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Different structures of the patellofemoral joint containing nociceptors can be sources of pain. The nociceptive afferent nerve supply of the patellofemoral joint is illustrated by the substance-P and the free nerve endings type IVa. Pain results when structures are irritated, inflamed, damaged, or overused. The underlying pathophysiologic mechanisms causing pain are variable and consist of mechanical, neural, and chemical factors. They can be present in combination and cumulative.