Why is it that when a tendon tears or breaks it is difficult to repair both physiologically and surgically?

Answer:
Because tendons and ligaments are essentially only colagen fibers, with very little active cells that can repair them inside.
And since they are flexible, the two ends of a severed tendon will not easily touch one another to allow for healing.

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If they tear away from the bone, they have to be re-attached ( touching ) in order to heal. They will heal, but it takes a long time. I had mine stapled back to make contact with the bone, and several months later they have re-attached.

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