Dynamic Ultrasound Assessment of Ulnar Nerve Instability in the Cubital Tunnel
Examination Protocol
According to recommendations (Griffith, Diagnostic Ultrasound: Musculoskeletal, 2025), ultrasound is the best imaging method for the cubital tunnel: effective, accurate, and dynamic. The examination is typically begun with the elbow in extension and at 90° flexion. The nerve is traced in the transverse plane throughout its course—from the proximal arm to the Guyon canal. The nerve caliber is measured immediately proximal to the canal and within it, using minimal transducer pressure to avoid nerve compression.
Dynamic Test Technique
The transducer is placed with light contact transversely over the cubital tunnel during slow passive elbow flexion. Continuous repositioning of the transducer is required to keep it over the medial epicondyle. Assessment is made for subluxation or dislocation of the ulnar nerve, as well as subluxation of the medial head of the triceps («double snap»). As the elbow flexes, the olecranon and roof of the cubital tunnel shift distally, allowing the nerve to subluxate.
Interpretation Criteria
| Parameter | Data (Griffith, 2025) |
|---|---|
| Normal nerve size in canal | 8 mm² (nerve size increased ~15% in canal compared to distal arm) |
| Threshold for «swollen» nerve | >15 mm²—«ulnar nerve swollen, consistent with CUTS» |
| Subluxation (at apex of medial epicondyle) | 25% of CUTS patients; 10% of normal |
| Dislocation (anterior to medial epicondyle) | 25% of CUTS patients; 10% of normal |
The ulnar nerve is accompanied by the recurrent ulnar artery and veins. During scanning, possible compression points are checked: proximally—arcade of Struthers, medial intermuscular septum; distally—beneath the aponeurosis of flexor-pronator muscles (flexor carpi ulnaris arcade) and between the ulnar and humeral heads of flexor carpi ulnaris (FCU). The canal contents are also inspected: osteophytes, synovitis, ganglion cyst, anconeus epitrochlearis, impingement of the medial triceps head.
Additional Techniques
Shear wave elastography may be useful in diagnosis: ulnar nerve stiffness >61 kPa is highly specific for CUTS (Griffith, 2025). Ulnar nerve instability in the cubital tunnel has also been described in asymptomatic volunteers (data cited in EFSUMB Guidelines, Part I, Fodor et al., 2022).
Anisotropy Artifact
When examining the nerve, anisotropy must be considered: if the beam does not strike perpendicular to the fiber orientation, the nerve appears hypoechoic, mimicking pathology. To eliminate the artifact, the transducer is tilted (heel-toe/angulation, 5–15°), restoring perpendicular beam incidence. If the «finding» disappears with tilting—it is an artifact, not pathology (Verified consensus answers from Medtrain, 2026).
Frequently asked questions
How should a dynamic test for instability be properly performed?
The transducer is placed with light contact transversely over the cubital tunnel and the elbow is slowly passively flexed from full extension to full flexion, continuously repositioning the transducer over the medial epicondyle, while tracking subluxation/dislocation of the nerve and triceps «double snap».
What is the normal size of the ulnar nerve in the cubital tunnel?
Normal size is 8 mm², with the nerve enlarged approximately 15% in the canal compared to the distal arm. When cross-sectional area exceeds 15 mm², the nerve is considered swollen, consistent with CUTS (Griffith, 2025).
How specific is instability for cubital tunnel syndrome?
Subluxation and dislocation occur in 25% of CUTS cases but also in 10% of normal individuals, so this finding is not strictly specific (Griffith, 2025).
Is there a role for elastography?
Yes, shear wave elastography may be helpful: ulnar nerve stiffness >61 kPa is highly specific for CUTS (Griffith, 2025).
How can anisotropy be distinguished from nerve pathology?
Tilt the transducer (heel-toe, 5–15°): if a hypoechoic area disappears with perpendicular beam incidence, it is an anisotropy artifact, not pathology.