Student Researcher

Meghan McGee

Project Title

The effects of annual hibernation on the structural properties of black bear femurs

Sponsor

MTU SURF Program, Michigan Space Grant Consortium

Abstract

It is known that bone strength is greatly reduced by disuse such as prolonged time in space and extended periods of bed rest due to injury, illness, and other causes.  This condition is known as disuse osteoporosis and is due to a lack of mechanical stimulation to the tissue, which leads to increased bone resorption and decreased bone formation.  It takes a remobilization period of approximately 2-3 times the length of the disuse period to regain lost bone strength. This remobilization period is not available for black bears, which experience annual disuse by hibernating for 6 months of the year.  It would be expected that the cumulative deficit in regained bone each year would contribute to a decline in bone strength with age in bears. However, it has been shown that unlike other species, black bears maintain normal bone formation during periods of disuse (hibernation) and are able to elevate bone formation during remobilization.  Furthermore, though their disuse and remobilization periods are approximately equal in length, their cortical bone material properties do not decline with age.  However, changes in bone architecture may have a stronger effect on whole bone strength than changes in material properties.  The purpose of this study was to determine if the structural properties of black bear bones are negatively affected by annual periods of disuse.  We hypothesized that the mechanical properties of black bear femurs would not decline with age because bone formation does not decline during disuse and is greatly elevated during remobilization. For this study, 14 black bear femurs were removed from hunter-killed bears.  Once cleaned of soft tissue, they were tested to failure in three-point bending while force and displacement were recorded.  After fracture, the bones were reconstructed and histological sections of the fracture were prepared.  Segments of the bone were also removed for later microstructural and compositional analysis.  The cross-sectional moment of inertia at the fracture was determined using a custom-written macro.  Ultimate stress and energy to failure were calculated from the load-displacement data and regressed against age (an indicator of annual disuse periods).  It was found that when males and females were considered separately, ultimate stress did not change with age for males or females.  Energy to failure also did not change with age for females, but significantly (p=.034) increased with age for males.  When males and females were grouped, ultimate stress significantly (p=.023) increased with age whereas energy to failure did not change with age.  This is further evidence that black bears possess a mechanism that helps them to avoid the deleterious effects of disuse on bone strength.  This study is part of a larger project that may lead to the development of therapies that help to combat disuse osteoporosis.

Advisor(s)

Dr. Tammy Haut Donahue

Assistant Professor, MEEM

Student Researcher

Sarah Magee

Project Title

Time Dependent and Failure Properties of Novel Anterior Cruciate Ligament Replacement Grafts

Sponsor

Linvatec Inc

Abstract

2005 Summer Bioengineering Conference, June 22-26, Vail Cascade Resort & Spa, Vail, Colorado Clinical Relevance: If the surgeon chooses to use an allograft to reconstruct a torn anterior cruciate ligament (ACL), then the Tutoplast® patellar (PT), achilles (AT) or folded anterior tibialis (ATT) tendons can be expected to provide structural properties similar to or better than the fresh frozen non-irradiated patellar tendon (FPT) graft. Introduction: Allograft tissue for ACL reconstruction is scarce because of limited donor availability. Other tendons could be used as an ACL graft if their structural properties were similar to currently used tendons such as the FPT graft. The purpose of this study was to compare the structural, mechanical, and viscoelastic properties of sterile, solvent dehydrated AT, PT and ATT grafts to the gold standard of the fresh frozen patellar tendon grafts. The AT, PT and ATT were prepared using a unique preservation technique to sterilize the graft as well as provide a longer ’shelf-life’. Methods and Materials: All grafts were harvested from middle-aged donors (average age = 54 yrs, range 25-76 yrs). The AT, PT and ATT samples were hydrated according to the manufacturer’s protocol (Tutogen Medical Corp.) and FPT samples were thawed at room temperature prior to testing. ATT samples were folded in half to form the graft. FPT samples were bisected longitudinally using an oscillating bone saw to form grafts. The cross-sectional area and length of each graft was measured. Tests were conducted on a servo-hydraulic uniaxial test machine (Instron Corp, Model 8872). Each graft was mounted in previously described grips [1-3]. ATT testing was performed by looping the middle of the tendon over a bar at the base of the materials testing machine and gripping the two free ends with a freeze clamp. PT and FPT testing proceeded by potting each bone block end into tensile fixtures. AT testing included potting the bone block into a tensile fixture while holding the other end with a freeze clamp. Grafts were pre-tensioned at an initial load of 75 N for 5-10 minutes under displacement control in series with an extension coil spring, which simulated the stiffness and travel of the Linvatec graft-tensioning device used intra-operatively (spring rate of 35 lb/in). To characterize the viscoelastic properties, each graft was preconditioned for 10 cycles at 0.1 Hz between 0 and 3% strain. The 11th cycle was held at 3% strain while the sample relaxed. This displacement was held constant while the decreasing load was measured at 4 Hz until the load changed less than 0.1 percent over 1 minute. After a sufficient time for recovery, as determined in preliminary testing (24 hours), the grafts were cycled between 20 and 100 N at a rate of 0.33 Hz until less than a 0.1% change in displacement was seen with consecutive cycles. This loading represented intra-operative in vivo preconditioning of the graft prior to tibial fixation. To characterize the failure properties, load-to-failure tests were performed following the creep test after full recovery of the graft (24 hrs). Each graft was preconditioned as described above and then pulled to failure at a strain rate of 2 percent/s [1,2]. Structural and mechanical properties were determined for each graft and compared using ANOVA. Results: The PT relaxed statistically slower than the ATT and AT, while the change in stress over the test period showed the ATT to be statistically greater than the 3 remaining grafts. Finally, there were no statistical differences between the 4 grafts for the stress at the end of  time dependent and failure properties of novel anterior cruciate ligament replacement grafts.

Dr. David Nelson

Professor, Biomed Engineering

Student Researcher

Katelin Engerer

Project Title

Tissue Characterization and Identification for Predicting Frequency Energy Deposition Rates

Sponsor

MTU SURF Program

Abstract

Models of Human exposure to electromagnetic energy of radio frequency (RF) are usually based on calculation of energy absorption per unit mass (the 'specific absorption rate' or SAR).  Standards for safe exposure to RF are based on the SAR, and not on temperature.  As the known bio-effects of RF exposure is heating, it would be useful to have model which predicts temperature increase resulting from exposure.    The ability to predict tissue temperature increases requires development of a detailed database of tissue properties (thermal and electric).  The objective of the current study was two-fold: 1) Incorporate physiological feedback in a working thermal model of human whole-body RF exposure.  This model, “ThermoReg,” uses a database derived from the National Library of Medicine’s “Visible Man” model.  It does not, however, include several tissues which are significant in determining human response to radiation absorption.  Specifically it does not include the hypothalamus, nor does it distinguish between skeletal and smooth muscle.   2) Develop a detailed model of the skin anatomy.  This is especially important for simulating thermal effects of high-frequency exposure (GHz range and higher), where RF energy penetration is shallow and differences in skin composition may effect heating significantly.      Results from the addition of the new tissue types show incorporation of hypothalamic feedback enables simulation of thermoregulatory responses to environmental heating.    The skin model is based on a histological study of the skin in the ear of the domestic rabbit, which is an accepted thermoregulatory model of the human skin.  Identification of the corresponding electrical and thermal properties is in progress.