BME 7021: Ligament and Tendon Notes

Introduction to Ligaments and Tendons

• Functions in Musculoskeletal System

  • Serve essential roles in the musculoskeletal system.

  • Transfer tensile loads.

  • Guide motion.

  • Stabilize diarthrodial (moving/synovial) joints.

• Ligaments

  • Connect bone to bone.

  • Primary role: Stabilize joints.

  • Examples:

    • Anterior Cruciate Ligament (ACL) in the knee.

    • Medial Collateral Ligament (MCL) in the knee.

• Tendons

  • Connect muscle to bone.

  • Primary role: Cause movement (translation).

  • Deal with much higher forces compared to ligaments.

  • Examples:

    • Achilles tendon (lower leg).

    • Patellar tendon (front of knee).

Ligament/Tendon Composition

• Water Content

  • $60-65\%$ of total weight.

• Solid Phase

  • Primarily Type I collagen:

    • Approximately $88\%$ in ligaments.

    • Approximately $95\%$ in tendons.

  • Remainder consists of glycoproteins.

Collagen Types

• Helical, Structural Types (I-III, V)

  • Type I: Found in tendon, ligament, skin, bone.

  • Type II: Found in cartilage, fibrocartilage (e.g., meniscus).

  • Type III: Important in wound healing and repair; also found in bowel, uterus, and blood vessels.

  • Type V: Regulates fibril diameter in tendons and ligaments.

• Minor Collagen Types

  • Types IX, X, XI: Co-polymerize with Type II in the fibrocartilage zone of insertion to minimize stress concentrations.

  • Type XII: Provides lubrication between collagen fibers.

Ground Substance

• Composition

  • Accounts for a small percentage of total dry weight.

  • Strong ability to attract and retain water.

  • Composed of water and proteoglycans (PGs).

• Role of Proteoglycans (PGs)

  • Water-retaining molecules (sugar-based).

  • Provide lubrication and spacing for the gliding of fibers.

  • Help reduce friction and heat generation.

• Elastin

  • Present in a few percent by weight.

  • Allows tissue to return to its pre-stretched length if not overloaded (similar to elastic material).

Tendon and Ligament Architecture (Hierarchical Structure)

• Macrostructure (Visible to the Naked Eye)

  • Whole tissue.

  • Fascicles (bundles of fibers).

• Microstructure (Visible Under Light Microscope)

  • Fibers.

• Ultrastructure (Visible with Electron Microscopy)

  • Fibrils: The basic building block of tendon and ligament ($50-500 \text{ nm}$ diameter).

  • Microfibrils:

    • Fibril-associated PGs (e.g., decorin, fibromodulin).

• Molecular Level

  • Collagen Molecule (Tropocollagen): Helical and non-helical domains, cross-linking ($1.5 \text{ nm}$ diameter).

Types of Tendon and Ligament Insertions into Bone

• Direct (Zonal) Insertion

  • Characterized by four distinct phases within a distance of less than $1 \text{ mm}$.

  • Phases are: Ligament/Tendon -> Fibrocartilage -> Mineralized Fibrocartilage -> Bone.

  • The gradual transition helps to minimize stress concentrations as bone is designed for compression, not tension.

• Indirect (Periosteal) Insertion

  • Superficial fibers connect to the periosteum (coating on the bone) at an acute angle.

  • Deep (Sharpey's) fibers anchor directly into the bone.

Material Characterization Definitions

• Elastic Material

  • A material that returns to its original shape when the external force is removed, provided the applied force does not exceed its elastic limit.

  • Example: Steel, rubber band (within limits).

  • Inelastic: Does not return to its original shape after force is removed (deforms).

• Viscous Material

  • A material whose viscosity is its resistance to deformation when under stress.

  • Example: Honey.

• Viscoelastic Material

  • A material that exhibits both viscous and elastic material behaviors (e.g., ligaments and tendons).

Failure Properties Determined in Tension

• Structural Properties (of Tendon or Bone-Ligament-Bone Complex)

  • Properties dependent on the tissue's dimensions.

  • Measured parameters:

    • Stiffness (($\text{Force} / \text{Distance}$)).

    • Maximum force (units: Newtons or pounds).

    • Energy ($\text{Force} \times \text{Distance}$).

    • Elongation (units: mm, inches).

  • Challenges: Isolated tissues can slip or show stress concentrations at grips.

• Material (Mechanical) Properties (of Normalized Tissue)

  • Properties independent of tissue dimensions because they are normalized by geometry.

  • Measured parameters:

    • Modulus (slope of the stress-strain curve) and maximum stress.

    • Strain energy density (SED) and strain.

  • Stress ($\sigma$): Force per unit cross-sectional area ($\text{psi} = \text{Force} / \text{Cross-Sectional Area}$).

  • Strain ($\varepsilon$): Elongation normalized by original length ($\text{Elongation} / \text{Original Length}$).

  • Strain Energy Density (SED): Area under the stress-strain curve (units: Joules/volume).

Typical Stress-Strain Curve for Tendon or Ligament

• Zone I: Toe Region

  • Initial, non-linear region.

  • Represents the straightening of crimped collagen fibers.

  • The tissue starts to respond to load, but the modulus is low.

• Zone II: Linear Region (Elastic Region)

  • Fibers are straightened and bear load efficiently.

  • The material exhibits linear elastic behavior, where it can return to its original length/shape if the load is removed.

  • Modulus or slope is determined in this region.

• Zone III: Region of Maximum Stress

  • The tissue approaches its ultimate strength.

  • Collagen fibers begin to fail or break.

  • The curve becomes non-linear again, often showing a decrease in slope.

  • Maximum stress typically ranges from $50-100 \text{ MPa}$.

  • Maximum strain typically ranges from $15-20\%$ .

• Zone IV: Failure Region

  • Complete rupture of the tissue.

  • The tissue will not fully return to its original shape/length.

Measurement of Cross-Sectional Area

• Importance: Accurate area measurements are critical for calculating stress values.

• Direct Methods (Contact)

  • Area micrometer, torque sensor, digital calipers.

  • Often require forcing the tissue into a regular shape (e.g., rectangular), which can alter its shape or structure and introduce artifacts.

• Indirect Methods (Non-contact/Optical)

  • Laser micrometer, shadow method, shadow amplitude/profile method.

  • Laser micrometer allows for precise and accurate area measurements and shape determination without contact or distortion.

Measurement of Strain Values

• Importance: Accurate length measurements are critical for calculating strain values.

• Initial Tissue Length Measurement

  • Contact Methods: Clip gage, calipers between bone ends.

  • Non-contact Methods: Video tracking of two reference markers.

    • Markers can be ink, elastin or Verhoeff stains, reflective tape/suture.

    • A camera records marker motions to calculate strain percentages.

• Methods for Measuring Tissue Strain

  • Grip-to-Grip (G-T-G) or Bone-to-Bone (B-T-B):

    • Measures overall deformation.

    • Can include measurement artifacts such as slippage (G-T-G) or bone bending (B-T-B).

  • Mid-substance Strain Measurement: Recommended if tissue composition varies by location.

    • Utilizes dye lines (ink, Verhoeff's stain, magic marker).

    • Advantage: Provides local strain in a region of similar material properties.

    • Disadvantage: Lines may only stain the surface, not reflecting strains throughout the tissue.

• Strain Variation: Strains often vary along the soft tissue length, with larger strains observed near the grips due to grip attempts to prevent slippage.

Factors Affecting Tissue Properties

• Specimen Orientation

  • Highest material properties are obtained when tissues are tested along their physiological axis rather than in the direction of attached bones.

  • Fibers are more uniformly loaded along the axis and less likely to peel off the insertion site.

  • However, aligning tissues with complex organization (e.g., ACL with major anteromedial and posterolateral bundles of different lengths and orientations) is difficult and often does not perfectly mimic in vivo motion.

• Temperature and Tonicity

  • Temperature: Dropping testing bath temperature from $37 \text{ deg C}$ (simulating in vivo) to $25 \text{ deg C}$ increases tissue stiffness by $33\%$. Therefore, testing at $37 \text{ deg C}$ is best.

  • Tonicity (Salt Solution):

    • Testing in a hypotonic salt solution (low ion concentration) causes tissue swelling.

    • Hypertonic solution (high ion concentration) has the opposite effect.

    • Best to test in phosphate-buffered saline, which mimics in vivo ion concentrations.

• Testing Rate

  • Traumatic in vivo rates can be very rapid (likely thousands percent per second), which are difficult to replicate in the lab.

  • Testing ligament-bone units at high rates (e.g., $~100\% / \text{s}$) increases material properties.

  • A $100\%$ increase in rate produces only a $10-15\%$ increase in modulus.

  • Higher testing rates increase the chance of soft tissue failures rather than bone avulsion fractures.

• Freezing

  • Necessary when specimens cannot be tested fresh. Controlled freezing to $-30 \text{º C}$ or below is best.

  • One cycle of freezing: Negligible effects on ligament-bone properties.

  • Repeated freezing cycles: Can create fissures in the bone attachment site and significantly reduce material properties.

• Irradiation

  • Used by tissue banks for sterilization against viral contaminants (e.g., HIV, hepatitis).

  • $3 \text{ to } 4 \text{ Mrad}$ of gamma irradiation is believed to inactivate HIV.

  • However, $4 \text{ Mrad}$ of gamma irradiation significantly decreases patellar tendon-bone maximum stress.

  • Higher levels of irradiation produce even greater reductions in tissue properties by heating up and damaging the tissues.