Anatomy of joints

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Anatomy of Joints (Arthrology)

1. Definition

A joint (articulation/junctura) is the site where two or more bones meet. Joints may be structurally simple (united by fibrous tissue) or anatomically elaborate (enclosed by a capsule with synovial fluid). They are studied both structurally and functionally.

2. Classification of Joints

A. Structural Classification

Type	Uniting Medium	Mobility	Examples
Fibrous	Fibrous connective tissue	None or slight	Skull sutures, gomphosis, syndesmosis
Cartilaginous	Hyaline or fibrocartilage	Slight	Epiphyseal plates, pubic symphysis, IVDs
Synovial	Joint cavity + synovial fluid	Freely mobile	Knee, hip, shoulder, elbow

B. Functional Classification

Term	Movement	Examples
Synarthrosis	Immobile	Skull sutures, sternomanubrial joint, gomphosis
Amphiarthrosis	Slightly mobile	Intervertebral disks, pubic symphysis
Diarthrosis	Freely mobile	All synovial joints

The diarthrosis is the most common type of joint. — Imaging Anatomy, Vol. 3

3. Fibrous Joints

Bones are united by fibrous connective tissue with no joint cavity. Three subtypes:

Sutures — found only between skull bones; interlocking edges held by Sharpey's fibers. In adults they fuse (synostosis).
Syndesmosis — bones connected by an interosseous membrane or ligament (e.g., tibiofibular syndesmosis, interosseous membrane of forearm).
Gomphosis — peg-and-socket joint; tooth root anchored into the jaw socket by the periodontal ligament.

Synarthrosis — skull sutures in infant vs adult

Fig. Synarthrosis: skull sutures in an infant (open) vs. adult (fused/synostosis) — Imaging Anatomy Vol. 3

4. Cartilaginous Joints

Bones are united by hyaline cartilage or fibrocartilage; no joint cavity. Two subtypes:

Synchondrosis (hyaline cartilage)

Temporary: Epiphyseal growth plates; Y-shaped triradiate cartilage of the acetabulum (ossifies ~age 25). Injury leads to limb deformity.
Permanent: Costochondral junctions; first sternocostal joint.

Symphysis (fibrocartilage disk)

Located in the midline of the body.
Strong fibrocartilage disk reinforced by periarticular ligaments.
Pubic symphysis: ends of pubic bones covered by hyaline cartilage attached to a fibrocartilage disk; functionally an amphiarthrosis.
Intervertebral disks: wide symphyses with nucleus pulposus + anulus fibrosus.
Manubriosternal joint: symphysis at the sternal angle.

Fig. Amphiarthrosis: pubic symphysis on cadaveric section and MRI — Imaging Anatomy Vol. 3

5. Synovial Joints

The most common and clinically important joint type. Key features:

Structural Components

Component	Description
Articular cartilage	Hyaline cartilage covers bone ends (except TMJ, which has fibrocartilage); avascular, aneural — nourished by synovial fluid
Joint (synovial) cavity	Space between articular surfaces; filled with synovial fluid
Articular capsule	Outer fibrous layer (dense CT, reinforced by ligaments) + inner synovial membrane
Synovial membrane	Lines the capsule and intra-articular structures (except articular surfaces); secretes synovial fluid
Synovial fluid	Viscous, lubricating ultrafiltrate of plasma + hyaluronic acid; nourishes avascular cartilage
Ligaments	Stabilize the joint; may be capsular (thickenings of fibrous capsule), extracapsular, or intracapsular
Articular disks/menisci	Fibrocartilage pads that improve congruity and shock absorption (e.g., knee menisci, TMJ disk)
Bursae	Fluid-filled sacs that reduce friction near joints
Fat pads	Intra-articular fat (e.g., infrapatellar fat pad) that fills dead space

Note: Cruciate ligaments are intracapsular but extra-articular (surrounded by synovial subtima); menisci are intra-articular (not covered by synovial intima, in direct contact with synovial fluid). — THIEME Atlas of General Anatomy

Development of Synovial Joints

Interzonal mesenchyme between developing bones differentiates:

Peripherally → joint capsule and ligaments
Centrally → undergoes cavitation (late embryogenesis to postnatal) → synovial cavity
Lining layer → synovial membrane (secretes fluid)

Movement in utero is required for normal joint formation; restricted fetal movement causes joint fixation. — The Developing Human, Moore

6. Types of Synovial Joints (by Shape)

Type	Axes of Movement	Movement	Examples
Plane (gliding)	0 (non-axial)	Gliding only	Intercarpal, intertarsal, facet joints
Hinge (ginglymus)	1 (uniaxial)	Flexion/extension	Elbow, interphalangeal joints, ankle
Pivot (trochoid)	1 (uniaxial)	Rotation only	Atlantoaxial, proximal radioulnar
Condyloid (ellipsoid)	2 (biaxial)	Flex/ext + ab/adduction	Metacarpophalangeal joints, wrist
Saddle (sellar)	2 (biaxial)	Flex/ext + ab/adduction	1st carpometacarpal (thumb CMC)
Ball-and-socket (spheroid)	3 (multiaxial)	All planes	Hip, shoulder (glenohumeral)

Fig. Osseous anatomy of the hand showing types of synovial joints — Imaging Anatomy Vol. 3

Pivot joint (atlantoaxial), plane joint (facet), hinge joint (elbow), proximal radioulnar pivot

Fig. Various synovial joint types on imaging — Imaging Anatomy Vol. 3

7. Articular Cartilage — Structure and Zones

Articular cartilage is hyaline cartilage, and its normal layered architecture is visible on high-resolution MRI:

Zone	Characteristics
Superficial (lamina splendens)	Tangential collagen fibers; dark on MRI; resists shear forces
Intermediate (transitional)	Oblique fibers; brightest layer on MRI
Deep (radial)	Perpendicular collagen fibers; highest proteoglycan content
Calcified cartilage	Interface with subchondral bone; separated from deep zone by the tidemark

Avascular and aneural — nourished entirely by diffusion from synovial fluid and, in part, from subchondral bone.
Limited regenerative capacity → damage progresses to osteoarthritis.
In osteoarthritis: proteoglycan + collagen loss → increased water content → cartilage softening → subchondral sclerosis and osteophyte formation.

Normal articular cartilage layers and early damage (patellofemoral MRI)

Fig. Three-layered articular cartilage on MRI — Imaging Anatomy Vol. 3

8. Knee Joint — A Paradigm of Synovial Joint Anatomy

The knee is the body's largest synovial joint and illustrates all key joint components:

Capsule extends 15 mm distal to the joint line.
Menisci (medial & lateral): fibrocartilage; deepen the tibial concavity; assist rotation.
- Peripheral 1/3 = red zone (vascular, repairable); inner 2/3 = white zone (avascular, poor healing).
- Medial meniscus is less mobile (attached to deep MCL) and tears more often.
Cruciate ligaments: intracapsular but extra-articular; ACL (anteromedial + posterolateral bundles), PCL (anterolateral + posteromedial bundles).
MCL: superficial (attaches 5–7 cm distal to joint) + deep components.
Bursae: suprapatellar pouch (extension of joint cavity), prepatellar bursa, infrapatellar bursa.
Screw-home mechanism: tibia externally rotates ~10° at full extension (locks the joint).

Fig. Anatomy of the knee — Miller's Review of Orthopaedics, after Gray's Atlas

Articular capsule structure and cruciate/meniscal relationships

Fig. Structure of the articular capsule showing intracapsular vs extracapsular ligaments — THIEME Atlas

9. Summary Table

Feature	Fibrous	Cartilaginous	Synovial
Joint cavity	No	No	Yes
Uniting tissue	Fibrous CT	Hyaline/fibrocartilage	Synovial fluid
Mobility	None–slight	Slight	Free
Articular cartilage	No	Yes (hyaline)	Yes (hyaline)
Synovial membrane	No	No	Yes
Capsule	No	No	Yes

Sources:

Imaging Anatomy: Text and Atlas, Vol. 3 — Bones, Joints, Vessels and Nerves, Classification and Joints & Cartilage sections
THIEME Atlas of General Anatomy and Musculoskeletal System, Structure of the Articular Capsule
The Developing Human: Clinically Oriented Embryology, Synovial Joints
Miller's Review of Orthopaedics, 9th ed., Arthrology

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