dip of the knees

例句：

1. 1.She looked down at her knee and noticed a dip in the skin.
   她低头看着膝盖，注意到皮肤上有一个凹陷。
2. 2. He had a deep crease on his knee, which he didn't notice until he sat down.
   他的膝盖上有一道深深的皱纹，他坐下后才注意到。
3. 3. The old woman smiled as she remembered the time when she used to have knee creases like this.
   这位老妇人微笑着回忆起过去她有这样膝盖皱纹的时候。