Balance and constraint in revision TKR: A classification for instability management.

Instability is an increasingly common cause and symptom of failure of Total Knee Arthroplasty (TKA). Patients seek 'Functional Stability', which is the sum of both a balanced joint and, if necessary, mechanical constraint. The objective of this paper is to classify the different types of TKA instability and their causes. Based on this classification, the authors give methodical recommendations for instability management. INSTABILITY CLASSIFICATION: Instability in revision TKA can be classified into 3 types based on the management of bone loss and ligamentous deficiency which directs the level of constraint required to achieve functional stability. TYPE 1: Bone deficiency: Revision with restoration of joint line and rebuilding the bony anatomy results in a balanced joint. No increased constraint is needed. TYPE 2: Ligament and soft tissue deficiency: Requires increased constraint to overcome instability. TYPE 3: Composite (Total) deficiency: (combined Type 1 and 2).The multiple causes of instability are outlined for each Instability type along with an algorithm for restoring the joint line and adding titrated constraint to restore functional stability.