Comparative anatomy of the porcine and human thoracic spines with reference to thoracoscopic surgical techniques.

BACKGROUND: This study compared porcine and human thoracic spine anatomies for a better understanding of how structures encountered during thoracoscopy differ between training with a porcine model and actual surgery in humans. METHODS: Parameters were measured including vertebral body height, width, and depth; disc height; rib spacing; spinal canal depth and width; and pedicle height and width. RESULTS: Although most porcine vertebral structures were smaller, porcine pedicle height was significantly greater than that of humans because the porcine pedicle houses a unique transverse foramen. The longus colli and psoas attach, respectively, to T5 and T13 in swine and to T3 and T12 in humans. In swine, the azygos vein generally was absent. The intercostal veins drained into the hemiazygos vein. CONCLUSIONS: Several thoracoscopically relevant anatomic differences between human and porcine spinal anatomies were identified. A thoracoscopic approach in a porcine model probably is best performed from the right side. The best general working area is between T6 and T10.

Penetration of cranial inner table with Gardner-Wells tongs.

The compression force exerted by Gardner-Wells tongs was compared with the force necessary to penetrate the cranial inner table with a Gardner-Wells tong pin. Load cells measured the force exerted by the spring-loaded Gardner-Wells tong pin on fresh cadaveric skull samples. Increasing forces were exerted until penetration of the inner table occurred. At the manufacturer's recommended, 1 mm indicator stem protrusion an average of 30 pounds of compressive force was exerted by the spring-loaded pin. The average force necessary to penetrate the inner table with the cadaveric specimens with the tong pin was 162 pounds. The force necessary to penetrate the inner table of the skull well exceeds that exerted by properly applied tongs, suggesting that the risk of inner table penetration is low.

Anterior lumbar microdiscectomy and interbody fusion for the treatment of recurrent disc herniation.

OBJECTIVE: To demonstrate the feasibility of anterior lumbar microdiscectomy in patients with recurrent, sequestered lumbar disc herniations. METHODS: Between 1997 and 1999, six patients underwent a muscle-sparing "minilaparotomy" approach and subsequent microscopic anterior lumbar microdiscectomy and fragmentectomy for recurrent lumbar disc extrusions at L5-S1 (n = 4) or L4-L5 (n = 2). A contralateral distraction plug permitted ipsilateral discectomy under microscopic magnification. Effective resection of the extruded disc fragments was accomplished by opening the posterior longitudinal ligament. Interbody fusion was performed by placing cylindrical threaded titanium cages (n = 4) or threaded allograft bone dowels (n = 2). RESULTS: There were no complications, and blood loss was minimal. Follow-up magnetic resonance imaging revealed complete resection of all herniated disc material. Plain x-rays revealed excellent interbody cage position. Radicular pain and neurological deficits resolved in all six patients (mean follow-up, 14 mo). CONCLUSION: Anterior lumbar microdiscectomy with interbody fusion provides a viable alternative for the treatment of recurrent lumbar disc herniations. Recurrent herniated disc fragments can be removed completely under direct microscopic visualization, and interbody fusion can be performed in the same setting.

Biomechanical effects of progressive anterior cervical decompression.

STUDY DESIGN: A repeated-measures in vitro flexibility test was performed. OBJECTIVES: To determine the biomechanical functions of tissues resected during anterior cervical decompression of various extents. SUMMARY OF BACKGROUND DATA: The biomechanical consequences of discectomy have been studied in vitro, and uncovertebral joint removal has been modeled numerically. No studies have assessed the relative biomechanical contributions of different anterior column structures. METHODS: In seven human cadaver C4-T1 specimens, 20 motion segments were studied. After each destructive step, including discectomy, unilateral uncinate process removal, bilateral uncinate process removal, and posterior longitudinal ligament transection, torques were applied to four-level specimens while the angular motion was measured at each level. RESULTS: Angular range of motion and neutral zone increased by variable but statistically significant amounts after each progressive resection, most notably in flexion and extension. Each resection step caused progressively larger shifts (up to 23 mm) in the location of the axis of rotation. Uncovertebral joint resection caused the most significant changes in the observed angular coupling. CONCLUSIONS: Anterior cervical decompression significantly increases the instability and alters the kinematics of cervical motion segments. Each structure resected contributes to normal stability and kinematics, so as many structures as possible should be left intact during anterior decompression without fusion. Because flexion and extension were the modes of motion that increased most significantly after decompression, the primary function of a grafting technique or fixation device should be to limit these motions.

Biomechanics of grade I degenerative lumbar spondylolisthesis. Part 1: in vitro model.

OBJECT: The authors sought to create and to evaluate an in vitro model of Grade I degenerative (closed-arch) spondylolisthesis. METHODS: The model of spondylolisthesis was created by two primary procedures: 1) resection of the disc; and 2) stripping of anterior and posterior longitudinal ligaments away from the vertebral bodies (VBs). In 13 vertebral levels obtained from three cadaveric lumbar spines, the tissues were resected sequentially in alternating order to determine the relative contribution of each resection to spinal instability. The entire specimens were loaded with nonconstraining torques and then individual levels were loaded with anteroposterior shear forces. The motion values were measured optoelectronically for each specimen at individual levels. CONCLUSIONS: The integrity of the disc was more important than attachment of the ligaments to the VB, but the resection of both structures was necessary to achieve substantial destabilization. The structures of the spine are highly resilient, and destabilization is difficult to achieve without performing extensive resection. Using the techniques described in this paper to alter normal spines, a level of spinal instability (Grade I; 25% slippage) that may represent spondylolisthesis can be modeled in vitro.

Biomechanics of grade I degenerative lumbar spondylolisthesis. Part 2: treatment with threaded interbody cages/dowels and pedicle screws.

OBJECT: The authors sought to determine the biomechanical effectiveness of threaded interbody cages or dowels compared with that achieved using pedicle screw instrumentation in resisting Grade I lumbar spine degenerative spondylolisthesis. METHODS: Thirty-three levels obtained from seven cadaveric lumbar spines were instrumented with cages or dowels, pedicle screw/rod instrumentation, or both. Entire specimens were loaded with nonconstraining torques. Each level was loaded with anteroposterior shear forces while an optical system was used to measure the specimen's motion at individual levels. Pedicle screw/rods outperformed interbody cages and dowels in treating spondylolisthesis. Cages or dowels alone provided only moderate biomechanical stability, and their effectiveness depended heavily on the integrity of the ligaments and remaining annulus, whereas the success of pedicle screw fixation relied predominantly on the integrity of the bone for solid fixation. Little biomechanical difference was demonstrated between cages and dowels; both devices were susceptible to loosening with cyclic fatigue. CONCLUSIONS: Biomechanically, cages or dowels alone were suboptimal for treating lumbar spondylolisthesis, especially compared with pedicle screw/rods. Threaded cages or dowels used together with pedicle screws/rods created the most stable construct.

The effects of dexamethasone on bone fusion in an experimental model of posterolateral lumbar spinal arthrodesis.

OBJECT: The use of corticosteroid agents during the healing phase after spinal arthrodesis remains controversial. Although anecdotal opinion suggests that corticosteroids may inhibit bone fusion, such an effect has not been substantiated in clinical trials or laboratory investigations. This study was undertaken to delineate the effect of exogenous corticosteroid administration on bone graft incorporation in an experimental model of posterolateral lumbar fusion. METHODS: An established, well-validated model of lumbar intertransverse process spinal fusion in the rabbit was used. Twenty-four adult New Zealand white rabbits underwent L5-6 bilateral posterolateral spinal fusion in which autogenous iliac crest bone graft was used. After surgery, the animals were randomized into two treatment groups: a control group (12 rabbits) that received intramuscular injections of normal saline twice daily and a dexamethasone group (12 rabbits) that received intramuscular dexamethasone (0.05 mg/kg) twice daily. After 42 days, the animals were killed and the integrity of the spinal fusions was assessed by radiography, manual palpation, and biomechanical testing. In seven (58%) of the 12 control rabbits, solid posterolateral fusion was achieved. In no dexamethasone-treated rabbits was successful fusion achieved (p = 0.003). Tensile strength and stiffness of excised spinal segments were significantly lower in dexamethasone-treated animals than in control animals (tensile strength 91.4+/-30.6 N and 145.3+/-48.2, respectively, p = 0.004; stiffness 31.4+/-11.6 and 45.0+/-15.2 N/mm, respectively, p = 0.02). CONCLUSIONS: The corticosteroid agent dexamethasone inhibited bone graft incorporation in a rabbit model of single-level posterolateral lumbar spinal fusion, inducing a significantly higher rate of nonunion, compared with that in saline-treated control animals.

Biomechanical comparison of anterior cervical plating and combined anterior/lateral mass plating.

BACKGROUND CONTEXT: Previous studies showed anterior plates of older design to be inadequate for stabilizing the cervical spine in all loading directions. No studies have investigated enhancement in stability obtained by combining anterior and posterior plates. PURPOSE: To determine which modes of loading are stabilized by anterior plating after a cervical burst fracture and to determine whether adding posterior plating further significantly stabilizes the construct. STUDY DESIGN/SETTING: A repeated-measures in vitro biomechanical flexibility experiment was performed to investigate how surgical destabilization and subsequent addition of hardware components alter spinal stability. PATIENT SAMPLE: Six human cadaveric specimens were studied. OUTCOME MEASURES: Angular range of motion (ROM) and neutral zone (NZ) were quantified during flexion, extension, lateral bending, and axial rotation. METHODS: Nonconstraining, nondestructive torques were applied while recording three-dimensional motion optoelectronically. Specimens were tested intact, destabilized by simulated burst fracture with posterior distraction, plated anteriorly with a unicortical locking system, and plated with a combined anterior/posterior construct. RESULTS: The anterior plate significantly (p<.05) reduced the ROM relative to normal in all modes of loading and significantly reduced the NZ in flexion and extension. Addition of the posterior plates further significantly reduced the ROM in all modes of loading and reduced the NZ in lateral bending. CONCLUSIONS: Anterior plating systems are capable of substantially stabilizing the cervical spine in all modes of loading after a burst fracture. The combined approach adds significant stability over anterior plating alone in treating this injury but may be unnecessary clinically. Further study is needed to assess the added clinical benefits of the combined approach and associated risks.

Pediatric spine and spinal cord trauma. State of the art for the third millennium.

The purpose of this work was to analyze the literature published in English and to review the experience of the Barrow Neurological Institute (BNI) with spine and spinal cord injury (SCI) in children. Standard computerized data bases were queried for information regarding SCI, spinal injury, spinal instability, and spinal cord regeneration to produce a review of the epidemiology, diagnosis, treatment, outcome and directions for future research. We also reviewed our experiences in the management of infants and children with spine injuries and SCIs and with spinal instability from all causes. A total of 132 articles were identified and obtained from the Medical Library at St. Joseph's Hospital and Medical Center in Phoenix, Ariz. and through interlibrary loan. All these articles were read, although not all were used in the final review. A review of all children with SCIs revealed that fractures treated over the past 20 years at the BNI were very rare in preadolescent children, who suffered mostly from ligamentous injury or SCI without radiographic abnormality. A total of 68 children aged 16 years or younger who had been treated over the past 15 years and who had undergone spinal fusions for trauma, congenital anomalies, or tumor resection were identified. Occipitocervical fusion is well tolerated in children as young as 11 months when internal stabilization with a threaded titanium rod is used. Posterior instrumentation, including pedicle screw fixation, is feasible in children as young as 4 years. Fusion techniques derived from the adult spinal instrumentation experience were found appropriate except for the youngest patients. Fusion in the newborn period was futile in our experience. The adolescent spine does not differ from the adult spine in terms of sensitivity or response to fixation. Children past the neonatal period can be successfully instrumented for spinal stability without apparent long-term sequelae. Related advances are needed in the area of prevention. Long-term advances in spinal cord regeneration can be expected from ongoing basic science investigations.

Reoperation for herniated thoracic discs.

OBJECT: In this review the authors address the surgical strategies required to resect residual herniated thoracic discs. METHODS: Data obtained in 15 patients who had undergone prior thoracic discectomy and who harbored residual or incompletely excised symptomatic thoracic discs were reviewed retrospectively. The surgical procedures that had failed to excise the herniated discs completely included 11 posterolateral approaches, one thoracotomy, and three thoracoscopy-guided surgical procedures. Of the lesions that were incompletely resected or residual, there were 13 central calcified, two soft. 12 extradural, and three intradural discs. Indications for reoperation were often multiple in each patient and included misidentification of the level of disc disease at the initial operation (five cases), abandoning the procedure because of intraoperative spinal cord injury (three cases), inadequate visualization of the pathological entity (eight cases), migration of a soft disc fragment within the spinal canal (one case), and intradural disc extension (three cases). The symptoms at the time of reoperation included myelopathy in 13 patients and radicular pain in two patients. The mean interval before reoperation was 150 days (range 1 day-4 years). The reoperation procedures included one thoracotomy and 14 video-assisted thoracoscopic procedures performed ipsilateral (11 cases) or contralateral (four cases) to the site of the initial surgery. The herniated disc material was excised completely in all 15 cases without causing new neurological deficits. Reoperation complications included atelectasis in three patients, intercostal neuralgia in two, a loosened screw that required removal in one, residual intradural disc herniation that required a second reoperation in one patient, and a cerebrospinal fluid leak in one patient. Of the 13 patients who experienced myelopathy prior to operation, 10 recovered neurological function and three stabilized. All patients with radicular pain improved. CONCLUSIONS: Calcified, large, broad-based, centrally located, or transdural thoracic disc herniations can be difficult to resect. These lesions require a ventral operative approach to visualize the dura adequately for a safe and complete resection.

Differential biomechanical effects of injury and wiring at C1-C2.

STUDY DESIGN: An in vitro study compared the biomechanics of the upper cervical spine among three groups of cadaveric specimens, each with a different source of instability: transverse-alar-apical ligament disruptions, odontoid fractures, or odontoidectomies. The responses of the three groups were again compared after a uniform posterior cable and graft fixation was applied to the specimens. OBJECTIVES: To quantify and compare the effects of different injuries on atlantoaxial stability and to determine whether a single fixation technique effectively treats each injury. SUMMARY OF BACKGROUND DATA: Previous biomechanical studies of atlantoaxial instability have been focused on mechanisms of injury or on comparison among fixation types. METHODS: Cables and pulleys applied torques to human cadaveric C0-C6 specimens quasistatically while an optical system tracked three-dimensional angular and translational motion at C0-C1 and C1-C2. Specimens were tested immediately after injury, after posterior cable and graft fixation, and after 6000 cycles of fatigue. RESULTS: Odontoidectomies increased C1-C2 angular and translational range of motion significantly more than odontoid fractures or ligament disruptions, especially during flexion-extension. Odontoid fractures produced a slightly larger increase in C1-C2 angular range of motion than ligament disruptions but a smaller increase in C0-C1 range of motion. The different injuries affected the lax zone and the position of C1-C2 axis of rotation differently. Restabilization by posterior cable and graft reduced motion only moderately for each injury type. All three fixated injuries were susceptible to loosening from fatigue. CONCLUSION: The three different injuries produce different spinal biomechanical responses. To best promote fusion, posterior cable and graft fixation should be used with an adjunctive stabilizing technique to treat all three injuries.

A biomechanical evaluation of occipitocervical instrumentation: screw compared with wire fixation.

OBJECT: The purpose of this study was to compare cable techniques used in occipitocervical fixation with two types of screw fixation. The authors hypothesized that screw fixation would provide superior immobilization compared with cable methods. METHODS: Ten cadaveric specimens were prepared for biomechanical analyses by using standard techniques. Angular and linear displacement data were recorded from the occiput to C-6 with infrared optical sensors after conditioning runs. Specimens underwent retesting after fatiguing. Six methods of fixation were analyzed: Steinmann pin with and without C-1 incorporation; Cotrel-Dubousett horseshoe with and without C-1 incorporation; Mayfield loop with C1-2 transarticular screw fixation; and a custom-designed occipitocervical transarticular screw-plate system. Sublaminar techniques were extended to include C-3 in the fusion construct, whereas transarticular techniques incorporated the occiput, C-1, and C-2 only. All methods of fixation provided significant immobilization in all specimens compared with the nonconstrained destabilized state. Despite incorporation of an additional vertebral segment, sublaminar techniques performed worse as a function of applied load than screw fixation techniques. Following fatiguing, these differences were more pronounced. The sublaminar techniques failed most prominently in flexion-extension and in axial rotation. On gross inspection, increased angular displacement associated with loosening of the sublaminar cables was observed. CONCLUSION: Occipitocervical fixation can be performed using a variety of techniques; all bestow significant immobilization compared with the destabilized spine. All methods tested in this study were susceptible to fatigue and loss of reduction and were weakest in resisting vertical settling. Screw fixation of the occiput-C2 reduces the number of vertebral segments that are necessary to incorporate into the fusion construct while providing superior immobilization and resistance to fatigue and vertical settling compared with sublaminar methods.

Stability of the craniovertebral junction after unilateral occipital condyle resection: a biomechanical study.

OBJECT: The authors sought to determine the biomechanics of the occipitoatlantal (occiput [Oc]-C1) and atlantoaxial (C1-2) motion segments after unilateral gradient condylectomy. METHODS: Six human cadaveric specimens (skull with attached upper cervical spine) underwent nondestructive biomechanical testing (physiological loads) during flexion-extension, lateral bending, and axial rotation. Axial translation from tension to compression was also studied across Oc-C2. Each specimen served as its own control and underwent baseline testing in the intact state. The specimens were then tested after progressive unilateral condylectomy (25% resection until completion), which was performed using frameless stereotactic guidance. At Oc-C1 for all motions that were tested, mobility increased significantly compared to baseline after a 50% condylectomy. Flexion-extension, lateral bending, and axial rotation increased 15.3%, 40.8%, and 28.1%, respectively. At C1-2, hypermobility during flexion-extension occurred after a 25% condylectomy, during axial rotation after 75% condylectomy, and during lateral bending after a 100% condylectomy. CONCLUSIONS: Resection of 50% or more of the occipital condyle produces statistically significant hypermobility at Oc-C1. After a 75% resection, the biomechanics of the Oc-C1 and C1-2 motion segments change considerably. Performing fusion of the craniovertebral junction should therefore be considered if half or more of one occipital condyle is resected.

Bilateral sagittal split mandibular osteotomies as an adjunct to the transoral approach to the anterior craniovertebral junction. Technical note.

Transoral approaches are used to expose the craniovertebral junction anteriorly. In patients in whom there is limited mandibular excursion, the placement of retractors and/or surgical instruments is difficult, and midline "stairstep split mandibulotomy" has been advocated as an adjunctive procedure. Although effective, this approach requires external splitting of the lip as well as median glossotomy or a lateral mucosal incision. The purpose of this study was to show that bilateral sagittal split mandibular osteotomies (BSSMOs), which are used in orthognathic surgery, represent a safer and more effective alternative to the stairstep split mandibulotomy when performed as an adjunct to the transoral approach because all incisions are intraoral and the plane of retraction is rostrocaudal instead of lateral. Hospital records and radiographic files of four patients who underwent BSSMO/transoral approach for odontoidectomy between 1994 and 1997 were reviewed retrospectively. There were three women and one boy (mean age 37.8 years, range 11-68 years). Predisposing conditions included rheumatoid arthritis (two patients), Klippel-Feil syndrome (one patient), and congenital occipitocervical instability (one patient). Jaw mobility was limited in all patients. In addition, one patient had macroglossia, another micrognathia, and another retrognathia. The BSSMO provided excellent exposure for resection of the odontoid process, as verified on follow-up magnetic resonance imaging or computerized tomography studies obtained in all patients. All mandibles were rigidly fixed by placing anterior mandibular border titanium plates and unicortical screws, and there was no incidence of nonunion or of lingual or inferior alveolar nerve injuries. The mean follow-up period was 26 months. The BSSMO is an excellent, less invasive adjunct to the transoral approach in patients with limited jaw mobility.

A new technique for determining 3-D joint angles: the tilt/twist method.

OBJECTIVE: To develop a new method of representing 3-D joint angles that is both physically meaningful and mathematically stable. DESIGN: The two halves of a joint are modeled as overlapping cylinders. This simple physical model is easily understood and yields mathematically stable angle equations. BACKGROUND: Two currently-used methods are the Euler/Cardan (joint coordinate system) method and the projection angle method. Both of these methods approach a singularity at 90 degrees that limits their use. The helical angle (attitude vector) method is mathematically stable but has limited physical meaning and is difficult to communicate. METHODS: Calculation of the tilt/twist angles is described. Tilt/twist angles are compared to Euler/Cardan, projection, and helical angles in terms of behavior and stability. RESULTS: Through a small range of angulation, tilt/twist angles match the specific projection and Euler/Cardan angles previously found to be appropriate for describing spinal motion. Through larger ranges, tilt/twist angles do not match the other angles studied. Although not as stable as helical angles, tilt/twist angles are twice as stable as Euler/Cardan and projection angles, reaching a singularity only at 180 degrees. CONCLUSIONS: Because of their mathematical stability and simple physical interpretation, tilt/twist angles are recommended as a standard in describing angular joint motion.