Instability is one of the leading causes of clinical faiure after total knee arthroplasty. Instability can be categorized according to four type: extension instabiity, flexion instability, genu recurvatum and global instabi;ity. Basically flexion and extenion gap should be equal. And also medial and lateral gap should be equal balance. we should know basic concepts, the effect of the ligament or capsular structure release. And also surgeon should understand of the nine gap- balancing permutaiion that can occur during revision TKA. After bony mechnical and rotational alignment correction, flexion gap correction first then adjust extension gap methode will be easier to adjust ligament balancing. Joint line elevation should be avoid if possible because this can lead to mid-flexion instability, decreased range of motion soft tissue impingement or anterior knee pain associated with patella infera.

Varus/valgus constrained components should be considered only in the presence of adequate inherent or to stabilize the knee until a ligament repair or reconstruction heal. In a situation of severe varus/valgus, or gobal instability where the knee cannot be stabilized other than through the implant, use of a rotating hinge or linked component is advocated.