In this paper, we explore the feasibility of developing a novel flexible pedicle screw (FPS) for enhanced spinal fixation of osteoporotic vertebrae. Vital for spinal fracture treatment, pedicle screws have been around since the early 20th century and have undergone multiple iterations to enhance internal spinal fixation. However, spinal fixation treatments tend to be problematic for osteoporotic patients due to multiple inopportune variables. The inherent rigid nature of the pedicle screw, along with the forced linear trajectory of the screw path, frequently leads to the placement of these screws in highly osteoporotic regions of the bone. This results in eventual screw slippage and causing neurological and respiratory problems for the patient. To address this problem, we focus on developing a novel FPS that is structurally capable of safely bending to fit curved trajectories drilled by a steerable drilling robot and bypass highly osteoporotic regions of the vertebral body. Afterwards, we simulate its morphability capabilities using finite element analysis (FEA). We then additively manufacture the FPS using stainless steel (SS) 316L alloy through direct metal laser sintering (DMLS). Finally, the fabricated FPS is experimentally evaluated for its bending performance and compared with the FEA results for verification. Results demonstrate the feasibility of additive manufacturing of FPS using DMLS approach and agreement of the developed FEA with the experiments.
Internal fixation with the use of locking plates is the standard surgical treatment for proximal humerus fractures, one of the most common fractures in the elderly. Screw cut-out through weak cancellous bone of the humeral head, which ultimately results in collapse of the fixed fracture, is the leading cause of failure and revision surgery. In an attempt to address this problem, surgeons often attach the plate with as many locking screws as possible into the proximal fragment. It is not thoroughly understood which screws and screw combinations play the most critical roles in fixation stability. This study conducted a detailed finite element analysis to evaluate critical parameters associated with screw cut-out failure. Several clinically relevant screw configurations and fracture gap sizes were modeled. Findings demonstrate that in perfectly reduced fracture cases, variation of the screw configurations had minor influence on mechanical stability of the fixation. The effects of screw configurations became substantial with the existence of a fracture gap. Interestingly, the use of a single anterior calcar screw was as effective as utilizing two screws to support the calcar. On the other hand, the variation in calcar screw configuration had minor influence on the fixation stability when all the proximal screws (A-D level) were filled. This study evaluates different screw configurations to further understand the influence of combined screw configurations and the individual screws on the fixation stability. Findings from this study may help decrease the risk for screw cut-out with proximal humerus varus collapse and the associated economic costs.
Recent advancements in additive manufacturing (AM) have motivated researchers to consider this fabrication technique as a solution for challenges in patient-specific orthopaedic needs. Although there is an increasing trend in the applications of AM in medical fields, there is a critical need to understand the biomechanical performance of AM implants. In particular, design opportunities, anisotropic material properties and resulting stability of AM implant constructs for large bone defects such as osteosarcoma, comminuted fractures and infections are unexplored. This study aims to evaluate metal AM for complex fracture fixation using both computational and experimental methods. In addition, this research highlights the role of AM in the entire workflow to fabricate metal AM fixation plates for treatment of comminuted proximal humerus fractures. A new AM-centric patient-specific implant design for reducing common postoperative complications such as varus collapse and screw cutout is investigated. Biocompatible 316L stainless steel specimens processed in laser-powder bed fusion (L-PBF) is subjected to tensile testing and post-hoc microhardness to obtain orthotropic material properties of the AM implants. Subsequently, risk of screw cut-out is evaluated using finite element modelling (FEM) of AM implant-bone constructs. Parallel experiments included synthetic bones that are evaluated using a 3D motion capture system. The biomechanical tests are analyzed to quantify the medial fracture gap displacement among study groups subject to different loading conditions. The outcomes of this study suggest that the proposed AM-centric fixation plate design reduces average varus collapse (i.e. medial fracture gap displacement) by 47.2 % and risk of screw cut-out by 14.6 % when compared to the conventional plate design. Findings from this study can be extended to other patient anatomy, loading conditions, and AM processes.
