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 109

O

O O

O

Catalys
Heat

O CH 2 C OCH 2 C

O

n

O

Glycolide

Polyglicolic acid

Fig. 6.2 The polymerization of glycolide to polyglycolic acid

O

O

Catalyst
Heat

O

O (CH 2 ) 5 C

n

Caprolactone Polycaprolactone

Fig. 6.3 Ring-opening polymerization of e -caprolactone to polycaprolactone

In addition to lactic acid copolymers, a combination of PLA and PGA is an
effective alternative in developing orthopedic implants. PLGA has excellent biode-
gradability, biocompatibility, and nontoxic properties [ 26 ]. Given the different deg-
radation rate of PLA and PGA degradation in the case of lactic acid-glycolic acid
copolymer (PLG) may be modified based on the PLA/PGA ratio. Copolymers of
PLG frequently used in orthopedic devices have ratios as PLA/PGA = 50:50, 75:25,
and 85:15.
PCL is one of the most flexible and easy-to-process biodegradable polymers, pre-
pared by ring opening polymerization of e-caprolactone in the presence of stannous
octoate as catalyst (Fig. 6.3 ). PCL exhibits several unusual properties not found
among the other aliphatic polyesters. These include its exceptionally low glass tran-
sition temperature ( T g) of −60°C, low melting temperature ( T m) of ~60°C, and high
thermal stability with a decomposition temperature ( T d) of ~350°C, while other
polyesters decompose at ~250°C. PCL is semicrystalline with a low modulus and
about 2 year's degradation time [ 27 ]. Due to the combination of crystallinity and
high olefinic character of PCL homopolymer the hydrolysis process is considerably
slower than the other poly(a-hydroxy esters) such as PGA and PLA.
Several properties of these polymers are detailed in Table 6.1 .
In order to obtain biodegradable composite materials for interference screws, the
same synthetic biodegradable polymers mentioned below are used as matrix rein-
forced with biodegradable ceramics (hydroxyapatite and tricalcium phosphate
(TCP) are used generally as ceramic filler). In the case of composite materials used
for interference screw, addition of the ceramic filler can have several advantages
because the filler is osteoconductive and increases bone bonding to the implant and
basic fillers may also buffer the acidic degradation products from the polymer,
reducing the chance of foreign body reactions and increasing the mechanical
properties.