General and Sports Anatomy - L.V. Kapilevich, K.V. Davletyarova 2008

Anatomy of the Musculoskeletal System
Arthrology

Arthrology is The Study of bone joints.

Classification of Joints

- interrupted (synovial joints);

- uninterrupted (formed by various types of Connective Tissue);

- semi-interrupted (cartilaginous joints);

By Structure (Figure 7):

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Figure 7. Types of bone connections.

I. Uninterrupted joints

✵ Fibrous joints: syndesmoses (connections of bones via ligaments and membranes);

✵ sutures (Skull): serrate, squamous, plane;

✵ gomphosis (tooth ROOT in the alveolus);

✵ Cartilaginous joints: synchondroses (temporary — epiphyseal Cartilage of tubular bones, and permanent — intervertebral discs);

✵ Bony synostoses — sacrum, etc.

II. Semi-interrupted joints: symphyses (pubic bones).

III. Interrupted joints

Synovial joints (elbow, hip, ankle, etc.).

By joint function:

1. immovable (skull sutures);

2. slightly movable (spinal cartilage layer);

3. freely movable (synovial joints).

Joints

Main Structural elements of joints: articular surfaces, articular capsule, joint cavity, synovial fluid (secreted by the epithelium). Articular surfaces are covered with a layer of hyaline cartilage (fibrocartilage is present only in the temporomandibular and sternoclavicular joints), which cushions friction and impacts.

The cartilage has irregularities — 1st-order curves (1 mm) and 2nd-order curves (0.05 mm), which flatten out under compression and prevent fluid outflow. The shapes of the cartilage surfaces match each other: if one bone has a convexity, the articulating bone has a concavity.

The articular capsule is firmly fused with the periosteum and consists of two layers: the outer fibrous layer (formed by Fibrous connective tissue, acting as a transition of the periosteum from one bone to the other) and the inner synovial layer made of loose connective tissue, lined on the inside with epithelium.

Accessory structures: synovial folds and villi (projections of the synovial layer filled with adipose tissue that act as Shock absorbers); intra-articular discs (cartilaginous plates that divide the joint into 2 chambers and enhance mobility); menisci (crescent-shaped plates with openings in the center, fused at their outer edges to the Joint Capsule, which improve bone congruency and shock absorption); labra (increase the surface area of bone contact, thereby reducing mechanical load).

Joint stabilization factors: ligamentous apparatus, Muscle tension, negative pressure within the joint cavity, and adhesion of articular surfaces (the adhesive effect of synovial fluid).

Joint shapes (Figure 8):

1. Hinge and pivot joints. Uniaxial joints (interphalangeal joints; humeroulnar, humeroradial joints).

2. Saddle joints. Biaxial joints shaped like a saddle (carpometacarpal joint of the thumb, sternoclavicular joint). Closely related in shape are the ellipsoid (radiocarpal) joint and the condylar joint (an intermediate type between them), namely the atlanto-occipital joint.

3. Ball-and-socket joints. Movements occur in 3 planes: flexion-extension, abduction-adduction, internal and external rotation, and circumduction. These include the shoulder and hip joints. In some joints, active movements are restricted along certain axes due to the absence of Muscles, leaving only passive movements possible (metacarpophalangeal joints).

4. Plane joints (gliding) - Articulations between the Carpal Bones.

Combined joints: two joints whose movements occur only simultaneously, such as the temporomandibular and radioulnar joints.

Bicameral joints: joints featuring intra-articular discs, such as the sternoclavicular and temporomandibular joints.

Simple joints: formed by the articulation of 2 bones; complex joints: formed by 3 or more bones.

The degree of joint mobility is inversely proportional to the congruency of the articulating surfaces and The ratio of their surface areas.

The range of mobility is highly variable and depends on sex (greater in females than in males), age (decreases with advancing age), ambient Temperature (decreases as temperature drops), and physical conditioning. Notably, strength training tends to reduce mobility.

Figure 8. Shapes of articular surfaces.

Joints in the Body

Skull.

The cranial bones are connected by syndesmoses (sutures), synchondroses, and diarthroses. In older age, syndesmoses and synchondroses undergo ossification and become synostoses.

Cranial sutures:

✵ serrate sutures (coronal suture between the frontal and parietal bones; sagittal suture between the parietal bones; lambdoid suture between the Occipital bone and the adjacent parietal and temporal bones);

✵ squamous sutures (between the temporal, sphenoid, and parietal bones);

✵ plane sutures (between the BONES OF THE facial Skeleton).

Synchondrosis: located at the skull base between the bodies of the occipital and sphenoid bones—the spheno-occipital synchondrosis.

Diarthroses:

1. Temporomandibular joints. A combined bicameral joint that is ellipsoid in shape and functionally approaches a ball-and-socket joint (owing to the intra-articular disc). It is reinforced by three ligaments: the lateral ligament, stylomandibular ligament, and sphenomandibular ligament. Permitted movements include depression and elevation of the Mandible, as well as forward, backward, and lateral excursions.

2. Atlanto-occipital joint (ellipsoid, 2 axes), supplemented by the atlantoaxial joint (pivot, 1 axis). They are reinforced by the cruciform ligament, the two alar ligaments of the atlas, and the anterior and posterior membranes.

JOINTS OF THE Trunk: The Spine

The vertebral bodies are connected by intervertebral discs, which consist of an annulus fibrosus (the peripheral part of the disc) and a Nucleus pulposus (the central part). The nuclei are elastic; they act to separate the vertebral bodies and serve as shock absorbers.

The discs account for 1/4 of the total height of the spine, decreasing with age. The widest discs are found in the lumbar region, becoming thinner superiorly (making up 1/3 of the spinal height in the lumbar region and 1/6 in the upper thoracic region), and are reinforced by the anterior and posterior intervertebral ligaments. The vertebral arches are connected by the ligamenta flava (very strong elastic fibers). The spinous processes are linked by interspinous ligaments, and the transverse processes by intertransverse ligaments. Running the entire length of the spine, the supraspinous ligament connects the spinous processes (continuing superiorly as the nuchal ligament).

The junction between the sacrum and the coccyx is a joint allowing forward and backward movement of about 2 cm, which transforms into a synchondrosis by the age of 40.

Rib-Vertebral Joints

The heads of the Ribs form joints with the bodies of the thoracic vertebrae (reinforced by the radiate ligament), while the rib tubercles articulate with the transverse processes (costotransverse joints, reinforced by the costotransverse ligament), which are cylindrical in shape. Together, both articulations form a combined joint with an axis of rotation passing through the neck of the rib.

Rib-Sternum Joints

These connections are established via cartilage. The cartilage of the 1st rib fuses directly with the sternum through cartilaginous tissue, whereas the cartilages of the remaining true ribs (up to the 7th rib inclusive) form flat sternocostal joints. These are reinforced by radiate and sternocostal ligaments.

Joints of the Upper Limb

Sternoclavicular joint. Formed by the sternal end of the clavicle and the clavicular notch of the sternum. It is a simple saddle joint featuring an articular disc, which allows it to function like a ball-and-socket joint. Range of motion is limited. It is reinforced by several ligaments: the interclavicular, costoclavicular, and the anterior and posterior sternoclavicular ligaments.

Acromioclavicular joint. A simple planar joint that may eventually ossify into a synchondrosis. It is reinforced by the coracoclavicular and acromioclavicular ligaments. Movements are highly restricted, consisting primarily of gliding around the sagittal axis.

Shoulder joint. Formed by the HEAD of the humerus and the glenoid cavity of the scapula. It is a ball-and-socket joint with poor congruence, though the glenoid labrum broadens the socket along its rim. It is reinforced by the coracohumeral ligament. Movements of the joint typically occur in conjunction with scapular movement, which collectively allows the limb to sweep through a hemisphere. The tendon of the long head of the biceps brachii passes through the joint cavity, providing additional stability. Within this joint, the humerus moves around three axes (flexion and extension, abduction and adduction, pronation and supination).

Elbow joint. Formed by the humerus, ulna, and radius. It comprises three articulations: the humeroulnar joint (ginglymus or hinge joint, connecting the trochlea of the humerus and the trochlear notch of the ulna), the humeroradial joint (spheroidal, between the capitulum of the humerus and the articular facet of the radius), and the proximal radioulnar joint (trochoid or pivot, between the head of the radius and the radial notch of the ulna). Because an interosseous membrane spans the gap between the radius and ulna, movement around the sagittal axis in the humeroradial joint is restricted. Consequently, flexion and extension of approximately 140⁰ occur via the humeroulnar and humeroradial joints combined, while pronation and supination of approximately 140⁰ (up to 180⁰) are facilitated by the humeroradial and radioulnar joints. The ligaments of the elbow include the ulnar and radial collateral ligaments, and the annular ligament of the radius.

Distal radioulnar joint. A pivot joint. It also participates in the pronation and supination of the forearm, forming a combined pivot joint with the proximal radioulnar articulation.

Radiocarpal joint. The articulation between the radius and three carpal bones: the scaphoid, lunate, and triquetrum. The articular disc located beneath the ulna fuses with the ulnar articular surface. This is a complex, ellipsoidal, biaxial joint permitting flexion, extension, abduction, and adduction.

Midcarpal joint. Located between the proximal and distal rows of carpal bones, featuring a complex, irregular (planar) articular surface.

The Ligamentous apparatus of the hand is highly intricate, with the primary ligaments being the collateral ligaments of the wrist (radial and ulnar). The transverse carpal ligament acts as a retinaculum, forming the carpal tunnel on the palmar side through which nerves and Blood Vessels pass.

Carpometacarpal joints. Situated between the distal row of carpal bones and the bases of the Metacarpal bones. All are planar and exhibit limited mobility (5-10⁰), with the exception of the thumb. The carpometacarpal joint of the thumb is a saddle joint with a separate joint cavity and two axes of rotation (40-60⁰). It can withstand loads of 60-100 kg, though dislocations may occur and frequently become recurrent.

Metacarpophalangeal joints. Spheroidal joints with three axes of rotation, although pronation and supination occur only passively. They are reinforced by collateral and palmar ligaments.

Interphalangeal joints. Hinge joints with a single axis of rotation. Flexion and extension range from 110-120° in the proximal joints to 80-90⁰ in the distal ones. Each joint is supported by three ligaments: medial, lateral, and palmar.

Joints of the Lower Limb

Sacroiliac joint. Formed by the hip bone and the sacrum. A simple planar joint with a range of motion of 3-5⁰, reinforced by a robust network of thick and strong ligaments.

Pubic Symphysis. Connects the surfaces of the two pubic bones. The intervening cartilage forms a disc containing a small central cavity, functioning as a hemiarthrosis. It is reinforced by the superior pubic ligament above and the arcuate pubic ligament below.

Hip joint. Formed by the femoral head and the acetabulum of the hip bone, augmented by the acetabular labrum. Intracapsular ligaments include the ligament of the head of the Femur and the transverse acetabular ligament. The joint is further reinforced by powerful extracapsular ligaments: the iliofemoral, pubofemoral, ischiofemoral, and the zona orbicularis. It is a simple ball-and-socket joint allowing three axes of rotation and circumduction. Flexion and extension: the active range of motion is ~120⁰, while the passive range is ~150-160⁰. Flexing the knee increases this range (compared to 84-87⁰ with an extended knee), due to the tension exerted by the hamstring muscles. Flexibility training for splits is typically asymmetrical—emphasizing posterior muscle stretching for the "front" leg and anterior muscles for the "back" leg. Abduction (40-60⁰) and adduction (15-30⁰) also increase when the knee joint is flexed. Pronation and supination range from 15-40⁰ and double when the hip joint is flexed. While it can be fixed passively (against the floor), maintaining this position unsupported in the air is impossible.

Knee joint. Formed by the femoral condyles, the Tibia, and the Patella. In full extension, it acts as a hinge joint (~130⁰ of flexion), with potential hyperextension of 10-12⁰. In a flexed position, it approximates an ellipsoidal joint, permitting pronation and supination around the vertical axis. Two menisci—medial and lateral—enhance joint congruence and serve as shock absorbers. The capsule is thin and lax. The ligamentous support includes the transverse ligament of the knee, the meniscofemoral ligaments, the collateral ligaments (tibial and fibular), the cruciate ligaments (anterior and posterior), the oblique popliteal ligament, and the patellar ligament.

Ankle joint. Talus and bones of the lower leg. A complex hinge joint allowing flexion (45-50⁰) and extension (15-25⁰), with adduction and abduction around the vertical axis by 12⁰ in the flexed position. Ligaments: medial (deltoid), 2 lateral (calcaneofibular, talofibular).

Subtalar joint. Talus and calcaneus in the posterior region. A simple cylindrical joint operating around the sagittal axis for pronation and supination. Reinforced by lateral talocalcaneal ligaments.

Talocalcaneonavicular joint. A complex ball-and-socket joint, primarily responsible for FOOT pronation and supination. Reinforced by the calcaneonavicular ligament, interosseous ligament (between the talus and calcaneus), and ligaments on the dorsal and plantar aspects.

The subtalar and talocalcaneonavicular joints form a combined joint.

Calcaneocuboid joint. A simple plane joint.

Cuneonavicular joint. A plane joint, reinforced by dorsal, plantar, and interosseous ligaments.

Tarsometatarsal joints. Plane joints, well reinforced by dorsal and plantar ligaments.

Metatarsophalangeal joints. Hinge joints with lateral and medial ligaments. Limited range of abduction and adduction.

Interphalangeal joints of the foot. Hinge joints, reinforced by plantar and collateral ligaments. Limited range of flexion and extension.

Arches of the foot

Provide shock-absorbing properties.

1. longitudinal arches (lateral and medial);

2. transverse arch.

Self-Assessment Questions:

1. What are the main structural elements of joints?

2. What types of bone connections exist?

3. Which joints are classified as hinge joints?