What causes pain at the front of the knee?

Dr Sarah Jarvis MBE Housemaid's knee is also known as prepatellar bursitis. It is caused by inflammation of a small fluid-filled sac (the bursa) in front of the kneecap. It more commonly occurs in people who spend long periods of time kneeling. Treatment is usually supportive only and the outlook is generally very good. https://patient.info/health/knee-pain-patellofemoral-pain/housemaids-knee-prepatellar-bursitis