Extensor Tendon Injuries of the Hand

Understanding Extensor Tendon Injuries of the Hand: A Comprehensive Guide

Extensor tendon injuries of the hand are common and can significantly impact hand function. This comprehensive guide, perfect for students, will break down the anatomy, biomechanics, testing, classification, and treatment strategies for these crucial injuries, drawing directly from expert medical knowledge.

Anatomy of the Hand's Extensor System

Understanding the intricate anatomy of the extensor tendons is fundamental to treating injuries effectively. These tendons originate from various muscle bellies and form a complex, layered system.

• Muscles Involved: The primary muscles include the extensor digitorum communis (EDC), extensor indicis proprius (EIP), extensor pollicis longus (EPL), extensor digiti minimi (EDM), abductor pollicis longus (APL), extensor pollicis brevis (EPB), and the wrist extensors (extensor carpi radialis longus and brevis (ECRL and ECRB), and extensor carpi ulnaris (ECU)). All are innervated by branches of the radial or posterior interosseous nerve.
• Anatomic Variations:
• Wrist Extensors: Accessory muscles like the extensor carpi radialis intermedius (ECRI) or anomalous tendons inserting on metacarpal bases are noted. These may be relevant for tendon transfers.
• Finger Extensors: Variations include the extensor medii proprius (EMP) (like an EIP for the middle finger), extensor indicis et medii communis (EIP variation for middle and index fingers), and the anomalous extensor digitorum brevis manus muscle (EDBM) on the dorsal hand. EDBM can be mistaken for a dorsal wrist ganglion cyst.
• Tendons at the Wrist: Extensor tendons pass through six synovial-lined fibro-osseous sheaths, or compartments. The fifth compartment is fibrous, and the sixth (ECU) has a subsheath, which if injured, can lead to symptomatic ECU tendon subluxation.
• Juncturae Tendinae: These are stout fibrous interconnections between the EDC tendons on the dorsal hand, proximal to the metacarpophalangeal (MP) joint. They can preserve finger extension even if an EDC is lacerated proximal to them but restrict independent extension of the middle and ring fingers.
• Sagittal Bands: Located at the MP joint level, these bands arise from the volar plate and intermetacarpal ligaments, centering the extensor tendon. Injury can lead to subluxation of the EDC tendon, compromising MP joint extension.
• Extensor Mechanism over Phalanges:
• Proximal Phalanx: The intrinsic tendons (lumbricals and interossei) form lateral bands, joining the extensor mechanism at the proximal third of the proximal phalanx. The EDC trifurcates proximal to the PIP joint into a central slip and two lateral bands.
• Middle Phalanx: The lateral bands are held in place by the triangular ligament (preventing palmar subluxation during PIP flexion) and transverse retinacular ligaments (preventing dorsal subluxation during extension). Distally, the lateral bands converge to form the terminal tendon, inserting on the base of the distal phalanx.

Biomechanics of Finger Extension

The movement of extensor tendons is crucial for hand function. The amplitude of tendon excursion for finger extensors and EPL is about 5 cm, mostly over the forearm, wrist, and dorsal hand. However, the excursion over the dorsum of the finger is very small.

• Impact of Small Gaps: Even small tendon gaps, overtightening of repairs, or minimal bony shortening/angulation can cause significant extensor lags or restrict flexion at the PIP and DIP joints.
• 1 mm of tendon lengthening over the proximal phalanx can cause a 12-degree PIP joint extensor lag.
• 1 mm of terminal tendon lengthening can cause a 25-degree DIP joint extensor lag.
• 1 mm of terminal tendon shortening can severely restrict DIP joint flexion.
• 2 mm of extensor tendon lengthening over the dorsum of the hand can produce a 7-degree MP joint extensor lag, though MP joints often have a