Osteocyte is the most abundant cell type in bone, and the only cell type located inside the mineralized matrix. The striking structural design of bone predicts an important role for osteocytes in determining bone structure and function. Osteocytes are connected with each other via gap junctions and form a three dimensional cellular network in mineralized bone matrix. Recently, it has been shown that osteocytes are not only passive bystanders, but also have an active regulatory role in whole body phosphate and calcium metabolism. Osteocytes are cells which sense mechanical loading in bone. They respond to mechanical stimuli by producing and secreting several bioactive substances including nitric oxide and prostaglandins and thus transmit messages of loading to effector cells, like osteoblasts and osteoclasts. Present data suggest that osteocytes actively inhibit osteoclastic bone resorption. Whenever osteocytes die, this inhibitory effect is turned off and osteoclasts are activated. This mechanism of action could explain targeted remodeling in the region of stress induced microcracks happened. Osteocytes can also modulate osteoblasts function. In conclusion, osteocytes sense mechanical stimuli, transmit signals through cellular network and regulate osteoblast and osteoclast function during bone remodeling.

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Three-dimensional cell culture is widely used for challenging to biomedical tissue reconstruction. A mass of the reconstructed tissue has been limited because of difficulty in supplying sufficient amount of oxygen deep into the densely cultured cells. Although many methods are available for supplying oxygen, we focused on utilizing microbubbles. The aims of the present study were therefore to develop a microbubble generator suitable for cell culture, and to examine its effects on cellular activity. A new microbubble generator was designed so that it could be autoclaved and settled in a culture vessel with medium. Filtered clean air was introduced to the generator, mechanically sheared by a rotating disk, and then supplied into the culture vessel. This method significantly elevated the dissolved oxygen level by approximately 1.5mg/l. Microscopic measurement showed that 66.8% of the bubbles was distributed within 5-20μm in diameter, meaning that the generated bubbles had a good morphological feature of microbubbles. With and without microbubbles, gel-embedded MC3T3-E1 osteoblastic cells were three-dimensionally incubated for 3 days. Cell viability assay showed that the introduction of microbubbles increased necrotic cell death, which might be due to hyperoxia as well as fluid shearing force. However, relative alkaline phosphatase activity was significantly enhanced by microbubbles. Although further examination is needed, microbubbles would have a potential to enhance osteoblastic cell activity, and could be utilized for effective aeration in dense three-dimensional cell culture.

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We developed a new estimation system using rapid prototyping technology that focuses on the relationship between the architecture and mechanical properties of trabecular bone. The system uses three-dimensional acrylic resin models of trabecular bone constructed from micro-CT data to predict the mechanical properties of trabecular bone. We used this method to clarify the relationship between loading orientation and bone fragility in the vertebral trabecular bone of ovariectomized (OVX) rats. Twenty 6-week-old female Wistar rats were randomly divided into two groups (OVX and normal groups). Five rats from each group were killed at 3 and 6 weeks following operations and the L4 vertebra was removed. Bone specimens from both groups were scanned using a micro-CT system. Subsequently, resin models of the bone were fabricated at 60× magnification from the micro-CT data sets using laser stereolithography. The resin models were subjected to compressive testing in three orthogonal orientations corresponding to the craniocaudal, mediolateral, and anteroposterior anatomic axes. The results showed that the elastic modulus and ultimate stress were lower in the models of OVX rats than in the normal rats, the mechanical properties of trabecular bone structure from OVX rats deteriorated with increasing time postoperatively, and the elastic moduli in the mediolateral and anteroposterior axes were especially reduced relative to the decrease in the craniocaudal axis in the OVX rats.

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Quantitative measurement of spinal rotational mobility is critical in assessing spinal instability. We have developed a system for intraoperative measurement of spinal mobility. The system consists of a motor-driven mechanical apparatus with a computerized controller, load cell, optical displacement transducer, and spinous process holders that allow handling of the spinal process without damaging the ligaments. In this study, we examined the ability of this measurement system to detect mobility and corresponding clinical instability of a destabilized model spine comprised of porcine lumbar segments. From the load-displacement curve, we determined three motion parameters: stiffness, neutral zone (NZ), and absorption energy (AE). To demonstrate the potential of the measurement system for clinical application, we applied five cycles of axial rotation to the segment ten times. The measurements were highly reproducible. To verify the utility of the measurement system, we tested destabilized functional spinal units in vitro. In this setting, stiffness and AE decreased and NZ increased with progressive destabilization of the model spine. Significant relationships were observed between the destabilized model and the biomechanical data.

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Adjacent segment degeneration (ASD) is an abnormal process that develops at spinal segments adjacent to the fused segment caused by biomechanical changes after spinal fusion. To avoid the adverse effects of spinal fusion on the adjacent segments, various flexible stabilization systems have been developed, of which the Graf system is one of the most widely used. We assessed the biomechanical influence of the Graf system and spinal fusion. The L3-L6 vertebrae were taken from porcine lumbar spines. A spinal motion tester displaced the end of the L3 vertebra to simulate continuous flexion/extension. Three cycles of flexion/extension were applied and the angular motion and intradiscal pressure were recorded during the third cycle for each test. The angular deformity at the flexible stabilized segment was suppressed until 4° of deformation; the deformity gradually increased after 4° and then finally equaled that of the intact spine. The maximum intradiscal pressure increased significantly in each segment fused and in the adjacent spinal segments using the Graf system. In conclusion, the Graf system reduces the motion of adjacent segments and may reduce the risk of ASD. Nevertheless, the relationship between ASD and the increase in the intradiscal pressure remains controversial.

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Spine is a basic element for maintenance of trunk of body and the physical motion and it has a structure which affords the rigidity as an attitude maintenance mechanism (or a stability) and the mobility as a joint mechanism simultaneously. And the thorax is a structure which affords the stability (rigidity) to protect the internal organs such as a heart and lungs and the mobility which makes the movement of the costal bone for breathing exercise. Therefore the spine-thorax is a compound structure which has stability and mobility, and this fundamental nature is based on mechanics. And its dysfunction is mainly based on the mechanical factors. When the dysfunction happens, a treatment is executed by a mechanical reconstruction. And if it is severe, a surgery is selected for the treatment. This paper develops a method of the mechanical simulation of the reconstructive surgery to help the diagnosis and the treatment.

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The purpose of this study was to investigate the transmission mechanism of the three-dimensional impact stress in the human knee joint. In this study, two fresh specimens were prepared to search the impact stress by two methods: the impact testing with drop tower apparatus and the finite element simulation. In the testing, the specimens were installed in the apparatus, and an impact load was applied to each specimen by dropping 1 kg weights. The mini-pressure transducers were implanted in the joints of the femur and tibia, and the stress transmission was measured. Under the intact load condition, the impact compressive stress values of 0.140 to 0.320 MPa were observed. In the impact simulation, the 3D modeling with the hexahedral finite elements was developed, and the way the impact load was passing through the cancellous bone could be visualized. The simulation results of the compressive stress in the cancellous bone and the strain in the cortical bone surface were in considerably agreement with the testing results. The stress distribution calculated from the simulation showed that the removal of the meniscus affected the load transmission mechanism. As for the reason, it was considered that when sliding interfaces between the femoral cartilage and tibial cartilage are reduced, the impact load cannot be transmitted to the interior of the joint, causing the stress shielding inside.

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Change in gait by footwear was investigated using a tactile sensor that measures displacement of the centroid and its moving velocity on the sole, and an equilateral-triangular force plate that measures the three orthogonal components of ground reaction force as well as plantar friction as the frictional coefficient between the flooring material and the sole. Gait pattern was demonstrably changed by footwear and was influenced by the kinematic constraint of the ankle joint and stiffness of the sole of the shoe. The time history of plantar friction displayed two peaks per step, synchronous with those of the vertical ground reaction force; this provides easy-to-understand information regarding the influence of footwear on gait pattern. In contrast, plantar friction between the two peaks was extremely low, approaching zero in places. This behavior can be explained by a sliding/rolling motion between the flooring material and the sole.

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3-D finite element models of two kinds of Stryker's PS type knee prostheses, Scorpio Superflex and NRG, were constructed using their CAD data with use of a nonlinear spring model and an analytical load data for deep squatting. Superflex model was formerly used in TKA, and NRG model is the latest version with a modified design of Post. Stress analysis was then performed by an explicit finite element method under continuous flexion motion from 0 to 135 degree. It was shown that only the condylar surfaces of the femoral component and the tibial insert contacted each other from 0 to 60 degree flexion for both the models, and the stress concentration in NRG was a little higher than that in Superflex. The simulation results also exhibited that severe stress concentration was generated at Post of the tibial UHMWPE insert due to Post/Cam contact. This kind of stress concentration may result in damage and failure of Post. It was shown that the design modification applied to NRG effectively reduced the stress concentration of Post.

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