Session: 03-03-02: Processing and Design of Materials and Components for Additive Manufacturing

Paper Number: 73086

Start Time: Wednesday, 05:30 PM

73086 - On Additive Manufacturing of Rib Fracture Fixation Implants: The Role of Lattice Design 

Rib fractures and chest flail injuries are life threatening injuries that require surgical treatment using metal (e.g. titanium) fracture reconstruction plates and screws. The implants are designed to provide chest wall stabilization (to allow for bone healing) while permitting large amounts of flexion for respiration. Previous studies have shown that traditionally manufactured titanium reconstruction plates are prone to early failure due to their relatively high and constant stiffness throughout the plate.  From a physiological standpoint, human ribs have a variable stiffness along their lengths. Current implant designs do not account for this variability and are much stiffer than the native bone.  As a result of this inherent limitation of traditional implants, the reconstruction often fails due to screw pullout at the ends of the implant. An additive manufacturing (AM) approach has the potential to play a key role in the design and fabrication of implants that exhibit optimized mechanical properties with no changes to external geometry.  Likewise, in this preliminary study, we designed and manufactured an array of miniature implants that had the same external geometry but had variable internal architecture. Using this approach, we hypothesized that we could decrease implant stiffness by 20%. We designed and manufactured 25 miniature implant test coupons that had an external geometry of 100 x 10 x 1.5 mm.  Five groups of latticed test plates were designed with a body centered cubic (BCC) lattice and porosities ranging from 36–86%. Porosity was altered by changing strut thickness between 0.225 – 0.425 mm and unit cell length between 1, 2, and 3 mm. The test plates were fabricated using the AM process of laser powder bed fusion, and half of all implants were heat-treated after fabrication while the other half was left untreated.  Flexural strength (4-point bending) tests were performed at a strain rate of 5 mm/min to characterize changes in bending stiffness, bending structural stiffness, proof load, bending strength, and maximum load. It was found that implant stiffness could be decreased by 15.7% (p = 0.068) by decreasing strut thickness from 0.425 to 0.225 mm and increasing unit cell length from 1 to 3 mm. Additionally, the structural thickness was 14.2% (p = 0.068) lower in the 0.225 mm strut and 3 mm unit cell than the 0.425 mm strut and 1 mm unit cell group. Both proof load and bending strength can be reduced by 12% (p = 0.016) by decreasing strut thickness from 0.425 to 0.225 mm, which correlates with a porosity increase of 42%. Furthermore, the maximum load was decreased by 14 % (p < 0.001) by decreasing strut thickness from 0.425 to 0.224 mm, and by 16.9% (p < 0.001) by also increasing unit cell length from 1 to 3 mm. Failure modes varied between heat treated and non-heat treated test plates, with 80% of heat treated test plates fracturing whereas the as-printed plates underwent significant plastic deformation but none experienced fracture. Heat treatment did not have a statistically significant impact on stiffness, however proof load and bending strength were improved. These results lead us to believe that reducing the porosity of the plate by decreasing lattice strut thickness and increasing unit cell length allows for the design and fabrication of plates with target stiffnesses that are comparable to human ribs. The results of this preliminary experiment will allow us to design full-sized rib fracture reconstruction plates that not only conform to individual patient bone geometry, but also contain a gradient lattice that varies mechanical properties to better represent the behavior of intact ribs.

 

Presenting Author: Lauren Judkins Pennsylvania State University

Authors:

Lauren Judkins Pennsylvania State University
Richa Gupta Pennsylvania State University
Christine Gabriele Pennsylvania State University
Charles Tomonto Johnson & Johnson 3D Printing
Michael W. Hast Pennsylvania State University
Guha Manogharan Pennsylvania State University