Development of Bioabsorbable Interference Screws: How Biomaterials Composition and Clinical and Retrieval Studies Influence the Innovative Screw Design and Manufacturing Processes - Biologically Responsive Biomaterials for Tissue Engineering - page 111

Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] has a definite composition, Ca/P weight ratio
of 2.151, Ca/P molar ratio of 1.67, and a crystallographic structure [ 28 ] . Since
hydroxyapatite is the main mineral constituent of teeth and bones, it seems to be a
very suitable biomaterial for the hard tissue implants and for scaffolds used in hard
tissue regeneration. Hydroxyapatite ceramics do not have any toxic effects.
Shikinami and Okuno [ 29 ] describe the possibility to increase the mechanical prop-
erties of biodegradable polymers using the combination with hydroxyapatite (HA)
and Yasunaga et al. [ 30 ] mention that the inclusion of HA into PLA has shown an
advantage for composite materials regarding the bone bonding ability.
TCP is another biodegradable ceramic that proved to be bioactive and useful for
obtaining composite biomaterials. TCP occurs in four polymorphs a , b , g, and super a ,
b-TCP being preferred to the other forms due to its chemical stability, mechanical
strength, and more predictable and consistent bioresorption rates [ 31 ] . The g poly-
morph is a high-pressure phase and the super a polymorph is observed only at tempera-
tures above 1,500°C. Because the degradation rate of b-TCP is higher than that of HA,
the partial degradation of calcium phosphates stimulates the bone-bioceramic bonding.
HA and b-TCP present differences in chemical composition and crystallographic struc-
ture. This aspect has a major impact on their physical characteristics [ 32 ] .
The composite materials used for interference screws keep all advantages of
bioresorbable polymers and assure new abilities like the regulation of the rate of
degradation and neutralization of the degradation by-products, less failure during
insertion, and improved bioactivity and osteointegration.
But one of the major problems for manufacturing of the composites is the
agglomeration of the HA and b-TCP powders in the polymeric matrix. In general,
fine particles tend to combine together, through electrostatic or van der Waals forces,
to form agglomerated particles. This agglomeration tends to decrease the mechani-
cal properties of the composite. So, the selected technique for preparing the com-
posites must have the ability to break down agglomerated powder and disperse it
into the polymeric matrix [ 33 ] .
Another advantage of preparation of composites would be that polymers are gen-
erally weak mechanically and the addition of a ceramic component would enhance
their strength and stiffness for more functional applications.
Different available composite interference screws are presented in Table 6.2 .
A quick market analysis revealed to us that all major companies on the market have
a composite interference screw and this is more expensive than similar screws made
by biodegradable polymers. Also, each company uses a proprietary composition of
composite materials, which differs from other competitors.
What changes can be made in the design and manufacturing of interference
screws, in order to improve graft fixation, is also under current research. Also, sev-
eral surface biofunctionalization issues could be very important for the interaction
phenomena between screw and tissue, with strong effects on the clinical perfor-
mance of the interference screws.
In our experience, the degradation process is affected by many implant-related
factors starting with biomaterial type and ending with thread design and size.
Also, most of the biodegradable screws used in current practice do not promote
bony regrowth.