Components of bone
Bone
Bone is a dynamic, living tissue composed of:
Types of bone tissue
* Woven bone
o Immature, irregular
o Found in fetus and early repair
* Lamellar bone
o Mature, organized
o Includes:
Circumferential
Concentric (osteons)
Interstitial
* Compact bone
o Dense, cortical
o Contains osteons (Haversian systems)
* Cancellous (trabecular) bone
o Spongy bone
o No osteons (exception in horses)
Bone matrix
* Inorganic:
o Calcium hydroxyapatite → hardness & compressive strength
* Organic:
o Collagen fibers → tensile strength
o Non-collagenous proteins → necessary for calcification
Bone cells
* Osteoblasts – bone formation
* Osteocytes – maintain bone
* Osteoclasts – bone resorption
Components of articular cartilage
Articular cartilage coats bone ends in joints and allows frictionless motion
Key properties
* Avascular
* Aneural
* Alymphatic
* Smooth, resilient, wear-resistant
Cellular component
* Chondrocytes
o Metabolically active
o Maintain extracellular matrix
o Produce collagen and proteoglycans
Extracellular matrix components
* Type II collagen (primary structural framework)
* Proteoglycans:
o Aggrecan (aggregating proteoglycan)
o Biglycan, decorin
* Glycoproteins
* Water and electrolytes
components of associated soft tissues
The joint is considered an organ, not just cartilage and bone
Components include:
* Subchondral bone
* Articular cartilage
* Ligaments
* Tendons
* Fibrous joint capsule
* Synovial membrane
o Lines joint
o Produces synovial fluid
o Secretes hyaluronan
o Becomes inflamed with cyclic trauma
Components of Synovial Fluid and Their Functions
Synovial fluid is produced by the synovial membrane
Major components
* Hyaluronan (HA)
* Water
* Electrolytes
* Lubricin
* Proteins
Functions
* Lubrication of articular surfaces
* Shock absorption
* Nutrient delivery to cartilage
* Removal of metabolic waste
* Maintenance of cartilage health
Enzymes That Contribute to Cartilage Degradation
During synovitis and osteoarthritis, the following mediators contribute to cartilage breakdown
Key degradative enzymes and mediators
* Matrix metalloproteinases (MMPs)
* Aggrecanases
* Prostaglandin E₂ (PGE₂)
* Free radicals
* Cytokines:
o Interleukin-1 (IL-1)
o Tumor necrosis factor-α (TNF-α)
Primary therapeutic target
* Synovitis, as it drives enzyme release and cartilage degeneration
Diagnostic Modalities Used to Characterize Equine Joint Disease
Clinical Examination
* Lameness (AAEP grading)
* Synovial effusion
* Pain on palpation or flexion
* Crepitation
* Reduced range of motion
Imaging Modalities
Radiography
* Gold standard for:
o Fractures
o Advanced osteoarthritis
* Limitations:
o Poor sensitivity for early cartilage or subchondral bone disease
Computed Tomography (CT)
* Detects early subchondral sclerosis and lysis
* Provides 3D visualization
* CT osteoabsorptiometry measures bone density distribution
Magnetic Resonance Imaging (MRI)
* Evaluates:
o Soft tissues
o Cartilage
o Subchondral bone
* Increasing clinical use
Ultrasonography
* Joint capsule
* Synovial effusion
* Periarticular soft tissues
Nuclear scintigraphy
* Detects increased bone turnover
* Useful for occult lesions
Synovial Fluid Analysis
* Conventional analysis
* Biomarkers (research and emerging clinical relevance)
Arthroscopy
* Gold standard for:
o Direct cartilage evaluation
o Correlation with clinical signs
Medications Used to Treat Joint Disease and Their Effects
Non-steroidal Anti-inflammatory Drugs (NSAIDs)
* Inhibit cyclooxygenase (COX)
o COX-1: constitutive (“good”)
o COX-2: inducible (“bad”)
Examples:
* Phenylbutazone
* Flunixin meglumine
* Firocoxib (Equioxx®) – COX-2 selective
Effects
* Reduce pain
* Decrease inflammation
* Target PGE₂ production
Intra-articular Corticosteroids
* Reduce synovitis
* Decrease inflammatory mediators
Examples:
* Betamethasone esters – no deleterious cartilage effects
* Triamcinolone acetonide – chondroprotective
* Methylprednisolone acetate – associated with cartilage degeneration
Hyaluronan (HA)
* Improves synovial fluid viscosity
* Anti-inflammatory effects
* Enhances lubrication
Polysulfated Glycosaminoglycans (PSGAGs)
* Modulate cartilage metabolism
* Reduce enzyme-mediated degradation
Biologic Therapies
* Platelet-rich plasma (PRP)
* Bone marrow supernatant
* Stem cells
* Anti-cytokine therapies
Effects of Exercise and Disuse on Equine Bones and Joints
Bone Adaptation Principles
* Goal: maintain an adequate strain environment
* Two processes:
o Bone modeling
Shape change
Resorption and formation uncoupled
o Bone remodeling
Replacement of bone
Resorption and formation coupled
Effects of Exercise
* Increases bone density
* Improves subchondral bone strength
* Promotes cartilage nutrition
* Normal repetitive loading:
o Rapid resorption
o Delayed formation
o Strong integration with parent bone
Effects of Disuse
* Decreased bone density
* Weakened subchondral bone
* Increased risk of injury upon return to work
* Detrimental to cartilage health
