Characterization of mouthguards: Impact performance.

BACKGROUND/AIM: It is difficult to characterize the impact behavior of mouthguards on the basis of their components. Impact behavior tests should be performed on mouthguard formed to simulate their intra-oral performance. The aim of this study was to compare the impact behavior of six models of mouthguards using a standardized experimental protocol. MATERIAL AND METHODS: Four commercially available mouth-formed mouthguards (SDI™, Gel Nano™, Opro Shield Gold™ and Kipsta R300™), one mouth-formed mouthguard prototype and one custom-made mouthguard were tested. The procedures recommended by the manufacturers (injecting procedure for custom-made mouthguard and "boil-and-bite" procedures for mouth-formed mouthguards) were used to adapt five samples per model on steel jaws. Impact performances were assessed according to labial aspect thickness and maximum contact load (FMax) during impact using a drop tower. RESULTS: SDI™ and Opro Shield Gold™ had the thinnest labial aspect thickness (P<.01), followed by the Gel Nano™ and the Kipsta R300™ (P<.01) with a thickness of about 3 mm. The prototype and custom-made mouthguard were thicker (almost 4 mm). The custom-made mouthguard, the Kipsta R300™ and the prototype had the best impact performances, but the labial aspect thickness of the Kipsta R300™ was significantly lower than that of the custom-made mouthguard and the prototype. Analysis of force curves and position of the mouthguard on the impacted zone showed that the Kipsta R300™ was less well adapted. CONCLUSION: Thickness and impact performance are not sufficient criteria to characterize performance of mouthguards.

Quantification of nonlinear elasticity for the evaluation of submillimeter crack length in cortical bone.

The objective of this study was to investigate the sensitivity of the nonlinear elastic properties of cortical bone to the presence of a single submillimetric crack. Nonlinear elasticity was measured by nonlinear resonant ultrasound spectroscopy (NRUS) in 14 human cortical bone specimens. The specimens were parallelepiped beams (50×2×2 mm(3)). A central notch of 500 µm was made to control crack initiation and propagation during four-point bending. The nonlinear hysteretic elastic and dissipative parameters αf and αQ, and Young׳s modulus Eus were measured in dry condition for undamaged (control) specimens and in dry and wet conditions for damaged specimens. The length of the crack was assessed using synchrotron radiation micro-computed tomography (SR-µCT) with a voxel size of 1.4 µm. The initial values of αf, measured on the intact specimens, were remarkably similar for all the specimens (αf =-5.5±1.5). After crack propagation, the nonlinear elastic coefficient αf increased significantly (p<0.006), with values ranging from -4.0 to -296.7. Conversely, no significant variation was observed for αQ and Eus. A more pronounced nonlinear elastic behavior was observed in hydrated specimens compared to dry specimens (p<0.001) after propagation of a single submillimetric crack. The nonlinear elastic parameter αf was found to be significantly correlated to the crack length both in dry (R=0.79, p<0.01) and wet (R=0.84, p<0.005) conditions. Altogether these results show that nonlinear elasticity assessed by NRUS is sensitive to a single submillimetric crack induced mechanically and suggest that the humidity must be strictly controlled during measurements.

Biomechanical evaluation of four femoral fixation configurations in a simulated anterior cruciate ligament replacement using a new generation of Ligament Advanced Reinforcement System (LARS™ AC).

BACKGROUND: Recent improvements in manufacturing of biomaterials have made available a new generation of artificial ligaments with better biocompatibility and design that have led to a new interest in using them for ACL reconstructions. PURPOSE: To evaluate the biomechanical characteristics of four femoral fixations using a Ligament Advanced Reinforcement System (LARS™ AC; LARS, Arc sur Tille, France) for anterior cruciate ligament replacement. METHOD: Six femoral ACL fixations in four configurations using fresh calf femurs with an interference titanium screw inserted inside to outside, an interference titanium screw inserted outside to inside, an interference titanium screw inserted inside to outside with a staple and a new transversal cortical suspension device developed by LARS™ were compared in a static loading and failure test. Output values were ultimate strength, graft slippage, mode of failure, energy to failure and stiffness. RESULTS: The transversal fixation performed with a significantly higher failure load than others (1804 N) (p < 0.001), whereas there were no significant differences between the three fixations with interference screws. There were no significant differences of stiffness between all fixations, and the transversal device had a significantly higher graft slippage (13.1 mm) than others (all p < 0.01). CONCLUSIONS: In this in vitro evaluation, the transversal fixation exhibited better biomechanical performance under static solicitations than others. The transversal device is expected to provide better clinical results than the well-established screw system fixations for femoral ACL fixation. CLINICAL RELEVANCE: Laboratory investigation (Level 2).

Characterization of mouth-formed mouthguards: thermal performance.

This study examined whether the thermo-modeling process suits the thermal properties of the material constituting mouth-formed mouthguards (MGs). Five mouth-formed MGs were compared: four commercially available MGs (SDI™, Gel Nano™, Opro Shield Gold™, and Kipsta R300™) and one prototype. Differential scanning calorimetry was used to determine melting (T(m)) and crystallization (T(c)) temperatures and specific fusion and crystallization enthalpies (∆H(f) and ∆H(c) (J/g)). MGs were modeled with recording of vestibular flange and occlusal cushion temperatures (Toccl). Tm ranged from 45.3°C to 53.0°C and Tc ranged from 40.9°C to 48.2°C. Specific heat of fusion ranged from 40.2 J/g to 62.0 J/g. Toccl was higher than Tm for all MGs except Kipsta R300™. Guidelines provided by manufacturers may not be adapted to thermal properties of the MG material. To ensure proper thermomodeling, heating and biting durations should be adjusted.

Non destructive characterization of cortical bone micro-damage by nonlinear resonant ultrasound spectroscopy.

The objective of the study was to evaluate the ability of a nonlinear ultrasound technique, the so-called nonlinear resonant ultrasound spectroscopy (NRUS) technique, for detecting early microdamage accumulation in cortical bone induced by four-point bending fatigue. Small parallelepiped beam-shaped human cortical bone specimens were subjected to cyclic four-point bending fatigue in several steps. The specimens were prepared to control damage localization during four-point bending fatigue cycling and to unambiguously identify resonant modes for NRUS measurements. NRUS measurements were achieved to follow the evolution of the nonlinear hysteretic elastic behavior during fatigue-induced damage. After each fatigue step, a small number of specimens was removed from the protocol and set apart to quantitatively assess the microcrack number density and length using synchrotron radiation micro-computed tomography (SR-µCT). The results showed a significant effect of damage steps on the nonlinear hysteretic elastic behavior. No significant change in the overall length of microcracks was observed in damaged regions compared to the load-free control regions. Only an increased number of shortest microcracks, those in the lowest quartile, was noticed. This was suggestive of newly formed microcracks during the early phases of damage accumulation. The variation of nonlinear hysteretic elastic behavior was significantly correlated to the variation of the density of short microcracks. Our results suggest that the nonlinear hysteretic elastic behavior is sensitive to early bone microdamage. Therefore NRUS technique can be used to monitor fatigue microdamage progression in in vitro experiments.

Biological and biomechanical evaluation of the ligament advanced reinforcement system (LARS AC) in a sheep model of anterior cruciate ligament replacement: a 3-month and 12-month study.

PURPOSE: The purposes of this study were to assess tissue ingrowth within the Ligament Advanced Reinforcement System (LARS) artificial ligament (LARS AC; LARS, Arc sur Tille, France) and to study the biomechanical characteristics of the reconstructed knees in a sheep model of anterior cruciate ligament (ACL) replacement. METHODS: Twenty-five female sheep underwent excision of the proximal third of the left ACL and intra-articular joint stabilization with a 44-strand polyethylene terephthalate ligament (mean ultimate tensile failure load, 2,500 N). Animals were killed either 3 or 12 months after surgery. Explanted knees were processed for histology (n = 10) or mechanical tests including tests of laxity and loading to failure in tension (n = 15). RESULTS: Well-vascularized tissue ingrowth within the artificial ligament was only observed in the portions of the ligament in contact with the host's tissues (native ligament and bone tunnels). Ligament wear was observed in 40% of explanted knees. The ultimate tensile failure loads of the operated knees at both time points were inferior to those of the contralateral, intact knees (144 ± 69 N at 3 months and 260 ± 126 N at 12 months versus 1,241 ± 270 N and 1,218 ± 189 N, respectively) (P < .01). In specimens with intact artificial ligaments, failure occurred by slippage from the bone tunnels in all specimens explanted 3 months postoperatively and in half of the specimens explanted 12 months postoperatively. CONCLUSIONS: This study provides evidence that the LARS AC has a satisfactory biointegration but that it is not suitable for ACL replacement if uniform tissue ingrowth is contemplated. Despite good clinical performance up to 1 year after implantation, none of the reconstructions approached the mechanical performance of the normal ACL in the ovine model. Partial tearing of the artificial ligament, which led to a significant decrease in ultimate tensile strength, was observed in 40% of cases in the ovine model. CLINICAL RELEVANCE: The LARS is not a suitable scaffold for ACL replacement. Further animal studies are needed to evaluate its potential for augmentation of ligament repair.

The effect of polystyrene sodium sulfonate grafting on polyethylene terephthalate artificial ligaments on in vitro mineralisation and in vivo bone tissue integration.

This study investigates the impact of polystyrene sodium sulfonate (PolyNaSS) grafting onto the osseo-integration of a polyethylene terephthalate artificial ligament (Ligament Advanced Reinforcement System, LARS™) used for Anterior Cruciate Ligament (ACL). The performance of grafted and non-grafted ligaments was assessed in vitro by culturing human osteoblasts under osteogenic induction and this demonstrated that the surface modification was capable of up-regulating the secretion of ALP and induced higher level of mineralisation as measured 6 weeks post-seeding by Micro-Computed Tomography. Grafted and non-grafted LARS™ were subsequently implanted in an ovine model for ACL reconstruction and the ligament-to-bone interface was evaluated by histology and biomechanical testings 3 and 12 months post-implantation. The grafted ligaments exhibited more frequent direct ligament-to-bone contact and bone formation in the core of the ligament at the later time point than the non-grafted specimens, the grafting also significantly reduced the fibrous encapsulation of the ligament 12 months post-implantation. However, this improved osseo-integration was not translated into a significant increase in the biomechanical pull-out loads. These results provide evidences that PolyNaSS grafting improved the osseo-integration of the artificial ligament within the bone tunnels. This might positively influence the outcome of the surgical reconstructions, as higher ligament stability is believed to limit micro-movement and therefore permits earlier and enhanced healing.

Quantitative ultrasound of cortical bone in the femoral neck predicts femur strength: results of a pilot study.

A significant risk of femoral neck (FN) fracture exists for men and women with an areal bone mineral density (aBMD) higher than the osteoporotic range, as measured with dual-energy X-ray absorptiometry (DXA). Separately measuring the cortical and trabecular FN compartments and combining the results would likely be a critical aspect of enhancing the diagnostic capabilities of a new technique. Because the cortical shell determines a large part of FN strength a novel quantitative ultrasound (QUS) technique that probes the FN cortical compartment was implemented. The sensitivity of the method to variations of FN cortical properties and FN strength was tested. Nine femurs (women, mean age 83 years) were subjected to QUS to measure the through transmission time-of-flight (TOF) at the FN and mechanical tests to assess strength. Quantitative computed tomography (QCT) scans were performed to enable analysis of the dependence of TOF on bone parameters. DXA was also performed for reference. An ultrasound wave propagating circumferentially in the cortical shell was measured in all specimens. Its TOF was not influenced by the properties of the trabecular compartment. Averaged TOF for nine FN measurement positions/orientations was significantly correlated to strength (R2 = 0.79) and FN cortical QCT variables: total BMD (R(2) = 0.54); regional BMD in the inferoanterior (R2 = 0.90) and superoanterior (R2 = 0.57) quadrants; and moment of inertia (R2 = 0.71). The results of this study demonstrate that QUS can perform a targeted measurement of the FN cortical compartment. Because the method involves mechanical guided waves, the QUS variable is related to the geometric and material properties of the cortical shell (cortical thickness, tissue elasticity, and porosity). This work opens the way to a multimodal QUS assessment of the proximal femur, combining our approach targeting the cortical shell with the existing modality sensitive to the trabecular compartment. In vivo feasibility of our approach has to be confirmed with experimental data in patients.

3D analysis from micro-MRI during in situ compression on cancellous bone.

A mini-compression jig was built to perform in situ tests on bovine trabecular bone monitored by micro-MRI. The MRI antenna provided an isotropic resolution of 78 microm that allows for a volume correlation method to be used. Three-dimensional displacement fields are then evaluated within the bone sample during the compression test. The performances of the correlation method are evaluated and discussed to validate the technique on trabecular bone. By considering correlation residuals and estimates of acquisition noise, the measured results are shown to be trustworthy. By analyzing average strain levels for different interrogation volumes along the loading direction, it is shown that the sample size is less than that of a representative volume element. This study shows the feasibility of the 3D-displacement and strain field analyses from micro-MRI images. Other biological tissues could be considered in future work.