Richard W. Porter

Spinal stenosis and neurogenic claudication.

Neurogenic claudication is diagnosed from a classical history and complementary spinal imaging. The abnormal signs may be few. It should be distinguished from intermittent claudication (peripheral vascular disease), referred pain from the back or root pain that is aggravated by walking, and psychological distress. Pathologically, a developmentally small canal is usually affected by multiple levels of segmental degenerative change, with venous pooling in the cauda equina between two levels of low pressure stenosis. There is probably then a failure of arterial vasodilatation of the congested roots in response to exercise, with symptoms in the legs when walking. Once established, symptoms tend neither to improve nor deteriorate. Conservative management is reasonable. Otherwise decompression at the most significant stenotic level is probably adequate to obtain a good surgical result.

An in vitro study of the biomechanical effects of flexible stabilization on the lumbar spine

Study Design Lumbar motion segments were tested in vitro to examine biomechanical changes after posterior fixation by a flexible device. Objectives To assess changes in load distribution and conformation of vertebral structures after a flexible stabilization. This should provide the foundations for a scientific understanding of the immediate effects of this surgical procedure. Methods Hooks were placed over the proximal spinous process and the distal laminas of a motion segment and connected by a polyester braid. Tension applied to the braid then generated a compression of the posterior elements. The force between the articular facets, the displacement of the posterior anulus fibrosus of the intervertebral disc, and the change in the relative position of the adjacent vertebrae were measured as the applied tension was increased. Results Facet joint force, disc bulge, and vertebral angulation increased with applied tension until a position of “locking” was achieved, apparently when the bony margin of the superior half of the facet joint contacted the inferior pars interarticularis. A tension of between 50 to 100 N in the braid was required for this. Facet joint force was less than 40% of this, and disc bulge was only 0.15 mm. The extension of the motion segment was between 2° and 8°. Conclusions The results suggest that if such a system is applied surgically, stabilization is produced by compaction of the bony margins of the facet joints. Only a relatively small proportion of the posteriorly applied load is carried by the facet joints themselves, and little angulatory change is expected with minimal disc bulge.

A comparison of methods for measuring trunk list. A simple plumbline is the best.

Study Design Trunk list was measured using three different techniques to compare accuracy, precision, and ease of use. Objective To obtain a reproducible technique for further studies of the nature, cause, and clinical relevance of trunk list. Summary of Background Data Gravity‐induced trunk list is a clinical sign that is frequently observed in patients with low back pain and has been associated with intervertebral disc lesions. Methods Patients with trunk list participated in a comparison of three techniques to determine list magnitude and direction. Paired measurements of trunk list were obtained from each patient using three techniques: a plumbline, a projected shadow, and the 3SPACE lsotrak (McDonnell Douglas Electronics Company, Colchester, VT). In addition, intra‐ and interobserver reliability of list measurement was assessed by comparison of paired measurements by each of two observers. Results List measurements assessed by the plumbline and the projected shadow techniques were not significantly different, but the lsotrak produced data that differed significantly (P < 0.05) from both of these techniques. Comparison of intra‐ and interobserver repeatability of list measurement using the plumbline technique indicated no significant difference between repeated measures by each observer or between two observers. Conclusions A plumbline is the most useful instrument for measuring static trunk list, but its limitations and the need for standardization of measurement technique must be recognized.

Laser Doppler Study of Porcine Cauda Equina Blood Flow: The Effect of Electrical Stimulation of the Rootlets During Single and Double Site, Low Pressure Compression of the Cauda Equina

Study Design This study involved a model of spinal claudication to assess the effect of single and double site compressions on blood flow in porcine cauda equina. Real-time monitoring of blood flow was achieved by using laser Doppler probes. Objectives To ascertain the difference between single and double site compressions on blood flow and nerve conduction in the cauda equina. Also, to provide possible explanations for the mechanisms underlying the pathophysiology of neurogenic claudication. Summary of Background Data The model used was based on that of K. Olmarker. Double as compared with single site compression of cauda equina showed impaired nerve impulse propagation and decreased blood flow. Method Pigs weighing 22–27 kg were anesthetized and the cauda equina was exposed by dorsal laminectomy of the sacral and first three coccygeal vertebrae. Polyethylene balloons were placed over the rootlets at the first and third coccygeal segments, and stimulating electrodes were positioned on the rootlets proximally to evoke motor activity. Electromyographic activity was monitored from tail musculature. Laser Doppler probes monitored blood flow in the cauda equina both between and distal to the two compression sites. Results Single site, low level compression did not affect blood flow whereas double site compression decreased it profoundly. Proximal stimulation caused a marked increase in blood flow, which was not sustained during prolonged compression. Electromyographic activity diminished concomitantly during this compression. Conclusions Low pressure, double site compression significantly reduces cauda equina blood flow and prevents the sustained increase in blood flow required to maintain normal neurologic rootlet function. Local blood flow failure may therefore be responsible for claudication symptoms.

The natural history of trunk list, its associated disability and the influence of McKenzie management

Abstract Lumbosacral list is a clinical sign that is frequently associated with low back pain and intervertebral disc lesions. This study examines the influence of McKenzie management on the natural history of trunk list. Patients with trunk list and low back pain were randomised into two groups: a control group receiving non-specific back massage and general back care advice, and a group treated according to the McKenzie protocol. Trunk list was measured over a period of 90 days and patients completed Oswestry Disability Questionnaires. There was a significantly greater resolution of list after 90 days in the group receiving McKenzie treatment compared to the control group. There was poor correlation between list magnitude and Oswestry scores. These data support previous observations that trunk list is not necessarily related to the degree of physical disability. The McKenzie method of assessment and treatment may assist in the resolution of trunk list, but it was ineffective in improving clinical condition.

Measurement of vertebral foraminal dimensions using three-dimensional computerized tomography.

The dimensions of lumbar intervertebral foramina were measured in cadaveric spines using three-dimensional computerized tomography. Six different image reconstruction protocols were used. The results were compared with measurements of the same foramina using calipers after dissection. All the three-dimensional computerized tomographic measurements underestimated the true foraminal dimensions. The best agreement, as well as the best images, were obtained using 4/3 slices and a reconstruction threshold of 300 Hounsfield Units. This method is not recommended for measurement of foraminal dimensions. However, there may be a use in assessment and planning surgical management.

Measurement of blood flow in tibial fracture patients using positron emission tomography

The quantification of local bone blood flow in man has not previously been possible, despite its importance in the study of normal and pathological bone. We report the use of positron emission tomography, using 15O-labelled water, to measure bone blood flow in patients with closed unilateral fractures of the tibia. We compared fractured and unfractured limbs; alterations in blood flow paralleled those found in animal models. There was increased tibial blood flow at the fracture site as early as 24 hours after fracture, reaching up to 14 times that in the normal limb at two weeks. Blood flow increase was less in displaced than in undisplaced fractures. The muscle to bone ratios of blood flow were similar to those in previous animal work using other techniques. Positron emission tomography will allow study of human bone blood flow in vivo in a wide variety of pathological conditions.

Trefoil configuration and developmental stenosis of the lumbar vertebral canal

The midsagittal and interpedicular diameters and the trefoil shape of lumbar vertebrae of known age at death were measured in skeletons from a population aged between 1 and 70 years. All the trefoil configurations were at L5 with the exception of one at L4. The overall prevalence was 25%, but this shape was not generally apparent until adulthood. The midsagittal diameter in the trefoil canals was found to be significantly smaller than that in the unaffected canals. This did not change significantly after six years of age indicating that the cause of the trefoil configuration is probably present early in life. The trefoil shape was no more common in the spines of the elderly subjects. Our findings indicate that the trefoil configuration of the lumbar vertebral canal has a developmental origin and is not a consequence of degenerative processes.

Changes of the lumbar spinal canal proximal to spina bifida occulta. An archaeologic study with clinical significance.

Study Design This archaeologic study, based on four populations, examines the incidence of spina bifida occulta in the lumbar spine and the size of the vertebral canal proximal to the lesion. Objectives To ascertain any significant change in the dimensions of the lumbar spinal canal of skeletons with spine bifida occulta. The incidence of the lesion also was compared in the separate genetic groups. Methods Central canals of 1760 lumbar vertebrae were examined. Silhouette, unmagnified pictures of the vertebral canals were measured by computerized image analysis. Results The mid-sagittal diameter at L4 and L5 and the cross-sectional area at L5 were found to be significantly larger proximal to the lesion compared with the unaffected spines. The overall incidence was 18%. Conclusions The capacity of the lumbar canal is greater proximal to spina bifida occulta. Therefore, delayed closure of the neural arch at a single segment has morphologic significance to the more proximal spine.

Nerve traction during correction of knee flexion deformity. A case report and calculation

We report the case of a child with cerebral palsy and spastic diplegia treated for bilateral fixed flexion of the knee by bilateral hamstring lengthening. An attempt to straighten the legs from 90 degrees to 20 degrees flexion damaged the sciatic nerve. There are no objective means of estimating how much deformity can be reduced safely. We present a method of calculating the extra strain in the sciatic nerve produced by reducing a flexion deformity. The result, combined with clinical judgement, provides guidelines for safe corrective surgery.