the knee cap

Surgeon Geoffrey Horne placed a metal band around the knee cap to help it knit back together.

外科医生杰弗里·霍恩在膝盖骨上放了一个金属箍以帮助其愈合。

《柯林斯英汉双解大词典》

With the changes of the knee angles, the moving track of upper and lower edge point of the knee cap is similarly a parabola;

髌骨上、下缘点随膝角变化的运动轨迹类似于抛物线形状；

The person who thus addressed him was a bent and lame old man, dressed almost like a peasant, who wore on his left knee a leather knee-cap, whence hung a moderately large bell.

和他说话的是一个腰驼腿瘸的老人，穿的衣服几乎象个乡巴佬，左膝上绑着一条皮带，上面吊个相当大的铃铛。