William A. Hyman

Principles of bone healing and biomechanics of external skeletal fixation.

External skeletal fixation is being used to treat an increasing number of orthopedic conditions in veterinary medicine. Study of the variables affecting the biomechanics of external fixation and bone healing is vital if patient morbidity is to be minimized. These are reviewed and incorporated into strategies that can be applied to decision making using external fixation in the clinical setting.

Fault Tree Analysis of Clinical Alarms

Clinical alarms have a deceptively simple purpose, which is to notify caregivers when a patient or a device needs their attention. This simple concept has been proven to be challenging as the number of available alarms has grown and been poorly integrated. When an anticipated notification is not received or an actual notification is not acted upon in a timely manner, patient harm can occur. In this regard, false alarms have been proven to be highly detrimental to the effective use of clinical alarms to enhance patient care. Equally problematical is the issue of false reliance in which a clinicians vigilance is degraded by the expectation that if anything bad happens, the system will notify him or her. Similarly, alarms have also been part of staff downsizing and shifting to lower expertise, wherein it is believed that the alarms are an appropriate substitute. Human factor issues associated with setting, observing, and responding to alarms have also been proven to be inadequately addressed. This article presents a fault tree analysis of the patient harm-related clinical alarms failures. This analysis can be used to understand, debate, and educate.

Modeling of failure mode in knee ligaments depending on the strain rate

BackgroundThe failure mechanism of the knee ligament (bone-ligament-bone complex) at different strain rates is an important subject in the biomechanics of the knee. This study reviews and summarizes the literature describing ligament injury as a function of stain rate, which has been published during the last 30 years.MethodsThree modes of injury are presented as a function of strain rate, and they are used to analyze the published cases. The number of avulsions is larger than that of ligament tearing in mode I. There is no significant difference between the number of avulsions and ligament tearing in mode II. Ligament tearing happens more frequently than avulsion in mode III.ResultsWhen the strain rate increases, the order of mode is mode I, II, III, I, and II. Analytical models of ligament behavior as a function of strain rate are also presented and used to provide an integrated framework for describing all of the failure regimes. In addition, this study showed the failure mechanisms with different specimens, ages, and strain rates.ConclusionThere have been several a numbers of studies of ligament failure under various conditions including widely varying strain rates. One issue in these studies is whether ligament failure occurs mid-ligament or at the bone attachment point, with assertions that this is a function of the strain rate. However, over the range of strain rates and other conditions reported, there has appeared to be discrepancies in the conclusions on the effect of strain rate. The analysis and model presented here provides a unifying assessment of the previous disparities, emphasizing the differential effect of strain rate on the relative strengths of the ligament and the attachment.

Theoretical evidence for the generation of high pressure in bone cells

Abstract A simple model of the mechanical behavior of the bone cell-bone matrix composite has been used to compute the hydrostatic pressure which may be developed within osteocytes when bone is placed under stress. Using current estimates of the elastic properties of long bones and single osteons it is shown that predicted pressures are within the range known to affect cellular function.

The Theory and Practice of Preventive Maintenance

Preventive Maintenance (PM) is a core function of clinical engineering, having as its objective the assurance of ongoing safety and performance of medical devices, and the preservation of the investment in the equipment through improved longevity. Despite its core role, however, the design and management of an effective PM program is not a simple matter. Key issues arise with respect to defining targeted PM procedures and frequencies based on real world assessment and data on failure modes, relative risk, effectiveness of specific PM functions, and incorporation of field experience. Assessment and improvement of the PM program is also an ongoing issue, with the goal of controlling or reducing cost while raising internal and external effectiveness. PEER-REVIEW PAPER

A Simplified Model of the Oxygen Supply Function of Capillary Blood Flow

An analytic model of the role of capillary blood flow in supplying oxygen to tissue has been developed. This model is less complicated than previously used models, yet may be more realistic. The fundamental point of departure here is that consideration of the radial concentration gradients in the capillary is avoided by assuming the capillary to be “well-stirred”. The source-sink role of the erythrocyte with respect to oxygen is retained in a distributed sense. The inhomogeneity of blood on the scale of the capillary as well as the phenomenon of bolus flow justify this approximation. A closed form solution to the resulting equations is readily obtained and compares favorably with the results of computer based computations of axial oxygen pressure profiles. This model is therefore expected to be of significant value in further work.

The effect of myoglobin on the oxygen concentration in skeletal muscle subjected to ischemia.

A mathematical model of oxygen transport in skeletal muscle is utilized to investigate myoglobins effect on the space-time distribution of oxygen in skeletal muscle subjected to ischemia.The model is formulated using the Krogh cylinder as the geometrical representation of the functional unit of transport, i.e., a capillary and the surrounding tissue it supplies. The model includes the convective and diffusive transport of oxygen within the capillary-tissue system, a nonlinear oxygen consumption rate, and the reversible reaction of oxygen with hemoglobin in capillary blood and with myoglobin in the muscle tissue.Myoglobins role in the maintenance of an adequate oxygen supply for use by the muscle is demonstrated by observing the transient capillary tissue systems response to a 60 sec capillary occlusion. Solutions are obtained for the model with and without myoglobins presence. A comparison of solutions is presented for two discrete points in the tissue region; namely, the so-called lethal corner which occurs at the venous end at the outer radius of the tissue cylinder and an average point found in the center of the tissue region. Three-dimensional oxygen distributions as a function of time are also presented.

The hemodynamics of arterial steal

Abstract Clinically significant conditions sometimes result in a flow reduction to one downstream branch of a major bifurcation as the result of a reduction in the resistance of the contralateral branch. This condition has been demonstrated at several major bifurcations in man and is known as “steal”. In this paper this phenomenon is modeled using the parameters of mean pressure, flow, and resistance in the upstream and two downstream vessels. The model is qualitatively confirmed in dogs by a pressure flow study at the external iliac trifurcation using manually controlled distal resistances. Flows were measured electromagnetically, and pressures by a fluid-filled catheter and diaphram LVDT transducer. As a result of this study a simple test is devised to determine if a site selected for vascular surgery has a potential for developing steal.

Research note: driver's brake reaction times with adaptive controls

This report presents the results of a preliminary study of the response in braking of adaptive hand controls such as those used by some physically handicapped drivers. The response time of a simulated pistol-grip controller with trigger-activated brakes was also tested. These results show significantly shorter response time for the hand-operated units than for the normal foot controls.

The distribution of induced electrical activity in bent long bone

Abstract An analytical model based on classical piezoelectric theory was used to related electrical effects to the stress-state in a bone cross-section for simple bending of a cylindrical bone shaft. The results demonstrate that the overall polarities of the polarization volume charge and the induced electrical potential compare favorably with available experimental findings. Both the polarization charge and the induced electrical potential are shown to be highly sensitive to the angle between the material and bone axes between 0 and 10 degrees. A reduction of the bone cross-sectional area results in a higher magnitude in the polarization charges and the induced electrical potential at all points in a cross-section. No appreciable difference in the predicted electrical effects is observed for the use of two relevant piezoelectric d -matrixes.