86% of a healthy tendon’s dry weight, while type III is found in lesser amounts. It is the high con­centration of collagen in combination with its parallel orientation that gives tendons their high tensile strength. Collagen chains are linked together to form brils that, in turn, are bound together by a proteoglycan matrix to form a fas­cicle, the primary unit in tendon structure. Fascicles, in turn, are bound by the endotenon, a layer of elastin-containing loose connective tis­sue that supports the blood, lymphatic, and neu­ral supply to the tendon unit. It is the endotenon that is contiguous with both the muscle bers and periosteum at the musculotendinous and teno­osseous junctions, respectively.

more helical structure such as the anterior and posterior cruciate ligaments (ACL and PCL) of the knee (Fig.8.3).

to a ligament, an injury or sprain occurs, whose severity (grades I–III) depends on the amount of stress applied. A grade I sprain represents the least traumatic episode where some ligamentous bers are torn on a microscopic level, while the overall structural integrity is maintained. An example of this is a common ankle sprain where traditionally the anterior talobular ligament (ATFL) is injured. In a grade II sprain, some bers are macroscopically torn in combination with microscopic damage, resulting in a stretch­ing of the ligament. While the biomechanical properties of the ligament are compromised in this scenario, some structural integrity of the lig­ament remains. An example of this injury is an injury to the medial collateral ligament (MCL) of the knee. When examining a patient with a grade II injury of the MCL, the knee demonstrates increased laxity to a valgus load but an endpoint is still felt, signifying some remaining structural integrity. In a grade III sprain or “tear/rupture,” the ligament completely fails with no structural integrity remaining. After rupture of the anterior cruciate ligament, there is an increase in anterior translation of the tibia relative to the femur with a force directed from posterior to anterior on the tibia. This represents a complete structural failure of the ligament.

Intra-articular (inside of a joint) and extra­articular (outside of a joint) ligaments differ in their response to acute trauma (Fig.8.4). This is inuenced by a difference in the local vascular supply of these entities, the presence of synovial uid within joints, the degree of ligamentous injury, and whether or not a signicant gap forms between the two ends of ligamentous rupture. Typically, extra-articular ligaments have a high potential for healing and gradually heal with pre­dominantly type I collagen within a 6- to 12-week time frame. Maturation of this ligament scar can sometimes take up to 1 year, despite histologic evidence of healing as early as 6 weeks. Contrastingly, intra-articular ligaments such as the anterior cruciate ligament have a poor healing potential due to the intra-articular milieu. In cases of complete disruption of this structure, dissocia­tion of the midsubstance “mop ends” results in signicant gap formation with inhibition of the