High-energy cutoff in the spectrum of strong-field nonsequential double ionization.

Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0.8 PW/cm2 and 1.1 PW/cm2, where PW is defined as 10(15) W/cm2). Numerical solutions of the full-dimensional time-dependent helium Schrödinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U(p) cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.

A biomechanical study of Jefferson fractures.

STUDY DESIGN: Fifteen specimens of the first cervical vertebra were tested by the application of pure tensile forces to failure. Seven specimens had intact transverse ligaments, and eight had transection of the transverse ligament before testing. Specimens were tested to failure by the rapid application of laterally directed tensile force to the ring; this force then was exerted through the lateral masses to simulate the mechanism of injury for this fracture as proposed by Jefferson. OBJECTIVES: To measure the biomechanical characteristics of the C1 ring, including the fracture patterns created with tensile loading, and to describe the influence of the transverse ligament on the behavior of the ring as it failed under tension. SUMMARY OF BACKGROUND DATA: Jefferson fractures have been reproduced in the laboratory by subjecting head and neck preparations to axial load. However, no previous detailed biomechanical studies of the fracture characteristics of the isolated C1 vertebra have been reported. METHODS: Specimens were tested to failure by rapid application of laterally directed tensile forces to the ring. RESULTS: Eleven two-part and three three-part fractures occurred. The mean tensile strength of the atlas was found to be 2,280 N. The average deformation required to fracture the C1 ring was 1.57 mm. The total energy absorbed by the ring averaged 1.99 N-m. There was no statistically difference between those specimens with the transverse ligament intact and those without a transverse ligament. CONCLUSIONS: The results of this study show that fractures of the C1 ring of greater than two parts can occur with pure tensile loading. The ring will fracture with as little as 1 mm of deformation.

A biomechanical study of anterior thoracolumbar screw fixation.

STUDY DESIGN: The pullout strength of unicortical and bicortical screws in thoracic and lumbar vertebral bodies was measured as a function of bone mineral density. OBJECTIVES: To determine the influence of bone mineral density and screw insertion technique on the stability of anterior thoracolumbar spine screw fixation. SUMMARY OF BACKGROUND DATA: No previous study has addressed the specific technique of screw insertion or stability of screw fixation in the anterior spine. METHODS: Fifty-one human thoracic vertebral bodies were tested in pullout with 6.55-mm cancellous screws inserted using unicortical and bicortical techniques. RESULTS: Pullout force increased exponentially with increasing bone mineral density for unicortical and bicortical screws. Bicortical screws were significantly stronger in resisting pullout than unicortical screws. CONCLUSION: Advancing an anterior vertebral body screw to engage the second cortex increases resistance to pullout by 25-44%, depending on vertebral bone mineral density. The difference in resistance between unicortical and bicortical techniques was smaller in specimens with low mineral densities.

Quantitative anatomy of the occiput and the biomechanics of occipital screw fixation.

STUDY DESIGN: The surgically relevant osseous anatomy of the human anatomy was carefully studied and described. The stability of cortical and cancellous screws placed in anatomic sites commonly used for internal fixation of the occiput was tested. OBJECTIVES: To define the bony anatomy of the occiput in quantitative terms and to measure the ability of cortical and cancellous screws inserted at sites commonly used for internal fixation. SUMMARY OF BACKGROUND DATA: To the authors' knowledge, no previous studies described the gross anatomy of the occiput in specific relation to the internal venous structures in the cranium and to the biomechanical strength of screw fixation in different areas of the occiput. METHODS: Thirty-seven human occiputs were carefully measured using calipers. Thin sections from six such specimens were analyzed with specific attention to cortical thicknesses. Stability of screws placed in various locations in the occiput were tested in axial pullout. RESULTS: The thickness of the occiput varied from extremely thin to a 0.1-mm thickness in the region of the cerebellar fossa and increased to a maximum of 8.3 mm at the level of the superior nuchal line and at the transverse sulcus. Results of pullout testing showed that the cancellous screws were as strong as the cortical screws in this area. In areas of the occiput thicker than 7 mm, unicortical fixation was as strong as bicortical fixation. CONCLUSION: There is a wide variation in thickness of the bone of the occiput. The strength of screw fixation was proportional to the bone's thickness.

Morphologic study of lumbar vertebral osteophytes.

BACKGROUND: Vertebral osteophytes are one of the principal radiographic diagnostic criteria for degenerative change in the lumbar spine. These osteophytes have been previously classified by morphologic features into two groups, the "traction spur" and the "claw spur." It has been stated that the traction spur is a sign of spinal instability. METHODS: Lumbar spines from 20 cadavers provided 120 vertebrae from T-11 to L-5 and 240 vertebral rims for study. The presence of osteophytes was determined by measurement with digital calipers. The type of osteophyte was then determined visually, using the Macnab classification. RESULTS: Sixty vertebral rims were found to have significant osteophytes. Twenty-four vertebral rims had osteophytes of the claw type only, and 11 had osteophytes of the traction type only. Of the eight vertebrae with traction osteophytes alone and with the adjacent vertebra available for study, none had such a corresponding osteophyte. CONCLUSION: Traction spurs and claw spurs frequently coexist on the same vertebral rim. This suggests that they may result from the same degenerative process and do not necessarily reflect the results of two distinct pathologic processes.

The trabecular anatomy of thoracolumbar vertebrae: implications for burst fractures.

The gross osseous anatomy of the spine has been the subject of much study but the internal architecture and trabecular anatomy of thoracolumbar vertebrate has not previously been well described. This study is based on high resolution x-ray images of 56 isolated thoracolumbar vertebrae and thick sections obtained from them. A previously undescribed array of trabeculae was documented which originated from the medial corner of the base of the pedicles and extended in a radial array throughout the vertebral body. This trabecular array persisted even in severely osteoporotic specimens. The cortex of the vertebral canal thinned abruptly near the base of the pedicle creating what may be a stress concentration at this site. This corresponded to the site of origin of the trabecular array. The presence of this possible stress concentration, in proximity to the trabecular array, may provide an explanation for the trapezoidal shape of the bony fragments that are frequently retropulsed into the spinal canal in axial load type burst fractures, common in this area of the spine.

Kinematic evaluation of lumbar fusion techniques.

STUDY DESIGN: Eight human cadaveric lumbosacral spines were biomechanically and kinematically tested in torsion and compression-flexion. They were retested after simulated posterolateral fusion, anterior lumbar interbody fusion, and circumferential fusion. OBJECTIVES: To analyze stiffness and motion in the anterior and posterior columns of the index and contiguous spinal motion units of anterior, posterolateral, and circumferential fusions. SUMMARY OF BACKGROUND DATA: Previous biomechanical studies have not incorporated analysis of motion with six degrees of freedom, consideration of contiguous levels, and comparisons of anterior and posterior column motion. METHODS: Eight human cadaveric lumbosacral spines were biomechanically tested in compression-flexion and torsion using an advanced biplanar radiography technique. Each specimen underwent either a simulated posterolateral fusion or anterior fusion followed by a circumferential fusion. Motion and stiffness at the level of the fusion and at contiguous levels were analyzed independently in the anterior and posterior columns of the spine. RESULTS: At the level of fusion, the simulated posterolateral and anterior fusions prevented more motion in torsion compared with compression-flexion. With all specimens, it was shown that circumferential fusions were stiffer than the intact specimen. Our comparison of motion in the anterior and posterior columns found no significant differences within the columns of a single vertebral motion segment. Compared with posterolateral fusions, anterior fusions were found to have the greatest effect in increasing motion at contiguous levels. The effect of circumferential fusions on adjacent level kinematics was not significantly greater than that of anterior fusions. CONCLUSION: There are major biomechanical differences between different fusion techniques. This information should be considered in patients undergoing lumbar spinal fusion.

Segmental variations of bone mineral density in the cervical spine.

STUDY DESIGN: This study assessed the variability of segmental bone mineral density in the lower cervical spine (C4 through C7). A mean segmental bone mineral density value at each level was determined for all specimens, and a mean coefficient of variation among the 17 specimens was calculated. OBJECTIVES: To quantify the degree of intersegmental bone mineral density variations within cadaveric lower cervical spine segments. SUMMARY OF BACKGROUND DATA: Bone mineral density studies in the thoracic and lumbar spine have shown a high degree of variability between spinal segments; however, the extent of segmental bone mineral density variability in the cervical spine is unknown. METHODS: Seventeen human cadaveric cervical spine specimens (C4 through C7) were scanned in a water bath using dual energy x-ray absorptiometry in a lateral direction. Segmental bone mineral density of the vertebral bodies of all specimens were analyzed with respect to differences between segments within each specimen. RESULTS: The mean coefficient of segmental bone mineral density variations within each specimen for all spines was 14.8% (range, 5.8%-22.9%). Bone mineral density mean values and ranges at each level were as follows: C4, 0.720 g/cm2 (range, 0.367-1.161 g/cm2); C5, 0.784 g/cm2 (range, 0.348-1.268 g/cm2); C6, 0.735 g/cm2 (range 0.367-1.450 g/cm2); C7, 0.590 g/cm2 (range, 0.340-1.040 g/cm2). Paired analysis of difference between all levels for 16 specimens demonstrated the bone mineral density at the C7 level to be significantly lower than at all other levels (P < 0.05). CONCLUSION: Our data show that significant interlevel bone mineral density variability exists in the lower cervical spine, and suggests that random single segment bone mineral density sampling or mean specimen bone mineral density values may not be relevant.

An in-vitro biomechanical comparison of the Orosco and AO locking plates for anterior cervical spine fixation.

The Orosco anterior cervical spine plate (H-plate) and the Morscher AO cervical locking plate (CSLP) were studied to determine their comparative in vitro mechanical properties in flexion. Human cadaver cervical spines were tested nondestructively in flexion as intact, destabilized, and stabilized specimens with both implants. Stabilized specimens were also subjected to a large angular displacement in an attempt to induce implant failure. Differences between intact, destabilized, and stabilized specimens were significant for both the CSLP and H-plate groups in flexion testing (p < 0.05). Large angular displacement testing resulted in plate/screw displacement with the H-plate but not with the CSLP. Small angular displacements in flexion was tolerated well by both implants, but only the CSLP maintained stability with large angular displacements.

Quantitative anatomy of the second cervical vertebra.

STUDY DESIGN: While the gross anatomy of the second cervical vertebra has been well described qualitatively, our evolving understanding of spinal pathology makes it necessary for the modern surgeon to have more quantitative information. OBJECTIVES: The authors' goal was to directly measure clinically relevant dimensions of the second cervical vertebra, using a collection of anatomic specimens. SUMMARY OF BACKGROUND DATA: Many existing studies have been based on analysis of radiographic images, which are subject to errors of magnification, rotation, and projection. Several recent studies of dens morphology have been based on direct measurements. These studies in general did not extend to the lateral masses or posterior ring, however. METHODS: Direct measurements using digital calipers and goniometer were taken from 51 dried human second cervical vertebrae. The maximum and minimum anteroposterior and lateral diameters of the dens, and the anteroposterior and lateral diameters of the end-plate and foramen and were measured. The heights of the dens and superior facet surfaces were measured relative to a plane defined by the anterior-most point of the inferior endplate and the inferior-most points of the inferior facet surfaces. RESULTS: The greatest variation was found in the dens angle, with specimen dens angles ranging from -2 degrees to 42 degrees. The most significant correlations were between the total height of the vertebra and the height to the base of the dens (r2 = 0.81), between the total height of the vertebra and the maximum lateral diameter of the dens (r2 = 0.77), and between the minimum lateral diameter of the dens and the maximum lateral diameter of the dens (r2 = 0.63). CONCLUSIONS: The dimensions measured in this study confirm a significant degree of anatomic variation in the second cervical vertebra, and show no important correlations among the various measured dimensions. In particular, no significant correlation was found between dens height and canal diameter. Dens angle in the sagittal plane was the most variable measurement.

Effects of retained diaphyseal plates on forearm bone density and grip strength.

Numerous complications have been attributed to elective plate removal following successful treatment of both bone diaphyseal forearm fractures, yet the effects of retained implants are not known. Fourteen patients were reviewed for residual forearm grip strength (FGS) and bone mineral density (BMD) following uneventful union. Patients were analyzed in two groups based upon time from fixation: group I, > 2 years but < or = 5 years, and group II, > 5 years. In group I, mean 2.6 years follow-up (n = 6), mean FGS ratios (patient values/age matched general population means) were 0.65 and 0.77, respectively, for plated and normal limbs (p = 0.08). The mean BMD ratios of plated/normal sides adjacent to the plate were 0.99 (ulna) and 1.02 (radius); these values were not significantly different from more proximal forearm BMD ratios (p = 0.92 ulna; p = 0.44, radius). In group II, mean 8.4 years (n = 8), mean FGS ratios were 0.97 and 1.09, respectively, for plated and normal limbs (p > 0.05). The BMD ratio was 1.04 adjacent to the plate; this was not significantly different from the ratios measured away from the plate (p > 0.4). We conclude that retained forearm plates can be well tolerated and that their routine removal is not indicated based on FGS or BMD.

The quantitative anatomy of the atlas.

STUDY DESIGN: This study is based on direct quantitative caliper measurements of 88 isolated anatomic specimens of the C1 vertebra. OBJECTIVES: The study was undertaken to establish the range and variability of the external dimensions of the atlas and to describe the cortical thicknesses and trabecular distribution of this unique vertebra. SUMMARY OF BACKGROUND DATA: Before this study, Francis in 1952 reported the total anterior and posterior diameter of 285 atlas vertebrae. Liu et al reported detailed external dimensions and facet joint surface morphologies on a total of three C1 vertebrae. METHODS: Measurements were made of overall dimensions, canal diameters, and the dimensions of the anterior and posterior arches of 88 dried human C1 vertebrae. Eight specimens were sectioned in the frontal plane, eight in the sagittal plane, and four in the coronal plane. The anatomy of these sections was documented by radiographic imaging. Cortical thicknesses on the sections were then obtained by direct measurement. RESULTS: The canal diameter ranged from 32 mm (SD 2 mm) in the sagittal plane, and 29 mm (SD 2 mm) in the lateral dimension. The mean thickness of the anterior ring was 6 mm (SD 1 mm) and posteriorly was 8 mm (SD 2 mm). Cortical bone was thinnest posteriorly. CONCLUSIONS: These measurements indicated remarkably constant dimensions for the ring itself, suggesting there may be significant functional restraints on the canal size of this unique vertebra. In contrast, a significant variability was noted in objective measurements of lateral mass height and sagittal plane widths of the entire bone.

Interspace distraction and graft subsidence after anterior lumbar fusion with femoral strut allograft.

The authors performed a retrospective review of 32 patients who had undergone a single-level anterior lumbar fusion with femoral strut allograft as an isolated procedure. The goal of the study was to use clinical radiographs to measure interspace distraction, graft subsidence, interspace collapse, the nature of allograft incorporation, and to correlate these results with successful arthrodesis. Results were categorized according to plain radiographic appearance and flexion/extension stability. Sixty-six percent of the group exhibited radiographic union with flexion/extension stability. Twenty-two percent exhibited stability on the flexion-extension analysis but less than complete arthrodesis was present. Twelve percent exhibited radiographic non-union and flexion-extension instability. Interspace distraction of 11 mm was obtained initially with a follow-up distraction of 5.5 mm. Graft subsidence was noted posteriorly in 27 patients with an average subsidence of 4 mm. The authors' conclusions were: 1) Interspace distraction can be achieved with anterior lumbar fusion if appropriate interbody grafts are used. 2) Despite a solid arthrodesis rate of only 66%, "functional arthrodesis" was achieved in 88%. In a retrospective review of patients who underwent anterior lumbar fusion with femoral strut allograft, interspace distraction, graft subsidence, and incorporation and arthrodesis status were measured. A solid arthrodesis was achieved in 66% of the patients, and functional arthrodesis in another 22%. Interspace distraction was maintained in 59% of cases.

Efficacy of radiographic evaluation of the cervical spine in emergency situations.

Evaluation of trauma to the cervical spine usually includes predominantly normal x-ray studies that prevent expeditious workup and treatment and increase cost. In a retrospective review of the cases of 1,686 consecutive trauma patients who had routine radiographs for possible injury to the cervical spine, injuries were detected in 32 (1.9%). In a similar consecutive group of 597 patients whose cases were more critically analyzed prospectively, 17 (2.8%) had suspected injuries; of these, five (0.8%) were true positives. All patients in the prospective group with suspected injuries had neck pain, neurologic deficit, or altered consciousness, suggesting that emergency radiographic screening according to more selective clinical criteria may be feasible.

The trabecular anatomy of the axis.

This article describes the internal anatomy of C2. Although some C2 specimens showed a high density of trabecular bone throughout, a feature noted in all specimens was a void or very hypodense area of bone located immediately beneath the dens. The observed nonuniform distribution of trabecular bone within the axis is considered to have an effect on internal fixation of this bone. The good cancellous bone quality consistently observed in the lateral masses and beneath the facets, as well as near the bony end plates, suggests that these areas may be reasonable sites for the insertion of internal fixation devices. The hypodense area observed in the upper portion of C2 would suggest that fixation devices inserted through this area may obtain relatively poor purchase and may be prone to cut-through failure.

Biomechanics of odontoid fracture fixation. Comparison of the one- and two-screw technique.

Direct anterior screw fixation of odontoid fractures has become more prevalent clinically. No biomechanical study, however, has determined whether one or two screws should be used. This study measured the stability of the odontoid process after fracture and internal fixation with one or two screws. Internal fixation of Type II odontoid fractures did not restore the original stability of the intact specimen, direct internal fixation with one or two screws provided 50% of the stability of the unfractured odontoid, and no significant differences between the one- and two-screw technique was found under loading to failure, although the two-screw technique provided increased stiffness in extension loading.

Discography causes end plate deflection.

Thoracolumbar cadaver spine segments were studied by biplane radiography techniques during intradiscal injection of radiographic contrast agent. In these radiographically nondegenerated discs, significant deformation of the end plate accompanied these in vitro discograms. End plate deflections averaged 0.3 mm. The intervertebral disc bulges averaged 0.5 mm. These results suggest that the end plate itself, or events within the bone, are possible pain sources during clinical discography.

A biomechanical study of odontoid fractures and fracture fixation.

The purpose of this study was to measure the stability of the odontoid process after fracture and subsequent screw fixation. To accomplish this, we mechanically reproduced Type II and Type III odontoid fractures on isolated C2 vertebrae by varying the direction of load. These fractures were subsequently stabilized with a single 3.5 mm screw and retested for multidirectional stability and load to failure. Reduced and instrumented specimens were found to have a stiffness equivalent to one half of that of the unfractured odontoid. Load to failure was also slightly less than one half of the original fracture force (average 160 lb). Screw failure was by a cut-out mechanism in all Type III fractures and by bending of the screw in all Type III fractures. Our findings, in conjunction with the existing literature, strongly suggest that Type III fractures result from extension forces, whereas Type II fractures result from lateral or oblique loading forces. Single screw fixation of an odontoid fracture will provide stability equal to approximately one half that of the unfractured bone.

The holding strength of cannulated screws compared with solid core screws in cortical and cancellous bone.

The comparative holding strength of cannulated screws (CS) versus solid core screws (SCS) has not been reported, although differences exist in the respective diameters of their outer thread and pilot drill holes. Our objectives were to characterize these differences and determine the holding power of CS compared with SCS in cortical and cancellous bone. The dimensions of the Synthes (Paoli, PA) 3.5-mm SCS, 3.5-mm CS, 6.5-mm SCS, and 7.0-mm CS were measured, and the cross-sectional area for thread purchase was calculated. Using adult canine femurs, small-fragment 3.5-mm SCS were inserted in cortical (midshaft) and cancellous (condyle) bone of one limb, and CS were placed in similar locations in the contralateral limb. The same technique was used for large-fragment CS and SCS. Pull-out testing was performed using an MTS machine (MTS Systems, Minneapolis, MN) with axial loads applied at 5 mm/s, and data were analyzed to determine the effects of screw type, location, and size. Differences in CS versus SCS design result in higher cross-sectional areas available for SCS thread purchase. Yet no significant differences exists between screw types (SCS vs. CS) in either cortical or cancellous bone. In cancellous bone, large-fragment screws required more force to pull out than did small screws (p = 0.000). The mean force required to pull out small-fragment screws was higher in cortical bone than in cancellous bone (p = 0.000). These data suggest that the clinical decision to use CS versus SCS should not be based on pull-out strength.

Cervical spine locking plate: in vitro biomechanical testing.

The AO cervical spine locking plate (CSLP) for anterior subaxial fixation was recently received increasing clinical acclaim, yet to date the in vitro mechanical properties of this implant have not been reported. To determine the in vitro biomechanical properties of this device, five fresh human cadaver cervical spines were subjected to nondestructive testing in flexion and torsion in three stages: stage 1: intact spine; stage 2: destabilized spine; stage 3: destabilized spine with CSLP. Stage 3 specimens were also subjected to large angular displacement testing to assess the integrity of the fixation. In flexion, mean spinous process displacement was 1.21 mm for stage 1, 3.19 mm for stage 2, and 1.37 mm, for stage 3. Mean torsional stiffness was 2.86 Nm/degree in stage 1, 1.82Nm/degree in stage 2, and 2.20Nm/degree in stage 3. Large angular displacement testing in stage 3 resulted in screw loosening from the bone in two specimens; no screw plate loosening occurred. In our severely destabilized in vitro model, the CSLP restored flexion stability but not rotational stability. This suggests that supplemented bracing or fixation may be required to restore torsional stability.