So what active biological process is causing an ongoing low grade inflammation around the intervertebral disc?
Believe it or not, the culprit behind all this may be a chronic bacterial infection – Proprionicbacterium acnes, which is found on the surface of the skin and within the oral cavity.
But how would the bacteria find their way into the centre of a contained structure such as the intervertebral disc? The first possible avenue is that P.acnes colonises within the intervertebral discs via the circulation (blood) system. As P.acnes resides on the skin or in the oral cavity, it may invade the circulatory system when skin or oral mucosa is broken.
Figure 1: Anatomical structures of the intervertebral disc
The second avenue is that specific immune cells, such as macrophages engulf P.acnes in the blood, transport them into intervertebral discs via the circulatory system, and release the bacterium upon cell death. More importantly, many macrophages will settle around the outer part of the disc after herniation, some of whichay contain devoured P.acnes. Therefore, macrophages may act as transport for P.acnes into the intervertebral disc. Because P.acnes has difficulty surviving and reproducing under aerobic conditions of blood, and because the outer portion of the disc is highly vascularised, the patient, in the past, must have ruptured the outer annulus fibrosis. It is interesting to note that P.acnes was found in the discs in 1/3 patients with visible annular tear, but not seen in the discs without annular tear, suggesting that the breach in outer annulus is a key determinant for P.acnes accessing the disc.
Once P.acnes has established itself within the anaerobic nucleus, inflammation in the adjacent bone may be “a secondary effect due to cytokine production or microbial metabolites (e.g. propionic acid) entering the vertebra through normal disc nutrition.” Importantly, these patients do not otherwise show signs of systemic illness or the classic evidence of infection (such as fever). ‘Propionibacterium acnes bacteria secrete propionic acid, which has the capacity to dissolve fatty bone marrow and bone. Over time it appears that P. acnes stimulates and perpetuates chronic inflammation within the adjacent bone and this is strongly associated with chronic lower back pain and Type 1 Modic (see Part 1 of this blog) changes seen on MRI. Furthermore, the presence of Modic changes have been associated with a significant increase in the rate of disc degeneration.
In the third and final part of our blog series, we will discuss how we can detect, assess and treat P.acnes.