Stabilizing Effects of a Particulate Demineralized Bone Matrix in the L4 Interbody Space with and without PEEK Cage - A Literature Review and Preliminary Results of a Cadaveric Biomechanical Study.

We reviewed the biological elements supporting the usefulness of a specifically designed particulate form of demineralized bone matrix (DBM) with spinal fusion, and report some limitations of its use described in the medical literature and in the interbody space using a cadaveric biomechanical model. A literature review and description of the techniques used to augment spinal fusion are presented, including a more thorough review of recent findings of cadaveric biomechanical flexibility studies using DBM alone at different percentage fills of the existing disc space and DBM with a polyetheretherketone (PEEK) interbody cage. The need for DBM was established by reviewing limitations of autografts and allografts in spinal fusion. Demineralized bone matrix used alone did not increase stability post discectomy at L4-L5, but was demonstrated to exhibit satisfactory stability when used with a PEEK interbody cage. There may be a future role for DBM that hardens and fills disc space more rigidly, overcoming this limitation to its use.

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.

Edge-bridged octahedral tungsten-oxygen-chlorine clusters: synthesis and characterization of two D(3d)-symmetric [W(6)O(6)Cl(12)](2-) isomers and [W(6)O(7)Cl(11)](3-).

Initial access to the chemistry of hexanuclear tungsten oxohalide clusters is provided through the reduction of WOCl(4) with bismuth metal at 360 degrees C. Reactions targeting W(6)O(6)Cl(10) produce an amorphous black solid, which, upon treatment with concentrated aqueous HCl, releases the edge-bridged octahedral cluster [alpha-W(6)O(6)Cl(12)](2-) into solution. The cluster exhibits a D(3d)-symmetry structure in which the six oxygen atoms bridge the edges between two opposing triangular faces of a trigonally compressed W(6) octahedron. Reactions incorporating additional bismuth metal yield a mixture of soluble clusters, including a 5:7 ratio of [alpha-W(6)O(6)Cl(12)](2-) and another D(3d)-symmetry isomer, [beta-W(6)O(6)Cl(12)](2-). The latter species displays a different core structure, in which the six oxygen atoms are situated on the edges comprising two opposing triangular faces of a trigonally elongated W(6) octahedron. Isolated as the BuN(+) salts, the two isomers can be separated by a process relying on the differences in crystal morphology. Cyclic voltammetry of acetonitrile solutions shows two reversible one-electron reductions for each cluster, the alpha isomer being slightly more easily reduced. Density functional theory calculations indicate that the two isomers of [W(6)O(6)Cl(12)](2-) are nearly identical in energy, with the beta isomer lying just 1.4 kcal/mol below the alpha isomer. The other major product isolated from the reaction with additional bismuth is [W(6)O(7)Cl(11)](3-), a cluster at least formally related to [beta-W(6)O(6)Cl(12)](2-) by substitution of an O(2)(-) ion for a core Cl(-) ion. In acetonitrile solution, this cluster displays a single reversible one-electron reduction. It is anticipated that the reactions elaborated here will lead to a general method for synthesizing metastable metal oxohalide clusters.

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.

Increase in spinal canal area after inverse laminoplasty: an anatomical study.

STUDY DESIGN: In vitro measurement of the area of the spinal canal in the rostral and caudal portions of lumbar vertebrae before and after application of a new technique called "inverse laminoplasty." OBJECTIVES: To quantify the normal area of the spinal canal in the rostral and caudal portions of lumbar vertebrae and the amount of enlargement gained after inverse laminoplasty. SUMMARY AND BACKGROUND DATA: Other types of laminoplasty have been proven to increase the area of the spinal canal. Inverse laminoplasty has been performed in 10 patients but has not been evaluated in vitro. METHODS: The transverse and anteroposterior diameter of the spinal canal was measured in 34 vertebrae from seven cadavers using digital calipers. In each vertebra, the laminae and spinous process were removed en bloc using a high-speed drill. The removed piece was inverted and reattached with titanium mini-plates. The area of the spinal canal was again measured and compared with the prelaminoplasty measurements using paired Student's t tests. RESULTS: The anteroposterior diameter and area of the spinal canal were significantly smaller before surgery in the rostral than in the caudal part of the vertebrae (P <10(-3)). The rostral and caudal areas of the spinal canal increased by 61% and 17%, respectively, after the laminae were inverted (P <10(-3)). CONCLUSION: Because inverse laminoplasty is simple and increases the area of the spinal canal, it may prove to be a useful surgical technique for the treatment of lumbar spinal stenosis. Further studies are needed to determine whether the technique is biomechanically sound and whether it helps prevent perineural scarring.

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.

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.

Lumbosacral junction fixation and fusion after complete L-5 spondylectomy. Case report.

The goals of surgery for metastatic disease of the lumbosacral spine are to relieve compression of the thecal sac and nerve roots, to resect malignant tissue, and to create a stable reconstruction of the spine. Reconstruction of the lumbosacral junction, specifically the L-5 vertebral body, is particularly challenging because the biomechanical properties of this level differ from other areas of the spine. A 40-year-old woman with intraductal breast carcinoma that metastasized to the L-5 vertebral body presented with progressive low-back pain, right-sided L-5 radiculopathy, and weakness. Magnetic resonance imaging revealed a pathological fracture of the L-5 vertebral body with compression of the cauda equina. The L-5 posterior arch, both facet joints and pedicles, and the posterior third of the vertebral body were removed via a posterior approach. A pedicle screw fixation system was applied from L-4 to S-1. The patient was repositioned, and a transabdominal approach was used to resect the anterior two thirds of the L-5 body, which was reconstructed using an allograft bone strut. An interference bone screw was placed through the inferior aspect of the allograft and screwed into the body of S-1 to provide stability for the reconstructive graft. The patient's clinical recovery was excellent. She was ambulating without difficulty when seen at 19-month follow-up examination. Complete spondylectomy by using this novel fusion technique was efficacious in the treatment of metastatic disease to the vertebral column.

The spinal lax zone and neutral zone: measurement techniques and parameter comparisons.

The neutral zone (NZ) is a well-established parameter that describes the loose region of the spinal range of motion (ROM) where the spine moves easily with minimal applied force. The loose region is of interest clinically because in vitro studies show that with the onset of instability, this region increases before the ROM increases. The upper boundary of the NZ has been described as the displacement at which ligamentous resistance just begins. However, we hypothesized that a different parameter that we have named the lax zone (LZ) more accurately describes the region of ligamentous laxity. We hypothesized that the NZ is a smaller subset of the LZ that is governed by frictional properties at the joint. Methods for determining the spinal LZ experimentally are introduced. To demonstrate that the LZ is distinct from the NZ, both parameters were quantified in six normal cadaveric human C5-C6 specimens for three different preload magnitudes and three different preload directions. LZ variations with changing preload closely matched variations in the location of the load-deformation curve elbow, whereas NZ values followed specimen resting position instead, verifying parameter independence. Reproducibility and interspecimen variability values were approximately equivalent for both parameters. Compared in a separate data set, NZ and LZ were both more sensitive than ROM in response to cervical discectomy. With discectomy, the absolute change in LZ was larger and the percentage change in LZ was smaller than in NZ. The NZ may be a more appropriate measure when investigating the biomechanical effect of alterations to joint articulations and when the loading environment is well controlled. The LZ may be a more appropriate measure when investigating the effect of ligament/disc alterations and when the loading environment cannot be controlled. Future work is needed to compare the sensitivities of the LZ and NZ in detecting spinal instability onset for various injuries.

Biomechanical comparison of C1-C2 posterior fixations. Cable, graft, and screw combinations.

STUDY DESIGN: Four combinations of cable-graft-screw fixation at C1-C2 were compared biomechanically in vitro using nondestructive flexibility testing. Each specimen was instrumented successively using each fixation combination. OBJECTIVES: To determine the relative amounts of movement at C1-C2 after instrumentation with various combinations of one or two transarticular screws and a posterior cable-secured graft. Also to determine the role of each component of the construct in resisting different types of loading. SUMMARY OF BACKGROUND DATA: Spinal stiffness increases after instrumentation with two transarticular screws plus a posterior wire-graft compared with a wire-graft alone. Other C1-C2 cable-graft-screw combinations have not been tested. METHODS: Eight human cadaveric occiput-C3 specimens were loaded nondestructively with pure moments, and nonconstrained motion at C1-C2 was measured. The instrumented states tested were a C1-C2 interposition graft attached with multistranded cable; a cable-graft plus one transarticular screw; two transarticular screws alone; and a cable-graft plus two transarticular screws. RESULTS: The transarticular screws prevented lateral bending and axial rotation better than the posterior cable-graft. The cable-graft prevented flexion and extension better than the screws. Increasing the number of fixation points often significantly decreased the rotation and translation (paired t test; P < 0.05). Axes of rotation shifted from their normal location toward the hardware. CONCLUSIONS: It is mechanically advantageous to include as many fixation points as possible when atlantoaxial instability is treated surgically.

Construction of local vertebral coordinate systems using a digitizing probe. Technical note.

STUDY DESIGN: When studying three-dimensional motion of multiple-vertebra spine segments in vitro, it is often desirable to report the kinematics at the individual vertebral levels in terms of each level's local coordinates systems. A novel technique is described for constructing local vertebral coordinate axes using a standard digitizing probe. OBJECTIVES: To describe a technique that was developed to allow researchers to relate vertebral landmarks to optical markers and to set the local coordinate axes of several vertebrae accurately through a short, simple procedure performed only once at the beginning of a spine testing experiment. SUMMARY OF BACKGROUND DATA: Other researchers have used radiographs and careful marker placement for establishing the coordinate systems of vertebrae and the relationships of anatomic landmarks to optical markers. The authors found no publications giving details of how vertebral coordinate systems are established from anatomic landmarks. METHODS: A digitizing probe is used to identify vertebral landmarks and to relate these landmarks to optical markers attached to the vertebrae. An algorithm is described whereby vertebral coordinate axes are constructed from the landmarks. RESULTS: The method described has been implemented successfully in a computerized in vitro spinal flexibility testing system that plots each individual motion segment's load-deformation curves in real time during experimentation. The proposed technique is less labor intensive and error prone than the earlier methods because landmarks are identified directly. CONCLUSIONS: The described technique quickly, easily, and accurately relates anatomic landmarks to optical markers and constructs local coordinate axes, two steps that are necessary before monitoring the kinematics of individual motion segments during multilevel spine testing.

Comparative mechanical properties of spinal cable and wire fixation systems.

STUDY DESIGN: Surgical spinal cable and wire fixation systems were tested mechanically using standardized methodologies. OBJECTIVES: To compare the relative mechanical properties and biomechanical performances of the different commercially available spinal wire and cable fixation devices, and to provide information that will help in selecting different cables for different clinical applications. SUMMARY OF BACKGROUND DATA: Spinal cables have become extensively used for spinal fixation; however, there are few published accounts delineating their mechanical properties. No reports have compared the relative properties of different cable systems. METHODS: Nine spinal cable and wire fixation systems were mechanically tested to compare their static tensile strength, stiffness, fatigue strength, creep, conformance, and abrasion properties. Titanium and stainless steel Codman cable, Danek cable, and AcroMed cable, polyethylene Smith & Nephew cable, and 20- and 22-gauge stainless steel monofilament Ethicon wire were tested using identical methodologies. The cable or wire was connected into loops with methods that simulated in vivo clinical applications. RESULTS: Under static tensile testing, titanium cables had 70% to 90% of the ultimate tensile strength of the comparable steel cables; the different cables were 100% to 600% stronger than monofilament wire; the ultimate strength of the polyethylene cable was similar to that of the strongest available steel cable. Fatigue testing delineated important differences among the different materials. For a given manufacturer, titanium cables were always more susceptible to fatigue than stainless steel cables of comparable diameter. Polyethylene cable withstood cyclical loading without breaking better than all of the metal cables and wires. The mechanisms of failure differed substantially among materials and types of tests. Polyethylene cables exhibited significant stretching or "creep" at loads that were much lower than the static failure loads. In contrast, no wire cable demonstrated creep. Monofilament wires demonstrated little creep. Polyethylene cables failed by elongating and loosening; wire cables failed by breaking. Monofilament wire and cables conformed least to a solid surface; polyethylene cable conformed the most and flattened out against solid surfaces. Abrasion properties depended on the surface characteristics of the implants. Polyethylene cable was abraded by (and eventually failed by wearing against) the simulated bone, a result that did not occur with any metal cables or wires. The steel and titanium cables and the monofilament wires all had an ability to abrade through simulated bone. CONCLUSIONS: Titanium, steel, and polyethylene cable systems all behave substantially differently mechanically compared with monofilament wire. The relative advantages and disadvantages of each particular products should be considered when selecting an implant for a specific clinical use.

Biomechanical effects of transthoracic microdiscectomy.

STUDY DESIGN: Nondestructive flexibility testing was performed to quantify biomechanical parameters of human cadaveric thoracic spines before and after microdiscectomy. OBJECTIVES: To assess the biomechanical differences between the normal thoracic spine and the thoracic spine after microdiscectomy and to determine whether microdiscectomy results in spinal instability. SUMMARY OF BACKGROUND DATA: Previous studies have investigated thoracic disc properties and the biomechanical effects of thoracic ligament or bone trauma. No studies were found assessing the effects of thoracic discectomy. METHODS: Eight motion segments (T4-T5 to T11-T12) from five human cadaveric thoracic spines were studied before and after microdiscectomy. Three-dimensional motion was recorded in response to nondestructive, nonconstraining pure moments. Parameters measured included the neutral zone, elastic zone, range of motion, rotational flexibility, and instantaneous axis of rotation. RESULTS: The neutral zone, elastic zone, and range of motion increased a small but significant (average P = 0.02 for range-of-motion increase) amount in all directions after thoracic microdiscectomy (mean bilateral range of motion increase, 2.1 degrees; range, 0.5-4.2 degrees). Flexibility increased slightly during lateral bending and flexion. The instantaneous axis of rotation location usually did not change, but sometimes shifted slightly away from the discectomy site after microdiscectomy. CONCLUSIONS: Thoracic microdiscectomy had small effects on the immediate mechanics and kinematics of the thoracic spine and did not overtly destabilize the motion segments.

Comparative pull-out strength of tapped and untapped pilot holes for bicortical anterior cervical screws.

STUDY DESIGN: This biomechanical study analyzed the axial pull-out strength of tapped versus untapped pilot holes for bicortical screws in the anterior cervical spine. OBJECTIVE: To determine which pilot hole preparation method was mechanically better. SUMMARY OF BACKGROUND DATA: Tapping pilot holes in the lumbar spine was previously shown significantly to reduce pull-out strength of pedicle screws. No study was found investigating the effect of tapping on pilot holes for anterior cervical bicortical screws. METHODS: Twenty-five unembalmed human cadaveric cervical vertebrae (C3-C7) were tested. Two identical pilot holes were drilled into each vertebra: one pilot hole was tapped, and the control pilot hole was not tapped. A fully threaded cortical bone screw was inserted into each pilot hole. Screw pull-out strength was determined using a servocontrolled hydraulic materials testing system and an axial load cell. Force-deformation and failure curves were obtained. RESULTS: There were no statistically significant differences between the axial pull-out strength of tapped and untapped pilot holes at any vertebral level. Mean force to-failure was 386 +/- 42 N in the untapped pilot holes and 397 +/- 48 N in the tapped pilot holes. CONCLUSIONS: Tapping a pilot hole for bicortical screws of the anterior cervical spine neither weakens nor strengthens the axial pull-out strength of fully threaded cortical bone screws. Tapping may be unnecessary; however, it may be desirable in patients with dense bone to cut the thread profile into the bone or if the screws have dull tips and threads.