Arthrofibrosis after Knee Replacement – Causes and Development of Disease:

1. Causes (Etiology) of Arthrofibrosis after 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.

Painful knee conditions like neuroma of the infrapatellar branch of the saphenous nerve, complex regional pain syndrome and kinesiophobia as well as other psychological variables can cause knee pain and reversible knee stiffness after total knee arthroplasty (pseudoarthrofibrosis). In such cases, the pain and stiffness resolve when the cause of pain is adequately treated.

2. Development (Pathogenesis) of Arthrofibrosis after 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 tissues in and around the knee  joint (synovium, capsule and periarticular soft tissues like quadriceps muscle). Sometimes, there is transformation of soft tissue into cartilage (metaplastic fibrocartilage formation – chondrometaplasia) and bone (heterotopic ossification). 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.

Arthrofibrosis following knee replacement can be “localized arthrofibrosis” involving only a part of the knee joint (Eg: between the patellar tendon and tibia) or “diffuse or generalized arthrofibrosis” involving the whole joint. Similarly, the scar tissue can form within the knee joint capsule (Intra-articular fibrosis) or can involve structures outside the knee joint capsule (Extra-articular fibrosis) like quadriceps tendon.

Some physicians restrict the use of the term arthrofibrosis only for patients who develop excessive scarring without a known cause (idiopathic arthrofibrosis). Some physicians use the term “primary arthrofibrosis” for idiopathic arthrofibrosis and use the term “secondary arthrofibrosis” for patients who develop excessive scaring and stiffness due to a specific cause.