Constrained total knee prostheses are used in knees with severe deformities and insufficiency of collaterals to provide stable and mobile knees.Dislocation after constrained knee prosthesis is an extremely rare and dreaded complication.When dislocation is associated with patellar tendon rupture,the management includes restoration of the extensor apparatus along with a stable knee.Repair of the patellar tendon is challenging due to poor soft tissue coverage in the area and a bulky repair can put tension on the wound closure.Ideal method of restoration of the extensor apparatus is a matter of debate.There are various modalities used ranging from primary end-to-end repair,augmentation by medial gastrocnemius flap,semitendinosus and synthetic implants and allograft tendoachilles.We report a rare case of a posterior dislocation of a constrained total knee arthroplasty in association with patellar tendon rupture due to a minor fall after a few weeks of surgery.The first episode was managed by reposition of the dislocation and V-Y plasty of the quadriceps and primary repair.The second episode of dislocation with re-rupture needed augmentation by semitendinosus along with the insertion of the thicker insert.The management of this complex problem along with the review of literature is discussed in this case report.