Knuckles: Anatomical Definition, Types of Joints, and Function

What is knuckle in anatomy?

In anatomical terms, "knuckle" is a layperson's term referring to the joints of the fingers, primarily the large joints connecting the hand to the fingers (metacarpophalangeal joints) and the smaller joints within the fingers themselves (interphalangeal joints).

The Anatomical Definition of a Knuckle

While "knuckle" is a widely understood term in common parlance, it is not a precise anatomical designation. Instead, it refers to the prominent joints that form the visible bumps on the posterior (back) aspect of the hand and fingers when a fist is made or the fingers are bent. These "knuckles" are crucial for the complex range of motion and dexterity of the human hand, enabling everything from powerful gripping to delicate fine motor tasks.

Anatomically, the term "knuckle" encompasses two main types of joints:

• Metacarpophalangeal (MCP) Joints: These are the most prominent knuckles, located where the fingers meet the palm.
• Interphalangeal (IP) Joints: These are the smaller knuckles located within the fingers themselves.

Metacarpophalangeal (MCP) Joints: The "Main Knuckles"

The metacarpophalangeal joints are the articulations between the distal heads of the five metacarpal bones in the palm of the hand and the proximal bases of the five proximal phalanges (the first bones of each finger). These are the joints that are most commonly referred to as "knuckles," particularly when forming a fist.

• Joint Type: MCP joints are classified as condyloid joints. This type of synovial joint allows for movement in two planes.
• Bones Involved: Each MCP joint involves one metacarpal bone and one proximal phalanx.
• Movements:
  • Flexion: Bending the fingers towards the palm.
  • Extension: Straightening the fingers.
  • Abduction: Spreading the fingers apart (away from the middle finger).
  • Adduction: Bringing the fingers together (towards the middle finger).
  • Circumduction: A combination of these movements, allowing for a circular motion.
• Function: These joints are fundamental for grip strength, allowing the fingers to wrap around objects, and for precise control over finger positioning for tasks like writing or typing. The thumb's MCP joint, while also a condyloid joint, has a more limited range of abduction and adduction compared to the other fingers.

Interphalangeal (IP) Joints: The "Finger Knuckles"

The interphalangeal joints are the articulations between the individual bones (phalanges) within each finger. There are two sets of IP joints in fingers 2-5, and one in the thumb.

Proximal Interphalangeal (PIP) Joints

• Location: These joints are located between the proximal phalanx and the middle phalanx of each finger (digits 2-5). They form the first set of smaller knuckles beyond the main MCP knuckles.
• Joint Type: PIP joints are hinge joints. This type of synovial joint allows for movement primarily in one plane.
• Movements:
  • Flexion: Bending the finger at the mid-joint.
  • Extension: Straightening the finger at the mid-joint.

Distal Interphalangeal (DIP) Joints

• Location: These joints are located between the middle phalanx and the distal phalanx (the fingertip bone) of each finger (digits 2-5). They form the smallest, most distal set of knuckles.
• Joint Type: DIP joints are also hinge joints.
• Movements:
  • Flexion: Bending the fingertip.
  • Extension: Straightening the fingertip.

Thumb's Interphalangeal (IP) Joint

• The thumb (digit 1) only has two phalanges (proximal and distal), unlike the other fingers which have three. Therefore, it possesses only one interphalangeal joint, which functions as a hinge joint, allowing for flexion and extension of the thumb's tip.

Anatomy Supporting Knuckle Function

The smooth and powerful function of the knuckles relies on a complex interplay of various anatomical structures:

• Bones: The metacarpals and phalanges provide the skeletal framework.
• Articular Cartilage: A smooth, slippery tissue covering the ends of the bones within the joint, reducing friction and allowing for fluid movement.
• Joint Capsule: A fibrous sac that encloses the joint, containing synovial fluid.
• Synovial Fluid: A viscous fluid within the joint capsule that lubricates the joint and nourishes the cartilage.
• Ligaments: Strong, fibrous bands that connect bones to bones, providing stability to the joints and preventing excessive or unwanted movements. Key ligaments include the collateral ligaments (on the sides of the joints, preventing sideways movement) and the palmar (volar) plates (thickened parts of the joint capsule on the palm side, preventing hyperextension).
• Tendons: Strong, cord-like tissues that connect muscles to bones, transmitting the force generated by muscle contraction to move the joints.
  • Extensor Tendons: Located on the back of the hand and fingers, responsible for extending (straightening) the knuckles.
  • Flexor Tendons: Located on the palm side, responsible for flexing (bending) the knuckles.
• Muscles: Both intrinsic muscles (originating and inserting within the hand, e.g., lumbricals and interossei) and extrinsic muscles (originating in the forearm and inserting into the hand) control the complex movements of the fingers and their knuckles.

Common Issues and Considerations for Knuckles

Given their frequent use and exposure, knuckles are susceptible to various conditions:

• Arthritis:
  • Osteoarthritis: Degenerative joint disease due to wear and tear of articular cartilage, common in older adults.
  • Rheumatoid Arthritis: An autoimmune disease causing inflammation of the joint lining, leading to pain, swelling, and potentially joint deformity.
• Sprains and Dislocations: Traumatic injuries can cause ligaments to stretch or tear (sprain) or bones to be forced out of their normal alignment (dislocation).
• Ganglion Cysts: Non-cancerous lumps that commonly develop near joints or tendons, often on the back of the wrist or fingers near the knuckles.
• Boxer's Fracture: A fracture of the neck of the fifth metacarpal bone, often sustained from punching an object with a closed fist, affecting the pinky finger's MCP joint.
• Knuckle Cracking: The sound produced by "cracking" knuckles is generally attributed to the rapid collapse of gas bubbles (cavitation) within the synovial fluid of the joint. While often perceived as harmful, research suggests it does not directly lead to arthritis.

Importance in Exercise and Function

Understanding the anatomy and biomechanics of the knuckles is critical for fitness professionals and enthusiasts alike:

• Grip Strength: The integrity and mobility of the MCP and IP joints are paramount for effective grip, whether for lifting weights, climbing, or performing daily tasks.
• Fine Motor Skills: The precise control offered by these joints allows for intricate movements required in sports like golf (grip), gymnastics, or any activity demanding dexterity.
• Impact Absorption: In activities involving striking (e.g., boxing, martial arts), the knuckles, particularly the MCP joints, are designed to absorb and distribute forces, though improper technique can lead to injury.
• Proprioception: The joints contain sensory receptors that provide feedback to the brain about the position and movement of the fingers, essential for coordinated actions.

In conclusion, while "knuckle" is a common term, its anatomical significance lies in the complex and highly functional metacarpophalangeal and interphalangeal joints that underpin the remarkable capabilities of the human hand.