Introduction A very specific group within the 80 percent of the population that suffers from low back pain at some stage in life are young cricket fast bowlers. Amongst them a high occurrence of unilateral L4 pars interarticularis fractures exists, which shows a strong statistical correlation to the presence of a contralateral volumetric increase in the Quadratus Lumborum (QL) muscle. However, there is no clear physical link between these two phenomena. To investigate this relationship, we have combined a mathematical model of the lumbar spine muscles with a finite element model of the fourth lumbar vertebra and analysed the stresses occurring in the L4 vertebra throughout the bowling motion.

Methods A mathematical model of the lumbar spine muscles has been developed previously at QUT. It contains 170 fascicles representing all major muscles in the lumbar region and allows for analysis of the forces and moments on the intervertebral joints caused by these muscles in any given posture. A Finite Element Model (FEM) of an L4 vertebra and intervertebral disc (IVD) was developed based on one created by Theo Smit and obtainable from the Internet through the BEL Repository of the Istituti Ortopedici Rizzoli, Bologna, Italy. Material properties were obtained from literature, while muscle forces, directions and attachment locations in the different postures came from the mathematical model. Six postures occurring in right-handed fast bowling were modelled to determine the differences in stresses between having symmetric and asymmetric QL muscles. The asymmetric condition consisted of a 30% increase in Physiological Cross-Sectional Area (PCSA) on the right side. In all cases it was assumed the left facet joints were ‘locked up’, to create a presumed worst-case scenario for the stress build-up in the pars.

Results It was found that when using muscle activation levels from literature an enlarged right-side QL did not increase the stresses in the left pars noticeably, in fact in some cases it even slightly reduced those stresses. When only the right-side QL muscle was activated, while all other muscles only provided passive muscle force, a 30% PCSA increase of this muscle produced an increase in maximum Von Mises and principal stresses in the left-side pars from typically 30 MPa to 40 MPa but only in the postures close to upright stance. In more extreme postures where the maximum stresses in the pars are higher, the increased PCSA of the right QL only led to small stress increases from typically 125 to 129 MPa.

Discussion Even in the worst-case scenario where only the right-side QL is active and the left-side facet joint is locked up, a PCSA increase of that muscle does not cause a large increase in stresses in postures where the stresses are high. Hence, this study has not demonstrated a clear physical link between asymmetric hypertrophy of QL and pars fractures. It may even suggest the hypertrophy is a response to postural overload attempting to reduce stresses in the pars. To clarify this, an improved FEM of the L3 and L4 vertebrae and IVDs, including all ligaments, is currently being developed. We believe that in the future this combination of models can be used for many more purposes where the influence of posture and musculature on the lumbar spine biomechanics needs investigation.

Introduction: An estimated 80% of all adults will experience back pain at sometime during their life. To aid in the understanding of how the spine functions as a mechanical system and assist clinicians in their diagnosis this study produced 3D models of the muscles in the lumbar spine region. The models show selected muscles at rest and during controlled activities.

Methods: The images were acquired on a Siemens Sonata 1.5T System using breathhold FISP sequences. Twenty slices of thickness 5mm and zero separation were acquired using an in-plane resolution of .68mm and Fast-Fourier-Transformed to 512 x 512. Single acquisitions were acquired per slice. Imaging time per posture (rest, extension, left rotation and right rotation) was approximately 17–20 seconds. All image series conformed to the DICOM Standard.

The code developed for this study was written in Interactive Data Language (IDL) Version 5.5 from Research Systems Inc (RSI).

Each slice from an image series was displayed to an Operator, who roughly selected the muscle(s) boundary. The user-selected points were then compared with the 24-neighbouring pixels, and the vertices moved to the minimum value in the 5x5 area, which corresponds to the muscle boundary. The adjusted region of interest was then displayed to the user for verification. Once the Operator had completed selection of the regions of interest in all slices, spatial smoothing was performed on the data, and 3D models of the muscles constructed.

Results: This analysis produces 3D images of the muscles in the lower back. The visualisation of the data enables different combinations of muscle and posture to be displayed. Typically, a muscle at rest is overlaid with one of the three controlled activities – extension, left or right extension. The 3D models can be displayed as either a meshed or solid object.

The 3D model is displayed in a window that enables an operator using a mouse to rotate, scale and/or translate the model.

To aid visualisation, the volume of each muscle of interest is calculated using the number of pixels within the region of interest, pixel spacing and slice thickness. The result, in mm³, is displayed alongside the 3D model.

Discussion: The refinement of MR Imaging techniques for subjects in a variety of postures, and the development of post processing techniques provides a useful tool for all in the understanding of the mechanics of the lumbar spine. It is envisaged that this tool with further analysis will assist in determining if there is a link between muscle volume during movement and lower back pain.

INTRODUCTION: An estimated 80% of all adults will experience back pain at some time during their life. To aid in the understanding of how the spine functions as a mechanical system and assist clinicians in their diagnosis this study produced 3D models of the muscles in the lumbar spine region. The models show selected muscles at rest and during controlled activities.

METHODS: The images were acquired on a Siemens Sonata 1.5T System using breathhold FISP sequences. Twenty slices of thickness 5 mm and zero separation were acquired using an in-plane resolution of .68 mm and Fast-Fourier-Transformed to 512 x 512. Single acquisitions were acquired per slice. Imaging time per posture (rest, extension, left rotation and right rotation) was approximately 17–20 seconds. All image series conformed to the DICOM Standard.

The code developed for this study was written in Interactive Data Language (IDL) Version 5.5 from Research Systems Inc (RSI).

Each slice from an image series was displayed to an Operator, who roughly selected the muscle(s) boundary. The user-selected points were then compared with the 24-neighbouring pixels, and the vertices moved to the minimum value in the 5x5 area, which corresponds to the muscle boundary. The adjusted region of interest was then displayed to the user for verification. Once the Operator had completed selection of the regions of interest in all slices, spatial smoothing was performed on the data, and 3D models of the muscles constructed. RESULTS: This analysis produces 3D images of the muscles in the lower back. The visualisation of the data enables different combinations of muscle and posture to be displayed. Typically, a muscle at rest is overlaid with one of the three controlled activities – extension, left or right extension. The 3D models can be displayed as either a meshed or solid object.

The 3D model is displayed in a window that enables an operator using a mouse to rotate, scale and/or translate the model.

To aid visualisation, the volume of each muscle of interest is calculated using the number of pixels within the region of interest, pixel spacing and slice thickness. The result, in mm³, is displayed alongside the 3D model.

DISCUSSION: The refinement of MR Imaging techniques for subjects in a variety of postures, and the development of post processing techniques provides a useful tool for all in the understanding of the mechanics of the lumbar spine. It is envisaged that this tool with further analysis will assist in determining if there is a link between muscle volume during movement and lower back pain.