A New Approach To Multiplanar, Real-Time Simulation Of Physiological Knee Loads And Synthetic Knee Components Augmented By Local Composition Control In Fused Filament Fabrication

Despite numerous advances in biomedical engineering, few developments in surgical simulation have been made outside of computational models. Cadavers remain the primary media on which surgical research and simulation is conducted. Most attempts to quantify the effects of orthopedic surgical methods fail to achieve statistical significance due to limited quantities of cadaver specimen, large variations among the cadaver population, and a lack of repeatability among measurement techniques. The general purpose of the research covered in this dissertation is to develop repeatable simulation of physiological loads and develop techniques to fabricate a synthetic-based replacement of cadaver specimens. Future work applying this study’s methods and technology is expected to produce a synthetic human joint that can assist in isolating the effects of surgical techniques by supporting repeatable measurement and response to physiological loads. This dissertation consists of three aims that collectively provide novel advancements in biomechanics, rapid prototyping, and materials science. Aim 1 of this dissertation is the development of the University of Texas Joint Load Simulator (UTJLS), which can apply physiological loads with synchronous application of ground reaction forces, joint kinematics, and muscle forces. Aim 2 advances technology and methods for Local Composition Control (LCC) in Fused Filament Fabrication (FFF) and investigates the viability of commercially available filaments for application as synthetic tissue. Aim 3 produced a synthetic femoral anterior cruciate ligament (ACL) tibial complex (FATC) using aim 2’s techniques and characterized its mechanical response to tensile loads applied in two different directions. Aim 1 includes a description of the design, configuration, capabilities, accuracy, and repeatability of the UTJLS during physiological loads applied to four cadaver knee specimens. The UTJLS is a musculoskeletal simulator consisting of two robotic manipulators and eight musculotendon actuators. Sensors include eight tension load cells, two force/torque systems, nine absolute encoders, and eight incremental encoders. A custom control system determines command output for position, force, and hybrid control and collects data at 2000 Hz. Controller configuration performed forward-dynamic control for all knee degrees of freedom except knee flexion. Actuator placement and specimen potting techniques uniquely replicate muscle paths. Real-time tests included 47 heel and toe squat maneuvers with and without musculotendon forces. Accuracy and repeatability standard deviations across specimen during squat simulations were equal or less than 8 N and 5 N for musculotendon actuators, 30 N and 13 N for ground reaction forces, and 4.4 N m and 1.9 N m for ground reaction moments. The UTJLS is the first of its design type. Controller flexibility and physical design supports axis constraints to match traditional testing rigs, absolute motion, and synchronous real-time simulation of multiplanar kinematics, ground reaction forces, and musculotendon forces. System degrees of freedom, range of motion, and speed support future testing of faster maneuvers, various joints, and kinetic chains of two connected joints. In pursuit of Aim 2, a series of FFF printers with a hotend containing an actuated element for LCC were developed. Printing functions were supported by custom software, custom slicer configurations, and custom firmware. Aim 2 not only provided the means of fabricating synthetic tissue but also provided insight into synthetic tissue design. Eight blends of high-temperature polylactic acid (HTPLA) and Ninjaflex® (NF), a thermoplastic elastomer, were characterized through standard D638 tensile tests and custom interface bond strength tests. Tests determined material properties including modulus of elasticity (E), Poisson’s ratio, yield stress (σY), percent elongation at yield (EL%Y), ultimate tensile stress (UTS), and percent elongation at failure (EL%F). Ranges of material properties for E, σY, and UTS were 17.3 to 3483 MPa, 0.53 to 52.0 MPa, and 23.7 to 66.8 MPa respectively. These compositions encompass a wide range of material stiffness overlapping the reported stiffness of biological tissues including trabecular bone, subchondral bone, tendon, ligaments, and some articular cartilage. Results from these tests provide insight into synthetic tissue design by informing the designer of the synthetic materials properties from which tissue composition can be selected. Bond strength tests revealed potential increases in synthetic tissue strength by providing insight into the benefits of including functional gradients in the tissue design. When comparing a binary interface between the softest and hardest material to a configuration of seven incremental interfaces, σIBS is expected to effectively increase by 25% to 95%. Strength of such an interface may be further increased by relief of stress concentrators due to incremental changes in stiffness. Aim 3 tested the methods of aim 2 for synthetic tissue design and fabrication. A series of synthetic knee designs were developed from magnetic resonance images (MRI) of a single human left knee. A model of an FATC with functional gradients at the ACL’s insertion sites was developed and configured for tensile testing along the Tibia’s mechanical axis and the ACL’s anatomical axis. A method for designing and implementing soluble and insoluble support structures was developed to support fabrication. The ACL component of printed specimen were assigned a composition of 22% HTPLA and 78% NF, which was shown in aim 2 to have an E equal to 290 MPa. Synthetic FATC stiffness and ultimate load ranged between 265 to 330 N and 41 to 70.5 N/mm, respectively. The largest stiffness and ultimate load of FATC specimen during tensile tests was no more than 39% and 50% of what was reported during in vitro tests with identical loading conditions respectively. Failure of these specimen to perform closer to the corresponding biological system was primarily attributed to anisotropy and bending loads introduced by tensile load direction and knee flexion. The methods of aim 2 provide numerous opportunities to improve upon these results through design modification utilizing the mixing printer’s control over geometry and local composition