Periarticular Connective Tissue - 2024 Student PDF

Periarticular Connective Tissue Andrew Sprague, PT, PhD, DPT Periarticular Connective Tissue  Periarticular Connective Tissue Components of Connective Tissue Fibrous Proteins Ground Substance Cells Types of Periarticular Connective Tissue Dense Connective Tissue...

Periarticular Connective Tissue Andrew Sprague, PT, PhD, DPT Periarticular Connective Tissue  Periarticular Connective Tissue Components of Connective Tissue Fibrous Proteins Ground Substance Cells Types of Periarticular Connective Tissue Dense Connective Tissue Structure; Function; Joint Capsule, Ligaments, Tendons Response to Aging, Injury, Articular Cartilage Loading and Unloading Fibrocartilage Bone Periarticular Connective Tissue Capsule, ligament, tendon, articular cartilage, and fibrocartilage Basic Components: 1. Fibrous proteins: Collagen and elastin 2. Ground substance: Glycosaminoglycans, water, and solutes 3. Cells: Fibroblasts, tenocytes, and chondrocytes The relative amount and types of these components dictate structure and function  Fibrous Proteins Collagen: Most common protein in the human body Collagen molecules  tropocollagen (threads)  fibrils  fibers Type I: Thick, strong fibers that resist tension well Ligaments, tendons, fascia, and joint capsule Type II: Thinner fibers that provide scaffold for other types of tissue, better resistance to compressive loads Hyaline cartilage Type III: Present in the same tissues as Type I but thinner and weaker. Associated with scar tissue and injury. Fibrous Proteins Elastin: Elastic (stretchy) proteins Resists tension but less well than collagen Helps maintain the structure of tissues - return to original shape after being loaded  Ground Substance Water-saturated matrix or gel that surrounds the fibrous and cellular components of connective tissue Glycosaminoglycans: Hydrophilic (water-loving) chains of polysaccharides (sugars) Provides hydration by bringing in water Helps provide resistance to compression Solutes: Ions and other materials that enter through diffusion to support the cells within the ground substance What are the consequences of reduced glycosaminoglycans? Cells Responsible for synthesizing the ground substance and fibrous proteins of the respective tissues Fibroblasts: Primary cells in ligaments, tendons and most other connective tissues Chondrocytes: Primary cells of articular cartilage and fibrocartilage Other cells break down and remove aging/injured components of the tissues Relatively few cells in periarticular connective tissue compared to other types of connective tissue Types of Periarticular Connective Tissue 1. Dense Connective Tissue Joint capsule, ligaments, and tendons 2. Articular Cartilage 3. Fibrocartilage  Dense Connective Tissue Few cells, low to moderate amounts of proteoglycans and elastin, high amounts of Type I collagen, and limited blood supply Irregular: Haphazard orientation of collagen fibers Adapted to resist multi-directional forces Regular: Orderly orientation of collagen fibers Adapted to resist forces in a single or relatively few directions What type of dense connective tissue is joint capsule? Ligament? Tendon? Primary Types of Dense Connective Tissue 1. Tendon 2. Joint Capsule 3. Ligament We will use tendon as a model for discussing the behavior of dense connective tissue  Tendon Composition Muscle  Bone Highly organized, parallel collagen fiber bundles 70-80% Type I Collagen Embedded in extracellular matrix Surrounded by epitenon/paratenon Reduces friction Tenocytes (tendon cells) sit between collagen fibers Aslan et al. 2008. Wang et al. 2012. Function of Tendon Transmit forces from muscle to bone Energy conservation (spring) Protect muscle from injury Adapted from Primal Pictures. 3D Atlas. Available at: Anatomy.tv Roberts & Konow 2013; Monte et al. 2020  Tendon Biomechanics Tension Compression Very well adapted to resist tension Not well suited for compression Viscoelastic structure Response to loading (function) is dependent on rate and duration of loading Adapted from: Netter. Atlas of Human Anatomy. 5 th ed. Wang et al. 2012; Docking et al. 2013; Khan & Scott, 2009 Tendon Mechanobiology Cells sense load  Trigger a biological response Tendons are metabolically active Respond to changes in their loading environment Tenocytes experience shear and compression forces during loading  remodeling Khan & Scott, 2009  Tendon Response to Load Underloading: Excessive Overload: Load Rest/Immobilization Overload + Inadequate Short term (1-2 weeks) Recovery unloading reduces tendon Leads to injury stiffness and collagen production Khan & Scott, 2009; Magnusson & Kjaer 2019  Joint Capsule Envelope or sac that surrounds and encloses a joint Fibrous (outer) layer – Irregular connective tissue Synovial (inner) layer Mechanosensitive – Adapts to loading demands Primarily Type I collagen but composition varies by joint  Function of Joint Capsule Enclose the joint to help maintain the articular environment Provides joint stability May be thickened in specific areas to form capsular ligaments Proprioceptive feedback Produces synovial fluid to lubricate and provide nutrition to joint Iliofemoral ligament (Y-ligament) is a capsular ligament Ligament Bone  Bone Similar hierarchical structure to tendon Densely arranged collagen fibers but not always in parallel Less collagen but more elastin and proteoglycans than tendon More metabolically active than tendon Tendon Ligament  Types of Ligaments Capsular Ligaments Thickenings of the joint capsule Oriented to resist multi-directional forces Intra- or Extra-capsular Ligaments More rope-like Capsular ligaments of hip Typically oriented to resist 1-2 forces Lateral collateral ligament (LCL) ACL and PCL outside joint inside of joint capsule (capsule resected) Function of Ligaments Resists excessive joint motion Guide joint motion Provide proprioceptive feedback  Dense Connective Tissue Injury Overuse: Repetitive microtrauma with inadequate recovery leads to damage accumulation (degeneration) Acute: Application of stress that exceeds the failure point of the structure May be preceded by degeneration that lowers failure point’ Can be complete or partial rupture Overuse Dense Connective Tissue Injury Load Tolerance Load Tolerance Injury Time/Recovery Time/Recovery = Net collagen degradation (breaking down) = Net collagen synthesis (building up) Magnusson & Kjaer 2019; Miller et al. 2005 Rate of Force Application Forces can be applied to ligaments and tendons at various speeds Not 100% predictive of injury High rate of loading ≈ Rupture Lower rate of loading ≈ Avulsion  Soft Tissue Healing Seconds to Hours Hours to Days Days to Weeks Weeks to >1 Year 1) Hemostasis (Bleeding) Phase Vessels constrict to slow bleeding Platelets stick together to form an immature clot Fibrin binds to clot to reinforce it and hold it in place 2) Inflammatory Phase Pro-inflammatory chemicals at wound site trigger inflammatory response Neutrophils and macrophages begin to remove: Bacteria and foreign materials Damaged cells 3) Proliferative (Repair) Phase Fibroblasts proliferate and lay down extracellular matrix and disorganized collagen fibers (Type III) Granulation tissue Angiogenesis: New blood vessels form 4) Remodeling Phase Weak, disorganized Type III collagen (scar tissue) replaced by stronger Type I collagen Collagen fibers begin to align to stresses placed on tissue If not mobilized, scar tissue can contract and limit joint motion  Importance of Mechanical Stress During Healing Moderate stress induces organization of collagen in a more parallel arrangement in the direction of applied forces Minimizes contraction of scar During early phases of healing, movement of joint appears to be Disorganized scar tissue  Organized Type I sufficient stress Smaller diameter Type III  Larger diameter Type I Effects of Temperature Viscoelastic properties change with heat to allow greater stress relaxation and creep Stress relaxation increases when tissues are heated >98.6° Less time is needed to reach a given strain in response to stretching when heated  Response to Prolonged Immobilization Arthrofibrosis: Excessive collagen production (scar tissue) and adhesion formation resulting in restricted joint Reduced tensile strength motion and pain. 50% loss after 8 weeks 12 to >18 months to restore Adaptive shortening of structures Especially joint capsule Changes occur even in uninjured structures Usher et al., Bone Res. 2019  Effects of Age and Aging Adolescent: Dense connective tissue stronger than bone More likely to experience avulsion fracture than rupture Old Age: Potentially degenerative changes Alterations to fibroblast metabolism/remodeling ability Reduction in stiffness, strength, and collagen content Appears to be more related to decreased activity and inactivity than aging itself Types of Periarticular Connective Tissue 1. Dense Connective Tissue Joint capsule, ligaments, and tendons 2. Articular Cartilage 3. Fibrocartilage  Cartilage Strong but elastic connective tissue Composed of chondrocytes, collagen, and matrix 70-85% water by weight Avascular and aneural Usually surrounded by perichondrium Contains blood vessels and nerves Provides nutrition and maintains tissue  Chondrocytes Mature cartilage cells May be in groups called Lacunae Produce and maintain the cartilaginous matrix Sparsely distributed Types of Cartilage 1. Articular (hyaline) cartilage Bluish white appearance Reduces friction and absorbs shock at joints Weakest of the three types of cartilage 2. Fibrocartilage Intervertebral discs, pubic symphysis, menisci Strongest of the 3 types of cartilage 3. Elastic cartilage External ear, arteries, lung tissue  Articular Cartilage Specialized type of hyaline cartilage on the load-bearing surfaces of joints Lacks a perichondrium Implications? Chondrocytes surrounded by Type II collagen Articular Cartilage Roles: Distribute and disperse compressive forces Reduce friction between joint surfaces 5-20x more slippery than ice on ice  Articular Cartilage Superficial tangential zone (STZ): Collagen fibers parallel to articular surface Highest collagen and lowest proteoglycan content Flat chondrocytes Middle Zone: Oblique collagen fibers Thickest layer Round chondrocytes  Articular Cartilage Deep Zone: Perpendicular collagen fibers Highest concentration of proteoglycans Calcified zone: Anchors cartilage to bone Tidemark Separates calcified zone from subchondral bone Diffusion barrier between cartilage and bone  Articular Cartilage Nutrition Nutrition provided by diffusion Deformation of articular surface during intermittent joint loading “Washes” synovial fluid in and out of cartilage Also provides lubrication Response to Reduced Loading Reduced nutrition: may lead to degenerative changes Reduced lubrication: Increases friction between joint surfaces  degenerative changes Response to Immobilization/Unloading Consequences of reduced loading + Decreased cartilage thickness Reduced chondrocyte density Reduced collagen content Softer and weaker  Response to Excessive Loading (Injury) Damage to collagen fiber network Fibrillation: Cartilage becomes less smooth Proteoglycan loss  decreased fluid content Decreased stiffness Loses ability to respond to compressive and shear forces Response to Impact Loading Occurs when loads are applied at a fast rate Cartilage becomes stiffer Unable to deform and redistribute loads fast enough Effect of Aging on Cartilage Imbalance between breakdown and repair ↓ Thickness Calcification of the cartilage tissue Increased cross-linking ↑ Stiffness ↑ Susceptibility to fatigue failure Reduced chondrocyte density Types of Periarticular Connective Tissue 1. Dense Connective Tissue Joint capsule, ligaments, and tendons 2. Articular Cartilage 3. Fibrocartilage  Fibrocartilage Mixture of dense connective tissue and articular cartilage Strength and shock absorption of articular cartilage + tensile strength of ligament and tendon Dense and multidirectional collagen network Type I collagen and moderate amounts of proteoglycans Resists multidirectional tensile, shear, and compressive forces Also in hip and shoulder labrum, TFCC (wrist) Fibrocartilage Function 1. Help supports and stabilize joints 2. Guides movement (arthrokinematics) 3. Dissipates forces Fibrocartilage Like articular cartilage, lacks a perichondrium Nutrition provided by diffusion of synovial fluid Intermittent loading Similar response to loading, unloading, and aging as articular cartilage  Composition of Bone Fibrous proteins: Primarily Type I Collagen Cells Osteoblasts – bone-building cells Osteocytes – mature bone cells Osteoclasts – resorb bones Ground substance Mostly calcium phosphate – provides rigidity Mineral salts crystallize in the framework created by the collagen – calcification Water, proteoglycans, and other materials Function of Bone Support Protection Assist in movement Mineral and fat storage and release Blood cell production Structure of Bone Comprised of two layers: 1) Compact (Cortical) Bone: Dense outer layer 2) Cancellous (Trabecular) Bone: Spongy, inner core  Compact (Cortical) Bone Exterior of bones Thick around the middle of the bone Thinner at ends of long bones Provides protection and support Resists stresses from weight and movement Osteon: Structural unit of compact bone Collagen and mineralized ground substance organized in concentric rings (lamellae) Arranged in the direction of the stresses they encounter Ex: parallel to the long axis of the bone in the shaft Surrounded by periosteum Supplies blood vessels  Cancellous (Trabecular) Bone More metabolically active than compact bone More flexible than compact bone Transmits and distributes forces along the long axis of the bone Wolff’s Law Bone constantly alters shape, strength, and density in response to external forces “Bone is laid down in areas of high stress and reabsorbed in areas of low stress” Immobilization or Weightbearing Inactivity Activity Pediatric Adult Mechanical Properties of Bone Subjected to several types of loading Greatest strength (failure stress) in compressive loading Different bones (or portions of a bone) are better suited for handling different types of stresses E.g. Weight-bearing long bones vs. flat bones  Fractures Named according to location, severity, and/or shape Open Fracture: Bones penetrate the skin Closed Fracture: Skin remains intact Displaced Fracture: Bones out of position relative to original orientation Stress Injury: Repetitive overload with inadequate recovery and/or nutrition May progress to a stress fracture Fracture Healing 1. Formation of Hematoma (Days 1-5) Fracture tears blood vessels, hematoma forms and clots Hematoma provides a temporary framework for the repair Neutrophils, macrophages, and osteoclasts migrate to hematoma Swelling and inflammation occur Cells remove the dead/damaged cells and tissue  Fracture Healing 2. Fibrocartilaginous Callus Formation (Days 5-11) Angiogenesis (new blood vessel) formation Fibrin-rich granulation tissue forms (strengthens clot) Mesenchymal stem cells differentiate into: Fibroblasts Chondroblasts Osteoblasts Fibrocartilaginous tissue bridges fracture gap Bone begins to form under the periosteal surface  Fracture Healing 3. Bony Callus Formation (Days 11-28) Ossification of callus Continued laying down of bone under periosteal surface Further angiogenesis Brings cells deeper into the fracture site  Fracture Healing 4. Bone Remodeling (Day 18+) Repeated remodeling Absorption by osteoclasts New bone formation by osteoblasts Lasts months to years Restoration of normal architecture Influenced by loading environment Response to Loading Responds favorably to compression loads Will grow stronger and in the direction of compression loads Increased deposition of mineral salts and production of collagen fibers Application of weight-bearing in the remodeling stage of fracture healing is critical Reduced compression may occur from lack of weight bearing or muscle force on bone Less deposition of bone, with same resorption rate results in reduced bone mass Bone will become mechanically weaker  Response to Unloading Occurs after fractures, comas and other diseases/conditions Reduced compression and tensile strain from lack of weight bearing and muscle force Less deposition of bone, with the same resorption rate results in reduced bone mass Bone will become mechanically weaker Greater impact on weight-bearing bones Bloomfield S.A., Med Sci Spots Exerc. 1997. Effects of Aging Resorption > Deposition Reduced Bone Mass Bone becomes stiffer (brittle) Mechanically weaker Changes with age may be delayed with activities that encourage muscle force and weight bearing loads  Osteopenia and Osteoporosis Resorption >>> Deposition Depletion of calcium Decreased estrogen and testosterone Osteopenia: Reduced bone mineral density Osteoporosis: Bone mineral density scores >2.5 standard deviations below normal Questions???