It is important for spine surgeons to be aware of the relative prevalence of SNs. Although most lesions appear to be asymptomatic, certain SNs can be symptomatic, causing back pain. It is known that next to upper respiratory tract infections, lower back pain is the most frequently diagnosed condition among all patients visiting the hospital, and that a remarkable 85 % of them are due to unexplained causes [37
]. As reported by Williams et al. [3
], multiple SNs are significantly associated with lumbar disc disease. Hence, it is possible that SNs may account for some of these unexplained cases of back pain, given that they are often considered to be incidental findings. It is, therefore, important for spine surgeons to appreciate the relationship between lower back pain and SNs, as they can be the source of significant lower back pain requiring surgical intervention when conservative methods fail [9
The pathogenesis of SNs is still a subject of debate. Given the evidence discussed in this manuscript, an axial load model for the development of SNs is more likely to be independently capable of causing the development of SNs in the majority of cases. It has been reported that axial forces may be capable of soliciting sufficient counteracting turgor pressure within the NP that can act on the cartilaginous endplate, causing deformity and herniation [39
The evidence in support of an axial trauma model is quite robust. However, certain preexisting conditions could also facilitate the ease with which herniation occurs due to axial forces. Hence, a combination of the models can be likely explanations for a particular presentation of SN. For example, a developmental insult can result in an indentation in the vertebral endplate into which NP herniation can readily occur if sufficient axial force is experienced or accumulated over time. Similar arguments can be made in favor of the pathological and degenerative models.
Hilton et al. [5
] proposed a strictly developmental pathogenesis for SNs. If this was true, one may expect to find SNs equally distributed in all regions of the vertebrae and not so highly localized to the thoracolumbar spine. In their study of postmortem thoracolumbar spines, they found no relationship between age and SNs. They, therefore, rejected a degenerative disease model and proposed a developmental/embryogenic model, arguing that SNs are already present during skeletal maturation; hence, the lack of a difference between specimens from their subject pool over and under the age of 50 years. To explain why SNs are not equally distributed in the spine, Hilton and colleagues [5
] proposed that the thoracolumbar spine is under greater stress than other regions of the vertebral column, and, therefore, is predisposed to SN formation caused by these developmental insults.
To the contrary, Vernon-Roberts et al. [10
] found SNs to be more prevalent in older spines (over 30 years) than younger ones, supporting the degenerative model. It is possible that the 20-year gap between the cut-off ages used in these studies affected the outcomes. Further studies are necessary to determine the point at which this discrepancy in SNs between old and young spines occurs, if at all.
A number of strategies to alleviate the symptoms of SNs have been reported in the literature. The quality of evidence for each of these treatment options is low, with no randomized studies. Of the four treatment modalities presented in this paper, the intervention with the strongest evidence of efficacy and utility to the patient was fusion surgery. Of the 21 cases of painful SNs reported by Peng et al. [30
], fusion surgery was reported to completely attenuate the pain due to SNs in all but three of their patients, who also had a reduction but incomplete relief of pain. Hasegawa et al. [31
] reported similar results in a case report. Fusion surgery was the only treatment modality found in the literature in which complete disappearance of pain was reported in a substantial majority of cases. Nevertheless, these results should be viewed in the context that the studies involved a small number of patients without a comparison group.