Efficacy of surface polishing clinically available internal fixation materials for ease of removal
Hayes, Jessica Susan
Removal of fracture fixation devices due to symptomatic or asymptomatic reasons is a costly procedure which is further increased when complications are encountered. One of the main difficulties faced by a surgeon is a timely and successful removal of the device due to excessive bone overgrowth. Due to the increased time required for removal of the bone from the device, complications such as increased surgery time, excessive blood loss, debris contamination and implant breakage are often encountered. Practically all internal fixation systems such as locking compression plate constructs, and intramedullary nails, are fabricated for clinics with a micro-rough surface. However, it is well known, that surface micro-topography can be a major factor in determining the type, and extent of tissue integration. Despite the knowledge that surface microtopography can be a major determinant of osseointegration, this avenue has only been investigated for applications requiring accelerated bony integration and has not previously been explored as a potential resolution to issues involving device removal. Thus, we hypothesise that reducing the surface micro-topography of clinically available materials will reduce the incidence of excessive bony over-growth, and consequently will ease implant removal. To investigate this hypothesis, we first characterised, using atomic force microscopy, scanning electron microscopy, contact angle, and white light profilometry, changes in the surface micro-topography of the clinically available materials commercially pure titanium (cpTi), titanium-6%aluminum-7%niobium (TAN), and titanium-15%molybdenum (Til5Mo) in their standard micro-rough form, as well as in their experimental electropolished, and paste polished form. Stainless steel (Ss) was included as the orthopaedic grade 'smooth9 surface control. Additionally, we employed X-ray photoelectron spectroscopy to examine the changes, if any, in the chemical composition of the surfaces. Overall, it was shown that both electropolishing and paste polishing techniques were successful at reducing the micro-roughness of the respective materials, as well as producing a smoothened surface morphologically. As all surfaces were anodised subsequent to polishing, alterations in the surface chemical composition were not evident. Initial in vitro analysis, which involved culturing rat calvarial cells on the various surfaces, showed that surface polishing did not significantly influence cell growth compared to standard micro-rough counterparts. Furthermore, viability on these surfaces was not found to be significantly different from standard micro-rough samples, indicating that overall, surface polishing does not affect the cytocompatibility of cpTi, TAN or Til5Mo. However, paste polished TAN was found to be the exception. Specifically, viability was significantly reduced on this surface compared to standard micro-rough TAN. While cell growth was not significantly affected on paste polished TAN, it was observed that initial cell attachment for this surface was lower compared to standard micro-rough TAN. Fluorescent labelling of the cytoskeletal components actin, tubulin and vinculin revealed that cell morphology was differentially influenced by the various surface morphologies. Generally, polished surfaces advocated a more well spread, elongated phenotype compared to the cuboidal phenotype noted for cells cultured on micro-rough surfaces. Paste polished TAN and standard micro-rough TAN samples also showed some degree of cytoskeletal disruption attributable to the inherent presence of beta-phase particles on their surface. Subsequent analysis of genotype, using real time PCR technology, revealed that for cpTi, TAN and Til5Mo samples, surface polishing essentially provokes its influence in different manners at initial stages of differentiation and consequently then similarly at the later stages of terminal differentiation, and that the magnitude of this effect is material dependent.. Subsequently, we applied surface polishing technology to two clinically relevant internal fixation systems, namely locking compression plates (LCP) and screws, and intramedullary nails (IM), to assess if surface polishing holds potential for reducing implant removal related morbidity. In our LCP model electropolished, paste polished and standard TAN cortical screws were evaluated in combination with electropolished, paste polished and standard cpTi plates, respectively, with stainless steel screws and plates included as a control system. Samples were implanted in a bilateral non-fracture sheep tibia model for 6, 12 and 18 months. At each timepoint, removal torque and percentage of bone contact were quantified for each screw type. Results indicate that both polishing techniques significantly reduce the torque removal required for cortical screws, compared to standard micro-rough cortical screws. Histomorphometric analyses indicated that polished constructs showed a trend for reduced bone contact, however, this was only found to be significantly different for paste polished screws. In our second in vivo model, IM nails fabricated from standard micro-rough TAN and paste polished TAN were implanted in a bilateral, non-fracture sheep model for 12 months. Control animals were implanted with Ss IM nails and standard micro-rough TAN IM nails in the contralateral tibia.