The Stryker/ORS Women’s Research Fellowship promotes women in science by providing an opportunity for a female ORS member who is a recent PhD in science or engineering to conduct research in the field of orthopaedic technology.
Amy L. Lenz, PhD is the recipient of the 2019 Fellowship and shares her experience. 


What is your specific area of research?

My research is focused on foot and ankle orthopaedic biomechanics.  My goal is to characterize healthy, diseased, and post-surgical foot and ankle morphology and in-vivo function to improve clinical treatment of ankle injuries leading to end-stage ankle osteoarthritis.  In collaboration with engineers, orthopaedic surgeons, computer scientists and physical therapists, we apply mechanical engineering principles to study in-vivo kinematics using bi-plane fluoroscopy and computationally assess morphology using statistical shape modeling.  The long-term goal is to define relationships between 3D morphology and in-vivo function to identify clinical indications for surgery with post-operative predictive outcomes.

What was the title of the project you submitted for the Stryker/ORS Women’s Research Fellowship?
“Advancing Total Ankle Replacement Through Morphometric and Kinematic Analyses”  

What were the specific aims of your project, and what were the results?
Aim 1. Quantify the 3D Shape of the Native Tibiotalar Joint and Subtalar Joint

Historically, our understanding of the morphology of the ankle joint complex (i.e., tibia, fibula, talus, and calcaneus) has been derived primarily from 2D measurements of plain film radiographs.  The advent of volumetric imaging, including weightbearing CT, has made it possible to generate 3D reconstructions of the bones of the ankle joint complex.  An augmented understanding of healthy anatomy could assist in the treatment of patients with conditions such as post-traumatic ankle osteoarthritis as well as provide information to assist in pre-operative planning for procedures such as total ankle replacement and ankle fusion.

Twenty-seven asymptomatic controls (age: 50.0 ± 7.3 years; height: 169.4 ± 6.4 cm; BMI: 25.3 ± 3.8 kg/m2; 7 males) underwent weight-bearing CT scans (Planmed Verity; 0.4 x 0.4 x 0.4 mm voxels) with IRB approval. For each subject, CT images were segmented to create 3D models of the tibia, fibula, talus and calcaneus (Amira, v6.0.1, Visage Imaging).  Surfaces representing right bones (tibia, fibula, talus, calcaneus) were reflected to appear as left bones and aligned using an iterative closest point algorithm as part of the preprocessing stage.  Procrustes analysis was used to remove scaling (i.e. size) from the shape modeling analysis.  Mean shapes for the tibia, fibula, talus and calcaneus were generated using statistical shape modeling (SSM) software (ShapeWorks, University of Utah). Correspondence particles from the SSM analysis were evaluated using principal component analysis (PCA). Significant (non-spurious) PCA modes of variation were identified using parallel analysis.  For each significant mode of variation for each bone, surface distances between the mean surface and those representing ± 2 standard deviations (SD) from the mean were quantified in PreView (v2.0, FEBio Software Suite, University of Utah), and visualized in PostView (v2.1.0, FEBio Software Suite, University of Utah).  For each participant the articulating surfaces of the tibiotalar, tibiofibular, talofibular and subtalar joints were isolated using the 2nd principal of curvature and a joint coverage calculation which identified regions of intersecting normal vectors. Then, identified articular surface nodal data points were used to calculate joint space distance, mean curvatures, and Gaussian curvatures for all participants using PostView (v2.1.0, FEBio Software Suite, University of Utah). Each participants’ talar or tibial model was compared with the SSM mean talar or tibial shape to determine common correspondence particle locations across all individuals. Joint congruency and distance were calculated at each correspondence particle across the population and viewed using MATLAB (MATLAB, R2017b, MathWorks, Natick, MA, USA).  Congruency was calculated at each point across the surface, with a value of 0 mm-1 representing two perfectly congruent surfaces at that point.

Seven PCA modes were determined to be significant for the tibia, fibula, talus and calcaneus. Anatomical variations per bone were observed across these significant modes.  Notably, the talar posterior process differed and the radius of curvature for the talar trochlea.  Tibial variation included a length difference of the medial malleolus and varying posterior curvature in the inferior articular surface of the tibiotalar joint.  Fibular variations included the articular slope and curvature on the distal fibula.  Calcaneal shape differences included an increased anterior/posterior length which corresponded to an overall decrease in calcaneal pitch and increased slope of the posterior facet articulating surface within the subtalar joint.  Joint coverage, space and congruency were also reported for the tibiotalar, tibiofibular, talofibular and subtalar joints to characterize joint parameters in a weightbearing position.

This aim resulted in three conference abstracts and two manuscripts currently in review and preparation.

Aim 2. Quantify Tibiotalar and Subtalar Kinematics in TAR Patients

Medical imaging techniques have been used to visualize two dimensional views of a TAR implant in-vivo but dynamic evaluation in precise 3D kinematics has not been conducted.  Biomechanical data could improve our clinical understanding of failures in total ankle replacement (TAR) patients, leading to better surgical approaches and implant designs.  Kinematics of the prosthetic tibiotalar joint in TAR patients have yet to be measured using dual fluoroscopy. With dual fluoroscopy, computed tomography images are acquired to track bone motion.  One challenge with this approach is dealing with metal artifact in the computed tomography (CT) images that distorts implant visualization and the surrounding bone to implant interfaces.  The first objective of this aim was to develop a methodology to measure in-vivo TAR kinematics using inputs of computer-aided design (CAD) models, dual fluoroscopy and CT imaging with metal artifact reduction. To develop this methodology, we created a hybrid three-dimensional (3D) model that contained both: (1) the segmented bone; and (2) the CAD models of the TAR components. We evaluated a patient following total ankle replacement to demonstrate feasibility.  The patient performed a self-selected overground walk during which dual fluoroscopy images were collected at 200 Hz.  In-vivo tracking verifications were performed during overground walking using a distance calculation between the implant articular surfaces to evaluate the model-based tracking 3D solution.  Tracking verification indicated realistic alignment of the hybrid models with an evenly distributed distance map pattern during the trial.  Articular surface distance calculations were reported as an average of 1.3 mm gap during the entirety of overground walking.  The successful implementation of our new tracking methodology with a hybrid model presents a new approach to evaluate in-vivo TAR kinematics.  Measurements of in-vivo kinematics could improve our clinical understanding of failures in TAR patients, leading to better long-term surgical outcomes.

Therefore, the second objective of this aim was to evaluate prosthetic tibiotalar and subtalar joint kinematics in participants with unilateral TAR and compare these results to data from healthy controls.  In this IRB-approved study, six asymptomatic participants with a unilateral Zimmer TAR and six healthy control participants were studied. Participants performed one trial of overground walking at a self-selected speed.  Kinematics were calculated using a landmark-based coordinate system and the Grood and Suntay method.  Kinematics were normalized based on gait events (heel-strike and toe off) and reported as a percentage of normalized stance.  Confidence intervals (CIs) visualized group profiles and statistical parametric mapping (SPM) evaluated differences between groups (p < 0.05).  SPM analysis yielded a p-value for reporting significance and a t* value reported directional changes when nearing significance.  Range of motion (ROM) was calculated between groups and statistically evaluated with a t-test (p < 0.05).

Sagittal plane tibiotalar TAR kinematics demonstrated a significant reduction in ROM, 12.8 ± 2.4°, compared to healthy controls 17.8 ± 4.6° (p = 0.05).  Tibiotalar TAR kinematics versus healthy controls were trending towards a significant reduction of dorsiflexion in late stance (Figure 1).  No statistical findings were noted for tibiotalar inversion/eversion angles.  TAR transverse kinematics qualitatively demonstrated increased variability and increased external rotation in late stance compared to controls.  Sagittal plane subtalar kinematics showed significant differences with increased TAR limb dorsiflexion from 70.3-87.5% normalized stance compared to healthy controls.  However, subtalar sagittal ROM was not significantly different between groups.  No statistical findings were noted for subtalar frontal plane kinematics.  Increased variability in subtalar TAR transverse kinematics was observed but no significant differences were seen compared to controls.

This aim resulted in two conference abstracts and two manuscripts currently with one in minor revisions and a second in preparation. 

What is the significance of your findings to the orthopaedic field and their potential impact on patient care?
Our statistical shape modeling results provided baseline measurements of a healthy control population that could be used as a reference for studying the progression of ankle osteoarthritis.  The modes of anatomic variation found herein could also inform the design of total ankle replacement prosthetics. As one example, current total ankle replacement implants have a single curvature for the talar dome, whereas our SSM data demonstrate that regardless of scale, the radius of curvature is a significant driver of anatomic variation across individuals.  Also, joint parameters established normative measurements of joint coverage, space and congruency throughout the tibiotalar, tibiofibular, talofibular and subtalar joints.  Future studies will compare these normative measurements to those quantified in patients with pathology to improve diagnosis and treatment decision making.

To our knowledge, this is the first study to investigate in-vivo tibiotalar and subtalar kinematics following TAR using dual fluoroscopy.  Altered TAR kinematics and ROM in the sagittal plane provides evidence that implant facilitated motion is altered compared to healthy individuals.  The bicondylar TAR design is intended to mimic normative motion in the sagittal and transverse planes.  However, we observed that participants did not utilize full sagittal plane motion, which may be related to implant design or secondary to habitual gait pattern tendencies persisting from pre-operative tibiotalar pain and limitations.  Altered TAR kinematics and compensatory subtalar motion may contribute to persisting postoperative pain, implant loosening, instability or secondary subtalar osteoarthritis. 

How has winning the fellowship impacted your career?
In the spring of 2019, I was faced with a challenging professional decision, to accept one of three engineering faculty offers, move and start a new chapter in life, or accept the Stryker/ORS Women’s Research Fellowship to continue the research that I was passionately pursuing as a postdoctoral researcher.  My decision to accept the fellowship and continue the positive momentum of research at the University of Utah changed my career trajectory in a very rewarding way.  The past year has enabled me to continue my research and at the end of my fellowship transition to a research faculty position at the University of Utah in Orthopaedics to grow a foot and ankle orthopaedic biomechanics program.  I am forever grateful for the support of this fellowship because it enabled me to seek and secure my dream job and successfully launch my career as an independent PI.      

Describe how will you continue your project?
I am continuing my work started in this fellowship in a few different research avenues.  First, I am pursuing funding to expand my statistical shape modeling work to characterize ankle osteoarthritis.  Secondly, I established a collaboration with Dr. Karen Kruger to continue investigating in-vivo foot and ankle kinematics with fluoroscopy imaging techniques for a multi-site study.  I look forward to continuing with the positive research momentum and establishing a strong foot and ankle orthopaedic biomechanics research program at the University of Utah.

In your spare time, what do you do for fun?
I love enjoying the outdoors with my amazing partner and our dogs.  He and I can often be found mountain biking, hiking, rock climbing and skiing in the mountains of Utah. 

What was the last book you read for fun? Would you recommend it?
I last read “The Slow Professor: Challenging the Culture of Speed in the Academy” by Maggie Berg and Barbara K. Seeber.  I would highly recommend it for any academic at any phase in their career because it provides a refreshing manifesto-like perspective that is often overlooked and not discussed about the rat-race of higher education.