The legacy of hephaestus: The first craniotomy

Hephaestus is best known as the Greek god of metalworking, fire, and fine arts. As the only Olympian deity not endowed with physical perfection, he has been considered misfortunate among the Olympians. However, textual analysis of his myths reveals that Hephaestus was highly regarded by Greeks for his manual skills and intelligence. Furthermore, one of the myths about Hephaestus indicates that he performed the first recorded craniotomy. This text asserts that Hephaestus intentionally performed the craniotomy to remove a mass growing inside Zeus\u27 head, thereby relieving him of an excruciating headache. The successful craniotomy resulted in the birth of the goddess Athena. From a neurosurgical perspective, the story is allegorical. Nonetheless, it represents the surgical management of intracranial ailments, which is thought to have been reported in Greece centuries later by Hippocrates. Copyright © 2010 by the Congress of Neurological Surgeons

Biomechanical contribution of the rib cage to thoracic stability

Study Design: In vitro assessment of rib cage biomechanics in the region of true ribs with the ribs intact then sequentially resected in 5 steps. Objective: To determine the contribution of the rib cage to thoracic spine stability and kinematics. Summary of Background Data: Previous in vitro studies of rib cage biomechanics have used animal spines or human cadaveric spines with ribs left unsecured, limiting the ability of the ribs to contribute to stability. Methods: Eight upper thoracic specimens that included 4 ribs and sternum were tested in special fixtures that disallowed relative movement of the distal ribs and their vertebrae. While applying 7.5 Nm pure moments in 3 planes, angular motion at the middle motion segment was studied in intact specimens and then (1) after splitting the sternum, (2) after removing the sternum, (3) after removing 50% of ribs, (4) after removing 75% of ribs, and (5) after disarticulating and completely removing ribs. Results: During flexion/extension, the sternum and anterior rib cage most contributed to stability. During lateral bending, the posterior rib cage most contributed to stability. During axial rotation, stability was directly related to the proportion of ribs remaining intact. On average, intact ribs accounted for 78% of thoracic stability. An intact rib cage shifted the axis of rotation unpredictably, but its position remained consistent after partial resection of the ribs. During lateral bending, coupled axial rotation was mild and unaffected by ribs. Conclusion: Because of testing methodology, the rib cage accounted for a greater percentage of thoracic stability than previously estimated. Different rib cage structures resisted motion in different loading planes. © 2011 Lippincott Williams & Wilkins

Stabilization of the Atlantoaxial Joint With C1-C3 Lateral Mass Screw Constructs: Biomechanical Comparison With Standard Technique

BACKGROUND: Anatomically and biomechanically, the atlantoaxial joint is unique compared with the remainder of the cervical spine. OBJECTIVE: To assess the in vitro stability provided by 2 C2 screw sparing techniques in a destabilized model of the atlantoaxial joint and compare with the gold standard system. METHODS: The 3-dimensional intervertebral motion of 7 human cadaveric cervical spine specimens was recorded stereophotogrammetrically while applying nonconstraining, nondestructive pure moments during flexion-extension, left and right axial rotation, and left and right lateral bending. Each specimen was tested in the intact state, followed by destabilization (odontoidectomy) and fixation as follows: (1) C1 and C3 lateral mass screws rods with sublaminar wiring of C2 (LC1-C3 + SW), (2) C1 and C3 lateral mass screws rods with a cross-link in the C1-2 interlaminar space (LC1-C3 + CL), (3) C1 and C3 lateral mass screw rods alone (negative control), and (4) C1 lateral mass and C2 pedicle screws rods augmented with C1-2 interspinous wire and graft (LC1-PC2, control group). RESULTS: Compared with the intact spine, each instrumented state significantly stabilized range of motion and lax zone at C1-2 (P \u3c .001, 1-way repeated-measures analysis of variance). LC1-C3 + SW was equivalent to LC1-PC2 during flexion and lateral bending and superior to LC1-C3 + CL during lateral bending, while LC1-C3 + CL was equivalent to LC1-PC2 only during flexion. In all other comparisons, LC1-PC2 was superior to both techniques. CONCLUSION: From a biomechanical perspective, both C2 screw sparing techniques provided sufficient stability to be regarded as an alternative for C1-2 fixation. However, because normal motion across C2-3 is sacrificed, these constructs should be used in patients with unfavorable anatomy for standard fixations. Copyright © 2010 by the Congress of Neurological Surgeons

Biomechanical comparison of costotransverse process screw fixation and pedicle screw fixation of the upper thoracic spine.

OBJECTIVE: To compare the biomechanics of costotransverse process screw fixation with those of pedicle screw fixation in a cadaveric model of the upper thoracic spine. METHODS: Ten human thoracic spines were instrumented across the T3-T4 segment with costotransverse and pedicle screws. Nonconstraining pure moments (maximum, 6.0 Nm) were applied to induce flexion, extension, lateral bending, and axial rotation. The range of motion, lax zone, and stiff zone were determined in each specimen in the normal state, after 3-column destabilization, and after instrumentation. After flexibility testing was completed, axial screw pull-out strength was assessed. RESULTS: In all directions of loading, both fixation techniques significantly decreased lax zone and range of motion at T3-T4 compared with the destabilized state (P \u3c .001). During all loading modes except lateral bending, pedicle screw fixation allowed significantly less range of motion than costotransverse screw fixation. Pedicle screws provided 62% greater resistance to axial pull-out than costotransverse screws. CONCLUSION: The costotransverse screw technique seems to provide only moderately stiff fixation of the destabilized thoracic spine. Pedicle screw fixation seems to have more favorable biomechanical properties. These data suggest that the costotransverse process construct is better used as a salvage procedure rather than as a primary fixation strategy

Biomechanics of a novel minimally invasive lumbar interspinous spacer: effects on kinematics, facet loads, and foramen height.

OBJECTIVE: To study the alteration to normal biomechanics after insertion of a lumbar interspinous spacer (ISS) in vitro by nondestructive cadaveric flexibility testing. METHODS: Seven human cadaveric specimens were studied before and after ISS placement at L1-L2. Angular range of motion, lax zone, stiff zone, sagittal instantaneous axis of rotation (IAR), foraminal height, and facet loads were compared between conditions. Flexion, extension, lateral bending, and axial rotation were induced using pure moments (7.5 Nm maximum) while recording motion optoelectronically. The IAR was measured during loading with a 400 N compressive follower. Foraminal height changes were calculated using rigid body methods. Facet loads were assessed from surface strain and neural network analysis. RESULTS: After ISS insertion, range of motion and stiff zone during extension were significantly reduced (P \u3c .01). Foraminal height was significantly reduced from flexion to extension in both normal and ISS-implanted conditions; there was significantly less reduction in foraminal height during extension with the ISS in place. The ISS reduced the mean facet load by 30% during flexion (P \u3c .02) and 69% during extension (P \u3c .015). The IAR after ISS implantation was less than 1 mm from the normal position (P \u3e .18). CONCLUSION: The primary biomechanical effect of the ISS was reduced extension with associated reduced facet loads and smaller decrease in foraminal height. The ISS had little effect on sagittal IAR or on motion or facet loads in other directions

Quantitative analysis of misplaced pedicle screws in the thoracic spine: How much pullout strength is lost?

Object. The object of this study was to investigate the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of thoracic pedicle screws. Methods. Forty human thoracic vertebrae (T6-11) from human cadavers were studied. Before pedicle screws were inserted, the specimens were separated into 4 groups according to the type of screw used: 1) standard pedicle screw (no cortical perforation); 2) screw with medial cortical perforation; 3) screw with lateral cortical perforation; and 4) airball screw (a screw that completely missed the vertebral body). Consistency among the groups for bone mineral density, pedicle diameter, and screw insertion depth was evaluated. Finally, each screw was pulled out at a constant displacement rate of 10 mm/minute while ultimate strength was recorded. Results. Compared with well-placed pedicle screws, medially misplaced screws had 8% greater mean pullout strength (p = 0.482) and laterally misplaced screws had 21% less mean pullout strength (p = 0.059). The difference in mean pullout strength between screws with medial and lateral cortical perforations was significant (p = 0.013). Airball screws had only 66% of the mean pullout strength of well-placed screws (p = 0.009) and had 16% lower mean pullout strength than laterally misplaced screws (p = 0.395). Conclusions. This in vitro study showed a significant difference in mean pullout strength between medial and lateral misplaced pedicle screws. Moreover, airball screws were associated with a significant loss of pullout strength

Biomechanics of dynamic rod segments for achieving transitional stiffness with lumbosacral fusion

Background: Transitioning from rigid to flexible hardware at the distal rostral or caudal lumbar or lumbosacral level hypothetically maintains motion at the transition level and protects the transition level and intact adjacent levels from stresses caused by fusion. Objective: To biomechanically compare transitional and rigid constructs with uninstrumented specimens in vitro. Methods: Human cadaveric L2-S1 segments were tested (1) intact, (2) after L5-S1 rigid pedicle screw-rod fixation, (3) after L4-S1 rigid pedicle screw-rod fixation, and (4) after hybrid fixation rigidly spanning L5-S1 and dynamically spanning L4-L5. Pure moments (maximum 7.5 Nm) induced flexion, extension, lateral bending, and axial rotation while motion was recorded optoelectronically. Additionally, specimens were studied in flexion/extension with a 400-N compressive follower load. Strain gauges on laminae were used to extract facet loads. Results: The range of motion at the transition segment (L4-L5) for the hybrid construct was significantly less than for the intact condition and significantly greater than for the rigid 2-level construct during lateral bending and axial rotation but not during flexion or extension. Sagittal axis of rotation at L4-L5 shifted significantly after rigid 2-level or hybrid fixation (P \u3c .003) but shifted significantly farther posterior and rostral with rigid fixation (P \u3c .02). Instrumentation altered L4-L5 facet load at more than the L3-L4 facet load. Conclusion: The effect of the dynamic rod segment on the kinematics of the transition level was less pronounced than that of a fully rigid construct in vitro with this particular rod system. This experimental model detected no biomechanical alterations at adjacent intact levels with hybrid or rigid systems. © 2013 by the Congress of Neurological Surgeons

Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines

Background context: Novel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation. Purpose: Tests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws. Study design/setting: In vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks. Patient sample: Seventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested. Outcome measures: Measures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure. Methods: For each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity. Results: In bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p\u3c.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156). Conclusions: Although dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity. © 2013 Elsevier Inc. All rights reserved

Biomechanical evaluation of posterior thoracic transpedicular discectomy Laboratory investigation

Object. The authors investigated the biomechanical properties of transpedicular discectomy in the thoracic spine and compared the effects on spinal stability of a partial and total facetectomy. Methods. Human thoracic specimens were tested while intact, after a transpedicular discectomy with partial facetectomy, and after an additional total facetectomy was incorporated. Nonconstraining pure moments were applied under load control (maximum 7.5 Nm) to induce flexion, extension, lateral bending, and axial rotation while spinal motion was measured at T8-9 optoelectronically. The range of motion (ROM) and lax zone were determined in each specimen and compared among conditions. Results. Transpedicular discectomy with and without a total facetectomy significantly increased the ROM and lax zone in all directions of loading compared with the intact spine (p \u3c 0.008). The segmental increase in ROM observed with the transpedicular discectomy was 25%. The additional total facetectomy created an insignificant 3% further increase in ROM compared with medial facetectomy (p \u3e 0.2). Conclusions. Transpedicular discectomy can be performed in the thoracic spine with a modest decrease in stability expected. Because the biomechanical behavior of a total facetectomy is equivalent to that of a medial facetectomy, the additional facet removal may be incorporated without further biomechanical consequences

Biomechanics of Thoracic Short Versus Long Fixation After 3-Column Injury: Laboratory Investigation

Object. Posterior screw-rod fixation for thoracic spine trauma usually involves fusion across long segments. Biomechanical data on screw-based short-segment fixation for thoracic fusion are lacking. The authors compared the effects of spanning short and long segments in the thoracic spine. Methods. Seven human spine segments (5 segments from T-2 to T-8; 2 segments from T-3 to T-9) were prepared. Pure-moment loading of 6 Nm was applied to induce flexion, extension, lateral bending, and axial rotation while 3D motion was measured optoelectronically. Normal specimens were tested, and then a wedge fracture was created on the middle vertebra after cutting the posterior ligaments. Five conditions of instrumentation were tested, as follows: Step A, 4-level fixation plus cross-link; Step B, 2-level fixation; Step C, 2-level fixation plus cross-link; Step D, 2-level fixation plus screws at fracture site (index); and Step E, 2-level fixation plus index screws plus cross-link. Results. Long-segment fixation restricted 2-level range of motion (ROM) during extension and lateral bending significantly better than the most rigid short-segment construct. Adding index screws in short-segment constructs significantly reduced ROM during flexion, lateral bending, and axial rotation (p \u3c 0.03). A cross-link reduced axial rotation ROM (p = 0.001), not affecting other loading directions (p \u3e 0.4). Conclusions. Thoracic short-segment fixation provides significantly less stability than long-segment fixation for the injury studied. Adding a cross-link to short fixation improved stability only during axial rotation. Adding a screw at the fracture site improved short-segment stability by an average of 25%