# Integration of a Three-Dimensional-Printed Titanium Implant in Human Tissues: Case Study
## 4 Discussion  
The 3D printing method for producing Ti6 Al4V titanium alloy implants has been recently used for clinical applications. Because 3D printing is an additive manufacturing technique, the porosity and roughness of a surface can be appropriately adjusted using this technique. Previous studies have reported that a titanium alloy implant with appropriate porosity or roughness can enhance tissue integration [11; 12; 15; 17; 18; 19; 20; 21]. However, to our knowledge, this potential has not yet been evaluated in a clinical application in humans. In this study, a retrieved 3D-printed titanium alloy implant from the human forearm was histologically analyzed for its osseointegration and soft tissue integration ability. The junctional region of this implant comprised a semi-circular patient-specific skirted plate and a mesh structure at the edge of the bone contact surface. Major considerations during the follow-up period included bone remodeling at the bony ends of the host and osteo/fibrosynthetic effects caused by the mesh structure or the roughness of the implant surface. An appropriate pore structure is important to overcome small gaps that are formed between the implant and the host bone. We observed that osteointegration with such small gaps was achieved at segments containing the mesh structure but not at those not containing the mesh structure. A few animal studies have previously focused on the osteointegration of a 3D-printed titanium implant [11; 12; 15; 20; 21; 27], and three of these specifically focused on using vertebral cages for flat \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Measured Segment** & **BIC (\%)** & **SIC (\%)** & **TC (\%)** \\ \hline Proximal & & & \\ P1 & 21.93 & 0 & 21.93 \\ P2 & 26.47 & 0 & 26.47 \\ P3 & 0 & 60.77 & 60.77 \\ P4 & 10.81 & 71.24 & 82.05 \\ P5 & 0 & 21.53 & 21.53 \\ \hline Overall & 8.47 & 37.5 & 45.97 \\ \hline Distal & & & \\ D1 & 0 & 34.13 & 34.13 \\ D2 & 0 & 50.81 & 50.81 \\ D3 & 0 & 0 & 0 \\ D4 & 8.91 & 8.01 & 16.92 \\ D5 & 0 & 1.74 & 1.74 \\ \hline Overall & 2.32 & 12.71 & 15.03 \\ \hline \hline \end{tabular} BIC: rate of bone-to-implant contact, SIC: rate of soft tissue-to-implant contact, TC: rate of total contact.  
\end{table} 
Table 1: Measurements for the 3D-printed implant and bone contact surfaces. bones [12; 15; 20]. However, it is difficult to compare the results of these studies with those of the present study, which used the long tubular bone. Another experimental animal study exhibited bone ingrowth at the cortical surface of flat bones and at the implant contact area [21; 27]. Herein, the total bone growth rate was less than 10%, and the osseointegration rate for the implant most proximal to the host bone bed was approximately 20%. Another study focused on osseointegration in the long bones of animals in accordance with the pore size of the implant produced through selective laser sintering, a 3D printing method [11]. Therein, the small and large implant pores had diameters of 700 \(\upmu\)m and 1500 \(\upmu\)m, respectively, and the osseointegration rates were 32.2% (72.4 mm/224.9 mm) and 40.6% (42.9 mm/105.7 mm), respectively. In the present study, the pore size of the meshed region of the implant was observed to be 2200 \(\upmu\)m (2.2 mm), which is greater than that of the large pores in the aforementioned study. Because of several differences between the two studies, such as pore size, recipient species (sheep vs. human), and implant design (exterior collar vs. internal contact surface), direct comparison of osseointegration rates did not yield significant results. However, implant osseointegration was nevertheless achieved upon overcoming small gaps between the implant and host bone, concurrent with previous studies. The novel design of the present implant enhanced the structural mechanical strength properties and thus can potentially protect against cortical atrophy. A half plate with a skirt at the base was utilized at the junctional site between the implant and the host bone.