Basic Sciences 2023, Jan, 25 | 12:00 am

Ligament

  • Ligament 

     

    FIG. 1.44 Tendon and ligament architecture.

    From Brinker MR, Miller MD: Fundamentals of orthopaedics,

    Philadelphia, 1999, Saunders, p 15.

  • Characteristics

    • Originates and inserts on bone

    • Stabilizes joints and prevents displacement of bones

    • Contains mechanoreceptors and nerve endings that facilitate joint proprioception

    • Like tendon, displays viscoelastic behavior

  • Structure and composition

    • Composition

      • Similar to that of tendon

        • Water: 60%–70% of total weight

        • Collagen: 80% of dry weight

          • 90% type I collagen; also types III, V, VI, XI, and XIV collagen

          • More collagen type I is seen at the origin and insertion, with collagen III seen midsubstance.

        • Elastin (1% dry weight)

        • Proteoglycans (1% dry weight)—function in water retention and contribute to viscoelastic behavior

      • Fibroblast

      • Primary cell, oriented longitudinally

      • Functions to synthesize ECM, collagen, and proteoglycans

  • Epiligament

    • Similar to that in epitenon; carries the neurovascular and lymphatic supply of tendons

  • Compared with tendon

    • Less total collagen but more type III collagen

    • More proteoglycans and therefore more water

    • Less organized collagen fibers that are more highly cross-linked and intertwined

    • “Uniform microvascularity”—receives supply at insertion site by the epiligamentous plexus

  • Insertion

 

  • Similar to that of tendon

  • Direct (fibrocartilaginous) insertion

    • Four layers: tendon, fibrocartilage, mineralized fibrocartilage, and bone

    • More common

    • Deep fibers attach at 90-degree angles

 

  • Injury

  • Indirect

     

  • Superficial fibers insert into the periosteum and deep fibers insert into bone via Sharpey fibers (perforating calcified collagen fibers).

     

    • Healing

  • Knee and ankle ligaments are most commonly injured

  • Ligaments do not plastically deform.

    • They “break, not bend.”

  • Midsubstance ligament tears are common in adults.

  • Avulsion injuries are more common in children.

  • Typically occurs between unmineralized and mineralized fibrocartilage layers

    • Increased number of collagen fibers but

      • Fewer mature cross-links (45% of normal at 1 year)

      • Decrease in mass and diameter

  • Three phases, as in bone

    • Inflammatory—early acute mediators (PMNs and then macrophages), with production of type III collagen and growth factors

    • Proliferative—around 1–3 weeks, with replacement of type

      III collagen by type I collagen (Think of macrophages as weakening the structure—weakest point.)

    • Remodeling and maturation

  • Factors that impair ligament healing

    • Intraarticular ligamentous injury

    • Old age, smoking, NSAID use

    • Diabetes mellitus

    • Alcohol use

    • Local injection of corticosteroids

  • Factors that improve ligament healing experimentally

    • Extraarticular ligamentous injury

    • Compromised immunity

    • IL-10 (antiinflammatory)

    • IL-1 receptor antagonists

    • Mesenchymal stem cells

    • Scaffolds (such as collagen–platelet-rich plasma hydrogels)

    • Neuropeptides

    • Calcitonin gene–related peptide

  • Immobilization

    • Adversely affects ligament strength: elastic modulus decreases

    • In rabbits, breaking strength reduced dramatically (66%) after 9 weeks of immobilization.

    • Effects reverse slowly upon remobilization.

    • Prolonged immobilization disrupts collagen structure, which may not return to normal within insertion sites.

  • Exercise

    • Improves mechanical and structural properties

    • Increases strength, stiffness, and failure load