Flexor Carpi Ulnaris Function and Management
Primary Function
The flexor carpi ulnaris (FCU) serves as a powerful wrist flexor and ulnar deviator, capable of generating forces between 40-135 N in active contraction, with its maximum force typically occurring near the neutral wrist position. 1
Anatomical and Functional Characteristics
Muscle Architecture
- The FCU consists of two distinct neuromuscular compartments: a humeral head and a larger ulnar head, which allows for potential surgical splitting while preserving some function 2
- The muscle demonstrates a potential active excursion of 4-7 cm during wrist movement from flexion to extension 1
- Passive forces at maximum active contraction range from 1-8 N, indicating variable baseline tension 1
Clinical Significance
- The FCU is an "ever-present" muscle of the anterior forearm flexor compartment, though rare anatomical variations can occur 3
- It serves as a useful local muscle flap for forearm and elbow reconstruction when needed 2
- The muscle's subcutaneous position makes it easily palpable for clinical examination 4
Management of FCU Pathology
Spastic Deformity Management
In spastic conditions requiring surgical intervention, simple tenotomy alone is inadequate and will result in recurrence—the procedure must include distal muscle belly dissection and excision of a tendon segment to prevent fibrous restoration. 5
- Previously tenotomized FCU muscles can regenerate sufficient strength (up to 110 N) through fibrous tendon restoration to cause recurrent flexion deformity 5
- The force-length characteristics of a restored FCU are comparable to non-operated spastic muscles 5
Tendon Transfer Considerations
- Pre-operative assessment of individual FCU function is essential, as there is substantial inter-individual variability in muscle performance 1
- Maximum active force location and passive tension vary significantly between patients, requiring tailored surgical planning 1
- When using the split FCU technique, the humeral compartment can be preserved to maintain ulnar deviation and palmar flexion while utilizing the ulnar compartment for reconstruction 2
Imaging for FCU Pathology
- MRI is the ideal imaging modality for evaluating FCU tendon injuries and surgical planning 6
- Ultrasound can effectively visualize the FCU tendon and identify anatomical variations, including rare accessory muscles 3
- Standard radiographs should be obtained first to exclude bony abnormalities, though they are typically normal in isolated tendon injuries 6
Important Clinical Pitfalls
- Avoid simple tenotomy without tendon segment excision in spastic conditions, as fibrous restoration will cause recurrence 5
- Be aware that rare accessory FCU muscles can exist and may have aberrant innervation (median nerve instead of ulnar nerve) 3
- Individual muscle function assessment is critical before tendon transfer surgery due to high variability in force generation and excursion patterns 1