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.