Individualized Determination of the Mechanical Fracture Environment After Tibial Exchange Nailing—A Simulation-Based Feasibility Study
https://www.frontiersin.org/journals/surgery/articles/10.3389/fsurg.2021.749209/full
Non-union rate after tibial fractures remains high. Apart from largely uncontrollable biologic, injury, and patient-specific factors, the mechanical fracture environment is a key determinant of healing. Our aim was to establish a patient-specific simulation workflow to determine the mechanical fracture environment and allow for an estimation of its healing potential. In a referred patient with failed nail-osteosynthesis after tibial-shaft fracture exchange nailing was performed. Post-operative CT-scans were used to construct a three-dimensional model of the treatment situation in an image processing and computer-aided design system. Resulting forces, computed in a simulation-driven workflow based on patient monitoring and motion capturing were used to simulate the mechanical fracture environment before and after exchange nailing. Implant stresses for the initial and revision situation, as well as interfragmentary movement, resulting hydrostatic, and octahedral shear strain were calculated and compared to the clinical course. The simulation model was able to adequately predict hardware stresses in the initial situation where mechanical implant failure occurred. Furthermore, hydrostatic and octahedral shear strain of the revision situation were calculated to be within published healing boundaries—accordingly the fracture healed uneventfully. Our workflow is able to determine the mechanical environment of a fracture fixation, calculate implant stresses, interfragmentary movement, and the resulting strain. Critical mechanical boundary conditions for fracture healing can be determined in relation to individual loading parameters. Based on this individualized treatment recommendations during the early post-operative phase in lower leg fractures are possible in order to prevent implant failure and non-union development.
Introduction: Despite current clinical advances diaphyseal tibial fractures are associated with delayed- and non-union rates of over 10% (1–3). The development of a healing delay is dependent on many factors that often cannot be adequately influenced once the fracture has occurred (4). Of high significance in aseptic cases are vascularity and mechanical fracture environment (5). To determine the relevant mechanical influences on fracture healing numerical modeling and computer simulation has gained increasing interest (6). Based on the initial ideas of Pauwels, Wolff, Perren, and Frost ever more precise mechanical fracture environment boundary conditions for influencing tissue differentiation can now be given (7–12). Despite the differences between the models owed in part to the specifics of the simulations and input characteristics, these approaches and their experimental validation in animal research underscore the great importance of the mechanical environment for fracture healing. Two of the most relevant parameters to determine tissue differentiation during the course of fracture healing are interfragmentary shear strain and tensile or compressive volumetric strain (13, 14). While the clinical use of these simulated parameters has been shown in the case of a patient treated with an external fixator (15), these boundary conditions for hydrostatic pressure and volumetric fracture strain have yet to be applied to a clinical case with internal osteosynthesis. Despite its theoretical relevance especially in cases where failed fracture healing and failure of implant material point toward a high mechanical influence this has not been performed. Current clinical management is largely based on general treatment principles and surgeon experience depending on the applied hardware.
The aim of this study was, thus, to establish a simulation workflow based on clinical imaging data to (1) determine the pre- and post-treatment mechanical fracture environment in a tibial fracture revision case, (2) simulate the associated volumetric strain and octahedral shear strain resulting from patient weight-bearing, and (3) provide a clinical proof-of-concept over the treatment course.
Materials and Methods: A 55-year old, female patient (height 152 cm, weight 73 kg) was treated for an open distal tibial shaft fracture (Figures 1A,B) with debridement, temporary external fixation, negative pressure wound therapy, unreamed intramedullary tibial nailing (8 mm diameter) (Figures 1C,D) and MESH graft skin closure at an external institution within the span of 4 weeks. Immediate post-operative full weight-bearing was ordered. Approximately 7 weeks after the tibial nail procedure was performed and without further trauma the patient suffered from an implant failure and refracture of the initial situation (Figures 1E,F). She was then referred for treatment to our institution, where after an initial hardware removal, temporary external fixation and histological and microbiological exclusion of infection, a reamed nailing procedure (9 mm nail diameter) and fibular plate osteosynthesis was performed (Figures 1G,H). Again, immediate post-operative full weight-bearing was performed and reached within the inpatient stay, as controlled with plantar pressure measurements (16). To estimate the non-union risk of the patient the Non-Union Risk Development (NURD) Score was calculated as 7 (17). Furthermore, the patient suffered from a severe Neurofibromatosis, that if counted as a chronic condition would increase the NURD score further to 9. The patient consented to the study. Only routine imaging data was used and the study was approved by the local ethics committee.
