Bioresorbable devices made of forged composites of hydroxyapatite (HA) particles and poly-L-lactide (PLLA): Part I. Basic characteristics

Compounds that had neither calcined nor sintered hydroxyapatite (u-HA) particles (particulate size 0.2-20 microns, averaging 3.0 microns, Ca/P = 1.69, and containing CO3(2-) uniformly distributed in a poly-L-lactide (PLLA, Mv: 400 KDa) matrix with a content of 20-50 wt% (with 10% increment) were reinforced into composites by a forging process, which was a unique compression molding, and were then machined on a lathe in order to produce practical radiopaque internal bone fixation devices having high mechanical strength which was maintained during bony union, total resorbability and bioactivity such as bone bonding capability and osteoconductivity. From the results of measurement of various mechanical properties, it was confirmed that the composites generally showed the highest mechanical strength among this type of reinforced bioceramic fibers or particles/bioresorbable polymer composite known to date. The bending strength (Sb) of about 270 MPa was far higher value than that for cortical bone, and the modulus (Eb) of 12 GPa was almost equivalent to that for cortical bone. In particular, the impact strength (Si) was extremely high at about two times the value (166 KJ/m2) of polycarbonate. The in vitro change in Sb, Mv (viscosity average molecular weight), Mw/Mn (molecular weight distribution) and crystallinity, and their relationship with each other was also examined by immersing samples in a phosphate buffer solution (PBS). An immediate decrease in the initial Mv could be found in composites with high u-HA contents (30-50 wt%), although a time-lag stage for degradation where the initial Mv hardly changes was apparent in cases of PLLA-only or in a composite with a low u-HA content (20 wt%). The Sb changed with corresponding decremental curves for the Mv and retained over 200 MPa for up to 24 weeks, the period of time necessary for full bony union, so that the composite satisfied initial mechanical strengths while maintaining them for as long as necessary for internal bone fixation devices. These results supported the idea that there is a difference in the degradation process such that PLLA alone required a period of time to achieve the possibility of hydrolysis into the inner side, whereas composites with high u-HA contents (30-50 wt%) immediately filled with water through to the inner side and were hydrolyzed homogeneously. Many hydroxyapatite crystals deposited and grew on the surface after 3-6 d and generously covered the surface with a fairly thick layer after 7 d of post-immersion in simulated body fluid (SBF) as evaluated by means of energy dispersive X-ray (EDX). This suggested the ability of the radiopaque composites to bond to bone. Since the composites were dense and had ultra-high strength, and the processability was so excellent, many kinds of fine and accurate screws, pins, plates, and other internal bone fixation devices for orthopedic, oral and maxillofacial, craniofacial, and plastic and reconstructive surgeries could be produced by machining treatment. These devices have potential applications for clinical use following the assessment of adaptation during in vivo studies.