Finite Element Analysis for Surgical Decision Support of Intramedullary Nail Fixation for Proximal Femur Fractures
Open Access
- Author:
- Tucker, Scott Michael
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 15, 2019
- Committee Members:
- Gregory Stephen Lewis, Dissertation Advisor/Co-Advisor
Judith Todd Copley, Committee Member
John Elfar, Outside Member
John Spence Reid, Special Member
Lisa M Shantz, Outside Member
Gregory Stephen Lewis, Committee Chair/Co-Chair - Keywords:
- Orthopaedic Biomechanics
Finite Element Analysis
Intramedullary Nail
Femur Fracture - Abstract:
- Intramedullary nails are the most commonly used implant for surgical fixation of proximal femur fractures. Although these fracture fixation procedures are generally successful with fewer than 15% of intramedullary nailing cases progressing to nonunion,1 hip fracture cases represent nearly 75% of the annual $19 billion spent on fracture management in the United States.2 Clinical and research efforts to reduce these costs focus on mitigating failure risk and optimizing patient outcomes to reduce the incidence of costly hospital readmissions and secondary surgeries. Surgeons play a role in optimizing nailing constructs through selection of appropriate implant features as well as recommendation of postsurgical return-to-function limitations. However, quantitative data to support implant selection criteria are generally lacking. Chapter 1 provides a broad overview of the biomechanics and biomaterials of fracture fixation, including clinical considerations, fracture healing biology, biomechanics, biomaterials, and experimental approaches to fracture biomechanics. Chapter 2 narrows the reader’s focus to a review of intramedullary nail-fixed long bone fractures with emphasis on the femur. Relevant literature was collected, reviewed, and organized to provide a comprehensive picture of recent discoveries regarding intramedullary nail biomechanics. A discussion of some limitations of application of biomechanical results to clinical practice is presented in Chapter 2 to remind readers that the laboratory environment often requires simplification of authentic scenarios for proper variable control. Finite element computational models of proximal femur fractures fixed by intramedullary nailing were developed, validated, and interpreted in Chapters 3 and 4. Specifically, a wide array of clinically relevant intramedullary nail design variable combinations was simulated for nine common femur fracture types under peak gait loading conditions with a generic femur model. Biomechanical influences of implant features such as nail diameter, nail length, use or disuse of distal fixation screws, and material stiffness were assessed for each fracture type. Custom-written code was developed to automate model assembly for a commercial finite element solver. Model validation was achieved through comparison of predicted construct stiffness to previously reported mechanical testing data with similar design variables. Reported outcome measures of peak implant stress and motion at the fracture site are surrogates of relative construct failure risk and offer insights into optimizing construct selection to reduce failure rates and promote fracture healing. The results of parametric modeling are summarized here and presented in full detail in Chapter 3 of this work. Filling the reamed endosteal canal with the largest fitting nail diameter reduced axial and shear interfragmentary motion for all modeled fracture types. Nail length was less predictive of shear interfragmentary motion for most simulated fracture types than other construct variables such as nail diameter, distal fixation screws, and nail material properties. Furthermore, gapping at the fracture site predisposed the construct to higher implant stresses and larger interfragmentary motions. The biomechanical outcomes from this computational study can directly aid in surgical decision-making for optimizing hip fracture fixation with intramedullary nailing. The work presented in Chapter 4 utilizes a subset of gapped fracture models from Chapter 3 to provide insight into safe loadbearing levels based on shear motion at the fracture site. Successful union following fracture fixation surgery is partially dependent on how the fixation construct resists shear motion as the patient returns to functional activities. Orthopaedic surgeons attempt to optimize these biomechanics via implant selection as well as the ability to recommend specific loadbearing limits for the patient postoperatively. We show the loadbearing percentage that maintains shear motions at the fracture site below a literature-reported threshold for each construct. Specifically, the data show that full weightbearing achieves the shear motion threshold for short nails with distal fixation screws for all tested nail diameters in a pertrochanteric and an intertrochanteric fracture type but not for a subtrochanteric fracture. Increasing nail diameter increases the safe loadbearing limit for nearly all tested configurations, while removing distal fixation screws has mixed results depending on if the nail is short or long. Some assumptions of nonspecific modeling are then challenged in Chapter 5 though introduction of patient-specific fracture type to the otherwise generic simulations. A patient image based fracture shape-specific model was generated and demonstrated a clinically relevant decrease in peak implant stress of over 50%, suggesting that simulation results are indeed sensitive to patient-specific modeling. This Chapter 5 pilot study provides motivation for future investigation into how results of generic models can be utilized, offers suggestions for directing future modeling efforts, and questions the functionality of simplistic fracture classification for certain applications. Results from such efforts have direct implications on surgical resident training, offering previously unavailable detail on the specific biomechanical influences of implant selection in parametric fashion, as well as utility in presurgical planning. Overall, the work presented in this dissertation provides a clinically oriented comprehensive understanding of the biomechanics of intramedullary nailing of proximal femur fractures.