Snehal S. Shetye

Pedicle screw placement in the lumbar spine: effect of trajectory and screw design on acute biomechanical purchase.

OBJECT Low bone mineral density in patients undergoing lumbar spinal surgery with screws is an especially difficult challenge because poor bone quality can severely compromise the maximum achievable purchase of the screws. A relatively new technique, the cortical bone screw trajectory, utilizes a medialized trajectory in the caudocephalad direction to engage a greater amount of cortical bone within the pars interarticularis and pedicle. The objectives of this cadaveric biomechanical study were to 1) evaluate a cortical screw system and compare its mechanical performance to the traditional pedicle screw system; 2) determine differences in bone quality associated with the cortical screw trajectory versus the normal pedicle screw insertion technique; 3) determine the cortical wall breach rate with both the cortical and traditional screw trajectories; and 4) determine the performance of the traditional screw in the cortical screw trajectory. METHODS Fourteen fresh frozen human lumbar spine sections (L1-5) were used in this study (mean age 57 ± 19 years). The experimental plan involved drilling and tapping screw holes for 2 trajectories under navigation (a traditional pedicle screw and a cortical screw) in both high-and low-quality vertebrae, measuring the bone quality associated with these trajectories, placing screws in the trajectories, and evaluating the competence of the screw purchase via 2 mechanical tests (pullout and toggle). The 3 experimental variants were 1) traditional pedicle screws placed in the traditional pedicle screw trajectory, 2) traditional pedicle screws placed in the cortical screw trajectory, and 3) cortical screws placed in the cortical screw trajectory. RESULTS A statistically significant increase in bone quality was observed for the cortical trajectories with a cortical screw (42%; p < 0.001) and traditional pedicle screw (48%; p < 0.001) when compared to the traditional trajectory with a traditional pedicle screw within the high-quality bone group. These significant differences were also found in the lowquality bone cohort. All mechanical parameter comparisons (screw type and trajectory) between high-quality and lowquality samples were significant (p < 0.01), and these data were all linearly correlated (r ≥ 0.65) to bone mineral density. Not all mechanical parameters determined from pullout and toggle testing were statistically significant between the 3 screw/trajectory combinations. The incidence of cortical wall breach with the cortical or traditional pedicle screw trajectories was not significantly different. CONCLUSIONS The data demonstrated that the cortical trajectory provides denser bone that allows for utilization of smaller screws to obtain mechanical purchase that is equivalent to long pedicle screws placed in traditional pedicle screw trajectories for both normal- and low-quality bone. Overall, this biomechanical study in cadavers provides evidence that the cortical screw trajectory represents a good option to obtain fixation for the lumbar spine with low-quality bone.

Experimental Characterization and Finite Element Implementation of Soft Tissue Nonlinear Viscoelasticity

Finite element (FE) models of articular joint structures do not typically implement the fully nonlinear viscoelastic behavior of the soft connective tissue components. Instead, contemporary whole joint FE models usually represent the transient soft tissue behavior with significantly simplified formulations that are computationally tractable. The resultant fidelity of these models is greatly compromised with respect to predictions under temporally varying static and dynamic loading regimes. In addition, models based upon experimentally derived nonlinear viscoelastic coefficients that do not account for the transient behavior during the loading event(s) may further reduce the models predictive accuracy. The current study provides the derivation and validation of a novel, phenomenological nonlinear viscoelastic formulation (based on the single integral nonlinear superposition formulation) that can be directly inputted into FE algorithms. This formulation and an accompanying experimental characterization technique, which incorporates relaxation manifested during the loading period of stress relaxation experiments, is compared to a previously published characterization method and validated against an independent analytical model. The results demonstrated that the static and dynamic FE approximations are in good agreement with the analytical solution. Additionally, the predictive accuracy of these approximations was observed to be highly dependent upon the experimental characterization technique. It is expected that implementation of the novel, computationally tractable nonlinear viscoelastic formulation and associated experimental characterization technique presented in the current study will greatly improve the predictive accuracy of the individual connective tissue components for whole joint FE simulations subjected to static and dynamic loading regimes.

Correlation of mechanical properties within the equine third metacarpal with trabecular bending and multi-density micro-computed tomography data.

Computed tomography (CT) data can be employed with respect to determining mechanical properties and has been used to predict parameters such as elastic modulus, yield strength, and ultimate strength of intact bone. Micro-computed tomography (muCT) possesses the resolution capable of detecting apparent bone density in extremely local regions and can characterize the trabecular structure. It has been asserted that this micro-structure is susceptible to micro-buckling and bending, which has a controversial role in predicting the global mechanical properties of bone. The current study measured the mechanical properties of relatively high apparent density bone from the equine distal third metacarpal. The mechanical properties were correlated with trabecular morphology parameters and apparent densities of localized regions obtained with muCT. These data were used to test two hypotheses: (1) accounting for trabecular bending using trabecular morphology parameters would provide better global mechanical property predictions than using only apparent density, and (2) regions of low apparent density dominate the overall mechanical behavior and provide greater correlation to the measured mechanical properties than regions of high apparent density. The data indicated that accounting for trabecular bending with morphological parameters resulted in stronger correlations to mechanical properties than correlations that relied only on apparent density (r2= 0.91 versus r2= 0.81). Low apparent density regions were more strongly correlated with mechanical properties than high apparent density regions (r2= 0.85 versus r2= 0.77), demonstrating the importance of selecting appropriate regions when attempting to predict mechanical properties from CT data.

Nonlinear Viscoelastic Characterization of the Porcine Spinal Cord

Although quasi-static and quasi-linear viscoelastic properties of the spinal cord have been reported previously, there are no published studies that have investigated the fully (strain-dependent) nonlinear viscoelastic properties of the spinal cord. In this study, stress relaxation experiments and dynamic cycling were performed on six fresh porcine lumbar cord specimens to examine their viscoelastic mechanical properties. The stress relaxation data were fitted to a modified superposition formulation and a novel finite ramp time correction technique was applied. The parameters obtained from this fitting methodology were used to predict the average dynamic cyclic viscoelastic behavior of the porcine cord. The data indicate that the porcine spinal cord exhibited fully nonlinear viscoelastic behavior. The average weighted root mean squared error for a Heaviside ramp fit was 2.8 kPa, which was significantly greater (p<0.001) than that of the nonlinear (comprehensive viscoelastic characterization method) fit (0.365 kPa). Further, the nonlinear mechanical parameters obtained were able to accurately predict the dynamic behavior, thus exemplifying the reliability of the obtained nonlinear parameters. These parameters will be important for future studies investigating various damage mechanisms of the spinal cord and studies developing high-resolution finite elements models of the spine.

Integrating dynamic stereo-radiography and surface-based motion data for subject-specific musculoskeletal dynamic modeling

This manuscript presents a new subject-specific musculoskeletal dynamic modeling approach that integrates high-accuracy dynamic stereo-radiography (DSX) joint kinematics and surface-based full-body motion data. We illustrate this approach by building a model in OpenSim for a patient who participated in a meniscus transplantation efficacy study, incorporating DSX data of the tibiofemoral joint kinematics. We compared this DSX-incorporated (DSXI) model to a default OpenSim model built using surface-measured data alone. The architectures and parameters of the two models were identical, while the differences in (time-averaged) tibiofemoral kinematics were of the order of magnitude of 10° in rotation and 10mm in translation. Model-predicted tibiofemoral compressive forces and knee muscle activations were compared against literature data acquired from instrumented total knee replacement components (Fregly et al., 2012) and the patients EMG recording. The comparison demonstrated that the incorporation of DSX data improves the veracity of musculoskeletal dynamic modeling.

Determination of mechanical properties of canine carpal ligaments.

OBJECTIVE To evaluate the mechanical properties of canine carpal ligaments for use in a finite element model of the canine antebrachium. SAMPLE POPULATION 26 forelimbs obtained from cadavers of 13 dogs euthanized for reasons unrelated to this study. PROCEDURES 6 ligaments (medial collateral, lateral collateral, palmar ulnocarpal, palmar radiocarpal, accessorometacarpal-V, and accessorometacarpal-IV) were evaluated. Quasistatic tensile tests were performed on all specimens (n = 8 specimens/ligament) by use of a servohydraulic materials testing system in conjunction with a 6-df load cell. Each specimen was preconditioned for 10 cycles by applying 2% strain by use of a Haversine waveform. Tension was subsequently applied to each specimen at a strain rate of 0.5%/s until ligament failure. RESULTS Significant differences in modulus of elasticity were detected among the ligaments. Elastic modulus did not differ significantly between the 2 accessorometacapal ligaments, between the 2 collateral ligaments, or between the 2 palmar carpal ligaments. Ligaments were classified into 3 groups (accessorometacarpal ligaments, intra-articular ligaments, and palmar carpal ligaments), and significant differences were detected among the 3 ligament groups. The accessorometacarpal ligaments had a relatively high elastic modulus, compared with results for the other ligaments. The medial and lateral collateral ligaments had the lowest elastic modulus of any of the ligaments tested. CONCLUSIONS AND CLINICAL RELEVANCE These results indicated a strong function-elastic modulus relationship for the 6 ligaments tested. The mechanical properties described here will be of use in creating a finite element model of the canine antebrachium.

Biaxial response of ovine spinal cord dura mater.

The dura mater performs a major functional role in the stability and mechanical response of the spinal cord complex. Computational techniques investigating the etiology of spinal cord injury require an accurate mechanical description of the dura mater. Previous studies investigating the mechanical response of the dura mater have reported conflicting results regarding the anisotropic stiffness of the dura in the longitudinal and circumferential direction. The aim of this study was to investigate the biaxial response of the dura mater in order to establish the tissue level mechanical behavior under physiological loading scenarios. To this end, square sections of the dura were tested in a custom biaxial setup under a comprehensive uniaxial and biaxial loading protocol. The resultant data were fit via a transversely isotropic continuum model and an anisotropic continuum constitutive model. The transversely isotropic formulation failed to accurately predict the dura mater׳s uniaxial behavior. The anisotropic formulation accurately predicted the uniaxial response in both longitudinal and circumferential directions. Significantly higher stiffness (p<0.0001) was observed in the circumferential direction as compared to the longitudinal direction. Further, the longitudinal direction displayed a significantly lower degree of nonlinearity (p<0.045) and significantly higher degree of collagen fiber dispersion (p<0.032) as compared to the circumferential direction. Results indicate that the dura mater has differential mechanical response in the longitudinal and circumferential directions and future studies should utilize an anisotropic two fiber family continuum model to accurately describe dura mater mechanics.

Cortical bone facet spacers for cervical spine decompression: effects on intervertebral kinetics and foraminal area.

OBJECTIVE Nerve root decompression to relieve pain and radiculopathy remains one of the main goals of fusion-promoting procedures in the subaxial cervical spine. The use of allograft facet spacers has been suggested as a potential alternative for performing foraminotomies to increase the space available for the cervical nerve roots while providing segmental stiffening. Therefore, the goal of this cadaveric biomechanical study was to determine the acute changes in kinetics and foraminal area after the insertion of cortical bone facet spacers into the subaxial cervical spine. METHODS Allograft spacers (2 mm in height) were placed bilaterally into cadaveric cervical spine specimens (C2-T1, age of donors 57.5 ± 9.5 years, n = 7) at 1 (C4-5) and 3 (C3-6) levels with and without laminectomies and posterior lateral mass screw fixation. Standard stereophotogrammetry under pure moment loading was used to assess spinal kinetics. In addition, the authors performed 3D principal component analysis of CT scans to determine changes in foraminal cross-sectional area (FCSA) available for the spinal nerve roots. RESULTS Generally, the introduction of 2-mm-height facet spacers to the cervical spine produced mild, statistically insignificant reductions in motion with particular exceptions at the levels of implantation. No significant adjacent-level motion effects in any bending plane were observed. The addition of the posterior instrumentation (PI) to the intact spines resulted in statistically significant reductions in motion at all cervical levels and bending planes. The same kinetic results were obtained when PI was added to spines that also had facet spacers at 3 levels and spines that had been destabilized by en bloc laminectomy. The addition of 2-mm facet spacers at C3-4, C4-5, and C5-6 did produce statistically significant increases in FCSA at those levels. CONCLUSIONS The addition of allograft cervical facet spacers should be considered a potential option to accomplish indirect foraminal decompression as measured in this cadaveric biomechanical study. However, 2-mm spacers without supplemental instrumentation do not provide significantly increased spinal segmental stability.

In Vitro Nonlinear Viscoelastic Characterization of the Porcine Spinal Cord

Approximately 12,400 new cases of spinal cord injuries (SCI) are reported in the United States every year. It has been estimated that the annual financial burden of SCI in the United States is approximately

Biaxial Response of Ovine Spinal Cord Dura Mater

7.736 billion. The mechanisms of mechanical damage to the spinal cord can be broadly classified into distraction, dislocation or contusion. Distraction injuries are predominantly caused by rapid acceleration-deceleration of the cervical spine. Vertebral burst fractures commonly result in contusion of the spinal cord and relative dislocation of adjacent vertebrae can inter-segmentally shear the spinal cord resulting in injury. Multiple studies have examined the quasi-static mechanical properties of the spinal cord [1–3]. However, considering that most spinal cord injuries occur during dynamic events with relatively high strain rates (ex: 10/s), alarmingly few studies have investigated the time-dependent mechanical characteristics of the spinal cord.Copyright