Sentence:  Once , then - for how many years or how few does not matter - my world was bound round by fences , when I was too small to reach the apple tree bough , to twist my <WSD>knee</WSD> over it and pull myself up . 
Meaning of the word marked with <WSD> tags:  hinge joint in the human leg connecting the tibia and fibula with the femur and protected in front by the patella