Over the last few decades, the importance of the sagittal plane and its contour has gained significant recognition. Through full-body stereoradiography, the understanding of compensatory mechanisms, and the concept of global balance and reciprocal change has expanded. There have been a few reports describing how cervical realignment surgery affects global spinal alignment (GSA) and global balance. Despite the research efforts, the concept of reciprocal change and global balance is still perplexing. Understanding the compensatory status and main drivers of deformity in a patient is vital because the compensatory mechanisms may resolve reciprocally following cervical realignment surgery. A meticulous preoperative evaluation of the whole-body alignment, including the pelvis and lower extremities, is paramount to appreciate optimal GSA in the correction of spinal malalignment. This study aims to summarize relevant literature on the reciprocal changes in the whole body caused by cervical realignment surgery and review recent perspectives regarding cervical compensatory mechanisms.
Over the last few decades, the importance of the sagittal plane and its contour has gained significant recognition. Through full-body stereoradiography, the understanding of compensatory mechanisms in patients with spinopelvic imbalance has progressed rapidly, and the concept of global balance and reciprocal change has expanded to the field of the cervical spine and lower extremities [
The essential function of global spinal alignment (GSA) is the maintenance of global balance, an upright posture, and a horizontal gaze [
Understanding the compensatory status of a patient is vital because the compensatory mechanisms may resolve reciprocally following cervical realignment surgery which correlates with improved patient outcome [
It is critical to understand the compensatory mechanisms and global balance of the spine beforehand as reciprocal change is a dynamic phenomenon. The sagittal balance reflects the spine’s shape, allowing individuals to maintain a standing position with minimal muscle force. The spine adapts to different changes in order to stay in balance. The normal aging process induces truncal stooping [
In patients with spinal pathologies, compensatory mechanisms from the thoracolumbar to the cervical spine and lower extremities occur in a staged fashion to maintain horizontal gaze and global balance [
For an overall assessment of a patient with spinal imbalance, radiography of the entire spine with a standardized position (hands resting on collar bones) is mandatory [
Global balance can be determined by the position of the GL, defined as a plumb line from the center of the acoustic meatus (CAM) [
In patients with spinal deformity, initial compensatory mechanisms usually initiate adjacent to the deformity. The next adjacent segments will be subsequently recruited after the exhaustion of adjacent compensatory reservoir to maintain an erect posture and balance [
After cervical realignment surgery, reciprocal changes ensue. If the alignment is inadequate, it intensifies the compensatory mechanisms. When realignment surgery is adequately performed, it often leads to the relaxation of compensatory mechanisms [
Traditionally, a balanced spine is defined as whether spinopelvic parameters, including the C7PL, are adequate. The C7PL was used to measure sagittal trunk balance as a virtual COG rather than GL, as it is generally concordant with GL in general populations and is a pragmatic tool to estimate sagittal trunk balance [
Only a handful of research has been conducted regarding how cervical realignment surgery reciprocally affects GSA [
The primary goal of cervical realignment surgery is to achieve OT concordance [
A significant chain of correlation has been demonstrated in asymptomatic subjects between C0–2 angle, C2–7 angle, and T1S [
Lee et al. [
To understand the clinical role of the odontoid parameters, we analyzed the correlation between patient-reported health-related quality of life and odontoid parameters in patients who underwent a multilevel posterior cervical fusion. First of all, the postoperative NDI showed a significant correlation with both OT (r = -0.37, p < 0.05) and OI (r = -0.40, p < 0.05). Secondly, a cutoff value of 20° for the T1S-CL corresponds to OT of 0° in a linear regression model (r2 = 0.702, p < 0.001). Lastly, a significant correlation between OI and ROM of both C1–2 (r = 0.37, p < 0.05) and C0–2 (r = 0.46, p < 0.01) has been observed.
Based on these results, we postulated that depending on OI of a patient, the clinical impact of anterior tilting of the dens may differ as the resulting OT is distinct [
Reciprocal changes following cervical realignment surgery in CD patients exhibit different patterns depending on whether they have an adequate compensatory reservoir in the thoracolumbar spine [
The cervical spine is still one of the most understudied and least understood parts of the spine. It is crucial to identify the drivers of CD and each compensatory mechanism connected with the deformity. Analyzing the compensatory mechanisms such as C0–2 hyperlordosis, posterior thoracolumbar malalignment, or thoracic hypokyphosis in isolation can be mistaken for a surgical indication or even a sign of deformity. Spine surgeons should recognize and accurately address the regional drivers of the deformities for optimal treatment. A meticulous preoperative evaluation of the whole-body alignment, including the pelvis and lower extremities, is paramount to appreciate optimal GSA in the correction of spinal malalignment. This study adds to the literature by advocating whole-body analysis for all CD patients. The proverb, “Do not miss the forest for the trees.” is helpful to understand the malalignment of the spine. Furthermore, it has been challenging for spine surgeons and researchers to predict reciprocal changes following realignment surgery. Expanding our ability to not only simulate postoperative alignment of the fused segments but also methodically and systematically predict reciprocal changes in the unfused segments is crucial. A future approach to CD needs to take reciprocal changes in the thoracolumbar spine, as well as the cervical spine to provide optimal planning of realignment surgery and achieve ideal cervical alignment.
The authors have nothing to disclose.
This study received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Conceptualization: JKL, SJH; Data curation: JKL; Formal analysis: JKL, SJH; Funding acquisition: SJH; Methodology: JKL; Project administration: SJH; Visualization: JKL, SJH, SHY, KJK; Writing - original draft: JKL; Writing - review & editing: JKL, SJH, SHY, KJK.
Compensatory mechanisms for age-related progressive kyphosis. This schematic illustration demonstrates the same person aging from left to right. When he ages, lumbar lordosis decreases, which results in a combination of different compensatory mechanisms to maintain horizontal gaze and global balance. TK, thoracic kyphosis.
Normative offset distances between bony landmarks and the gravity line. Positive values denote locations anterior to the gravity line and negative values indicate locations posterior to the gravity line. CAM, center of the acoustic meatus; CI, confidence interval.
Compensatory mechanisms in patients with symptomatic cervical kyphosis. PL, plumb line; CK, cervical kyphosis; LL, lumbar lordosis; PI, pelvic incidence; TK, thoracic kyphosis.
Schematic drawings illustrating the different spatial orientations of the dens with an identical C2 slope and different odontoid incidence values. (A) A dens with a straight curvature is conducive to a small odontoid incidence, prone to anterior tilting of the center of the dens. (B) A dens with a lordotic curvature can maintain the center of the dens more posteriorly.
Schematic drawing of the odontoid parameters. (A) Odontoid incidence (OI): the angle between the line perpendicular to the C2 endplate at its midpoint and the line connecting this point to the center of the odontoid process (the center of a circle with an anterior/posterior border and the apex of the dens as a tangent). Odontoid tilt: the angle created by a line running from the C2 endplate midpoint to the center of the odontoid process and the vertical axis (VRL) C2 slope: the angle between the C2 endplate and a horizontal line (HRL). (B) Inverse illustration demonstrating similarity with the pelvic parameters.
(A) Compensated cervical kyphosis patient with the COG PL located on the femoral head, but the C7 PL located markedly posteriorly. Cervical malalignment was corrected to achieve global sagittal balance and OT concordance. However, the lower-extremity alignment parameters did not change significantly. (B) Decompensated cervical kyphosis patient with the COG PL located markedly anteriorly, but the C7 PL located on the femoral head. Sagittal PLs indicated cervical sagittal imbalance and OT discordance before surgery. No significant changes were found in lumbopelvic alignment or the lower-extremity alignment parameters. COG, center of gravity; PL, plumb line; OT, occiput-trunk.