Nobuo Sakai

Roles of adsorbed film and gel layer in hydration lubrication for articular cartilage

Abstract To maintain low friction and low wear in natural synovial joints, adsorbed film formation on articular cartilage surface appears to play an important role in mixed or boundary lubrication regime where local direct contact occurs. Furthermore, the proteoglycan gel layer at the uppermost superficial zone in articular cartilage appears to play an important role in preserving low friction and low wear by the hydration lubrication mechanism even after removal of adsorbed film. However, the interaction or synergistic action between adsorbed film and hydrated film/surface has not yet been clarified. To examine the roles of adsorbed film and gel layer on articular cartilage surface in hydration lubrication, the changes in friction were observed in the reciprocating test of articular cartilage against a glass plate, at repeated rubbing including restarting after interrupting—unloading process. It is noticed that at restarting immediately after loading, hydration lubrication is expected to become effective. The lubricating roles of protein-adsorbed film in hydration lubrication are discussed on the basis of experimental results in both cases with and without surface gel layer. The comparison of albumin and γ -globulin is described in connection with the existence of a gel layer.

Superior lubricity in articular cartilage and artificial hydrogel cartilage

In healthy natural synovial joints, the extremely low friction and minimum wear are maintained by their superior load-carrying capacity and lubricating ability. This superior lubricating performance appears to be actualized not by single lubrication mode but by synergistic combination of multimode mechanisms such as fluid film, biphasic, hydration, gel film and/or boundary lubrication. On the contrary, in most artificial joints composed of ultra-high molecular weight polyethylene against metal or ceramic-mating material, boundary and/or mixed lubrication modes prevail and thus local direct contact brings down high friction and high-wear problems. To extend the durability of artificial joint, the reduction in friction and wear by improvement in lubrication mechanism is required as an effective design solution. In this paper, at the start, the mechanism of superior lubricity for articular cartilage is examined from the viewpoints of biphasic and boundary lubrication mechanism. Subsequently, the proposal of biomimetic artificial hydrogel cartilage is put forward to improve the lubricating modes in artificial joints. The tribological behaviours in two kinds of poly(vinyl alcohol) hydrogels are compared with that of natural cartilage. The importance in lubrication mechanism in artificial hydrogel cartilage is discussed.

Effectiveness of adsorbed film and gel layer in hydration lubrication as adaptive multimode lubrication mechanism for articular cartilage

With various daily activities, the effectiveness of adsorbed film formed on a gel-like layer at the uppermost superficial articular cartilage in natural synovial joints becomes important to control the friction and wear of articular cartilage in mixed or boundary lubrication regime as an adaptive multimode lubrication mechanism. Furthermore, in the case where the adsorbed film has been removed, the proteoglycan gel layer is expected to preserve low friction and protect against the wear of bulk cartilage tissue with an effective hydration lubrication mechanism. Besides, it is indicated that the biphasic lubrication plays an important role in lowering of friction in articular cartilage containing a large amount of water. At the present stage, however, the detailed relationship between adsorbed films and hydrated gel layers has not yet been elucidated. In this article, the frictional behaviours of articular cartilage on a glass plate were observed in the reciprocating tests with the restarting process after interruption and unloading. The lubricating effectiveness of adsorbed films in hyaluronate (HA) solutions was examined using intact and partially damaged cartilage specimens. The role of albumin and γ-globulin in relation to the surface conditions of gel layer is discussed. The restarting friction immediately after reloading became lower as a result of recovery of the effective interstitial fluid pressurization and hydration and adsorbed films have a significant effect on the frictional behaviour at local contacts. To clarify the molecular phenomena taking place under rubbing condition, in situ observations of the forming adsorbed film were conducted. The measurements were performed using the fluorescent staining method for protein and HA molecules at low contact pressures.

Development of a novel endoscopic manipulation system: the Endoscopic Operation Robot ver.3.

BACKGROUND AND AIMS The next generation of flexible endoscopy platforms such as The Master and Slave Transluminal Endoscopic Robot (MASTER) is primarily for remote control manipulation of forceps, but manipulation of the flexible endoscope itself still depends on conventional techniques. We have developed the Endoscopic Operation Robot (EOR) ver.3, which incorporates haptic feedback to provide complete remote control flexible-endoscope manipulation. The present study aimed to evaluate the performance of the EOR ver.3. METHOD A colonoscopy training model was used with scope insertion to the cecum. Force during insertion and insertion time (seconds) to the cecum were evaluated. The data were compared by colon zone and experience level (trainee or expert). RESULTS The mean insertion time into the cecum was 118.54 ± 89.42 seconds. Stronger force and torque were required for deeper insertion of the scope. Expert and trainee endoscopists differed in the insertion time to the cecum, maximum counterclockwise torque, mean clockwise torque, and mean counterclockwise torque. CONCLUSION The EOR ver.3 has operability with which endoscopists can easily familiarize themselves.

Elasto-plastic contact analysis of an ultra-high molecular weight polyethylene tibial component based on geometrical measurement from a retrieved knee prosthesis

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of these implants. Especially in retrieved knee prostheses with anatomical design, the predominant types of wear on UHMWPE tibial components are delamination and pitting. These fatigue wear patterns of UHMWPE are believed to result from repeated plastic deformation owing to high contact stresses. In this study, the elasto-plastic contact analysis of the UHWMPE tibial insert, based on geometrical measurement for retrieved knee prosthesis, was performed using the finite element method (FEM) to investigate the plastic deformation behaviour in the UHMWPE tibial component. The results suggest that the maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial component. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at macroscopic level.

Effects of mucosal thickness on the stress distribution and denture stability of mandibular implant-supported overdentures with unsplinted attachments in vitro

The aim of this study was to compare the effects of mucosal thickness on the stress pattern around implants and movement of implant-supported overdentures with ball/female and three different types of magnetic attachments. After insertion of two root-form implants into a mandibular model, the surface of the model was covered with a 1.5- or 3-mm layer of impression material to simulate the oral mucosa, and removable overdentures were fabricated on each model. A 50-N vertical force was applied to the right first molar, and the resultant stress distribution and denture movement were measured. In the 1.5-mm mucosal model, the magnetic attachments showed significantly lower bending moments than did the ball attachment. The denture base displacement was the lowest on a magnetic attachment. In this study, use of magnetic attachments could be advantageous for mandibular implant-supported overdentures based on lower stress and better denture stability especially in the thin mucosal model.

Relationship between magnitude of immediate loading and peri‐implant osteogenesis in dogs

OBJECTIVES The purpose of this study was to investigate the influence of the magnitude of immediate loading on peri-implant bone in an animal model of dental implantation. MATERIAL AND METHODS Eight weeks after the extraction of maxillary and mandibular premolars, three implants were inserted bilaterally in the mandibles of six Beagle dogs. One implant was unloaded (UL) as a control, and two implants were loaded immediately with 10 N (mild loading: ML) or 50 N (excessive loading: EL) laterally using a cyclic loading device twice a week for 3 weeks. Fluorescent bone markers were injected to examine bone formation around the implants. The animals were sacrificed 3 weeks after implantation. Peri-implant osteogenesis was assessed by histomorphometric procedures, i.e., measuring bone-implant contact (BIC) and bone density (BD). RESULTS The UL and ML groups had no peri-implant infection, and newly formed bone was observed over a wide area from the implant neck toward the tip, and in direct contact with the implant surface. In contrast, in the EL group, newly formed bone was rarely observed around the implant neck and there were signs of infection. Both BIC and BD in the ML group were significantly greater than those in the other groups. BIC and BD in the EL group were significantly lower than those in the other groups. CONCLUSION A suitable magnitude of load applied immediately after dental implantation promotes peri-implant osteogenesis.

Biphasic and boundary lubrication mechanisms in artificial hydrogel cartilage: A review

Various studies on the application of artificial hydrogel cartilage to cartilage substitutes and artificial joints have been conducted. It is expected in clinical application of artificial hydrogel cartilage that not only soft-elastohydrodynamic lubrication but biphasic, hydration, gel-film and boundary lubrication mechanisms will be effective to sustain extremely low friction and minimal wear in daily activities similar to healthy natural synovial joints with adaptive multimode lubrication. In this review article, the effectiveness of biphasic lubrication and boundary lubrication in hydrogels in thin film condition is focused in relation to the structures and properties of hydrogels. As examples, the tribological behaviors in three kinds of poly(vinyl alcohol) hydrogels with high water content are compared, and the importance of lubrication mechanism in biomimetic artificial hydrogel cartilage is discussed to extend the durability of cartilage substitute.

Basic Study on Superconducting Magnetic Bearing (SMB) With Superconducting Coil

Our group has developed a superconducting magnetic bearing (SMB) with superconducting coil for magnetically levitated conveyers used for clean environments. The SMB is composed of a levitated permanent magnet (PM) and a supporting superconductor with superconducting coil, a control coil and a Hall sensor. The damping for the SMB with superconducting coil is compared with the SMB without superconducting coil. PD control is applied to the SMB and the damping characteristics are also discussed.

Finite Element Analysis of Articular Cartilage Model Considering the Configuration and Biphasic Property of the Tissue

Articular cartilage tissue has high water content from 70 to 80% and shows biphasic behavior in which both solid and fluid properties should be considered. Furthermore, the mechanical behavior of cartilage shows depth-dependence. Therefore it is necessary to consider not only the average tissue property but also the local one to explain mechanical and functional behavior.