Ligaments are like ropes in that they offer little resistance to compression, but they are strong in tension.

A structure such as a ligament, whose mechanical properties depend on the orientation of the force applied, is anisotropic. To stabilize the joint, most ligaments work in pairs. In the ankle, the lateral talofibular ligament is balanced by the deltoid ligament on the medial side.

A ligament’s resistance to tension differs slightly compared with that of a tendon, the tissue that attaches a muscle to bone. A ten- don is uniformly stiff, and it does not elon- gate much when pulled. This stiffness en- sures that the entire tug of the muscle is used to move the joint and that no force is wasted by simply elongating the tendon. A ligament does, however, have some built-in laxity in response to low tension forces. This lower stiffness allows the joint to withstand small deforming forces without damage—just as a tree may bend and not break in response to wind. At higher tension, however, the liga- ment becomes stiffer, thus keeping the joint stable. The structural properties of tissue can be described by measuring the relationship between stress and strain (Fig. 1).

Figure 1
The mechanical response of the tissue can be illustrated by plotting a load versus elongation curve or a stress versus strain curve.The slope of this curve defines the stiffness of the tissue. The load-elongation curve shown in the graph is typical of that of ligaments, with an initial region of low stiffness (the toe region) and a second region of high stiffness (linear region).
(Reproduced from Woo SLY, An KN, Frank CB, et al: Anatomy, biology, and biomechanics of tendon and ligament, in Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Mulsculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 581-616.)

Stress is the deforming force, defined as the amount of load per unit of cross-sectional area of the tissue. Strain is defined as the amount of tis- sue elongation divided by the original tissue length—a percentage of the deformation. When a material is stressed, a given strain will be observed.

Consider, for example, an individual walking over rocky terrain. Whenever weight is disproportionately borne on the medial side of the foot, an inversion force is placed on the lateral ankle. When the forces are small, the anterior talofibular ligament stretches in response to the load and springs back to its normal length once the force abates. When the forces are increased and the joint threatens to give way, the ligament becomes taut and barely elongates at all. Of course, when the force exceeds the ligament’s own strength, such as that produced when a jumping basketball player lands un- evenly on another player’s foot, the ligament will tear and no longer stabilize the joint.

It is worth noting that the extent of ligament disruption does not always predict the degree of clinical instability that an individual will perceive. There are instances in which the lateral ankle ligaments are completely torn and yet there may not be much instability, as the individual can use his or her muscles (the peroneus longus and brevis) to actively stabilize the joint. Conversely, in some cases, even once the torn ligament heals, the individual may still sense subtle instability. This perceived instability may occur because the nerves with- in the ligament that sense movement and dis- placement can be damaged in a severe sprain. When these nerves are injured, the individual may lose proprioception and report a poor sense of control, even if instability is not objectively confirmed.