Connective Tissue Biomechanics

Question 1

Connective Tissue Introduction

Answer 1

• building block of bone, ligament, tendon, cartilage, joint capsules, IVD, nerves, etc
• tissues affected by: lifespan, injury, pathology, physical activity, hydration, sex
• understanding biologic and mechanical nature of these tissues provides insights necessary for prevention and management of injuries to these structures-better balance stress and recovery in these tissues

Question 2

Characteristics of Connective Tissue

Answer 2

• framework upon which other tissues build functional units
• much different than epithelial tissues where cells are tightly adherent to one another
• has much different function than organs
• consist of dispersed cells that typically lack intercellular contact
• most are vascularized but less well than muscles or nerve tissues; cartilage is not vascularized
• EC spaces in CTs are more abundant and contain blood vessels
• CT represents 15-20% of total body weight and contains 20-25% of body’s total water content

Question 3

Functions of Connective Tissues

Answer 3

• connect epithelium to rest of body (basal lamina)
• connect and protect (tendon and ligament)
• provide structure and protect (bone)
• store energy (fat)
• transport materials (blood)
• repair following injury (scar tissue)

Question 4

How Connective Tissue is Organized

Answer 4

• EC space is ECM: secondary accumulation of specialized cells
• these products include protein fibers and ground substances
• cellular: permanent components include fibroblasts, myofibroblasts, macrophages, mast cells, adipose cells; transient components include lymphocytes, plasma cells, basophils, neutrophils, eosinophils, and monocytes
• ECM: comprises functional characteristic of different fibers of connective tissue-fibers=collagen fibers, elastin fibers, reticular fibers; ground substances=mainly water, glycoaminoglycans (GAGs) and proteoglycans (PGs)

Question 5

Collagen

Answer 5

• most abundant CT
• greatest tensile strength (also withstands compression, torsion, etc)
• 20+ types
• type I and III most abundant

Question 6

Type I Collagen

Answer 6

• most common type
• accounts for ~90% of all collagen in body
• great tensile strength
• major type of collagen found in skin, tendon, bone, synovium and is also found in ligaments and muscle

Question 7

Elastin

Answer 7

• much greater elasticity than collagen
• allows tissues to stretch functionally: increases ability to withstand stress and undergo strain
• interwoven with collagen: prevents injury by increasing elasticity of tissues
• produced by fibroblasts and smooth muscle cells
• comprised of 2 parts: central elastin core and fibrillin microfibrils
• central elastin core: protein which forms fibrils which then intertwine to produce elastin fiber
• fibrillin microfibrils: surround central core, function to organize elastin fibrils into elastin fibers
• elastin fibers coil randomly in resting state, elongate in response to applied tensile force, recoil once load is released
• major CT in ligaments but found in many tissues throughout the body

Question 8

Reticular Fibers

Answer 8

• found primarily in loose CT
• typically located at boundary of CT and epithelium
• also found surrounding muscles, blood vessels, and nerves
• similar to collagen fibers: contain collagen fibrils but made up primarily of type III collagen; contain more sugar groups
• have the least tensile strength of collagen and elastin
• produced by fibroblasts

Question 9

Ground Substance

Answer 9

• occupies space between CT cells and fibers
• viscous, clear, gel-like substance
• GAG and PG are major components
• has high water content and little morphologic structure
• permits diffusion of oxygen, nutrients, cellular waste products; between the blood vessels and the cellular components of the CT
• gel-like consistency inhibits movement of large molecules and bacteria
• helps maintain interfiber distance within CT: critical for healthy CT
• hyaluronic acid is most abundant of the 7 GAGs found in our CTs
• GAGs have strong negative charge which attracts water –> bind to form gel-like consistency
• water is bound to GAG –> GAG bound to PG

Question 10

Classifying Connective Tissue

Answer 10

• broadly classified into 3 groups: supportive connective tissue, connective tissue proper, and fluid connective tissue
• supportive: offers strength to structures; high levels in bone, cartilage, etc
• proper: serves to connect and protect and is divided into loose (much in muscle sheaths, epithelial tissue, fascia) and dense regular (tendons and ligaments) or dense irregular (joint capsules, periosteum, aponeuroses)
• fluid: transportation medium

Question 11

Wolff’s Law

Answer 11

• mechanical stress is the basis for bone architecture
• remodeling occurs in response to regular mechanical stress or lack thereof
• bone deposited in areas of high stress and resorbed in areas from sites of little stress
• considerable research over 100+ years supports this observation
• notion regarding mechanical stress applies also to other CT in MS system: tendon, ligament, cartilage
• health of all CT influenced by complex interaction of hormonal, metabolic, and biomechanical factors
• use it or lose it
• key to successful rehab is balancing positive mechanical stress to each patient’s ability to recover metabolically
• forces PTs to continually eval and re-eval patients with the following questions
• are we prescribing adequate mechanical stress to maximize rebuilding/modeling of a damaged tissue?
• is the pt recovering from each dose of the mechanical stress?
• have we applied mechanical stress appropriate for the phase of tissue regeneration (acute, subacute/proliferation, chronic tissue regeneration/remodeling)?

Question 12

Connective Tissue and the Musculoskeletal System

Answer 12

• bone is specialized connective tissue
• unique secondary to mineralization of ECM
• type I collagen is primary structural component of bone
• type V also very present
• I and V make up 90% of organic structure in bone matrix

Question 13

Bones as Structures and Organs

Answer 13

• structures: provide rigid framework to withstand mechanical loads; serve as levers for locomotor function; protect internal organs
• organs: contain hematopoietic tissue; storage for Ca, P, Mg, Na; help maintain mineral homeostasis, help maintain blood
• both factors influenced by: age, site and shape of bone, dietary habits,presence of disease, sex

Question 14

Biochemistry of Bone

Answer 14

• organic substances: 30%
• inorganic substance: 60%
• water: 10%

Question 15

Organic Substances of Bone

Answer 15

• bone cells: osteoprogenitor, osteocytes, osteoblasts, osteoclasts; collagen fibers and non-collagenous proteins-95% of organic matrix
• matrix includes small quantities of reticular fibers and amorphous substances like hyaluronic acid and chondroitin sulfate

Question 16

Inorganic Substances in Bone

Answer 16

• calcium
• phosphorous
• magnesium
• sodium
• hydroxyl
• carbonate
• fluoride

Question 17

Enzymes

Answer 17

• many enzyme systems in bone
• necessary for metabolism within bone
• i.e. bone alkaline phosphatase plays role in production of organic matrix before calcification (produced by osteoblasts)

Question 18

Anatomical Structure

Answer 18

• also called osseous tissue
• strong and resilient
• flexible collagen fibers
• hardened via calcium and salt deposits

Question 19

Regions of Skeletal System

Answer 19

• axial: cranium and vertebral column including hyoid, sternum, ribs, sacrum, and coccyx
• appendicular: UE and LE including scapula, clavicle, and illium

Question 20

Bony Structural Classification

Answer 20

• long or tubular: humerus, femur
• short or cuboid: carpals, tarsals
• flat: cranium, scapula
• irregular: vertebrae, sphenoid, ethmoid
• each bone consists of cortical bone and porous cancellous or trabecular bone

Question 21

Types of Bone

Answer 21

• compact: dense/cortical; forms outer shell of all bones
• cancellous: spongy/trabecular; surrounded by compact bone; found throughout inside of bone; composed of network of trabeculae; larger spaces in this network store primarily water, fat, and marrow

Question 22

General Structure of Long Bones

Answer 22

• diaphysis
• metaphysis
• epiphysis
• epiphyseal plate
• periosteum
• medullary cavity

Question 23

Periosteum

Answer 23

• thin fibrous layer covering outer surface of diaphysis
• metaphysis, and epiphysis
• rich blood and nerve supply
• not present in areas covered by articular cartilage

Question 24

Medullary Cavity

Answer 24

• runs throughout diaphysis
• ends at epiphyseal plates
• lined with compact bone
• filled with bone marrow

Question 25

Marrow

Answer 25

• red marrow consists of RBCs in various stages of development
• reticular cells and fibers also present, providing supporting network
• present in proximal and distal diaphysis of long bones as well as in marrow cavity
• amount of red marrow decreases as one matures

Question 26

Micro Structure of Bone

Answer 26

• mature bone made up of haversian systems: aka osteons-number of circular districts run entire length of diaphysis in long bones parallel to bones long axis
• each haversian system has central hole surrounded by rings
• haversian canal is center of ring
• lamellae are the concentric rings
• lacunae are small cavities in gaps between each lamelle that house osteocytes
• canaliculi are additional canals through lamellae which cross lamellae at right angles and join lamellae within the same haversian system
• volkman’s canals are large canals that pass through the bone aka perforating canals typically at 90% to haversian canals; pathway for blood and nerve supply which enters bone and can be differentiated from canaliculi as they pass through numerous haversian systems

Question 27

Osteoprogenitor Cells

Answer 27

• located in deeper layer of periosteum
• also line cavities of endosteum, Volkman’s canals, and haversian canals
• in immature bone divide and differentiate into osteoblasts
• in mature bone called resting cells; lay dormant until stimulated to differentiate into osteoblasts
• similar to stem cells

Question 28

Osteoblasts

Answer 28

• bone forming cells essential for osteogenesis
• create and secrete matrix necessary for ossification which then lays foundation for calcification
• respond to mechanical stimulation–>bone growth and remodeling

Question 29

Osteocytes

Answer 29

• morphs from osteoblast after local formation of bone matrix, collagen, and ground substance
• a mature bone cell located in lacunae
• functions to maintain bone matrix

Question 30

Osteoclasts

Answer 30

• large multinucleated
• capable of resorbing or removing bone
• remove organic matrix and calcium simultaneously
• responsible for deossification of bone

Question 31

Blood Supply to Long Bones

Answer 31

• three distinct system exist
• afferent: nutrient and metaphyseal arteries supply inner 2/3 of cortex; periosteal arteries supply outer 1/3
• efferent vascular system: transports venous blood
• intermediate vascular system: capillary beds within cortex

Question 32

Biomechanical Properties of Bones

Answer 32

• cortical bone stiffer than cancellous: 1.5-2% strain to failure for cortical and 50% for cancellous; cancellous has greater capacity for energy storage (elastic)
• tensile and compressive strength proportional to cross-sectional area
• stress fractures arise from secondary repeated minor trauma-related to tissue fatigue

Question 33

Remodeling of Bone

Answer 33

• accomplished by simultaneous osteoblastic and osteoclastic activity
• must maintain bone balance: typically positive bone balance for children and often negative for elderly
• bone deposition in areas of stress and reabsorption from sites with little stress
• disuse results in bone loss known as osteopenia
• bone atrophy may occur with bed rest, prolonged inactivity, space travel, sedentary lifestyle
• genetically determined baseline mass for normal function: certain level of physical activity bone health
• decreased physical activity=bone loss
• weight bearing PA=bone growth
• too much intense PA problematic and optimal levels exist for each individual

Question 34

Activity, Aging, and Bone Remodeling

Answer 34

• muscles help to dampen high tensile and compressive forces: eccentric mm action very important
• immobility leads to ~1% bone mass decrease per week
• for typical adult 35+ stress/strain tolerance, fracture toughness, and bending strength each decrease 2-3% every 10 years

Question 35

Bone Remodeling and Exercise

Answer 35

• minimal essential strain: threshold stimulus for formation of new bone
• must be weight bearing to stimulate increases in bone density
• resistance exercise: variable results in studies when looking at bone mineral density; none focus on rate and magnitude of loading
• aerobic: high intensity repetitive activities like rowing, stair climbing, running have been moderately successful in demonstrating bone mineral density increases

Question 36

Women and Bone Remodeling

Answer 36

• osteoporosis major societal health concern
• linked to menopause, age, and activity
• dietary habits as young adults play role
• female athletic triad: disordered eating, amenorrhea, and osteoporosis
• female athlete can have one, two, or all three parts of triad

Question 37

Designing Interventions to Stimulate Bone Growth

Answer 37

• specificity of loading: perform exercise that stress part of skeleton where increases in bone density are desired; axial loading
• exercise selection: force vectors through spine and hip, multiple muscles, multi-joint, use greater absolute loads
• progressive overload: gradually increase training loads
• cross training: vary selection of exercises to vary distribution of forces