114 - In Vivo Analysis of Wear Debris Produced from Titanium Plasma-Sprayed Implants: A Micro–Computed Tomographic Investigation

Introduction: Titanium plasma-spray (TPS) coatings have been shown to improve the rate and magnitude of osseous incorporation of orthopaedic implants. However, maintenance of coating integrity secondary to implantation and in vivo loading remains a critical concern. Clinically, metallic wear particulates can elicit a macrophage reaction, leading to implant loosening, pseudoarthrosis and increased risk of device failure.

Aims/

Objectives: Using an in-vivo ovine model and micro-computed tomography (micro-CT) assays, the purpose of the current investigation was to quantify size and numbers of residual titanium particles produced from TPS coated dowels following biomechanical extraction from ovine femoral and humeral sites following osseous ingrowth.

Methods: Twelve skeletally mature sheep were equally randomized into postoperative periods of 6 and 12 weeks. Four types of dowel implants--PEEK, titanium plasma-sprayed PEEK (TPS PEEK), titanium, and titanium plasma-sprayed titanium (TPS titanium) were implanted into cylindrical metaphyseal defects in the distal femurs and proximal humeri (n=48 sites). Half the specimens underwent destructive pullout testing followed by micro-CT of the bone void tunnel and remaining half quantitative micro-CT and histomorphometry to calculate bone volume and direct appositional osseointegration. Micro-CT scanning analyzed the bone void tunnel for metallic wear debris particulates (n=16 samples; TPS PEEK=6, TPS titanium= 5, and titanium=5). The number of particles per specimen and particle size in pixels (1 pixel = 15 µm) were quantified and compared between treatment groups using a One-Way ANOVA and Bonferroni post hoc test at each post-operative time interval. Significance was indicated at p≤0.05.

Results: The greatest number of titanium particles was quantified in the TPS PEEK group (24.3±5.0), followed by TPS titanium (18.0±11.3), and titanium (4.7±4.7) at 6 weeks (p≤0.05 for TPS PEEK vs. Titanium). Similar trends were observed at 12 weeks; however, fewer particles were detected when compared to 6 weeks (11.7±6.7, 8.7±7.5, and 0.5±0.7 for TPS PEEK, TPS titanium, and titanium, respectively) (p>0.05). Particle size was comparable between treatment groups at 6 weeks for the TPS PEEK (45.5±13.7 µm), TPS titanium (48.5±11.3 µm), and titanium (49.4±10.0 µm) groups. Similar particle sizes were measured at 12 weeks (47.8±5.3, 46.7±19.7, and 15.0±0.0 µm for the TPS PEEK, TPS titanium, and titanium groups, respectively). No significant differences between particle size were observed between treatment groups at both time intervals (p>0.05).

Conclusions: Previous in vitro studies reported an inordinately high number of particles following device implantation. The current project serves as the first in vivo analysis to document wear particulate numbers and size distributions localized at the implantation site following implant extraction. The values calculated were negligible ( < 100 particles) compared to those associated with a significant histiocytic reaction (>1x10^8). The use of novel 3D printing and plasma spray coating technologies of implants requires an assessment of the implant surface, potential for wear debris and surface coating delamination. Wear particle assessments may be overestimated based on in vitro testing. In vivo modeling provides a biologic environment consistent with the clinical conditions to assess surface coating technologies.