: HandLab Clinical Pearls are intended to be an informal sharing of practical clinical ideas; not formal evidence-based conclusions of fact.