Development of Distal Ulna Small Locking Bone Plate Using Fused Deposition Modeling

Abstract

Partial or full discontinuity in bone tissues is characterized by bone fractures that are mainly caused due to accidents, aging and impact of stress on bone. Distal radius and ulna fractures constitute one of the most common fractures of the bone injuries. Small locking plate is a type of bone plate that is commonly preferred by orthopedic surgeons for healing of such bone fractures. The healing of these fractures highly depends on the biocompatibility, stability in biological conditions, biodegradability, technical functionality and shelf-life of biodegradable biomaterials. Prolonged existence of metallic bone plates in human body results in stress shielding, release of toxic ions, excessive corrosion in human body and requirement of operation for removal of bone plate. Polymer materials are being explored for bone implants due to their light weight, biocompatibility, biodegradability and absence of stress shielding. Fabrication of customized patient specific implants by conventional manufacturing techniques results in high production time with reduced dimensional accuracy. In the era of Additive Manufacturing, Fused Deposition Modeling (FDM) is a powerful technique for fabrication of biomedical implants as it results in porous parts that cause infiltration of nutrients for improved bone ingrowth. Poly Lactic Acid (PLA) is a biocompatible and biodegradable polymer that is corrosion resistant, environment friendly, recyclable, easy to fabricate and can degrade within human body however, FDM based PLA structures lack mechanical and biological properties that can be enhanced by application of coatings. Polydopamine (PDM) is a biocompatible coating that undergoes covalent interactions with the substrate, enhancing the mechanical properties. In the present study, PDM has been used for development of PDM coated PLA bone plates with adequate mechanical and biological properties. The mechanical behavior of a structure is greatly influenced by the design of the respective structure. The distal ulna small locking bone plates were fabricated using FDM based experimental setup at varying infill patterns. The direct immersion process of coating PLA bone plates with PDM has been discussed. The effect of infill pattern on coating deposition and modification of mechanical properties was studied. The bone plates were fabricated at varying printing parameters and the effect of PDM coating was investigated on their tensile and flexural strengths, under varying coating conditions. The study involved characterization of uncoated and PDM coated bone plates using Scanning Electron Microscopy/ Energy Dispersive Spectroscopy, X-Ray Diffraction, Raman Spectroscopy along with measurement of surface roughness and water contact angle. The parametric based experimental investigation has been performed on maximum tensile and flexural strengths of bone plates using Response Surface Methodology (RSM) based Design of Experiments (DOE). The statistical analysis of printing process parameters and coating parameters on maximum tensile strength and maximum flexural strength have been discussed. The work involves RSM based optimization of printing and coating parameters for development of distal ulna-based bone plate with excellent mechanical strength. The statistical models were developed for output responses and confirmation experiments have been performed for validation of developed models. The high degradation rate can result in rapid decline of mechanical properties, which makes implant inadequate for structural support during degradation process. The unnecessary slow degradation rate can result in poor bone growth, hindering new bone regeneration. The porous bone plates fabricated by FDM provide the flexibility of altering the porosity that further affects the degradation rate of the fabricated structures. The developed bone plates have been examined for their degradation, mechanical and biological behaviour after preservation in Simulated Body Fluid and Hank’s Balanced Salt Solution for 90 days and 180 days. The DOE technique requires the conduct of a large number of experiments and the complexity increases with the number of experiments. As a result, Machine Learning (ML) has been used for forecasting the mechanical properties of orthopedic implants with varying process parameters. Furthermore, the optimization of process parameter values was performed using ML based optimization algorithms. The findings suggested that the deposition of PDM coating improved the tensile strength of bone plates from 41.673 MPa to 61.073 MPa. Similar improvement was found for flexural strength of bone plates. Also, the PDM deposition slowed down the degradation of bone plates, which could result in complete healing of bone fractures. Thus, it can be inferred that PDM coated PLA bone plates fabricated using FDM possess excellent mechanical and biological properties for fixation of distal ulna bone fractures.