Influence of Staphylococcus epidermidis biofilm on the mechanical strength of soft tissue allograft.

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.

Mechanical evaluation of canine sacroiliac joint stabilization using two short screws.

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.

Compression generated by cortical screws in an artificial bone model of an equine medial femoral condylar cyst.

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.

Mapping Bacterial Biofilm on Features of Orthopedic Implants In Vitro.

Implant-associated infection is a major complication of orthopedic surgery. One of the most common organisms identified in periprosthetic joint infections is Staphylococcus aureus, a biofilm-forming pathogen. Orthopedic implants are composed of a variety of materials, such as titanium, polyethylene and stainless steel, which are at risk for colonization by bacterial biofilms. Little is known about how larger surface features of orthopedic hardware (such as ridges, holes, edges, etc.) influence biofilm formation and attachment. To study how biofilms might form on actual components, we submerged multiple orthopedic implants of various shapes, sizes, roughness and material type in brain heart infusion broth inoculated with Staphylococcus aureus SAP231, a bioluminescent USA300 strain. Implants were incubated for 72 h with daily media exchanges. After incubation, implants were imaged using an in vitro imaging system (IVIS) and the metabolic signal produced by biofilms was quantified by image analysis. Scanning electron microscopy was then used to image different areas of the implants to complement the IVIS imaging. Rough surfaces had the greatest luminescence compared to edges or smooth surfaces on a single implant and across all implants when the images were merged. The luminescence of edges was also significantly greater than smooth surfaces. These data suggest implant roughness, as well as large-scale surface features, may be at greater risk of biofilm colonization.

Combined center of rotation of angulation-based leveling osteotomy and tibial tuberosity transposition: An ex vivo mechanical study.

OBJECTIVE: To describe the technique of combined center of rotation of angulation (CORA)-based leveling osteotomy (CBLO) with tibial tuberosity transposition (TTT) and to compare the load to failure between CBLO combined with TTT and CBLO or TTT alone. STUDY DESIGN: Ex vivo study. SAMPLE POPULATION: Twelve pairs of cadaveric pelvic limbs. METHODS: Six pairs of cadaveric tibia were tested in each group (CBLO-TTT versus CBLO) and (CBLO-TTT versus TTT) with each limb randomly assigned to a treatment group. Construct stability was determined by applying a tensile force to each patellar tendon until failure occurred. Load at failure and mode of failure were recorded for each specimen. RESULTS: No difference in mean load to failure was identified between CBLO-TTT (897 N) and CBLO alone (943 N) (P = .81). There was also no difference in the mean load to failure between the CBLO-TTT (928 N) and TTT alone (1046 N) (P = .12). CONCLUSION: Performing a TTT in combination with a CBLO does not weaken the construct failure to load when compared with each procedure performed alone. CLINICAL SIGNIFICANCE: A combined CBLO and TTT could be considered a viable option for concurrent management of a cranial cruciate ligament deficient stifle and medial patella luxation.

The failure mode of a mechanically loaded equine medial femoral condyle analog with a void and the impact of lag and neutral screw placement.

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.

Kinematics of a Novel Canine Cervical Fusion System.

OBJECTIVE: The aim of this study was to determine the biomechanical behaviour of a novel distraction-fusion system, consisting of an intervertebral distraction screw, pedicle locking screws and connecting rods, in the canine caudal cervical spine. STUDY DESIGN: Biomechanical study in cadaveric canine cervicothoracic (C3-T3) spines (n = 6). Cadaveric spines were harvested, stripped of musculature, mounted on a four-point bending jig, and tested using non-destructive four-point bending loads in extension (0-100 N), flexion (0-60 N) and lateral bending (0-40 N). Angular displacement was recorded from reflective optical trackers rigidly secured to C5, C6 and C7. Data for primary and coupled motions were collected from intact spines and following surgical stabilization (after ventral annulotomy and nucleotomy) with the new implant system. RESULTS: As compared with the intact spine, instrumentation significantly reduced motion at the operated level (C5-C6) with a concomitant non-significant increase at the adjacent level (C6-C7). CONCLUSION: The combination of a locking pedicle screw-rod system and intervertebral spacer provides an alternative solution for surgical distraction-stabilization in the canine caudal cervical spine and supports the feasibility of using this new implant system in the management of disc-associated cervical spondylomyelopathy in dogs. The increase in motion at C6-C7 may suggest the potential for adjacent level effects and clinical trials should be designed to address this.

Impact of microwave ablation treatment on the biomechanical properties of the distal radius in the dog: A cadaveric study.

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.

In Vivo Aortic Magnetic Resonance Elastography in Abdominal Aortic Aneurysm: A Validation in an Animal Model.

OBJECTIVES: 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. MATERIALS AND METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.

A Cadaveric Evaluation of Pin and Tension Band Configuration Strength for Tibial Tuberosity Osteotomy Fixation.

OBJECTIVE: The purpose of this study was to compare the load at failure and mode of failure of four constructs used to stabilize a tibial tuberosity osteotomy, including two vertically aligned pins (V), two horizontally aligned pins (H), two vertically aligned pins with a tension band wire (V-TB) and two horizontally aligned pins with a tension band wire (H-TB). STUDY DESIGN: Eighteen pairs of cadaveric tibiae were randomized to receive a TB or no TB. One limb was randomized to be in the H or V group. The contralateral limb was then assigned to the opposite configuration. One pair of limbs was used as a control. A tensile force was applied to the patellar ligament until construct failure. RESULTS: There was no significant difference between the mean load at failure of the H (595 N) and V (556 N) groups or between H-TB (1032 N) and V-TB groups (1034 N) (p = 0.487 and p = 0.238, respectively). The TB constructs were significantly stronger than the pin only constructs (p < 0.001). The mode of failure was similar for the pin only constructs, regardless of pin orientation. The TB constructs and control tibias failed at similar loads, most commonly by patellar ligament rupture. CONCLUSION: The use of vertically aligned pins versus horizontally aligned pins does not affect construct strength. These results support the placement of pins in a vertically or horizontally aligned fashion. When performing a tibial tuberosity osteotomy, the addition of a TB adds significant strength to the construct.

Measurement of Shoulder Abduction Angles in Dogs: An Ex Vivo Study of Accuracy and Repeatability.

OBJECTIVE: The aim of this study was to determine the accuracy and repeatability of the shoulder abduction test and to assess the effect of transection of the medial shoulder support structures in canine cadavers. MATERIALS AND METHODS: The shoulder abduction angle was measured by three separate observers, both with the shoulder extended and at a neutral angle. Shoulder abduction was then measured, using craniocaudal fluoroscopic images. Arthroscopy was performed in all shoulder joints, with the medial support structures transected in one shoulder of each dog. The three observers again measured shoulder abduction angles in all dogs. Shoulder abduction was measured again using fluoroscopy. Accuracy and repeatability of the abduction test were assessed using linear mixed models. RESULTS: All three observers had different measured abduction angles when compared with fluoroscopy (p < 0.01); however, the experienced surgeon had an error of only 2.9°. Inter-observer repeatability was poor, with all three observers having different abduction measurements (p < 0.001). Intra-observer repeatability, however, indicated no differences on repeated measurements (p = 0.26). Placing the shoulder at a neutral standing angle, and transection of support structures caused an average increase in abduction by 8.2° (p < 0.001) and 4.4° respectively. CONCLUSION: Significant variation exists between observers performing this test, increased accuracy seen in the more experienced observer. Shoulder flexion angle can significantly affect measured abduction angles.

A Biomechanical Analysis of Lateral Interbody Construct and Supplemental Fixation in Adjacent-Segment Disease of the Lumbar Spine.

OBJECTIVE: To analyze the stability of lateral lumbar interbody fusion (LLIF) and compare various methods of supplemental fixation in adjacent-segment disease. METHODS: Four fresh-frozen human cadaveric lumbar spines (L1 to sacrum) were used for motion analysis in extension, flexion, and lateral bending. The L4-L5 level was secured with a lateral interbody cage and pedicle screws to simulate a fused segment. The adjacent segment (L3-L4) was evaluated with flexibility testing sequentially under the following conditions: native disc (control), LLIF cage, cage with lateral plate, pedicle screws with z-rod, and single-rod construct. The difference in mean displacement (millimeters) between groups was studied by the analysis of variance and post-hoc Tukey test. RESULTS: Mean displacement (millimeters) on averaging motion in all planes was 0.741 for native disc, 0.273 for cage, 0.183 for cage with plate, 0.086 for pedicle screws and z-rod, and 0.106 for the single-rod construct. All 4 constructs led to a significant reduction (P < 0.001) in displacement in extension and flexion, as compared with native disc. There was no demonstrable superiority between the 4 constructs as the mean displacements were not significantly different from each other. CONCLUSIONS: LLIF with and without supplemental fixation reduced motion significantly at the adjacent segment as compared with intact disc. There was a trend toward increasing rigidity with supplemental fixation (plate and pedicle screw constructs). Further biomechanical studies with larger sample sizes are needed to confirm these initial findings.

Biomechanical Comparison of Four Methods of Fixation of a Polymeric Cranial Cruciate Ligament in the Canine Femur and Tibia.

OBJECTIVE: The aim of this study was to compare the biomechanical properties of four different methods of artificial cranial cruciate ligament fixation in canine cadaveric tibias and femurs. METHODS: Femurs and tibias from skeletally mature large breed canine cadavers were assigned into four fixation groups: group 1, 4.5-mm interference screw (IS); group 2, 4.5-mm IS and 4.0-mm screw and spiked washer (SW); group 3, 5.0-mm IS; group 4, 5.0-mm IS + SW. RESULTS: The mean ultimate load was significantly greater for femur fixations than for tibias, when a SW was added, and for 5.0-mm IS compared with 4.5-mm sizes. There was also a significant interaction between SW and IS size. A SW significantly increased stiffness, a 5.0-mm IS in femurs provided more stiffness than 4.5-mm IS and was greater than 5.0-mm IS in tibias. In tibias, a 4.5-mm IS was stiffer than a 5.0-mm IS and a 4.5 IS + SW had greater stiffness than a 5.0-mm IS + SW. Groups 1 to 3 and tibias in group 4 failed by artificial ligament pullout. Nine femurs in group 4 failed by fracture, 5 by artificial ligament pullout, and 1 by artificial ligament tearing. CLINICAL SIGNIFICANCE: A 5.0-mm IS + SW provided superior artificial ligament fixation strength in femurs and tibias compared with a 4.5-mm IS without SW. Overall, artificial ligament fixation with 5.0-mm IS in femurs had the mechanical characteristics that most closely matched those reported in normal canine cranial cruciate ligaments.

Kinematic behavior of a novel pedicle screw-rod fixation system for the canine lumbosacral joint.

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.

FiberSecure suture compared to braided polyester suture.

Reliability of wound closure is limited primarily by the capacity of tissues to support conventional sutures (or staples), not by strength of either material per se. We developed FiberSecure™ for closures to surpass tissue strength. We assessed and compared the mechanical and histological performance of FiberSecure™ suture versus commercially available braided polyester suture (Mersilene) in the closure of abdominal muscle incisions in miniature swine at approximately 3 months postsurgery. Four incisions were closed in the external oblique muscle of eight Sinclair minipigs. Two wounds were closed with FiberSecure™ suture size 0 and the remaining two with Mersilene suture size 0. At 90 days, specimens were removed for biomechanics and histology. In destructive tensile testing, in the 16 abdominal muscle specimens for the FiberSecure™ suture, muscle tear was not near the suture implantation region, which remained intact. Wound strength met or exceeded strength of neighboring tissue in FiberSecure™ groups, which had peak force of 55.7 ± 22.1 N (mean ± SD) and peak stress of 579.0 ± 159.2 KPa (mean ± SD). For Mersilene, 3 of the 16 samples tore at the suture site and the remaining samples tore through the abdominal muscle not near the implantation region. The wound strength was similar to surrounding tissue, and these specimens had peak force of 51.8 ± 21.7 N and peak stress of 550.3 ± 239.4 KPa (mean ± SD). No significant difference was observed in peak force or stress between groups (p > 0.05), most repairs having met or exceeded native tissue strength by this time point. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1126-1130, 2017.

In vivo magnetic resonance elastography to estimate left ventricular stiffness in a myocardial infarction induced porcine model.

PURPOSE: To estimate change in left ventricular (LV) end-systolic and end-diastolic myocardial stiffness (MS) in pigs induced with myocardial infarction (MI) with disease progression using cardiac magnetic resonance elastography (MRE) and to compare it against ex vivo mechanical testing, LV circumferential strain, and magnetic resonance imaging (MRI) relaxometry parameters (T1 , T2 , and extracellular volume fraction [ECV]). MATERIALS AND METHODS: MRI (1.5T) was performed on seven pigs, before surgery (Bx), and 10 (D10), and 21 (D21) days after creating MI. Cardiac MRE-derived MS was measured in infarcted region (MIR) and remote region (RR), and validated against mechanical testing-derived MS obtained postsacrifice on D21. Circumferential strain and MRI relaxometry parameters (T2 , T1 , and ECV) were also obtained. Multiparametric analysis was performed to determine correlation between cardiac MRE-derived MS and 1) strain, 2) relaxometry parameters, and 3) mechanical testing. RESULTS: Mean diastolic (D10: 5.09 ± 0.6 kPa; D21: 5.45 ± 0.7 kPa) and systolic (D10: 5.72 ± 0.8 kPa; D21: 6.34 ± 1.0 kPa) MS in MIR were significantly higher (P < 0.01) compared to mean diastolic (D10: 3.97 ± 0.4 kPa; D21: 4.12 ± 0.2 kPa) and systolic (D10: 5.08 ± 0.6 kPa; and D21: 5.16 ± 0.6 kPa) MS in RR. The increase in cardiac MRE-derived MS at D21 (MIR) was consistent and correlated strongly with mechanical testing-derived MS (r(diastolic) = 0.86; r(systolic) = 0.89). Diastolic MS in MIR demonstrated a negative correlation with strain (r = 0.58). Additionally, cardiac MRE-derived MS demonstrated good correlations with post-contrast T1 (r(diastolic) = -0.549; r(systolic) = -0.741) and ECV (r(diastolic) = 0.548; r(systolic) = 0.703), and no correlation with T2 . CONCLUSION: As MI progressed, cardiac MRE-derived MS increased in MIR compared to RR, which significantly correlated with mechanical testing-derived MS, T1 and ECV. LEVEL OF EVIDENCE: 1 J. Magn. Reson. Imaging 2017;45:1024-1033.

Biomechanical Comparison of 2 Veterinary Locking Plates to Monocortical Screw/Polymethylmethacrylate Fixation in Canine Cadaveric Cervical Vertebral Column.

OBJECTIVE: To compare the biomechanical properties of 2 veterinary locking plates and monocortical screws/polymethylmethacrylate (PMMA) fixation in canine cadaveric cervical vertebral columns. STUDY DESIGN: Biomechanical cadaveric study. MATERIALS: Nineteen cervical vertebral columns (C2-C7) from large breed, skeletally mature, canine cadavers were used. A cortical ring was placed as a disk spacer at C4-C5 in all specimens. Seven vertebral columns were plated at C4-C5 with two 4-hole, 3.5 mm string of pearls plates (SOP) and 6 vertebral columns were plated with two 6-hole, 2.4 mm titanium locking reconstruction plates (Ti recon plate). All screws were placed monocortically. Six vertebral columns had monocortical titanium screws and PMMA (Ti screws/PMMA) placed, tested as part of a prior study. METHODS: Stiffness testing in 3 directions was performed of the unaltered C4-C5 vertebral motion unit and repeated after placement of the disk spacer and implants. Data were compared using a linear mixed model that incorporated data from previously tested spines (Ti screw/PMMA). RESULTS: The mean (95% CI) stiffness (N/m) in extension for SOP was 407 N/mm (330-503), for Ti recon plate was 284 N/mm (198-407) and for Ti screws/PMMA was 365 N/mm (314-428); in flexion for SOP was 250 N/mm (178-354), for Ti recon plate was 147 N/mm (106-204) and for Ti screws/PMMA was 311 (235-416); in lateral bending for SOP was 528 N/mm (441-633), for Ti recon plate was 633 N/mm (545-735) and for Ti screws/PMMA was 327 N/mm (257-412). There were no significant differences in stiffness between the 3 fixations for any outcome. CONCLUSION: Monocortical fixation with two 3.5 mm SOP or two 2.4 mm Ti recon plates may be an alternate fixation to monocortical screws and PMMA.

Evaluation of Three Human Cervical Fusion Implants for Use in the Canine Cervical Vertebral Column.

OBJECTIVE: To assess technical feasibility and mechanical properties of 3 locking plate designs (Zero-P, Zero-P VA, and Uniplate 2) for use in the canine cervical spine. STUDY DESIGN: Prospective ex vivo study. ANIMALS: Cadaver cervical spines from skeletally mature large breed dogs (n = 18). METHODS: Specimens were screened using radiography and allocated into balanced groups based on bone density. Stiffness of intact C4-C5 vertebral motion units was measured in extension, flexion, and lateral bending using nondestructive 4-point bend testing. Uniplate 2 was then implanted at C4-C5 and mechanical testing was repeated. Mechanical test data were compared against those from 6 spines implanted with monocortical screws, an allograft ring spacer, and PMMA. RESULTS: The Zero-P and Zero-P VA systems could not be surgically implanted due to anatomical constraints in the vertebral column sizes of the canine cervical spines used in this study. Fixation with Uniplate 2 or with screws/PMMA significantly increased stiffness of the C4-C5 vertebral motion units compared to unaltered specimens (P < .001) in extension. Stiffness of the titanium screw/PMMA fixation was significantly greater than the Uniplate 2 construct in extension. Flexion and lateral bending could not be evaluated in 3 of 6 specimens in the Uniplate 2 group due to failure at the bone/implant interface during extension testing. CONCLUSION: Fixation with Uniplate 2 was biomechanically inferior to screws/PMMA. Particularly concerning was the incidence of vertebral fracture after several testing cycles. Based on our results, Zero-P, Zero-P VA, and Uniplate 2 cannot be recommended for use in dogs requiring cervical fusion.

Biomechanical Evaluation of 6.5-mm Cannulated Screws.

Although biomechanical and clinical evidence exists regarding smaller compression screws, biomechanical data regarding the larger headless screws are not currently available. Headed and headless 6.5-mm cannulated compression screws were examined, with analysis of interfragmentary compression, insertion torque, and resistance of the construct to a shear force. No significant differences were seen between the maximum insertion torque of the headless or headed screws. Maximum and steady-state compression forces were also not significantly different between groups. Countersinking the headless model 2 mm led to a 77.01% decrease in steady-state compression levels. Shear testing did not reveal any significant differences in peak load at ultimate failure, specimen stiffness, or final block displacement, although a trend to increased peak load and stiffness was seen with the headless specimens.

Fixation of Metacarpal Shaft Fractures: Biomechanical Comparison of Intramedullary Nail Crossed K-Wires and Plate-Screw Constructs.

OBJECTIVES: Metacarpal (MC) fractures are very common, accounting for 18% of all fractures distal to the elbow. Many MC fractures can be treated non-operatively; however, some are treated most effectively with surgical stabilization, for which there are multiple methods. It was postulated that plates would have a significantly higher (P < 0.05) load to failure than crossed K(XK)-wires and that intramedullary metacarpal nails (IMNs) and XK-wires would have equivalent load to failure. METHODS: Mid-diaphyseal transverse fractures were created in 36 synthetic metacarpals and stabilized using nails, XK-wires or non-locking plates. Three-point bending was performed with continuous recording of load and displacement. Statistical analysis was performed using single factor ANOVA and Scheffe's test. Statistical significance was defined as P < 0.05. RESULTS: Biomechanical testing revealed significant differences between groups in load-to-failure. Average load to failure was significantly greater in the plate (1669 ± 322 N) than the XK-wire (146 ± 56 N) or IMN (110 ± 43 N) groups. The loads to failure of the K-wires and nails were equivalent. Plates were 11 and 15 times stronger in three-point bending than the K-wires and nails, respectively. There was no statistically significant difference between strengths of the K-wires and nails. CONCLUSIONS: Although plates are the most stable means of fixation of midshaft metacarpal fractures, if minimally-invasive techniques are indicated, intramedullary nails may provide equivalent stability as commonly-used XK-wires. Although some studies have shown favorable clinical outcomes with IMNs, additional clinical correlation of these biomechanical results to fracture healing and outcomes is needed.