Human Anatomy and Physiology - N. I. Fedyukovich 2003

Bones and Joints
Bone Articulations

The BONES OF THE human Skeleton are integrated into a single functional system (the passive part of The Musculoskeletal System) through various types of articulations. All bone joints are divided into three types: continuous, discontinuous, and symphyses. Depending on the type of tissue connecting the bones, continuous joints are classified into fibrous, osseous, and synchondroses (cartilaginous joints) (Fig. 9).

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Fig. 9. Types of Bone Articulations (diagram):

A — joint; B — fibrous joint; C — synchondrosis (cartilaginous joint); D — symphysis (hemiarthrosis); 1 — periosteum; 2 — bone; 3 — Fibrous Connective Tissue; 4 — Cartilage; 5 — synovial membrane; 6 — fibrous membrane; 7 — articular cartilage; 8 — articular cavity; 9 — cleft in the interpubic disc; 10 — interpubic disc

Fibrous joints are characterized by high strength and low mobility. They include syndesmoses (ligaments and interosseous membranes), sutures, and gomphoses.

Ligaments are thick bundles or sheets formed by Dense Fibrous Connective tissue with a high content of Collagen fibers. In most cases, ligaments connect two bones and reinforce joints, limiting their movement and withstanding significant loads.

Interosseous membranes connect the diaphyses of long bones and serve as attachment sites for Muscles. These membranes have openings through which Blood Vessels and nerves pass.

A variety of fibrous joints are cranial sutures, which, depending on the configuration of the articulating bone edges, can be serrate, squamous, or plane. In all types of sutures, thin layers of connective tissue lie between the joined bones.

Gomphosis is a special type of fibrous joint found where a tooth articulates with the bone of the dental alveolus. A thin layer of connective tissue, the periodontium, lies between the tooth and the bony wall.

Synchondroses are bone articulations formed by cartilaginous tissue. They are characterized by elasticity and strength, and they perform a Shock-absorbing function.

The replacement of the cartilaginous layer between bones with Bone tissue is called synostosis. Mobility in such joints disappears, while strength increases.

Discontinuous (synovial or articular) joints are the most mobile bone articulations. They offer high mobility and a wide range of movements. Characteristic Features of a joint include the presence of articular surfaces, a joint cavity, synovial fluid, and a capsule. The articular surfaces of bones are covered with hyaline cartilage ranging from 0.25 to 6 mm in thickness, depending on the load on the joint. The joint cavity is a slit-like space between the articular surfaces of the bones, enclosed on all sides by the Joint Capsule and containing a small amount of synovial fluid.

The articular capsule encloses the connecting ends of the bones, forming a sealed sac whose walls consist of two layers: an outer fibrous layer and an inner synovial membrane.

The outer fibrous layer consists of dense fibrous Connective tissue with longitudinally oriented fibers, providing the articular capsule with significant strength. In some joints, the fibrous layer may form thickenings (capsular ligaments) that reinforce the joint capsule.

The inner layer (synovial membrane) has small projections (villi rich in Blood Vessels) that significantly increase its surface area. The synovial membrane produces fluid that lubricates the articulating surfaces, eliminating friction between them. Additionally, this membrane absorbs fluid, ensuring a continuous metabolic process.

In case of incongruence of the articular surfaces, cartilaginous plates of various shapes—articular discs and menisci—are situated between them. They can shift during movement, smooth out irregularities of the articulating surfaces, and perform a shock-absorbing function.

In some cases (for example, the shoulder joint), an articular labrum is located along the edge of the articular surface of one of the bones, which deepens it, increases the joint area, and provides greater congruence between the articulating surfaces.

Depending on The Structure of the articulating surfaces, movements in the joints can occur around various axes. Flexion and extension are movements around the frontal axis; abduction and adduction are around the sagittal axis; rotation is around the longitudinal axis; circumduction is around all axes. The range and volume of motion in the joints depend on the difference in angular degrees of the articulating surfaces. The greater this difference, the wider the range of motion.

Joints can differ from one another in the number of articulating bones and the shape of their articular surfaces.

A joint formed by only two articular surfaces is called a simple joint, while a joint consisting of three or more articular surfaces is called a compound joint.

A distinction is made between complex and combined joints. The former are characterized by the presence of an articular disc or meniscus between the articulating surfaces; the latter consist of two anatomically isolated joints that function together (such as the temporomandibular joint).

According to the shape of their articular surfaces, joints are classified into cylindrical, ellipsoidal, and spherical (Fig. 10).

Fig. 10. Shapes of joints:

1 — hinge; 2 — ellipsoidal; 3 — saddle; 4 — ball-and-socket

Variations of the aforementioned joint shapes also occur. For example, a hinge joint is a variation of a pivot joint, while cotyloid and plane joints are variations of a ball-and-socket joint. The shape of the articular surfaces determines the axes around which movement occurs in a given joint. With pivot-shaped articular surfaces, movement occurs around a single axis; with ellipsoid surfaces, around two axes; and with ball-and-socket surfaces, around three or more mutually perpendicular axes. Thus, There is a clear relationship between the shape of the articular surfaces and the number of axes of motion. Consequently, joints are classified as uniaxial, biaxial, and triaxial (multiaxial).

Uniaxial joints include pivot and hinge joints. For example, in a pivot joint, rotation occurs around a vertical axis that coincides with the longitudinal axis of the bone (such as the Rotation of the first cervical vertebra, the atlas, along with the Skull around the odontoid process of the second vertebra, the axis). In hinge joints, movement occurs around a single transverse axis, such as flexion and extension in the interphalangeal joints. A variation of the hinge joint is the cochlear (helical) joint, where movement follows a spiral path (for example, the humeroulnar joint).

Biaxial joints include ellipsoid, saddle, and condylar joints. In an ellipsoid joint, movements occur around mutually perpendicular axes (for example, the radiocarpal joint)—flexion and extension around the frontal axis, and adduction and abduction around the sagittal axis.

In a saddle joint (such as the carpometacarpal joint of the thumb), the movements are similar to those in an ellipsoid joint, allowing not only abduction and adduction, but also opposition of the thumb to the other fingers.

A condylar joint (such as the knee joint) represents a transitional form between a hinge joint and an ellipsoid joint. It features two convex articular heads that resemble an ellipse, known as condyles. In a condylar joint, movement is possible around the frontal axis (flexion and extension) and around the longitudinal axis (rotation).

Triaxial (multiaxial) joints include ball-and-socket, cotyloid, and plane joints. In a ball-and-socket joint, flexion and extension, adduction and abduction, as well as rotation occur. Due to the significant difference in size between the articular surfaces (the HEAD of the joint and the socket), the ball-and-socket (shoulder) joint is the most mobile of all joints.

The cotyloid joint (such as the hip joint) is a type of ball-and-socket joint. It differs from the latter in the greater depth of its socket. Due to the small difference in the angular dimensions of the articular surfaces, the range of motion in this joint is limited.

In plane joints, movements occur around three axes, but the range of motion is limited due to the slight curvature and small size of the articular surfaces. Plane joints include the zygapophysial (intervertebral) and tarsometatarsal joints.