Instability is a common complication of total knee replacements, accounting for 21.2% of revision surgeries.9 Instability is generally viewed as a complication of surgical technique resulting from a variety of factors, including:

  • Ligamentous imbalance and incompetence
  • Malalignment of prosthesis
  • Deficient extensor mechanism
  • Inadequate prosthetic design or patient unsuitable for implant design
  • Patient may require various levels of constraint such as cruciate substituting implant
  • Surgical error

There are several types and classifications:

  • Patellofemoral
  • Medial and lateral
  • Tibiofemoral

Although frequently a clinical diagnosis, radiographs may demonstrate various types of instability. Subtle rotational malalignment of components requires CT for more accurate analysis.

Tibiofemoral instability due to incompetent posterior cruciate ligament with dislocated total knee replacement.

Medial instability with marked valgus alignment.

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