(L-443) Contact Response of 3D-Printed Implants with Additive Surface Features

Introduction:: 3D-printing affords designers the benefits of biomimetic structures to optimize fusion rates with inclusion of teeth and/or outer framework to help resist migration and enhance structural integrity. However, little is known how these features may alter the propensity for subsidence. While clinical subsidence is a dynamic cyclic event and not due to a single catastrophic overloading condition, the current study addresses implant/substrate contact under static uniform conditions to elucidate the effects of traditional surface features upon surface engagement by the implant surface. The investigators hypothesize that an open structure allows for increased surface engagement and load transfer via increased interdigitation with the underlying surface.


Materials and Methods:: Randomized and equal allocation of twenty 20 PCF (Sawbones USA, Vashon Island, WA) bone blocks were cut into 30 mm X 30 mm X full height blocks and assigned to one of four 3D-printed intervertebral body implant groups possessing equal dimensions and matrix configuration but differing in surface geometry: from the same manufacturer; Base Matrix (BM, Xenix Medical, Orlando, FL), BM+Frame (BMF), BM+Teeth (BMT) and BM+Frame+Teeth (BMFT) (Figure 1A). For the devices containing a circumferential frame, the inner matrix was thinned to maintain comparable surface area to isolate the surface features of Frame and Teeth. Ultralow Fuji film (Sensor Products Inc., Madison, NJ) was placed on the surface of the bone blocks. Static compression testing was performed using each implant type to a load of -350 N at a rate of 10 N/s and sustained for 15 seconds (Figure 1B). The exposed film was analyzed using Topaq software (Sensor Products Inc., Madison, NJ). Mean pressure and surface engagement were normalized to the BM implant (BM=1) and compared using a 1-way ANOVA with a Holm-Sidak post-hoc tests.


Results, Conclusions, and Discussions:: The Topaq software converted the exposed Fuji film to a pressure map to extract surface engagement and pressure data (Figure 1C). The BM and BM+Frame designs displayed decreased mean pressures (P < 0.0001) and were not different from each other (Figure 2A). More specifically, designs incorporating Teeth upon the surface resulted in increased pressures. With respect to surface engagement, the BM design displayed statistically increased engagement compared to all other designs incorporating a Frame and/or Teeth (P < 0.03 for all, Figure 2B). Designs incorporating a Frame and/or Teeth were not statistically different from each other. The BM and BM+Frame designs displayed a statistically decreased pressure as compared to all other designs. The interdigitation of the matrix without the presence of the Teeth feature likely permits increased surface engagement as the BM and surrounding frame allow for increased contact with the substrate and can therefore result in decreased pressure as compared to point contact in the case of Teeth residing on the substrate. It should be noted that in addition to decreased pressure, the BM design also demonstrated statistically increased surface engagement as compared to all other designs. The combination of increased engagement and decreased pressure in the case of the BM design is a favorable mechanical environment for subsequent fusion through load transfer without increasing the propensity for subsidence due to the increased contact forces. Subsidence is a phenomenon that occurs under cyclic conditions and is influenced by anatomical and physiological factors as well as the device design. Using implants of identical matrix supplemented with/without a circumferential Frame provided reduced pressures in comparison to the inclusion of Teeth upon the device surface. Furthermore, in the case of surface engagement, it was determined that the inclusion of a circumferential Frame with/without Teeth reduced the load transfer ability as evidenced by the significant reduction in engagement with the underlying substrate. The BM design without features likely allows for increased interdigitation with underlying surfaces, especially if one considers the non-uniform endplate surface under clinical applications.


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