Axial-torsional Tests Research Articles

Study on development and biomechanics test of plastic rod-hook plate

To develop a new type of internal fixation device which can be used to treat the minor avulsion fracture of the medial malleolus, lateral malleolus, the base of the fifth metatarsal, and the ulnar styloid process, and investigate the reliability and effectiveness of the device through biomechanical test. Eighty human's bone specimens with complete medial malleolus, lateral malleolus, the base of the fifth metatarsal, and the ulnar styloid process were selected and measured the anatomic indexes (the height, width, and thickness of medial malleolus, lateral malleolus, the base of the fifth metatarsal, and the ulnar styloid process). The CT three-dimensional reconstruction data of 200 healthy adults which including medial malleolus, lateral malleolus, the base of the fifth metatarsal, and the ulnar styloid process was also selected and measured the anatomic indexes by Mimics software. The plastic rod-hook plate was designed according to the measured results and prepared. Forty fresh porcine lower limb specimens were randomly divided into groups A and B (20 in each group), and 8 adult lower limb specimens including 4 left and 4 right were also randomly divided into groups A and B (4 in each group). All specimens were prepared for avulsion fracture of medial malleolus. Then, the fractures were fixed with plastic rod-hook plate in group A and wire anchor in group B. The load and axial torsion test of ankle joint were carried out by universal biomechanical testing machine. According to the anatomical characteristics, a plastic rod-hook plate was designed successfully. The biomechanical test results between animal and human specimens were consistent. There was a linear relationship between load and displacement in the ankle distal load test. The displacement when loaded to the maximum load was significantly lower in group A than in group B ( P<0.05). The torsion angle and torque were significantly higher in group A than in group B when the internal fixation failed in the axial torsion test of the ankle joint ( P<0.05), and the torsion angle was significantly smaller in group A than in group B when the torque was 1 N·m ( P<0.05), and the maximum torque was also significantly higher in group A than in group B ( P<0.05). However, there was no significant difference in torsion angle between the two groups in the maximum torque ( P>0.05). The biomechanical properties of plastic rod-hook plate is obviously better than wire anchor, and the fixation of avulsion fracture with plastic rod-hook plate is easy to operate, which is expected to be used in the clinical treatment of minor avulsion fractures such as medial malleolus, lateral malleolus, base of the fifth metatarsal, and ulna styloid process.

Development of scanning electron microscope-compatible multiaxial miniature testing system

Knowledge of deformation and failure mechanisms at micro- to nano-length scales is important for the prediction of material behavior as well as the development of new materials with desired properties. In situ multiaxial testing with scanning electron microscopes (SEM) can reveal physical deformation mechanisms under realistic multiaxial loading conditions. Although in situ SEM testing has gained traction in recent years, there is currently no multiaxial in situ SEM testing stage available with axial-torsional loading capabilities which can generally be used in any SEM. In this study, we report the development of a multiaxial miniature testing system (MMTS) with a unique capability for performing axial-torsional testing of a tubular specimen with a 1–2 mm outer diameter, inside most SEMs. The different challenges of developing a multiaxial in situ SEM testing stage, such as small load frame size, appropriate specimen position, high vacuum compatibility of MMTS load frame components, as well as the development of installation accessories, were addressed. A custom SEM stage door was developed for the MMTS load frame. Verification tests have confirmed the successful development of the MMTS for in situ SEM testing. In addition, digital image correlation was used with recorded SEM images during the test to determine the surface strain.

Friction between pearlitic steel surfaces

Experiments rubbing two pearlitic rail steel surfaces against each other were done using an axial-torsion test rig. After the experiments were completed, the surfaces were observed under stereomicroscope, and the wear debris were examined. The aim of the experiments was to evaluate the variation of friction characteristics between two surfaces of pearlitic rail steel, mimicking the friction which occurs between the faces of a crack during rolling contact fatigue (RCF) loading. For both dry and wet conditions, the sliding velocity, the angles of rotation, and the average normal force were varied. It was found that the most significant effect of changing angles of rotation is on the formation of ridges on the contact surface with large angles, leading to a higher friction coefficient. The presence of water reduces the friction coefficient and leads to less deformation and wear in the contact surface.

Mechanical performance of lumbar intervertebral body fusion devices: An analysis of data submitted to the Food and Drug Administration

Lumbar intervertebral body fusion devices (L-IBFDs) are intended to provide stability to promote fusion in patients with a variety of lumbar pathologies. Different L-IBFD designs have been developed to accommodate various surgical approaches for lumbar interbody fusion procedures including anterior, lateral, posterior, and transforaminal lumbar interbody fusions (ALIF, LLIF, PLIF, and TLIF, respectively). Due to design differences, there is a potential for mechanical performance differences between ALIF, LLIF, PLIF, and TLIF devices. To evaluate this, mechanical performance and device dimension data were collected from 124 Traditional 510(k) submissions to the FDA for L-IBFDs cleared for marketing from 2007 through 2016. From these submissions, mechanical test results were aggregated for seven commonly performed tests: static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077, and subsidence testing per ASTM F2267. The Kruskal-Wallis test and Wilcoxon signed-rank test were used to determine if device type (ALIF, LLIF, PLIF, TLIF) had a significant effect on mechanical performance parameters (static testing: stiffness and yield strength; dynamic testing: runout load; subsidence testing: stiffness [Kp]). Generally, ALIFs and LLIFs were found to be stiffer, stronger, and had higher subsidence resistance than PLIF and TLIF designs. These results are likely due to the larger footprints of the ALIF and LLIF devices. The relative mechanical performance and subsidence resistance can be considered when determining the appropriate surgical approach and implant for a given patient. Overall, the mechanical performance data presented here can be utilized for future L-IBFD development and design verification.

A new assembly of locking compression plate for treatment of femoral shaft comminuted fracture: a biomechanical study

Objective To compare the biomechanical characteristics of new assembly of locking compression plate (NALCP) and locking compression plate (LCP) in internal fixation of femoral shaft comminuted fractures. Methods The preparation of a femoral shaft wedge fracture model (AO type 32-C2.1), six pairs of (12) femoral specimens were collected and divided into two groups randomly, with six in each group. The Group A was made up of the new assembly of locking compression plate fixation model (NALCP), and Group B the locking compression plate fixation model(LCP). The biomechanical properties of steel plates in two groups were tested by axial loading and torsional loading tests. The relative maximum displacement of fracture blocks in two groups on the X, Y and Z axes (the coronal axis was set as X axis, through the medial and lateral femur; the transverse axis was set as Z axis, through the femoral intercondylar fossa, perpendicular to the X axis; the sagittal axis was set as Y axis, perpendicular to the X and Z axis), the maximum strain, and the average strain of the steel plate were recorded. Strain distribution nephogram was produced, and the axial loading fatigue test results of Group A were recorded. Results Axial loading test: the relative maximum displacement of fracture in Group A on X, Y and Z axis were smaller than those in Group B (P 0.05). Torsional loading test: The relative maximum displacement of fracture in Group A on X and Z axis was smaller than that of Group B (P 0.05); the main strain of plate in Group A was greater than that in Group B (P 0.05). There was no obvious difference in strain distribution between the two groups. In Group A, the fatigue test of axial cyclic loading was performed for 1 million times, and the NALCP was intact without deformation, loosening, or rupture. Conclusion NALCP can provide strong mechanical stability for comminuted femoral fracture. The design of bridge steel plate is reasonable, which can effectively avoid stress concentration, reduce the stress shielding of steel plate, and facilitate bone healing. Key words: Femoral fractures; Fracture fixation, internal; Biomechanics

Short cracks growth in low cycle fatigue under multiaxial in-phase loading

Crack propagation in full plastic regions is one of the main aspects of fatigue life design for components subjected to high strain concentrations. Residual life assessment for those components, in which high stress concentrations cause cyclic yielding of the material, can be considered as a crack propagation problem by assuming crack growth from the first load cycle. The aim of this paper is to study the crack growth behaviour of short cracks in low cycle fatigue under a multiaxial loading condition. In particular, a series of experiments in LCF regime at room temperature was performed to determine crack growth during axial, torsional and axial-torsional tests. Crack advancement was checked with the plastic replica technique, during test interruptions. Experimental results were compared, in terms of crack growth rates and fatigue life assessment, with those analytically calculated, considering different multiaxial fatigue parameters introduced in an exponential crack growth law and an approach based on the multiaxial cyclic J-Integral concept.

Modeling of kinematic hardening at large biaxial deformations in pearlitic rail steel

Using an Axial-Torsion testing machine, pearlitic R260 steel specimens are twisted until fracture under different axial loads. A well established framework for finite elastoplasticity with kinematic hardening is used to model the deformation of the specimens. In particular, we evaluate the ability of different kinematic hardening laws to predict the observed biaxial load versus displacement response. It is found that the combination of Armstrong–Frederick dynamic recovery and Burlet–Cailletaud radial evanescence saturation is efficient even for the large strains achieved in this study. The results are less conclusive on the appropriateness of replacing the Armstrong–Frederick with an Ohno–Wang type of kinematic hardening law.

Dynamic characteristics of expanded polystyrene composite soil under traffic loadings considering initial consolidation state

As a type of artificial filling material, the shear modulus and damping ratio of expanded polystyrene (EPS) composite soil are two key parameters used to analyze the dynamic stability of an embankment. Nineteen combined axial–torsional tests are conducted on hollow cylinder specimens of EPS composite soil to study its dynamic characteristics under the complex stress path induced by the simulated traffic loadings. The characteristics of skeleton curve, dynamic shear modulus and damping ratio for EPS composite soil are analyzed. It is found that EPS composite soil is characterized by typical dynamic nonlinearity, which is influenced by the mixing ratio and the initial stress state. The increasing cement content can effectively improve the dynamic strength of EPS composite soil. EPS bead content has a slight influence on the initial shear modulus of EPS composite soil as well as the cyclic stress-strain curve in the linear elastic stage. However, the increasing EPS bead content obviously reduces the dynamic strength. The initial shear modulus increases with increasing initial minor principal stress for the isotropic and anisotropic consolidated specimens. The characteristics of modulus attenuation are significantly influenced by the initial minor principal stress, EPS bead content and initial rotation angle of the major principal stress axis. The “structural damping” effect induced by the weak interface formed between EPS beads and cemented soil is an important component of the damping mechanism for the EPS composite soil. Based on the experimental results, this paper provides the empirical models to describe the skeleton curve, modulus attenuation and damping growth characteristics for EPS composite soil.

Biomechanical assessment of newly-designed proximal femoral medial buttress plate for treatment of reverse oblique femoral intertrochanteric fracture

To evaluate the biomechanical properties of proximal femoral medial buttress plate (PFMBP) for fixing the reverse oblique intertrochanteric fractures by comparing with proximal femoral locking compression plate (PFLCP) and proximal femoral nail antirotation (PFNA). Eighteen synthetic femoral bone models (Synbone) were divided into 3 groups (group PFLCP, group PFNA, and group PFMBP), 6 models in each group; an AO 31-A3.1 reverse oblique femoral intertrochanteric fracture was made based on the same criterion. After being fixed and embeded, the axial load testing, torsion testing, and axial load-to-failure testing were performed on each model. The axial displacement of different loads, torque of different torsion angles, and failure load of each model were recorded, and the stiffness of axial load and torsion were calculated. The axial stiffness in groups PFLCP, PFNA, and PFMBP were (109.42±30.14), (119.13±29.14), and (162.05±22.05) N/mm respectively, showing significant differences between groups ( P<0.05). There were significant differences in torque between different torsion angles in the same group, as well as in the torque between groups at the same torsional angle ( P<0.05). The torsion stiffness in groups PFLCP, PFNA, and PFMBP were (1.45±0.44), (1.10±0.13), and (1.36±0.32) N·mm/deg respectively; there were significant differences when compared groups PFLCP and PFMBP with group PFNA ( P<0.05), but no significant difference was found between group PFLCP and group PFMBP ( P>0.05). The failure loads of groups PFLCP, PFNA, and PFMBP were (1 408.88± 0.17), (1 696.56±0.52), and (2 154.65±0.10) N respectively, showing significant differences between groups ( P<0.05). The newly-designed PFMBP is better than PFNA and PFLCP in axial load stiffness and torsion stiffness for fixing reverse oblique intertrochanteric fracture by biomechanical test, indicating that reconstruction of medial stability is a key element for unstable intertrochanteric fracture.

Mechanical performance of cervical intervertebral body fusion devices: A systematic analysis of data submitted to the Food and Drug Administration

Cervical intervertebral body fusion devices (IBFDs) are utilized to provide stability while fusion occurs in patients with cervical pathology. For a manufacturer to market a new cervical IBFD in the United States, substantial equivalence to a cervical IBFD previously cleared by FDA must be established through the 510(k) regulatory pathway. Mechanical performance data are typically provided as part of the 510(k) process for IBFDs. We reviewed all Traditional 510(k) submissions for cervical IBFDs deemed substantially equivalent and cleared for marketing from 2007 through 2014. To reduce sources of variability in test methods and results, analysis was restricted to cervical IBFD designs without integrated fixation, coatings, or expandable features. Mechanical testing reports were analyzed and results were aggregated for seven commonly performed tests (static and dynamic axial compression, compression-shear, and torsion testing per ASTM F2077, and subsidence testing per ASTM F2267), and percentile distributions of performance measurements were calculated. Eighty-three (83) submissions met the criteria for inclusion in this analysis. The median device yield strength was 10,117N for static axial compression, 3680N for static compression-shear, and 8.6Nm for static torsion. Median runout load was 2600N for dynamic axial compression, 1400N for dynamic compression-shear, and ±1.5Nm for dynamic torsion. In subsidence testing, median block stiffness (Kp) was 424N/mm. The mechanical performance data presented here will aid in the development of future cervical IBFDs by providing a means for comparison for design verification purposes.

Metaphyseal locking plate as an external fixator for open tibial fracture: Clinical outcomes and biomechanical assessment

ObjectiveThis study aimed to evaluate the outcome of using a metaphyseal locking plate as a definitive external fixator for treating open tibial fractures based on biomechanical experiments and analysis of clinical results. MethodsA metaphyseal locking plate was used as an external fixator in 54 open tibial fractures in 52 patients. The mean follow-up was 38 months (range, 20–52 months). Moreover, static axial compression and torsional tests were performed to evaluate the strength of the fixation techniques. ResultsThe average fracture healing time was 34.5 weeks (range, 12–78 weeks). At 4 weeks postoperatively and at the final follow-up, the average Hospital for Special Surgery knee score was 85 (range, 81–100) and 94 (range, 88–100), respectively, and the American Orthopaedic Foot and Ankle Society score was 88 (range, 80–100) and 96 (range, 90–100), respectively. Based on the static test result, the axial stiffness was significantly different among groups (p=0.002), whereas the torsional stiffness showed no significant difference (p=0.068). ConclusionsClinical outcomes show that the use of locking plate as a definitive external fixator is an alternative choice for tibial fractures after obtaining appropriate fracture reduction. However, external locked plating constructs were not as strong as standard locked plating constructs. Therefore, the use of external locked plating constructs as a definitive treatment warrants further biomechanical study for construct strength improvement.

Investigation of Kryptonite™ bone cement in hybrid screw configurations of locking plate humeral midshaft fixation: A study of surrogate bone model

PurposeLocking compression plates (LCP) allow trauma and orthopedic surgeons to have a variety of options for utilizing locking and non-locking screw features. In this study, the hybrid constructs of mixed unicortical and bicortical screws were investigated for humerus midshaft fractures. The locking and non-locking features were also incorporated into the hybrid LCP constructs. Kryptonite™ bone cement is biocompatible with low exothermic properties and strong metal adhesion. This novel bone cement was incorporated into the non-locking screw feature to compare its mechanical effect with that of a traditional locking screw feature. MethodsA total of 24 synthetic bones (Sawbones® Inc., USA) were equally divided into three groups (n=8). The control group obtained traditional LCP fixation (JSM Medimax Inc., India) with bicortical screws, while the hybrid locking constructs employed a mix of bicortical and unicortical screws. The two bicortical screws in hybrid constructs were placed at the end holes of the LCP constructs and the unicortical screws were placed into the remaining holes. The hybrid locking (HL) group contained locking unicortical screws, whereas the hybrid non-locking group (HNK) utilized non-locking screw features incorporating the Kryptonite™ bone cement in the plate/screw bone construct. Specimens were tested by dynamic and static analysis. The eight total constructs were equally divided into two subgroups (n=4) to conduct axial compression and torsion test individually. The low cyclic test was conducted for dynamic analysis, which included 10,000 cycles at 1Hz frequency with a cyclic loading of 0–500N of axial force for the axial compression test and 0–20° of angular displacement for the torsion test. The static analysis was run by a failure test with a nondestructive strain rate of 0.1mm/s for the axial compression test and 0.5°s−1 for the torsional test. The construct stiffness of axial compression and torsion were derived from the linear portion of the load-displacement curves. The yield strength of axial compression and torsion was determined by using offset methods. The results of stiffness and yield strength were compared by using both one-way ANOVA and Scheffe and Games-Howell post hoc tests to analyze statically significant differences among the three groups. ResultsSpecimens completed 10,000 cycles in the dynamic analysis of axial and torsional cyclic tests without major deformation. To compare with the control group in static analysis, the HL and HNK groups achieved positive effect in axial stiffness, 12.3% and 10.5% greater than the control group respectively. HL obtained axial yield strength about 12% less than the control group. The HNK group was equivalent with the control group in axial yield strength. The torsional stiffness and yield strength were found similar in three groups, which indicated torsional equivalence among them. The stiffness and yield strength from axial compression and torsion were found statically non-significant among three groups. ConclusionsHybrid LCP constructs were initially showed to maintain equivalent axial and torsional stability with the traditional method. Kryptonite™ hosted mechanical and biological advantages for internal plate/screws (PLT) fixation.

Biomechanical analysis of the rotation-self oriented distraction trauma fixation system

Objective To analyze the mechanical features of rotation-self oriented distraction trauma fixation system (RTS) in fixation of spinal fractures of anterior and median columns. Methods Five fresh calf specimens of thoracolumbar vertebrae (T11 to L6) were made models of spinal fractures of anterior and median columns according to the Gurr's methods. The biomechanical stabilities of the models were measured by axial compression, stretching and torsion tests in the models before (control group) and after (experimental group) fixation with RTS. Next, RTS was fixed onto a titanium rod by a locking force of 10.0 to 11.5 N • m to test its gripping and anti-torsion strengths. The mechanical properties of RTS were compared with similar overseas products. Results The axial compression test showed no significant difference in displacement under 100 to 500 N load between the control and experimental groups (P > 0.05) . The anteflexion test showed no significant difference in displacement under 20 to 100 N load between the control and experimental groups (P > 0.05). The extension test showed significantly greater displacement under 20 to 100 N load in the experimental group than in the control group (P 0.05). RTS was basically similar to the overseas products in the grasping force, but superior in the anti-torsion force. Conclusion Since the fine biomechanical performance of RTS can lead to rigid three-dimensional orthopedic fixation, this self-designed internal fixation system may have a prospect of wide clinical application for spinal fractures of anterior and median columns. Key words: Spinal fractures; Fracture fixation, internal; Bone nails; Biomechanics

Biomechanical study of improved memory alloy embracing fixators in treatment of periprosthetic femoral fracture due to hip arthroplasty

Objective To study the mechanics of improved memory alloy fixators for salvage of periprosthetic femoral fracture (PFF) after hip arthroplasty in the elderly. Methods Thirty countrymen fresh cadaveric femurs with no pathological defect, fracture, deformity or tumor were randomly divided into experimental group and control group with 15 femurs in each according to the random number table. A model of Vancouver type B1 periprosthetic femoral fracture following hip arthroplasty was induced. The fracture was treated with modified memory alloy embracing fixators in experimental group; instead general memory alloy embracing fixators in control group. All specimens were tested biomechanically. Results Under the same mechanical loading, the two groups showed respective 30% and 48% maximum differences in stress value and displacement. Results in three-point bending test did not differ significantly between the two groups (P>0.05), but there were significant differences in axial compression and torsion test (P<0.05 or 0.01). Conclusion The improved memory alloy embracing fixators present better resistance to compression and torsion compared to the general fixators. Key words: Internal fixators; Arthroplasty, replacement, hip; Periprosthetic fractures; Memory, alloy

A new alternative to expandable pedicle screws: Expandable poly-ether-ether-ketone shell

Screw pullout is a very common problem in the fixation of sacrum with pedicle screws. The principal cause of this problem is that the cyclic micro motions in the fixation of sacrum are higher than the other regions of the vertebrae that limit the osteo-integration between bone and screw. In addition to that, the bone quality is very poor at sacrum region. This study investigated a possible solution to the pullout problem without the expandable screws' handicaps. Newly designed poly-ether-ether-ketone expandable shell and classical pedicle screws were biomechanically compared. Torsion test, pullout tests, fatigue tests, flexion/extension moment test, axial gripping capacity tests and torsional gripping capacity tests were conducted in accordance with ASTM F543, F1798 and F1717. Standard polyurethane foam and calf vertebrae were used as embedding medium for pullout tests. Classical pedicle screw pullout load on polyurethane foam was 564.8 N compared to the failure load for calf vertebrae's 1264 N. Under the same test conditions, expandable poly-ether-ether-ketone shell system's pullout loads from polyurethane foam and calf vertebrae were 1196.3 and 1890 N, respectively. The pullout values for expandable poly-ether-ether-ketone shell were 33% and 53% higher than classical pedicle screw on polyurethane foam and calf vertebrae, respectively. The expandable poly-ether-ether-ketone shell exhibited endurance on its 90% of yield load. Contrary to poly-ether-ether-ketone shell, classical pedicle screw exhibited endurance on 70% of its yield load. Expandable poly-ether-ether-ketone shell exhibited much higher pullout performance than classical pedicle screw. Fatigue performance of expandable poly-ether-ether-ketone shell is also higher than classical pedicle screw due to damping the micro motion capacity of the poly-ether-ether-ketone. Expandable poly-ether-ether-ketone shell is a safe alternative to all other expandable pedicle screw systems on mechanical perspective.

Preliminary biomechanical study of different acetabular reinforcement devices for acetabular reconstruction.

BackgroundAcetabular reinforcement devices (ARDs) are frequently used as load-sharing devices to allow allograft incorporation in revision hip arthroplasty with massive acetabular bone loss. The key to a successful reconstruction is robust fixation of the device to the host acetabulum. Interlocking fixation is expected to improve the initial stability of the postoperative construct. However, all commercially available ARDs are designed with non-locking fixation. This study investigates the efficacy of standard ARDs modified with locking screw mechanisms for improving stability in acetabular reconstruction.MethodsThree types of ARDs were examined to evaluate the postoperative compression and angular stability: i) standard commercial ARDs, ii) standard ARDs modified with monoaxial and iii) standard ARDs modified with polyaxial locking screw mechanisms. All ARDs were implanted into osteomized synthetic pelvis with pelvic discontinuity. Axial compression and torsion tests were then performed using a servohydraulic material testing machine that measured load (angle) versus displacement (torque). Initial stability was compared among the groups.ResultsEquipping ARDs with interlocking mechanisms effectively improved the initial stability at the device/bone interface compared to standard non-locked ARDs. In both compression and torsion experiments, the monoaxial interlocking construct demonstrated the highest construct stiffness (672.6 ± 84.1 N/mm in compression and 13.3 ± 1.0 N·m/degree in torsion), whereas the non-locked construct had the lowest construct stiffness (381.4 ± 117.2 N/mm in compression and 6.9 ± 2.1 N·m/degree in torsion) (P < 0.05).ConclusionsOur study demonstrates the potential benefit of adding a locking mechanism to an ARD. Polyaxial ARDs provide the surgeon with more flexibility in placing the screws at the cost of reduced mechanical performance. This in vitro study provides a preliminary evaluation of biomechanical performance for ARDs with or without interlocking mechanisms, actual clinical trial deserves to be further investigated in future studies.

Comparison of three fixations for tibial plateau fractures by biomechanical study and radiographic observation

Aim: The aim of this study was to compare the fixation effects of three fixation devices for tibial plateau fracture (AO/OTA classification 41 A1). Methods: Sixteen human cadaver tibial specimens were randomly divided into four groups. An A1 fracture model was established. The fractures were subsequently fixed by axial controlled intramedullary nail, external fixation and steel plate fixation. Each specimen was subjected to axial compression, torsion test and three-point bending test. Then a rat model was used to evaluate the therapeutic effect of these three fixations by evaluation of callus formation time and healing time. Results: It was found that the axial controlled intramedullary nail group obtained superior biomechanical properties of resistance ability of bending, torsional and axial compressive, compared with external fixation and steel plate group. In animal experiments, the axial controlled intramedullary nail group had a significant shorter callus occurrence and healing time than steel plate and external fixator group. Conclusion: The axial controlled intramedullary nail fixation has a superior biomechanical characteristic and fixation effect for tibial plateau fractures than steel plate and external fixator.

Interpedicular kinematics in an in vitro biomechanical assessment of a bilateral lumbar spondylolytic defect

BackgroundA spondylolytic defect in lumbar vertebra is a common condition during early childhood and adolescence, and is considered a precursor to spondylolisthesis. This study examined whether a bilateral spondylolytic defect in lumbar spine intrinsically results in increased intervertebral translations during different bending motions. MethodsSeven fresh frozen cadaveric kangaroo lumbar (L1–L6) spine specimens were tested in a kinematic spine simulator; first in their intact state, followed by creating a bilateral spondylolytic defect at L4 and retesting. In addition to recording global and segmental range of motions, the pedicles at L3, L4, and L5 vertebrae were digitized bilaterally and virtually tracked throughout testing. Interpedicular kinematic metrics were employed to capture any changes in translatory motions during flexion–extension, bilateral bending, and axial torsion testing modes. FindingsFollowing the defect, range of motion at the defect level (L4–L5) increased significantly in all the three motion planes. At L4–L5, normalized interpedicular displacement increased significantly in flexion–extension (median change +156%) and bilateral bending (median change +58%) motions, but changes in bending-plane and out-of-plane intervertebral translations were not significant in any of the testing modes. InterpretationIn the absence of any significant changes in bending-plane and out-of-plane intervertebral translations at L4–L5, changes in interpedicular displacement would directly correspond with the stretching of posterior annulus of the L4–L5 intervertebral disc. A bilateral spondylolytic defect at L4 may result in significant overstretching of the posterior annulus of the L4–L5 disc during flexion–extension and bilateral bending motions.

Varization open-wedge osteotomy of the distal femur: comparison between locking plate and angle blade plate constructs.

Biomechanical properties of locking plate and angle blade plate constructs in opening-wedge distal femur osteotomy were compared, with special attention to the impact of opposite cortex breakage and buttressing. Forty-eight synthetic femora underwent a lateral opening-wedge distal femur osteotomy, which was fixed either with a locking plate or with an angle blade plate. Two main groups were constituted based on the integrity of the opposite medial cortex-intact medial cortex and fractured medial cortex (FMC). Axial compression and torsion tests were performed comparing stiffness of all bone-implant constructs. In the group FMC, a special subgroup has been tested taking into consideration the effect of placing an additional 6.5-mm cancellous screw buttressing the medial cortex. Statistical level of significance was fixed in 0.05 using a confidence interval of 95%. Intact medial cortex significantly increased the stiffness for both implants tested in this study. In the FMC group, angle blade plate constructs reached higher stiffness mean values for both torsion and axial compression. The insertion of a buttressing screw on the medial cortex increased the stiffness of all tested bone-plate constructs. This additional screw favoured mostly locking plate constructs implementing their stiffness to torsion to mean values comparable to those reached by angle blade plate constructs. Angle blade plate constructs reached higher stiffness mean values than locking plates for both axial compression and torsion loads. The addition of a buttressing screw on the opposite cortex increased the stability of all bone-implant constructs to levels similar to those showed with an intact medial cortex. The present study findings support the clinical use of angle blade plates, alone or associated with a medial buttressing screw, as an alternative fixation method for supracondylar open-wedge osteotomies of the femur.

Biomechanical evaluation of fracture fixation constructs using a variable-angle locked periprosthetic femur plate system

BackgroundIn the United States there are more than 230,000 total hip replacements annually, and periprosthetic femoral fractures occur in 0.1–4.5% of those patients. The majority of these fractures occur at the tip of the stem (Vancouver type B1). The purpose of this study was to compare the biomechanically stability and strength of three fixation constructs and identify the most desirable construct. MethodsFifteen medium adult synthetic femurs were implanted with a hip prosthesis and were osteotomized in an oblique plane at the level of the implant tip to simulate a Vancouver type B1 periprosthetic fracture. Fractures were fixed with a non-contact bridging periprosthetic proximal femur plate (Zimmer Inc., Warsaw, IN). Three proximal fixation methods were used: Group 1, bicortical screws; Group 2, unicortical screws and one cerclage cable; and Group 3, three cerclage cables. Distally, all groups had bicortical screws. Biomechanical testing was performed using an axial–torsional testing machine in three different loading modalities (axial compression, lateral bending, and torsional/sagittal bending), next in axial cyclic loading to 10,000 cycles, again in the three loading modalities, and finally to failure in torsional/sagittal bending. ResultsGroup 1 had significantly greater load to failure and was significantly stiffer in torsional/sagittal bending than Groups 2 and 3. After cyclic loading, Group 2 had significantly greater axial stiffness than Groups 1 and 3. There was no difference between the three groups in lateral bending stiffness. The average energy absorbed during cyclic loading was significantly lower in Group 2 than in Groups 1 and 3. ConclusionsBicortical screw placement achieved the highest load to failure and the highest torsional/sagittal bending stiffness. Additional unicortical screws improved axial stiffness when using cable fixation. Lateral bending was not influenced by differences in proximal fixation. Clinical relevanceTo treat periprosthetic fractures, bicortical screw placement should be attempted to maximize load to failure and torsional/sagittal bending stiffness.