human anatomy The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a he ...

, the annular ligaments of the fingers, often referred to as A pulleys, are the

annular Annulus (or anulus) or annular indicates a ring- or donut-shaped area or structure. It may refer to: Human anatomy * '' Anulus fibrosus disci intervertebralis'', spinal structure * Annulus of Zinn, a.k.a. annular tendon or ''anulus tendineus co ...

part of the fibrous sheathes of the

finger A finger is a limb of the body and a type of digit, an organ of manipulation and sensation found in the hands of most of the Tetrapods, so also with humans and other primates. Most land vertebrates have five fingers ( Pentadactyly). Chambers ...

s. Four or five such annular pulleys, together with three cruciate pulleys, form a fibro-osseous tunnel on the palmar aspect of the hand through which passes the

deep Deep or The Deep may refer to: Places United States * Deep Creek (Appomattox River tributary), Virginia * Deep Creek (Great Salt Lake), Idaho and Utah * Deep Creek (Mahantango Creek tributary), Pennsylvania * Deep Creek (Mojave River tributary), ...

and superficial flexor tendons. The annular and cruciate ligaments serve to govern the flexor mechanism of the hand and wrist, providing critical constraints to the flexor tendons to prevent bowstringing upon contraction and excursion of extrinsic flexor musculo-tendinous units. The first annular pulley (A1 pulley), near the head of the

metacarpal bone In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...

, lies in the flexor groove in the

deep transverse metacarpal ligament The deep transverse metacarpal ligament (also called the deep transverse palmar ligament) is a narrow fibrous band which runs across the palmar surfaces of the heads of the second, third, fourth and fifth metacarpal bones, connecting them togethe ...

. As a general rule, the A1, A3, and A5 pulleys in the fingers are "joint pulleys" that originate from the volar plate on the volar aspect of the

metacarpophalangeal The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cav ...

, proximal interphalangeal, and distal interphalangeal joints, respectively. The A2 and A4 pulleys arise from the periosteum on the proximal half of the proximal phalanx and the mid portion of the middle phalanx, respectively. The first annular pulley (A1 pulley), near the head of the

, lies in the flexor groove in the

. In the thumb there are two annular pulleys and a single oblique pulley. ("Mechanisms of Finger Flexion") Together, the A pulleys form a continuous tunnel and, because each A pulley's attachments on the bone is wider than its roof, its shape prevents the pulley from pinching its neighbours at extremes of flexion. The short roof also minimises pressure on the tendon under tension, instead distributing pressure throughout the tunnel.

Notes

References

* Fingers Ligaments Upper limb anatomy {{ligament-stub