Ulnar Nerve Laceration at Guyon's Canal: Why the Grip Failed When the Big Muscles Still Work
Ms. K is a 29-year-old cellist who sustained a glass laceration at the level of Guyon's canal 8 weeks ago; the ulnar artery and nerve were repaired primarily. She presents with clawing of the ring and small fingers, a positive Froment's sign, key pinch of 2 kg versus 6 kg on the left, and no active finger abduction or adduction. Sensation is diminished over the small finger and the ulnar border of the ring finger. Long finger flexors and extensors are 5/5 and wrist motion is full. She asks why her grip is weak when "all the big muscles still work."
1. Explain the clawing pattern to her in anatomical terms, including why the index and long fingers are spared.
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The claw is a balance problem, not a strength problem. At rest, the extensor digitorum communis pulls the MCP joints into hyperextension while the FDP pulls the interphalangeal joints into flexion. The lumbricals normally counteract both: they flex the MCP and extend the IP joints. With the ulnar nerve out, the third and fourth lumbricals are gone, so the ring and small fingers drift into MCP hyperextension with IP flexion - the classic ulnar claw.
Why the index and long fingers do not claw: their lumbricals are median-innervated and are still working, so the MCP–IP balance is preserved. This is a useful teaching point for her: the pattern itself tells you which nerve is out, and it also tells you the claw is not a tendon problem.
Why the fingers still flex and extend: the long flexors and extensors are intact - they are the "big muscles" she is thinking of. What she lost is the fine control layer that positions the MCP joints before the long muscles fire, which is why the fingers look wrong even though they move.
2. Explain why her grip and key pinch fell so much when the long flexors are graded 5/5, and identify which specific functions she has lost.
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Grip dynamometry depends on the intrinsics more than patients expect. The interossei and ulnar lumbricals contribute MCP flexion torque and, critically, they hold the MCPs in flexion while the long flexors work. Without them, the MCPs drift into extension during grip, which puts the long flexors at a mechanical disadvantage - the finger rolls open instead of wrapping. The 2 kg key pinch has a second cause: the adductor pollicis is ulnar-innervated, so pinch force through the thumb is gone, and Froment's sign shows she is substituting FPL (median) and flexing the IP joint to compensate.
Functions lost or degraded: finger abduction and adduction (interossei), so no spreading the fingers on the string board; sustained MCP flexion during a held chord; thumb adduction for bow grip and for pinching; precision control of the small finger, worsened by sensory loss.
Reasoning: a normal manual muscle test of the long flexors and a poor dynamometer reading are not contradictory - they measure different things. The dynamometer measures the integrated output of a hand that is now mechanically poorly positioned.
3. Given her goals as a cellist, what would you prioritize in intervention and what would you tell her about prognosis for the sensory loss?
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Priorities, in order:
- Prevent fixed deformity. The claw position left unmanaged becomes a contracture of the MCP collateral ligaments and IP volar plates. An anti-claw orthosis that blocks MCP hyperextension while leaving the IP joints free is the priority, worn during practice and at rest.
- Preserve and rebuild what remains. Median-innervated intrinsics, thumb opposition, and long flexor coordination are all trainable. Splint-free practice should emphasize positioning the MCPs in flexion before flexing the IPs.
- Sensory re-education for the small finger. She needs protective sensibility education now, plus graded tactile discrimination work as Tinel advances. String placement on the fingerboard depends on this, and she should not practice for long unobserved periods while sensation is absent.
- Functional adaptation for playing. Bow control and string stopping should be reintroduced in short, specific bouts with the orthosis, and she should be warned that fatigue will degrade her hand position before it degrades her sound.
Prognosis conversation: recovery after nerve repair is measured in months and is never complete; sensation returns before fine discrimination, and the small finger will likely remain the least reliable. Set the expectation that she will need to compensate with visual and auditory feedback, and that the orthosis schedule is what protects her joint motion while the nerve recovers.