Update on pathology and surgical treatment for adult spinal deformity.

Update on pathologies of adult spinal deformity (ASD): With advancement of aging society, ASD has become one of the most notable topics of spinal disorders owing to its significant impact on health related quality of life. Treatment for ASD is challenging due to complex nature of deformity and high prevalence of comorbidities. Spino-pelvic harmony that is evaluated by pelvic incidence (PI) minus lumbar lordosis (LL) is the most important concept, which allows us to understand pathology of ASD more deeply. Proposed optimum "PI minus LL" is within ±10°. However, according to analysis of patients having good surgical outcomes, minimum requirement of postoperative "PI minus LL" is calculated by following equation: "PI minus LL" = 0.41PI - 11.12 (r = 0.45, p = 0.0059). "PI minus LL" is not fixed but flexible value reflecting the specific setting of the individual PI. To date, little is known about dynamic global sagittal alignment that is susceptible to compensatory mechanisms. Gait analysis revealed that compensated sagittal balance by pelvic retroversion in static standing was lost immediately after walking due to alignment change of the pelvis and worsened over time. Dynamic assessment of sagittal balance is recommended. Update on surgical strategies for ASD: We classified ASD into following 5 types in terms of curve patterns, global balance, and curve flexibility: Type 1, well-balanced scoliosis with flexible kyphosis is indicated for corrective posterior spinal fusion (PSF) without any release procedures; Type 2, poor-balanced scoliosis with flexible kyphosis is well corrected by aggressive intervertebral release with PSF; Type 3, fixed sagittal imbalance without coronal deformity is candidate for pedicle subtraction osteotomy; Type 4, fixed sagittal imbalance with coronal deformity is indicated for vertebral column resection; and Type 5, severe scoliosis without marked global sagittal malalignment can be treated by corrective anterior spinal fusion. Minimally invasive lateral access surgery can be solution for reduction of surgical morbidity.