An accurate and sensitive imaging method for diagnosing arthrofibrosis and identifying the affected tissues is urgently needed. Due to the current lack of diagnostic biomarkers the diagnosis of arthrofibrosis has been based on non-specific symptoms, particularly a loss of range of motion (ROM) in the joint, and pain. However, a significant amount of scar tissue can be present in a joint and have minimal impact on ROM. After infection is ruled out (or treated), tests for inflammation can be useful, together with imaging.
An ultrasound-guided injection of anaesthetic into the infrapatellar fat pad (IFP) is a useful method to determine the extent of IFP involvement as a pain generator. In our experience when a knee is painful and fibrotic IFP is typically inflamed due to repeated scissoring in the joint.
Test for Inflammation
Following a diagnosis of arthrofibrosis there should be a determination of the degree of inflammation present. Traditional inflammatory markers such as CRP and ESR are typically not sensitive enough in arthrofibrosis.
Where indicated, an inflammatory cytokine panel blood test is useful for guiding treatment, particularly TNF-α, IL-1 and IL-6. Inflammatory cytokines are proteins the body makes that play an important role in inflammation. Testing levels of systemic inflammatory cytokines, including IL-1, IL-6, TNF-a is important for understanding an individual’s immune status, and which treatments are appropriate. Companies that offer these tests include ARUP Laboratories and the Mayo Institute in the US, and NutriPATH in Australia. Abnormal test results may indicate that a rheumatologist should be consulted.
Other blood tests for a range of autoimmune diseases are also recommended.
Test for Infection
Where inflammation is active needle aspiration of synovial fluid is necessary to conduct PCR testing for infection, with or without culture tests. Culture-based tests are not sufficiently sensitive to detect all types of infection.
In cases of prior injury or surgical procedure it is important to test the synovial fluid in the affected joint for infection. Synovial fluid is extracted by needle aspiration of the joint, often after flushing with sterile saline solution. Infections tests are particularly important for people with a prosthesis or metal “hardware” that microorganisms can adhere to.
Chronic low-grade infections often do not have the classic symptoms of redness, heat and swelling that are typical of acute infections, and may be missed by surgeons. This, together with the need for highly sensitive infection tests, means that chronic low-grade infections are notoriously difficult to diagnose. The traditional culture test is not adequately sensitive.
In addition to a culture test a molecular test called a PCR and sequencing should be performed on the synovial fluid. This test is more sensitive for identifying low numbers of bacteria that may be present in the joint. Next generation sequencing is a more advanced and powerful molecular test that is capable of identifying multiple microorganisms if these are present.
Identification of microorganisms using molecular techniques permits the appropriate antibiotic (or other medication) to be prescribed.
It is also necessary to rule out infection prior to taking anti-inflammatory medications, since these increase the risk of infection.
At present, the most effective imaging methods for arthrofibrosis include magnetic resonance imaging (MRI), ultrasound and PET/CT. X-rays and CT (without a radioactive tracer) are not effective for imaging scar tissue but can detect badly distorted soft tissues, such as patella baja. X-ray and CT may be used to rule out mechanical issues such as a loose prosthesis. However, the usefulness of these methods is limited where a metal prosthesis is present due to imaging artefacts
Sagittal MRI is the most commonly used technique to image fibrosis and inflammation of the IFP . While MRI is helpful, radiologists may lack the necessary training to report on the presence of scar tissue in early arthrofibrosis. Due to image artefacts from metal special methods are required to image joints containing a prosthesis, however, these methods are now widely available in modern imaging centres.
In our experience, where a prosthesis is present ultrasound can be used to image arthrofibrosis, providing the operator is experienced. Imaging tissues in the middle of knees, such as the deep infrapatellar fat pad, is difficult but is often possible.
Positron emission tomography–computed tomography (PET/CT) is a nuclear medicine imaging technique. Using a radioactive tracer for inflammation can accurately image inflammation in a joint. Since inflammation can cause arthrofibrosis this method may be useful. However, the cause of inflammation can’t be determined, and a positive signal can be caused by non-fibrotic conditions, so this method is not diagnostic. A bone scan can also detect regions of active bone remodelling following surgery such as a joint replacement. This may detect regions of active remodelling after healing should be complete, indicating abnormal healing that may arise from arthrofibrosis.
Dragoo, J. L., Johnson, C. & McConnell, J. Evaluation and Treatment of Disorders of the Infrapatellar Fat Pad. Sports Med 42, 51-67, doi:10.2165/11595680-000000000-00000 (2012).
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