Continuous Articulations (Synarthroses)
When the connective tissue between bones maintains a solid, continuous layer, a continuous joint or synarthrosis (Synarthrosis) is formed. Based on the type of intervening tissue, they are divided into three main types:
- Fibrous joints (Junctura fibrosa). Bones are held together by dense connective tissue fibers. This group includes syndesmoses (ligaments and gomphosis, such as tooth roots in dental alveoli), interosseous membranes (between the tibia and fibula, radius and ulna, and in the obturator foramen), and sutures (Sutura). Skull sutures vary: plain, squamous, limbic, serrate, denticulate, and schindylesis. In newborns, temporary fibrous connections called fontanelles (fonticuli cranii) are present.
- Cartilaginous joints (Junctura cartilaginous). Connections formed by continuous cartilage tissue are called synchondroses. They can be permanent (in the thoracic spine) or temporary (epiphyseal plates). When a narrow, slit-like cavity forms within the cartilage mass, it is a symphysis (Symphysis). Symphyses are more mobile than synchondroses but are not true synovial joints. Epiphyseal cartilage is responsible for longitudinal bone growth.
- Bony joints (Synostosis). These form when fibrous or cartilaginous tissue is replaced by bone (e.g., the age-related ossification of cranial sutures).
Structure of Discontinuous Articulations (Diarthroses)
When a fluid-filled cavity develops within the connective tissue between articulating bones, a synovial joint (Diarthrosis, Junctura synovialis) is formed. Every synovial joint features obligate (essential) elements:
- Articular surfaces (facies articularis), covered with hyaline (rarely fibrocartilage) cartilage.
- Articular capsule (capsula articularis), consisting of an outer fibrous membrane and an inner synovial membrane. The synovial membrane often forms folds and villi.
- Articular cavity (cavitas articularis).
- Synovial fluid (synovia), which acts as a lubricant and provides nutrition for the avascular cartilage.
To ensure complex biomechanics, accessory (non-essential) elements may be present. Articular lips (labrum articulare), discs, and menisci increase joint surface congruence. Ligaments (ligamenta), which are often thickenings of the capsule, are classified as intracapsular, capsular, or extracapsular. Synovial bursae and tendon sheaths may also be associated with the joint.
Biomechanical Classification of Joints
Joint movement analysis is directly related to its structural morphology. Based on the number of articulating bones, joints are classified as simple (two bones) or complex (three or more bones). If the joint cavity is divided into compartments by an intra-articular cartilage disc or meniscus, it is considered compound (or complex). When multiple anatomically isolated joints must function simultaneously (e.g., temporomandibular joints), they are termed combined.
Movements occur around three cardinal axes: flexion/extension (frontal axis), abduction/adduction (sagittal axis), and rotation (vertical axis). Based on articular surface geometry, joints are divided into:
- Uniaxial: pivot (trochoid) joints (for rotation) and hinge (ginglymus) joints (for flexion and extension).
- Biaxial: condyloid, saddle, and ellipsoid joints.
- Multiaxial: ball-and-socket (spheroidal), cotyloid (cup-shaped), and plane joints. In plane joints, gliding movements can occur along multiple axes, but their range of motion is extremely limited.