Biomedical Engineering Reference
In-Depth Information
6.4
Biodegradable Interference Screw Manufacturing
Processes
Weiler et al. [ 48 ] showed that torque to failure depends not only on the screw
design, but also on the biomaterials composition (e.g., pure PLLA implants have a
higher stiffness compared with PDLLA implants). Also, it is proved that addition
of TCP modifies the mechanical properties of the implant, which becomes brittle;
in order to control this aspect, the polymer must be submitted to different process-
ing steps.
Despite the fact that details about the processes used for obtaining the bioresorb-
able raw materials are presented in the literature [ 75 ], one must take into account
that the manufacturing processes and their corresponding process parameters
influence the mechanical properties of the interference screw, which are different
from those of the raw material.
However, the producers keep confidential the information about the exact com-
position and manufacturing processes of bioresorbable interference screw. Also, a
small modification of manufacturing process parameters will affect the resorption
rate. For instance, injection molding process determines a crystalline structure of
the implant, which causes a slower resorption rate. On the other hand, machining
allows a better tissue growth due to a rougher surface of the implant.
Even though the scientific literature reports injection molding, injection transfer
molding, and compression molding as typical processes for manufacturing biore-
sorbable interference screws, the information referring to the process parameters
(injection temperature, pressure, mold temperature, mold cooling, etc.) remains
confidential for the manufacturers.
Researchers at Fraunhofer Institute for Manufacturing Engineering and Applied
Materials Research in Bremen—IFAM [ 76 ]—reported in 2011 the development of
a new granulate PLA and hydroxyapatite material that can be used for manufactur-
ing interference screws using a powder injection molding process. Thus, the
screws are manufactured directly using the new molding process, requiring no
post-processing operations. Another proprietary manufacturing process is Micro
Particle Dispersion used from DePuy Mitek for manufacturing Milagro
Interference Screw using a biocomposite material (Polylactic/polyglycolic acid
PLGA—reinforced with b -TCP).
Rapid Prototyping (RP) processes can be used indirectly in manufacturing inter-
ference screw by building the mold in which material is cast. To the best of our
knowledge, there is no information in the literature regarding the direct manufactur-
ing of a bioresorbable interference screw using an RP process, despite the advance-
ment in this field, which now provides the possibility to use a larger range of
processing materials. In this field, the most part of the research is now oriented in
producing scaffolds with controlled porosity for tissue engineering using rapid pro-
totyping processes [ 77, 78 ] .
Based on the literature analysis and experimental research on other bioresorb-
able interference screws made by composite materials, we have selected as a material
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