CORR Insights®: What Are the Medium-term Reciprocal Changes in Cervical Sagittal Alignment After Posterior Correction for Lenke 5C Adolescent Idiopathic Scoliosis?

Where Are We Now? The sagittal alignment of the spine is crucial to treatment outcomes because of its relationship with truncal balance and quality of life [3]. A harmonious spinal alignment is natural and provides the functional stability for most upright daily activities. Our understanding of proper sagittal alignment primarily comes from adult deformity surgery, with insights extending to the cervical spine in recent years [2]. The cervical spine, with its highly dynamic nature, is influenced by the degree of thoracic kyphosis and lumbar lordosis. In cases of forward sagittal balance due to loss of lumbar lordosis, cervical hyperlordosis may occur to maintain a horizontal gaze. After fusion surgery, spinal alignment adjusts based on the nonfused part of the spine, and cervical sagittal alignment adapts to maintain a horizontal gaze. Surgical correction of sagittal malalignment in patients with adult spinal deformity may result in transient postoperative cervical kyphosis. Sudden correction of the global kyphotic alignment will pull the body’s axis posteriorly. To compensate, cervical hyperlordosis will reduce or even become kyphotic. Lee et al. [9] showed that drastic lumbar lordosis correction after T10-pelvis fusions may reach up to 15.3° of change in cervical sagittal alignment. However, as the nonfused thoracic spine adapts with more kyphosis because of a new lumbar alignment, cervical kyphosis also decreases. The impact of cervical alignment on disability in the presence of thoracolumbar scoliosis remains unclear. Increased neck disability is associated with a cervical spine fused in a positive sagittal alignment [12]. Despite reduced cervical spine lordosis with increasing thoracic lordosis, there seems to be no effect on health-related quality of life parameters in patients with adult idiopathic scoliosis (AIS) and nonfused cervical spines [1]. With aging, the adult spine and pelvis are more prone to imbalance and decompensation, leading to poorer quality of life. Global balance is often not a concern for adolescents as they have yet to experience degeneration and pain because of their compensatory mechanisms. Nevertheless, it is important to consider alignment goals in fusion surgery for patients with AIS as improper sagittal alignment restoration may lead to future problems in adulthood. In AIS, the relationship between cervical alignment and the thoracic and lumbar spine resembles that of adults. Thoracic scoliosis often presents with reduced kyphosis in the sagittal plane [7, 10], leading to a loss of cervical lordosis or even kyphosis [8]. Realignment of the thoracic spine can spontaneously restore cervical lordosis. With increased thoracic kyphosis after fusion surgery for AIS, a similar degree of improvement in cervical lordosis is often achieved [6]. Up to 60% of the gains in thoracic kyphosis are transferred to gains in lordosis of the distal cervical spine. Translation of effects to the cervical spine occurs irrespective of the degree of coronal rotation as seen from posterior correction and spinal fusion of Lenke Type 6C curves [11], which are those with major thoracolumbar/lumbar curve and structural main thoracic curve. Correction of the thoracic and lumbar sagittal alignment tends to reduce cervical kyphosis. In this month’s Clinical Orthopaedics and Related Research®, Zhao et al. [13] studied the relationships between Lenke 5C curves (a major thoracolumbar/lumbar curve and a nonstructural main thoracic curve) undergoing fusion surgery and their outcomes in terms of cervical sagittal alignment. Among the patients with cervical kyphosis preoperatively, the authors observed an overall increase in cervical lordosis after surgery with a mean difference of 4°. The changes were seen mainly in the preexisting thoracic hypokyphosis group. In the preoperative normokyphotic thoracic spine, correction did not change the cervical alignment. This shows that changes in thoracic kyphosis directly influence postoperative cervical alignment change. The study highlights a close dynamic relationship between cervical, thoracic, and lumbar sagittal alignment. With only surgical realignment of the lumbar spine, compensatory changes occur in both the cervical and thoracic spine. Surgeons should take note of any preexisting sagittal hypokyphosis and cervical kyphosis. Lumbar sagittal realignment may result in a similar degree of restoring thoracic kyphosis and cervical lordosis. This may have implications on early disc degeneration and future neck pain. Where Do We Need to Go? This study and previous research indicate that cervical alignment dynamically adjusts to thoracic and lumbar sagittal alignment. However, this relationship is only evident in the upright standing position, one of the many positions in which humans spend their time. Further investigation is needed to understand how dynamic changes occur with different postures [5]. Additionally, the clinical implications of the cervical response require further exploration. Attention should be given to the risks of proximal junctional kyphosis and postoperative pain associated with cervical kyphosis in the immediate postoperative stage. Changes in unfused segments occur when patients adapt to the standing posture after surgery. Studying dynamic alignment changes with variations in fusion alignment will provide insights into expected cervical outcomes and help establish alignment goals. Understanding the onset of disc degeneration and the risk of early degeneration and pain with different cervical alignments is a crucial clinical question. In addition to the uncertainties of thoracic realignment in AIS surgery, there are knowledge gaps concerning the development and prognosis of cervical malalignment. Bracing has been linked to flatback deformities in the thoracic and lumbar spine, suggesting potential for growth modulation during the pediatric growth spurt [4]. Further study is needed to determine whether interventions can halt the progression of these deformities. Investigating the onset of cervical kyphosis is also important, as it is unclear whether there are inherent risks associated with cervical kyphosis. How Do We Get There? The main objectives for future study include understanding how cervical alignment impacts long-term outcomes and how much sagittal correction should be achieved during AIS surgery. Long-term longitudinal follow-up of patients with or without cervical kyphosis and those who have had appropriate or inappropriate surgical correction is essential. These studies will identify the intraoperative thoracic and lumbar alignment targets to achieve appropriate cervical sagittal alignment. This alignment must be subjected to patient job requirements and lifestyle needs as standing posture should not be the sole reference. Preoperative cervical mobility should also be considered. To aid preoperative planning, a decision-making tool incorporating pelvic parameters, lumbar and thoracic alignment goals, and cervical sagittal alignment outcomes is necessary. There is a complex interplay of variables compounded by uncertainties surrounding growth. Alignment goals will need to be age-dependent, which is particularly challenging for prepubertal patients. Utilizing machine-learning algorithms and artificial intelligence may help us develop a tool for this purpose. Read This Next This retrospective review of 186 patients with adult spinal deformity demonstrated that cervical kyphosis is dynamic [9]. Read this paper to learn more about cervical spine alignment changes with Lenke 6C curves [11]. This prospective study outlines how curves and alignment change with different postures [5].