Can tendonitis bi-lateral cause a serious injury to the knee 27 years later after previous injury has completely healed?

Tendinitis: Tendinitis is not a severe injury even though it can feel like it. Treatment depends on location and cause.

Not necessarily: No. But the problem previously could have been a precursor to your problem now. Overuse tendinitis can be from being overweight or over active on your patellar tendon. Later you can develop arthritis in your knee, especially under your kneecap. Reducing your weight, avoiding jumping, running, squatting and kneeling, wearing good cushioned arch supported shoes should help.