Extensor tendon injuries of the hand can significantly impact hand function, making everyday tasks challenging. Understanding these injuries, their specific locations, and appropriate treatments is crucial for effective recovery. This comprehensive guide, perfect for students, will break down the anatomy, biomechanics, diagnostic methods, and treatment protocols for various extensor tendon injuries.
Understanding the Anatomy of Hand Extensors
To grasp extensor tendon injuries, a solid understanding of the hand's intricate extensor anatomy is essential. This system involves muscles, tendons, and supporting structures that enable finger and wrist extension.
Extensor Muscles and Tendons
The extensor tendons originate from various muscle bellies in the forearm and hand. These include:
- Finger Extensors: Extensor Digitorum Communis (EDC), Extensor Indicis Proprius (EIP), Extensor Pollicis Longus (EPL), Extensor Digiti Minimi (EDM).
- Wrist Extensors: Extensor Carpi Radialis Longus (ECRL), Extensor Carpi Radialis Brevis (ECRB), Extensor Carpi Ulnaris (ECU).
- Thumb Extensors: Abductor Pollicis Longus (APL), Extensor Pollicis Brevis (EPB).
All these muscles are innervated by branches from the radial nerve or the posterior interosseous nerve. Variations like accessory muscles (e.g., extensor carpi radialis intermedius, extensor medii proprius) and the extensor digitorum brevis manus (EDBM) are also noted, which can sometimes be mistaken for dorsal wrist ganglion cysts.
Tendon Compartments at the Wrist
At the wrist level, extensor tendons pass through six individual, synovial-lined, fibro-osseous sheaths, known as the extensor tendon compartments. The fifth compartment, containing the EDM, is uniquely fibrous. The sixth compartment, for the ECU, has a subsheath critical for its stability; rupture can lead to symptomatic subluxation.
Juncturae Tendinae
On the dorsal hand, proximal to the metacarpophalangeal (MP) joint, fibrous interconnections called juncturae tendinae exist between the communis tendons. These bands, especially stout between the ring, small, and middle fingers, can help preserve finger extension even if an EDC is lacerated proximal to them. However, they also restrict independent extension of the ring and middle fingers.
Sagittal Bands: MP Joint Support
The extensor tendon at the MP joint is held centrally by sagittal bands, which originate from the volar plate and intermetacarpal ligaments. Injury to these bands can cause the EDC tendon to subluxate or dislocate, compromising MP joint extension. There is no direct tendinous insertion of the EDC onto the proximal phalanx; extension is transmitted via these bands.
Extensor Mechanism over the Phalanges
Over the proximal phalanx, the extensor mechanism is an intricate, layered system. Intrinsic tendons (lumbricals and interossei) form the 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 (attaching to the middle phalanx) and two lateral bands (continuing to the distal phalanx).
Ligaments of the PIP Joint
Over the middle phalanx, the lateral bands are maintained by the triangular ligament (preventing palmar subluxation during PIP flexion) and transverse retinacular ligaments (preventing dorsal subluxation during extension). The triangular ligament is crucial; its incompetence can contribute to a boutonnière deformity. The terminal tendon forms where the lateral bands converge, inserting onto the base of the distal phalanx.
Flashcards
Tap to flip · Swipe to navigate
Biomechanics of Finger Extension
Understanding how fingers extend reveals why even small injuries can have dramatic effects on movement.
Tendon Excursion
The amplitude of tendon excursion for finger extensors is about 5 cm, mostly occurring over the forearm, wrist, and dorsal hand. Over the dorsum of the finger, however, excursion is extremely small. This explains why minor issues can lead to significant problems:
- Small tendon gaps or overtightening: Can cause dramatic extensor lags or restrict flexion.
- Bony shortening or angulation: Can also result in notable functional deficits.
Sensitivity to Tendon Lengthening
The finger's extensor mechanism is highly sensitive to changes in tendon length, especially distally:
- Proximal phalanx: 1 mm of tendon lengthening can produce a 12-degree PIP joint extensor lag.
- Terminal tendon: 1 mm of lengthening can cause a 25-degree DIP joint extensor lag. Conversely, 1 mm of shortening can severely restrict DIP joint flexion.
- Dorsum of the hand (MP joint): 2 mm of extensor tendon lengthening can produce about a 7-degree MP joint extensor lag. However, MP joints often have a