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AN2.1-6 | General features of bones & Joints — Part 2

Sesamoid Bones (AN2.3)

A sesamoid bone develops within a tendon where it crosses a joint. Special features:

Sesamoid Bones (AN2.3)

Figure: Sesamoid Bones (AN2.3)

Illustration of sesamoid bones: patella increasing quadriceps lever arm at the knee, pisiform in FCU tendon at the wrist, sesamoids under the first metatarsal head, and the general mechanical function of sesamoid bones in altering tendon pull direction
  • Purpose: Alters the direction of pull of the tendon, reduces friction, and protects the tendon from wear.
  • The patella is the largest sesamoid bone — it increases the lever arm of the quadriceps by keeping the patellar tendon away from the axis of knee movement.
  • Other examples: pisiform (in the tendon of flexor carpi ulnaris), fabella (behind the lateral condyle of femur, inconsistent), sesamoids in flexor hallucis brevis tendons (under the first metatarsal head).
  • Sesamoid bones ossify late and may be confused with fracture fragments on X-rays. A key differentiator: sesamoid bones have smooth, rounded cortical margins; fracture fragments have irregular, sharp edges.
  • The pisiform is unique — it is both a sesamoid bone AND a carpal bone (part of the proximal row).

Types of Cartilage (AN2.4)

Three Types of Cartilage — Comparison

Feature Hyaline Cartilage Elastic Cartilage Fibrocartilage
Matrix Glassy, homogeneous, type II collagen Type II collagen + dense elastic fibres Thick type I collagen bundles
Perichondrium Present Present Absent
Flexibility Moderate High (elastic recoil) Low (resistant to compression)
Blood supply Avascular Avascular Avascular
Distribution Articular surfaces, trachea, nasal septum, costal cartilages Auricle, epiglottis, Eustachian tube Intervertebral discs, pubic symphysis, menisci, TMJ disc
Clinical relevance Poor healing; degeneration in osteoarthritis Cauliflower ear from perichondrial haematoma Disc prolapse; meniscal tears

Cartilage is a specialised connective tissue with a firm matrix. Three types:

Types of Cartilage (AN2.4)

Figure: Types of Cartilage (AN2.4)

Multi-panel comparison of three cartilage types: hyaline cartilage histology and distribution, elastic cartilage with elastic fibre network, fibrocartilage with collagen bundles, and summary comparison of key distinguishing features

1. Hyaline cartilage — the most common type.
• Structure: Chondrocytes in lacunae within a glassy, homogeneous matrix rich in type II collagen.
• Distribution: Articular surfaces of synovial joints, tracheal and bronchial rings, nasal septum, costal cartilages, epiphyseal plates.
• Clinical: Has no blood supply (avascular), no nerves, and no lymphatics — nourished by diffusion from synovial fluid or perichondrium. This is why cartilage injuries heal poorly.

2. Elastic cartilage — identical to hyaline but with abundant elastin fibres.
• Distribution: Pinna of ear, epiglottis, corniculate and cuneiform cartilages of larynx, walls of Eustachian tube.
• Key property: Springs back to shape after deformation.

3. Fibrocartilage — contains dense type I collagen fibres in addition to the cartilage matrix.
• Distribution: Intervertebral discs (annulus fibrosus), pubic symphysis, menisci of knee, labrum of hip and shoulder, TMJ disc.
• Key property: Resists compressive and tensile forces simultaneously.
• Has NO perichondrium (unlike the other two types).

Mnemonic for fibrocartilage locations: "IMPuLSe" — Intervertebral disc, Menisci, Pubic symphysis, Labrum, Symphysis menti.

SELF-CHECK

A patient has a torn meniscus of the knee. Which type of cartilage is primarily found in the meniscus?

A. Hyaline cartilage

B. Elastic cartilage

C. Fibrocartilage

D. Calcified cartilage

Reveal Answer

Answer: C. Fibrocartilage

The menisci of the knee are composed of fibrocartilage, which can resist both compressive and shearing forces. This is why meniscal tears are common in rotational injuries — the cartilage reaches its limit of tensile strength.

Classification of Joints (AN2.5)

Joints (articulations) are classified structurally into three major groups:

Classification of Joints (AN2.5)

Figure: Classification of Joints (AN2.5)

Multi-panel classification of joints: fibrous joints (suture, syndesmosis, gomphosis), cartilaginous joints (synchondrosis, symphysis), synovial joint structure, and six types of synovial joints (hinge, pivot, ball-and-socket, condyloid, saddle, plane) with movement axes

1. Fibrous joints — bones united by fibrous tissue, no joint cavity.
• Sutures (skull) — immovable (synarthrosis). Types: serrate (coronal), squamous (temporal), plane (internasal).
• Syndesmosis — bones connected by an interosseous membrane or ligament (inferior tibiofibular joint, interosseous membrane of forearm). Allows slight movement.
• Gomphosis — peg-in-socket (teeth in alveolar bone).

2. Cartilaginous joints — bones united by cartilage.
• Primary (synchondrosis) — united by hyaline cartilage, temporary (epiphyseal plates) or permanent (first sternocostal joint). Immovable.
• Secondary (symphysis) — united by fibrocartilage disc. Slightly movable (amphiarthrosis). Examples: pubic symphysis, intervertebral discs, manubriosternal joint.

3. Synovial joints — most common and most mobile. Features: joint cavity, articular cartilage, synovial membrane, capsule, synovial fluid.
Types by shape of articular surface:
• Plane (gliding) — flat surfaces, sliding movements (intercarpal joints)
• Hinge (ginglymus) — uniaxial, flexion/extension (elbow, interphalangeal)
• Pivot (trochoid) — uniaxial, rotation (superior radioulnar, atlantoaxial)
• Condyloid (ellipsoid) — biaxial, flex/ext + abd/add (wrist, MCP joints)
• Saddle (sellar) — biaxial, reciprocally concave-convex (1st carpometacarpal)
• Ball-and-socket (spheroidal) — multiaxial, all movements including rotation (hip, shoulder)