Bursitis Causing Your Knee Pain? The bursa is fluid-filled sac located between bone, muscle, tendons, and skin; with the purpose of reducing rubbing and friction in these regions. Excess stress or injury to the knee joint will cause muscles and connective tissue to tighten, thereby constricting and squeezing the bursa sac. As a result, knee bursitis (irritation of the bursa sac) will develop.
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