A 55 years old gentleman presented to Accident and Emergency department with right knee pain and swelling after a twisting injury to the right knee which he sustained while he was playing golf. On examination he had joint effusion with restriction of the knee
extension and positive anterior drawer test.
The right knee radiograph showed joint effusion and tri-compartmental degenerative changes.
The MRI right knee was initially reported by a senior musculoskeletal resident who reported ACL rupture, tri-compartmental degenerative changes with posterior horn medial meniscal root avulsion tear. This meniscal tear was subsequently reported by a musculoskeletal consultant radiologist as air-fluid level within the degenerated meniscus in keeping with a vacuum phenomenon.
Background:
“Vacuum Phenomenon” (VP) is the term for a gas accumulation, mainly of nitrogen, in a closed joint which is liberated from surrounding tissues secondary to a negative pressure created within the joint after applying traction force [1-3]. This phenomenon is usually observed in patients with spondylosis and spinal disc degenerative disease on CT [3].
Resnick reported this gaseous collection in 2-3% in all patients and 20% of elderly patients with degenerative intervertebral disc disease [4]. However, this could also be seen in different spinal and intervertebral disc diseases such as vertebral osteomyelitis, Schmorl nodule formation, vertebral collapse with osteonecrosis [3]. A few cases of this phenomenon have also been reported in temporo-mandibular joint with or without any discernable disease [5, 6]. Sakamoto reported 1.3% (12 cases) VP within the total of 914 consecutive three Tesla MRI scans from 875 patients (524 men; mean age, 35 years). Half of the 12 cases resembled a meniscal tear. These were better appreciated on Gradient-recalled echo (GRE) localizer images, with 3D SPACE images being the next most effective [7]. Same appearances were also observed in a case report within the lateral compartment in a patient who sustained a lateral tibial plateau fracture [8].
Imaging Perspective:
VP can be seen on the radiograph as an air density but more readily depicted on the CT with gas attenuating values ranging from -800 to -400 Hounsfield Units [5, 9]. On MRI, the gas collection appears as low signal on both T1 and T2 weighted sequences, similar to MRI signal of calcium deposition and fibrous tissue which makes the diagnosis of this phenomenon challenging.
Outcome:
Vacuum phenomenon is rather a presentation of an underlying disease or condition such as degeneration or trauma. Therefore, the treatment should be directed towards the underlying cause of this phenomenon.
Teaching Point:
VP within the knee is a rare entity which could be seen in both traumatic and non-traumatic scenarios such as degeneration. This phenomenon could mimic meniscal tear; therefore, radiologists should be aware of its radiological presentation in order to avoid erroneous radiological diagnosis and subsequently unnecessary surgical intervention.
Medial meniscus vacuum phenomenon with ACL rupture
Based on the X-ray and MRI images provided by the patient, degenerative changes can be observed in the right knee joint's cartilage surface, with slight narrowing of the joint space. On MRI, a linear low-signal area is visible in the posterior horn of the medial meniscus. This linear low signal is consistently low on all routine sequences, suggesting a possible “vacuum phenomenon” artifact rather than a true tear. Joint effusion is also noted, correlating with the patient’s clinical presentation of swelling and effusion. Moreover, given the patient’s positive anterior drawer test, an anterior cruciate ligament (ACL) injury should be considered. However, the provided MRI sequences do not show a definite complete tear, indicating the possibility of partial fiber injury or sprain.
Taking into account the patient’s age, history of twisting injury, the distribution of the low-signal artifact on the X-ray and MRI, and the mild degenerative changes on X-ray, the most likely diagnosis is:
Vacuum Phenomenon
This phenomenon is relatively uncommon in the knee but can occur in degenerative conditions or after external traction, and it can be easily confused with a meniscal tear. The current imaging findings do not support a definite complete meniscal tear, but the possibility of a small partial tear or partial ligament injury cannot be fully excluded. If symptoms persist or worsen, further clinical evaluation is recommended.
The vacuum phenomenon itself does not require targeted treatment; management focuses on the underlying condition (e.g., degenerative changes, trauma, ligament sprain). Based on this, the following measures are recommended:
Throughout the rehabilitation process, closely monitor knee pain, swelling, and range of motion. If symptoms worsen, discontinue exercising immediately and seek medical evaluation to rule out new injuries or potential complications.
Disclaimer: This report provides a reference assessment based on the available imaging and clinical information and does not replace a professional, in-person medical diagnosis or treatment recommendation. Please consult a licensed physician or specialist for a specific treatment plan.
Medial meniscus vacuum phenomenon with ACL rupture
