摘要:

Study Design.Caudal vertebrae were obtained from male and female mice from two transgenic lines expressing an erythroid-specific human growth hormone transgene construct, and gender-matched, age-matched, non-transgenic control mice.Objective.To characterize the effect of human growth hormone transgene expression on the biomechanical structural properties of caudal vertebrae in compression.Summary of Background Data.An increase in trabecular and cortical bone deposition caused by erythroid-specific human growth hormone transgene expression was demonstrated previously.Methods.Compression tests were performed on individual caudal vertebrae (Ca4, Ca5, Ca6) obtained from male and female mice from two transgenic lines (TG420 and TG450) and nontransgenic control mice. Two age groups were evaluated: 12 weeks old and 6 months old. The data were used to obtain axial stiffness, maximum load, and energy to failure.Results.Vertebrae from male TG420 transgenic mice produced significantly larger values for maximum load, energy to failure, and axial stiffness at both 12 weeks and 6 months in comparison with their age-matched non-transgenic male controls. Vertebrae from female TG420 transgenic mice produced similar responses at 6 months. Vertebrae from male TG450 transgenic mice showed significant increases in maximum load and energy to failure at 6 months. In general, the biomechanical properties of vertebrae were significantly larger in the 6-month age group than in the 12-week age group, and this increase was significantly greater in the transgenic mice than in the gender-matched control mice during the same time span. This process was also influenced by transgenic genotype and gender.Conclusions.Erythroid-specific production of human growth hormone in transgenic mice resulted in significant increases in biomechanical properties of their caudal vertebrae in compression. The changes in the biomechanical properties were influenced by genotype, age, and gender.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.A three-group design with consistent pullout strength measures.Objectives.To determine pullout strength of three fixation types (unicortical screws, bicortical screws, wires) and to investigate their correlation with respect to occipital morphology.Summary of Background Data.A secured, multidirectional occipitocervical fusion requires internal fixation. Devices secured at occipital protuberance were suggested to offer the greatest pullout strength because of this region's thickness.Methods.Twelve fresh human cadaveric occiputs were sketched with a grid delineating 21 fixation sites. Each site was drilled and hand-tapped. Four specimens were instrumented with unicortical screws on one side of the midline and bicortical screws on the other. Another four were instrumented with bicortical screws and wires, and the remaining four were instrumented with unicortical screws and wires. Two points on each specimen were secured with identical fixation to examine side-to-side symmetry. An MTS materials testing apparatus (MTS Systems Corporation, Eden Prairie, MN) was used to displace the fixators. Pullout strengths at different anatomic locations were recorded.Results.The greatest pullout strength was at the occipital protuberance for all fixation types. The bicortical pullout strength was 50% greater than unicortical. The wire pullout strength was not significantly different from that of the unicortical screw (P0.05). Seventy-eight percent of wires broke at 1100 N. Unicortical pullout strength at occipital protuberance was comparable with that of the bicortical screw at other locations.Conclusions.Unicortical screw fixation at occipital protuberance offers acceptable pullout strength without the potential complications of bicortical screws or wire fixation.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.A high-resolution strain measurement technique was applied to axially loaded parasagittal sections from thoracic spinal segments.Objectives.To establish a new experimental technique, develop data analysis procedures, characterize intrasample shear strain distributions, and measure intersample variability within a group of morphologically diverse samples.Summary of Background Data.Compression of intact vertebral bodies yields structural stiffness and strength, but not strain patterns within the trabecular bone. Finite element models yield trabecular strains but require uncertain boundary conditions and material properties.Methods.Six spinal segments (T8-T10) were sliced in parasagittal sections 6-mm thick. Axial compression was applied in 25-N increments up to sample failure, then the load was removed. Contact radiographs of the samples were made at each loading level. Strain distributions within the central vertebral body were measured from the contact radiographs by an image correlation procedure.Results.Intrasample shear strain probability distributions were log-normal at all load levels. Shear strains were concentrated directly inferior to the superior endplate and adjacent to the anterior cortex, in regions where fractures are commonly seen clinically. Load removal restored overall sample shape, but measurable residual strains remained.Conclusions.This experimental model is a suitable means of studying low-energy vertebral fractures. The methods of data interpretation are consistent and reliable, and strain patterns correlate with clinical fracture patterns. Quantification of intersample variability provides guidelines for the design of future experiments, and the strain patterns form a basis for validation of finite element models. The results imply that strain uniformity is an important criterion in assessing risk of vertebral failure.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique.Objectives.To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement.Background Data.An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affectin situloads incurred by pedicle screws.Methods.Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture.Results.Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws.Conclusions.In situpedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.The biomechanical characteristics of the Leeds-Keio artificial ligament (Neoligament LTD, England) were assessed in the posterior stabilization of degenerative lumbar spondylolisthesis in a porcine vertebral model.Objectives.To evaluate the biomechanical properties of this nonrigid system immediately after instrumentation and during cyclic loading.Summary of Background Data.Mochida et al reported an innovative method of posterior stabilization in which the Leeds-Keio artificial ligament was used as a nonrigid instrument. Since 1990, this system has been used successfully to treat a select group of patients with degenerative lumbar spondylolisthesis.Methods.Five porcine motion segments (L4-L5) were tested in the intact, destabilized, and Leeds-Keio-instrumented conditions. Specimens were loaded in flexion and extension, and construct stiffness was measured during the initial loading cycle and at 250 cycle intervals for 1500 cycles.Results.Biomechanical stability in the model of degenerative lumbar spondylolisthesis was inferior to that in the controls. The instability in the Leeds-Keio group significantly improved immediately after placement and after 1500 cycles.Conclusions.This system is effective in initially stabilizing an unstable degenerative lumbar spondylolisthesis model and maintains its stabilizing effect during cyclic loading. Therefore, it can offer patients with degenerative lumbar spondylolisthesis in the late static stage several advantages over other treatment methods.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.This is a retrospective study of the morphologic limitations of posterior decompression for ossification of the posterior longitudinal ligament in the cervical spine.Objectives.To determine the morphologic limitations of the posterior approach in the management of ossification of the posterior longitudinal ligament.Summary of Background Data.Thirty-eight patients who underwent laminoplasty by midsagittal splitting for ossification of the posterior longitudinal ligament were reviewed. Fifteen patients were included in the noncontact group, in which the spinal cord was free of the ossified lesion after posterior decompression. Twenty-three patients were included in the contact group, in which the spinal cord was not free of the ossified lesion even after posterior decompression.Methods.The preoperative sagittal alignment of the cervical spine and preoperative maximal thickness of ossification were compared between the two groups. In addition, the morphologic limitations of posterior decompression for ossification of the posterior longitudinal ligament were investigated.Results.The following factors were found to contribute significantly to contact between the spinal cord and ossification of the posterior longitudinal ligament after posterior decompression: 1) lordosis of less than 10° or kyphosis in the preoperative sagittal alignment, and 2) preoperative maximal thickness of ossification of more than 7 mm.Conclusions.Patients who exhibit significant risk factors for continued contact of the spinal cord should be morphologically considered for anterior decompression.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.This study involved weekly radiographic examination of pinealectomized rats, hamsters, and chickens to observe the development of scoliosis.Objectives.To determine whether pinealectomy produces scoliosis in animals more closely related phylogenetically to humans than to chickens, namely rats and hamsters, which are representative of mammals.Summary of Background Data.Pinealectomy in 3-day-old chickens has consistently resulted in the development of scoliosis with many characteristics similar to those seen in patients with adolescent idiopathic scoliosis. It has not been determined whether this phenomenon is restricted solely to chickens or is applicable to other animals, especially those more closely related to humans.Methods.The pineal gland was removed from young rats, hamsters, and chickens. All animals underwent radiography weekly to detect the development of any scoliosis. Weight and length measurements were also taken weekly, and serum melatonin levels were determined at the time the animals were killed.Results.Scoliosis was not observed in either the rats or the hamsters. In contrast, scoliosis developed in 10 of 21 chickens. Serum melatonin levels in all pinealectomized animals were zero.Conclusions.In contrast to the chickens, pinealectomy does not seem to cause scoliosis in either young rats or hamsters. The reasons for this discrepancy may include differences in the physiology and spinal morphology of the rat and hamster in comparison with the chicken. In the pinealectomized chickens, the results also suggest that future scoliosis development might be indicated by a significant increase in size when they are compared with pinealectomized chickens that do not develop scoliosis. Such differences in growth rates also distinguish patients with adolescent idiopathic scoliosis.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Study Design.Independent evaluation by two observers of 132 lumbar discs in 45 patients with chronic low back pain investigated by both magnetic resonance imaging and discography.Objectives.To assess some of the fundamental differences between lumbar disc protrusions, disc bulges, and discs with normal contour but abnormal signal intensity on T2-weighted magnetic resonance images.Summary of Background Data.Moderate interobserver agreement has been reported when the morphologic termsnormal, bulge, protrusion,andextrusionare used. The validity of this nomenclature remains unknown.Methods.Discs were evaluated on magnetic resonance images for central and peripheral signal characteristics, height, contour, and nerve root compression. Discograms were classified according to degrees of disc degeneration, disruption, and pain reproduction.Results.Loss of intervertebral height or abnormal signal intensity on magnetic resonance imagery was significantly associated with disc disruptions extending into or beyond the outer anulus on discograms. All 23 protrusions (100%) and 12 of 15 disc bulges (80%) were associated with Stage 2 or 3 anular disruptions and, in most instances, similar or exact reproduction of pain during disc injection. There was no significant difference between disc protrusions, disc bulges, and discs with normal contour but abnormal signal, with respect to degree of disc degeneration, extent of disruptions, or presence of discogenic pain.Conclusions.In patients with chronic low back pain, loss of disc height or abnormal signal intensity is highly predictive of symptomatic tears extending into or beyond the outer anulus. Disc bulges and disc protrusions do not represent discs with significantly different internal architecture, based on the findings of discography, and are no more suggestive of symptomatic tears than discs showing normal contour but decreased height or abnormal signal intensity.

ISSN:0362-2436    

出版商:OVID    

年代:1999

数据来源: OVID