Biomedical Engineering IV: Recent Developments: Proceeding of the Fourth Southern Biomedical Engineering Conference

Biomedical Engineering IV: Recent Developments contains the proceedings of the Fourth Southern Biomedical Engineering Conference held in Jackson, Mississippi on October 11-12, 1985. The purpose of the annual conference is to bring together scientists, engineers, veterinarians, dental and medical personnel, and graduate and undergraduate students of the southern states for the dissemination of advances in biomedical engineering research. Organized into the 12 sessions of the conference, this book begins with a description of biomaterials, instrumentation, modeling, robotics, and corrosion. Other chapters elucidate soft tissue and orthopedics biomechanics, as well as clinical engineering.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483139470

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Mississippi

Session 1

BIOMATERIALS I

Outline

Chapter 1: EFFECT OF STORAGE ON THE ELECTRICAL PROPERTIES OF BONE

Chapter 2: EFFECTS OF PLA SURFACE MICRO COATINGS ON BONE INGROWTH INTO POROUS CORALLINE HYDROXYAPATITE

Chapter 3: An In Vivo Tissue Response to Ti-6Al-4V/Co-Cr-Mo Implants

Chapter 4: ENHANCEMENT OF THE TI-6AL-4V/UHMWPE WEAR COUPLE THROUGH NITROGEN ION IMPLANTATION

Chapter 5: DEVELOPMENT OF ALUMINO-CALCIUM-PHOSPHOROUS OXIDE (ALCAP) CERAMIC CEMENTS

Chapter 6: TOTALLY RESORBABLE HIGH-STRENGTH BONE PLATE FOR INTERNAL FRACTURE FIXATION

EFFECT OF STORAGE ON THE ELECTRICAL PROPERTIES OF BONE

Subrata Saha, Ph.D and Paul Allen Williams, B.S.,     Biomechanics Laboratory, Dept. of Orthopaedic Surgery, Louisiana State University Medical Center, P.O. Box 33932, Shreveport, LA 71130-3932

ABSTRACT

In this study the effects of various storage environments on the electrical properties of bone were evaluated. Cortical bone specimens from canine femora and tibiae were prepared and divided into three groups with one group maintained at room temperature (24°C), a second group stored in a refrigerator at 3°C, and the third group stored in a freezer at −10°C to −20°C. In each group, both the resistance and the capacitance decreased with time, the percentage of change being maximum for the samples stored in the freezer. This suggests that storage of bone specimens in a refrigerator or freezer with repeated thawing at room temperture does effect the electrical properties of bone, with the effect being dependent on the method of storage.

KEYWORDS

Electrical Properties

Bone

Storage Medium

Resistance

Capacitance

INTRODUCTION

Although Orthopaedic surgeons are increasingly using electrical stimulation to treat non-unions and congenital pseudoarthrosis, the mechanism of action of bio-electricity is still unknown. For a better understanding of the role of electrical stimulation in bone remodeling and for an analysis of the distribution of direct or induced current in bone, we need accurate data on the electrical properties of bone. Although some investigators have measured electrical properties in vivo, this creates uncertainties regarding the current paths between a pair of electrodes placed in such a material and the nature of the tissue-electrode interface (Singh and Saha, 1984). Therefore, in vitro measurement techniques on standardized bone specimens have been utilized to characterize the electrical properties of bone.

With in vitro measurement methods it is important to know how various factors and parameters effect the measured value. Previously, Reddy and Saha (1984) have shown that the electrical properties of bone are anisotropic in nature and frequency dependent. Saha, Reddy, and Albright (1984) have shown that the electrical properties of bone are dependent on the moisture content, temperature, pH, time of exposure to the air, and measurement procedures. Other authors (Kosterich, Foster, Pollack, 1984; Singh and Saha, 1984) have shown that the electrical properties of bone are dependent on the conductivity of the immersion fluid, perserving solution, principles and techniques of measurement, and others. However, the effect of the environment in which the bone sample is stored, when not being measured, has not been properly investigated.

The fact that in most studies the bones are stored frozen or refrigerated prior to testing indicates a need to know the effect of this type of storage on the electrical properties of bone. The objective of this study was to evaluate and determine if such storage changes the electrical properties of bone, and to compare this with other type of storage environments. Three storage environments were chosen which were room temperature, a refrigerator, and a freezer.

METHODS AND PROCEDURES

Canine femora and tibiae were used in the study. The bones were removed soon after the sacrifice of the animal and wrapped in towels soaked in lactated Ringer’s solution to prevent the bones from drying. Two to three centimeter long specimens were then machined from the mid-diaphysis of each bone. Each specimen was then further machined in the axial direction to produce two to four matched specimens from each bone. During the entire machining process the bone were kept moist at all times. After machining, a total number of eleven bone specimens were individually placed into containers with lactated Ringer’s solution and an added bacteriostatic agent.

After the specimens were prepared, the resistance and capacitance were measured using a LCR meter (HP model 4262A), as described before (Saha, Reddy, and Albright, 1984). All measurements were made at 1 kHz. This initial measurment was made approximately two and a half hours after the sacrifice of the animal. The measurments were repeated throughout the day. At the end of the first day the samples were divided into three groups. The first group was maintained at room temperature (24°C); the second group was stored in a refrigerator at 3°C; and the third group was stored at −10° to −20°C. The next day the samples were removed from their storage environment and allowed to thaw and equilibrate to room temperature. Then the resistance and capacitance were measured repeatedly through the course of the day, being placed back into their respective storage environments at the end of the day. The procedure was repeated for upto four days with the times in which the bone specimens were removed from their environments and placed back being the same.

The electrical properties were measured using chlorided silver-metal electrodes. Surface moisture was removed from the bone prior to the measurement and a layer of conductive gel (AquasonicR 100, Parker lab) was applied to the bone surface and to the electrodes. The amount of time between the removal of the sample from the solution and the measurement was kept constant for each measurement due to the effect of exposure time (Saha, Reddy, and Albright, 1984).

RESULTS

Figure 1 shows the change in resistance for the three groups. The values for each day were calculated as the mean for the hourly readings for that day. As is shown, the resistance of the samples maintained at room temperature decreased slightly, similar to that of the specimens stored at refrigerator temperature. The resistance of the frozen specimens decreased at a noticeably faster rate than those stored at room temperature or in the refrigerator. The resistance of one sample at room temperature began to increase at day 5 while that of other specimens continued to decrease which was the reason for the large standard deviation noted. The reason for this increase is still unknown.

Fig. 1 Change in resistance with time for bone specimens stored in different environment.

Figure 2 shows the change in capacitance versus time for the three groups of specimens. The values for each day were calculated by the same method as those for the resistance. The capacitance of the refrigerated specimens decreased at a slower rate than did the capacitance of those stored at room temperature or those in the freezer. The capacitance of the frozen specimens decreased by approximately 50% after the first night of storage and then it did not change to any noticeable extent. The capacitance of the room temperature specimens decreased until they reached approximately 50% of their original values and then the values paralleled those for the frozen specimens.

Fig. 2 Change in capacitance with time for bone specimens stored in different environment.

DISCUSSION

Previously, other authors have reported changes in other physical properties of bone over time, when preserved in various ways. Steinberg and coworkers (1976) found decreases in strain related potentials in adult rat femora for 4–7 days, after the bone had been excised. Elwood and Smith (1984) have reported decreases in the zeta-potentials of bone during storage.

Although we have reported our results at one frequency (1KHz), it is possible that the nature of change in resistance and capacitance at other frequencies may be different. Also, Elwood and Smith (1984) found that different storage methods utilizing different fluids in which the bone is stored in, minimized the effect of storage.

From our study we have shown that the resistance and capacitance of bone is effected by the method in which it is stored and the rate of change is dependent on the storage method. Further studies are in progress to evaluate if the change in electrical properties can be minimized by different storage methods, other than those reported here. We also plan to study the effect of storage methods on frequency dependence of the electrical properties of bone.

ACKNOWLEDGMENT

This research was partially supported by National Science Foundation grant No. ECS - 8312680.

REFERENCES

Elwood, W. K., Smith, S. D. Effects of Refrigerated (4°C) and Deepfreez (−80°C) storage in Buffered HEPES pH 7.4 on the Zeta - Potentials of Bone. J. Bioelectricity. 1984; 3:385–407.

Kosterich, J. D., Foster, K. R., Pollack, S. R. Dielectric Properties of Fluid-Saturated Bone: The Effect of Variation in Conductivity of Immersion Fluid. IEEE Trans. Bio-med. Engng. 1984; 31:347–369.

Reddy, G. N., Saha, S. Electrical and Dielectric Properties of Wet Bone as a Function of Frequency. IEEE Trans. Bio-med. Engng. 1984; 31:296–303.

Saha, S., Reddy, G. N., Albright, J. A. Factors Affecting the Measurment of Bone Impedance. Med. Biol. Eng. Comp. 1984; 22:123–129.

Singh, S., Saha, S. Electrical Properties of Bone: A Review. Clin. Orthop. Rel. Res. 1984; 186:249–271.

Steinberg, M. E., Finnegan, W. J., Labosky, D. A., Black, J. Temporal and Thermal Effects on Deformation Potentials in Bone. Calcif. Tiss. Res. 1976; 21:135–144.

EFFECTS OF PLA SURFACE MICRO COATINGS ON BONE INGROWTH INTO POROUS CORALLINE HYDROXYAPATITE

Peggy L. Woodard*, Jon Swenson* and Allan F. Tencer*,     *Division of Orthopedic Surgery, University of Texas Medical Branch, Galveston, Texas 77550

Abstract

In this study, a porous synthetic hydroxyapatite was microcoated with three different thicknesses of PLA and compared to uncoated samples in an in vivo model to determine the effect of coating on bone ingrowth. Coatings were mixtures of chloroform and DL-polylactic acid in ratios of 3:1, 10:1, and 30:1. These coated specimens, along with uncoated specimens, were implanted transcortically into the tibiae of New Zealand White rabbits. At 12 weeks the specimens with 3:1 and 10:1 coatings seemed to inhibit bone ingrowth as measured from interface shear tests (p<.025), and to a lesser degree so did 30:1. However, at 24 weeks, possibly due to degradation of the coating, interface shear strength in specimens with all coating thicknesses was not statistically different from the shear strength in uncoated specimens.

Key Words

Bone graft

coated ceramic

in-vivo bone regeneration

Introduction

For many years there has been a search for bone grafting materials as effective as cancellous autograft but available in larger quantities and without the additional trauma to the patient in harvesting the materials. Xenogenic materials (Salama and Grazit, 1978) and ceramics (Groves and co-workers, 1971; White and co-workers, 1975) have been suggested as alternatives to iliac crest autograft. White and co-workers (1975) described the replamineform process for conversion of the calcium carbonate material of various corals into pure hydroxyapatite without altering the pore structure. Because of its pore structure, the coral genus Goniopora (CHAG) after conversion has been used successfully as a bone defect filler in dogs (White and co-workers, 1975) and in humans (Holmes and co-workers, 1984). However, this material exhibits low compressive strength and is prone to brittle failure (Tencer and co-workers, 1984). Internal microcoating with polymers has been shown to improve the mechanical properties of CHAG while maintaining its pore dimensions (Tencer and co-workers, 1984). Our objective in this study was to determine if polymer coating of CHAG affected bone ingrowth into the implant in vivo.

METHODS

SURGICAL

Preparation of Implants

Blocks of coralline hydroxyapatite (Interpore 600, Interpore International, Irvine, CA) were cut and shaped into cylinders. These sections were dip coated in mixtures of chloroform and DL-polylactic acid in ratios by weight of 3:1, 10:1, and 30:1, and cured for 24 hours under vacuum. The samples were then gas sterilized using ethylene oxide. The cylinders were slightly oversized to allow them to be hand filed at the time of surgical implantation into cylinders that would fit snugly into 4.76 mm diameter holes that were drilled in rabbit tibiae. The length of the cylinder varied according to the location of the implantation site on the tibia, but averaged 7 mm.

Surgical Implantation

Four implant sites were drilled with a 4.76 mm center drill in the proximal tibia on the anterior medial surface contralaterally in adult New Zealand White rabbits. One sample of CHAG with each coating thickness and one uncoated (UC) sample were fitted into the four holes in a random arrangement. However the implant arrangement was the same for both legs of each rabbit to allow correlation of mechanical and histomorphometric tests (now in progress). After recovering from anesthesia the rabbits moved about normally with little or no effect from the implants.

Surgical Removal

One leg of each rabbit was recovered for mechanical testing and the other was preserved for histomorphometric studies. The right leg was disarticulated at the knee, cleaned of excess muscle tissue, tagged for identification, wrapped in wet gauze, and frozen until mechanical push-out tests could be performed. The left leg was infused with 1000 cc of heparinized saline or until the afferent returning was completely clear. After completion of the infusion of heparinized saline, an equal amount of 10% N formalin solution was infused. The left tibia was then retrieved in the same manner as the right, tagged for identification and kept in formalin solution for histomorphometric studies. The rabbits were then sacrificed by an intracardiac injection of 10 cc of KCl.

MECHANICAL

Preparation of the Bone for Mechanical Testing

First the length of the tibia and the distances from the proximal surface of the tibia to the midpoint of the cross section of each of the implants was measured. Using an Isomet saw with a diamond wheel, the tibia was cut transversely at the midpoint between each of the implants. The implant was covered with parafilm to protect it from drying. A ring of modeling clay was put around the implant to protect it from impregnation by methylmethacrylate (MMA) and then it was press fit, using the clay, onto a chuck. The mounting chuck was slipped into a lubricated mold and a mixture of acrylic liquid and acrylic powder (MMA; Lang, Chicago, IL) was poured in and allowed to cure, embedding the bone surrounding the specimen but keeping the implant and the interface free. The MMA embedded sample was removed from the mold and cut to remove all bone except the flat portion surrounding the implant. The specimen/MMA disk was removed from the molding mount and mounted on a platform by centering the implant over a 4.79 mm diameter hole with the medullary canal side of the bone facing down. The hole was only .040 mm in diameter greater than the push out plunger to insure interfacial shear loading. The specimen was also epoxyed in place to the platform to prevent slipping during testing.

Mechanical Testing

The platform was mounted to a jig manufactured in our laboratory especially for push-out tests which allowed bi-axial alignment (Figure 1). The jig was fixed to the ram of the testing machine (MTS Systems Corporation, Minneapolis, MN). A plunger of 4.75 mm diameter was fixed to the load cell of the MTS and centered over the jig. The MTS was set to raise the ram at a displacement rate of 12.8 mm/min so that the plunger sheared the implant from the bone into the hole on the platform. This shearing force was recorded graphically on the X-Y recorder along with the distance moved by the ram. The test was ended when the measured force reached a peak showing maximum shear force, and began to drop. The sample was removed from the platform and examined to confirm that the shearing did in fact occur at the bone-implant interface Several measurements of bone thickness were then taken around the interface site using a micrometer so that the surface area of the interface could be calculated.

Figure 1 Mechanical Push-Out Jig and Plunger. Plunger is seated above CHAG sample and is used to shear sample from surrounding bone.

Results

Push-Out Tests

At three weeks statistical significance was not determined since there were too few specimens in the groups, but it appeared that the 3:1 coated specimen, the CHAG with the thickest PLA coating, did seem to produce lower shear stresses compared to the other coatings (Figure 2). At 12 weeks, interface shear stress of uncoated specimens increased dramatically to 26 MPa. Specimens with 30:1 coatings, the thinnest, had a mean shear stress value of 17 MPa. Specimens with 3:1 and 10:1 coatings were essentially identical (8.8 MPa and 9.1 MPa), showing significantly less shear stress than 30:1 or UC. At 24 weeks all of the implants approached the same interface shear stress. This may be due to degradation of the coatings, bone ingrowth or bonding of bone to hydroxyapatite. Further studies are planned with degradable coatings to study these mechanisms. The shear stresses measured at 24 weeks for all the implants constitute approximately 147 percent of that found in normal cortical bone (Bobyn and co-workers, 1980).

Figure 2 - Interface shear stress between cortex and implants of various coating thicknesses shown at periods post-implant as compared to normal bone ( Bobyn and co-workers, 1980). (Please note values shown for % Normal Bone are all × 10².)

Conclusions

In early stages after implantation, CHAG with a degradable polymer microcoating does reduce interface shear stress. However, in late stages shear strengths in coated specimens are comparable to uncoated samples. The coating increases the ease of handling of the material during implantation and provides improved mechanical properties.

References

1. Bobyn, J. D., Pilliar, R. M., Cameron, H. U., Weatherly, G. C. Clin Orthop and Rel Res. 1980; 150:263–270.

2. Groves, G. A., Hentrich, B. L., Stein, H. G., Bajpai, P. K. J. Biomed Mat Res Symp. 1971; 2:91–95.

3. Holmes, R. E., Mooney, V., Bucholz, R. W., Tencer, A. F. Clin Orthop Rel Res. 1984;