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Biomedical subjects

D R Carter

Publications and source records attributed to D R Carter.

156 records · Page 9Linked to original sources
PubMedDate not supplied

The role of loading memory in bone adaptation simulations.

The concept that bone responds to a time-averaged value of its current mechanical loading forms the basis for many computational bone adaptation algorithms. Some mathematical formulations have incorporated a quantification of the loading experienced during a single "average" day and thus implicitly assume that bone responds abruptly to changes in its loading history. To better reflect the time delays inherent in bone cell recruitment and activation processes, we included a fading memory of past loading. Implementing an exponentially fading memory with time constants of 5, 20, and 100 days, we simulated bone adaptations to abrupt and gradual changes in mechanical loading. Both an idealized single degree-of-freedom model and a finite element model of the proximal femur were studied. A time constant of 5 days produced time-dependent density changes that were negligibly different from those of the standard approach without memory. Models with higher time constants produced significant transient time lags (up to 8.1% difference) in the predicted short-term (3 months) bone density changes. A time constant of 100 days produced overshoots (by approximately 1%) of the eventual steady-state. All models predicted comparable long-term (after several years) steady-state adaptations. Future experimental analyses will be necessary to better determine appropriate fading memory time constants for bone under various loading conditions.

Adaptation, Physiological↗
PubMedDate not supplied

Effect of cutting flute design on cortical bone screw insertion torque and pullout strength.

OBJECTIVE: To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws. DESIGN: Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured. SETTING: The study was conducted at an experimental biomechanics laboratory associated with a university medical center. OUTCOME MEASUREMENTS: Insertion torque and pullout strength were normalized by the local bone mineral density. RESULTS: The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05). CONCLUSIONS: Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.

Biomechanical Phenomena↗
PubMedDate not supplied

The anatomy of the subglottic larynx.

There is little concensus concerning the boundaries of the subglottic space. To better define the subglottis, 50 head and neck surgeons were asked to define its anatomic limits. In addition, the larynx of each of 50 adult cadavers was removed and four measurements in the anterior subglottis were taken. The results revealed significantly smaller subglottic dimensions for women than for men. In most women, the cricothyroid membrane was situated less than 1 cm from the glottis in the anterior midline.

Adult↗
PubMedDate not supplied

Anatomy of the subglottic larynx.

There is little concensus concerning the boundaries of the subglottic space. To better define the subglottis, 50 head and neck surgeons were asked to define its anatomic limits. In addition, the larynx of each of 25 male and 25 female adult cadavers was removed, and four measurements in the anterior subglottis were taken. The results revealed significantly smaller subglottic dimensions for the women than for the men.

Adolescent↗
PubMedDate not supplied

Endoscopic nomenclature for bronchopulmonary anatomy.

Development of the flexible fiberoptic bronchoscope (FFB) has allowed visualization of the peripheral tracheobronchial tree to the level of fifth-order bronchi. Examination of this area requires an extended anatomic nomenclature to describe the pathology of findings. A system of description is presented that combines the concepts of Jackson and Huber, Boyden, Ikeda, and Weibel into a practical endoscopic shorthand lettering designation for endobronchial anatomy. Using the system, a sub-sub-subsegmental (fifth order) bronchus in the right lung could be designated RB1b1 alpha. The principles are described and illustrated.

Bronchi↗
PubMedDate not supplied

Biomechanics of hyperextension injuries to the cervical spine in football.

The biomechanics of cervical spine hyperextension injuries in football players were assessed by using quasi-static-free body analyses. Three situations, i.e., "cases," corresponding to the loading conditions created by three different helmet designs, were examined with the following assumptions: (1) the helmet rim is cut high enough posteriorly so that it does not impinge upon the posterior cervical spine, (2) the helmet rim impacts at the level of the fourth cervical vertebra, and (3) the posterior rim of the helmet strikes the shoulder pads. The results of the analyses suggest that the most dangerous hyperextension situation is Case 1 which leads to high forces and possibly serious injury to the upper cervical spine. The impact of the posterior rim of the helmet at the fourth cervical vertebra (Case 2) significantly reduces these forces. This finding directly conflicts with the so-called "guillotine" mechanism of injury. The impact of the posterior rim of the helmet on the shoulder pads (Case 3) creates the least hazardous loading conditions.

Athletic Injuries↗
PubMedDate not supplied

Mechanobiology of tendon adaptation to compressive loading through fibrocartilaginous metaplasia.

Tendons that wrap around bones often undergo fibrocartilaginous metaplasia. In this paper, we examine the biomechanical causes and consequences of this metaplasia. We propose an adaptation rule in which tissue permeability changes in response to local cyclic hydrostatic pressures associated with physical activity. The proposed rule predicts the development of a low-permeability region corresponding to the fibrocartilaginous region in a representative wrap-around tendon. A poroelastic finite element model is used to examine the time-dependent fluid pressures and compressive stresses and strains in the solid constituents of the tendon's extrafibrillar matrix. The low permeability in the adapted fibrocartilaginous region maintains fluid pressures, protecting the solid constituents of the tendon's extracellular matrix from high compressive stresses and strains that could disrupt the matrix organization. Adaptation through fibrocartilaginous metaplasia therefore allows wrap-around tendons to function effectively over a lifetime without sustaining excessive mechanical damage due to cyclic compressive loading.

Adaptation, Physiological↗
PubMedDate not supplied

Mechanobiology in the development, maintenance, and degeneration of articular cartilage.

During skeletal development, the establishment of a layer of cartilage at the ends of long bones is intimately linked to the process of endochondral ossification. Previous in vivo studies and computer models suggest that mechanobiological factors can play a key role in modulating cartilage growth and ossification. Specifically, intermittent hydrostatic pressure is thought to maintain cartilage, and shear stresses encourage cartilage destruction and ossification. In the present investigation we examined the combined effects of hydrostatic pressure and shear stress--in the form of an osteogenic index--on the development of a layer of articular cartilage, using an idealized finite element computer model. The results of our analyses provide further support for the view that mechanobiological factors play a key role in regulating the distribution of cartilage thickness and in maintaining a stable cartilage layer at maturity. The model predicts that joints that experience higher contact pressures will have thicker cartilage layers. These predictions are consistent with observations of cartilage thickness in both humans and animals. Variations in articular mechanical load are predicted to modulate cartilage thickness. These results are consistent with the view that the mechanobiological factors responsible for the development of diarthrodial joints eventually lead to cartilage degeneration and osteoarthritis (OA) with aging.

Aging↗
PubMedDate not supplied

Time-dependent effects of intermittent hydrostatic pressure on articular chondrocyte type II collagen and aggrecan mRNA expression.

The normal loading of joints during daily activities causes the articular cartilage to be exposed to high levels of intermittent hydrostatic pressure. This study quantified effects of intermittent hydrostatic pressure on expression of mRNA for important extracellular matrix constituents. Normal adult bovine articular chondrocytes were isolated and tested in primary culture, either as high-density monolayers or formed aggregates. Loaded cells were exposed to 10 MPa of intermittent hydrostatic pressure at a frequency of 1 Hz for periods of 2, 4, 8, 12, and 24 hrs. Other cells were intermittently loaded for a period of 4 hrs per day for 4 days. Semiquantitative reverse transcription polymerase chain reaction assays were used to assess mRNA signal levels for collagen types II and I and aggrecan. The results showed that type II collagen mRNA signal levels exhibited a biphasic pattern, with an initial increase of approximately five-fold at 4 and 8 hrs that subsequently decreased by 24 hrs. In contrast, aggrecan mRNA signal increased progressively up to three-fold throughout the loading period. Changing the loading profile to 4 hrs per day for 4 days increased the mRNA signal levels for type II collagen nine-fold and for aggrecan twenty-fold when compared to unloaded cultures. These data suggest that specific mechanical loading protocols may be required to optimally promote repair and regeneration of diseased joints.

Aggrecans↗
PubMedDate not supplied

Tendon and ligament adaptation to exercise, immobilization, and remobilization.

This study provides a theoretical and computational basis for understanding and predicting how tendons and ligaments adapt to exercise, immobilization, and remobilization. In a previous study, we introduced a model that described the growth and development of tendons and ligaments. In this study, we use the same model to predict changes in the cross-sectional area, modulus, and strength of tendons and ligaments due to increased or decreased loading. The model predictions are consistent with the results of experimental exercise and immobilization studies performed by other investigators. These results suggest that the same fundamental principles guide both development and adaptation. A basic understanding of these principles can contribute both to prevention of tendon and ligament injuries and to more effective rehabilitation when injury does occur.

Achilles Tendon↗
PubMedDate not supplied

A model of mechanobiologic and metabolic influences on bone adaptation.

Bone adaptation, the process through which bone mass is modified in the body, plays a key role in the development of osteoporosis. Bone adaptation is known to be influenced by both mechanical and metabolic stimuli. Previous studies have concentrated on changes in bone adaptation caused by mechanical stimuli (mechanobiologic influences), yet current treatments for osteoporosis depend significantly on metabolic influences. We develop a theoretical model of bone adaptation that accounts for both mechanobiologic and metabolic influences. We demonstrate the utility of this model using a simulation of the cellular processes of bone adaptation on a representative volume of cancellous bone. Our long-term objective is the development of a more comprehensive computational model that will aid in the study of osteoporosis and other bone diseases.

Adaptation, Physiological↗