Anatomy
The bones of the knee joint include the femur (thigh bone), tibia (shin bone) and patella (kneecap). The joint cavity is lined with a very thin synovial membrane.
Knee joint:
A. Patella (Kneecap)
B. Tibiae (Shinbone)
C. Meniscus lateralis (Outer meniscus)
D. Femur (Femur)
Cause of the problem
Twisting and other trauma to the knee joint releases various ‘inflammatory substances’ (inflammatory proteins), triggering an ‘inflammation’ where the synovial membrane (synovialis) thickens and produces fluid, causing the joint to swell (Nieboer MF, et al. 2023). In some cases, the fluid oozes into the back of the knee (Baker’s cyst).
Symptoms
Swelling of the joint. Pain when moving the knee joint. Difficulty bending the knee fully.
Examination
Joint swelling should be assessed by a professional. Diagnosis is usually made by general clinical examination where damage to other structures (ligaments, menisci) cannot be detected. Small accumulations in the knee and popliteal fossa (Baker’s cysts) can only be detected by ultrasound
See ultrasound scan here: Baker’s cyst in the popliteal fossa with fluid and thickened synovium (mucosa) with increased vessel growth (Doppler activity).
If the joint swelling occurs after an acute injury and a diagnosis cannot be made, the knee should be re-examined 2 weeks later to avoid overlooking an ACL injury, which can be difficult to diagnose in the acute stage.
A Baker’s cyst is a symptom that something has happened to the knee that causes too much synovial fluid to be produced. It is, of course, crucial to rule out a treatable knee injury as the cause of the joint swelling and/or Baker’s cyst.
Treatment
Relief. If the swelling does not subside despite relief, this can be supplemented with medical treatment in the form of arthritis pills (NSAIDs) or injection of adrenal cortex hormone into the joint, possibly preceded by drainage of the synovial fluid and Baker’s cyst (Fredberg U, Bolvig L. 2001).
Usually the Baker’s cyst communicates with the knee joint and thin, yellowish synovial fluid can be discharged from the Baker’s cyst. On an ultrasound scan, the injected adrenal cortex hormone can be seen to distribute itself in both the knee joint and the Baker’s cyst.
If gelatinous fluid is discharged from the Baker’s cyst, the communication to the knee joint is probably interrupted and it is therefore necessary to drain and possibly inject both the knee joint and Baker’s cyst.
Complications
If the progress is not smooth, you should consider whether the diagnosis is correct or if there are complications.
In particular, the following should be considered:
