The Metacarpophalangeal (MP) joint is the articulation point where the hand bones meet the finger bones, commonly known as the knuckles. These joints act as the bridge between the palm’s structure and the fingers’ mobility. There are five such joints in each hand. The integrity of the MP joints is necessary for performing fine motor tasks, from writing to grasping objects.
Anatomy and Location
The MP joints are located at the base of the fingers, visible on the back of the hand when a fist is made. Each joint is formed by the rounded, convex head of a metacarpal bone fitting into the shallow, concave base of the corresponding proximal phalanx. This arrangement classifies the MP joint as a condyloid synovial joint, permitting movement across two planes.
The joint is enclosed by a fibrous joint capsule, which contains the lubricating synovial fluid and provides stability. The medial and lateral aspects of this capsule are reinforced by strong collateral ligaments. These ligaments act as the primary stabilizers, becoming tight when the finger is bent (flexed) to prevent excessive side-to-side movement.
Function and Movement
The MP joints are highly flexible, allowing the fingers to move in multiple directions necessary for complex hand functions. The primary movements permitted are flexion (bending the fingers toward the palm) and extension (straightening the fingers). These actions are driven by muscles in the forearm and hand.
The condyloid structure allows for abduction (spreading the fingers apart) and adduction (bringing them back together). This range of motion, including circumduction, is fundamental for achieving the precision grip used when holding a pen or manipulating small tools. The MP joint of the thumb allows opposition against the other fingers for a strong pinch.
Common MP Joint Conditions
The MP joints are susceptible to conditions that cause pain, stiffness, and loss of function, with arthritis being prevalent. Rheumatoid arthritis (RA) is the most frequent inflammatory condition, typically causing destruction of the cartilage and soft tissues. This chronic inflammation often leads to a characteristic deformity called ulnar drift, where the fingers shift toward the pinkie finger.
Osteoarthritis (OA), resulting from wear and tear or previous trauma, also affects the MP joints, though less commonly than RA. Symptoms of OA include deep, aching joint pain that worsens with gripping and pinching activities, accompanied by swelling at the knuckles. Post-traumatic arthritis may develop years after an injury, such as a fracture that extended into the joint surface.
Acute injuries often involve damage to the supporting soft tissue structures. A common example is a sprain or tear of the ulnar collateral ligament (UCL) of the thumb MP joint, often referred to as Skier’s Thumb. A dislocation occurs when the ends of the bones are forced out of their normal alignment, causing immediate pain, swelling, and a visible deformity.
Diagnosis and Treatment Approaches
Evaluation of an MP joint problem begins with a physical examination to assess tenderness, swelling, range of motion, and joint stability. Imaging techniques confirm the diagnosis and determine the extent of the damage. Plain X-rays are the first line of imaging, revealing joint space narrowing, bone spurs, and signs of bony erosion or fractures.
For soft tissue injuries, such as ligament or tendon tears, a magnetic resonance imaging (MRI) scan or ultrasound may be necessary to visualize the non-bony structures. Initial non-surgical management involves rest, ice application, and the use of nonsteroidal anti-inflammatory drugs (NSAIDs) to control pain and inflammation. Splinting or supportive taping can also be employed to immobilize the joint and allow healing.
Physical therapy, including specialized hand exercises, helps to maintain or improve the flexibility and strength of the affected joint. When non-surgical methods fail, or in cases of severe joint destruction, surgical intervention may be considered. Options include joint replacement (arthroplasty) using implants to preserve motion, or joint fusion (arthrodesis), which is sometimes used for the thumb to ensure stability during pinching.