Continuous connections (Synarthroses)
If the connective tissue between bones maintains a continuous layer, a continuous connection is formed — a synarthrosis (Synarthrosis). Depending on the type of intermediate tissue, they are divided into three types:
- Fibrous (Junctura fibrosa). Bones are held together by dense fibers. This group includes syndesmoses (ligaments and gomphosis of tooth roots in the alveoli), interosseous membranes (on the leg, forearm, and in the obturator foramen), and sutures (Sutura). Skull sutures are diverse: plane, squamous, limbate, serrated, denticulate, and schindylesis. In newborns, temporary fibrous connections are present — fontanelles (fonticuli cranii).
- Cartilaginous (Junctura cartilaginea). Connections formed by continuous cartilage tissue are called synchondroses. They are permanent (in the thoracic spine) and temporary. If a narrow slit-like space forms within the thickness of the cartilage, it is a symphysis (Symphysis). Symphyses are more mobile than synchondroses but are not true joints. Epiphyseal cartilage is responsible for bone growth.
- Bony (Synostosis). These form when fibrous or cartilaginous tissue is replaced by bone (e.g., age-related ossification of skull sutures).
Structure of a discontinuous connection (Diarthrosis)
If a cavity forms inside the connective tissue, a joint arises (Diarthrosis, Junctura synovialis). Any joint is characterized by the presence of obligate elements:
- Articular surfaces (facies articularis), covered with hyaline (rarely fibrocartilage) cartilage.
- Articular capsule (capsula articularis), consisting of fibrous and synovial membranes. The synovial membrane forms folds and villi.
- Articular cavity (cavitas articularis).
- Synovial fluid (synovia), which acts as a lubricant and nourishes the cartilage.
To ensure complex biomechanics, accessory elements exist. Articular lips (labrum articulare), discs, and menisci increase surface congruity. Ligaments (ligamenta), which are derivatives of the capsule, are divided into intracapsular, capsular, and extracapsular. Synovial bursae and tendon sheaths may also be present.
Biomechanical classification of joints
The analysis of movements in a joint is directly related to its structure. According to the number of articulating bones, joints are simple (two bones) and complex (three or more). If the joint cavity is divided into chambers by an intra-articular cartilage, it is called compound (complex). If several anatomically isolated joints function only simultaneously (e.g., temporomandibular), they are considered combined.
Movements occur around three axes: flexion/extension (frontal axis), abduction/adduction (sagittal axis), and rotation (vertical axis). According to the shape of their surfaces, joints are divided into:
- Uniaxial: cylindrical (for rotation) and hinge (for flexion and extension).
- Biaxial: condylar, saddle, and ellipsoid.
- Multiaxial: ball-and-socket, cotyloid (spheroidal/cup-shaped), and plane. In plane joints, movements are possible along all axes, but their range is extremely small.
Briefly: answers to topical questions
Around which specific axes are movements possible in biaxial joints?
In biaxial joints, movements are possible around two axes.
- Condylar joints (articulatio bicondylaris) — flexion and extension around the frontal axis, as well as rotation around the vertical axis, which appears after preliminary flexion.
- Saddle joints (articulatio sellaris) — flexion and extension around the frontal axis, abduction and adduction around the sagittal axis.
- Ellipsoid joints (articulatio ellipsoidea) — flexion and extension around the frontal axis, abduction and adduction around the sagittal axis.
What substances are included in the chemical composition of synovial fluid?
The chemical composition of synovial fluid (synovia) includes water, proteins, fats, salts, enzymes, and hyaluronic acid.
Synovial fluid is a blood transudate. Hyaluronic acid within its composition is a non-sulfated glycosaminoglycan consisting of repeating units of D-glucuronic acid and N-acetyl-D-glucosamine. It provides water binding, viscosity, and lubrication of articular surfaces.
Which joints in the human body are examples of combined joints?
Examples of combined joints (articulationes combinatoriae) in the human body are the temporomandibular, atlantoaxial, and radioulnar joints.
- Temporomandibular joints — the right and left joints are anatomically isolated but work synchronously.
- Atlantoaxial joints — movements in them are performed simultaneously.
- Radioulnar joints — the proximal radioulnar joint functions together with the distal one, ensuring the rotation of the radius.
Which specific human joints are plane joints according to the shape of their surfaces?
According to the shape of the articular surfaces, the following human joints are plane (articulatio plana):
- Zygapophysial (intervertebral) joints (articulationes zygapophysiales) — particularly in the cervical region.
- Sacroiliac joint — formed by the auricular surfaces of the sacrum and the hip bone.
- Acromioclavicular joint — connects the clavicle and the acromion of the scapula.
- Intercarpal joints — located between the carpal bones.
- Carpometacarpal joints — between the distal row of carpal bones and the metacarpal bones.
The algorithm for describing any joint consists of 6 steps: 1) articular surfaces, 2) accessory elements, 3) capsule attachment, 4) ligaments, 5) auxiliary elements (bursae), 6) biomechanical classification.
Class and exam questions
How does a symphysis differ from a synchondrosis?
Symphysis has a slit-like space within the thickness of the cartilage, providing it with slightly greater mobility, whereas synchondrosis is entirely continuous cartilage tissue.
What is the difference between a compound (complex) joint and a combined joint?
A compound (complex) joint has a disc or meniscus inside the cavity, dividing it into chambers. A combined joint consists of two or more anatomically isolated joints in which movements can occur only simultaneously.
What is an amphiarthrosis?
This is a low-mobility, "tight" joint with high surface congruity, a very dense capsule, and powerful ligaments, such as the sacroiliac joint.
What else to study on this topic
- Morphology of fibrous sutures: from squamous to schindylesis
- Classification of joint ligaments (intra-, extra-, and capsular)
- Temporary and permanent synchondroses
- Mechanics of multiaxial joints and their limitations
Study the complete topic in the Anatomy course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "General Arthrology" and 1,363 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 295 video lessons by subject
Other topics in the "Arthrology" section
Free video lessons: AnatomyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.