Sentence:  At first the patient felt stronger , and the hemoglobin rose to 13.8 gm. , but on Oct. 20 , 1958 , he complained of `` caving in '' in his <WSD>knees</WSD> . 
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