pathogenetic reasons for Modic changes was published, a bacterial, a mechanical, and a rheumatological cause. Bacteria as a cause The bacterial cause is where the development of Modic change is due to an infection in the adjacent disc. A tear in the outer fibers of a disc may well initiate this infection. A disc herniation will result in a new local capillarization around the extruded material as the body attempts to remove the extruded material and inflammation with macrophages which are present in abundance. The mucous membrane of the gums is slightly damaged during ordinary tooth brushing, and anaerobic bacteria Propionibacterium acnes type 2 from the mouth enters the bloodstream. The anaerobic bacteria can enter the disc through the breach, causing a low virulent and slowly developing infection. Since the disc is an avascular structure, it is an ideal environment for the growth of anaerobic bacteria. Propionibacterium acnes bacteria secrete propionic acid, which can dissolve fatty bone marrow and bone. Therefore, it was hypothesized that diffusion of propionic acid from the disc into the vertebrae results in bone oedema, i.e. Modic changes. The first study by Stirling and Lambert found Propionibacterium acnes in the extruded disc material in patients undergoing surgery inspired the bacterial theory. The discovery has led to many studies trying to identify bacteria in the disc material. The methods used in the studies to determine the presence of bacteria vary significantly from very primitive methods such as simple culturing to highly sophisticated methods, where independent research groups utilizing staining techniques with fluorescence in situ hybridization were able to visualize bacteria from an ongoing infection as well as associated inflammatory cells. These studies were carried out in real time and therefore demonstrated an ongoing bacterial infection. The results of these studies have been reproduced in several systematic reviews, concluding that 30-34 % of patients with a disc herniation have bacteria in the extruded disc material. Mechanical Mechanical Modic changes might initially reflect bleeding, oedema, and vascularization following trauma or oedema associated with the repair process after micro-fractures within the endplate and the vertebral bone. Another possibility is that the toxic nucleus tissue (from the center of the discs) invades the endplate and vertebral bone through fractures in the endplates and causes an inflammatory response. It may be nucleus material entering the vertebrae, but Crock et al. also suggested that after a disc is damaged, irritating substances are produced,