The spine has several important functions including load transmission, permission of limited motion, and protection of the spinal cord. The vertebrae form functional spinal units, which represent the smallest segment that has characteristics of the entire spinal column. Discs and paired facet joints within each functional unit form a three-joint complex between which loads are transmitted. Surrounding the spinal motion segment are ligaments, composed of elastin and collagen, and joint capsules which restrict motion to within normal limits. Ligaments have variable strengths and act via different lever arm lengths to contribute to spinal stability. As a consequence of the longer moment arm from the spinous process to the instantaneous axis of rotation, inherently weaker ligaments (interspinous and supraspinous) are able to provide resistance to excessive flexion. Degenerative processes of the spine are a normal result of aging and occur on a spectrum. During the second decade of life, the intervertebral disc demonstrates histologic evidence of nucleus pulposus degradation caused by reduced end plate blood supply. As disc height decreases, the functional unit is capable of an increased range of axial rotation which subjects the posterior facet capsules to greater mechanical loads. A concurrent change in load transmission across the end plates and translation of the instantaneous axis of rotation further increase the degenerative processes at adjacent structures. The behavior of the functional unit is impacted by these processes and is reflected by changes in the stress-strain relationship. Back pain and other clinical symptoms may occur as a result of the biomechanical alterations of degeneration.
Aging ; Arm ; Axis, Cervical Vertebra ; Back Pain ; Capsules ; Collagen ; Elastin ; Intervertebral Disc ; Joint Capsule ; Ligaments ; Spinal Cord ; Spine ; Zygapophyseal Joint

Lumbar spinal steroid injections (LSSI) are universally used as preferred diagnostic or therapeutic treatment options before major spinal surgeries. Some recent studies have reported higher risks of surgical-site infection (SSI) for spinal surgeries performed after injections, while others have overlooked such associations. The purpose of this study is to systematically review the literature and perform a meta-analysis to evaluate the associations between preoperative LSSI and postoperative infection following subsequent lumbar decompression and fusion procedures. Three databases, namely PubMed, Scopus, and Cochrane Library, were searched for relevant studies that reported the association of spinal surgery SSI with spinal injections. After the comprehensive sequential screening of the titles, abstracts, and full articles, nine studies were included in a systematic review, and eight studies were included in the meta-analysis. Studies were critically appraised for bias using the validated MINOR (methodological index for non-randomized studies) score. The odds ratio (OR) and 95% confidence interval (CI) were calculated. Subgroup analysis was performed according to the time between LSSI and surgery and the type of lumbar spine surgery. Meta-analysis showed that preoperative LSSI within 30 days of lumbar spine surgery was associated with significantly higher postoperative infection compared with the control group (OR,1.79; 95% CI, 1.08–2.96). Based on subgroup analysis, lumbar spine fusion surgery within 30 days of preoperative LSSI was associated with significantly high-infection rates (OR, 2.67; 95% CI, 2.12–3.35), while no association was found between preoperative LSSI and postoperative infection for lumbar spine decompression surgeries. In summary, given the absence of high-level studies in the literature, careful clinical interpretation of the results should be performed. The overall risk of SSI was slightly higher if the spinal surgery was performed within 30 days after LSSIs. The risk was higher for lumbar fusion cases but not for decompression-only procedures.