Both ligaments and tendons are fibrous, dense regular collagenous connective tissues. Ligaments join bone to bone in a joint whilst tendons join muscles to bone.
Function of Ligament
1. to resist external load. Prevents excessive motion
2. to guide joint motion. Therefore, guide relative movements of the 2 bones
3. to passively control the maximum range of movement. Therefore, augment mechanical stability of the joint. Hence, the ligament length, stiffness, location and orientation with respect to the articulating bones affect the motion patterns.
4. to provide motor control
Function of Tendon
1. transmit tensile forces from muscle to bone
2. advantage mechanical pulley enables muscle belly to be an optimal distance from joint without requiring an extended length of muscle between origin and insertion.
3. to maintain body posture
4. to provide motor control
In the diagram (Reid, 1992) below depicts the structure of either the tendon or ligament . They are split into smaller fascicles and each fascicle contains the basic fibril and the fibroblasts. It is the crimp/waviness of the fibril that contributes significantly to the non-linear stress-strain relationship of ligaments and tendons. The fibroblasts are the biological cells that produce the ligament or tendon.
Both tendons and ligaments are made up of mainly 70% water, 20% collagen and about 2% of ground substance. However, collagen contributes to 70% of its dry weight. Collagen is a fibrous protein with high tensile strength.
However, the arrangement of collagen fibres differs between tendons and ligaments. Ligaments are organized in a dense but random/loose-packed network unlike tendon that have an orderly, parallel, linearly packed arrangement. This facilitates tendons to handle high unidirectional (uniaxial) tensile loads during activity whilst ligaments generally sustain tensile loads in a predominant direction but also bear smaller tensile loads in other directions.
A. Ligaments- densely packed collagen in a random-packed network but towards a general primary direction.
B. Tendons- linear arrangement(Reid, 1992)
Tendons and extremity ligaments have very little protein elastin. Ligament have a higher ratio of fibrillar Type I collagen to Type III collagen (elastic). Thus, ligament’s mechanical properties provide it with not only the flexibility but also the tensile properties to resist force.
It provides mechanical properties in compression. Ligaments buckle under compression and do not provide any support.
Innervation and mechanoreceptors
Within tendons there are 4 types of nerve endings: type I or Ruffuni corpuscles monitor pressure; two types of mechanoreceptors – type II or Vater-Pacini corpuscles and type III or Golgi tendon organs; type IV free nerve endings monitor pain. Together they provide essential afferent sensory neural information that controls the movements of the body via the central nervous system (Jozsa and Kannus, 1997). The ACL is richly innervated by mechanoreceptors (Schultz et al, 1984). It has been proposed that the sensory information provides neuromuscular control and hence, contribute to dynamic stability of the joint.
Outer structure and Insertion into Bone
Both tendons and ligaments are surrounded by loose areolar connective tissue. Whilst this tissue around ligament is not significant, in tendons, this tissue known as paratenon is more structured and forms a sheath that protects it and promotes gliding.
Clinical implication: introduction of passive motion for healing/repair to prevent adhesion between the sheath. (study of 3 groups of dogs-complete immobilization; delayed immobilization and early passive motion. The latter group’s tendon strength and motion improved more quickly than the rest.)
Load refers to the external force applied to the collagen. Stress refers to the amount of resistance within the collagen to the load. Strain is the amount of deformation that occurs in the collagen because of the load (Sandrey 2000) .