During a hand examination, if flexors of the 4th and 5th digits at the DIP are weak while the 2nd and 3rd digits are normal, the lesion is most consistent with injury to which nerve?

During a hand examination, if flexors of the 4th and 5th digits at the DIP are weak while the 2nd and 3rd digits are normal, the lesion is most consistent with injury to which nerve?

• A. Median nerve C8–T1
• B. Median nerve C5–C6
• C. Ulnar nerve C5–C6
• D. Ulnar nerve C8–T1

Answer: (D)

Flexor digitorum profundus (FDP) is responsible for bending the DIP joints. The FDP to digits 4 and 5 is innervated by the ulnar nerve (C8–T1), while the FDP to digits 2 and 3 is innervated by the median nerve via the anterior interosseous branch (also at C8–T1). If the DIP joints of the 4th and 5th digits are weak while the 2nd and 3rd digits work normally, the problem lies with the ulnar nerve’s supply to those distal flexors, rather than the median nerve. This pattern points to an ulnar nerve injury at the C8–T1 level.

Flexor digitorum profundus (FDP) is responsible for bending the DIP joints. The FDP to digits 4 and 5 is innervated by the ulnar nerve (C8–T1), while the FDP to digits 2 and 3 is innervated by the median nerve via the anterior interosseous branch (also at C8–T1). If the DIP joints of the 4th and 5th digits are weak while the 2nd and 3rd digits work normally, the problem lies with the ulnar nerve’s supply to those distal flexors, rather than the median nerve. This pattern points to an ulnar nerve injury at the C8–T1 level.