Joints in the human body enable a wide array of movements, from the most intricate finger gestures to large-scale body motions. While some joints, like the hinge joints of the elbow or knee, allow clear bending and straightening, others are more subtle in their function. Among these less conspicuous yet important connections are planar joints, which facilitate a unique type of motion important for flexibility and stability.
Defining Planar Joints
Planar joints, also known as gliding joints or plane joints, are a specific type of synovial joint characterized by the flat or slightly curved surfaces of the bones that articulate with each other. These relatively flat surfaces allow the bones to glide across one another. The name “planar” directly refers to this flat, plane-like arrangement of bone surfaces.
As synovial joints, planar joints possess features that enable their movement. Each planar joint is enclosed within a joint capsule, which contains the joint components. Inside this capsule, a lubricating fluid called synovial fluid reduces friction between the articulating bones. Additionally, the bone surfaces within the joint are covered by articular cartilage, a smooth, rubbery tissue that further aids in gliding motion and acts as a shock absorber.
Common Locations in the Body
Planar joints are found in various locations throughout the human body, contributing to both broad movements and subtle adjustments. A prominent example includes the intercarpal joints, located between carpal bones of the wrist. Similarly, the intertarsal joints, situated between the tarsal bones in the ankle, exemplify planar joints. These joints in the wrists and ankles enable the complex flexibility needed for fine motor skills and stable locomotion.
Another significant planar joint is the acromioclavicular joint, which connects the acromion of the scapula (shoulder blade) to the clavicle (collarbone). This joint contributes to the overall mobility of the shoulder girdle. The facet joints, specifically the zygapophyseal joints, are planar joints found between the vertebrae of the spine. These spinal joints allow for limited movement between adjacent vertebrae, contributing to flexibility and stability of the vertebral column.
Understanding Their Movement
The primary movement characteristic of planar joints is a gliding motion. This movement occurs when one flat bone surface slides over another, without significant angular change between the bones. Unlike hinge joints that allow movement around a single axis, planar joints are considered non-axial, meaning their movement does not occur around a fixed axis of rotation.
While the movement at an individual planar joint may be limited, the collective action of multiple planar joints can result in a greater range of motion. For instance, the numerous planar joints in the wrist and foot work together to allow for complex and fluid movements. This collective gliding motion also plays a role in shock absorption, distributing forces across multiple small joints. The flat surfaces of these joints are designed to facilitate this smooth, translational movement.