Causes of Arthrofibrosis (Etiology) following Total Knee Replacement:
The cause of the arthrofibrosis following total knee replacement remains unclear. The following factors have been proposed by various investigators as potential risk factors but the literature is controversial about many of these factors. These factors include genetic predisposition for formation of scar tissue following injury or surgery, immune response (T cell mediated), metal allergy, abnormal new bone formation in soft tissues (heterotopic ossification) of knee or other sites, female gender, African Americans, younger patients (< 45 years), obesity, diabetes mellitus, high blood pressure (hypertension), smoking / narcotic drug abuse (opioid), psychological issues like depression, unmotivated patients, inflammatory arthritis like rheumatoid arthritis, fibromyalgia and hemophilia.
Development of Arthrofibrosis (Pathogenesis) following Total Knee Replacement:
Scarring is part of normal wound healing response after any surgery. Like any other surgery, total knee replacement heals with some amount of scarring. Not all scar tissue are symptomatic. Only when the scar tissue is excessive and leads to restriction of motion, the diagnosis of arthrofibrosis is considered.
The molecular steps and the mediators involved in the development of arthrofibrosis are unclear. Various theories have been proposed.
There is chronic inflammation leading to excessive scar tissue formation (fibrosis) and thickening of the soft tissue in and around the knee (synovium, capsule and periarticular soft tissues like quadricpes muscle). Sometimes, there is transformation of soft tissue into cartilage (metaplastic fibrocartilage formation – chondrometaplasia) and bone (heterotopic ossification). Arthrofibrosis following knee replacement can be localized involving only part of a joint (Eg: between the patellar tendon and tibia) or diffuse (generalized) involving the whole joint. The severity of bone formation can range from small deposits of calcium (punctate calcification) to complete bridging of knee with bone (complete bony ankylosis).
Arthrofibrosis affects 1.2% to 17% of the knees after the first total knee replacement. Heterotopic ossification affects 0.9% to 26% of the knees after total knee replacement.
Causes of Stiffness following Total Knee Replacement:
In addition to arthrofibrosis and heterotopic ossification of the knee, stiffness following total knee replacement can arise from many other reasons. These causes need to be excluded before making a diagnosis of arthrofibrosis and treatment planning.