Leveraging Mechanical Compliance in the Design of Orthopaedic Fracture Fixation Plates

Restricted (Penn State Only)
- Author:
- Huxman, Connor
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 04, 2024
- Committee Members:
- April Armstrong, Outside Field Member
Gary F Updegrove, Special Member
Gregory Lewis, Outside Unit Member
Jared Butler, Chair & Dissertation Advisor
Mary Frecker, Major Field Member
Robert Kunz, Professor in Charge/Director of Graduate Studies - Keywords:
- fracture fixation
compliant mechanisms
analytical modeling
finite element analysis
orthopaedic implant design
orthopaedic biomechanics
bone healing
bridge plating - Abstract:
- Surgical treatment of long bone fractures, such as in the humerus, femur, and tibia, is commonly performed using fracture fixation plates. A major limitation of traditional rigid plates, however, is their high axial stiffness, which has been shown to result in suppressed and asymmetric interfragmentary motion and can lead to nonunion in 5-15% of cases. The last half century of experimental and clinical research has investigated how mechanical factors affect fracture healing, highlighting the need for controlled axial motion between bone fragments to stimulate secondary healing. Several efforts have been made to design dynamic plates that can provide axial micromotion while still maintaining necessary strength under physiological loads. However, many of these plates do so at the expense of simplicity, requiring assembly of multiple components and materials. In this work, a new approach to designing dynamic fracture fixation plates is developed which incorporates compliant mechanisms, devices that get their motion from elastic deflection of flexible members. Compliant mechanisms offer significant potential in this space due to their reduced part count, decreased friction and wear, and ability to store strain energy. The objective of this research is to advance both compliant mechanism design and modeling methods as well as the current state of fracture fixation by developing technologies that could improve the strength and rate of bone healing. Specifically, this work develops new implant designs, compliant mechanism design approaches, analytical models, finite element models, and experimental approaches, culminating in a novel fracture fixation technology. First, a review of the mechanobiological factors for bone healing is conducted, followed by systematically reviewing previous dynamic plating technologies and identifying trends, limitations, and opportunities. New analytical models are then developed and validated for two types of flexures, fixed-clamped (straight) and serpentine (winding switchback) flexures, with utility in compliant orthopaedic implants. Next, finite element models are proposed and validated for predicting the performance of flexure-based plates and the response of such plates, bone, and screws under physiological load. Advanced finite element models are used to parametrically evaluate construct performance in response to changes in plate geometry and material, highlighting how the stiffnesses, strength, and flexure manufacturability will vary to accommodate varying levels of prescribed axial motion. Experimental testing is then conducted to compare novel compliant and rigid plates and their axial stiffness, strength, high-cycle durability, and symmetry of provided interfragmentary motion in a humeral diaphyseal shaft fracture model. Finally, extended applications of flexure-based implants are demonstrated through the design of new devices for long bone compression plating as well as anterior fracture fixation of patella fractures. This work demonstrates that the proposed compliant mechanism design approaches and models can be successfully leveraged in the design of fracture fixation implants as well as other flexure-based linear motion mechanisms. The described analytical models demonstrate excellent agreement to experimental data and allow researchers and designers to efficiently select flexure parameters for mechanisms across a wide range of applications. The described flexure-based implant technology demonstrates its ability to provide significantly greater and more symmetric interfragmentary motion into the range known to induce bone healing. This has the potential to provide surgeons with a new plating technology that enhances and controls stimulatory motion without changing the size, shape, material, or surgical technique. Overall, this dissertation illustrates how cross-functional engineering design with clinical collaboration can unlock innovation and lead to both new research methods and technologies with commercial potential.