Total knee arthroplasty (TKA) instability is a common cause of failure of TKA and accounts for a high percentage of revision surgeries. TKA instability can be a result of an unrecognized ligamentous injury during the index surgery, a traumatic event, or more commonly, component malposition or malalignment during the index procedure.

Classic history and presentation: Patients will typically present subtlety with pain or instability that may be described as "buckling" events, recurrent knee effusions, or periarticular tendonitis. Patients may describe a total joint that "doesn't feel right" or feels unstable. Rarely, patients might present acutely with a prosthetic knee dislocation. However, true symptomatology will likely exist on a spectrum between the scenarios above.

Prevalence:

• Age – No bias
• Sex / gender – No bias

Risk factors: Risk factors for instability include previous revision surgeries, implant malposition, connective tissue disorders, extreme preoperative deformity, hip / foot deformities, obesity, inflammatory arthropathies, ligamentous deficiency / injury, or extensor mechanism injury.

Pathophysiology: The pathophysiology of this presentation stems from the biomechanical interaction between the implanted components and the patient's remaining native tissues. When there is an alignment mismatch in the axial, coronal, or sagittal planes between the remaining ligamentous / bony structures and the implanted components, then instability can result. For late instability, wearing of the components can lead to gradual malalignment and subsequent failure / instability.

Classification: A true validated classification system is not globally utilized; however, the type and timing of instability is typically discussed in terms of the following: early instability, late instability, extension instability, flexion instability, mid-flexion instability, and global instability.