Vibration platform for mice to deliver precise, low intensity mechanical signals to the musculoskeleton.

OBJECTIVE: Low intensity vibration as a therapeutic and training modality has received increased attention despite the lack of clear mechanistic pathways. Thus, to determine mechanisms underpinning vibration-induced musculoskeletal adaptations, a vibration platform for mice was designed, constructed, and validated. METHODS: Critical aspects of the platform include use of off-the-shelf components to (1) tailor individual parameter selection (acceleration and frequency), (2) produce low error across the plate's surface and throughout the range of vibration parameters, and (3) utilize accelerometer feedback to ensure fidelity within and between bouts of vibration. The vibration device is controlled by a centrally-mounted linear actuator on the underside of the platform that is modulated by accelerometer feedback. RESULTS: Triaxial accelerometers confirmed that vibrations were purely vertical and acceleration responses were within 5% of target stimuli for all accelerations (0.2-1.0 g) and frequencies (25-90 Hz). The platform produced acceleration responses with ≤4% error between 25-90 Hz. Vibration modes were not detected indicating that the circular plate produced uniform stimuli across the platform (error ≤1.1%, P≥0.23) and mouse body mass did not affect the platform's performance (P≥0.43). CONCLUSIONS: Our vibration device for mice improves upon existing devices and enables precise, low intensity mechanical signals to be applied with confidence.

Biomechanical analysis of pedicle screw thread differential design in an osteoporotic cadaver model.

BACKGROUND: Pedicle screw fixation, the standard surgical care for posterior stabilization in the thoraco-lumbar spine has a high rate of failure in osteoporotic individuals. Screw design factors and insertion techniques have been shown to influence the biomechanical performance of pedicle screws. Our objective was to investigate the biomechanical characteristics of pedicle screw fixation in osteoporotic bone by comparing standard screws with newly designed differential crest thickness dual lead screws. METHODS: An in-vitro spinal-level paired factorial study design was used to examine thoraco-lumbar spine biomechanical outcomes for differential pedicle screw thread designs. Six cadaveric human spines (T8-L5) were tested for six groups (n=20) consisting of 2 different crest thickness and 3 different insertion techniques. Bone mineral density was assessed and peak insertion torque measured while placing one screw of new design and control on the contralateral side. Screw pullout properties were measured from classical American Society for Testing and Materials protocols. FINDINGS: The screws designed specifically for osteoporotic bone showed significantly larger insertion torque compared with the standard screw design irrespective of insertion technique. Much of the variability in pullout failure and stiffness was explained by bone mineral density. The osteoporotic screws of different crest thickness were statistically similar to each other in all outcome measures. INTERPRETATION: Compared with standard pedicle screws, the dual lead osteoporotic-specific pedicle screws demonstrated significantly larger insertion torques and similar pullout properties. Non-significant increased biomechanical strength was observed for thin crest compared to thick crest dual lead pedicle screws indicating their enhanced purchase in osteoporotic bone.

Tensile mechanics of the developing cervical spine.

This study examined the effect of spinal development (developmental age) on the tensile mechanics of the cervical spine. A total of 68 isolated functional spinal units were subjected to tensile loading to document their mechanical response (tensile stiffness and ultimate failure load). Cadaveric baboon specimens, ranging in age from 2 to 26 human-equivalent years, were used due to the limited availability of human tissues in the pediatric age range. Statistically significant correlation was found between developmental age and both tensile stiffness and ultimate failure load. Furthermore, differences in these properties were observed as a function of spinal level. In addition to providing age-related data for the developing spine, our findings suggest that reasonable scaling relationships exist between the adult and the child spine. These relationships provide a basis for scaling adult properties to the child, which may abet the development of pediatric neck injury tolerance values.

Retinal anatomy of a new species of bresiliid shrimp from a hydrothermal vent field on the Mid-Atlantic Ridge.

A new species of shrimp (Rimicaris sp.) was recently collected from the Snake Pit hydrothermal vent field on the Mid-Atlantic Ridge. Until the discovery in 1989 that the deep-sea, hydrothermal vent species, Rimicaris exoculata, possessed a hypertrophied dorsal eye, everyone believed that animals recovered from vent environments were blind. Like R. exoculata, Rimicaris sp., a small orange bresiliid shrimp, has an enlarged dorsal eye specialized for detecting light in a very dim environment instead of the expected compound eye. The individual lenses characteristic of a compound eye adapted for imaging have been replaced in Rimicaris sp. by a smooth cornea underlain by a massive array of photosensitive membrane. The number of ommatidia in this species is about the same as in shrimp species that live at the surface; however, the photoreceptors are larger in the deep-sea species and the shape of the photoreceptors is markedly different. The light-sensitive region of the photoreceptor is much larger than those of other shrimp and the rest of the receptor is much smaller than normal. All screening pigment has moved out of the path of incident light to a position below the retina, and the reflecting pigment cells have adapted to form a bright white diffusing screen between and behind the photoreceptors. The ultrastructure of the microvillar array comprising the rhabdom is typical for decapod crustaceans; however, there is a much greater volume density of rhabdom (80% to 85%) than normal. There is no ultrastructural evidence for cyclic rhabdom shedding or renewal. Rimicaris sp. has apparently adapted its visual system to detect the very dim light emitted from the throats of the black smoker chimneys around which it lives.