As you review wrist anatomy, note:
- the carpal tunnel, through which the median nerve passes. Therapists employ a special maneuver, Phalen's test, to detect median nerve entrapment or compression in the carpal tunnel. In one study (Gellman, Gelberman, Tan, & Botte, 1986), Phalen's test (Hertling & Kessler, 1996, Fig. 17-25, pp. 544,567) detected median nerve involvement with specificity and sensitivity of 0.71 and 0.50, respectively. Draw on your knowledge of anatomy and function to explain the rationale for Phalen's test. Ponder why it sometimes yields false positive and false negative results.
- the tunnel of Guyon (Hertling & Kessler, 1996, p.254), through which pass the ulnar artery and ulnar nerve.
- the dorsal tendon compartments
Identify the tendons that, encased in synovial tendon sheaths, comprise six numbered dorsal compartments of interests to hand therapists:
- abductor pollicis longus and extensor pollicis brevis
- extensor carpi radialis longus and brevis
- extensor pollicis longus
- extensor digitorum comunis (four tendons) and extensor indicis
- extensor digiti minimi
- extensor carpi ulnaris
To understand the muscular synergies involved in opening the hand (and in the next problem, which involves closing the hand), you should examine the extensor mechanism and the muscles that attach to it. Refer to a popular anatomy atlas like Netter (1997, Plate 433 - Flexor and extensor tendons in fingers), to your text's reproductions of Netter's drawings of the extensor mechanism (Smith, Weiss, & Lehmkuhl, 1996, Fig 6-12), or to adaptations of Netter's drawings.
The extensor digitorum "is mechanically capable of extending the MCP, PIP, and DIP joints but not at the same time. When the extensor digitorum contracts alone, ... the MCP joints extend but the IP joints remain semiflexed in a clawhand position (Smith, Weiss, & Lehmkuhl, 1996, p.205-6)." The authors also explain that the extensor digitorum combines with the lumbricales in a muscle synergy to open the hand. Unless the hand must be opened forcefully or against resistance, the interosseous muscles are inactive (pp. 206-207).
- How can the lumbricales contribute to hand opening?
- How can the interossei contribute to forceful opening of the hand?
According to Smith, Weiss, and Lehmkuhl (1996, p. 201), "forceful closure of the hand or power grip elicits high-level activity of the flexor digitorum superficialis, the interossei, and the flexor digitorum profundus."
Explain why we use the interosseous muscles in hand closure, even though they can contribute to PIP and DIP extension.
The first CMC joint also flexes and extends in a plane that is parallel to the palm. Some therapists refer to extension in this plane as "radial abduction."
Opposition and its antagonistic movement, reposition, involve an automatic rotation of the first metacarpal that occurs when certain movements are combined. You can verify this if you:
extend and adduct the first CMC: the metacarpal rotates the opposite direction as the joint moves toward reposition.
Note that you cannot passively rotate the CMC after you have placed it in full opposition or full reposition. Those are the joint's close packed positions. The joint's capsular fibers are elongated as the joint approaches either end of its range of motion. Once they capsule is maximally elongated and its ligamentous fibers are maximally, the surfaces cannot rotate further.