The metacarpophalangeal (MCP) joint, commonly known as the knuckle, connects the metacarpal bones of the palm to the finger bones (phalanges). These joints are fundamental for the hand’s diverse functions, enabling both powerful gripping and precise fine motor tasks. They provide stability and flexibility, acting as transitions between the palm and fingers.
Anatomy of the MCP Joint
Each finger, excluding the thumb, has one MCP joint; the thumb has its own single MCP joint. These joints form where the rounded, convex head of a metacarpal bone articulates with the concave base of a proximal phalanx. This configuration allows for a broad range of finger motion.
Hyaline cartilage covers the articulating surfaces of both the metacarpal head and the proximal phalanx. This tissue reduces friction and cushions the joint, absorbing forces during movement. A fibrous joint capsule surrounds each MCP joint, offering structural support and stability.
Various ligaments strengthen the joint capsule, including proper collateral ligaments and accessory collateral ligaments on the sides of the joint. These ligaments extend from the metacarpal head to the proximal phalanx, contributing to joint stability. A palmar (or volar) ligament reinforces the front aspect of the joint capsule, helping prevent hyperextension.
Movements of the MCP Joint
MCP joints are condyloid synovial joints, allowing movement in multiple planes. This enables a wide range of motion and hand dexterity. Primary movements include flexion, where fingers bend towards the palm, and extension, where they straighten away.
MCP joints also facilitate abduction (spreading fingers apart) and adduction (bringing them together). The MCP joints of fingers 2-5 allow for limited circumduction, a circular movement combining these actions. The thumb’s MCP joint allows flexion but has a more restricted range of extension, abduction, and adduction.
Common Conditions Affecting the MCP Joint
MCP joints can be affected by various conditions impacting their function and stability. Arthritis is common, involving loss of the protective cartilage layer on bone ends. Osteoarthritis, or “wear and tear” arthritis, often affects the thumb and index finger MCP joints due to repetitive stress.
Rheumatoid arthritis, an autoimmune disorder, frequently involves MCP joints, causing inflammation and destruction of joint tissues. This can lead to pain, swelling, and deformities. Other forms of arthritis, like gout and psoriatic arthritis, can also affect these joints.
Beyond arthritis, MCP joints are susceptible to injuries like sprains and dislocations. Sprains involve damage to supporting ligaments. Dislocations occur when joint bones are forced out of normal alignment. These conditions can result from trauma or forceful movements, compromising joint integrity and limiting hand function.