Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery

Analysis of pelvic incidence from 3-dimensional images of a normal population

STUDY DESIGN

Pelvic incidence (PI) was measured in 3 dimensions from computed tomographic (CT) images of normal subjects using a computerized method.

OBJECTIVE

To obtain the angle of PI from 3-dimensional (3D) images and analyze its distribution in a normal population.

SUMMARY OF BACKGROUND DATA

The sagittal alignment of the pelvis is usually evaluated in 2-dimensional (2D) sagittal radiographs. The purpose of this study is to measure and analyze PI, which represents a key parameter of sagittal alignment, in 3D images.

METHODS

A computerized method, based on image-processing techniques, was used to automatically determine the anatomical references required to measure PI, that is, the centers of the femoral heads in 3 dimensions and the center and inclination of the sacral end plate in 3 dimensions. Multiplanar image reformation was applied to obtain perfect sagittal views with all anatomical structures completely in line with the hip axis from which PI was calculated.

RESULTS

PI was successfully obtained in CT images of 370 normal subjects (mean age: 41.5 years; range: 1-87 years). The mean PI (± standard deviation) was equal to 46.6° (±9.2°) for male subjects, 47.6° (±10.7°) for female subjects, and 47.1° (±10.0°) for both sexes. No statistically significant differences were obtained between the sexes, and statistically significant correlation was obtained between PI and age.

CONCLUSION

In this study, the sagittal alignment of the pelvis was evaluated in terms of PI completely in 3 dimensions. The results show that computerized measurements of PI in 3 dimensions are less variable than manual measurements. The large span of PI values for normal subjects indicates that the natural variation of PI is relatively large.

Criteria to restore the sagittal balance in deformity and degenerative spondylolisthesis

PURPOSE

To present in a single source the relevant information needed to assess spinopelvic balance and alignment, and to estimate the amount of correction needed in a patient during surgical treatment.

METHODS

Narrative literature review

RESULTS

Sagittal balance can be evaluated by global balance estimates (sagittal vertical axis and T1 tilt). Other important parameters are the relationship between pelvic incidence and lumbar lordosis (spinopelvic harmony), between pelvic incidence and difference of thoracic kyphosis and lumbar lordosis (spinopelvic balance), excess of pelvic tilt, knee flexion and thoracic compensatory hypokyphosis. Different methods to calculate the amount of surgical correction needed in patients with sagittal imbalance have been based on combinations of these parameters.

CONCLUSIONS

Relevant parameters of sagittal imbalance have been identified and correlated with clinical outcomes. Methods for calculation of surgical correction of imbalance have been proposed, but not validated in patients with mid-term follow-up.

Sacro-femoral-pubic angle: a coronal parameter to estimate pelvic tilt

INTRODUCTION

Pelvic tilt is an established measure of position which has been tied to sagittal plane spinal deformity. Increased tilt is noted in the setting of the aging spine and sagittal malalignment syndromes such as flatback (compensatory mechanism). However, the femoral heads are often poorly visualized on sagittal films of scoliosis series in adults, limiting the ability to determine pelvic incidence and tilt. There is a need to establish a coronal plane (better visualization) pelvic parameter which correlates closely with pelvic tilt.

METHODS

This is a retrospective review of 71 adult patients (47 females and 24 males) with full-length standing spine radiographs. Visualization of all spinal and pelvic landmarks was available coronally and sagittally (including pelvis and acetabuli). Pelvic tilt was calculated through validated digital analysis software (SpineView(®)). A new parameter, the sacro-femoral-pubic angle (midpoint of S1 endplate to centroid of acetabuli to superior border of the pubic symphysis) was analyzed for correlation (and predictive ability) with sagittal pelvic tilt.

RESULTS

The sacro-femoral-pubic angle (SFP angle) was highly correlated to PT, and according to this analysis, pelvic tilt could be estimated by the formula: PT = 75 - (SFP angle). A Pearson's correlation coefficient of 0.74 (p < 0.005) and predictive ability of 76% accuracy was obtained (±7.5°). The correlation and predictive ability was greater for males compared to females (male: r = 0.87 and predictive model = 93%; female: r = 0.67 and predictive model = 67%).

CONCLUSION

The pelvic tilt is an essential measure in the context of radiographic evaluation of spinal deformity and malalignment. Given the routinely excellent visibility of coronal films this study established the SFP as a coronal parameter which can reliably estimate pelvic tilt. The high correlation and predictive ability of the SFP angle should prompt further study and clinical application when lateral radiographs do not permit assessment of pelvic parameters.

Sagittal spino-pelvic alignment failures following three column thoracic osteotomy for adult spinal deformity

PURPOSE

Three column thoracic osteotomy (TCTO) is effective to correct rigid thoracic deformities, however, reasons for residual postoperative spinal deformity are poorly defined. Our objective was to evaluate risk factors for poor spino-pelvic alignment (SPA) following TCTO for adult spinal deformity (ASD).

METHODS

Multicenter, retrospective radiographic analysis of ASD patients treated with TCTO. Radiographic measures included: correction at the osteotomy site, thoracic kyphosis (TK), lumbar lordosis (LL), sagittal vertical axis (SVA), pelvic tilt (PT), and pelvic incidence (PI). Final SVA and PT were assessed to determine if ideal SPA (SVA < 4 cm, PT < 25°) was achieved. Differences between the ideal (IDEAL) and failed (FAIL) SPA groups were evaluated.

RESULTS

A total of 41 consecutive ASD patients treated with TCTO were evaluated. TCTO significantly decreased TK, maximum coronal Cobb angle, SVA and PT (P < 0.05). Ideal SPA was achieved in 32 (78%) and failed in 9 (22%) patients. The IDEAL and FAIL groups had similar total fusion levels and similar focal, SVA and PT correction (P > 0.05). FAIL group had larger pre- and post-operative SVA, PT and PI and a smaller LL than IDEAL (P < 0.05).

CONCLUSIONS

Poor SPA occurred in 22% of TCTO patients despite similar operative procedures and deformity correction as patients in the IDEAL group. Greater pre-operative PT and SVA predicted failed post-operative SPA. Alternative or additional correction procedures should be considered when planning TCTO for patients with large sagittal global malalignment, otherwise patients are at risk for suboptimal correction and poor outcomes.

[Sagittal deformity. Basic principles of surgical strategies]

There is a large body of literature supporting the importance of restoring sagittal balance to the spine. The main message is this: regardless of the specific surgical strategy and treatment or pathology, rebalancing results in a positive patient outcome. Complex deformity patients need to be evaluated with attention to the global balance and the operative planning and strategy must be adapted accordingly. Spinal fusions are not always considered within the framework of sagittal balance. Unsuccessful outcome including continued pain, adjacent level disease, accelerated degenerative changes of the spine, pseudarthrosis and hip and knee changes, may then ensue. Certainly, those patients need to be re-evaluated with attention to the global balance of the spine. The reason for the outcome may be sagittal imbalance and osteotomy techniques as well as fusion extension may be needed. The postoperative outcome can only be improved when the sagittal balance is already considered in the planning and treatment strategy during initial correction surgery. Concerning sagittal balance a paradigm shift seems to occur.