Arthrology, derived from the ancient Greek word arthros (meaning “articulated”), is the study of the structures that hold bones together, allowing them to move in varying degrees or fix them in place according to the design and the function of the joint.
The term articulation applies to any joint of bones, whether it moves freely or does not move.
Within some joints, such as the knees and elbows, are fluid-filled sacs called bursas, which help reduce friction between the tendons and bones; The inflammation in these sacs is called bursitis.
Syndesmosis joints are stabilized by connective tissue called ligaments or interosseous membranes, ranging from bundles of collagen fibers that restrict movement and hold a joint in place to elastic yarns that can stretch repeatedly and return to their original forms.
The shoulder joint, or corticohumeral ligament, extends from the coracoid process of the scapula to the greater tubercle of the humerus.
The pubofemoral ligament extends from the pubic bone to the femur.
The knee joint (oblique popliteal ligament), where the tendon of the semimembranosus muscle expands to cross the back of the knee joint.
The three types of joints are the following:
Fibrous: Fibrous tissue binds rigidly to bones in the form of articulation called synarthrosis, which is characterized by no movement. The sutures of the skull are fibrous joints.
Any slight movement in a joint depends on the length of the fibers that connect the bones.
Cartilaginous: This type of joint can be found in two ways:
The synchondrosis joint involves a hyaline (rigid) cartilage that does not allow movement.
The most common example is the epiphyseal plate of the long bone. Other examples are the joint between the ribs, the costal cartilage, and the sternum.
The joints of the symphysis are produced when the fibrocartilage fuses the bones so that the pressure can cause a slight movement, called antriarthrosis.
Examples include the intervertebral discs and the symphysis pubis.
Synovial: Also known as diarthrosis or joints that move freely, this type of joint involves a synovial cavity, which contains joint fluid secreted by the synovial membrane to lubricate the opposite surfaces of the bone covered by soft articular cartilage.
The synovial membrane is covered by a fibrous layer of the joint capsule that is continuous with the periosteum of the bone.
The ligaments surrounding the joint strengthen the capsule and keep the bones in place, preventing dislocation.
In some synovial joints, such as the knee, fibrocartilage pads called meniscus (singular: meniscus) are developed in the cavity, dividing it into two parts. These menisci stabilize the joint and act as shock absorbers on the knees.
There are six classifications of mobile or synovial joints:
Sliding: Curved or flat surfaces slide against each other, such as between the carpal bones at the wrist or between the tarsal bones at the ankle.
Hinge: A convex surface joints with a concave surface, allowing movements at right angles in a plane, such as the elbows, knees, and joints between the bones of the fingers.
Pivot (or rotary): A bone pivots or revolves around a stationary bone, like the atlas that rotates around the odontoid process of the shaft in the upper part of the spine.
Condition: The oval head of one bone fits into a shallow depression in another to allow five movements: flexion, extension, abduction, adduction, and bypass.
Examples are the carpal-metacarpal joint in the wrist and the tarsal-metatarsal joint in the ankle.
Saddle: Each of the adjacent bones has the shape of a harness (the technical term is reciprocally concave-convex).
The chair joints resemble the condyle joints but have greater freedom of movement. An example is the carpometacarpal joint of the thumb.
Ball and socket: The round head of one bone fits into a cup-shaped cavity in the other bone, allowing movement in many directions, as long as the bones are not separated or forced, such as the shoulder joint between the humerus, the scapula, and the hip joints.
Types of movement of the joint:
- Flexion: A decrease in the angle between two bones.
- Extension: An increase in the rise between two bones.
- Abduction: Moving away from the midline of the body.
- Adduction: Movement towards the midline of the body.
- Rotation: Rotating around an axis.
- Pronation: Down or palm down.
- Supination: Up or palm up.
- Eversion: Turning the sole outwards.
- Investment: Turning the sole inward.