The most likely cause is patellofemoral pain syndrome or a maltracking knee cap.It occurs because the knee cap tracks or moves to the outside of the knee.The knee cap travels down a groove in your thigh bone (the femur ) and if the knee cap tracks laterally there is a potential for irritation , rubbing or even flaking or divoting of the cartilage surface of the two bones.
The reason this occurs commonly with moving up and down stairs is that the knee cap tracks closer to the femur and there is a greater chance of maltracking.
Biomechanics ie.the position of the knee cap in the groove, a tight hamstring or iliotibial band , weak quadricep muscles , especially the inside quadriceps and tight ligaments around the knee cap all contribute to producing pain , inflammation and occasionally cracking noises called crepitus in the knee.
The good news is that this is a condition that our Physiotherapists can help you with. In the absence of any more serious underlying pathology we have a very good record in addressing these types of injuries. In the meantime refer to our website under the tab Conditions , then knee and patellofemoral pain .Contact us on (03) 9670 3996 if this is something that happens to you and it is not getting better.