Bone and Joint Biology

• Bone Functions: Support, mineral homeostasis, marrow housing

• Bone Types: Long (endochondral), flat (intramembranous)

• Macrostructure:
  Cortical → dense, load-bearing
  Trabecular → porous, marrow-rich, weak in osteoporosis

• Cells:
  Osteoblasts (matrix formation, Runx2, osterix)
  Osteocytes (mechanosensors, RANKL secretion)
  Osteoclasts (resorption, RANKL/M-CSF, inhibited by OPG)

• Matrix: Mineral (HA, TCP) + collagen (type I) + growth factors (BMP, TGF-β, IGF)

• Bone Homeostasis: Balance of formation/resorption → disrupted in osteoporosis, osteopetrosis

• Fracture Healing:
  Primary → direct, stable fixation
  Secondary → hematoma, callus, endochondral ossification, remodelling

• Therapies: Bisphosphonates, PTH (intermittent), anti-RANKL, calcitonin

• Synovial Joint: Cavity, capsule, cartilage, synovium (Type A & B cells), synovial fluid (HA, lubricin); proprioception (A fibers), pain (C fibers).

• Non-Synovial Joints:
  Symphysis (fibrocartilage, e.g. pubic symphysis)
  Synchondrosis (cartilage-only, e.g. costal, cranial base)
  Syndesmosis (fibrous, e.g. distal tibiofibular)

  Bone

Functions: Provides mechanical support, regulates mineral homeostasis, harbors bone marrow elements.

Types:

• Long      bones: Formed via endochondral ossification from a cartilage model.

• Flat      bones: Formed via intramembranous ossification directly from      mesenchymal tissue.

Anatomy:

• Diaphysis:     Cortical bone tube enclosing the medullary canal with trabecular bone;      surfaces consist of periosteum and endosteum.

• Metaphysis:     Transition zone between epiphysis and diaphysis; composed of loose      trabecular bone.

• Epiphysis:     Articular end containing subchondral bone and the growth plate.

Vascular and neural supply: Neurovascular bundles enter through the periosteum and run within Haversian and Volkmann canals. Inner two-thirds of cortical bone are supplied by the nutrient artery, while the outer one-third is nourished by periosteal vessels.

Macrostructure:

• Cortical      bone: Dense, load-bearing; serves as boundary in metaphysis/epiphysis.

• Trabecular      bone: Porous, marrow-containing; architecture compromised in      osteoporosis.

Microstructure:

• Woven      bone: Primary bone, irregular collagen alignment.

• Lamellar      bone: Secondary bone, organized structure.

• Lacunar–canalicular      system: Provides osteocyte interconnections.

Extracellular matrix:

• Mineral      (60–70%): Hydroxyapatite, tricalcium phosphate; provides compressive      strength and mineral reservoir.

• Organic      (20–25%): 90% type I collagen, other collagens, adhesive proteins      (fibronectin, vitronectin), matrix proteins, proteoglycans, growth factors      (BMP, TGF-β, IGF).

Cells:

• Osteoblasts:     Synthesize bone matrix, regulate osteoclasts; differentiation via Runx2      and osterix.

• Osteocytes:     Mechanosensors, secrete RANKL, maintain bone homeostasis.

• Osteoclasts:     Multinucleated, perform bone resorption; activated by RANKL and M-CSF,      inhibited by OPG.

Bone homeostasis: Maintained by the balance of osteoblast and osteoclast activity. Renewal occurs via Howship’s lacunae in trabecular bone and osteons in cortical bone.

Disease and treatment:

• Remodeling      impaired in conditions like osteoporosis and osteopetrosis.

• Therapies:     Bisphosphonates, intermittent PTH, anti-RANKL agents, calcitonin.

Fracture healing:

• Primary      healing: Direct bone formation under stable fixation.

• Secondary      healing: Hematoma, inflammation, cartilage callus, endochondral      ossification, and remodeling.

• Growth      factors (BMP, TGF-β, IGF) and angiogenesis play critical roles.

2. Synovial Joint

Structure: Joint cavity, articular cartilage, capsule, ligaments, tendons.

Development: Arises from mesenchymal condensation; apoptosis within interzone forms the cavity.

Components:

• Articular      cartilage: Provides low-friction motion.

• Ligaments:     Provide stability.

• Capsule:     Encloses the joint.

• Synovium:     Contains type A cells (macrophage-like) and type B cells (fibroblast-like,      producing hyaluronan); provides nutrition and synovial fluid.

• Synovial      fluid: Plasma ultrafiltrate rich in hyaluronic acid and lubricin.

Innervation:

• Type      A fibers (proprioception).

• Type      C fibers (pain).

Function: Enables wide range of motion between bones with minimal friction.

3. Non-Synovial Joints

Types:

• Symphysis:     Fibrocartilaginous disc between bones (e.g., intervertebral disc, pubic      symphysis); stability, load transfer, limited mobility.

• Synchondrosis:     Cartilage-covered joint surfaces without synovium; limited motion (e.g.,      sternomanubrial, costal cartilage, cranial base).

• Syndesmosis:     Fibrous connection without cartilage interface; limited movement (e.g.,      distal tibiofibular joint).

References

1. Bolamperti S, Villa I, Rubinacci A. Bone remodeling: an operational process ensuring survival and bone mechanical competence. Bone Res. 2022 Jul 18;10(1):48. doi: 10.1038/s41413-022-00219-8. PMID: 35851054; PMCID: PMC9293977.

2. Abdulrahman SJ, Abdulhadi MA, Turki Jalil A, Falah D, Merza MS, Almulla AF, Ali A, Ali RT. Conjugated linoleic acid and glucosamine supplements may prevent bone loss in aging by regulating the RANKL/RANK/OPG pathway. Mol Biol Rep. 2023 Dec;50(12):10579-10588. doi: 10.1007/s11033-023-08839-x. Epub 2023 Nov 6. PMID: 37932498.