Tendon and Ligament Structure

Tendons and ligaments are both dense regular connective tissue, built mainly from type I collagen, which gives tensile strength, within a ground substance of proteoglycans and water that contributes to their viscoelastic behaviour. Water makes up much of the wet weight and, held in place by the proteoglycans, helps the tissue resist and redistribute load. The cells, tenocytes in tendon and fibroblasts in ligament, are relatively few and maintain and repair the matrix. Ligaments contain more elastin than tendons, most of all the spinal ligaments, which lets them stretch and recoil.

A tendon is built in a hierarchy. Collagen molecules assemble into fibrils, which bundle into fibres, then into fascicles, and finally into the whole tendon, with each level held by thin connective tissue layers, the endotenon around fascicles and the epitenon around the tendon, and a surrounding paratenon or, in some sites, a synovial sheath. At rest the collagen has a wavy, crimped pattern that straightens out under the first part of a load.

The collagen in tendons runs highly parallel, suited to the single direction of muscle pull, whereas in ligaments it is a little less ordered, to resist load coming from several directions across a joint. Where a tendon or ligament meets bone, the enthesis, the attachment may be fibrous or fibrocartilaginous; a fibrocartilaginous enthesis passes through a transitional zone of unmineralised and then mineralised fibrocartilage, a gradual change that spreads load but is a common site of pathology.

The way these tissues respond to load is captured by the stress-strain curve. The first, toe region is the crimp straightening out, which happens easily and at low stiffness. In the linear region that follows, the collagen itself is stretched and the tissue is stiffer, and as a rough guide it behaves elastically up to around four per cent strain, returning to its original length when the load is removed. Beyond that, fibres begin to fail, with rupture at roughly eight to ten per cent, though these thresholds are illustrative and vary with the tendon or ligament, age, disease and loading rate.

These tissues are also viscoelastic, so their behaviour depends on the rate and history of loading. They show creep, a gradual lengthening under a constant load, stress relaxation, a gradual fall in tension when held at a constant length, and hysteresis, an energy loss between loading and unloading. Tendons in particular exploit their elasticity to store and return energy, acting like springs, as the Achilles does with each stride in running, which makes movement more economical.

Vascularity varies by tissue and region, and some areas, such as the mid-portion of the Achilles, are relatively hypovascular, which may contribute to their slower healing. Tendons and ligaments also carry mechanoreceptors that contribute to joint position sense, which is why ligament injury impairs proprioception. When they are injured, healing runs through inflammation, proliferation and remodelling; it is slow and commonly lays down structurally inferior scar, with initially weaker type III collagen, that may never fully restore the original structure and mechanics. Given appropriate, progressive load, however, healthy tendons adapt, and depending on the loading stimulus this may alter collagen turnover, stiffness, modulus or cross-sectional area so the tendon can tolerate more force; this capacity to remodel in response to loading is the basis of loading-based rehabilitation. Compared with muscle, these tissues have a modest blood supply and a slow metabolism, so both adaptation and healing take months rather than weeks and have to be progressed patiently.

Tendon and ligament problems are among the commonest presentations in sport. Tendinopathy is persistent load-related pain, often with thickening and impaired function, of a tendon such as the Achilles or patellar tendon; its contributors are multifactorial, and because these tissues remodel slowly it responds to progressive loading rather than rest. A tendon can also rupture suddenly, as at the Achilles, and a ligament sprain follows a force that stresses a joint beyond its normal range, causing pain, swelling and, in higher grades, instability, as with an ankle or anterior cruciate ligament injury. Because ligaments carry position sense, their injury blunts proprioception, so balance and neuromuscular retraining sit alongside strengthening in rehabilitation.

• •Tendons join muscle to bone; ligaments join bone to bone; both are mainly type I collagen.
• •Tendon hierarchy: collagen, fibril, fibre, fascicle, tendon, with a crimped resting pattern.
• •The toe region of the stress-strain curve is crimp straightening; the tissue is elastic to a few per cent strain.
• •Tendons and ligaments are viscoelastic, showing creep, stress relaxation and hysteresis.
• •Vascularity is region specific; healing is slow and often leaves structurally inferior scar.
• •Tendons adapt to load, the basis of progressive loading in rehabilitation.