To develop a method for evaluating viscosity and elasticity of the cornea and to examine the effect that both properties have on hysteresis.
A three-component spring and dashpot model was created in ...Simulink in Matlab to represent the purely elastic and viscoelastic behavior of the cornea during a measurement using device called an ocular response analyzer (ORA). Values for elasticity and viscosity were varied while sinusoidal stress was applied to the model. The simulated stresses were used to determine how hysteresis is affected by the individual components of elasticity, viscosity, and maximum stress. To validate the model, high-speed photography was used to measure induced strain in a corneal phantom during ORA measurement. This measured strain was compared with the strains simulated by the model.
When the spring in the viscoelastic portion of the model was stiffened, hysteresis decreased. When the spring in the purely elastic element was stiffened, hysteresis increased. If both springs were stiffened together, hysteresis peaked strongly as a function of the viscosity of the viscoelastic element. Below the peak value, lower elasticity was associated with higher hysteresis. Above the peak value, higher elasticity was associated with higher hysteresis. In addition, hysteresis increased as the air maximum pressure was increased. Measurements from phantom corresponded to predictions from the model.
A viscoelastic model is presented to illustrate how changing viscosity and elasticity may affect hysteresis. Low hysteresis can be associated with either high elasticity or low elasticity, depending on the viscosity, a finding consistent with clinical reports.
Objective
To determine the biomechanical behavior of a novel distraction‐stabilization system, consisting of an intervertebral distraction bolt, polyaxial screws, and connecting rods, in the canine ...lumbosacral spine.
Study design
Biomechanical study.
Sample population
Cadaveric canine lumbosacral spines (L4‐Cd3) (N = 8).
Methods
Cadaveric lumbosacral spines were harvested, stripped of musculature, mounted on a 4‐point bending jig, and tested in extension, flexion, and lateral bending using nondestructive compressive axial loads (0‐150 N). Angular displacement was recorded from reflective optical trackers rigidly secured to L6, L7, and S1. Data for primary and coupled motion were collected from intact spines, after destabilization at L7‐S1, and following surgical stabilization with the new implant system.
Results
As compared with the intact spine, laminectomy resulted in a modest increase in angular displacement at L6‐L7 and a marked increase at L7‐S1. Instrumentation significantly reduced motion at the operated level (L7‐S1) with a concomitant increase at the adjacent level (L6‐L7).
Conclusion
The combination of a polyaxial pedicle screw‐rod system and intervertebral spacer provides a versatile solution of surgical stabilization of the lumbosacral joint following surgical decompression in the canine lumbosacral spine. The increase in motion at L6‐L7 may suggest the potential for adjacent level effects and clinical trials should be designed to address this question.
Clinical relevance
These results support the feasibility of using this new implant system for the management of degenerative lumbosacral disease in dogs. The increase in motion at L6‐L7 may suggest the potential for adjacent level effects and clinical trials should be designed to address this question.
Objective
To assess the feasibility and mechanical stability of sacroiliac (SI) joint stabilization using 2 short 3.5 mm cortical screws, each spanning an average of 23% of the width of the sacral ...body.
Study design
Cadaveric experimental study.
Sample population
Twenty‐four canine pelvis specimens.
Methods
Pelvis specimens were prepared by disarticulation of the left SI joint and osteotomy of the left pubis and left ischium, and stabilized using a single long lag screw (LLS), 2 short lag screws (SLS) or 2 short positional screws (SPS). Computed tomography (CT) imaging was used to determine standardized screw lengths for each group and was repeated following implant insertion. Specimens were secured within a servohydraulic test frame and loaded through the acetabulum to simulate weight bearing under displacement control at 4 mm/min for 20 mm total displacement. Group mechanical testing data were compared.
Results
Peak load, yield load, and stiffness were more than 2 times greater in both the SLS and SPS groups when compared with the LLS group. No mechanical difference was identified between the short‐screw groups.
Conclusion
Sacroiliac luxation fixation using 2 short screws created a stronger, stiffer construct when compared with fixation using a single lag screw spanning 60% of the width of the sacral body. No mechanical advantage was observed between short screws inserted in positional vs. lag fashion.
Clinical significance
Sacroiliac luxation fixation using 2 short screws creates a mechanically superior construct with a larger region of acceptable implant positioning and potentially reduced risk of iatrogenic injury compared with conventional fixation.
We sought to determine the impact of bacterial inoculation and length of exposure on the mechanical integrity of soft tissue tendon grafts. Cultures of Staphylococcus epidermidis were inoculated on ...human tibialis posterior cadaveric tendon to grow biofilms. A low inoculum in 10% growth medium was incubated for 30 min to replicate conditions of clinical infection. Growth conditions assessed included inoculum concentrations of 100, 1000, 10,000 colony‐forming units (CFUs). Tests using the MTS Bionix system were performed to assess the influence of bacterial biofilms on tendon strength. Load‐to‐failure testing was performed on the tendons, and the ultimate tensile strength was obtained from the maximal force and the cross‐sectional area. Displacements of tendon origin to maximal displacement were normalized to tendon length to obtain strain values. Tendon force‐displacement and stress‐strain relationships were calculated, and Young's modulus was determined. Elastic modulus and ultimate tensile strength decreased with increasing bioburden. Young's modulus was greater in uninoculated controls compared to tendons inoculated at 10,000 CFU (p = 0.0011) but unaffected by bacterial concentrations of 100 and 1000 CFU (p = 0.054, p = 0.078). Increasing bioburden was associated with decreased peak load to failure (p = 0.043) but was most significant compared to the control under the 10,000 and 1000 CFU growth conditions (p = 0.0005, p = 0.049). The presence of S. epidermidis increased elasticity and decreased ultimate tensile stress of human cadaveric tendons, with increasing effect noted with increasing bioburden.
Clinical Relevance
Understanding the impact of subclinical and chronic infection on mechanical strength following ACLR may reduce graft failure and prolong the integrity of the reconstruction.
Using maximum diameter of an abdominal aortic aneurysm (AAA) alone for management can lead to delayed interventions or unnecessary urgent repairs. Abdominal aortic aneurysm stiffness plays an ...important role in its expansion and rupture. In vivo aortic magnetic resonance elastography (MRE) was developed to spatially measure AAA stiffness in previous pilot studies and has not been thoroughly validated and evaluated for its potential clinical value. This study aims to evaluate noninvasive in vivo aortic MRE-derived stiffness in an AAA porcine model and investigate the relationships between MRE-derived AAA stiffness and (1) histopathology, (2) uniaxial tensile test, and (3) burst testing for assessing MRE's potential in evaluating AAA rupture risk.
Abdominal aortic aneurysm was induced in 31 Yorkshire pigs (n = 226 stiffness measurements). Animals were randomly divided into 3 cohorts: 2-week, 4-week, and 4-week-burst. Aortic MRE was sequentially performed. Histopathologic analyses were performed to quantify elastin, collagen, and mineral densities. Uniaxial tensile test and burst testing were conducted to measure peak stress and burst pressure for assessing the ultimate wall strength.
Magnetic resonance elastography-derived AAA stiffness was significantly higher than the normal aorta. Significant reduction in elastin and collagen densities as well as increased mineralization was observed in AAAs. Uniaxial tensile test and burst testing revealed reduced ultimate wall strength. Magnetic resonance elastography-derived aortic stiffness correlated to elastin density (ρ = -0.68; P < 0.0001; n = 60) and mineralization (ρ = 0.59; P < 0.0001; n = 60). Inverse correlations were observed between aortic stiffness and peak stress (ρ = -0.32; P = 0.0495; n = 38) as well as burst pressure (ρ = -0.55; P = 0.0116; n = 20).
Noninvasive in vivo aortic MRE successfully detected aortic wall stiffening, confirming the extracellular matrix remodeling observed in the histopathologic analyses. These mural changes diminished wall strength. Inverse correlation between MRE-derived aortic stiffness and aortic wall strength suggests that MRE-derived stiffness can be a potential biomarker for clinically assessing AAA wall status and rupture potential.
Objective
To determine the failure method of simulated equine medial femoral condyle (MFC) subchondral bone defects under compression and the influence of screw placement on failure resistance.
Study ...Design
In vitro study.
Sample Population
Composite disks (CD) simulating the moduli of yearling bone in the MFC.
Methods
Four CD conditions were tested, all with a 12.7 mm void (n = 6 per condition): intact (no void), void only, void with a 4.5 mm screw placed in neutral fashion, and void with a 4.5 mm screw placed in lag fashion. Composite disks of each condition were tested under monotonic compression to 6000 N and cyclic compression to 10 000 cycles. Observable failure, load at first observable failure, and displacement at peak 2000 N load were compared among conditions.
Results
Specimens failed by cracking at the superior aspect of the void or the screw exit hole. After monotonic loading, cracks were observed 6/6 CD with a void, 6/6 CD with a void/lag screw, and 5/6 CD with a void/neutral screw. After cyclical testing, cracks were noted only on the superior aspect of 6/6 CD with a void and 3/6 CD with a void/lag screw. Displacement at peak load was 0.06 mm (intact), 0.32 mm (void), 0.24 mm (void/lag screw), and 0.11 mm (void/neutral screw).
Conclusion
Model MFC voids failed by superior cracking that was resisted by lag and neutral screw placement.
Clinical Significance
Neutral screws may be an acceptable treatment for subchondral lucencies in the MFC.
Objective
Determine compression generated by lag and neutral screws over 12 h using two bone analogs.
Study design
Experimental study.
Sample population
Bone analogs were made of composite synthetic ...bone (CSB) or three‐dimensional printed polylactic acid (PLA). Analogs had a 2 mm exterior shell with a 10 mm thick internal layer of open‐cell material.
Methods
Bone analogs were opposed, making a 4‐sided box with open ends. A central channel contained the sensor and the screws passed through it to engage both paired analogs. Four screw/analog conditions were tested: neutral and lag screw with bicortical engagement, neutral and lag screw with unicortical engagement. All screws were tightened to 2 Nm torque and compression values recorded at 0, 0.5, 1, 2, 6, and 12 h (six trials per condition). Medians were compared across groups for statistical significance.
Results
There was no difference in median compression between lag and neutral bicortical screws. For PLA, greater median compression was generated by neutral (median 437 N) and lag (median 379 N) bicortical screws compared to neutral unicortical screws (median 208 N, p < .001); lag bicortical screws generated greater median compression than lag unicortical screws (median 265 N, p = .012). For CSB, lag bicortical screws (median 293 N) generated greater median compression than neutral unicortical screws (median 228 N, p = .008).
Conclusion
Lag and neutral screws generated similar compression. Bicortical screws had higher median compression than unicortical screws in bone analogs.
Clinical significance
Neutral screws generate compression in cancellous bone analogs that can be increased with bicortical bone engagement.
Spine loads associated with lifting a 9-kg weight were estimated at three torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees), and lumbosacral motion segments were cyclically loaded using ...these loads until failure or to a maximum of 10,020 cycles.
To simulate the postures and loads experienced by the lumbar spine during repetitive lifting of moderate weights in different torso flexion postures, and to analyze the fatigue failure response of lumbosacral motion segments.
Previous fatigue failure studies of lumbar motion segments have not reproduced the combination of spinal postures, loads, and load rates anticipated in different torso flexion postures during lifting tasks characteristic of those in occupational settings.
Twelve fresh human lumbosacral spines were dissected into three motion segments each (L1-L2, L3-L4, and L5-S1). Motion segments within each spine were randomly assigned to a simulated torso flexion angle (0 degrees, 22.5 degrees, or 45 degrees) using a partially balanced incomplete block experimental design. Spinal load and load rate were determined for each torso flexion angle using previously collected data from an EMG-assisted biomechanical model. Motion segments were creep loaded for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading). Specimens were inspected to determine failure mechanisms.
The degree of torso flexion had a dramatic impact on cycles to failure. Motion segments experiencing the 0 degrees torso flexion condition averaged 8,253 cycles to failure (+/-2,895), while the 22.5 degrees torso flexion angle averaged 3,257 (+/-4,443) cycles to failure, and motion segments at the 45 degrees torso flexion angle lasted only 263 cycles (+/-646), on average. The difference was significant at P < 0.0001, and torso flexion accounted for 50% of the total variance in cycles to failure.
Fatigue failure of spinal tissues can occur rapidly when the torso is fully flexed during occupational lifting tasks; however, many thousands of cycles can be tolerated in a neutral posture. Future lifting recommendations should be sensitive to rapid development of fatigue failure in torso flexion.
Objective
To determine whether microwave ablation (MWA) modifies the biomechanical properties of the normal distal radius in the dog to better estimate the clinical impact of MWA as a tool for the ...treatment of neoplastic bone lesions.
Study design
Biomechanical experimental study.
Sample population
Sixteen pairs of dog forelimbs from 16 canine cadavers.
Methods
From each pair of forelimbs, one radius was randomly assigned to an MWA group, and the other radius was randomly assigned to a control group. Bone tunnels were created in each distal radial epiphysis for a length of 6 cm toward the middiaphysis. In the MWA group, the ablation probe was inserted into the bone tunnel for a series of three ablation treatments. Specimens were then tested in three‐point bending to acute failure with the middle point located 3 cm from the distal articular surface (middle of the ablated zone). Load and displacement were continuously recorded to determine maximum displacement and peak load before failure. Data were analyzed with noninferiority tests.
Results
The mean peak loads for the control group and MWA group were 1641.9 N and 1590.9 N, respectively. Microwave ablation‐treated radii were not biomechanically inferior to control radii (P < .0001).
Conclusion
Microwave ablation of normal cadaveric dog distal radii did not affect the maximum displacement and peak load before failure.
Clinical significance
Microwave ablation does not affect biomechanical bending properties of the distal radius in the dog. Future studies, both cadaveric and in vivo, are required to evaluate the impact of MWA on neoplastic bone.
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