The vacuum phenomenon (VP) is classified as an abnormal accumulation of gas from the blood serum,
in any anatomical area related to the osteoarticular system.
It is predominantly composed of Ni with traces of O2 and CO2.
The mechanism of its appearance is the result of the transfer of gas from the blood to the surrounding soft tissue due to a negative pressure gradient that sucks gases from the blood stream.
It must be distinguished from other pathologies that generate and accumulate air with positive pressure,
as occurs in infections caused by gas-producing germs (Table 1) or after surgical interventions ( Fig. 1 ).
Table 1: Gas collection in osteoarticular system
Radiologic signs
|
PV
|
Infection
|
Homogeneity
|
Very homogeneous
|
Less homogeneous
|
Shape
|
Linear,
extended
|
Oval or irregular
|
Possible accompanying radiological findings
|
Fracture/callus,
degenerative changes
|
Gas in nearby tissues
|
Variation with posture
|
Increases in hyperextension
|
Does not change
|
Depending on the pathological significance and the affected area,
VP is classified into 4 groups:
A.
Intradiscal
B.
Intraosseous
C.
Intraarticular
D.
Others
A.
INTRADISCAL:
Is the most common subtype and it manifests as a horizontal air line in the disc space.
We can find a vertical component when air is displaced inside Schmorl nodules or through fractured vertebral plates .
Two subtypes are described:
-
Type 1 (Primary or degenerative): Dehydration and / or disc defects.
It is associated with senility and usually affects usually lumbar segments ( Fig. 2 ).
-
Type 2 (Secondary or traumatic): Collapse of the adjacent vertebra ( Fig. 3 ).
Greater variability in age range and location.
B.
INTRAOSSEOUS
According to its mechanism of appearance it is subclassified in:
-
Primary: It frequently appears inside vertebrae related to osteopenia and osteonecrosis (Kümmel disease).
Usually associated with pathologies such as DM,
alcoholism or chronic corticosteroid treatment.
A suction effect associated with the collapse of the pathological vertebral segment has been implicated ( Fig. 4 ).
-
Secondary: Associated with PV from a degenerate disc,
by migration of gas through an eroded platform into the vertebral body ( Fig. 5 ).
When the intraosseous gas appears surrounded by a thin sclerotic ridge it is referred to as pneumatocyst.
It is usually located juxtaarticular,
and in order of frequency has been described in the iliac bone,
sacrum,
humeral head,
distal third of the clavicle,
vertebral body (most frequently C6) and scapula.
The eventual visualization of air-fluid levels inside pneumatocyst suggests it could correspond to a subcortical geoda,
filled with gas that migrated from an adjacent intra articular VP,
although its pathophysiology continues to be uncertain ( Fig. 6).
C.
INTRAARTICULAR
C1.
PHYSIOLOGICAL:
Due to its peculiar pathophysiological mechanism,
it is a phenomenon that frequently appears as an incidental finding without a pathological significance ( Fig. 7 )
-
Joints in which movement involves distractive forces (gleno-humeral,
acromio-clavicular,
sternum-clavicular,
sacroiliac and pubis).
-
Other articulations with a wide range of movements (medial patellofemoral and femorotibial compartments of the knee,
ankle and temporo-mandibular joint).
C2.
PATHOLOGICAL:
The cause-effect relationship is not clearly established,
although it usually appears related to other pathologies including ( Fig. 8 ,
Fig. 9 ):
-
Degenerative.
-
Inflammatory.
-
Traumatic: distractive forces in unstable joints,
but not correlated to the degree,
severity or internal repercussion of the trauma.
VP has also been described in pseudoarthrosis for poorly consolidated fractures.
D) OTHERS (GAS EMIGRATION):
While VP typically occurs within areas of negative pressure there is frequent diffusion of the gas to surrounding structures after compression.
As examples of emigration,
it is common to observe intradiscal gas extending into the medullary canal through herniated discs,
and subcondral cysts and geodes with air content that comes from adjacent joints,
affected by arthritis or post inflammatory pathology.
Another well known finding is "reversible" VP seen in the spine,
which can be physiological or pathological according to its origin,
it is characterized by a cyclic filling and emptying of gas and / or serum,
depending on the degree of extension or flexion of the column ( Fig. 10 ).