Patients undergoing total knee replacement surgery experience severe post-operative pain. Continuous femoral nerve block combined with selective tibial nerve block provides optimal pain control in the anterior and posterior aspect of the knee respectively. However, in patients with pre-existing neuropathy of the sciatic nerve, the blockade of tibial nerve is contraindicated because of increased risk of nerve damage that may lead to exacerbation of neurologic symptoms. In such patients where selective tibial nerve block cannot be offered, ultrasound guided posterior knee capsular injection may be an alternative technique for controlling posterior knee pain after surgery. Posterior capsular injection by surgeons intraoperatively, when combined with femoral nerve block has been shown to provide pain control after total knee replacement. With posterior capsular injection, the main trunk of the tibial nerve is spared and only the terminal nerve endings innervating the posterior knee joint is blocked providing effective pain control. In this study, the efficacy of ultrasound guided posterior capsular injections performed pre-operatively will be compared to tibial nerve block in controlling knee pain after surgery.