Influence of sagittal balance on spinal lumbar loads: A numerical approach

Abstract

Background

Pathological deformities involving the sagittal alignment of the spine may lead to loss of spine stability and imbalance. The effect of different patterns of sagittal balance on the loads acting in the spine was only marginally investigated, although it would be of critical importance in the clinical management of spinal disorders.

Methods

Optimization-based finite element models of the human spine in the standing position able to predict the loads acting in the lumbar spine and the activation of the spinal muscles were developed and used to explore a wide range of sagittal balance conditions, covering both inter-subject variability and pathological imbalance. 1000 two-dimensional randomized spine models with simplified geometry were generated by varying anatomical parameters such as lumbar lordosis, sacral slope, and C7 plumb line. Muscular loads were calculated by means of an optimization procedure aimed to minimize total muscular stress.

Findings

The simulation of a physiological spine in the standing position predicted average disk stresses ranging from 0.38 to 0.5 MPa, in good agreement with in vivo measurements. The C7 plumb line and the parameters describing the lumbar spine were found to be the strongest determinants of the lumbar loads and muscle activity. Marginal relevance was found concerning the thoracic and cervical parameters.

Interpretation

The present modeling approach was found to be able to capture correlations between sagittal parameters and the loads acting in the lumbar spine. The method represents a good platform for future improvements aimed at patient-specific modeling to support pre-operative surgical planning.

Introduction

The development of bipedalism in humans determined a series of peculiar changes in the spinal anatomy and biomechanics in comparison to non-human primates, which could not be observed in other animals adopting the erect position such as birds, kangaroos and dinosaurs (Le Huec et al., 2011b). These changes include the verticalization of the pelvis (Berge, 1998) and the development of the spinal sagittal curvatures, i.e. lumbar lordosis and thoracic kyphosis, which are specific to humans but can be developed by non-human primates trained to walk bipedally (Preuschoft et al., 1988, Wagner et al., 2012). In its physiological conditions, the human spine exploits its curved shape in the sagittal plane to achieve equilibrium and stability with a minimal activation of the back musculature (Skoyles, 2006).

Despite the general effectiveness of the human spinal anatomy and biomechanics, pathological deformities (Roussouly and Pinheiro-Franco, 2011), both congenital and degenerative, may lead to loss of spine stability and sagittal imbalance. Classification systems based on radiographic assessment have been introduced and are currently used for the characterization of the sagittal balance of patients suffering from various spinal disorders (Roussouly et al., 2005, Wang et al., 2012). The correct use of these systems is critical in the planning of back surgeries, in which the restoration of a physiological balance was shown to be a strong determinant for the clinical outcome, especially in the presence of spondylolisthesis (Bourghli et al., 2011, Lamartina et al., 2012).

In recent times, a great amount of resources have been dedicated to epidemiological and clinical investigations of the relevance of spinal sagittal balance, supported by the increasingly widespread use of the EOS imaging system which allows for low-dose X-ray imaging of the spine in standing position (Dubousset et al., 2008). Nevertheless, data about the importance of sagittal balance in biomechanical terms are still lacking. To our knowledge, the effect of different patterns of sagittal imbalance on the loads acting in the spine was only marginally investigated (e.g. in Harrison et al., 2005, Keller et al., 2005, Kiefer et al., 1998), although it would be of critical importance in the management of spinal disorders. For example, the estimation of the amount of restoration to be achieved with surgical correction of the imbalance is still based on empirical formulas (reviewed in Lamartina et al., 2012), which have been proven to give acceptable results but could be optimized with a more scientific approach in which the post-operative spinal loads, stresses and muscle activation could be predicted. Methods which would allow for such an approach are currently already available, and include finite element analysis and numerical optimization, widely used for the investigation of other topics regarding spine biomechanics (Fagan et al., 2002).

This paper is therefore aimed at the development of optimization-based finite element models of the human spine in the standing position which can predict the loads acting in the lumbar spine and the activation of the spinal muscles. The models have been subsequently used to explore a wide range of sagittal balance conditions, covering both the inter-subject variability and pathological imbalance, in order to determine possible correlations between specific characteristics of the spinal shape and the resulting spinal biomechanics in terms of loads and muscle activation. Future uses of the models will include pre-operative planning of surgical imbalance correction.