Janet E. Macintosh’s research while affiliated with University of Newcastle Australia and other places

A modeling study was undertaken to determine the effects of flexion on the forces exerted by the lumbar back muscles. Twenty-nine fascicles of the lumbar multifidus and erector spinae were plotted onto tracings of radiographs of nine normal volunteers in the flexion position. Moment arms and force vectors of each fascicle were calculated. The model revealed that moment arms decreased slightly in length resulting in no more than an 18% decrease in maximum extensor moments exerted across the lumbar spine. Compression loads were not significantly different from those generated in the upright posture. However, there were major changes in shear forces, in particular a reversal from a net anterior to a net posterior shear force at the L5/S1 segment. Flexion causes substantial elongation of the back muscles, which must therefore reduce their maximum active tension. However, if increases in passive tension are considered it emerges that the compression forces and moments exerted by the back muscles in full flexion are not significantly different from those produced in the upright posture.

The maximal, axial torque generated by the lumbar back muscles was determined by modelling the action of the 49 fascicles of longissimus thoracis, iliocostalis lumborum and the lumbar multifidus on radiographs of the lumbar spine of nine young male subjects in upright standing and in full lumbar flexion. No single fascicle exerted more than 2 Nm of axial torque in the upright posture, and the collective torque of all muscles acting a segment did not exceed 5 Nm. All torques were considerably less in full flexion. The lumbar back muscles exert very little torque on the lumbar spine, and contribute only about 5% of the total torque involved in trunk rotation. None of the lumbar back muscles can be considered a rotator. The oblique abdominal muscles are the principal rotators of the trunk. Preventative and rehabilitation programmes concerned with torsion injuries should focus on the abdominal muscles rather than the back muscles for stability in axial rotation.

A model of the lumbar back muscles was constructed incorporating 49 fascicles of the lumbar erector spinae and multifidus. The attachment sites and sizes of fascicles were based on previous anatomic studies, and the fascicles were modeled on radiographs of nine normal volunteers in the upright position. Calculations revealed that the thoracic fibers of the lumbar erector spinae contribute 50% of the total extensor moment exerted on L4 and L5; multifidus contributes some 20%; and the remainder is exerted by the lumbar fibers of erector spinae. At upper lumbar levels, the thoracic fibers of the lumbar erector spinae contribute between 70% and 86% of the total extensor moment. In the upright posture, the lumbar back muscles exert a net posterior shear force on segments L1 to L4, but exert an anterior shear force on L5. Collectively, all the back muscles exert large compression forces on all segments. A force coefficient of 46 Ncm-2 was determined to apply for the back muscles. These results have a bearing on the appreciation of the effects on the back muscles of surgery and physiotherapy.

The attachments and orientation of every fascicle of the lumbar erector spinae were determined in five cadavers and recorded radiographically. Little variation was found in the sites of muscle attachment, which enabled the construction of maps whereby these sites could be plotted on clinical radiographs or models of the lumbar spine. When all fascicles were plotted on 21 clinical radiographs using the maps previously developed, no significant difference in the orientation of fascicles was found compared with that observed in cadavers. This result vindicates the technique used to plot the location of individual fascicles of the lumbar back muscles.

The lumbar erector spinae consists of two muscles--iliocostalis lumborum and longissimus thoracis--each with distinct thoracic and lumbar parts. The thoracic parts consist of tiny muscle bellies with segmental origins from the thorax and long caudal tendons that form the erector spinae aponeurosis. The lumbar fibers arise from the lumbar accessory processes and the L1-4 transverse processes, and insert independently of the erector spinae aponeurosis into the ilium. The intrinsic lumbar fibers of the erector spinae are poorly described in the literature, and the existence of the iliocostalis lumborum pars lumborum has rarely been recognized even though it constitutes a substantial portion of the total muscle mass acting directly on the lumbar vertebrae.

Prompted by clinical failures of percutaneous radiofrequency neurotomy in the treatment of back pain and neck pain, we performed a study to determine the shape and size of lesions made by radiofrequency electrodes. Experimental lesions were made in egg white and fresh meat at temperatures recommended in clinical practice. The cardinal finding was that lesions do not extend distal to the tip of the electrode. They only extend radially around the electrode tip in the shape of an oblate spheroid, with a maximal effective radius of only 2 mm. Consequently, if electrodes are directed perpendicularly onto a nerve, the nerve may not be encompassed by the lesion generated. Some of the clinical failures of percutaneous medial branch neurotomy ("facet rhizotomy") may be due to this phenomenon. We suggest modified techniques for medial branch neurotomy in which the electrodes are introduced parallel to the target nerve whereupon it is more readily encompassed by the radial spread of the lesion.

The back muscles alone are unable to provide the extensor moment required to lift large weights, and must be aided by another source of anti-flexion moments. It has been postulated that contraction of the abdominal muscles can provide an extension moment by developing tension in the thoracolumbar fascia (TLF). Anatomical studies and a biomechanical analysis, however, reveal that the anti-flexion moment generated in this way is only very small. Too little of the abdominal musculature attaches to the TLF to generate a significant tension in it. Previous calculations of the forces in the TLF have overestimated the tension developed in it because of erroneous assumptions and interpretations of the relevant anatomy. Whatever the role played by the TLF in lifting it must be essentially independent of abdominal mechanisms.

The possible actions of the lumbar multifidus were determined by plotting the points of attachment and orientation of each of its component fascicles on radiographs of 5 cadavers and 21 living subjects. Subsequent analysis revealed that the principal action of multifidus is posterior sagittal rotation (extension without posterior translation) of the lumbar vertebrae. It has no translatory action. Any axial rotation exerted by the lumbar multifidus is only a minor, secondary action which must be coupled with posterior sagittal rotation. This extension balances the flexion moment generated by the abdominal muscles which rotate the trunk. The constancy of the sites of attachment of the multifidus allows each of its fascicles to be plotted accurately on radiographs or computer diagrams which can be used to produce highly detailed analyses or models of the forces exerted by the multifidus on the lumbar spine.

Dissection studies revealed that the fibres of the lumbar multifidus are divided by distinct cleavage planes into five bands. Each band arises from a lumbar spinous process, and is innervated unisegmentally. The lumbar multifidus is therefore composed of five myotomes arranged such that the fibres that move a particular segment are innervated by the nerve of that segment. Target points are described that enable electromyography to be performed on paraspinal muscles of known unisegmental innervation.

The thoracolumbar fascia was studied by dissection in ten adult human cadavers. The posterior layer of this fascia was found to consist of two laminae. The superficial lamina is formed by the aponeurosis of latissimus dorsi. The deep lamina consists of bands of fibers passing caudolaterally from the midline. Both laminae form a retinaculum over the back muscles, and the deep lamina constitutes a series of accessory posterior ligaments that anchor the L2 to L5 spinous processes to the ilium and resist flexion of the lumbar spine. The function of these ligaments is enhanced by the contraction of the back muscles and the action of certain, restricted portions of the abdominal muscles.