September
Ligaments
are tough connective structures that connect bone to bone. The fundamental difference between ligament and tendon is that ligaments are weaker and not as stiff as tendons, due to a less regular collagen arrangement, because ligaments must withstand forces in varying directions, whereas the direction of force on a tendon is constant.
Composition Water makes up 60-80% of weight. Collagen makes up 70% of dry weight. Type I collagen makes up 90%, type III is the next most prevalent. Type III is seen in higher amounts in injured ligaments. Elastin makes up 1% of dry weight (higher in spinal ligaments). Fibroblasts are the dominant cell population.
Ultrastructure Bands of collagen fibrils in a more interwoven structure than tendons.
Insertion into bone Is either direct (femoral insertion of MCL) or indirect (tibial insertion). Direct insertions are characterized by attachment of collagen fibres at right angles to the bone, through four zones. Zone I is the ligament. Zone II is fibrocartilaginous. Zone III is mineralized fibrocartilage. Zone IV is bone. There is a tidemark between zones II and III. Indirect insertions are characterized by a superficial insertion into periosteum and a deep insertion via Sharpey’s fibres into the bone.
Factors affecting ligament properties
Estrogen decreases load to failure – may account for increased rates of ACL injury in females. Aging leads to decreases in stiffness and ultimate load. Immobilization decreases load to failure, and return to normal strength takes a much longer than the period of immobilization.
Healing Intrinsic healing responses are seen in extra-articular ligaments. Intra-articular healing isn’t seen.
Tendons
Tendons are regular connective tissue structures that connect muscle to bone. Compared with ligaments tendons deform less under applied load, which makes for more efficient load transfer. Tendons can be divided into two main categories – those that pull in a straight line and are surround by a paratenon (e.g. Achilles tendon) and those that pull around bends and are surrounded by a tendon sheath (e.g. flexor tendons). The former heal from granulation tissue from the paratenon; the latter can heal via intrinsic healing mechanisms from tenocytes (if passive motion is used) or heal via granulation tissue from the sheath (if the tendon is immobilized).
Factors affecting tendon properties
Flexor tendons are stronger and stiffer than extensor tendons in the digits. They have a higher collagen content. Exercise improves load to failure and stiffness. Fluroquinolones cause a decrease in proteoglycan production, increased oxidative stress and damage to collagen fibres and a decrease in decorin production.
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