📝 STANDARD DESCRIPTION:
The Flexor Assistant
- Origin: C8, T1, T2 (most caudal roots)
- Path: Diverges caudally from median → passes caudal to medial epicondyle ('funny bone') → groove between FCU and Ulnaris Lateralis
- Motor: Flexor Carpi Ulnaris, FDP (ulnar head), Interosseous muscles (fetlock support)
- Sensory: Lateral foot, caudal forearm
Clinical Signs: 'Knuckling' (fetlock sinking due to suspensory weakness), anesthesia lateral foot.
📚 DETAILED DESCRIPTION:
THE LATERAL SPECIALIST
The Ulnar nerve is the partner to the Median, handling the 'outer' (lateral) duties of the limb—carpal flexion via ulnar side and critical fetlock support via intrinsic muscles.
ORIGIN & SEPARATION:
Derived from most caudal roots (C8-T2). Separates from median nerve in arm and diverges caudally toward elbow.
TOPOGRAPHIC LANDMARKS:
1. Medial Epicondyle: Passes caudal to this bony prominence—bovine equivalent of the 'funny bone.' Here it is superficial and can be palpated/rolled against bone.
2. Antebrachial Groove: Enters forearm between two heads of Flexor Carpi Ulnaris (FCU), descending in groove between FCU and Ulnaris Lateralis.
3. Division: Just proximal to accessory carpal bone, splits into:
• Dorsal Branch: Cutaneous to lateral carpus/metacarpus
• Palmar Branch: Joins median nerve branches in digital nerves
MOTOR SUPPLY:
• Flexor Carpi Ulnaris (FCU): Powerful ulnar flexor of carpus, counterbalancing FCR
• Flexor Digitorum Profundus (FDP) - Ulnar head: Assists median-supplied heads in digital flexion
• Interosseous Muscles: Critical suspensory function—these are the suspensory muscles of the fetlock providing passive support via check ligaments
Functional Synthesis: Provides fine-tuning of flexion and crucial intrinsic support of the fetlock.
SENSORY DISTRIBUTION:
• Dorsal branch: Lateral aspect of carpus and forearm
• Palmar branch: Lateral aspect of the foot and caudal forearm
CLINICAL PRESENTATION:
Motor Deficit:
'Knuckling' of the fetlock—subtle compared to radial paralysis. Because Ulnar innervates the interossei (suspensory apparatus), paralysis weakens suspensory tone causing the fetlock to sink or knuckle slightly during weight bearing.
Sensory Deficit (Diagnostic):
Anesthesia of lateral aspect of foot and caudal forearm—distinctive signature differentiating from median paralysis (medial anesthesia).
NERVE BLOCK PROTOCOL:
Site: Caudal distal forearm, ~10cm proximal to accessory carpal bone
Landmark: Palpable groove between FCU and Ulnaris Lateralis
Technique: Superficial injection (5-10mL). Needle placed in groove.
Standard Practice: Rarely performed alone—almost always paired with Median block for complete palmar foot desensitization ('High Volar' equivalent).
// COMPARATIVE ANALYSIS
| DOG |
Dorsal branch larger, supplies 5th digit heavily. 'Claw paralysis' from ulnar lesion described. |
| HORSE |
Similar groove location. Used in 'Low 4-point' block at button of splint bone level. |
CLINICAL RELEVANCE
Suspensory Failure: Combined median/ulnar lesions cause complete loss of deep flexor function and suspensory collapse—fetlock drops to ground. Chronic Neuropathy: cattle with chronic ulnar damage develop asymmetric hoof wear on lateral digit (IV) due to proprioceptive loss and abnormal landing.
