Date Published: November 6, 2019
Publisher: Springer International Publishing
Author(s): Lorenzo Dall’Ava, Harry Hothi, Johann Henckel, Anna Di Laura, Paul Shearing, Alister Hart.
The design freedom allowed by three-dimensional (3D) printing enables the production of acetabular off-the-shelf cups with complex porous structures. The only studies on these designs are limited to clinical outcomes. Our aim was to analyse and compare the designs of different 3D printed cups from multiple manufacturers (Delta TT, Trident II Tritanium and Mpact 3D Metal).
We analysed the outer surface of the cups using scanning electron microscopy (SEM) and assessed clinically relevant morphometric features of the lattice structures using micro-computed tomography (micro-CT). Dimensions related to the cup wall (solid, lattice and overall thickness) were also measured. Roundness and roughness of the internal cup surface were analysed with coordinate measuring machine (CMM) and optical profilometry.
SEM showed partially molten titanium beads on all cups, significantly smaller on Trident II (27 μm vs ~ 70 μm, p < 0.0001). We found a spread of pore sizes, with median values of 0.521, 0.841 and 1.004 mm for Trident II, Delta TT and Mpact, respectively. Trident II was also significantly less porous (63%, p < 0.0001) than the others (Delta TT 72.3%, Mpact 76.4%), and showed the thinnest lattice region of the cup wall (1.038 mm, p < 0.0001), while Mpact exhibited the thicker solid region (4.880 mm, p < 0.0044). Similar roundness and roughness of the internal cup surfaces were found. This was the first study to compare the designs of different 3D printed cups. A variability in the morphology of the outer surface of the cups and lattice structures was found. The existence of titanium beads on 3D printed parts is a known by-product of the manufacturing process; however, their prevalence on acetabular cups used in patients is an interesting finding, since these beads may potentially be released in the body.
More than 90,000 total hip arthroplasties (THA) were performed in the United Kingdom in 2017, with almost 70% using uncemented acetabular cups . Although the majority of these implants were manufactured using traditional technologies [2, 3], around 10% of off-the-shelf designs are now produced using three-dimensional (3D) printing methods [4, 5].
A schematic flowchart of the study design is shown in Fig. 1.
Fig. 1Flow chart of the study design
The results are presented according to the location on the cup, starting with the analysis performed on the outer surface and continuing with the investigation of the internal surface of the three acetabular cup designs.
This study is the first to compare different designs of 3D printed off-the-shelf acetabular cups from multiple manufacturers. We found a variability in the morphology of the outer surface due to the different 3D printing process (EBM vs LRM), with evidence of partially molten beads on all the cups. Differences in the lattice structures were also present, with a variability in the pore sizes and porosity of the cups, as well as in the thickness of the cup wall.
This was the first study to compare different designs of 3D printed off-the-shelf acetabular cups from multiple manufacturers. We found a variability in the morphology of the outer surface of the cups and the properties of the porous structure. Although the existence of partially molten surface beads on 3D printed parts is a known by-product, their prevalence on these acetabular cups was interesting and the related clinical implications, if any, need to be investigated.