Instability After Total Knee Arthroplasty

Instability after TKA is categorized by the arc of motion in which it occurs:

Flexion instability: The most common form, often underdiagnosed. The knee is stable in extension but collapses or bows backward when the patient tries to descend stairs or rise from a chair. Caused by a mismatch between the flexion gap (space at 90° of bend) and the extension gap — typically from overly aggressive posterior cruciate resection, too small a tibial insert, or excessive tibial slope. The patient may describe the knee as "unstable" without true dislocation.

Extension (global) instability: The knee buckles in near-full extension and with normal walking. Caused by collateral ligament incompetence, flexion/extension gap mismatch (both gaps too large), or component subsidence. Severe global instability may cause recurrent dislocation.

Asymmetric (coronal) instability: Varus or valgus instability from medial or lateral collateral ligament incompetence, component malposition, or residual malalignment. The knee buckles medially or laterally, especially on uneven ground.

Patellofemoral instability: Patellar tilt, subluxation, or maltracking causing anterior knee pain, popping, and occasional giving way. Caused by component malrotation (tibial or femoral external rotation), medialized tibial tubercle, or inadequate patellar resurfacing.

Additional contributing factors include:

• Component malposition: Tibial internal rotation is the most common rotational error and a leading cause of multiple instability patterns.
• Polyethylene wear: Progressive bearing surface loss reduces insert thickness and creates effective gap laxity over time.
• Ligament injury or failure during the original surgery or as a result of progressive wear.

Treatment options

Frequently Asked Questions