The ovariectomized sheep as a model for human bone loss.

The aim of this study was to determine the effects of ovariectomy on the trabeculae of ovine iliac bone, with a view to the further development of a model for human osteoporosis. Seven ovariectomized (OVX) and five control Dorset ewes were observed for one year. Iliac crest bone samples were obtained at the time of ovariectomy ("time 0") and again when the animals were killed 12 months later. At "time 0" and at 3-month intervals thereafter samples of blood and urine were collected for the assay of serum 17beta-oestradiol and osteocalcin (OC) and urinary deoxypyridinium (D-PYR). One year after ovariectomy, trabecular bone volume and thickness were reduced (P<0.05) and trabecular separation was increased (P<0.05) as compared with the controls. In OVX sheep, serum 17beta-oestradiol concentrations were significantly lower at the end of the experiment than at "time 0", while serum OC and urinary D-PYR concentrations were significantly higher (P<0.001). The results suggest that the OVX sheep is a valid model for changes in trabecular bone architecture associated with oestrogen deficiency, especially in women experiencing early menopause.

Structure, metallurgy, and mechanical properties of a porous tantalum foam.

This study evaluated a porous tantalum biomaterial (Hedrocel) designed to function as a scaffold for osseous ingrowth. Samples were characterized for structure, Vickers microhardness, compressive cantilever bending, and tensile properties, as well as compressive and cantilever bending fatigue. The structure consisted of regularly arranged cells having struts with a vitreous carbon core with layers of CVI deposited crystalline tantalum. Microhardness values ranged from 240-393, compressive strength was 60 +/- 18 MPa, tensile strength was 63 +/- 6 MPa, and bending strength was 110 +/- 14 MPa. The compressive fatigue endurance limit was 23 MPa at 5 x 10(6) cycles with samples exhibiting significant plastic deformation. SEM examination showed cracking at strut junctions 45 degrees to the axis of the applied load. The cantilever bending fatigue endurance limit was 35 MPa at 5 x 10(6) cycles, and SEM examination showed failure due to cracking of the struts on the tension side of the sample. While properties were variable due to morphology, results indicate that the material provides structural support while bone ingrowth is occurring. These findings, coupled with the superior biocompatibility of tantalum, makes the material a candidate for a number of clinical applications and warrants further and continued laboratory and clinical investigation.

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.

Tensile fatigue of 4-META cement bonding three base metal alloys to enamel and comparison to other resin cements.

The tensile median fatigue limits and fracture mode of 4-META cement were evaluated after bonding Ni-Cr, Ni-Cr-Be, and Co-Cr alloys to enamel. Alloy surfaces, 6 mm in diameter, were grit blasted with 50 microns Al2O3 and cemented to etched bovine enamel under a 2 kg load. Samples were cycled in tension to failure or 10(6) cycles at 5 Hz in Ringer's solution at 37 degrees C. Two-point strategy was used to determine median fatigue limits (S50). Fracture modes were evaluated by SEM on samples failing before 10(6) cycles. Results indicated differences between all sample groups where S50 (Ni-Cr-Be) > S50 (Co-Cr) > S50 (Ni-Cr). Failure analysis revealed mixed cohesive fractures near both interfaces with small areas of delamination within the cement. Comparison to reported median fatigue limits of two commercially available cements were discussed.

Histological and electron microscopic analysis of tissue response to synthetic composite bone graft in the canine.

A synthetic composite bone graft material, composed of fibrillar collagen, hydroxyapatite, and tricalcium phosphate, was evaluated in canine femoral shaft defects and compared to ungrafted defects and defects grafted with autogenous bone. The results of mechanical testing of the grafted femora in torsion at 1 year postoperatively were previously reported. This report details the histological and microscopic features of the graft sites. The results of this study suggest that the healing of the graft site is not yet complete at 1 year and that healing may be more complete in defects that received the synthetic graft as compared to ungrafted sites and autogenous bone graft. The bone tissue appears to respond to the graft particles as if they were bone particles.

Torsional properties of healed canine diaphyseal defects grafted with a fibrillar collagen and hydroxyapatite/tricalcium phosphate composite.

The need for alternatives to autogenous bone grafts is widely recognized. This study compared the torsional strength of canine femora 1 year after grafting with one of three forms of a collagen/hydroxyapatite/tricalcium phosphate bone grafting material (COLLAGRAFTTM), autogenous bone, or no graft. The groups were compared to each other and to the unoperated contralateral femora. Results of torsional testing were evaluated for torsional strength, torsional displacement, total energy to fracture and White fracture mode. Data analysis showed lower torsional strength of the operated vs. unoperated femora with the exception of morsellized COLLAGRAFTTM material, which had higher strength. However, the only difference in the operated groups was that the morsellized COLLAGRAFTTM had greater strength than several groups including the autogenous bone group. There was no difference found in angular displacement between any of the groups. However, there was a difference in the energy to fracture in both strip forms of the COLLAGRAFTTM. The final conclusion is that in this model, grafting with COLLAGRAFTTM provided torsional properties at one year postoperatively at least equivalent to autogenous bone.

Tensile fatigue of two composite cements bonding three base metal alloys to bovine enamel.

Tensile fatigue endurance limits were determined for three base metals (Ni-Cr, Ni-Cr-Be, and Co-Cr) bonded to bovine enamel using two composite cements: a Bis-GMA/phosphate ester composite cement which relies on a sand-blasted metal surface, and a Bis-GMA composite luting cement which relies on electrolytically etched metal surfaces. Samples were tested to failure or to 10(6) cycles at 5 hz in Ringer's solution at 37 degrees C, and endurance limits were determined using a two-point test strategy. SEM evaluation was performed on fractured samples to determine failure mode. Statistical analysis of the results showed no difference between cements when using Ni-Cr-Be; however when using Co-Cr, the Bis-GMA/phosphate ester cement produced greater values than the Bis-GMA cement that relied on electrolytical etching. The opposite result occurred when a Ni-Cr alloy was tested with both cements. Evaluation of the results for each cement with the three different alloys showed statistical significant differences. SEM fracture analysis revealed a mixed failure pattern with apparent adhesive fracture from both the composite-enamel and composite-metal interfaces and cohesive failure throughout the cement.

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.

Holding power of cortical screws after power tapping and hand tapping.

Paired equine third metacarpal bones were drilled and tapped for 4.5 mm and 5.5 mm cortical screws. Tapping was done by hand or with an air-driven reversible orthopedic drill. Screws were inserted and subjected to extraction forces to failure of the osseous threads or the screws. There was no difference in holding power of either screw size between hand-tapped and power-tapped holes.

Mechanical properties of 3M staples in bone block models.

The use of power driven staples is an accepted method of fixation for small bones of the hand and wrist and of the foot and ankle. The staples must be applied to flat or nearly flat surfaces. A synthetic model was developed to evaluate the ability of the 3M power driven staple system to resist failure due to pulling out of the staples in cantilever bending, tension, and torsion. Three different staple configurations and two leg lengths were evaluated in each of the test modes. Results indicated statistically significant differences between one and two staple configurations and between longer and shorter leg lengths. There was clear superiority of the two staple configurations and of longer leg lengths.

Torsional properties of implant grade titanium.

The purpose of this research was to evaluate the torsional strength and ductility of CP titanium in the as received condition, heat treated below the alpha----beta transition temperature, and glass bead blasted. Results were compared to as-received samples of implant quality 316L stainless steel. Little or no effect of any of the treatments was noted. The torsional strength of all titanium samples was comparable to 316L stainless and the torsional ductility of the titanium was significantly greater than 316L stainless (p less than .01). These results are significantly different than those obtained when torsion testing bone screws of the same material. A hypothesis is proposed for the differences in these results.

The mechanical integrity of healed diaphyseal bone defects grafted with calcium hydroxyapatite/calcium 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.

Development of a custom-fit mouthstick appliance.

Quadriplegics rely on mouth-controlled devices to perform a variety of tasks and to establish some degree of self-sufficiency. The most functional mouthstick appliances have custom-fitted mouthpieces which are fabricated by dental professionals, and in some cases are cost-prohibitive. An inexpensive lightweight mouthstick which incorporates a new thermoplastic mouthpiece and can be custom-fitted by the user is described.

Metallurgical evaluation of retrieved implants and correlation of failures to patient record data.

This study sought to establish an orthopedic implant retrieval program according to ASTM F561-87 (the American Society for Testing and Materials) and to describe the results of evaluation of failed implants. A total of 275 fracture fixation and prosthetic devices have been retrieved to date. Results of metallurgical evaluation showed no deviations in ASTM specifications among the failed implants evaluated. The cause of failure in most cases was fatigue following nonunion or delay in healing.

Crack propagation in conventional and high copper dental amalgam as a function of strain rate.

Dental amalgam prepared from high copper and conventional alloys was evaluated for its compressive strength as a function of deformation rate. Results showed that the compressive strength rose to a maximum as deformation rate increased and then began to fall at higher deformation rates. Crack propagation predominated through the gamma 1 phase and around copper containing particles at lower strain rates. Cracks also occurred through copper containing original particles where the stress at fracture was sufficiently high.