Ligaments and Tendons

Load and Material Properties: Joint Structures

Function of Ligaments

• Stability to joint
• Loading patterns:
  • Tensile
  • Compression
  • Bending
  • Torsion
• Anatomical structures: Hook of hamate, Flexor retinaculum (transverse carpal ligament), Pisiform bone, Tubercle of trapezium, Carpal tunnel entrance, Ulna, Radius.

Ligaments Composition

• Dense, irregular connective tissue
• Extra-cellular matrix: Contains crimped bundles of collagen fibrils, proteoglycans, elastin, water.
• Usually contain more Type I collagen than elastin.
• Bundled together in parallel, but bundles themselves are oriented in multiple directions → resists forces from different directions.
• Poor blood supply, which means they do not heal/repair well, and are slower to adapt to stresses, including exercise.
• Side note: Type II collagen – mainly found in cartilage and intervertebral discs – provides compression strength.

Ligaments Loading Behaviors

• Amount and direction of loading are important to the health and strength of the ligament.
• Factors that determine the strength of a ligament under load:
  • Size and shape: A greater number of fibers oriented in the direction of the load and the wider/thicker they are → the stronger the ligament.
  • Speed of loading: Increase in strength and stiffness with an increased speed of loading.
  • Remodel capabilities: Intermittent tension (application and release of a tensile force) → increased thickness, strength & stiffness.

Normal Loading: Ligament

• Anisotropic in nature

• Examples of bending load on ligaments:

  • Varus-valgus loading on collateral ligaments
  • Spinal ligaments

• Exhibit viscoelastic qualities of creep, stress relaxation, and hysteresis.

• When loaded, ligaments initially uncrimp (low resistance) as they elongate and then stretch (high resistance) until they reach a point of failure (rupture).

• Normal activities function at low resistance levels.

• Slow loading rate on the bone-ligament complex of a joint → bone is more likely to fail or break first via avulsions.

• Increased loading rate → the ligament is likely to rupture first as bone becomes stronger.

Decreased Load: Ligament

• Decreased load → decreased strength and stiffness → implications for immobilization.
• Immobilized ligaments weaken rapidly and can take more than 12 months to recover their mechanical properties.
  • 8 weeks of immobilization may require 1 year of recovery.
• Decreases in strength and stiffness are found with increased age.

Overload: Ligament

• When a ligament is loaded, microfailure can take place before the yield point is reached (Grade I sprain) -- when the yield point is reached, gross failure occurs.
• Loading an unstable joint (ligament rupture) produces “abnormally high stresses on the joint cartilage”.
• Creeping of the ligament is greatest during the first 6 - 8 hours of loading and can continue at a lower rate for months (implications for splinting).

Loading: Ligaments and Tendons

• Relation of Grades 1, 2, and 3 of sprains to stress-strain curve
• Plastic region – grade I and II ligament sprains and tendon sprains

Tendon: Function

• Attach muscle to bone or fascia
• To transmit tensile loads from muscle to bone

Tendons Composition

• Collagen (60\% dw) including type I (III, IV, V, VI, XII, XIV)
• Proteoglycan (0.5\% dw) including decorin, versican, lumican
• Glycoproteins (5\% dw) including tenascin, COMP, elastin

Tendons Composition

• Similar to muscle, have the hierarchy of structural units:
  • Collagen molecules → microfibrils → fibrils → fibers → fascicles → tendon unit which is enclosed or surrounded by epitenon
• Parallel arrangement of the components to the long axis of the tendon → suited for high tensile loading provided by muscle contraction
• Fibers are mostly made up of Type I collagen (key to tensile strength) but also have other types of collagen, proteoglycans, and glycoproteins.

Loading Behaviors

• Mechano-responsive, meaning they alter their structure and biological behaviors in response to the various mechanical loading conditions placed on them.
• Like ligaments, tendon strength & stiffness is related to size, shape, and speed of loading.
  • Tendons are stronger in the middle than at their ends.
• Tendons contain more collagen and less elastin than ligaments - hence can transmit forces better but are stiffer (stretch less).
  • Tendons also have greater tensile strength than ligaments – maybe due to adaptation from constant muscle pull.
• Collagen fibers in ligaments can be in several directions providing resistance to loading in different directions, but collagen fibers in tendons are straight and parallel and are subjected to tensile forces by muscle.
  • So the tensile strength of ligaments is relatively lesser than that of tendons, but ligaments can also withstand forces in other directions relatively better than tendons.
• Increases in tensile forces cause increases in Type I collagen in ligaments and tendons.

Loading Behaviors

• Anisotropic and viscoelastic (creep, stress-relaxation).
• At low strain rates, tendons are more deformable and tend to absorb more mechanical energy but are less effective in carrying mechanical loads.
• At high strain rates, tendons become stiffer and less deformable and more effective in transmitting large muscular loads to bone.

Loading Behaviors

• The amount of stress on a tendon increases as the muscle contracts.

• Cross-sectional area and length of the tendon determine the amount of force it can resist and the amount of elongation possible, and therefore, affect the amount of stress on the tendon.

  • Under normal conditions, tendons with a larger cross-sectional area should be able to withstand larger forces than tendons with a smaller cross-sectional area (Achilles tendon vs. Palmar Longus tendon).
  • But when a tendon is healing, it may have lesser strength than its smaller counterpart due to less collagen and smaller fibrils.

• Exposure to corticosteroids, nutritional deficiencies, hormonal imbalance, dialysis, chronic loading into the high linear region of the stress-strain curve with inadequate time for recovery, and sudden large loads may predispose the tendon to injury at previously physiological levels of loading.

  • In other words, the same load now produces more deformation in the tendon.

Loading Behaviors

• Loading characteristics under normal, immobilization, and overloading, like other tissues.

  • Tendons readily adapt to changes in the magnitude and direction of loading.
  • Tendons exhibit creep when subjected to tensile loading, most often when stress is created via muscle contraction.
  • Tendons subject to continuous compressive loading will alter their composition to resemble cartilage, and their tensile strength may decline.
  • Conversely, tendons subject to tensile loads, especially physiological loads over long periods of time, will increase in size, collagen concentration, and collagen cross-linking.
  • Tendons subjected to immobilization show atrophy at the Myotendinous junction (MTJ) with a decrease in collagen concentration.

• Progressive loading programs are successfully used to treat tendon dysfunction, presumably through inducing changes in tendon composition.

Loading Behaviors

• Ligaments and tendons can deform more than cartilage, and cartilage is able to deform more than bone. However, total deformation also depends on the size (length, width, and depth) of the structure.

• Failure of Ligaments and Tendons:

  • Failure in the middle of the structure through a disruption of connective tissue is called a rupture.
  • If failure occurs at the bony attachment of the ligament or tendon, it is called an avulsion.
  • When failure occurs within the bony tissue, it is called a fracture.
  • Muscle strains occur on the muscle side of the muscle-tendon junction (MTJ).
  • The bone-tendon junction is weaker than MTJ.

• Slow loading rates tend to create avulsions or fractures, whereas fast loading rates create mid-substance tears or ruptures.

Additional video - Ligaments and Tendons

• Ligament: Nearly parallel bundles of collagen fibers, Fibroblasts
• Tendon: Parallel bundles of collagen fibers, Fibroblasts

Joint Capsule Composition

• Type I collagen

Function

• May help limit or control specific movements
• Joint support

Loading Behaviors

• Resists loading and forces from different directions
• Same as for any of the other connective tissue for unloading or overloading
  • Effects of immobilization with decreased nutrition and change in composition of structure
  • Creep, change in viscosity, and hysteresis

Cartilage

• See video lectures posted

Specific Adaptation to Imposed Demand (SAID Principle)

Too Little

• Cellular structures change to reflect a lack of loading.

• Lessens or changes the composition to adjust for the lower load.

• Example: Immobilization

  • Muscle – atrophy
  • Joints – lack of nutrition, change in collagen structures
  • Ligaments & tendons – can alter the ability to withstand load (weakens)
  • Bone – demineralization

• Time between loading and failure is decreased.

• The energy-absorbing capacity of the bone-ligament complex is decreased.

• Physical stress theory by Mueller & Maluf examines the potential effects that physical stress has on tissue adaptation.

Too Much

• Injury

  • Muscle – strains
  • Joints – cartilage breakdown // instability
  • Ligaments – sprains
  • Tendons – tendinitis // tendonosis
  • Bone – fracture

Just Right

• ‘Controlled and/or purposeful loading’ is needed to load to preserve and/or build healthy tissue.

Conclusion

• The relationship between form and function is essential for therapists to remember during rehabilitation after injury.
  • All tissues will adapt to increased load through changes in structural and/or material properties (form follows function).
  • The load must be gradual and progressive; as the tissue adapts to the new loading conditions, the load must change to induce further adaptation.
• For example, when a bone is broken, the fracture may be the main injury that dictates subsequent treatment, but a lack of motion and decreased loading will also affect cartilage, ligaments, the joint capsule, tendons, and muscles.
• The ideal rehabilitation protocol considers the behavior of all the affected structures and includes interventions tailored to induce adaptations in each structure.
• This means understanding the time course and nature of adaptation of each tissue to altered loading conditions. * *