Arthrofibrosis is suspected when the normal postoperative pain, restricted motion and inflammation do not settle at predictable time points as they would do normally. In some patients, the knee may do well for the first few weeks and then deteriorates. The diagnosis of arthrofibrosis is mainly clinical after excluding all other causes of stiffness following total knee replacement. There are no specific laboratory tests available for routine use to diagnose arthrofibrosis. Radiology in knee arthrofibrosis following total knee replacement is useful in excluding other causes of stiffness.
The X-Rays (plain radiographs) may show useful findings like bone formation in the soft tissues (heterotopic ossification) and patella situated lower than its normal location (patella baja). Other than these, there are no specific radiological findings suggestive of arthrofibrosis following total knee replacement.
Ultrasound examination can show thickening and formation of new blood vessels (neovascularity) of the innermost layer of the knee capsule (synovial thickening) and the fat behind the tendon of knee cap (Hoffa’s fat pad).
CT scan with or without angiogram can be useful in patients with heterotopic ossification, especially if it is situated behind the knee (posteriorly) to asses its proximity to nerve and blood vessels.
MRI with optimized conventional pulse sequences and advanced metal artifact reduction techniques can afford improved visualization of bone, implant and soft tissue for diagnosis and treatment planning. Typical findings are best demonstrated on proton-density and STIR images. Scar tissue demonstrates low signal intensity on T1 weighted images and inhomogeneous intermediate-to-low signal intensity (dark / black lesions) on T2 weighted images (both fat-suppressed and non-fat-suppressed). Scar tissue as well as new bone formation in soft tissue appear as dark (intermediate to low signal intensity) on all MRI pulse sequences.
Bone scan may show diffusely increased uptake.
Biopsy of soft tissue (histopathology) shows high density of scar forming cells (fibroblasts) which may help in diagnosis and classification of arthrofibrosis into different grades of severity. One author suggested a special type of biopsy technique [indirect immunohistochemistry for β-catenin-positive fibroblasts (myofibroblasts)] is useful in diagnosis of incipient arthrofibrosis.
All these tests described above are not used routinely. As mentioned earlier, the diagnosis of arthrofibrosis is mainly clinical after excluding all other causes of stiffness following total knee replacement.