There are many different possible causes of joint stiffness after total knee replacement (TKR). The most common one is stiffness and loss of motion before the surgery. The quadriceps muscle along the front of the thigh and knee gets tight or contracted. This type of extension contracture keeps the knee from bending all the way.
A second problem is body type. Short, stocky patients often have problems regaining their full motion. The situation is made worse if the person is obese with thick thighs preventing the leg from bending more.
Patients who don't do their rehab program can also end up with a stiff, contracted joint. In other cases there are technical reasons for this kind of result. If the implant is too large for the patient or the plastic insert is too thick, motion is limited.
And finally, sometimes we just don't know what causes the problem. Some patients just form an excess amount of scar tissue, a condition called arthrofibrosis. In such cases, further surgery and intense rehab are needed to restore and maintain motion.