Ulnar Nerve in the Cubital Tunnel: Cross-Sectional Area and Subluxation on Ultrasound
Anatomy of the Cubital Tunnel
Distally, the ulnar nerve may be compressed at the entrance to the true cubital tunnel, formed by the humeral and ulnar heads of the flexor carpi ulnaris, connected by the arcuate ligament (Jacobson, 2026). The roof of the tunnel is formed by the cubital retinaculum (an extension of the flexor carpi ulnaris fascia and Osborne's ligament), approximately 4 mm wide and difficult to visualize on ultrasound; the floor and walls are formed by adjacent bones, the joint capsule, and the medial collateral ligament. The ulnar nerve normally slightly enlarges within the tunnel (Griffith, 2025). Its location is between the medial epicondyle of the humerus and the olecranon.
Cross-Sectional Area (CSA)
Assessment of CSA is a key quantitative criterion. Below are the threshold values from the sources.
| Parameter | Value | Source |
|---|---|---|
| Normal average CSA | 9 mm² | Griffith, 2025 |
| Pathology threshold (in most studies) | 10 mm² | Griffith, 2025 |
| CSA at the point of maximum enlargement — pathology | >9 mm² | Jacobson, 2026 |
| Ratio to proximal segment — pathology | >2.8 | Jacobson, 2026 |
| Confident diagnosis of CUTS (considering variability) | >15 mm² | Griffith, 2025 |
Due to the wide variability in the size of the ulnar nerve, cubital tunnel syndrome (CUTS) can be confidently diagnosed with CSA >15 mm² (Griffith, 2025).
Ultrasound Signs of Cubital Tunnel Syndrome
Sonographic diagnosis relies on the visualization of hypoechoic enlargement of the ulnar nerve just proximal to the cubital tunnel, usually transitioning to normal caliber within the tunnel (Jacobson, 2026). The best diagnostic sign is a swollen ulnar nerve within the tunnel (Griffith, 2025).
On longitudinal scanning, a fusiform appearance is noted due to local edema. On transverse sections, an increase in CSA with loss of the fascicular pattern is observed: a normal nerve (~8 mm²) maintains a fascicular structure, while moderate enlargement (12–16 mm²) results in loss of the fascicular pattern (Griffith, 2025).
Ulnar Nerve Subluxation
Transient displacement (subluxation/dislocation) of the ulnar nerve is documented during dynamic assessment; snapping triceps syndrome and subluxation with anconeus epitrochlearis are also described (Jacobson, 2026, video materials 4.24–4.28). [note: specific criteria for subluxation are not provided in the given fragments]
Additional Methods
Radiography may reveal osteophyte formation in hypertrophic osteoarthritis, post-traumatic deformity, chronic inflammatory arthropathy, gouty tophi. On MRI (T1WI/T2WI+FS) — edema of the ulnar nerve (>15 mm²) (Griffith, 2025).
Frequently asked questions
What CSA value of the ulnar nerve is considered pathological?
According to Jacobson (2026) — CSA >9 mm² at the point of maximum enlargement. According to Griffith (2025), the normal average CSA is 9 mm², and most studies use a threshold of 10 mm².
At what CSA can cubital tunnel syndrome be confidently diagnosed?
At CSA >15 mm², as the size of the ulnar nerve varies widely (Griffith, 2025).
What does a ratio >2.8 mean?
An increase in nerve size with a ratio >2.8 compared to the proximal segment is considered pathological (Jacobson, 2026).
Where to look for the zone of maximum nerve thickening?
Hypoechoic enlargement is detected just proximal to the cubital tunnel, usually transitioning to normal caliber within the tunnel (Jacobson, 2026).
What qualitative sign confirms the lesion besides CSA?
Loss of the nerve's fascicular pattern and fusiform appearance on longitudinal scanning (Griffith, 2025).