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BACKGROUND: Lag screw cut-out failure following fixation of unstable intertrochanteric fractures in osteoporotic bone remains an unsolved challenge. This study tested if resistance to cut-out failure can be improved by using a dual lag screw implant in place of a single lag screw implant. Migration behavior and cut-out resistance of a single and a dual lag screw implant were comparatively evaluated in surrogate specimens using an established laboratory model of hip screw cut-out failure. METHODS: Five dual lag screw implants (Endovis, Citieffe) and five single lag screw implants (DHS, Synthes) were tested in the Hip Implant Performance Simulator (HIPS) of the Legacy Biomechanics Laboratory. This model simulated osteoporotic bone, an unstable fracture, and biaxial rocking motion representative of hip loading during normal gait. All constructs were loaded up to 20,000 cycles of 1.45 kN peak magnitude under biaxial rocking motion. The migration kinematics was continuously monitored with 6-degrees of freedom motion tracking system and the number of cycles to implant cut-out was recorded. RESULTS: The dual lag screw implant exhibited significantly less migration and sustained more loading cycles in comparison to the DHS single lag screw. All DHS constructs failed before 20,000 cycles, on average at 6,638 +/- 2,837 cycles either by cut-out or permanent screw bending. At failure, DHS constructs exhibited 10.8 +/- 2.3 degrees varus collapse and 15.5 +/- 9.5 degrees rotation around the lag screw axis. Four out of five dual screws constructs sustained 20,000 loading cycles. One dual screw specimens sustained cut-out by medial migration of the distal screw after 10,054 cycles. At test end, varus collapse and neck rotation in dual screws implants advanced to 3.7 +/- 1.7 degrees and 1.6 +/- 1.0 degrees , respectively. CONCLUSION: The single and double lag screw implants demonstrated a significantly different migration resistance in surrogate specimens under gait loading simulation with the HIPS model. In this model, the double screw construct provided significantly greater resistance against varus collapse and neck rotation in comparison to a standard DHS lag screw implant.

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In vitro comparative testing of fracture fixation implants is limited by the highly variable material properties of cadaveric bone. Bone surrogate specimens are often employed to avoid this confounding variable. Although validated surrogate models of normal bone (NB) exist, no validated bone model simulating weak, osteoporotic bone (OPB) is available. This study presents an osteoporotic long-bone model designed to match the lower cumulative range of mechanical properties found in large series of cadaveric femora reported in the literature. Five key structural properties were identified from the literature: torsional rigidity and strength, bending rigidity and strength, and screw pull-out strength. An OPB surrogate was designed to meet the low range for each of these parameters, and was mechanically tested. For comparison, the same parameters were determined for surrogates of NB. The OPB surrogate had a torsional rigidity and torsional strength within the lower 2% and 16%, respectively, of the literature based cumulative range reported for cadaveric femurs. Its bending rigidity and bending strength was within the lower 11% and 8% of the literature-based range, respectively. Its pull-out strength was within the lower 2% to 16% of the literature based range. With all five structural properties being within the lower 16% of the cumulative range reported for native femurs, the OPB surrogate reflected the diminished structural properties seen in osteoporotic femora. In comparison, surrogates of NB demonstrated structural properties within 23-118% of the literature-based range. These results support the need and utility of the OPB surrogate for comparative testing of implants for fixation of femoral shaft fractures in OPB.

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In vitro models of traumatic brain injury (TBI) are indispensable to explore the effects of mechanotrauma on neurological injury cascades and injury thresholds. This study characterizes a novel in vitro model of neural shear injury, which for the first time subjects organotypic cultures to inertia-driven shear strain. In this model, organotypic cultures preserved a high level of biological heterogeneity and spatial cytoarchitecture, while inertia-driven shear strain represented a tissue-level insult typical for closed head TBI in vivo. For neural injury simulation, organotypic hippocampal cultures derived from rats were inserted in an inertial loading module, which was subjected to impacts at five graded impact velocities ranging from 2 to 10 m/sec. The mechanical insult was quantified by measuring the transient shear deformation of the culture surface during impact with a high-speed camera. The resultant cell death was quantified with propidium iodide (PI) staining 24 hours following shear injury. Increasing impact velocities of 2, 4.6, 6.6, 8.1, and 10.4 m/sec caused graded peak shear elongation of 2.0 +/- 0.9%, 5.4 +/- 3.8%, 15.1 +/- 14.6%, 25.4 +/- 14.7%, and 56.3 +/- 51.3%, respectively. Cell death in response to impact velocities of 6.6 m/sec or less was not significantly higher than baseline cell death in sham cultures (4.4 +/- 1.5%). Higher impact velocities of 8.1 and 10.4 m/sec resulted in a significant increase in cell death to 19.9 +/- 12.9% and 36.7 +/- 14.2%, respectively (p < 0.001). The neural shear injury model delivered a gradable, defined mechanotrauma and thereby provides a novel tool for investigation of biological injury cascades in organotypic cultures.

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BACKGROUND: Malrotation of the tibial and femoral components has been recognized to be a clinical complication affecting the performance and durability of total knee arthroplasty. This study used a novel strain acquisition technique to determine the effect of tibio-femoral component malrotation on tibial torque and strain distribution of the proximal tibial cortex with a cemented fixed-bearing posterior-stabilized knee. METHODS: Using electronic speckle pattern interferometry, strain on the proximal tibia of human cadaveric knees was obtained in response to 1500N axial loading for neutrally aligned tibial and femoral components, and for 10 degrees internal and external malrotation between the tibial and femoral components. Local strain gage measurements were combined with full-field optical strain measurements to quantify effects on tibial cortex strain and strain distributions caused by the 10 degrees malrotations. In addition, tibial torque was measured for incremental degrees of tibio-femoral malrotation. FINDINGS: Tibio-femoral malrotations as small as 2 degrees caused tibial torque in excess of 4 Nm. At 10 degrees malrotation, tibial torque significantly increased to over 8 Nm (P<0.001) as compared to neutrally aligned components. Local strain gage results significantly increased from 500 muepsilon to 632 muepsilon compressive strain in response to 10 degrees external malrotation, and to 1000 muepsilon compressive strain in response to 10 degrees internal malrotation. Full-field optical strain reports yielded the highest strain of 2153 muepsilon for 10 degrees internal malrotation 30 mm below the joint line. INTERPRETATION: Laser-based strain measurement technology provides novel capabilities to capture cortex strain fields. The sensitivity of cortex strain and torsion to small amounts of tibio-femoral malrotation may explain factors contributing to aseptic implant loosening of the tibial component.

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Mobile and fixed-bearing knee prostheses are likely to generate distinct strain gradients in the proximal tibia. The resulting strain distribution in the proximal tibia governs bone remodeling and affects implant integration and stability. We determined the effects of fixed and mobile-bearing total knee prostheses on strain distribution at the proximal tibia. This mobile-bearing prosthesis was evaluated in cadaveric specimens under axial and torsional loading. Strain on the proximal tibial cortex was measured with rosette strain gages and an optical full-field strain acquisition system. Tibial torsion in response to combined axial and torsional loading was documented. There was no difference in cortex strain between the fixed and the mobile-bearing prostheses under 1.5 kN axial loading. Superimposing 10 degrees tibial internal rotation induced 22% less compressive strain in the mobile-bearing prosthesis compared with the fixed-bearing prosthesis. Under 10 degrees tibial external rotation, the mobile-bearing prosthesis induced 33% less compressive strain than the fixed-bearing prosthesis. Optically acquired strain fields showed peak compressive strain at the anteromedial aspect 30 mm below the joint line. The mobile-bearing prosthesis reduced torque in the proximal tibia during knee rotation by 68-73% compared with the fixed-bearing prosthesis. Our data suggest that the particular mobile-bearing prosthesis tested potentially reduces elevated strain levels in the proximal tibia.

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BACKGROUND: Articulated external fixation has been proposed as a method to protect ligament reconstructions while allowing aggressive and early postoperative rehabilitation after knee dislocation. However, the ability of these fixators to protect and stabilize the knee joint has not been clearly determined. HYPOTHESIS: Articulated external fixation can reduce anteroposterior translation in the cruciate-deficient knee and reduce cruciate ligament strain in cases of intact or reconstructed ligaments. STUDY DESIGN: Controlled laboratory study. METHODS: Knee stability was assessed by 3 standard clinical stability tests (Lachman, anterior drawer, and posterior drawer) on 7 human cadaveric lower extremities. Instrumented forces of 100 N were applied to the tibia to measure cruciate ligament forces and tibiofemoral displacement in intact and cruciate-deficient specimens with and without articulated external fixation to determine the degree to which a fixator can protect cruciate ligaments and stabilize the knee. Articulated external fixation was applied using monolateral and bilateral fixators to comparatively analyze the effectiveness of each construct. Statistical analysis was performed using 2-tailed, paired Student t tests. RESULTS: Application of the monolateral articulated external fixator to specimens with intact ligaments significantly reduced cruciate ligament forces by 1.0 N (P = .011), 1.7 N (P = .046), and 1.4 N (P = .009) for Lachman, anterior drawer, and posterior drawer tests, respectively. In the cruciate ligament-deficient knees, the application of a monolateral fixator significantly reduced tibiofemoral translation by 49%, 70%, and 46% for Lachman, anterior drawer, and posterior drawer tests, respectively. No significant differences between the monolateral and bilateral fixator frames, in terms of ligament protection and joint stabilization, were observed. CONCLUSION AND CLINICAL RELEVANCE: Articulated external fixation of the knee can reduce stress in the cruciate ligaments after multiligament reconstructions and can decrease anteroposterior translation in the cruciate-deficient knee.

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This biomechanical study reports strain gradients in patellofemoral joint cross-sections of seven porcine specimens in response to 1% unconfined axial compression subsequent to specific amounts of off-set strain. Strain distributions were quantified with a customized laser-based electronic speckle pattern interferometry (ESPI) system in a non-contact manner, delivering high-resolution, high-sensitivity strain maps over entire patellofemoral cartilage cross-sections. Strain reports were evaluated to determine differences in strain magnitudes between the superficial, middle, and deep cartilage layers in femoral and patellar cartilage. In addition, the effect of 5%, 10%, 15%, and 20% off-set strain on depth-dependent strain gradients was quantified. Regardless of the amount of off-set strain, the superficial layer of femoral cartilage absorbed the most strain, and the deep layer absorbed the least strain. These depth-dependent strain gradients were most pronounced for 5% off-set strain, at which the superficial layer absorbed on average 5.7 and 23.7 times more strain as compared to the middle and deep layers, respectively. For increased off-set strain levels, strain gradients became less pronounced. At 20% off-set strain, differences in layer-specific strain were not statistically significant, with the superficial layer showing a 1.4 fold higher strain as the deep layer. Patellar cartilage exhibited similar strain gradients and effects of off-set strain, although the patellar strain was on average 19% larger as compared to corresponding femoral strain reports. This study quantified for the first time continuous strain gradients over patellofemoral cartilage cross-sections. Next to provision of a detailed functional characterization of normal diarthrodial joints, this novel experimental approach holds considerable attraction to investigate joint degenerative processes.

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OBJECTIVES: To establish a laboratory model of implant cutout, which can evaluate the effect of implant design on cutout resistance in a clinically realistic "worst case" scenario. SETTING: Orthopaedic biomechanics laboratory. DESIGN: Implant cutout was simulated in an unstable pertrochanteric fracture model, which accounted for dynamic loading, osteoporotic bone, and a defined implant offset. For model characterization, lag screw cutout was simulated in human cadaveric specimens and in polyurethane foam surrogates. Subsequently, foam surrogates were used to determine differences in cutout resistance between 2 common lag screws (dynamic hip screw, Gamma) and 2 novel blade-type implant designs (dynamic helical hip system, trochanteric fixation nail). MAIN OUTCOME MEASURES: Implant migration was continuously recorded with a spatial motion tracking system as a function of the applied loading cycles. In addition, the total number of loading cycles to cutout failure was determined for specific load amplitudes. RESULTS: Implant migration in polyurethane surrogates closely correlated with that in cadaveric specimens, but yielded higher reproducibility and consistent cutout failure. The cutout model was able to delineate significant differences in cutout resistance between specific implant designs. At any of 4 load amplitudes (0.8 kN, 1.0 kN, 1.2 kN, 1.4 kN) dynamic hip screw lag screws failed earliest. The gamma nail lag screw could sustain significantly more loading cycles than the dynamic hip screw. Of all implants, trochanteric fixation nail implants demonstrated the highest cutout resistance. CONCLUSIONS: Implant design can significantly affect the fixation strength and cutout resistance of implants for pertrochanteric fracture fixation. The novel cutout model can predict differences in cutout resistance between distinct implant designs.

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OBJECTIVE: To measure changes in knee kinematics after the application of articulated external fixators along a previously described knee flexion/extension axis and 16 specific "off-axis" fixator hinge configurations. DESIGN: Cadaver, biomechanical study. SETTING: Biomechanics laboratory. PARTICIPANTS: Nine fresh cadaver knee specimens. INTERVENTION: Each specimen was mounted on a custom-built frame that constrained the knee to move about a fixed flexion/extension axis. Passive knee motion was induced, and the resulting flexion moment was measured. Data were collected for the on-axis fixator position and 16 distinct rotational and translational off-axis positions. In addition, effects of tibial translation and rotation were investigated. MAIN OUTCOME: Range of motion (ROM) attainable within a moment envelope of +/-1 N-m and average energy required to impart movement. RESULTS: The average ROM for unconstrained knees was 122 degree. Constraining the knee to rotation around an on-axis aligned hinge significantly reduced the ROM by 35% to 79 degree. The 5-mm posterior translated hinge was the only alignment to show on average a slightly larger ROM (86 degree) than the on-axis hinge. All other hinge alignments showed decreased average ROM compared with the on-axis position. Tibiofemoral alignments significantly affected the obtainable ROM for the on-axis aligned hinge. CONCLUSION: It was not possible to replicate precisely the complex kinematics of the knee using a single axis fixator over the entire ROM. Using the axis of rotation previously defined in the literature, however, it was possible to obtain a limited ROM of the knee without placing excessive forces on the periarticular structures.

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