Eric Abel

A finite element analysis of hollow stemmed hip prostheses as a means of reducing stress shielding of the femur

Stress shielding of the femur is known to be a principal factor in aseptic loosening of hip replacements. This paper considers the use of a hollow stemmed hip implant for reducing the effects of stress shielding, while maintaining acceptably low levels of stress in the cement. Using finite element modelling, the stresses in the proximal femur using different shapes of hollow stem were compared with those produced using comparable sizes of solid stem with different values of elastic modulus. A reduction in stress shielding could be achieved with a hollow stem. A cylindrical hollow stem design was then optimised in order to control the maximum allowable stress in the cement, the minimum allowable stresses in the bone, and a combination of the two. The resulting stems achieved an increase in proximal bone stress of about 15% for the first case and 32% for a model using high strength cement, compared with solid stems of the same nominal outside diameter. The gains of these theoretically optimised designs dropped off rapidly further down the stem. Linearly tapered hollow stems reached a 22% gain, which could be a good compromise between acceptable cement stresses and ease of manufacture.

Ossicular replacement prosthesis

A fixed footplate in a well aerated middle ear devoid of ossicles is generally acknowledged to be a difficult otologic reconstructive problem. A rapid method employing readily available materials is presented with several case reports. The method employs a combined alloplastic-tissue prosthesis which is easily assembled and inserted.

Mean platelet volume is a useful parameter: a reproducible routine method using a modified Coulter Thrombocytometer

The principal physiological function of platelets is to promote haemostasis but they also contribute to thrombosis and atherogenesis. Platelet volume is a marker and possibly a determinant of platelet function in that large platelets are more active than normal sized platelets. Mean platelet volume (MPV), a measure of platelet size, reflects changes in either the level of platelet stimulation or the rate of platelet production. For these reasons, we have developed a sensitive instrument to measure platelet volume, which we believe is more reliable and specific than previously used instruments. It is based on a computer - interfaced Coulter Thrombocytometer and a pulse analyser including a high - speed baseline restorer. We have developed a reproducible method to assay MPV and from the histogram derived the median (MED) and the skewness (SK) values. We have looked at the effects of anticoagulant used and time elapse prior to assay. A normal range has been established for MPV which correlates directly with MED and inversely with SK. The MPV decreases with age but there is no difference between genders. We have demonstrated a negative correlation between whole blood platelet number and MPV and MED, and a direct relationship with the SK of the histogram of the platelet volume.The principal physiological function of platelets is to promote haemostasis but they also contribute to thrombosis and atherogenesis. Platelet volume is a marker and possibly a determinant of platelet function in that large platelets are more active than normal sized platelets. Mean platelet volume (MPV), a measure of platelet size, reflects changes in either the level of platelet stimulation or the rate of platelet production. For these reasons, we have developed a sensitive instrument to measure platelet volume, which we believe is more reliable and specific than previously used instruments. It is based on a computer-interfaced Coulter Thrombocytometer and a pulse analyser including a high-speed baseline restorer. We have developed a reproducible method to assay MPV and from the histogram derived the median (MED) and the skewness (SK) values. We have looked at the effects of anticoagulant used and time elapse prior to assay. A normal range has been established for MPV which correlates directly with MED and inversely with SK. The MPV decreases with age but there is no difference between genders. We have demonstrated a negative correlation between whole blood platelet number and MPV and MED, and a direct relationship with the SK of the histogram of the platelet volume.

Issues concerning the measurement of transformation temperatures of NiTi alloys

The transformation between martensite and austenite in shape memory alloy is characterized by four transformation temperatures: martensite start temperature (Ms), martensite finish temperature (Mf), austenite start temperature (As) and austenite finish temperature (Af). In actuator designs that rely on the shape memory effect, it is important to obtain an accurate measure of these transformation temperatures, especially As and Af, because they determine the functional temperature range of the actuator. Several methods of determining these temperatures have been reported, but their accuracy and coherence are not clear. The transformation temperatures of NiTi wire under different heat treatment conditions were measured by the three commonly used methods: differential scanning calorimetry (DSC); an electrical resistance method, which uses a sudden change in resistance as an indication of transformation; and an applied loading method, where a macroscopic displacement indicates the transformation. The results show that the transformation temperatures measured by DSC do not correspond to those measured by the other two methods, which are similar. The applied loading method is most effective for providing practical information about the stress-dependent transformation temperatures. The electrical resistance method gives clearly determined points for Ms and Mf in the cooling resistance–temperature curve, but As and Af are not clearly identifiable in the heating process.

The development and validation of an algorithm for real‐time computerised fetal heart rate monitoring in labour

Objective To develop and validate a computerised algorithm for the interpretation of the characteristics of fetal heart rate monitoring in labour.

Interscale wavelet maximum - a fine to coarse algorithm for wavelet analysis of the EMG interference pattern

A method has been developed, interscale wavelet maximum (ISWM), for characterising the electromyogram (EMG) interference pattern to assist in the diagnosis of neuromuscular disease. EMG signals are decomposed with the redundant dyadic wavelet transform and wavelet maxima (WM) are found. Thresholding methods are applied to remove WM due to noise and background activity. An efficient fine-to-coarse algorithm identifies the WM tree structure for the motor unit action potential rising edges. The WM for each tree are summed at each scale; the largest value is the ISWM. Highly significant differences in ISWM values have been found between healthy, myopathic, and neuropathic subjects that could make the technique a useful diagnostic tool.

An anatomically shaped incus prosthesis for reconstruction of the ossicular chain

An anatomically shaped incus replica prosthesis has been designed to reconstruct the ossicular chain. A series of in vitro studies on human temporal bones evaluated the acoustic performance of this prosthesis and compared it with a Causse partial ossicular replacement prosthesis (PORP). Pure tones in the frequency range 0. 125-8 kHz stimulated the tympanic membrane at sound pressure levels of 80, 90 and 100 dB. Measurements of the stapes footplate velocity were made with a laser interferometer. The acoustic function of the ossicular chain reconstructed with the incus replica prosthesis was found to be within 10 dB of that of the original intact ossicular chain, when both the upper and lower joints of the implant were rigidly fixed in place. It was shown that a rigid mechanical contact between the ossicular prosthesis and ossicles is a prerequisite for effective sound transmission. The anatomically shaped incus prosthesis gave a 15-dB improvement on the PORP at frequencies below 1.5 kHz.

Preliminary Assessment of Remote Photoelectric Excitation of an Actuator for a Hearing Implant

Sound modulated infrared (IR) light, as an alternative signal (and energy) transmission method, was evaluated for remote powering of a hearing implant via an implanted photodiode receiver. One human cadaveric temporal bone was used to test this concept by locating an IR source (LED) in the ear canal 4 mm away from the eardrum and a photodiode in the middle ear cavity. The photodiode output was directly used to drive a piezo-ceramic disc actuator which was located externally for facilitating the test. The displacement frequency response of the actuator was measured by a laser vibrometer. It was found that, at a LED current of 8.6 mA, the actuator displacement was about 22.4 nm, equivalent to a stapes displacement from about 91 dB sound-pressure-level (SPL) stimulation at the eardrum. The IR light transmittance through the eardrum (compared to through an air path) was about 67%. The LED optical power was 0.77 mW (its maximal irradiance was about 3.95 mW/cm2) which is a safe to the human body. Current consumption can be reduced by a factor of 3-5 by using a more efficient LED and photodiode

Assessment of multi-layer piezoelectric actuator technology for middle-ear implants

Abstract A small multi-layer piezoelectric actuator, which contains a stack of piezoelectric crystals in a thin-layered compact structure, was evaluated for its suitability as a stimulator in middle ear hearing implants. Using laser Doppler vibrometry for non-contact vibration measurement, the actuator was found to have a useful bandwidth spanning the range of audio frequencies. During simulated surgical implantation on human temporal bones, one side of the actuator was cemented to the medial wall of the attic of the middle ear and the other side was cemented to the body of the incus. Stapes displacement from actuator excitation at 3.5 V rms was equivalent to that from acoustic stimulation at 90 dB SPL below 1 kHz, increasing to 120 dB at 8 kHz, which is more than adequate stimulation of the ossicular chain. The total harmonic distortion (THD) was less than 2.5% over the audio frequency range. The corresponding power consumption was 0.7 mW per volt of excitation at 1 kHz, which is low enough for the actuator to be used in a partially implantable hearing implant. The surgical implantation and positioning of this type of actuator are straightforward due to the simple linear motion it provides.

A comparison of methods for the training of NiTi two-way shape memory alloy

The creation of an effective two-way shape memory alloy (TWSMA) requires appropriate heat treatment and optimal training considerations. In particular, the training method used plays a key role. This work investigates different training methods for producing NiTi TWSMA wires with the hot shape of an arc and the cold shape of a straight line. These methods are shape memory cycling, constrained cycling of deformed martensite, pseudoelastic cycling and combined shape memory and pseudoelastic cycling. In order to give a meaningful evaluation of their performance that is relevant to training TWSMA for practical applications, these training methods are assessed in terms of maximum two-way strain, changes in the original hot shape together with the transformation temperatures after the training process, and the effective production of the cold shape. It was found that only the combined shape memory and pseudoelastic cycling provides an effective training method for creating NiTi TWSMA with a non-uniaxial two-way shape change. The undesirable side effects of training are that the NiTi TWSMA wire loses partial memory of the original hot shape and its transformation temperatures shift to lower values. There also exists an optimal number of training cycles, and possibly an optimal training load for obtaining the best cold shape memory and the greatest two-way recoverable strain. These findings give future directions to advance the training technology for TWSMA.