Inferior Vertebra Research Articles

Spinal shape changes resulting from scoliotic spine surgical instrumentation expressed as intervertebral rotations and centers of rotation

This paper reports the changes in spinal shape resulting from scoliotic spine surgical instrumentation expressed as intervertebral rotations and centers of rotation. The objective is to test the hypothesis that the type of spinal instrumentation system (Cotrel–Dubousset versus Colorado) does not influence these motion parameters. Intervertebral rotations and centers of rotation of the scoliotic spines were computed from the pre- and post-operative radiographs of 82 patients undergoing spinal correction. The three-dimensional (3D) reconstruction of six anatomical landmarks was achieved for each of the thoracic and lumbar vertebrae. A least-squares approach based on singular value decomposition was used to calculate the rigid body transformation parameters. Average centers of rotation for all intervertebral levels are located in the neural canal at the mid-sagittal plane and approximately at the superior endplate level of the inferior vertebra. Intervertebral rotations have components in all planes: 6.7° (frontal), 5.5° (sagittal) and 4.5° (transverse) RMS for all intervertebral levels. Nearly all intervertebral rotations and centers of rotation are not significantly different for the two instrumentation systems. Various intervertebral rotations and 3D reconstruction errors were simulated on a theoretical model of a lumbar functional unit to assess the proposed method. Intervertebral rotation errors were 1.7° when simulating 3D errors of 3 mm on the position of the landmarks. Maximum errors for the position of centers of rotation were below 1 cm in the case of intervertebral rotations larger than 2.5° (most cases), but were larger (38 mm) for small intervertebral rotations (<1°). The type of instrumentation system did not influence intervertebral rotations and centers of rotation. These results provide valuable data for the development and validation of simulation models for surgical instrumentation of idiopathic scoliosis.

BIOMECHANICAL EFFECT OF THORACIC POSTERIOR VERTEBRAL ELEMENTS ON PATTERNS OF THE LOCI OF INSTANTANEOUS AXES OF ROTATION IN SAGITTAL PLANE

The objective of this study was to investigate the effect of the thoracic posterior vertebral elements on the kinematics of T10–T11 motion segment in sagittal plane by assessing the locations and loci of the instantaneous axes of rotation (IARs) under flexion and extension pure moments using finite element (FE) method. The IAR has proven to be a useful parameter of vertebral motion and it is an indicator of spinal instability. An anatomically accurate FE model of thoracic T10–T11 functional spinal unit (FSU) was used to characterize the loci of centers of rotation for the intact T10–T11 FSU and disc body unit (without posterior vertebral elements) under flexion and extension pure moments. The centers of rotation predicted by the intact model and disc body unit of thoracic T10–T11 for both flexion and extension were directly below the geometrical center of the moving vertebra. However, the loci of the IARs were significantly affected by the posterior vertebral elements. The loci of instantaneous axes of rotation for the intact model were tracked superoanteriorly and inferoposteriorly for flexion and extension with rotation, respectively. While, for the disc body unit, the loci were detected to diverge lateroinferiorly from the mid-height of the intervertebral disc, they converge medio-inferiorly toward the superior endplate of the inferior vertebra T11 with increased moment. These findings may offer an insight to better understanding the kinematics of the human thoracic spine and provide clinically relevant information for the evaluation of spinal stability and implant devices functionality.

Thoracolumbar spine mechanics contrasted under compression and shear loading

The mechanical properties of the human spine have been studied extensively in compression, but there remains a lack of fundamental data in shear. The overall goal of this study was to contrast the mechanics of the thoracolumbar functional spinal unit (FSU) under compression and shear-type loads by evaluating endplate deformation, disc pressures, and kinematics between the different loading types. Eleven T12-L1 and one L1-L2 human FSUs were tested. Compression loads consisted of pure compression, extension–compression, flexion–compression, lateral left and right compression applied individually to a maximum of 500 N. Shear loading consisted of posterior, anterior, left, and right shear to a maximum of 500 N. Intervertebral motions, disc pressure, and vertebral body deformations were recorded for all loads. The deformations were measured using strain gauge rosettes at three points on the inferior vertebral body and one on the superior endplate of the inferior vertebra. The disc pressures and endplate deformations measured were significantly less in shear loading compared to compression and did not change significantly with the type of compression load. Vertebral rim strains were generally greater under shear loading compared with compression. The mechanics of load transfer in compression was the production of high disc pressures which were not linearly correlated with the central endplate deformation. In shear, the mechanism appears to be via the annulus fibrosus without the development of significant disc pressure. These differences between compression and shear loading may have implications for injury mechanisms in the thoracolumbar spine.

Quantitative morphology of the human and porcine mid-lumbar interspinous ligament

SummaryAnimal models are essential in spine research for evaluating implants and for studying spinal mechanics. Several studies have compared the geometrical characteristics of animal and human vertebrae, but few studies have compared the structure of the spinal ligaments. The purpose of this study was to systematically quantify the collagen fibre orientation of the porcine and human interspinous ligament and thereby allow clearer interpretation of function. Human and porcine lumbar spine segments were loaded with a 10 Nm pure-flexion moment and chemically fixed. The sagittal plane collagen fibre orientation in the mid-lumbar interspinous ligaments was quantified by examining histological sections using a plane-polarized light macroscope and custom analysis software. The specimens showed collagen fibres in a posterior-cranial orientation originating from the superior aspect of the spinous process of the inferior vertebra and merging into the supraspinous ligament. There were not any statistically significant differences in interspinous ligament collagen fibre orientation between the human and porcine specimens. The middle and ventral spaces between the spinous processes of the human specimens contained loose disorganized collagen, skeletal muscle, and voids. The main load-bearing component of porcine and human interspinous ligament at the midlumbar level appears to be the dorsal portion, which is oriented at approximately 77-79 degrees with respect to the mid-disc plane. This dorsal aspect has a long moment arm and therefore is well suited to prevent excessive flexion. The similarity of the interspinous ligament morphology suggests that the porcine lumbar spine is a good model of the human lumbar spine.

The effect of nucleotomy on lumbar spine mechanics in compression and shear loading.

An in vitro biomechanical investigation on human cadaveric specimens was conducted before and after nucleotomy. Endplate and vertebral body deformation patterns were measured under compression and shear loading, in addition to kinematics and disc pressure. The working hypotheses of this study were that in compression, nucleotomy results in an altered deformation pattern of the endplate and that in shear, nucleotomy does not result in an altered endplate deformation pattern or disc pressure. The pressure distributions within the intervertebral disc have been studied in compression loading but not in shear loading. Severe degeneration and surgical nucleotomy result in small nuclear pressure and altered loading distribution in compression. The effect of these changes on the vertebral endplate and the response under shear loads are not well understood. Five L3-L4 and two L4-L5 functional spinal units were tested under compression and shear loading, intact and after nucleotomy. Vertebral body deformations, intradiscal pressure, and intervertebral kinematics were measured. A series of compression-type (maximum 1000 N) and shear-type (maximum 500 N) loads were applied. With nucleotomy, the disc pressure and the endplate strains decreased under compression, but the vertebral rim strains did not change. In shear, the vertebral rim and endplate strains did not change with nucleotomy. Disc pressure was lower in shear than in compression. Nucleotomy resulted in decreased disc pressure, decreased endplate deformation, and modified loading patterns onto the inferior vertebra in compression loading. However, nucleotomy did not appreciably affect the behavior of the disc in shear loading.

IDIOPATHIC RETROPERITONEAL HEMATOMA MIMICKING CYSTIC TUMOR ASSOCIATED WITH HEMORRHAGIC RENAL CYST

A 54-year-old woman was admitted to the hospital for evaluation of a retroperitoneal cystic mass detected incidentally during a complete followup physical examination for hypertension. CT revealed a 3 3 3 cm. cystic tumor between the inferior vena cava and abdominal aorta, and a 1 3 1 cm. enhanced mass in the left kidney (fig. 1). Magnetic resonance imaging showed that the inferior vena cava and abdominal aorta were deviated laterally by the mass in front of the vertebra, and a high density tumor in the left kidney was found (fig. 2). Angiography demonstrated no feeding arteries to the retroperitoneal mass or ruptured vessels. Preoperative diagnosis was cystic retroperitoneal and left renal tumors. To rule out lymphocele and retroperitoneal hematoma resection of the retroperitoneal mass and left partial nephrectomy were performed. The retroperitoneal mass was adherent to the inferior vena cava, abdominal aorta and vertebra, and had more capsular vessels than expected. Fluid collection of the retroperitoneal mass and left renal mass revealed an old hematoma and hematoma clot, respectively. Histologically, there were features of fibrous scar with fibrin and monocyte infiltration, while the left renal mass had no evidence of malignancy. Final diagnosis was idiopathic retroperitoneal hematoma and benign hemorrhagic renal cyst. No history of gastrointestinal upset, surgery or trauma was noted, and the cause of retroperitoneal hematoma and origin of the bleeding could not be identified. Convalescence was uneventful, although hypertension did not subside. The patient was well without evidence of disease recurrence at 1-year followup.

Lateral exit-zone stenosis and lumbar radiculopathy.

Hypertrophy of the superior facet of the inferior vertebra, resulting in a compression of the nerve root at the lateral foraminal exit, is a recognized cause of radicular symptoms, particularly in patients in whom previous lumbar spine surgery has failed. The lesion-specific presenting symptoms, imaging findings, and surgical treatment of this lesion, however, have received little attention. The authors prospectively studied a series of eight consecutive patients, in whom a diagnosis of lumbar stenosis at the lateral foraminal exit had been made, to elucidate the common presenting signs and symptoms of this disorder, as well as to evaluate the success of the operative treatment. The eight patients were selected from a group of 250 consecutive patients who presented to a tertiary-care hospital and in whom a diagnosis of long-standing lumbar radiculopathy had been made. In all cases the diagnosis was confirmed by imaging studies and by intraoperative findings. The authors performed decompressive procedures on the nerve root via a medial facet-sparing approach. The authors conclude that this lesion presents with characteristic physical findings and on imaging studies that distinguish it from other causes of radiculopathy, and they propose a lesion-specific, facet-sparing surgical technique that has yielded excellent results.

Complications in three-column cervical spine injuries requiring anterior-posterior stabilization.

A study was undertaken to elicit the hidden factors that, when identified, would signal the presence of cervical spine instability. Data were derived from the records and radiographs of 21 patients having sustained traumatic injury to the lower cervical spine (C3-C7) and who failed a single-stage posterior stabilization procedure necessitating a second (or combined) anterior-posterior arthrodesis. Mechanism of injury most frequently identified in this group was the distraction-flexion (locked facets) pattern (nine patients) and the "tear drop" compression-flexion injury pattern (seven patients). All 21 patients underwent a posterior wiring and bone graft stabilization procedure with persistent postoperative instability. Thus, failure to recognize the presence of "three-column" instability, the sine qua non of this group, resulted in the failure of posterior tension band stabilization as a means of gaining cervical spine stability. Three-column cervical spine instability is suspected in the presence of: 1) retrolisthesis and angulation of the superior vertebra on the next inferior vertebra; 2) distraction of the posterior interspinous ligaments sufficient to allow subluxation or dislocation of the facets; in conjunction with 3) a "shear" dislocation of one vertebra on another. Anterior shearing force through the disc space is capable of disrupting the intervertebral disc, along with disruption of the anterior and posterior longitudinal ligaments, each contributing to the presence of anterior and middle column cervical spine instability.

Intervertebral motion during vibration.

Vibration exposure is widely recognized as a risk factor for low back pain. An experimental protocol was designed to quantify the intervertebral motion response in human subjects to sinusoidal vertical vibration at 5 and 8 Hz, and at a variety of acceleration levels. Intervertebral motion in the mid-sagittal plane was measured using a transducer linkage system attached to pins placed directly into the spinous processes of adjacent vertebrae. The postures of the subjects were carefully controlled. The effects of forward flexion, arm support, gravitational load, and sitting on a cushion were evaluated. The rigid body motion of the superior vertebra with respect to the inferior vertebra was expressed in terms of relative sagittal plane rotation, axial translation, and anterior-posterior shear translation. It was found that the lumbar motion segments exhibited coupled periodic behaviour in response to sinusoidal vertical vibration, with up to 1 mm peak-to-peak displacement in the axial direction. The greatest intervertebral motion occurred when the subject was exposed to 5 Hz vibration as compared to 8 Hz. For a constant frequency of 5 Hz excitation, the peak-to-peak amplitudes of the computed motions tended to increase as the acceleration level increased. In the flexed posture, with no arm support, the active trunk musculature helped reduce the intervertebral motion. Additional gravitational load on the shoulders caused increased relative axial displacement. A polyethylene foam cushion placed on the seat reduced vibration transmission at 5 Hz excitation and consequently decreased the intervertebral motion.

Carbon and glass fiber-reinforced uhmwpe for endoprostheses

This paper reports the changes in spinal shape resulting from scoliotic spine surgical instrumentation expressed as intervertebral rotations and centers of rotation. The objective is to test the hypothesis that the type of spinal instrumentation system (Cotrel–Dubousset versus Colorado) does not influence these motion parameters. Intervertebral rotations and centers of rotation of the scoliotic spines were computed from the pre- and post-operative radiographs of 82 patients undergoing spinal correction. The three-dimensional (3D) reconstruction of six anatomical landmarks was achieved for each of the thoracic and lumbar vertebrae. A least-squares approach based on singular value decomposition was used to calculate the rigid body transformation parameters. Average centers of rotation for all intervertebral levels are located in the neural canal at the mid-sagittal plane and approximately at the superior endplate level of the inferior vertebra. Intervertebral rotations have components in all planes: 6.7° (frontal), 5.5° (sagittal) and 4.5° (transverse) RMS for all intervertebral levels. Nearly all intervertebral rotations and centers of rotation are not significantly different for the two instrumentation systems. Various intervertebral rotations and 3D reconstruction errors were simulated on a theoretical model of a lumbar functional unit to assess the proposed method. Intervertebral rotation errors were 1.7° when simulating 3D errors of 3 mm on the position of the landmarks. Maximum errors for the position of centers of rotation were below 1 cm in the case of intervertebral rotations larger than 2.5° (most cases), but were larger (38 mm) for small intervertebral rotations (<1°). The type of instrumentation system did not influence intervertebral rotations and centers of rotation. These results provide valuable data for the development and validation of simulation models for surgical instrumentation of idiopathic scoliosis.

Sciatica caused by nerve root entrapment in the lateral recess: the superior facet syndrome.

✓ Fifteen patients with intense sciatic pain in whom surgical exploration disclosed no evidence of a herniated disc were found to have an entrapped L-5 or S-1 nerve root in a stenotic lateral recess beneath the superior articular facet of the inferior vertebra. Neurological abnormalities were infrequent. A conspicuously positive Lasègue sign was the most definite and consistent finding. Roentgenograms of the spine were not helpful, and myelography was negative or showed minimal changes because of the normal ventrodorsal diameter of the spinal canal and the lack of filling of the narrow lateral recesses. Electromyography was positive in the five patients studied. Surgical unroofing of the lateral recess with removal of the overhanging horizontal portion of the superior facet decompressed the incarcerated nerve root and relieved symptoms.