Old tendons injure more easily. That, we know. What we didn’t know—but do now, thanks to a new study from researchers in the U.K.—is what, within the aging tendon itself, weakens. The scientists have learned that it has much to do with a stretchy, sticky, weblike structure within the tendon that loses its resistance over time. That new discovery, they say, could help lead to more targeted treatment to battle tendon problems in old horses.

“Tendons are a bit like a rope, with lots of separate strong strands (fascicles), which are held together by a surrounding soft, sticky material,” called the interfascicular matrix, said Hazel Screen, CEng, MIMechE, MIPEM, professor of biomedical engineering and chair of the Queen Mary University of London School of Engineering and Materials Science Division of Bioengineering and Biomaterials.

This sticky matrix acts as a support system for the fascicles to allow them to slide alongside each other and coil back up after a “load”—an energetic use of the tendon—is removed. Screen said injury risk increases when that supporting matrix starts to break down.

“We have discovered that many of the problems we see in old or injured tendons are not associated with the strong fascicle strands, but with surrounding interfascicular matrix,” she said. “We have seen that it becomes less stretchy and less elastic as a horse or human ages, and that stops the tendon from being able to stretch and recoil so effectively, meaning it gets injured more easily.”

Comparing Tendons

The research group also detected a distinct difference in the effects of aging between two kinds of tendons: “active” tendons—what scientists call the “energy storing” tendons—and “passive” ones—what they call the “positional” tendons. Energy storing tendons, such as the superficial digital flexor tendon (SDFT) in the back of the lower leg, receive high forces and work like elastic springs during exercise. Positional tendons, such as the common digital extensor tendon (CDET) in the front of the lower leg, serve mainly to stabilize and keep the leg in position.

Screen’s team found that the energy storing tendons’ matrix, and even the fascicles, lose their ability to resist fatigue and use as they age, whereas the positional tendons’ matrix and fascicles don’t. That knowledge could help scientists better understand what weakens tendons, she said, by comparing the two kinds.

Screen and her fellow researchers examined SDFTs and CDETs from eight horses euthanized for reasons other than leg injuries. Four of the horses were classified as young (between 3 and 7 years old), and the other four were considered old (between 17 and 20 years). The scientists put the freshly extracted tendons to mechanical laboratory testing, loading them with machines until the tendon “failed,” or ruptured. They counted the number of loading cycles it took for each tendon to fail and compared the results.

They found that old SDFTs failed much sooner than young ones—at, on average, 65% fewer cycles to failure, Screen said. By contrast, they didn’t detect significant differences in cycles to failure with the positional CDETs, young or old.

The team didn’t investigate the effects of exercise on these horses, however. How much the horses had already used their tendons could affect their resistance levels, but that was outside the scope of the study.

Nonetheless, Screen said her experience allows her to speculate: “The horse that has exercised throughout life will have used that interfascicular matrix for healthy tendon function consistently,” she said. “The cells in the body are good at responding to use by renewing tissues, so the interfascicular matrix in this tendon is likely to be better renewed and maintained. However, the interfascicular matrix will also have been subjected to a lot more use over the life time of the horse, and all materials (tissues are no exception) become damaged with use.

“By contrast, if a horse has had less use of the tendon, and the interfascicular matrix has been less heavily loaded (just walking or trotting in a field) there is less stimulus to the cells to encourage renewal and repair of that material and keep it healthy,” she continued. “Certainly if you suddenly starting asking such a horse to exercise heavily, I would expect a high injury risk, as the interfascicular matrix had not had time to adapt.”

Other Factors

Genetics and other factors probably also play a role in tissue makeup and its ability to deal with loading, Screen added.

“Our data points to the idea that the interfascicular matrix is a highly active, continually maintained, and renewed region of tendon, which responds to the loading (use) stimulus applied,” she said. “Certainly our data agree with the current thinking that if you suddenly change the exercise requirements on a horse, you are at risk of injury, but we think this is likely to be focused in the interfascicular matrix, with damage and a cellular response all initiating here.”

Further research based on these findings could lead to more targeted imaging of the matrix for better diagnostics and follow-up, Screen said.

The study, “Fascicles and the interfascicular matrix show decreased fatigue life with aging in energy storing tendons,” was published in Acta Biomaterialia.