What is a ligament?

Histology

Ligaments are made out of dense regular connective tissue. This implies two of the histological properties that this tissue has:

• Its fibers are densely packed next to one another and leave very little open space in between.
• The fibers are arranged parallel (regularly) to each other. Since ligaments are stretched in a predictable direction, this arrangement is parallel to that direction and gives ligaments maximum strength to resist mechanical distress and prevent injuries.

The fibers that comprise the ligaments are either elastic or collagen fibers, and both types are synthetized by fibroblasts, which are the main type of cells in connective tissue. Based on which type of fiber is mostly present in the tissue, there are white and yellow ligaments.

White ligaments can be rigid when the majority of the extracellular matrix is made out of type I collagen fibers, like the anterior cruciate ligament in the knee. On the other hand, yellow (elastic) ligaments are mostly comprised of elastic fibers, like the ligamentum flavum in the spinal column

Because of this histological difference, yellow ligaments are much more stretchy than white ligaments. For instance, a sudden 30 degree inward rotation of the knee can cause the anterior cruciate ligament to tear, while the ligamentum flavum can handle any possible angle of spinal flexion without tearing. The elasticity of the ligamentum flavum is therefore very important to support the vertebral column and assisting it to resume the upright posture after flexion or extension.

Histologically, like every other tissue, ligaments also consist of the cells and extracellular matrix (ECM) which they produce. Fibroblasts are the dominant cells and they produce the fibers that comprise the ECM. The fibers within ligaments are densely packed and arranged in parallel, which is why this tissue is called dense regular connective tissue. 

This dense fibrillar arrangement provides maximum strength for the ligaments and increases the resistance to the stretching forces that occur during movement. If the main fibrillar component were composed of collagen fibers, the ligament would be more rigid and resistant to stretching.

This is useful for supporting the joints and preventing the articulating surfaces from dislocating. On the other hand, if the fibers are mostly elastic, the ligament will be able to stretch more and allow wider motion range, but also during that stretching, to generate the elastic force needed to resume the primary position.

Articulation ligaments

These ligaments are also called “true” ligaments because they connect two articulating bones as opposed to peritoneal ligaments which connect viscera to viscera, or viscera to the abdominal wall. Articulation ligaments participate in the stabilization of joints based on their position within the joint, but can also limit certain movements. 

Besides connecting the two bones and stabilizing their joint, ligaments have additional functions.

• Extend the surface that the bone provides for the attachment of the muscles (interosseous ligamentsin the leg and the forearm)
• Prevent tendons from popping up when a certain joint is flexed or extended (flexor retinaculum in the wrist)
• Some ligaments are different shapes, such as oval plates called the articular discs stuck between the bones (triangular disc of the wrist joint, intervertebral disc of the vertebral column). 

Note that the articular discs allow a more even distribution of forces between the articulating surfaces of the bones and increase the stability of the joints.

Peritoneal ligaments

Peritoneal ligaments are, of course, the folds of peritoneum. Their major functions are the stabilization of the organs within the abdominal cavity and transmitting neurovascular elements to specific organs. They do so by connecting the viscera to the abdominal wall, or simply viscera to viscera.

There are many examples of these ligaments in the gastrointestinal tract and uterus – we will mention a few of them. 

• The gastrosplenic ligament connects the stomach and the spleen and carries the short gastric artery and the left gastroepiploic artery. 
• The broad ligament of the uterus stabilizes the uterus by connecting it to the pelvic wall. It can be divided into three components: the mesovarium, mesosalpinx, and mesometrium
• The splenocolic ligament connects the capsule of the spleen to the transverse colon
• The round ligament of uterus extends from the uterine horns to the labia majora. It maintains the anteversion of the uterus during pregnancy.
• The suspensory ligament of the penis supports the penis when erect by attaching it to the pubic symphysis.
• The triangular ligament of the pelvis, also referred to as the urogenital diaphragm, is a layer of pelvis that separates the deep perineal sac from the upper pelvis.

Peridontal ligaments

Besides articulation and peritoneal ligaments, another major group is the periodontal ligaments. They connect a tooth to the alveolar bone. During mastication, they distribute the mechanical pressure from the teeth to the alveolar bone and support it. More about dental anatomy and periodontal ligaments you can find in the article about the anatomy of the teeth and this interesting video tutorial.

Innervation

Ligaments have a very rich sensory neural supply. Since both free nerve endings and encapsulated sensory ends are present in ligaments, especially in articulation ligaments, they react immediately if an excessive or inconvenient movement is produced.

The sensation that we would feel after these nerves are stimulated is a very sharp pain, indicating that these sensory fibers innervate proprioceptors and nociceptors. The most frequently described example is the anterior cruciate ligament of the knee. We all know that unpleasant feeling when our knee quickly rotates inward.