What tests can be used to determine which muscles in the extensor compartment of the forearm are functioning properly?

Functional Testing of the Extensor Compartment Muscles of the Forearm

To determine which muscles in the extensor compartment of the forearm are functioning properly, specific manual muscle testing should be performed for each muscle, focusing on their unique actions and anatomical functions.

Anatomical Overview of the Extensor Compartment

The dorsal compartment of the forearm contains nine muscles divided into two groups:

Superficial Group:

• Extensor digitorum communis
• Extensor digiti minimi
• Extensor carpi ulnaris
• Anconeus

Deep Group:

• Supinator
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
• Extensor indicis

Specific Muscle Testing Techniques

1. Extensor Digitorum Communis

• Test: Ask the patient to extend all fingers at the metacarpophalangeal (MCP) joints against resistance
• Observation: Assess extension strength and look for any inability to extend individual digits
• Note: Weakness may indicate radial nerve pathology or muscle injury

2. Extensor Digiti Minimi

• Test: Have the patient extend the little finger against resistance
• Observation: Isolate the fifth digit extension and compare with contralateral side

3. Extensor Carpi Ulnaris

• Test: Ask the patient to extend and deviate the wrist ulnarly against resistance
• Observation: Assess strength and look for tendon prominence on the ulnar aspect of the wrist

4. Extensor Carpi Radialis (Longus and Brevis)

• Test: Have the patient extend and radially deviate the wrist against resistance
• Observation: Evaluate strength and look for tendon prominence on the radial aspect of the wrist

5. Extensor Pollicis Longus

• Test: Ask the patient to extend the interphalangeal (IP) joint of the thumb against resistance
• Observation: Look for extension of the distal phalanx of the thumb

6. Extensor Pollicis Brevis

• Test: Have the patient extend the MCP joint of the thumb against resistance
• Observation: Look for extension of the proximal phalanx of the thumb

7. Abductor Pollicis Longus

• Test: Ask the patient to abduct the thumb in the plane of the palm against resistance
• Observation: Assess thumb movement away from the palm

8. Extensor Indicis

• Test: Have the patient extend the index finger alone against resistance
• Observation: Evaluate isolated index finger extension

9. Supinator

• Test: Ask the patient to supinate the forearm against resistance with the elbow flexed at 90°
• Observation: Assess strength of forearm rotation from pronation to supination

Advanced Diagnostic Testing

If manual muscle testing is inconclusive or if more detailed assessment is needed:

Imaging Studies:

• Radiographs: Initial imaging to rule out fractures or bony abnormalities that might affect muscle function 1
• MRI: Most appropriate for suspected tendon or ligament injuries when radiographs are normal or indeterminate 1
• Ultrasound: Useful for examining tendon abnormalities, particularly for conditions like De Quervain's tenosynovitis affecting the first dorsal compartment 1, 2

Special Tests:

1. Finkelstein Test:

   • Tests for first dorsal compartment tendinopathy (abductor pollicis longus and extensor pollicis brevis)
   • Have the patient make a fist with the thumb inside, then ulnarly deviate the wrist
   • Pain indicates positive test

2. Compartment Pressure Testing:

   • For suspected chronic exertional compartment syndrome
   • Measures intracompartmental pressure before, during, and after exercise 3

Clinical Pearls and Pitfalls

• Common Pitfall: Testing individual extensor muscles can be challenging due to their synergistic actions. Ensure proper isolation of the muscle being tested.
• Important Note: The extensor tendons pass through six anatomical compartments at the wrist (sometimes up to eight in anatomical variants) 4, 5
• Caution: Tendon ruptures may occur with fractures of the radial styloid and should be suspected in such cases 6
• Anatomical Variants: Be aware that anatomical variations exist, such as extensor digitorum tendons arising from different muscles 7

Interpretation of Findings

• Weakness in specific muscle groups may indicate:

  • Radial nerve injury (most common cause of extensor weakness)
  • Posterior interosseous nerve syndrome
  • Tendon rupture or laceration
  • Compartment syndrome
  • Epicondylitis (lateral or medial)

• Compare findings with the contralateral limb to establish baseline function and identify asymmetries

By systematically testing each muscle of the extensor compartment using these specific techniques, clinicians can accurately determine which muscles are functioning properly and identify any deficits requiring further evaluation or treatment.