Characterization of degradation kinetics of additively manufactured PLGA under variable mechanical loading paradigms.

Controlled degradation of biodegradable poly-lactic-co-glycolic acid (PLGA) trauma implants may increase interfragmentary loading which is known to accelerate fracture healing. Additive manufacturing allows us to tune the mechanical properties of PLGA scaffolds; however, little is known about this novel approach. The purpose of this study was to use in vitro and in vivo models to determine the degradative kinetics of additively manufactured test coupons fabricated with PLGA. We hypothesized that 1) increases in infill density would lead to improved initial mechanical properties, and 2) loss of mechanical properties would be constant as a function of time, regardless of implant design. Porous and solid test coupons were fabricated using 85:15 PLGA filament. Coupons were either incubated in serum or implanted subcutaneously in rats for up to 16 weeks. Samples were tested in tension, compression, torsion, and bending on a universal test frame. Variables of interest included, but were not limited to: stiffness, and ultimate force for each unique test. Infill density was the driving factor in test coupon mechanical properties, whereas differences in lattice architecture led to minimal changes. We observed moderate levels of degradation after 8 weeks, and significant decreases for all specimens after 16 weeks. Results from this study suggest substantial degradation of 3-D printed PLGA implants occurs during the 8- to 16-week window, which may be desirable for bone fracture repair applications. This study represents initial findings that will help us better understand the complicated interactions between overall implant design, porosity, and implant biodegradation.

Functionally graded 3D printed plates for rib fracture fixation.

BACKGROUND: Design freedom offered by additive manufacturing allows for the implementation of functional gradients - where mechanical stiffness is decreased along the length of the implant. It is unclear if such changes will influence failure mechanisms in the context of rib fracture repair. We hypothesized that our novel functionally graded rib implants would be less stiff than controls and decrease occurrence of secondary fracture at implant ends. METHODS: Five novel additively manufactured rib implants were tested along with a clinically used Control implant. Fracture reconstructions were modeled with custom synthetic rib bones with a transverse B1 fracture. Ribs were compressed in a cyclic two-point bend test for 360,000 cycles followed by a ramp to failure test. Differences in cyclic stiffness, 3D interfragmentary motions, ramp-to-failure stiffness, maximum load, and work to failure were determined. FINDINGS: The Control group had lower construct stiffness (0.76 ± 0.28 N/mm), compared to all novel implant designs (means: 1.35-1.61 N/mm, p < 0.05) and rotated significantly more about the bending axis (2.7° ± 1.3°) than the additively manufactured groups (means between 1.2° - 1.6°, p < 0.05). All constructs failed via bone fracture at the most posterior screw hole. Experimental implants were stiffer than Controls, and there were few significant differences between functional gradient groups. INTERPRETATION: Additively manufactured, functionally graded designs have the potential to change the form and function of trauma implants. Here, the impact of functional gradients was limited because implants had small cross-sectional areas.