The morphology of cervical deformities: a two-step cluster analysis to identify cervical deformity patterns

OBJECTIVE

Cervical deformity (CD) is difficult to define due to the high variability in normal cervical alignment based on postural- and thoracolumbar-driven changes to cervical alignment. The purpose of this study was to identify whether patterns of sagittal deformity could be established based on neutral and dynamic alignment, as shown on radiographs.

METHODS

This study is a retrospective review of a prospective, multicenter database of CD patients who underwent surgery from 2013 to 2015. Their radiographs were reviewed by 12 individuals using a consensus-based method to identify severe sagittal CD. Radiographic parameters correlating with health-related quality of life were introduced in a two-step cluster analysis (a combination of hierarchical cluster and k-means cluster) to identify patterns of sagittal deformity. A comparison of lateral and lateral extension radiographs between clusters was performed using an ANOVA in a post hoc analysis.

RESULTS

Overall, 75 patients were identified as having severe CD due to sagittal malalignment, and they formed the basis of this study. Their mean age was 64 years, their body mass index was 29 kg/m2, and 66% were female. There were significant correlations between focal alignment/flexibility of maximum kyphosis, cervical lordosis, and thoracic slope minus cervical lordosis (TS-CL) flexibility (r = 0.27, 0.31, and -0.36, respectively). Cluster analysis revealed 3 distinct groups based on alignment and flexibility. Group 1 (a pattern involving a flat neck with lack of compensation) had a large TS-CL mismatch despite flexibility in cervical lordosis; group 2 (a pattern involving focal deformity) had focal kyphosis between 2 adjacent levels but no large regional cervical kyphosis under the setting of a low T1 slope (T1S); and group 3 (a pattern involving a cervicothoracic deformity) had a very large T1S with a compensatory hyperlordosis of the cervical spine.

CONCLUSIONS

Three distinct patterns of CD were identified in this cohort: flat neck, focal deformity, and cervicothoracic deformity. One key element to understanding the difference between these groups was the alignment seen on extension radiographs. This information is a first step in developing a classification system that can guide the surgical treatment for CD and the choice of fusion level.

Cervical Spine Deformity Correction Techniques

Cervical kyphotic deformity can be a debilitating condition with symptoms ranging from mechanical neck pain, radiculopathy, and myelopathy to impaired swallowing and horizontal gaze. Surgical correction of cervical kyphosis has the potential to halt progression of neurological and clinical deterioration and even restore function. There are various operative approaches and deformity correction techniques. Choosing the optimal strategy is predicated on a fundamental understanding of spine biomechanics. Preoperative characterization of cervical malalignment, assessment of deformity rigidity, and defining postoperative clinical and radiographic objectives are paramount to formulating a surgical plan that balances clinical benefit with morbidity. This review of cervical deformity treatment provides an overview of the biomechanics of cervical kyphosis, radiographic classification, algorithm-based management, surgical techniques, and current surgical outcome studies.

Compensatory Mechanisms for Kyphotic Change in the Cervical Spine According to Alignment Analysis of the Cases after Anterior Cervical Corpectomy and Fusion

BACKGROUND

Compensatory mechanisms for cervical kyphosis are unclear. Few alignment analyses have targeted ongoing cervical kyphosis and detailed the effects of compensatory alignment changes.

METHODS

We analyzed the radiographic alignment parameters of 31 patients (21 men and 10 women) with postoperative kyphotic changes after anterior cervical corpectomy and fusion (ACCF) between 2006 and 2015. This analysis included lordotic angle of the fusion area, fusion area length, cervical lordosis angle (CL), O-C7 angle (O-C7a), and cervical sagittal vertical axis (cSVA) as basic parameters and occipito-C2 angle (O-C2a), adjacent cranial angle, adjacent caudal angle, and T1 slope as compensatory parameters at 2 time points after surgery.

RESULTS

Alignment analysis revealed that CL was significantly decreased by 5.0 ± 7.7° (P < 0.01) and O-C7a was changed by only -0.2 ± 6.8° (P = 0.75). An inverse correlation was found between ΔCL and ΔO-C2a (ρ = -0.40), with a nearly 1:1 relationship in the scatter diagram. ΔT1 slope had no direct compensatory correlation with ΔCL (P = 0.28) but was strongly correlated with ΔcSVA (ρ = 0.78). The scatter diagram of ΔcSVA and ΔT1 slope showed compensatory relevance and a shifted point to its collapse as the T1 slope lost control of cSVA; thereafter, both parameters incessantly increased, and ΔT1 and ΔcSVA became positive.

CONCLUSIONS

When CL decreased after ACCF, ΔO-C2 immediately compensated for the CL loss that could lead to failure to obtain horizontal gaze. If cSVA increased, Δcaudal adjacent angle and ΔT1 slope (extension below the kyphosis) compensated for the horizontal offset translation. The noncompensatory status (ΔcSVA and ΔT1 positive) may necessitate further correction surgery in which the caudal fused level is beyond T1.

McGregor's slope and slope of line of sight: two surrogate markers for Chin-Brow vertical angle in the setting of cervical spine pathology

BACKGROUND CONTEXT

Chin-Brow Vertical Angle (CBVA) is not routinely measured on radiographs even though it is a strong assessor of horizontal gaze.

STUDY DESIGN

Retrospective cohort study of patients with full-body stereoradiographs and a primary cervical diagnosis at the time of presentation.

PURPOSE

Assess the utility of McGregor's Slope (McGS) and Slope of Line of Sight (SLS) as surrogate markers for the CBVA in cervical spine pathology.

METHODS

A retrospective review of patients with full-body stereoradiographs was performed. Patients were ≥18 years of age with a primary cervical diagnosis. Analysis of CBVA, McGS, and SLS was conducted as markers of horizontal gaze. Sagittal alignment was characterized by: pelvic tilt (PT), pelvic incidence minus lumbar lordosis (PI-LL), T1-pelvic angle (TPA), sagittal vertical axis (SVA), T2-T12 thoracic kyphosis, C2-C7 SVA (cSVA), C2-C7 Cervical lordosis, T1-Slope minus Cervical Lordosis (TS-CL), and C2-Slope (C2S). A subgroup analysis was performed in patients with cervical deformity. Independent sample t tests and paired t tests compared radiographic alignment. Pearson correlations characterized linear relationships. Linear regression analysis identified relationships between the parameters.

RESULTS

In all, 329 patients were identified with primary cervical spine diagnoses. Chin-Brow Vertical Angle was visible in 171 patients (52.0%), McGS in 281 (85.4%), and SLS in 259 (78.7%). Of the 171 patients with visible CBVA, the mean CBVA was 2.30±7.7, mean McGS was 5.02±8.1, and mean SLS was -1.588±2.03. Chin-Brow Vertical Angle strongly correlated with McGS (r=0.83) and SLS (r=0.89) with p<.001. McGregor's Slope positively correlated with SLS (r=0.89, p=.001).

CONCLUSIONS

This study demonstrates that McGS and SLS serve as strong, positive correlates for CBVA. The reported mean differences between these measurements provide a useful conversion, broadening CBVA's use as a radiographic assessment of horizontal gaze.

Limited morbidity and possible radiographic benefit of C2 vs. subaxial cervical upper-most instrumented vertebrae

Background

The study aims to evaluate differences in alignment and clinical outcomes between surgical cervical deformity (CD) patients with a subaxial upper-most instrumented vertebra (UIV) and patients with a UIV at C2. Use of CD-corrective instrumentation in the subaxial cervical spine is considered risky due to narrow subaxial pedicles and vertebral artery anatomy. While C2 fixation provides increased stability, the literature lacks guidelines indicating extension of CD-corrective fusion from the subaxial spine to C2.

Methods

Included: operative CD patients with baseline (BL) and 1-year postop (1Y) radiographic data, cervical UIV ≥ C2. Patients were grouped by UIV: C2 or subaxial (C3-C7) and propensity score matched (PSM) for BL cSVA. Mean comparison tests assessed differences in BL and 1Y patient-related, radiographic, and surgical data between UIV groups, and BL-1Y changes in alignment and clinical outcomes.

Results

Following PSM, 31 C2 UIV and 31 subaxial UIV patients undergoing CD-corrective surgery were included. Groups did not differ in BL comorbidity burden (P=0.175) or cSVA (P=0.401). C2 patients were older (64 vs. 58 yrs, P=0.010) and had longer fusions (9 vs. 6 levels, P=0.002). Overall, patients showed BL-1Y improvements in TS-CL (P<0.001), cSVA (P=0.005), McGS (P=0.004). Cervical flexibility was maintained at 1Y regardless of UIV, assessed by CL flexion (-0.2° vs. 6.0°, P=0.115) and extension (13.9° vs. 9.9°, P=0.366). While both subaxial and C2 patients showed BL-1Y improvements in McGS (both P<0.030), C2 patients improved to a larger degree (7.3° vs. 6.2°). Between UIV groups, there were no differences in BL-1Y changes in HRQLs, overall complication rates, or operative complication rates (all P>0.05).

Conclusions

C2 UIV patients showed similar cervical range of motion and baseline to 1-year functional outcomes as patients with a subaxial UIV. C2 UIV patients also showed greater baseline to 1-year horizontal gaze improvement and had complication profiles similar to subaxial UIV patients, demonstrating the radiographic benefit and minimal functional loss associated with extending fusion constructs to C2. In the treatment of adult cervical deformities, extension of the reconstruction construct to the axis may allow for certain clinical benefits with less morbidity than previously acknowledged.

Adult cervical deformity: radiographic and osteotomy classifications

Cervical spine deformity represents a broad spectrum of pathologies that are both complex in etiology and debilitating towards quality of life for patients. Despite advances in the understanding of drivers and outcomes of cervical spine deformity, only one classification system and one system of nomenclature for osteotomy techniques currently exist. Moreover, there is a lack of standardization regarding the indications for each technique. This article reviews the adult cervical deformity (ACD) and current classification and nomenclature for osteotomy techniques, highlighting the need for further work to develop a unified approach for each case and improve communication amongst the spine community with respect to ACD.

Quantitative Correction of Cervical Sagittal Deformity in Atlanto-Axial Dislocation

STUDY DESIGN

A mechanism-based reasoning and retrospective cohort study.

OBJECTIVE

To establish a quantitative relationship between the change of clivo-axial angle (CXA) and the change of subaxial cervical lordosis (CL) in correction surgery of atlantoaxial dislocation (AAD).

SUMMARY OF BACKGROUND DATA

The evolving understanding of mechanism has dramatically improved the treatment modality of AAD. Cervical sagittal alignment is another important aspect of the surgery, in addition to reduction of odontoid process. However, a quantitative reference for correction surgery has not been established.

METHODS

Frankfort-axial angle (FXA) was introduced. Based on two assumptions, (1) sagittal alignment of the spine aims at keeping horizontal gaze and (2) deformities at craniovertebral junction make little impact on slope of T1 vertebra, we deduced that the change of CXA equaled change of CL (ΔCXA = -ΔCL). We retrospectively reviewed our case cohort to validate this finding with linear regression analysis.

RESULTS

Sixteen cases (eight male and eight female, mean age 40.4±12.5 yr old) were included. Liner fitting equation for ΔFXA and ΔCXA is y=1.005x (coefficient of determination, R=0.966; significance of the estimated coefficients P<0.001, t-statistics) and that for ΔCL and ΔCXA is y=-1.023x (R=0.976, P<0.001). These results support our deduction that ΔCXA = -ΔCL, which can be used as a guidance of quantitative correction of sagittal deformity in AAD.

CONCLUSION

Correction of CXA will influence the subaxial cervical lordosis (ΔCXA = -ΔCL) of AAD patients. This equation can serve as a quantitative reference for preoperative planning and intraoperative refining of the correction of cervical sagittal deformity in AAD.

LEVEL OF EVIDENCE

4.

Cervical Spine Deformity: Indications, Considerations, and Surgical Outcomes

Cervical spinal deformity (CSD) in adult patients is a relatively uncommon yet debilitating condition with diverse etiologies and clinical manifestations. Similar to thoracolumbar deformity, CSD can be broadly divided into scoliosis and kyphosis. Severe forms of CSD can lead to pain; neurologic deterioration, including myelopathy; and cervical spine-specific symptoms such as difficulty with horizontal gaze, dysphagia, and dyspnea. Recently, an increased interest is shown in systematically studying CSD with introduction of classification schemes and treatment algorithms. Both major and minor complications after surgical intervention have been analyzed and juxtaposed to patient-reported outcomes. An ongoing effort exists to better understand the relationship between cervical and thoracolumbar spinal alignment, most importantly in the sagittal plane.

The Effect of Bimaxillary Surgery on the Airway Morphology in Class III Patients: Taking Into Account the Head Posture During Walking

OBJECTIVES

To assess changes in head and neck posture, airway, hyoid, tongue, and soft palate in skeletal class III individuals whose cephalograms were taken using dynamic natural head positioning method with an inclinometer.

MATERIALS AND METHODS

This study was conducted on 20 class III patients operated with double jaw surgery. Natural head positions were determined before and 6 months after the surgery. Craniocervical, pharyngeal airway, tongue, and hyoid positions were measured. In the analysis of the data, paired t test, independent sample t test and correlation tests were used.

RESULTS

There was a statistically significant change in the craniocervical measurements with the operation and the head position was further extended (P < 0.05). Significant changes were observed in the soft palate angles upon surgery (P < 0.026) and the increase in oropharyngeal airway area was statistically significant (P < 0.001). There was a statistically significant increase in tongue length (TL) and distance from vertical reference plane to tongue tip (TTi/VRP) measurements (P < 0.001 and P < 0.012, respectively). There was a statistically significant decrease in distance from horizontal reference plane to tongue tip (TTi/HRP) and tongue dorsum (TD/HRP) measurements (P < 0.033 and P < 0.003, respectively). The only significant change among the hyoid bone position-related measurements was distance between hyoidale and epiglottis (P < 0.013).

CONCLUSIONS

Hyoid bone location, tongue measurements, and posterior pharyngeal airway were found to be related to each other and they have been found to be effective on the head and neck posture. Head position will be affected by possible surgical alternatives.

Effects of Different Head Positioning Methods on Facial Soft Tissue Analysis Using Stereophotogrammetry

PURPOSE

The purpose of this study was to compare the sagittal tilt of the head in different head positioning techniques using an inclinometer and facial stereophotogrammetric measurements.

MATERIALS AND METHODS

The study was carried out in 45 participants (26 female, 19 male). Participants' head positioning was obtained with dynamic walking, Frankfort horizontal plane (FHP'), self-balance plus mirror, and subjective photographic positioning methods. All pitch values were recorded by an inclinometer and stereophotogrammetric images were obtained. Facial analysis included distances of the glabella (G'), pronasale (Pn), soft tissue A point (A'), upper lip (Ls), lower lip (Li), soft tissue B point (B)', and soft tissue pogonion (Pog') to the true vertical line (TVL) and face height and lip length measurements.

RESULTS

Participants' head positions were observed to be more forward in the FHP' head positioning technique compared with other methods, whereas a more backward head position was recorded with subjective head positioning, and the difference was significant (P < .001). There were no relevant differences in pitch values between the self-balance plus mirror and dynamic walking methods. G'-TVL (P < .000), Pn-TVL (P < .029), A'-TVL (P < .039), Ls-TVL (P < .001), Li-TVL (P < .037), B'-TVL (P < .003), and Pog'-TVL (P < .000) in the profile view and face height, lower face height, and lower lip length values in the frontal view (P < .001) differed significantly by head positioning method.

CONCLUSIONS

The dynamic walking and self-balance plus mirror head positioning methods offered similar and advisable natural head position results, whereas FHP' head positioning was questionable for an accurate determination of natural head position. Facial soft tissue measurements, such as face height, lower face height, lower lip length, and projection of structures such as the G', Pn, lips, and chin, varied based on head positioning method.

Cervical spine alignment following surgery for adolescent idiopathic scoliosis (AIS): a pre-to-post analysis of 81 patients

Background

Several studies have emphasized the importance of restoring thoracic kyphosis (TK) in the setting of AIS, but very few have discussed changes in cervical spine alignment following surgery. Aim of this study was to evaluate reciprocal cervical alignment change after modification of global and regional thoracolumbar alignment with surgery in the setting of adolescent idiopathic scoliosis (AIS).

Methods

Baseline and 2-yrs follow-up radiographs of AIS patients (n = 81) were analysed measuring cervical parameters (upper cervical: C2-C0, McGregor Slope; lower cervical: C2-C7, C2-C7 sagittal vertical axis (SVA), C2-T3, C2-T3SVA, C2-T1Harrison (C2-T1Ha), T1 Slope (T1S)), thoracic, lumbar, pelvic and global alignment parameters. Post-operatively, patients were grouped twice; based on changes in TK and SVA. Cervical alignment was compared between groups. Pearson correlation was conducted to examine the relationship between changes in TK, SVA, and cervical alignment.

Results

Stratification by change in TK, revealed significant alteration of lower cervical alignment T1S [p < 0.001]), C2-T3 [p = 0.019], C2-T1Ha [p = 0.043]), but there was no reciprocal change in the upper cervical spine. Stratification by SVA revealed a significant coexisting change in the lower cervical spine (T1S [p < 0.001], C2-C7SVA [p = 0.034], C2-T3 [p = 0.023], C2-T3SVA [p = 0.001]). SVA change was not associated to a change in the upper cervical spine. The correlation analysis showed that with a post-operative increase in TK, the cervical spine became more lordotic. Changes in TK were significantly correlated with: ΔT1S, ΔC2-C7, ΔC2-T3, and ΔC2-T3SVA. Similarly, increased cervical kyphosis was found when SVA was decreased post-operatively. Furthermore, there was a significant correlation between change of SVA and both ΔC2-T3 and ΔC2-T3SVA.

Conclusions

In surgically treated AIS patients, changes in global and regional alignment of the thoracolumbar and cervical spinal segments exhibit interdependence. Thus, surgical planning with regard to sagittal deformity in AIS patients should account for the post-operative impact on cervical alignment.

Global spinal deformity from the upper cervical perspective. What is "Abnormal" in the upper cervical spine?

Hypothesis

Reciprocal changes in the upper cervical spine correlate with adult TL deformity modifiers.

Design

This was a retrospective review.

Introduction

The upper cervical spine has remarkable adaptability to wide ranges of thoracolumbar (TL) deformity.

Methods

Patients >18 years with adult spinal deformity (ASD) and complete radiographic data at baseline (BL) and 1 year were identified. Patients were grouped into component types of the Roussouly classification system (Type 1: Pelvic incidence [PI] <45° and lumbar lordosis [LL] apex below L4; Type 2: PI <45° and LL apex above L4; Type 3:45°<PI <65°; and Type 4: PI >65°). Patients were categorized by increasing severity of Schwab modifiers at BL (0, +, and ++) and further grouped by regional malalignment moving cranially (P: pelvic only; LP: lumbopelvic; TL: thoracic and LP; C: subaxial and TL). Analysis of variance and Pearson's r assessed changes in BL upper cervical parameters (C0-2, C0 slope, McGregor's Slope [MGS], and CBVA) across groups.

Results

A total of 343 ASD patients were analyzed. When grouped by BL Schwab and Roussouly, Group P had the lowest BL disability compared to other Groups, while Roussouley Type 1 correlated with higher BL disability compared to Type 2. Moving cranially up the spine, Group P, Group LP, and Group TL did not differ in C0-2 angle, C0 slope, MGS, or CBVA. Group C had a significantly smaller C0-C2, and more negative MGS, C0 slope, and CBVA than noncervical groups. Type 1 trended slightly higher CBVA and MGS than types 2-4, but no differences in cervical lordosis, C0-C2, or C0S were found. MGS (r = -0.131, P = 0.015), CBVA (r = -0.473, P < 0.001), and C0S (r = -0.099, P = 0.042) correlated most strongly with sagittal vertical axis (SVA) compared to other Schwab modifiers. We found SVA > 34 mm predicted a 1 unit (°°) decrease in MGS (odds ratio [OR]: 0.970 [0.948-0.993], P = 0.010), while cervical SVA >51 mm predicted a 1 unit increase in MGS (OR: 1.25 [1.12-1.38], P < 0.001).

Conclusions

Our study suggests that upper cervical alignment remains relatively stable through most broad variations of adult TL deformity. Changes in SVA correlated most with upper cervical changes.

Cervical mismatch: the normative value of T1 slope minus cervical lordosis and its ability to predict ideal cervical lordosis

OBJECTIVE

Numerous studies have attempted to delineate the normative value for T1S−CL (T1 slope minus cervical lordosis) as a marker for both cervical deformity and a goal for correction similar to how PI-LL (pelvic incidence–lumbar lordosis) mismatch informs decision making in thoracolumbar adult spinal deformity (ASD). The goal of this study was to define the relationship between T1 slope (T1S) and cervical lordosis (CL).

METHODS

This is a retrospective review of a prospective database. Surgical ASD cases were initially analyzed. Analysis across the sagittal parameters was performed. Linear regression analysis based on T1S was used to provide a clinically applicable equation to predict CL. Findings were validated using the postoperative alignment of the ASD patients. Further validation was then performed using a second, normative database. The range of normal alignment associated with horizontal gaze was derived from a multilinear regression on data from asymptomatic patients.

RESULTS

A total of 103 patients (mean age 54.7 years) were included. Analysis revealed a strong correlation between T1S and C0–7 lordosis (r = 0.886), C2–7 lordosis (r = 0.815), and C0–2 lordosis (r = 0.732). There was no significant correlation between T1S and T1S−CL. Linear regression analysis revealed that T1S−CL assumed a constant value of 16.5° (R2 = 0.664, standard error 2°). These findings were validated on the postoperative imaging (mean absolute error [MAE] 5.9°). The equation was then applied to the normative database (MAE 6.7° controlling for McGregor slope [MGS] between −5° and 15°). A multilinear regression between C2–7, T1S, and MGS demonstrated a range of T1S−CL between 14.5° and 26.5° was necessary to maintain horizontal gaze.

CONCLUSIONS

Normative CL can be predicted via the formula CL = T1S − 16.5° ± 2°. This implies a threshold of deformity and aids in providing a goal for surgical correction. Just as pelvic incidence (PI) can be used to determine the ideal LL, T1S can be used to predict ideal CL. This formula also implies that a kyphotic cervical alignment is to be expected for individuals with a T1S < 16.5°.

The Effect of Aging on Cervical Parameters in a Normative North American Population

STUDY DESIGN

Retrospective cohort study.

OBJECTIVES

To investigate age-based changes in cervical alignment parameters in an asymptomatic population.

METHODS

Retrospective review of a prospective study of 118 asymptomatic subjects who underwent biplanar imaging with 3-dimensional capabilities. Demographic and health-related quality of life data was collected prior to imaging. Patients were stratified into 5 age groups: <35 years, 35-44 years, 45-54 years, 55-64 years, and ≥65 years. Radiographic measurements of the cervical spine and spinopelvic parameters were compared between age groups. The normal distribution of parameters was assessed followed by analysis of variance for comparison of variance between age groups.

RESULTS

C2-C7 lordosis, C0-C7 lordosis, and T1 slope demonstrated significant increases with age. C0-C7 lordosis was significantly less in subjects <35 years compared with ≥55 years. Significant differences in T1 slope were identified in patients <35 versus ≥65, 35-44 versus ≥65, and 45-54 versus ≥65 years. T1 slope demonstrated a positive correlation with age. Horizontal gaze parameters did not change linearly with age and mean averages of all age groups were within 10° of one another. Cervical kyphosis was present in approximately half of subjects who were <55 compared with approximately 10% of subjects ≥55 years. Differences in pelvic tilt, pelvic incidence-lumbar lordosis, and C7-S1 sagittal vertical axis were identified with age.

CONCLUSIONS

C0-C7 lordosis, C2-C7 lordosis, and T1 slope demonstrate age-based changes while other cervical and horizontal gaze parameters remain relatively constant with age.

Cervical and postural strategies for maintaining horizontal gaze in asymptomatic adults

PURPOSE

To investigate the different cervical strategies for maintaining horizontal gaze in asymptomatic subjects.

METHODS

One hundred and forty-four asymptomatic adults filled the SF-36 quality of life questionnaire and underwent full-body biplanar radiographs. Chin brow vertical angle (CBVA) and postural and cervical parameters were measured. Subjects were grouped according to cervical spine curvature (C2-C7 angle): kyphotic (< - 5°), straight [- 5°, 5°], lordotic (> 5°). Demographics, SF-36 component scores and CBVA were compared between groups. All other parameters were compared between groups, while controlling for confounding factors (ANCOVA). A correlation test was conducted between all cervical parameters.

RESULTS

32% of subjects had kyphotic (- 12° ± 7°), 27% straight (0° ± 3°) and 41% lordotic (12° ± 7°) cervical spines. While demographic and SF-36 data did not differ between groups, CBVA differed between lordotic and kyphotic groups (2° vs. 6.5°, p = 0.002). Sagittal vertical axis (SVA) and thoracic kyphosis (TK) were lower in the kyphotic group (SVA: K = - 26 ± 20 mm vs. L = - 2 ± 21 mm, p < 0.001; TK: K = 40° ± 6° vs. L = 51° ± 8°, p < 0.001). C2 slope (K = 29° ± 6° vs. L = 18° ± 6°, p < 0.001), C0-C2 (K = 42° ± 8° vs. L = 30° ± 8°, p < 0.001) and C1-C2 (K = 33° ± 6° vs. L = 28° ± 6°, p = 0.004) were higher in the kyphotic group. Significant correlations were found between almost all cervical parameters and C2-C7 angle.

CONCLUSIONS

Subjects with cervical kyphosis presented with more posterior global alignment and lower TK than subjects with lordosis. In order to maintain horizontal gaze, subjects with cervical kyphosis presented with a more lordotic upper cervical spine than subjects with cervical lordosis. Subjects with straight cervical curvature presented with an intermediate sagittal alignment. These slides can be retrieved under Electronic Supplementary Material.

Radiologically defining horizontal gaze using EOS imaging-a prospective study of healthy subjects and a retrospective audit

BACKGROUND CONTEXT

As sagittal alignment of the cervical spine is important for maintaining horizontal gaze, it is important to determine the former for surgical correction. However, horizontal gaze remains poorly-defined from a radiological point of view.

PURPOSE

The objective of this study was to establish radiological criteria to define horizontal gaze.

STUDY DESIGN/SETTING

This study was conducted at a tertiary health-care institution over a 1-month period.

PATIENT SAMPLE

A prospective cohort of healthy patients was used to determine the best radiological criteria for defining horizontal gaze. A retrospective cohort of patients without rigid spinal deformities was used to audit the incidence of horizontal gaze.

OUTCOME MEASURES

Two categories of radiological parameters for determining horizontal gaze were tested: (1) the vertical offset distances of key identifiable structures from the horizontal gaze axis and (2) imaginary lines convergent with the horizontal gaze axis.

MATERIALS AND METHODS

Sixty-seven healthy subjects underwent whole-body EOS radiographs taken in a directed standing posture. Horizontal gaze was radiologically defined using each parameter, as represented by their means, 95% confidence intervals (CIs), and associated 2 standard deviations (SDs). Subsequently, applying the radiological criteria, we conducted a retrospective audit of such radiographs (before the implementation of a strict radioimaging standardization).

RESULTS

The mean age of our prospective cohort was 46.8 years, whereas that of our retrospective cohort was 37.2 years. Gender was evenly distributed across both cohorts. The four parameters with the lowest 95% CI and 2 SD were the distance offsets of the midpoint of the hard palate (A) and the base of the sella turcica (B), the horizontal convergents formed by the tangential line to the hard palate (C), and the line joining the center of the orbital orifice with the internal occipital protuberance (D). In the prospective cohort, good sensitivity (>98%) was attained when two or more parameters were used. Audit using Criterion B+D yielded compliance rates of 76.7%, a figure much closer to that of A+B+C+D (74.8%). From a practical viewpoint, Criterion B+D were most suitable for clinical use and could be simplified to the "3-6-12 rule" as a form of cursory assessment. Verbal instructions in the absence of stringent postural checks only ensured that ~75% of subjects achieved horizontal gaze.

CONCLUSIONS

Fulfillment of Criterion B+D is sufficient to evaluate for horizontal gaze. Further criteria can be added to increase sensitivity. Verbal instructions alone yield high rates of inaccuracy when attempting to image patients in horizontal gaze. Apart from improving methods for obtaining radiographs, a radiological definition of horizontal gaze should be routinely applied for better evaluation of sagittal spinal alignment.

Cervical Spine Deformity-Part 1: Biomechanics, Radiographic Parameters, and Classification

Cervical spine deformities can have a significant negative impact on the quality of life by causing pain, myelopathy, radiculopathy, sensorimotor deficits, as well as inability to maintain horizontal gaze in severe cases. Many different surgical options exist for operative management of cervical spine deformities. However, selecting the correct approach that ensures the optimal clinical outcome can be challenging and is often controversial. We aim to provide an overview of cervical spine deformity in a 3-part series covering topics including the biomechanics, radiographic parameters, classification, treatment algorithms, surgical techniques, clinical outcome, and complication avoidance with a review of pertinent literature.

The Influence of Natural Head Position on the Cervical Sagittal Alignment

Introduction

This study investigated the relationship between the parameters related to the natural head position and cervical segmental angles and alignment of patients with neck pain.

Material and Methods

The lateral radiographs of the cervical spine were collected from 103 patients and were used to retrospectively analyze the correlation between the natural head position, cervical local sagittal angles, and alignment. Sagittal measurements were as follows: cervical curvature classification, slope of McGregor's line (McGS), local sagittal angles (C0–C2 angle, C2–C5 angle, C5–C7 angle, and C2–C7 angle), T1 slope, center of gravity of the head to sagittal vertical axis (CG–C7 SVA), and local sagittal alignment (C0–C2 SVA and C2–C7 SVA).

Results

McGS was significantly correlated to C0–C2 angle (r = 0.57), C0–C2 SVA (r = −0.53), C2–C7 SVA (r = −0.28), and CG–C7 SVA (r = −0.47). CG–C7 SVA was also significantly correlated to curvature type (r = 0.27), C5–C7 angle (r = −0.37), and C2–C7 angle (r = −0.39).

Conclusions

A backward shift with an extended head position may accompany a relatively normal curvature of the cervical spine. The effect of posture control in relieving abnormal mechanical state of the cervical spine needs to be further confirmed by biomechanical analysis.

An assessment of the most reliable method to estimate the sagittal alignment of the cervical spine: analysis of a prospective cohort of 138 cases

OBJECTIVE

Although there is increasing recognition of the importance of cervical spinal sagittal balance, there is a lack of consensus as to the optimal method to accurately assess the cervical sagittal alignment. Cervical alignment is important for surgical decision making. Sagittal balance of the cervical spine is generally assessed using one of two methods; namely, measuring the angle between C-2 and C-7, and drawing a line between C-2 and C-7. Here, the best method to assess sagittal alignment of the cervical spine is investigated.

METHODS

Data from 138 patients enrolled in a randomized controlled trial (Procon) were analyzed. Two investigators independently measured the angle between C-2 and C-7 by using Harrison's posterior tangent method, and also estimated the shape of the sagittal curve by using a modified Toyama method. The mean angles of each quantitative assessment of the sagittal alignment were calculated and the results were compared. The interrater reliability for both methods was estimated using Cronbach's alpha.

RESULTS

For both methods the interrater reliability was high: for the posterior tangent method it was 0.907 and for the modified Toyama technique it was 0.984. For a lordotic cervical spine, defined by the modified Toyama method, the mean angle (defined by Harrison's posterior tangent method) was 23.4° ± 9.9° (range 0.4°–52.4°), for a kyphotic cervical spine it was −2.2° ± 9.2° (range −16.1° to 16.9°), and for a straight cervical spine it was 10.5° ± 8.2° (range −11° to 36°).

CONCLUSIONS

An absolute measurement of the angle between C-2 and C-7 does not unequivocally define the sagittal cervical alignment. As can be seen from the minimum and maximum values, even a positive angle between C-2 and C-7 could be present in a kyphotic spine. For this purpose, the modified Toyama method (drawing a line from the posterior inferior part of the vertebral body of C-2 to the posterior upper part of the vertebral body of C-7 without any measurements) is a better tool for a global assessment of cervical sagittal alignment.

Clinical trial registration no.: ISRCTN41681847 (https://www.isrctn.com)

Recent and Emerging Advances in Spinal Deformity

BACKGROUND: Over the last several decades, significant advances have occurred in the assessment and management of spinal deformity.

OBJECTIVE: The primary focus of this narrative review is on recent advances in adult thoracic, thoracolumbar, and lumbar deformities, with additional discussions of advances in cervical deformity and pediatric deformity.

METHODS: A review of recent literature was conducted.

RESULTS: Advances in adult thoracic, thoracolumbar, and lumbar deformities reviewed include the growing applications of stereoradiography, development of new radiographic measures and improved understanding of radiographic alignment objectives, increasingly sophisticated tools for radiographic analysis, strategies to reduce the occurrence of common complications, and advances in minimally invasive techniques. In addition, discussion is provided on the rapidly advancing applications of predictive analytics and outcomes assessments that are intended to improve the ability to predict risk and outcomes. Advances in the rapidly evolving field of cervical deformity focus on better understanding of how cervical alignment is impacted by thoracolumbar regional alignment and global alignment and how this can affect surgical planning. Discussion is also provided on initial progress toward development of a comprehensive cervical deformity classification system. Pediatric deformity assessment has been substantially improved with low radiation-based 3-D imaging, and promising clinical outcomes data are beginning to emerge on the use of growth-friendly implants.

CONCLUSION: It is ultimately through the reviewed and other recent and ongoing advances that care for patients with spinal deformity will continue to evolve, enabling better informed treatment decisions, more meaningful patient counseling, reduced complications, and achievement of desired clinical outcomes.

Standing sagittal alignment of the whole axial skeleton with reference to the gravity line in humans

Human beings stand upright with the chain of balance beginning at the feet, progressing to the lower limbs (ankles, knees, hip joints, pelvis), each of the spinal segments, and then ending at the cranium to achieve horizontal gaze and balance using minimum muscle activity. The details of the alignment and balance of the chain, however, are not clearly understood, due to the lack of information regarding the three‐dimensional (3D) orientation of all bony elements in relation to the gravity line (GL). We performed a clinical study to clarify the standing sagittal alignment of whole axial skeletons in reference to the GL using the EOS slot‐scanning 3D X‐ray imaging system with simultaneous force plate measurement in a healthy human population. The GL was defined as a vertical line drawn through the centre of vertical pressure measured by the force plate. The present study yielded a complete set of physiological alignment measurements of the standing axial skeleton from the database of 136 healthy subjects (a mean age of 39.7 years, 20–69 years; men: 40, women: 96). The mean offset of centre of the acoustic meati from the GL was 0.0 cm. The offset of the cervical and thoracic vertebrae was posterior to the GL with the apex of thoracic kyphosis at T7, 5.0 cm posterior to the GL. The sagittal alignment changed to lordosis at the level of L2. The apex of the lumbar lordosis was L4, 0.6 cm anterior to the GL, and the centre of the base of the sacrum (CBS) was just posterior to the GL. The hip axis (HA) was 1.4 cm anterior to the GL. The knee joint was 2.4 cm posterior and the ankle joint was 4.8 cm posterior to the GL. L4‐, L5‐ and the CBS‐offset in subjects in the age decades of 40s, 50s and 60s were significantly posterior to those of subjects in their 20s. The L5‐ and CBS‐offset in subjects in their 50s and 60s were also significantly posterior to those in subjects in their 30s. HA was never posterior to the GL. In the global alignment, there was a positive correlation between offset of C7 vertebra from the sagittal vertical axis (a vertical line drawn through the posterior superior corner of the sacrum in the sagittal plane) and age, but no correlation was detected between the centre of the acoustic meati‐GL offset and age. Cervical lordosis (CL), pelvic tilt (PT), pelvic incidence, hip extension, knee flexion and ankle dorsiflexion increased significantly with age. Our results revealed that aging induces trunk stooping, but the global alignment is compensated for by an increase in the CL, PT and knee flexion, with the main function of CL and PT to maintain a horizontal gaze in a healthy population.

Cranial Center of Mass Compared to C7 Plumb Line Alignment in Adult Spinal Deformity

OBJECTIVE

To report the cranial center of mass (CCOM) progression in surgically treated patients for adult spinal deformity (ASD). The C7 plumb line/sagittal vertical axis (SVA) has important relationships with patient-reported outcomes; however, this has not yet been defined for CCOM.

METHODS

Patients with consecutive ASD who were undergoing surgery greater than 5 levels of fusion between 2007 and 2012 and had radiographic, clinical, and outcomes data spanning ≥2 years were analyzed, retrospectively. Radiographic parameters were obtained preoperatively and at 6 weeks, 1 year, and 2 years postoperatively. Statistical analysis included descriptives (measures of central tendency, dispersion, frequencies), independent Student t tests, χ(2) square, Pearson correlation, and Kaplan-Meyer curve.

RESULTS

Fifty-eight patients (10 male, 48 female) with a mean age of 60.5 years (range, 27-81 years) were reviewed. The mean preoperative SVA was 7.40 cm (SD = 5.51; 37/58 [63.8%] malalignment), and mean CCOM was 10.0 cm (SD = 6.58; 47/58 [81%] malalignment). Six-week postoperative SVA and CCOM was -0.17 cm (SD = 3.3) and 2.5 cm (SD = 4.11), respectively. SVA malalignment was 12.7% and CCOM malalignment was 38.2% at 6 weeks postop. Six week (absolute), 6-week change, and patient number at 6 weeks who were CCOM malaligned was significant compared with SVA (P = 0.003, P < 0.001, P < 0.001, respectively). SRS appearance worsened as preoperative SVA and CCOM increased (P < 0.05), and 2-year SRS appearance and mental health was worsened as 2-year SVA and CCOM increased (P < 0.05). SVA malalignment was 8 and 10 at 1 and 2 years, respectively, and CCOM malalignment was 24 and 32, respectively. Kaplan-Meier curve demonstrates persistent malalignment of CCOM at 6 weeks if not corrected.

CONCLUSION

CCOM alignment restoration is an important parameter in ASD, and malalignment is consistent over time.