Initial Bending Strength Research Articles

New bioresorbable pin for the reduction of small bony fragments: design, mechanical properties and in vitro degradation

The design, material properties, and in vivo degradation characteristics of a new resorbable pin for the reductions of small bony fragments are described. The Polypin ®, made of 70:30 poly( L, DL-lactide), had an initial bending strength of 155–163 MPa, as measured by a three-point bending test. Ethylene oxide (EO)- and gamma-sterilization did not substantially affect its initial mechanical properties. The initial molecular weight ( M w ) of 523,000 to 600,000, however, decreased 60–75% after gamma-sterilization. Incubation of the EO-sterilized pins in 37 °C saline solution produced a complete loss of bending strength at 18 months. An accelerated test at 70 °C led to a complete loss of strength after only 96 h. Degradation of the gamma-sterilized pin at 70 °C was about 30% faster than that of the EO-sterilized pin. Bending strength and molecular weight were unaffected by storage at room temperature for 46 months. The relatively slow strength loss rate of the Polypin ® potentially extends the application of resorbable devices to slow-healing fractures. The new pin design allows application of light interfragmentary compression, thus reducing the risk of pin loosening, and an X-ray marker is provided.

A new type of fiber-bonded-ceramic material synthesized from pre-oxidized Si-Ti-C-O fiber

Two types of fiber-bonded-ceramic material (FBC2123 or FBC1873) were synthesized from preoxidized Si-Ti-C-O fibers with oxide layers of 150 to 500 nm in thickness at 2123 K or 1873 K under 50 to 70 MPa. The interstices in both types of the materials were packed by an oxide material, which had existed on the surface of the pre-oxidized Si-Ti-C-O fiber. So, the dense, fiber-bonded-ceramic materials with small amount of the oxide matrix were obtained. During hot-pressing, carbon in excess of the non-stoichiometric ratio was released from the fiber and formed an interfacial layer on the surface of the fiber, beneath the pre-existing oxide material. Both FBC2123 and FBC1873 showed fibrous fracture patterns with high fracture energies at temperatures up to 1573 K and 1773 K, respectively. FBC2123 exhibited some plasticity in air at a temperature of 1673 K or over, due to the existence of amorphous silica in the matrix, and then a reduction in bending strength was observed at 1773 K in air. On the other hand, FBC1873 maintained its initial bending strength up to 1773 K in air, which is attributed to reduced crystallization of Si-Ti-C-O fiber and to the formation of cristobalite in the matrix.

Absorbable self-reinforced polyglycolide (SR-PGA) screws for the fixation of fractures and osteotomies: Strength and strength retention in vitro and in vivo

The initial mechanical strength and strength retention in vitro and in vivo of novel absorbable and self-reinforced polyglycolide (SR-PGA) screws for the fixation of fractures and osteotomies were determined. The SR-PGA screws showed initial bending strength values comparable with those of yield strength of standard stainless steel. The SR-PGA screws lost all their bending and shear strengths in 6 weeks in vivo. The strength retention properties of SR-PGA screws are sufficient for safe fixation of relatively rapidly consolidating fractures and osteotomies of cancellous bone that are not exposed to hard mechanical stresses.

In vitro and in vivo studies on bioabsorbable ultra-high-strength poly(L-lactide) rods.

Ultra-high-strength poly(L-lactide) (PLLA) rods were fabricated using a drawing technique. Rods with a diameter of 3.2 mm and a draw ratio of 2.5:1 showed initial bending strength and modulus values of 240 MPa and 13 GPa, respectively. The purpose of this study was to investigate the in vitro and in vivo degradation of PLLA rods with a draw ratio of 2.5:1. The greater the rod diameter, the longer the bending strength was maintained in phosphate buffered saline at 37 degrees C. The bending strength retention of rods (diam. 3.2 mm) implanted in the subcutis of rabbits was almost equal to that of rods in the in vitro study, while those rods implanted in the medullary cavity of rabbit femora showed a slightly lower bending strength retention. Molecular weight was reduced to the greatest extent in the medullary cavity, followed by in the subcutis and in vitro. The weight of PLLA rods in the medullary cavity was reduced by 22% at 52 weeks and by 70% at 78 weeks after implantation. Histologically, no inflammatory or foreign body reaction was observed in the medullary cavity for 52 weeks. The drawn PLLA rods maintained a bending strength exceeding that of human cortical bone in the medullary canal for a period of 8 weeks, suggesting that the drawn PLLA rods may be useful in the repair of fractured human bones.

Absorbable self-reinforced polylactide (SR-PLA) composite rods for fracture fixation: strength and strength retention in the bone and subcutaneous tissue of rabbits

The strength and strength retention of self-reinforced (SR) absorbable polylactic acid composite rods were evaluated after intramedullary and subcutaneous implantation in rabbits. Rods made of poly-l-lactic acid (SR-PLLA) and of poly-dl-lactic acid + poly-l-lactic acid composite (SR-PDLLA/PLLA) were used. The molecular mass (Mv) of PLLA was 260.000 and that of PDLLA 100.000. The bending and shear strengths were measured after a follow-up of 1–48 weeks. The initial bending strength of the SR-PLLA rods was 250–271 MPa and the shear strength was 94 98 MPa. After intramedullary and subcutaneous implantation of 12 weeks the bending strength of the SR-PLLA implants was 100 MPa. At 36 weeks the bending strength had decreased to the level of the strength of cancellous bone (10–20 MPa). There were no changes in the shear strength during 12 weeks hydrolysis. The initial bending strength of SR-PDLLA/PLLA implants was 209 MPa and during the follow-up the implants lost their bending and shear strength faster than the SR-PLLA implants. The present investigation gave us the impetus to continue the studies with the fixation of experimental cortical bone osteotomies with SR-PLLA intramedullary rods.

Ultra‐high‐strength absorbable self‐reinforced polyglycolide (SR‐PGA) composite rods for internal fixation of bone fractures: In vitro and in vivo study

The ultra-high-strength, self-reinforced (SR) absorbable polymeric composites, consisting of reinforcement elements, like fibers, and of matrix polymer which have the same chemical element composition as reinforcement, were defined. A method to manufacture self-reinforced, absorbable polyglycolide (SR-PGA) rods of polyglycolide sutures (Dexon) by sintering them partially together at elevated temperature and pressure was presented. The rods with nominal diameters of 1.5 mm, 2.0 mm, 3.2 mm, and 4.5 mm showed initial bending modulus and strength values of 8-15 GPa and 220-405 MPa, respectively. Their initial shear strengths were 165-255 MPa. The smallest rods (diam. 1.5 mm) lost their mechanical strength after implantation in the subcutis of rabbits in 4-5 weeks while the thickest rods retained their strength over 8 weeks. The ultra-high-strength SR-PGA rods were concluded to be suitable for fixation of cancellous bone fractures, osteotomies, and epiphyseal plate fractures where the fixation is not exposed to excessive mechanical stresses and where the loads are predominantly of a shear nature.

Fatigue of Ceramics (Part 2)

To investigate the fatigue strength in the S-N curve of ceramic materials, the high speed cyclic fatigue testing technique was developed using the resonant bending method, and the cyclic fatigue test up to 1010 cycles was conducted. To compare with the resonant bending fatigue tests at 3kHz, the alternating cantilever bending fatigue tests were carried out at 20Hz, 0.3Hz and 0.03Hz with the same specimen using an electro-hydraulic fatigue testing machine. Cyclic fatigue at room temperature depended mainly on the number of cycles accumulated rather than on time. The distributions for the fatigue life were presented in Weibull plots with three parameters. From the predicted fatigue life limit at each stress amplitude, the fatigue strength limit was estimated to be about 40% of the initial bending strength.

Strength and strength retention vitro, of absorbable, self-reinforced polyglycolide (PGA) rods for fracture fixation

The initial shear strength and changes in flexural strength of self-reinforced, absorbable polyglycolide (PGA) composite rods, submerged in distilled water (at 37°C) for a period of 6 wk, were investigated. The recently developed self-reinforced absorbable material consists of an absorbable polymeric matrix reinforced with fibres of the same polymer. The initial shear strength of self-reinforced cylindrical PGA rods with a diameter of 3.2 mm was 250 MPa and the initial flexural strength of the rods was 370 MPa. During the first week of immersion the level of flexural strength decreased very little i.e. to 320 MPa. The loss of flexural strength increased after 1 wk immersion. However, after 3 wk it was 90 MPa. After 5 wk the flexural strength decreased to the level of strength of cancellous bone i.e. 10–20 MPa. The γ-irradiation of the PGA rods (total dosage 2.5 Mrad) decreased the initial bending strength to 300 MPa but the hydrolytic behaviour of the rods was not changed. The in vitro strength and the strength retention of self-reinforced PGA rods are clearly better than the corresponding values for self-reinforced glycolide/lactide copolymer rods which we developed recently. Self-reinforced PGA rods are now used routinely in Helsinki University Central Hospital in the treatment of certain types of cancellous bone fracture.