Physiology of Ligaments

 

Introduction

Ligaments are tough, fibrous bands of connective tissue that attach bones to bones, providing static stability and support to joints.

The primary function of ligaments is to limit excessive movement of joints, protecting them from injury. They also provide proprioceptive feedback, allowing the brain to sense the position and movement of joints.

 

Structure

Collagen fibers are arranged in bundles or fascicles, which are surrounded by a thin layer of connective tissue. Multiple fascicles are then bundled together to form a larger unit - the ligament proper - which is surrounded by a vascularised epiligament sheath.

The collagen fibers are classified based on their composition and arrangement:

  • Type I - most abundant and provide high tensile strength
  • Type III - less abundant and contribute to flexibility

Other proteins involved include elastin, proteoglycans and glycoproteins.

Within a ligament, collagen fibers run in multiple directions; in a tendon, they are much more parallel aligned, allowing more stiffness to transmit the muscle's contraction force.

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Reproduced from MDPI Open Access Journals

 

The attachment of ligaments to bone includes four layers that provide a firm anchor:

  1. Ligament layer composed of collagen fibers and other extracellular matrix components; provide structural support and allow the ligament to resist tensile forces
  2. Fibrocartilage layer composed of chondrocytes and an extracellular matrix rich in collagen fibers; helps to absorb shock and distribute load across the bone
  3. Mineralized fibrocartilage layer characterized by the presence of calcified collagen fibers; provides a strong and rigid attachment, helping to anchor it firmly to the bone surface
  4. Bone layer in which the collagen fibers of the ligament blend with the mineralized fibrocartilage and bone tissue to form a seamless transition between the ligament and bone (SHARPEY’S )

 

Biomechanics

Collagen fascicles are arranged in a longitudinal fashion along the axis of the ligament with subtle crimping, which enables them to resist tensile forces that act along the length of the ligament. This also helps to distribute forces evenly across the ligament, reducing the risk of localised stress concentrations that could lead to injury or failure.

When a ligament is subjected to tensile forces, the collagen fibers and fascicle crimping start to elongate, resulting in strain. The amount of strain that a ligament can withstand before it starts to fail depends on various factors such as the composition of collagen fibers, their arrangement, and the amount and direction of the applied force.

Collagen fibers have a nonlinear stress-strain relationship, which means that the amount of strain increases faster than the applied stress. This nonlinear behavior of ligaments allows them to absorb energy and prevent sudden failure during high-force impacts or sudden movements.

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Stress-strain curve for tendons and ligaments (reproduced from ResearchGate)

 

Injuries

Sprains are common and can occur due to a sudden impact, twisting, or stretching of the joint beyond its normal range of motion. They can become damaged or torn (partially or completely). Ligaments are poorly vascular, meaning they have minimal direct blood supply. Instead, they receive nourishment and oxygen from the synovial fluid that surrounds the joint and the bony / capsular attachments. This is why injuries can take a long time to heal.

Treatment

Depends on the severity of the injury. In mild cases, rest, ice, compression, and elevation (RICE) and analgesia can help to reduce pain and swelling. Untreated injuries can lead to long-term joint instability and chronic pain.

  • Early immobilisation of the joint is recommended with a brace / splint / cast to allow the ligament to heal
  • Early weight-bearing is recommended to encourage the damaged collagen fibers to heal along lines of physical load
  • Physical therapy may also be necessary to restore strength and flexibility to the joint
  • In severe cases, surgery may be required to repair or reconstruct the damaged ligament

 

FRCS exam questions

  1. Describe the structural composition of a ligament and its functions in joint stability.
  2. Explain the biomechanics of ligament injuries and how they occur.
  3. Compare and contrast the healing processes of a torn ligament and a fractured bone.
  4. Discuss the differences between partial and complete ligament tear and the implication of treatment.

 

References

  • Frank, C. B. (2004). Ligament structure, physiology and function. Journal of Musculoskeletal & Neuronal Interactions, 4(2), 199–201. https://europepmc.org/article/med/15615126
  • Katz, A., & Erickson, S. J. (2003). Ligament Pathology. 179–200. https://doi.org/10.1007/978-3-642-59363-5_11
  • Kerkhoffs, G. M., Rowe, B. H., Assendelft, W. J., Kelly, K. D., Struijs, P. A., & van Dijk, C. N. (2002). Immobilisation and functional treatment for acute lateral ankle ligament injuries in adults. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.CD003762/ABSTRACT
  • Ligament - Physiopedia. (n.d.). Retrieved May 7, 2023, from https://www.physio-pedia.com/Ligament
  • MCQ Question Development. (n.d.). Retrieved May 7, 2023, from https://www.jcie.org.uk/content/content.aspx?ID=5
  • Yang, G., Rothrauff, B. B., & Tuan, R. S. (2013). Tendon and Ligament Regeneration and Repair: Clinical Relevance and Developmental Paradigm. Birth Defects Research. Part C, Embryo Today : Reviews, 99(3), 203. https://doi.org/10.1002/BDRC.21041