Clinical Correlations to Specific Phenotypes and Measurements With Classification Systems: Lumbosacral Spine

Adult spinal deformity

Adult spinal deformity (ASD) has conventionally been defined as aberrant curvature or alignment of the spinal vertebrae. While the vast majority of spinal deformity falls into the category of degenerative scoliosis, there are frequent incidences of ASD in patients with persistent adolescent idiopathic scoliosis. As such, increased precedence is being placed on examining other, atypical deformities as novel research continues to unravel the complex, heterogeneous nature of the various disorders that fall within the diagnosis. In 2006, the SRS and Schwab classifications were developed to address such issues and have subsequently been combined in routine clinical use. The SRS-Schwab classification leverages full-spine (free-standing) radiographs to assess the hip joints and femoral heads in relation to a patient’s underlying deformity. In 2008, Kuntz described the CKIV Classification, which utilized some of the parameters in the SRS classification, such as thoracic kyphosis (TK) and lumbar lordosis (LL), but placed precedence on developing an age-dependent system that was further subcategorized by abnormal characteristics (location, severity, pattern) as well as overall global spinal alignment ( Table 1 ). Finally, in 2012, the SRS-Schwab classification was modified to include updated pelvic parameters, such as pelvic tilt (PT), pelvic incidence-lumbar lordosis (PI-LL) mismatch, and Cobb angle, as these factors had been previously correlated with clinical outcomes, such as disability, pain, and quality of life scores ( Table 2 ).

The currently used classification systems allow for an understanding of the impact of disease, mostly by leveraging radiographic findings, yet fail to accurately prognosticate the likelihood of complication rates. The most commonly utilized system is the SRS classification, and while it provides pertinent information regarding previously unconsidered factors (pelvic tilt, incidence, etc.) it fails to take into consideration other potentially relevant factors (comorbidities, treatment, alignment, and patient goals). Perhaps the most commonly utilized system is the pelvic incidence/lumbar lordosis (PI-LL) mismatch calculation, which was described by the creators of the Schwab classification ( Fig. 1 ). The PI-LL mismatch is a quick, simple calculation that is commonly applied for evaluation and preoperative planning. Furthermore, the PI-LL mismatch is a diagnostic marker in other pathologies, such as adjacent segment disease after lumbar fusion or pelvic dissociation. A PI-LL mismatch < 10 degrees has traditionally been associated with improved postoperative health-related quality of life scores, though some recent evidence may suggest that differences up to 20 degrees may lead to acceptable outcomes as well.

Recent literature indicates the need to reevaluate our current parameters to identify associations with perioperative complication rates in ASD patients. While studies, such as Lafage et al., have described age-specific ASD classification systems, there still is ample opportunity to improve upon the current ASD schemes, as some researchers believe the patients’ symptomology should have a larger role in their classification and treatment. The current heterogeneity in the described classification systems at the time of the writing of this chapter further serves as evidence supporting the need for a more comprehensive system.

Lumbar facet arthropathy

Lumbar facet arthropathy (LFA) is a form of osteoarthritis that occurs in the lumbar zygapophysial joints. LFA is often portrayed as a result of degenerative processes, such as excessive loading or abnormal movement patterns, but may occur as a result of trauma as well. Generally speaking, LFA occurs as the synovium in the facet joint begins to fail, leading to erosions, decreased disc space, and subchondral osteosclerosis. LFA presents most commonly at L4-L5, though all levels of the lumbar vertebral column may be involved, and often results in complaints of chronic low back pain or radiculopathy due to spinal cord compression. Three classification systems are currently utilized to describe lumbar facet arthropathy ( Tables 3–5 ).

Despite expansive efforts concerning LFA classification schema development, there is no current consensus on any of these classifications. A study by Kettler et al. in 2006, which set kappa or ICC at > 0.40 (moderate agreement), was carried out to determine which system to recommend statistically for clinical use. Their efforts yielded two classifications that met their recommendation standards, namely the Pathria and Weishaupt systems. Additionally, they included the Grogan system, which is also utilized in clinical care, despite the that the classification did not meet study standards. As a whole, the agreement set on this study is low compared to the research community’s standards. The heterogeneity in classification systems as well as the lack of agreement at the time of the writing of this chapter necessitates further evaluation. Examination of which of the aforementioned systems correlates “better” with patient symptoms or clinical outcomes could be completed in a comparative analysis similar to that described by Kettler et al. in 2006.

Endplate degeneration

Modic changes (MC) are currently defined as signal changes in vertebral subchondral bone marrow present on magnetic resonance. The sentinel study by M.T. Modic et al., first described Modic changes in 1989 using T1-weighted sagittal MRI. These changes have been strongly associated with degenerative spinal pathologies, specifically degenerative disc disease. Type 1 MC has been correlated with low back pain (LBP) in numerous studies although its role in the pathogenies of LBP is under scrutiny. To date, three main types of MC have been delineated ( Fig. 2 ).

Currently, only Modic type 1 has been associated with low back pain in study populations, despite recent evidence the Modic type 2 changes may be more prevalent in individuals with greater fat mass. According to a study by Mitra et al., Modic changes represent a pathologic progression from acute (Modic type 1) to other chronic forms with Modic type 2 resulting in improvement of the patients’ symptoms. Overall, most hypotheses about the pathogenesis and role of Modic changes in degeneration are speculative, despite the classification’s widespread use in the research community. Continued analysis of the role of type 3 changes in spinal pathologies, with specific attention to the progression of degenerative etiologies, is necessary to elucidate the relationship with patient symptoms. Furthermore, it is important to note that recent studies have proposed a combined classification system that utilizes not only Modic changes, but Pfirrmann grading, high-intensity zones, and loss of disc height as well. Continued analysis of algorithmic approaches that leverage multiple classical scoring systems may provide better prognostic utility when evaluating the spine for degeneration.

Schmorl’s nodes

Schmorl’s nodes (SN) are a subclassification of herniation that results in the nucleus pulpous (NP) being pushed through the cartilage and endplate. In most cases, SN are asymptomatic findings on imaging modalities but may become painful over time. Despite numerous postulates as to the pathogenesis of SN (autoimmunity, trauma, degeneration, etc.), no single theory is currently accepted. Previous studies indicated SN is associated with the degeneration of the vertebral discs. With the wide variability of reported prevalence data, 3.8%–76%, precedence has been placed not only on examining the different etiologies of SN but on classifying it as well. Currently, at least one study has proposed a classification system. The classification is broken into five categories based on the contour, morphology, and typology of the lesion.

Despite continued research, such as that by Mok et al. which described the association between SN and disc degeneration, the clinical relevance of SN remains uncertain as the explanatory mechanism or pathogenesis to lower back pain has not been fully elucidated. Additionally, conflicting evidence about the progression of SN to disc degenerative symptomology exists, making prognostication inherently difficult. The initial results of the classification system described by Samartzis et al are promising with an excellent intraobserver (kappa = 0.88) and good interobserver reliability (mean kappa = 0.79). Despite the initial findings, it is important to note that the intraobserver reliability was calculated with a small observer sample size ( n = 2) and is a logical starting place for future analysis. Additionally, further studies investigating the prognostic utility and reliability of the aforementioned system as well as mechanistic evaluations of the pathophysiology of SN is necessary.