I would first like to discuss the various factors which may be responsible.
· Limited range of motion of before surgery
· Underlying diagnosis like rheumatoid arthritis which can cause decreased range of motion after surgery
· If the knee has been operated before replacement
Intraoperative factors .Inadvertent mistakes can be committed during surgery
· Mistake in flexion-extension gap balancing
· Oversizing or malpositioning of components
· Decreased femoral or tibial resection
· Inadequate resection of posterior osteophytes
· Arthrofibrosis that is formation of excessive scar tissue
· Reflex sympathetic dystrophy or complex regional pain syndrome
· Heterotopic ossification that is excessive new bone formation leading to block of joint motion
· Inadequate physical therapy post surgery and decreased motivation from patient
Treatment depends upon the cause
If there is a mistake committed during surgery causing stiffness , revision surgery may be required.
Otherwise intensive physical therapy should be employed initially. If it fails manipulation under anesthesia may be required. If stiffness persists even after these arthroscopic or modified open release of adhesions in the knee joint should be considered. Surgical release of adhesions should be considered only if more than 3 months have passed since surgery.In your case as more than 6 months have passed you may benefit by surgical release.You need to discuss this possibility with your doctor.
Infection should be ruled out and if an infection is found to be present than primary focus should be control of infection.
In case knee has developed RSD or CRPS pain management specialist should be involved in care.Since you have pain only on bending more than 90 degrees, this is unlikely to be the cause in your case.
I suggest that you discuss the issue with your surgeon with the information I have provided and plan out the treatment you may require for this in consultation with him.