Radiopacity Enhancements in Polymeric Implant Biomaterials: A Comprehensive Literature Review

Abstract

Polymers as biomaterials possess favorable properties, which include corrosion resistance, light weight, biocompatibility, ease of processing, low cost, and an ability to be easily tailored to meet specific applications. However, their inherent low X-ray attenuation, resulting from the low atomic numbers of their constituent elements, i.e., hydrogen (1), carbon (6), nitrogen (7), and oxygen (8), makes them difficult to visualize radiographically. Imparting radiopacity to radiolucent polymeric implants is necessary to enable noninvasive evaluation of implantable medical devices using conventional imaging methods. Numerous studies have undertaken this by blending various polymers with contrast agents consisting of heavy elements. The selection of an appropriate contrast agent is important, primarily to ensure that it does not cause detrimental effects to the relevant mechanical and physical properties of the polymer depending upon the intended application. Furthermore, its biocompatibility with adjacent tissues and its excretion from the body require thorough evaluation. We aimed to summarize the current knowledge on contrast agents incorporated into synthetic polymers in the context of implantable medical devices. While a single review was found that discussed radiopacity in polymeric biomaterials, the publication is outdated and does not address contemporary polymers employed in implant applications. Our review provides an up-to-date overview of contrast agents incorporated into synthetic medical polymers, encompassing both temporary and permanent implants. We expect that our results will significantly inform and guide the strategic selection of contrast agents, considering the specific requirements of implantable polymeric medical devices.

Keywords: biocompatibility; contrast agent; implant; polymers; radiolucent; radiopacity.

Figure 1

Hounsfield scale of different hard and soft tissues in the human body. Reproduced with permission from ref (34). Copyright 2020 MDPI.

Figure 2

Atomic numbers of elements commonly contained in contrast agents.

Figure 3

X-ray radiograph of specimens of commercial radiopaque bone cements, Palacos R (ZrO₂), Simplex P (BaSO₄), and nonpacified Palacos + IDX containing A, B, C = ZrO₂ (5, 10, 15 wt %); D, E = BaSO₄ (5, 10 wt %); F, G, H = IDX (5, 10, 15 wt %); and an aluminum wedge, all imaged with 0.1 m water phantom. Reproduced with permission from ref (12). Copyright 2004 John Wiley and Sons.

Figure 4

Anterior marker wire and posterior ball marker (shown with black arrows) enabled determination of fracture of this UHMWPE bearing. Reproduced with permission from ref (62). Copyright 2013 Elsevier.

Figure 5

(A) Digital radiograph of radiolucent UHMWPE cable (1), UHMWPE cable incorporated with Bi₂O₃ particles in different views (2–4), and a titanium cable (5–6) relative to an aluminum step wedge (B) radiograph of the radiopaque UHMWPE cable implanted in a sheep spine. Reproduced with permission from ref (26). Copyright 2017 John Wiley and Sons.

Figure 6

Micro-CT images of radiolucent PLLA bone screws (a, c) and radiopaque PLLA + 20 wt %. Fe₂O₃ particles (b, d) at 2 and 4 weeks, respectively, were implanted in white rabbits. Reproduced with permission from ref (40). Copyright 2015 W-J. Chang, Y.-H. Pan, J-J. Tzeng, T.-L. Wu, T.-H. Fong, S.-W. Feng, and H-M. Huang.