Flexor Pollicis Longus Exam

• Origin: Radius (anterior surface of middle 1/2) and adjacent interosseous membrane
  Ulna (coronoid process, lateral border [variable])
  Humerus (medial epicondyle [variable])
• Insertion: Thumb (base of distal phalanx, palmar surface)
• Innervation: Cervical root(s): C7 and C8
  Nerve: median nerve (anterior interosseous branch)

When assessing a finger injury for signs of flexor pollicis longus (FPL) laceration, the goal is to determine whether the FPL tendon is completely lacerated, partially transected, or intact.

The first step in the examination is to observe the resting flexion posture of the thumb IP joint. When the normal hand is resting on the exam table with the forearm supinated and the wrist in mild dorsal flexion, the fingers are typically in a flexed position and the thumb IP joint is slightly flexed. If a complete laceration of the FPL tendon is present, then the IP joint will likely be in neutral or slightly extended. Partial lacerations of either flexor pollicis longus tendon (FPL) may also disrupt the normal finger flexion cascade. This initial evaluation can be augmented by performing a Thompson test. During this maneuver, pressure is applied to the FPL muscle belly in the distal radial volar forearm. If the FPL is intact, the pressure on the muscle belly causes slight thumb IP flexion.

If the patient can cooperate with the request for active thumb flexion, then the second step is to ask the patient to actively flex the thumb IP joint. Observing active flexion of the IP joint will help identify the normal function of the FPL flexor tendon.

The third step is to perform muscle testing of the flexor pollicis longus tendon if possible.  In the examination of an uninjured FPL musculotendinous unit, the 0 to 5 muscle testing grading system is applied. In this system, zero indicates a total loss of flexor pollicis longus (FPL) contraction, while a grade of 5 represents normal FPL function capable of contracting against standard resistance. Detailed information on graded muscle testing is provided below. Typically, full muscle testing is impractical in cases of acute laceration due to pain and tenderness. The examiner may have to rely on the observation that the laceration occurred in the palmar section containing the FPL. Nevertheless, the examiner should assess the contraction of the potentially injured musculotendinous unit as comprehensively as possible. The examination's primary aim is to preoperatively determine whether the tendon is completely, partially cut, or intact. To evaluate the FPL musculotendinous unit, rest the patient's hand and upper extremity on the table with the forearm in supination and the wrist in a neutral or mildly dorsiflexed position. Start with the thumb in a neutral resting posture. Stabilize the proximal thumb with one hand while resisting flexion of the IP joint with other hand. Instruct for the patient to “Bend the tip of your thumb. Hold it. Don’t let me straighten it.”

Definition of Positive Result in FPL Muscle Testing: A normal result is a positive one. During a normal muscle test, the examiner should observe a normal muscle contraction that can move the joint or tendon against full normal resistance.

Definition of Negative Result in FPL Muscle Testing: The FPL tendon should be observed and palpated and compared to the uninjured side. In muscle testing, an abnormal result is a negative one. During a partially abnormal muscle test, the examiner should observe an abnormal muscle contraction that can move the joint or a tendon but not against normal resistance. In a complete denervation injury, such as a high median nerve complete laceration, there may be no evidence of any muscle contraction, and the muscle testing grade will be zero.

In a patient with a laceration of the FPL in sections 1, 8, or 9, the IP joint may not actively flex at all due to a complete transection (cut) of the FPL tendon. This results in an abnormal or negative muscle testing or possibly a grade 3 due to muscle belly contraction without active thumb IP joint flexion. However, this observation can indicate a complete FPL laceration requiring surgical repair. Thus, this negative muscle testing exam will be positive for a complete index FPL laceration. Likewise, an abnormal FPL Thompson test, loss of resting thumb IP joint flexion, and/or loss of thumb active IP joint flexion may indicate a complete FPL tendon laceration.