Causes / typical injury mechanism: Total knee arthroplasty (TKA) remains one of the most successful arthroplasty procedures, with reports in literature of 90% survivorship at 15-20 years. Although great advances have been made in the field of joint reconstruction, aseptic loosening after TKA remains a common cause for revision surgery.

Classic history and presentation: Early in the course of post-TKA aseptic loosening, patients might not report any pain at all; however, with progression of this pathology, patients will report atraumatic onset of pain localized to the replaced joint and surrounding tissues with weight-bearing and activity (usually painless at rest). The patient may report classic start-up pain, which is described as pain during the beginning of ambulation that may improve temporarily but then increase with longer periods of ambulation.

Prevalence:

• Age – No age bias.
• Sex / gender – No predilection toward any gender.

Risk factors:

• Polyethylene implant composition (early conventional polyethylene was found to be less wear resistant compared to newer ultra-high molecular weight and highly crosslinked polyethylene liners).
• Older polyethylene sterilization methods in oxygen-rich environments led to decreased wear resistance.
• Unpolished tibial trays can lead to backside of the polyethylene liner.
• Presence of debris in joint causing third-body wear.
• High activity level of the patient.
• Component malposition at index surgery, leading to preferential and abnormal loading pattern.
• TKA instability.

Pathophysiology: The microscopic polyethylene wear particles activate the body's macrophages. Activated macrophages recruit further macrophages to the area, eliciting an inflammatory response and subsequent release of secondarily osteolytic (bone breakdown) factors such as tumor necrosis factor (TNF)-a and interleukin (IL)-6, which activate osteoclasts to resorb bone around the implant. Resorption of bone allows for subsequent micromotion (loosening) of the implant, which creates further debris, inciting further inflammatory response and further loosening.