Lyle D. Zardiackas

Computer simulation of trabecular remodeling using a simplified structural model

A simplified three-dimensional simulation of trabecular bone remodeling has been developed. The model utilizes 441 planar structural units to represent approximately 50 mm3 of initial bone volume with 199 basic multicellular units (BMUs). The simulation takes into account trabecular perforation in the structural model. The cases of male bone remodeling with no menopause and female bone remodeling with menopause are examined from the period of simulated age 25-80 years. Menopause is arbitrarily started at age 45 and extends for 7.5 years. Zero-, first-, and second-order BMU activation responses are employed to examine how the bone would be affected by the method of increase of BMU activation during menopause. At age 80, the female bone remodeling simulation produced a bone volume loss of approximately 49% for all three activation responses. This compared to a 38% bone volume loss for the case of no menopause. For the menopause simulations, an average of about 40% of the total bone loss was due to perforation.

Cortisone, vitamin A, and wound healing: The importance of measuring wound surface area

Traditional methods of measuring wound strength have often ignored one of the most important variables, i.e., wound surface area. Using a readily reproducible method of determining wound surface area, the effect of topical vitamin A on steroid-treated rats was studied. Findings revealed a significant (P less than 0.001) increase in wound strength with topical vitamin A application. No difference could be detected between steroid-treated and saline-treated rats when wound surface area was taken into account. Measurement of wound surface area is essential for meaningful results in wound healing studies.

Response of canine bone to a synthetic bone graft material

A model simulating a spiral diaphyseal fracture with butterfly fragments and bone loss was utilized to evaluate an hydroxyapatite/tricalcium phosphate, and collagen composite bone graft substitute in twelve dogs. The resultant grafted and contralateral control femora were tested in torsion at one year. This study examines the histological response to the graft material as well as crack propagation and fracture surface morphology using light microscopy and SEM. SEM and gross evaluation of the grafted bones revealed that 8/12 had fractured through bone outside the osteotomy site and all fractures included bone outside the graft site. No graft material was demonstrated at the points of initiation or termination of fracture for any of the bones. It was apparent that recorticalization had begun to occur at the graft site but the canal had not yet fully formed. The HA/TCP was seen to be tightly bound in tissue which had the appearance of new bone. Bone was found to be in direct apposition to the surface of the ceramic and within pores with no intervening soft tissue. Much of the new bone had remodeled into well organized Haversian systems with some patchy areas of woven bone and osteoid seen with polarized light illumination.

Comparison of the Corrosion Fatigue Characteristics of 23Mn-21Cr-1Mo Low Nickel, 22Cr-13Ni-5Mn, and 18Cr-14Ni-2.5Mo Stainless Steels

The purpose of this study was to evaluate and compare the notched corrosion fatigue (CF) characteristics of a series of stainless steel alloys including 18Cr-14Ni-2.5Mo (ASTM F 138), 22Cr-13Ni-5Mn (ASTM F 1314), and 23Mn-21Cr-1Mo low nickel stainless steel (ASTM F 2229). Evaluation of alloy composition, microstructure, static single cycle mechanical properties, stress corrosion cracking, and smooth sample corrosion fatigue was performed on the same lots of each material in a previous study in our laboratories. Notched CF tensile samples of each alloy were machined using low stress grind techniques to a 2.5 mm notch root diameter (Kt=3.2) and tested at 1 Hz according to ASTM F 1801 in Ringers solution at 37°C. A minimum of three samples was tested at five tension-tension sinusoidal load levels including a run-out level at 106 cycles. SEM analysis was performed on the fractured surfaces of representative samples of each alloy to characterize and compare the failure mechanisms. Previous evaluation of corrosion fatigue on smooth samples of these alloys found no significant differences in the fracture modes or cycles to failure between the samples run in distilled deionized water and those run in Ringers solution indicating corrosion fatigue mechanisms were not in effect. However, a significant lowering of fatigue strength was observed for the notched samples of each alloy compared to the smooth samples previously tested. This suggests that even though the notch sensitivity of these alloys under static single cycle conditions has been shown to be acceptable, a notch under dynamic fatigue mechanisms may cause a substantial reduction in strength and implant life. Despite this reduction in fatigue strength, in high strength applications where the implant must be bent during surgery or may be scratched during implantation BioDur 108 proved to be the best alternative. For lower strength applications, 316LS revealed the least fatigue notch sensitivity of any of the alloys tested.

The mechanical integrity of healed diaphyseal bone defects grafted with calcium hydroxyapatite/calciu triphosphate ceramic in a new animal model

The need for an animal model to test bone graft materials simulating a weight bearing clinical situation is identified. The concept, design and operative detail of a new model is described. This model involved the creation of a mid-diaphyseal wedge defect in the femur of the adult beagle which separated both cortices, plating with a six-hole dynamic compression plate, and allowed immediate full weight bearing. At six months plates were removed and immediate weight bearing was allowed for an additional six months to sacrifice. The initial animal project utilizing this model to evaluate a hydroxyapatite based synthetic graft material was performed using 12 dogs. In addition to the operative procedure, the retrieval testing in torsion of 12 healed grafted bones and their 12 contralateral unoperated controls is described and evaluated. Results showed no statistically significant difference between the torsional strength of test and control femurs (p less than or equal to 0.05). In addition, the future development of the model is discussed.

Comparison of the Corrosion Fatigue Characteristics of CP Ti-Grade 4, Ti-6Al-4V ELI, Ti-6Al-7Nb, and Ti-15Mo

The purpose of this study was to evaluate and compare the corrosion fatigue (CF) characteristics of a series of titanium alloys including Grade 4 CP Ti (ASTM F 67), Ti-6Al-4V (ASTM F 136), Ti-6Al-7Nb (ASTM F 1295), and Ti-15 Mo (ASTM F 2066). Evaluation of alloy composition, microstructure, and Vickers microhardness was performed to ensure that each material met the required specification. Smooth and notched CF tensile samples of each alloy were machined using low stress grind techniques and tested at 1Hz according to ASTM F 1801 in both Ringers solution and distilled/deionized water at 37°C. Smooth CF samples had a 10 mm gauge length and a 2.5 mm gauge diameter, and notched CF samples had a 2.5 mm notch root diameter (Kt=3.2). A minimum of three samples was tested at five tension-tension sinusoidal load levels including a run-out level at 106 cycles. SEM analysis was performed on the fractured surfaces of representative samples of each alloy to characterize and compare the failure mechanisms. Fatigue results revealed no differences between the smooth and notched samples of each alloy run in distilled water and those run in Ringers solution. Results indicated corrosion fatigue mechanisms were not contributing to the fractures under these conditions. However, a significant reduction of fatigue strength was observed for the notched samples of each alloy compared to the smooth samples in identical solutions. These results suggest that even though there is no notch sensitivity in these alloys under static conditions, a notch under dynamic fatigue mechanisms may cause a substantial reduction in cycles to implant fracture. SEM analysis showed typical fatigue fracture morphologies on both the smooth and notched samples. Notch samples, however, did show more defined fluting in terracing due to the higher tri-axial stress state at the root of the notch. In conclusion, corrosion fatigue mechanisms were not contributing to the fracture of these alloys under the given conditions, but the presence of a notch significantly reduced the fatigue strength of all alloys tested.

A comparison of the stress corrosion cracking susceptibility of commercially pure titanium grade 4 in Ringer's solution and in distilled water: a fracture mechanics approach.

From the results of laboratory investigations reported in the literature, it has been suggested that stress corrosion cracking (SCC) mechanisms may contribute to early failures in titanium alloys that have elevated oxygen concentrations. However, the susceptibility of titanium alloys to SCC in physiological environments remains unclear. In this study, a fracture mechanics approach was used to examine the SCC susceptibility of CP titanium grade 4 in Ringers solution and distilled de-ionized (DI) water, at 37°C. The study duration was 26 weeks, simulating the non-union declaration of a plated fracture. Four wedge loads were used corresponding to 86-95% of the alloys ligament yield load. The longest cracks were measured to be 0.18 mm and 0.10 mm in Ringers solution and DI water, respectively. SEM analysis revealed no evidence of extensive fluting and quasi-cleavage fracture features which, in literature reports, were attributed to SCC. We thus postulate that the Ringers solution accelerated the wedge-loaded crack growth without producing the critical stresses needed to change the fracture mechanism. Regression analysis of the crack length results led to a significant best-fit relationship between crack growth velocity (independent variable) and test electrolyte, initial wedge load, and time of immersion of specimen in electrolyte (dependent variables).

Comparison of Stress Corrosion Cracking Characteristics of CP Ti, Ti-6Al-7Nb, Ti-6Al-4V, and Ti-15Mo

The purpose of this research was to evaluate and compare the stress corrosion cracking (SCC) of Ti-15 Mo (ASTM F2066), Ti-6Al-7Nb (ASTM1295), Ti-6Al-4V ELI (ASTM F136), and Grade 4 CPTi (ASTM F67). Evaluation of alloy composition, microstructure, Vickers microhardness, and tensile properties was performed to determine compliance with the appropriate ASTM specification. For SCC testing, smooth tensile samples with a gauge length of 10 mm and a gauge diameter of 4 mm and notched samples with a notch root diameter of 4 mm (Kt=3.2) were prepared using low stress grinding procedures. Three smooth and three notched samples were tested in distilled H2O and Ringers solution at 37°C. Testing was performed using the slow extension rate method at a stroke rate of 10−5 mm/s according to the guidelines established in ASTM G129. The ratio of the percent elongation (PER) and reduction of area (ROAR) of smooth and notched samples tested in Ringers solution and distilled water were evaluated. The fracture surfaces of representative samples were also examined for fracture mode identification using SEM. Results showed that all alloys complied with the appropriate ASTM specification. SCC may be considered to occur in a material if the ductile properties in an aggressive media are inferior to the ductile properties in a non-aggressive media. Evaluations of the PER and ROAR ratios in smooth and notched samples for each alloy in Ringers and distilled water showed no indication of SCC failure mechanisms. SEM examination of the fracture surfaces showed no differences in the fracture morphology regardless of the testing solution. These results were consistent with the mechanical testing data. It is therefore concluded that SCC mechanisms were not operating or contributing to the fracture of these alloys under the conditions evaluated.

Comparison of Stress Corrosion Cracking and Corrosion Fatigue (Anodized and Non-Anodized Grade 4 CP Ti)

In light of the possible effects of anodization on stress corrosion cracking (SCC) and corrosion fatigue (CF) of CP titanium, a research project has been completed recently in our laboratories to evaluate and compare SCC and CF of anodized versus non-anodized samples from a single lot of Grade 4 CPTi. Initial evaluation of alloy composition, microstructure, Vickers microhardness, and mechanical properties including the tensile and yield strength, % elongation and reduction of area was performed. After these tests ensured the material met the standards of ASTM Standard Specification for Unalloyed Titanium for Surgical Implant Applications (F 67), samples were prepared using low stress grinding techniques. Samples were divided into two groups, and the non-anodized SCC and CF testing was initiated. The surface of the second group of samples was anodized by Synthes to provide a green/gold surface consistent with standard production processing and then tested using the identical methodology as used for the non-anodized samples. Results of the slow strain rate SCC testing on both smooth and notched anodized and non-anodized samples in both distilled de-ionized water and Ringers solution at 37°C showed no effect of anodization. Results of corrosion fatigue testing indicated that, while there was no effect of anodization on corrosion fatigue, there was a significant effect of the notch on the fatigue characteristics regardless of the two different surface conditions.