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Objective We used NexGen? Legacy? Constrained Condylar Knee (CCK) prosthesis as primary Total Knee Arthroplasy (TKA) in severe knee deformity and to compare its post operative outcomes in terms of clinical, radiological, functional situation, pain relief, and patient satisfaction with respect to expectation; as compared to pre operative status. Materials and Method We performed a retrospective study of 46 (48 knees) patients of a mean age of 61 years with severe knee deformity who underwent TKA with NexGen? Legacy? CCK prosthesis during the period of December 2007 and February 2012. The indications of use CCK prosthesis were severe valgus with incompetent medial collateral ligament in 34 knees, severe flexion contracture with inability to achieve knee balancing in flexion and extension by posterior soft tissue release in 11 knees, Charcot arthritis with severe varus and bone loss in 2 knees, traumatic osteoarthritis with severe varus and ligamentous instability in 1 knee. The mean duration of follow-up was 71 months (range 40–90 months). The New Knee Society scoring (NKSS) system and the Hospital for Special Surgery (HSS) score were used as the tool to study the functional and clinical situation. Patients satisfaction, patients expectation, and respective scores were considered for analysis. Visual Analogue Scale (VAS) was used for pain. Factors affecting patients satisfaction were analyzed separately. We assessed the radiological situation using Knee Society criteria. Results For the total 48 knees, 1 case of loosening because of the short-stem tibial component at 3 months postoperatively underwent revision, the 6-year survival was 97.9%. There were no components infection occurred within 6 years. Parameters of NKSS and HSS system showed significant change postoperatively in comparison to their preoperative scores. The NKSS score improved from 30.52±14.22 to 89.69±8.37 from preoperative score to the mean 71 months follow-up. The HSS score improved from 35.95±15.74 to 71.40±25.16. Patient satisfaction was recorded to be improved significantly from 8.38±4.015 to 37.88±4.646 along with significant improvement in patient expectation from 10.31±1.662 to 14.31±1.138. Pain score was decreased from 6.29±2.25 to 1.21±1.09. Total functional score was improved from 31.46±11.43 to 86.42±8.87, range of motion (ROM) from 42.42°±23.57° to 95.31°±23.45°, the flexion contracture from 5.31° ± 7.87° to 0.92° ± 1.80°. Preoperative radiographic study showed excessive valgus (≥7°) in 37 knees, varus deformity in 2 knees, the postoperative femorotibial alignment was valgus 3.88° ±1.76° in 48 knees. A/P radiographic review showed there were 4 radiolucent lines (RLL) in 48 tibial components. Conclusion CCK TKA is a satisfactory modality for the treatment of the severe unstable primary knee that cannot be balanced by soft tissue. It is comparable to other widely studied prostheses designs with respect to functional regain, pain relief, improvement in ROM, and patient satisfaction.