Rule of Nine Test for Radial Nerve Entrapment
The Rule of Nine test is a clinical diagnostic tool that divides the anterior forearm into 9 equal squares to localize tenderness corresponding to the path of the posterior interosseous nerve through the radial tunnel, with tenderness in the lateral column of 3 squares indicating radial tunnel syndrome. 1
How to Perform the Test
Setup and Positioning
- Position the patient with the forearm exposed and accessible
- Visualize the anterior aspect of the forearm from elbow to wrist
- Divide the forearm into 9 equal squares arranged in a 3x3 grid pattern (3 columns × 3 rows)
Technique
- Systematically palpate each of the 9 squares
- Apply consistent pressure to elicit tenderness
- Document which squares produce pain or tenderness
- A positive test occurs when tenderness is elicited in the lateral column of 3 squares, which corresponds to the anatomical path of the posterior interosseous nerve through the radial tunnel 1
Clinical Context and Diagnostic Value
Purpose
The Rule of Nine test was developed specifically to improve diagnostic accuracy in distinguishing radial tunnel syndrome from lateral epicondylitis, which present similarly but require different treatment approaches 1. This distinction is clinically critical because misdiagnosis leads to inappropriate management and persistent symptoms.
Anatomical Basis
Cadaveric studies demonstrate consistent mapping of the posterior interosseous nerve to the lateral column of squares on the anterior forearm 1. This anatomical consistency provides the foundation for the test's reliability in identifying nerve compression within the radial tunnel.
Integration with Other Tests
While the Rule of Nine test provides spatial localization, it should be combined with other clinical assessments:
- Resisted wrist extension test: Perform with elbow extended and forearm pronated (EP position) for maximum sensitivity (100%) and specificity 2
- Upper limb neural tension test 2b (ULNT2b): The standard sequence is more effective at reproducing symptoms and paresthesias than variations 3
- Consider that 63% of patients with lateral epicondylitis have associated superficial radial nerve sensory disturbance 2
Important Clinical Considerations
Differential Diagnosis
Radial tunnel syndrome frequently coexists with or mimics lateral epicondylitis. Key distinguishing features:
- Radial tunnel syndrome: tenderness 3-4 cm distal to lateral epicondyle in the lateral forearm column
- Lateral epicondylitis: tenderness directly over the lateral epicondyle
Treatment Implications
If conservative therapy fails after 4-6 months and the Rule of Nine test is positive, surgical decompression via anterolateral approach achieves success rates of 67-92% 4. The anterolateral approach through the internervous plane between brachioradialis and brachialis muscles provides superior healing and extensibility 4.
Pitfalls to Avoid
- Do not rely solely on this test; combine with electrodiagnostic studies when diagnosis remains uncertain
- Recognize that corticosteroid injections may decrease the incidence of associated sensory disturbances, potentially masking nerve involvement 2
- Avoid testing with the elbow flexed and forearm supinated, as this position has only 24% sensitivity for detecting lateral epicondylitis 2