Effect of Disk Fenestration on Sagittal Kinematics of the Canine C5-C6 Intervertebral Space

OBJECTIVE: To determine the effect that ventral intervertebral disk fenestration has on the sagittal range of motion about the C5-C6 intervertebral space. STUDY DESIGN: A repeated measures in vitro mechanical study of spinal range of motion under controlled loading conditions before and after fenestration. SAMPLE POPULATION: A total of 10 canine cervical vertebral specimens (C4-C7) collected from clinically normal animals within 12 hours of euthanasia. METHODS: Specimens were loaded as cantilever beams fixed at C7. Weights (1 to 5 kg) were progressively applied to C4 to produce flexion or extension in the sagittal plane. Radiographs were taken at each load, 3 times before and 3 times after fenestration of the C5-C6 disk. The positions of radiodense markers embedded in the vertebrae were used to calculate flexion and extension angles and range of motion. RESULTS: Range of motion (difference between flexion and extension) and flexion and extension angles (individually) significantly increased after fenestration (P < .0001). CONCLUSION: Ventral fenestration produces sagittal instability of the C5-C6 disk space. CLINICAL RELEVANCE: In surgical fusion of caudal cervical intervertebral spaces in dogs, radiographically normal disks adjacent to the affected space are often fenestrated to facilitate distraction before surgical stabilization. This study shows that ventral fenestration produces instability of a caudal cervical intervertebral space in the model used. Such instability may contribute to the development of the secondary instability ("domino lesions") seen in some surgically treated dogs.     

A morphometric investigation on breed-specific features affecting sagittal rotational and lateral bending mobility in the canine cervical spine (c3-c7)

Vertebral and inter-vertebral parameters obtained in large breeds (n = 74), small breeds (n = 35), and Dachshunds (n = 30) were compared to reveal potential differences in the range of motion of the cervical spine between these three groups of breeds. Body size normalized dimensions of vertebral and inter-vertebral parameters and correlations between these indicate large canine breeds to have a tendency towards higher range of motion in sagittal rotation and lateral bending compared with Dachshunds and small breeds. Higher mobility in large breeds is based on significantly (P < 0.05) lower vertebral endplate heights and widths, shorter vertebral bodies and longer inter-vertebral discs, wider but shorter cranial and caudal articular surfaces, larger differences in width between caudal and cranial joining facets (compared with Dachshunds from C3/4 to C6/7, compared with small breeds from C4/5 to C5/6), and larger differences in length between caudal and cranial joining facets. Large differences in width between caudal and cranial joining facets were associated with small distances between the most medial (C3/4 to C6/7) and lateral (C3/4 to C5/6) aspects of the articular surfaces as well as with small differences in length between caudal and cranial joining facets (C3/4 to C5/6). This suggests that from C3/4 to C5/6 a higher range of motion in lateral bending is coupled to a lower range of motion in sagittal rotation. The present findings contribute also to explain the higher incidence of degenerative lesions of the cervical spine in large dogs.     

Effects of anatomic conformation on three-dimensional motion of the caudal lumbar and lumbosacral portions of the vertebral column of dogs

OBJECTIVE: To determine the association between the 3-dimensional (3-D) motion pattern of the caudal lumbar and lumbosacral portions of the canine vertebral column and the morphology of vertebrae, facet joints, and intervertebral disks. SAMPLE POPULATION: Vertebral columns of 9 German Shepherd Dogs and 16 dogs of other breeds with similar body weights and body conditions. PROCEDURE: Different morphometric parameters of the vertebral column were assessed by computed tomography (CT) and magnetic resonance imaging. Anatomic conformation and the 3-D motion pattern were compared, and correlation coefficients were calculated. RESULTS: Total range of motion for flexion and extension was mainly associated with the facet joint angle, the facet joint angle difference between levels of the vertebral column in the transverse plane on CT images, disk height, and lever arm length. CONCLUSIONS AND CLINICAL RELEVANCE: Motion is a complex process that is influenced by the entire 3-D conformation of the lumbar portion of the vertebral column. In vivo dynamic measurements of the 3-D motion pattern of the lumbar and lumbosacral portions of the vertebral column will be necessary to further assess biomechanics that could lead to disk degeneration in dogs.     

Morphologic and morphometric magnetic resonance imaging features of Doberman Pinschers with and without clinical signs of cervical spondylomyelopathy

OBJECTIVE: To compare morphologic and morphometric features of the cervical vertebral column and spinal cord of Doberman Pinschers with and without clinical signs of cervical spondylomyelopathy (CSM; wobbler syndrome) detected via magnetic resonance imaging (MRI). ANIMALS: 16 clinically normal and 16 CSM-affected Doberman Pinschers. PROCEDURES: For each dog, MRI of the cervical vertebral column (in neutral and traction positions) was performed. Morphologically, MRI abnormalities were classified according to a spinal cord compression scale. Foraminal stenosis and intervertebral disk degeneration and protrusion were also recorded. Morphometric measurements of the vertebral canal and spinal cord were obtained in sagittal and transverse MRI planes. RESULTS: 4 of 16 clinically normal and 15 of 16 CSM-affected dogs had spinal cord compression. Twelve clinically normal and all CSM-affected dogs had disk degeneration. Foraminal stenosis was detected in 11 clinically normal and 14 CSM-affected dogs. Vertebral canal and spinal cord areas were consistently smaller in CSM-affected dogs, compared with clinically normal dogs. In neutral and traction positions, the intervertebral disks of CSM-affected dogs were wider than those of clinically normal dogs but the amount of disk distraction was similar between groups. CONCLUSIONS AND CLINICAL RELEVANCE: The incidence of intervertebral disk degeneration and foraminal stenosis in clinically normal Doberman Pinschers was high; cervical spinal cord compression may be present without concurrent clinical signs. A combination of static factors (ie, a relatively stenotic vertebral canal and wider intervertebral disks) distinguished CSM-affected dogs from clinically normal dogs and appears to be a key feature in the pathogenesis of CSM.     

RELATIONSHIP OF CERVICAL SPINAL CORD DIAMETER TO VERTEBRAL DIMENSIONS: A RADIOGRAPHIC STUDY OF NORMAL DOGS

Cervical spinal cord abnormalities are often unapparent on myelographic studies, because no normal values for cervical spinal cord diameter are currently available. The purpose of this study was to establish, myelographically, the normal sagittal diameter of the cervical spinal cord in large and small breed dogs and its relationship to the sagittal diameter of the vertebral canal and sagittal height/length of the corresponding vertebral bodies. Forty-one adult dogs underwent cervical radiography and myelography. Spinal cord and vertebral canal sagittal diameter, vertebral body height at C2 to 5, body length at C3 to 5, and dorsal spine length of C2 were measured on lateral views. Ratios of spinal cord:vertebral canal diameter, spinal cord:body height, and spinal cord:body length/spine were calculated, and a normal range was determined for small and large breed dogs. The spinal cord:vertebral canal ratios showed that small breeds have a higher cervical cord-to-canal ratio than large breeds. The mean values and ranges of 14 ratios are reported. The ratios of spinal cord:body length at C2 to 4 in small breeds and spinal cord:body height at C3 to 5 in large breeds were found to be the most accurate for assessing spinal cord sagittal diameter. These normal ranges would allow quantitative and objective evaluation of the cervical spinal cord by myelography and early identification of dogs with altered spinal cord diameter, which could be further evaluated by means of alternative imaging techniques.     

RELIABILITY OF T2‐WEIGHTED SAGITTAL MAGNETIC RESONANCE IMAGES FOR DETERMINING THE LOCATION OF COMPRESSIVE DISk HERNIATION IN DOGS

Magnetic resonance imaging is used commonly to diagnose intervertebral disk herniation in dogs. It is common to locate areas of suspected compression on sagittal T2‐weighted (T2‐W) images and then obtain limited transverse images in these areas to reduce the acquisition time (a step‐by‐step approach). Our objective was to assess the frequency of correct localization of spinal cord compression due to disk herniation using only the sagittal images. The results from isolated readings of the sagittal T2‐W images alone or combined with a single‐shot fast spin echo (SSFSE) slab in 118 dogs were compared with a gold standard, based on a consensual reading of all images available, including complete transverse images across the entire spinal segments under study. The sites of compression were localized correctly from the sagittal images in 89.8% of dogs. If only the most significant lesions were accounted for, the percentage increased up to 95.2%. In 54.9% of the readings with incorrect localization, the actual compressive site was immediately adjacent to the one suspected from review of the sagittal images. The frequency of correct localization was higher in the cervical region, and was increased by examination of the SSFSE slab. The most common cause of disagreement was the presence of multiple degenerate bulging disks. Based on these results we recommend obtaining transverse images across the entire segment when multiple bulging disks are present. It is also recommended to obtain transverse images across the spaces immediately adjacent to the suspected site of herniation from review of the sagittal images.     

Association of cauda equina compression on magnetic resonance images and clinical signs in dogs with degenerative lumbosacral stenosis

Magnetic resonance imaging (MRI) was used to examine the lumbosacral spine of 27 dogs with degenerative lumbosacral stenosis. Four normal dogs were also similarly imaged. Compression of the soft-tissue structures within the vertebral canal at the lumbosacral space was assessed in two ways: by measuring dorsoventral diameter on T1-weighted sagittal images and cross-sectional area on transverse images. The severity of the clinical signs was compared to the severity of cauda equina compression. No significant correlation was found. It is concluded that degree of compression as determined by MRI at time of presentation is independent of disease severity.     

Magnetic resonance imaging of the ligamentous structures of the occipitoatlantoaxial region in the dog

Our objectives were to establish a magnetic resonance (MR) protocol for the examination of, and then describe, the normal ligaments and the supporting structures of the occipitoatlantoaxial region. This was done in 10 cadaver dogs. In addition, MR images of three patients with cervical pain localized to the occipitoatlantoaxial region are included to provide examples of ligamentous abnormalities. All ligaments were hypointense in all pulse sequences. The apical, dorsal atlantoaxial, and dorsal longitudinal vertebral ligaments were seen best in the sagittal T1W and PD-weighted images. The transverse ligament was best visualized in the transverse plane in all pulse sequences and appeared to be confluent with the dorsal longitudinal vertebral ligament dorsal to the dens in the sagittal plane. A 20° dorsal plane reconstructed image in 0.6-mm slice thickness was necessary to visualize the alar ligaments, which were visible in 9/10 dogs. The dorsal longitudinal vertebral ligament appeared continuous with the apical ligament and tectorial membrane. Abnormalities in clinically affected dogs included thickening of the alar ligaments, absence of transverse ligament and elongation, and irregularity of the apical ligament.     

MRI features of cervical articular process degenerative joint disease in Great Dane dogs with cervical spondylomyelopathy

Cervical spondylomyelopathy or Wobbler syndrome commonly affects the cervical vertebral column of Great Dane dogs. Degenerative changes affecting the articular process joints are a frequent finding in these patients; however, the correlation between these changes and other features of cervical spondylomyelopathy are uncertain. We described and graded the degenerative changes evident in the cervical articular process joints from 13 Great Danes dogs with cervical spondylomyelopathy using MR imaging, and evaluated the relationship between individual features of cervical articular process joint degeneration and the presence of spinal cord compression, vertebral foraminal stenosis, intramedullary spinal cord changes, and intervertebral disc degenerative changes. Degenerative changes affecting the articular process joints were common, with only 13 of 94 (14%) having no degenerative changes. The most severe changes were evident between C4-C5 and C7-T1 intervertebral spaces. Reduction or loss of the hyperintense synovial fluid signal on T2-weighted MR images was the most frequent feature associated with articular process joint degenerative changes. Degenerative changes of the articular process joints affecting the synovial fluid or articular surface, or causing lateral hypertrophic tissue, were positively correlated with lateral spinal cord compression and vertebral foraminal stenosis. Dorsal hypertrophic tissue was positively correlated with dorsal spinal cord compression. Disc-associated spinal cord compression was recognized less frequently.     

MAGNETIC RESONANCE IMAGING OF CANINE DEGENERATIVE LUMBAR SPINE DISEASES

Degenerative lumbar spine diseases, i.e., sacrolumbar stenosis, intervertebral disk degeneration and protrusion and spondylosis deformans of the canine lumbar spine were studied in eleven canine patients and three healthy controls using radiography and 0.02 T and 0.04 T low field magnetic resonance imaging. The T1 and T2 weighted images were obtained in sagittal and transverse planes. The loss of hydration of nucleus pulposus, taken as a sign of degeneration in the intervertebral disks, could be evaluated in both T1 and T2 weighted images. As a noninvasive method magnetic resonance imaging gave more exact information about the condition of intervertebral disks than did radiography. Sacrolumbar stenosis and compression of the spinal cord or cauda equina and surrounding tissue could be evaluated without contrast medium.     

Vacuum disk and facet phenomenon in a dog with cauda equina syndrome

A 7-year-old castrated male Rottweiler was evaluated for an 18-month history of bilateral hind limb lameness, signs of pain on rising, and mild ataxia. A gas-filled lumbosacral disk space (vacuum disk phenomenon) was revealed by use of computed tomography. Additional smaller gas bubbles were in the periphery of the disk and between the degenerated L5-L6 dorsal articular facets (vacuum facet phenomenon). Vacuum disk and facet phenomena have been described in humans with degenerative disk and vertebral articular facet joint disease; gas attracted from surrounding tissues accumulates within clefts of the degenerated disk or in unstable facet joints. Vacuum disk phenomenon has been reported in dogs and indicates degenerative disk disease. The phenomenon is more commonly observed by use of computed tomography, compared with conventional radiography, and can be elicited by hyperextension of the vertebral column.     

Intervertebral biomechanics of locking compression plate monocortical fixation of the canine cervical spine

Objective: To evaluate the use of a locking compression plate (LCP) with monocortical screw purchase for stabilization of the canine cervical spine. Study Design: Experimental study. Animals: Cadaveric canine cervical spine specimens (n=7). Methods: Flexion and extension bending moments were applied to canine cadaveric specimens (C3–C6) in 4‐point bending, before and after creation of a ventral slot at C4–C5, and after fixation with a 5 hole, 3.5 mm LCP with monocortical screw placement. Screw placement and penetration into the vertebral canal were determined by radiography. Range of motion, stiffness, and energy for passive physiologic loads were determined for the C3–C4, C4–C5, and C5–C6 vertebral motion units (VMU). Monotonic failure properties were determined for cervical extension. Effects of treatments on biomechanical variables were assessed using repeated measures analysis of variance and least square means (P≤.05). Results: The ventral slot procedure increased range of motion at the treated VMU. Plate fixation decreased range of motion, increased stiffness, and decreased energy at the treated VMU. No changes were observed at adjacent VMUs. None of the screws penetrated the vertebral canal. Mean (± SD) yield bending moment of plate stabilized, slotted spines was 15.6 ± 4.6 N m. Conclusion: LCP fixation with monocortical screws stabilized the canine cervical spine.     

In vitro biomechanical comparison of cervical disk arthroplasty, ventral slot procedure, and smooth pins with polymethylmethacrylate fixation at treated and adjacent canine cervical motion units

Objective— To compare the biomechanical properties of cervical arthroplasty to a ventral slot procedure and pin‐polymethylmethacrylate (pin‐PMMA) fixation. Sample Population— Fresh cadaveric cervical (C2–T1) spines from 6 large dogs. Methods— Four spinal conditions were studied in each spinal specimen: intact, disk arthroplasty, ventral slot, and fixation with smooth pin‐PMMA at C5–C6 intervertebral space. Axial compression, torsion, flexion–extension, and lateral bending moments were sequentially tested on each specimen for the 4 spinal conditions. Data from the C3–C4, C4–C5, C5–C6, and C6–C7 vertebral motion units (VMUs) were compared among treatments. Results— In axial compression and torsion, the ventral slot procedure allowed significantly less motion than intact, pin‐PMMA, and arthroplasty groups at C5–C6. In lateral bending and flexion–extension, pin‐PMMA had the least motion of C5–C6, followed by the arthroplasty group, intact spine, and ventral slot, all of which were significantly different from each other. Overall, the artificial disk was better able to mimic the behavior of the intact specimens compared with the ventral slot and pin‐PMMA, producing similar displacements in axial compression and rotation in torsion, but more limited motion than intact in flexion–extension and in lateral bending. Conclusion— Cervical spine specimens with an implanted prosthesis have biomechanical behaviors more similar to an intact spine compared with spinal specimens with ventral slot and pin‐PMMA procedures. Cervical arthroplasty may then preserve some of the motion in the affected area after neural decompression while providing distraction. Clinical Relevance— Cervical arthroplasty should be further investigated in vivo to determine if it is a viable alternative to the ventral slot or pin‐PMMA procedures for surgical treatment of cervical diseases in dogs and in particular for treatment of disk‐associated caudal cervical spondylomyelopathy.     

MAGNETIC RESONANCE IMAGING OF ARTICULAR PROCESS JOINT GEOMETRY AND INTERVERTEBRAL DISK DEGENERATION IN THE CAUDAL LUMBAR SPINE (L5–S1) OF DOGS WITH CLINICAL SIGNS OF CAUDA EQUINA COMPRESSION

The geometry of the lumbosacral region has been suspected to play a role in the development of degenerative lumbosacral stenosis in the dog. In this study, 50 dogs (21 German Shepherd dogs and 29 dogs of other breeds) with clinical signs of cauda equina compression were studied by magnetic resonance (MR) imaging. The orientation of the articular process joints in the L5-S1 region and the angle difference between two adjacent motion segments were calculated. Intervertebral disc degeneration of the same region was identified and classified in four stages. A positive association between MR-imaging stage and articular process joint angle difference in the transverse plane was found in the two groups of animals. German Shepherd dogs and dogs of other breeds had different geometry of the lumbosacral region with different articular process joint angles in the transverse plane and statistically different stages of disc degeneration.     

Effect of a ventral slot procedure and of smooth or positive-profile threaded pins with polymethylmethacrylate fixation on intervertebral biomechanics at treated and adjacent canine cervical vertebral motion units

OBJECTIVE: To investigate the biomechanics of cervical vertebral motion units (VMUs) before and after a ventral slot procedure and after subsequent pin-poly-methylmethacrylate (pin-PMMA) fixation and to assess the use of smooth and positive-profile threaded (PPT) pins in pin-PMMA fixation and intravertebral pin placement. SAMPLE POPULATION: Cervical portions (C3 through C6 vertebrae) of 14 cadaveric canine vertebral columns. PROCEDURE: Flexion and extension bending moments were applied to specimens before and after creation of a ventral slot across the C4-C5 intervertebral space and after subsequent smooth or PPT pin-PMMA fixation at that site. Data for the C3-C4, C4-C5, and C5-C6 VMUs were compared among treatments and between pin types, and pin protrusion was compared between pin types. RESULTS: Compared with values in intact specimens, ventral slot treatment increased neutral zone range of motion (NZ-ROM) by 98% at the treated VMUs and appeared to decrease overall ROM at adjacent VMUs; pin-PMMA fixation decreased NZ-ROM by 92% at the treated VMUs and increased overall NZ-ROM by 19% to 24% at adjacentVMUs. Specimens fixed with PPT pins were 82% (flexion) and 80% (extension) stiffer than smooth-pin-fixed specimens. Overall, 41% of pins protruded into foramina; PPT pins were more likely to protrude into transverse foramina. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that fixation of a cervical VMU alters the biomechanics of adjacent VMUs and may contribute to intervertebral degeneration of adjacent intervertebral disks. Use of threaded pins may lower the incidence of pin loosening and implant failure but enhances the likelihood of transverse foramina penetration.     

Kinematic study of back movement in clinically sound malinois dogs with consideration of the effect of radiographic changes in the lumbosacral junction

Objective— To determine thoracolumbar spinal movement in dogs and the influence of subclinical radiographic changes involving the lumbosacral junction. Study Design— Experimental study. Animals— Clinically sound Malinois dogs (n=22). Methods— Kinematic analysis of markers on the spinal processes of C7, T6, T13, L3, L7, and S3 was performed while dogs were walking on a treadmill. Range of motion (ROM) in the transverse and vertical direction and the time of occurrence (TOO) of the maximal marker position were calculated. ROM and TOO of angulations formed by the corresponding markers were calculated. Initial kinematic analysis was performed without knowledge of the radiographic changes, and then data were reanalyzed to determine whether vertebral changes influenced back motion. Based on the results of radiographic analysis of the lumbosacral junction, dogs were divided into 3 groups: 1=no radiographic changes; 2=shortened L7 vertebra; and 3=transitional vertebrae, spondylosis, subluxations, and spondylarthrosis of the lumbosacral junction. ROM and TOO were compared using ANOVA for repeated measures and a Bonferroni's post hoc test; P<.05 was considered significant. Results— The highest transverse ROM was achieved by markers T6, T13, and L3, and in the vertical direction by S3; however, there were no significant differences in ROM in horizontal angulations. In the sagittal plane, T13–L3–L7 had a lower angulation than L3–L7–S3. In Group 3, transverse ROM for C7 was significantly higher than in Group 1; the horizontal angular maximum of T13–L3–L7 occurred significantly earlier. Conclusions— Significant kinematic changes were detected between clinically sound dogs with radiographic lumbosacral changes and dogs with no radiographic abnormalities. Clinical Relevance— Kinematic data from clinically sound dogs can be used for comparison with data from dogs with gait disturbances associated with orthopedic or neurologic disease or changes associated with therapy.     

Morphometry of the thoracic spine in German shepherd dog: a computed tomographic study

Computed tomographic images of the thoracic spine of 13 German shepherd dogs were examined in order to determine the thoracic spine morphometry. Examinations were carried out in the transverse plane both intervertebral and mid-vertebral levels of the each thoracic vertebrae. The dorsoventral and interpedicular diameters of the spinal canal, the dorsoventral and transverse diameters of the vertebral body, the dorsoventral and transverse diameters of the spinal cord and also the cross-section area of the spinal canal were measured. The maximum values were found to be at the level of C7-T1. The shapes of the spinal canal and cord were circular in middle part, the shape became transverse oval in the cranial and caudal parts of the thoracic spine. The most significant correlation between the diameters was found to be in male dogs, except between dorsoventral diameters of the spinal canal and that of the vertebral body and between dorsoventral diameters of the spinal canal and transverse diameters of the vertebral body.     

Magnetic Resonance Imaging (MRI) Anatomy of the Ovine Lumbar Spine

Although the ovine spine is a useful research model for intervertebral disc pathology and vertebral surgery, there is little peer‐reviewed information regarding the MRI anatomy of the ovine spine. To describe the lumbar spine MRI anatomy, 10 lumbar segments of cadaver ewes were imaged by 1.5‐Tesla MR. Sagittal and transverse sequences were performed in T1 and T2 weighting (T1W, T2W), and the images were compared to gross anatomic sagittal and transverse sections performed through frozen spines. MRI was able to define most anatomic structures of the ovine spine in a similar way as can be imaged in humans. In both T1W and T2W, the signals of ovine IVDs were similar to those observed in humans. Salient anatomic features were identified: (1) a 2‐ to 3‐mm linear zone of hypersignal was noticed on both extremities of the vertebral body parallel to the vertebral plates in sagittal planes; (2) the tendon of the crura of the diaphragm appeared as a hypointense circular structure between hypaxial muscles and the aorta and caudal vena cava; (3) dorsal and ventral longitudinal ligaments and ligamentum flavum were poorly imaged; (4) no ilio‐lumbar ligament was present; (5) the spinal cord ended between S1–S2 level, and the peripheral white matter and central grey matter were easily distinguished on T1W and T2W images. This study provides useful reference images to researchers working with ovine models.     

Assessment of the utility of using intra- and intervertebral minimum sagittal diameter ratios in the diagnosis of cervical vertebral malformation in horses.

Cervical vertebral malformation is one of the most common causes of ataxia in horses. The most important factor in the diagnosis of cervical vertebral malformation is the identification of cervical vertebral canal stenosis, but published data for minimum sagittal diameter ratios in adult horses are only available for C4-C7 intravertebral sites. Intra- and intervertebral sagittal diameter ratios at C2-C7 were evaluated in 26 ataxic horses, for which a complete clinical and neuropathological evaluation was undertaken. Eight of these horses were diagnosed with cervical vertebral malformation. In these horses the majority of compressive lesions were intervertebral. The mean sagittal diameter ratios of horses with cervical vertebral malformation were significantly smaller than those of horses without cervical vertebral malformation, and for an individual horse in our study, the site with the smallest intervertebral sagittal diameter ratio was always the site at which the spinal cord was compressed. Mean sagittal diameter ratio intravertebral site measurements of horses with cervical vertebral malformation were smaller than those of horses without cervical vertebral malformation; however, the site of compression could not be predicted from the data. For our dataset, horses with a sagittal diameter ratio of < or = 0.485 at any inter- or intravertebral site could be correctly classified as having cervical vertebral malformation, and sagittal diameter ratio measurements were an effective tool to identify at least one site of compression in an individual case.