Locking of knee joint can be caused by -a) Osgood shalterb) Loose body in knee jointc) Tuberculosis of kneed) Medial meniscal partial tear
Pathological locking of knee joint

It has already been explained that physiological locking of knee occur in full extension. It is due to action of muscle and geometry of articular surfaces of the knee.
However some pathology of knee joint can cause locking of the knee, i.e., the knee is locked in partial flexion and there is inability to extend the knee for the last few degrees. This is frequently known as locking of the knee joint.
Locking occurs because the fragments (bone or cartilage) get nipped between the condyles of tibia and femur and do not allow the extension : Knee is held in flexion.
Causes of locking are : -


Meniscal tear
Osteochondral fracture
Loose body in the knee
Osteophytes fracture in osteoarthritis
Fractures tibial spine fragment                                                                                                                                         


Hamstring spasm also does not allow the knee to get extended. However, this is not true locking as the pathology is outside the knee joint. Therefore, it is known as pseudolocking.
c
d
bd
ac