The diagnostic value of contrast-enhanced magnetic resonance imaging in the detection of experimentally induced anular tears in sheep.
METHODS
An investigation of the visualization of experimental anular tears using contrast-enhanced magnetic resonance imaging.


OBJECTIVE
To investigate how different kinds of experimentally induced anular tears can be visualized on contrast-enhanced magnetic resonance imaging.


BACKGROUND
Because the outer part of the anulus is innervated, tears of this part of disc are considered one cause for lumbar back pain. Moreover, clinical and experimental studies suggest that anular injuries may lead to a progressive degeneration of the entire disc. In the human disc, vascularized anular tears associated with disc degeneration can be visualized with contrast-enhanced magnetic resonance imaging, but acute peripheral anular injuries, probably caused by sudden trauma, have not been studied with this method.


METHODS
Two adjacent lumbar discs in adult sheep (n = 11) were injured with a scalpel blade. The L2-L3 discs were injured superficially, whereas in the L3-L4 discs, the incision reached the nucleus pulposus (full-thickness injury). In seven animals, only a stab incision was made to the disc, and in four animals, a small fragment (5 x 2 x 3 mm) of anulus was cut and removed. The animals were killed 3 weeks (acute injury, n = 5) and 3 months (subacute injury, n = 6) after surgery. Five minutes before death, gadolinium-diethylenetriaminepentaacetic acid was injected intravenously. After death, the whole lumbar spines were excised and 1.5-T high-field magnetic resonance imaging was immediately performed. Thereafter, the disc samples were examined histologically to determine the existence of blood capillaries.


RESULTS
In all injured discs, the injured area was macroscopically visible. Histologically, blood capillaries, lamellar destruction, and granulation tissue were clearly seen in every injured anulus. Contrast-enhanced magnetic resonance imaging showed that the superficial injuries were only occasionally visible in magnetic resonance imaging (3 of 11), whereas the full-thickness injuries were visible in a majority of the discs (8 of 11). In magnetic resonance imaging, the size of the injury did not relate to the enhancement intensity. The subacute injuries, particularly the full-thickness injuries, were more often visualized than the acute ones.


CONCLUSIONS
Even though macroscopically visible and histologically evident, it was not always possible to demonstrate experimental anulus injuries by contrast-enhanced magnetic resonance imaging. This experimental study shows that further research work is needed to develop more sensitive methods to detect peripheral, relatively small, but probably clinically important disc injuries.
Lappalainen
Anu K
AK
Faculty of Veterinary Medicine, Department of Clinical Veterinary Sciences, University of Helsinki, Finland. anu.k.lappalainen@helsinki.fi
Kääpä
Eeva
E
Lamminen
Antti
A
Laitinen
Outi M
OM
Grönblad
Mats
M
D000818:Animals; D003287:Contrast Media / Q000493:pharmacokinetics; D019786:Gadolinium DTPA; D007089:Image Enhancement / Q000379:methods / Q000592:standards; D007403:Intervertebral Disc / Q000293:injuries*; D008159:Lumbar Vertebrae / Q000293:injuries*; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000592:standards; D012680:Sensitivity and Specificity; D012756:Sheep; D013124:Spinal Injuries / Q000175:diagnosis*
D003287:Contrast Media; D019786:Gadolinium DTPA