Advanced Biomechanics of Physical Activity (KIN 831) Lecture 2 Biomechanics of Tendons and Ligaments * Material included in this presentation is derived primarily from two sources: Enoka, R. M. (1994). Neuromechanical basis of kinesiology. (2nd ed.). Champaign, Il: Human Kinetics. Nordin, M. & Frankel, V. H. (2001). Basic Biomechanics of the Musculoskeletal System. (3rd ed.). Philadelphia: Lippincott Williams & Wilkins.

What do you know about the macroscopic structure and function of tendons and ligaments?

What do you know about the microscopic structure and function of tendons and ligaments?

Functions of Ligaments and Joint Capsules connect bone to bone act as static restraint to: help with joint stability guide joint motion prevent excessive motion

Functions of Tendons connect muscle to bone transmit tensile loads from muscle to bone to: produce joint torque stabilize joint during isometric contractions and in opposition to other torques cause joint motion during isotonic contractions act as a dynamic joint restraint interact with ligaments and joint capsule to mitigate loads that they receive ----------------------------------------------------- Interesting points: tendon extends the reach of muscle tendon may conserve muscle tissue mass (i.e., muscle tissue not required to extend from origin to insertion)

Tendons and Ligaments Dense connective tissues (parallel-fibered collagenous tissues) Sparsely vascularized Composed primarily of collagen (fibrous protein which gives tendons and ligaments strength and flexibility) Consist of relatively few cells or fibroblasts (≈ 20% of total tissue volume) Contain abundant extracellular matrix ≈80% of total tissue volume ≈70% of extracellular matrix is water and ≈30% solids (collagen (≈75% of extracellular matrix), ground substance, and small amount of elastin) Structure and chemical composition identical to other animal species (extrapolate behavior from animals)

Tendons and Ligaments Tendons Ligaments Join muscle to bone Organization of collagen fibers to accommodate specialized function Fibers longitudinal and parallel Transmit tensile muscle forces Ligaments Join bone to bone Organization of collagen fibers to accommodate specialized function Fibers generally longitudinal and parallel, some oblique and spiral Primarily transmit forces in functional direction, but also multidirectional

How can you make string able to support a large load?

How do manufacturers of string make it able to support a large load?

Collagen Molecule Synthesized by within fibroblast as procollagen (precursor to collagen) Consists of 3 polypeptide chains ( chains) each coiled in left hand helix 3  chains combined in a right handed triple helix Bonding (cross-linking) between  chains enhances strength of collagen molecules Develops extracellularly into collagen molecules

Collagen Groups of 5 collagen molecules form microfibrils Cross links formed between collagen molecules that aggregate at the fibril level Cross links between collagen molecules give strength to tissues (e.g., tendons and ligaments) they compose Fibrils aggregate further to form collagen fibers Fibers aggregate to form bundles

Collagen Fiber Arrangement in Tendons and Ligaments

Macroscopic and Microscopic Structure of Tendon and Ligaments

Macroscopic and Microscopic Structure of Tendon and Ligaments

Macroscopic and Microscopic Structure of Tendon and Ligaments Epitendidium -outer covering Fascicle - bundle of fibrils Fibril - basic load bearing unit of tendon and ligaments Microfibril - 5 rows of triple helixes in parallel (see figure)

Schematic illustration depicting the hierarchical structure of collagen in ligament midsubstance

Macroscopic and Microscopic Structure of Tendon

Schematic representation of the microarchitecture of a tendon

Structural hierarchy of a tendon Structural hierarchy of a tendon. Connective tissue layers or sheaths envelop the collagen fascicles (endotenon), bundles of fascicles (epitenon), and the entire tendon (paratenon)

Macroscopic and Microscopic Structure of Tendon and Ligaments Collagen molecule - triple helix in series; 5 rows stacked side-by side (parallel) Triple helix - cross links occur both between and within rows of triple helixes  strength (# and state of cross links influence strength)  determined by age, gender, and activity level

Elastin tendons and ligaments contain protein elastin influences elastic properties of tendons and ligaments (↑ elastin  ↑ elasticity) proportion varies by function little in tendons and extremity ligaments much present in ligamentum flavum between laminae of vertabrae protect spinal nerve roots pre-stress the motion segment provide intrinsic stability to spine

Ground Substance in Tendons and Ligaments amorphous material in which structural elements occur in connective tissues, composed of proteoglycans, plasma constituents, metabolites, water, and ions between cells and fibers Ground Substance in Tendons and Ligaments Proteoglycans act as cement-like substance between collagen microfibrils contributing to overall strength of tendons and ligaments

Water and Proteoglycans Forms a gel Viscosity decreases with activity Thixotrophy (property seen in catsup) Increased ability to accommodate higher velocity stretches Advantage of a warm-up

Vascularization of Tendons and Ligaments Dual Pathway for Tendons Vascular (tendon surrounded by paratenon) receives blood supply from vessels in perimysium, periosteal insertion, and surrounding tissues Avascular (tendon surrounded by tendon sheath) Synovial diffusion Healing and repair in the absence of blood supply Ligaments Vascularity Originates from ligament insertion sites Small size and limited blood flow ---------------------------------------------------------------- Take home message: Amount of tissue vascularization is directly related to rate of tissue metabolism and healing Tendons and ligaments have limited vascularization

Macroscopic and Microscopic Structure of Tendon and Ligaments Tendons surrounded by loose connective tissue (paratenon) Paratenon forms sheath Protects tendon Enhances gliding Epitenon Synovial-like membrane beneath paratenon in locations of high friction Absent in low friction locations Surrounds several fiber bundles Endotendon Surrounds each fiber bundle Joins musculotendinous junction into perimysium Ligaments surrounded by very loosely structured connective tissue (not named) Vascularity Originates from ligament insertion sites Small size and limited blood flow

Tendon Insertion in Bone

What comes to mind when you hear the word “toe”?

Load Deformation Relationships in Collagenous Tissues Toe - collagen fibrils stretched to line up, from zigzag to straighten linear region - elastic capability of tissue; elastic modulus failure region - fibers disrupted Hysteresis – failure to return to resting length

Stress-Strain Relationship in Collagenous Tissues

Collagen Fibers – Unloaded (Toe) and Loaded (Elastic Region)

Typical Load-Elongation Curve

Load-Elongation Curve of Ligaments with High Levels of Elastin Elastin (protein) scarcely present in tendons and extremity ligaments Ligamentum flavum: Substantial proportion of elastin Connect laminae of adjacent vertebrae Function to protect spinal nerve roots Provide intrinsic stability to spine

Load-Deformation Relationships for Connective Tissues * 1kN = 224.8 pounds Note that text gives value of failure of ACL between 76.4 and 87.67 lbs (340-390 N)

Is there any movement in isometric contractions?

Physiological Loading of Tendons and Ligaments P (max) of ligaments and tendons not achieved during normal activities normally 30% of P (max) achieved upper limit during running and jumping  2 - 5 % P (max)

Ligament and Tendon Injury Mechanisms Injury mechanisms similar in tendons and ligaments Microfailures take place before yield point After yield point, gross failure results and joint begins to displace abnormally Joint displacement can also damage surrounding structures (e.g., joint capsule, other ligaments, blood vessels)

Anterior Drawer Loading the ACL to Failure

Anterior Drawer Loading the ACL to Failure Microfailure begins before physiological loading range is exceded

What is the numerical categorization system used by athletic trainers to differentiate between levels of ligamentous injury?

Categorization of Ligamentous Injury Negligible clinical symptoms, some pain, microfailure of some collagen fibers Severe pain, clinical detection of some joint instability, progressive collagen fiber failure resulting in partial ligament rupture, strength and stiffness may decrease 50% or more, muscle guarding, perform clinical testing under anesthesia

Categorization of Ligamentous Injury 3. Severe pain, joint completely unstable, most collagen fibers ruptured, loading joint produces abnormally high stress on the articular cartilage  correlated with osteoarthritis

Additional Factors in Injuries to Tendons Amount of force of contraction produced by muscle attached to tendon Tensile stress on tendon directly related to force of muscle contraction High levels of tensile stress can be produced by eccentric contraction, possibly reaching failure

Additional Factors in Injuries to Tendons Cross sectional area of tendon in relation to cross sectional area of its muscle Cross sectional area of muscle directly related to force of contraction Cross sectional area of tendon directly related to tensile strength Tensile strength of healthy tendon may be more than twice that of force of muscle contraction (clinically, muscle ruptures more common than tendon ruptures) Large muscles usually have large tendons

Viscoelastic Behavior (Rate Dependency) in Tendons and Ligaments Increased strain  increased slope of stress-strain curve (i.e., greater stiffness at higher strain) Higher strain rate  more energy stored, require more force to rupture, undergo greater elongation

Typical loading (top and unloading curves (bottom) from tensile testing of knee ligaments. The two nonlinear curves, called the area of historesis, represents the energy losses within the tissue.

Two Standard Tests of Viscoelastic Behavior* Stress-relaxation test Loading halted in safe region of stress-strain curve Strain kept constant over extended period of time Stress decreases rapidly at first, then gradually Decrease in stress less pronounced with repeat tests *Viscoelastic – variation in mechanical properties of tissue with different rates of loading

If you were asked to develop a creep test, what would you use to make measurements?

Two Standard Tests of Viscoelastic Behavior 2. Creep test Loading halted in safe region of stress-strain curve Stress kept constant over extended period of time Strain increases rapidly at first, then gradually Clinically used in casting club foot and bracing in scoliosis

Schematic creep curve for ligament

Influence of Loading Rates on Bone-Ligament-Bone Complex At slow loading rates (60 sec.; much slower than in vivo injury mechanism), avulsion produced At fast loading rates (0.6 sec.; simulates in vivo injury mechanism), ligamentous injury typical

Factors Affecting Biomechanical Properties of Tendons and Ligaments Maturation and aging Up to 20 years of age, number and quality of cross-links in collagen molecules increases  increased tensile strength Collagen fibril diameter increased  increased tensile strength After maturation, Collagen content of tendon and ligaments decreases  decreased tensile strength

Factors Affecting Biomechanical Properties of Tendons and Ligaments Pregnancy and postpartum period Clinical observation – increased laxity of tendons and ligaments in pubic area during latter stages of pregnancy and during early postpartum period  hormonal influence Research studies of rats– increased laxity of tendons and pubic symphasis during latter stages of pregnancy and during postpartum period; stiffness of these structures later returned

Factors Affecting Biomechanical Properties of Tendons and Ligaments Pregnancy and postpartum period (continued) Hormones may have influence on ligament laxity in women at various stages of menstrual cycle  influence ligamentous injury rates in females (e.g., higher incidence of injury in women in basketball and soccer in comparison to men)

Factors Affecting Biomechanical Properties of Tendons and Ligaments Mobilization and immobilization Tendons and ligaments remodel in response to mechanical demands Become stronger and stiffer when subjected to increased stress Become weaker and less stiff when stress removed Physical training found to increase tensile strength of tendons and ligament-bone interface

Factors Affecting Biomechanical Properties of Tendons and Ligaments Mobilization and immobilization Immobilization found to decrease tensile strength of ligaments Immobilization decreased mechanical properties of bone-ligament-bone complex in knee of primates (8 weeks of casting) Considerable reconditioning required in primate knees to regain former complex strength (approx. 12 months) (see figure)

Influence of Immobilization on Primate ACL Ligament

Influence of Immobilization on Primate ACL Ligament

Factors Affecting Biomechanical Properties of Tendons and Ligaments Nonsteroidal Anti-Inflammatory Drugs (NSAID) (e.g., aspirin, acetaminophen, indomethacin) In animal studies, short term administration of NSAIDs (indomethacin) found to increase the rate of biomechanical restoration of tissues (tendons)