A novel closed-loop electromechanical stimulator to enhance osseointegration with immediate loading of dental implant restorations.

The degree of osseomechanical integration of dental implants is acutely sensitive to their mechanical environment. Bone, both as a tissue and structure, adapts its mass and architecture in response to loading conditions. Therefore, application of predefined controlled loads may be considered as a treatment option to promote early maturation of bone/implant interface prior to or in conjunction with crown/prosthesis attachment. Although many studies have established that the magnitude, rate of the applied strain, and frequency have significant effects on the osteogenic response, the actual specific relationships between strain parameters and frequency have not yet been fully defined. The purpose of this study was to develop a stimulator to apply defined mechanical stimuli to individual dental implants in vivo immediately after implantation, exploring the hypothesis that immediate controlled loading could enhance implant integration. An electromechanical device was developed, based on load values obtained using a two-dimensional finite element analysis of the bone/implant interface generating 1000 to 4000 pe and operated at 30 and 3 Hz respectively. The device was then tested in a cadaveric pig mandible, and periosteal bone surface strains were recorded for potential future comparison with a three-dimensional finite element model to determine loading regimens to optimize interface strains and iterate the device for clinical use.

Analysis of glenoid fixation for a reversed anatomy fixed-fulcrum shoulder replacement.

The restoration of pain-free stable function in gleno-humeral arthritic cases in various situations such as rotator cuff deficiency, old trauma and failed total shoulder arthroplasty is a challenging clinical dilemma. The Bayley-Walker shoulder has been designed specifically for very difficult cases where surface replacement devices do not provide sufficient stability. This device is a fixed-fulcrum reverse anatomy implant where the centre of rotation is placed medially and distally with respect to the normal shoulder, to increase the lever arm of the abductor muscles. An important problem in devices of this type is obtaining secure and long-lasting fixation of the glenoid component. In this design, fixation is achieved using a tapered screw for engagement with cortical bone and HA coating for subsequent osseointegration. This study presents the results from a three-dimensional finite element analysis conducted on this component for two load cases at 60 degrees and 90 degrees abduction. The results showed that most of the forces were transmitted from the component to the cortical bone of the scapula, the remaining load being transmitted through cancellous bone. Histology from a retrieved case shows evidence of bone remodelling. The retrieval case obtained some time after implantation showed new bone formation had occurred around the threads onto the HA coating. Fixing the component in this way at multiple locations in cortical bone may overcome the problems of glenoid loosening historically associated with cemented constrained devices.

Evaluation of a testing method for the fatigue performance of total knee tibial trays.

Catastrophic failure of tibial baseplates due to fatigue fracture have occurred in patients. The International Standards Organisation (ISO) have proposed an endurance test to ensure a safety level for trays with respect to this problem. At present the magnitude of the applied load has not been defined. The purpose of this study is to evaluate the ISO test by comparing the results of two tibial trays: one with a clinical history of fracture of the tibial baseplate in some cases (Kinematic) and one without such a record (Kinemax). A load of 2000 N was selected, representing the peak force during the normal walking cycle. Finite element analysis (FEA) predicted that both the Kinematic and Kinemax trays would rapidly fail for two defined contact positions. Mechanical testing performed on both these components confirmed the FEA predictions. However, under service conditions, the Kinematic tray has been reported to have a failure of only 1-2%, the failure occurring at a mean of 54 months for early fracture and 7 years for late fracture. There are no published reports of failed Kinemax trays. It is concluded that the ISO test is an exaggeration of reality when a physiological load is used. As a result of our study we propose that a load of 500 N would effectively screen components with respect to the ISO test conditions for the likelihood of fracture.