Simulation of impact test for determining "health" of percutaneous bone anchored implants.

There is an ongoing requirement for a clinically relevant, noninvasive technique to monitor the integrity of percutaneous implants used for dental restorations, bone-anchored hearing aids, and to retain extra-oral prostheses (ear, eye, nose, etc). Because of the limitations of conventional diagnostic techniques (CT, MRI), mechanical techniques that measure the dynamic response of the implant-abutment system are being developed. This paper documents a finite element analysis that simulates a transient response to mechanical impact testing using contact elements. The detailed model allows for a specific interface between the implant and bone and characterizes potential clinical situations including loss of bone margin height, loss of osseointegration, and development of a soft connective tissue layer at the bone-implant interface. The results also show that the expected difference in interface stiffness between soft connective tissue and osseointegrated bone will cause easily measurable changes in the response of the implant/abutment system. With respect to the loss of bone margin height, changes in the order of 0.2 mm should be detectable, suggesting that this technique is at least as sensitive as radiography. A partial loss of osseointegration, while not being as readily evident as a bone margin loss, would still be detectable for losses as small as 0.5 mm.

Mandibular kinematics associated with simulated low-velocity rear-end impacts.

Rear-end-impact motor vehicle accidents may result in cervical and temporomandibular-related pain complaints. Head kimematics in simulated low-impact rear-end impacts have been investigated but mandibular kinematics have not been described. Thirty healthy adult subjects underwent three impacts (4.5 m s(-2) expected, 10.0 m s(-2) unexpected, and 10.0 m s(-2) expected). Onset time and peak magnitude of angular head acceleration, angular mandibular acceleration and angular mandibular displacement were measured. Significant mandibular opening acceleration was not identified with rearward head rotation. The peak magnitude of mandibular closing angular acceleration approximately doubled with increased impact magnitude. No differences in peak angular mandibular acceleration regarding expectation were identified. Gender differences were detected in the fast unexpected impact. The peak time for the angular mandibular acceleration (mandibular closure) was approximately 84-120 ms later than peak rearward angular head acceleration for all impacts. Onset and peak times for angular mandibular acceleration (mandibular closure) were similar to the onset and peak times for forward head acceleration. There was also a positive correlation between the magnitude of the forward angular acceleration of the head and angular acceleration of the mandible for the slow (0.65, P = 0.015) and fast expected (0.844, P = 0.001) impacts. The average angular mandibular angular displacement (mandibular closure) was approximately 6 degrees . The hyperextension hypothesis regarding mechanism of temporomandibular joint injury in low-impact rear-end collisions cannot be supported.

Cutaneous cleansers.

Skin cleansers may be an important adjunct to the regimen of those who use cosmetics, have sensitive or compromised skin, or utilize topical therapies. Cleansers emulsify dirt, oil and microorganisms on the skin surface so that they can be easily removed. During cleansing, there is a complex interaction between the cleanser, the moisture skin barrier, and skin pH. Cleansing, with water soap or a liquid cleanser, will affect the moisture skin barrier. Soap will bring about the greatest changes to the barrier and increase skin pH. Liquid facial cleansers are gentler, effecting less disruption of the barrier, with minimal change to skin pH, and can provide people with a cleanser that is a combination of surfactant classes, moisturizers and acidic pH in order to minimize disruption to the skin barrier.

Intra-operative measurement of vertebral bone strength.

A significant complication of surgical correction of the deformed spine is pull out of the vertebral hooks or screws. This complication can be partly attributed to poor bone stock. Currently, there are few methods available for surgeons to assess the mechanical strength and stiffness of the vertebra, and even fewer methods that provide in-vivo measurements. An "upbiter" is often used to form a seat in the lamina for the hook. A typical upbiter was modified and instrumented with strain gauges to investigate the feasibility of determining the forces and displacements needed to cut through laminar bone during surgical procedures. The calibration showed that the system was repeatable and could highly correlated with applied force (R2=0.98) and displacement (R2=0.99) with resolutions of 0.72 N and 0.40 mm respectively. This system was tested on three females, on lamina ranging from T4 to T12 regions. The average maximum force for bone failure was at 470+/-128N.

Computer modelling of hooks for use as intra-operative force sensors.

There are a number of forces applied during scoliosis surgery, the magnitude and direction of which remains unknown. There is little literature concerning the in vivo distribution of forces along the spine. Computer modelling (ANSYS) was used to investigate the possibility of using an instrumented hook to intra-operatively measure the antero-posterior and distraction/compression forces applied by the surgeon during corrective scoliosis surgery. Three hook designs were evaluated based on specific design criteria. ANSYS provided the preliminary analysis to determine the strain distribution in these hooks. One design, the "membrane" design, was selected and a prototype was manufactured. Preliminary tests demonstrate that this prototype will be able to differentiate between the four major forces applied during the surgical correction.