Extensor hood

The extensor hood is a complex, triangular aponeurotic structure located on the dorsal (back) aspect of each finger. It represents the proximal part of the broader extensor expansion of the hand, though it's important to note that these two terms are often used interchangeably in anatomical literature. Its primary role is to coordinate extension across the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints.

This intricate structure is formed by the flattening of the extensor digitorum tendon as it crosses the MCP joint. The extensor hood gains significant reinforcement from contributions of the intrinsic hand muscles, specifically the lumbricals and interossei (palmar, dorsal). Additionally, the tendons of the extensor digiti minimi (for the little finger), extensor indicis (for the index finger), and in the thumb, the extensor pollicis brevis, also integrate with these hoods.

Each extensor hood is triangular and comprises several key anatomical components:

• Sagittal bands: These are transversely oriented fibrous structures within the extensor hood, positioned at the level of the MCP joint. They originate from the volar plate and the deep transverse metacarpal ligament, encircle the MCP joint, and insert dorsally into the extensor digitorum tendon. Their main function is to centrally align and stabilize the extensor tendon over the MCP joint, preventing it from slipping radially or ulnarly during finger movement.

• Central slip: This direct continuation of the extensor digitorum tendon inserts onto the base of the middle phalanx and is primarily responsible for extending the PIP joint.

• Lateral slips and bands: The lateral slips diverge from the central slip, running along each side of the finger. They join with the lateral bands from the interossei and reunite distally to form the terminal tendon, which inserts on the distal phalanx, enabling extension of the DIP joint.

• Transverse and oblique fibers: The transverse fibers run horizontally across the dorsal aspect of the proximal phalanx, connecting the sagittal bands and further stabilizing the extensor tendon at the MCP joint. The oblique fibers originate from the intrinsic muscles (lumbricals and interossei) and course obliquely to join the lateral bands. These fibers contribute significantly to the extension of both the PIP and DIP joints. Together, they reinforce the extensor mechanism distally, maintaining the alignment of the lateral bands and preventing their displacement towards the palm (volar subluxation).

• Retinacular ligaments: The retinacular ligaments in the extensor hood of the hand, primarily the oblique retinacular ligament, are connective tissue bands that run obliquely from the flexor tendon sheath at the proximal phalanx, past the proximal interphalangeal (PIP) joint, to merge with the lateral bands near the distal interphalangeal (DIP) joint. Their main function is to provide a stabilizing link between the PIP and DIP joints, helping coordinate extension and preventing volar subluxation of the lateral bands during finger motion.