Optimization of rib surgery parameters for the correction of scoliotic deformities using approximation models

An In Vitro Biomechanical Study on Ovine Rib Flexibility With Increasing Deconstruction-As an Alternative to Rib Resection for Costoplasty

STUDY DESIGN

An in vitro biomechanical study into the effect of rib deconstruction on the flexibility in ovine ribs, using three-point bending.

OBJECTIVE

To examine the feasibility and possible effectiveness of a more conservative costoplasty using an in vitro animal model.

SUMMARY OF BACKGROUND DATA

Costoplasty remains useful in the treatment of adolescent idiopathic scoliosis, rib hump, and associated chest wall deformities. However, traditional costoplasty increases morbidity and blood loss.

METHODS

Ribs 2-10 were dissected from four fresh half ovine rib cages. The ribs were randomly allocated to Groups 1 to 4. The ribs underwent deconstruction, 10 mm from the lateral tubercle and 30 mm long, according to their group: Group 1 = control; Group 2 = convex cortical bone removed; Group 3 = convex cortical and cancellous bone removed; Group 4 = all but the ventral cortex is removed. Flexibility was tested by loading the concave side of each rib while fixed at the rib head and equidistant from the center of the resected area. The ribs were deformed at 0.5 mm/s up to a maximum load of 9.99 kg or fracturing. Load was plotted against displacement to find the load/displacement coefficient for each group. Statistical analysis was by an analysis of variance with Tukey's honestly significant difference post hoc testing.

RESULTS

The load/displacement coefficients were as follows: Group 1 = 131.93 (±27.52) N/mm, Group 2 = 93.36 (±40.71) N/mm, Group 3 = 88.66 (±25.84) N/mm, and Group 4 = 29.69 (±29.11) N/mm. Group 4 was significantly less stiff than Groups 1, 2, and 3 (p < .01). No ribs in Groups 1, 2, and 4 fractured. Five of 8 ribs in Group 3 fractured during loading.

CONCLUSIONS

Deconstructing the rib down to the concave side significantly increases the flexibility by approximately 4.5 times. Despite large removal of bone, it retains the ability to withstand 10 kg of load without fracture.

Characterization of the centroidal geometry of human ribs

While a number of studies have quantified overall ribcage morphology (breadth, depth, kyphosis/lordosis) and rib cross-sectional geometry in humans, few studies have characterized the centroidal geometry of individual ribs. In this study, a novel model is introduced to describe the centroidal path of a rib (i.e., the sequence of centroids connecting adjacent cross-sections) in terms of several physically-meaningful and intuitive geometric parameters. Surface reconstructions of rib levels 2-10 from 16 adult male cadavers (aged 31-75 years) were first extracted from CT scans, and the centroidal path was calculated in 3D for each rib using a custom numerical method. The projection of the centroidal path onto the plane of best fit (i.e., the "in-plane" centroidal path) was then modeled using two geometric primitives (a circle and a semiellipse) connected to give C1 continuity. Two additional parameters were used to describe the deviation of the centroidal path from this plane; further, the radius of curvature was calculated at various points along the rib length. This model was fit to each of the 144 extracted ribs, and average trends in rib size and shape with rib level were reported. In general, upper ribs (levels 2-5) had centroidal paths which were closer to circular, while lower ribs (levels 6-10) tended to be more elliptical; further the centroidal curvature at the posterior extremity was less pronounced for lower ribs. Lower ribs also tended to exhibit larger deviations from the best-fit plane. The rib dimensions and trends with subject stature were found to be consistent with findings previously reported in the literature. This model addresses a critical need in the biomechanics literature for the accurate characterization of rib geometry, and can be extended to a larger population as a simple and accurate way to represent the centroidal shape of human ribs.

Musculoskeletal modelling of muscle activation and applied external forces for the correction of scoliosis

BACKGROUND

This study uses biomechanical modelling and computational optimization to investigate muscle activation in combination with applied external forces as a treatment for scoliosis. Bracing, which incorporates applied external forces, is the most popular non surgical treatment for scoliosis. Non surgical treatments which make use of muscle activation include electrical stimulation, postural control, and therapeutic exercises. Electrical stimulation has been largely dismissed as a viable treatment for scoliosis, although previous studies have suggested that it can potentially deliver similarly effective corrective forces to the spine as bracing.

METHODS

The potential of muscle activation for scoliosis correction was investigated over different curvatures both with and without the addition of externally applied forces. The five King's classifications of scoliosis were investigated over a range of Cobb angles. A biomechanical model of the spine was used to represent various scoliotic curvatures. Optimization was applied to the model to reduce the curves using combinations of both deep and superficial muscle activation and applied external forces.

RESULTS

Simulating applied external forces in combination with muscle activation at low Cobb angles (< 20 degrees) over the 5 King's classifications, it was possible to reduce the magnitude of the curve by up to 85% for classification 4, 75% for classifications 3 and 5, 65% for classification 2, and 60% for classification 1. The reduction in curvature was less at larger Cobb angles. For King's classifications 1 and 2, the serratus, latissimus dorsi, and trapezius muscles were consistently recruited by the optimization algorithm for activation across all Cobb angles. When muscle activation and external forces were applied in combination, lower levels of muscle activation or less external force was required to reduce the curvature of the spine, when compared with either muscle activation or external force applied in isolation.

CONCLUSIONS

The results of this study suggest that activation of superficial and deep muscles may be effective in reducing spinal curvature at low Cobb angles when muscle groups are selected for activation based on the curve type. The findings further suggest the potential for a hybrid treatment involving combined muscle activation and applied external forces at larger Cobb angles.

The Use of Finite Element Models to Assist Understanding and Treatment For Scoliosis: A Review Paper

INTRODUCTION

Scoliosis is a complex spinal deformity whose etiology is still unknown, and its treatment presents many challenges. Finite element modeling (FEM) is one of the analytical techniques that has been used to elucidate the mechanism of scoliosis and the effects of various treatments.

METHODS

A literature review on the application of FEM in scoliosis evaluation and treatment has been undertaken. A literature search was performed in each of three major electronic databases (Google Scholar, Web of Science, and Ovid) using the key words "scoliosis" and "finite element methods/model". Articles using FEM and having a potential impact on clinical practice were included.

RESULTS

A total of 132 abstracts were retrieved. The query returned 105 articles in which the abstracts appeared to correspond to this review's focus, and 85 papers were retained. The current state of the art of FEM related to the biomechanical analysis of scoliosis is discussed in 4 sections: the etiology of adolescent idiopathic scoliosis, brace treatment, instrumentation treatment, and sensitivity studies of FEM. The limitations of FEM and suggested future work are also discussed.

Biomechanical modelling of growth modulation following rib shortening or lengthening in adolescent idiopathic scoliosis

A biomechanical model was developed to evaluate the long-term correction resulting from rib shortening or lengthening in adolescent idiopathic scoliosis (AIS). A finite element model of the trunk, personalised to the geometry of a scoliotic patient, was used to simulate rib surgery. Stress relaxation of ligaments following surgery was integrated into the model, as well as longitudinal growth of vertebral bodies and ribs and its modulation due to mechanical stresses. Simulations were performed in an iterative fashion over 24 months. A concave side rib shortening, inducing load patterns on the vertebral end-plates that could act against the scoliosis progression, was tested. A fractional factorial experimental design of 16 runs documented the effects of six modelling parameters. Wedging of the apical vertebra in the frontal plane decreased from 5.2 degrees initially to a mean value of 3.8 degrees after 24 months. The wedging decrease in the thoracic apical region was reflected by changes in the spine curvature, with a Cobb angle decrease from 46 degrees to 44 degrees immediately after the surgery and to a mean of 41 degrees after 24 months. However, both rib hump and vertebral axial rotation increased, on average, by 4 degrees at the curve apex. The most significant parameters were the growth sensitivity to stress in ribs and vertebrae and the rate of stress relaxation of intercostal ligaments. The results confirmed the potential of long-term correction of spinal curvature resulting from the rib shortening on the concavity. This modelling approach could be used for further design of less invasive surgery, taking into account residual growth, for scoliosis correction.