[Ligament instability in total knee arthroplasty--causal analysis].

Ligament instability is a common reason for revision total knee arthroplasty (TKA). A meticulous analysis of the type of instability is very important in order to revise such a knee successfully. The objective of this study was to analyze the different reasons for ligament instability in revision TKA. A total of 135 knee revisions performed by one surgeon were analyzed pre- (clinical and x-ray) and intraoperatively for the cause of failure. X-ray analysis included the assessment of each component for position. Intraopertive analysis included stability testing in extension and 30 degrees , and 90 degrees of flexion, wear pattern, patella motion (shifting and tilting) and patella height. In 32.6 % of all cases, ligament instability was the primary reason for revision. In another 21.6%, ligament instability was identified as a secondary reason for revision. Analysis of the different instability forms showed combined instability in extension and flexion as the most common cause, followed by isolated instability in flexion (31.8%) and isolated instability in extension (9.1%). The high correlation between instability and malpositioning of the prostheses was obvious. Often, an isolated femoral malposition, in particular for rotation, was found, as well as an isolated malposition of the tibia component. In summary, ligament instability is a common reason for revision TKA. Many different forms of instability can be found either as isolated or combined instability types. Correct anatomical positioning of the components and balanced ligaments in the different extension and flexion positions are important for good clinical results, a stable joint, good function and longevity.