On additive manufacturing of rib fracture fixation implants: The role of lattice design

Rib fractures and chest flail injuries are life threatening injuries that often require surgical treatment using metal (e.g. titanium) fracture reconstruction plates and screws. Current implant designs do not account for the variable stiffness present in human ribs and are much stiffer than the native bone, causing undesirable clinical outcomes. In this preliminary study, 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 lattice strut thickness between 0.225 – 0.425 mm and unit cell length between 1, 2, and 3 mm. The test plates were fabricated using an established laser powder bed fusion additive manufacturing process. Flexural strength (4-point bending) tests were performed at a strain rate of 1.3 mm/min to characterize changes in bending stiffness and strength. 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. The results of this preliminary experiment serve as guidelines for the design of full-sized rib fracture reconstruction plates that contain a gradient lattice with varied mechanical properties to better match the behavior of intact ribs.