Population-based evaluation of knee mechanics considering inter-subject and surgical alignment variability

Investigators: P. Laz and P. Rullkoetter
                   D. Dennis and R. Kim, Colorado Joint Replacement
Supported by: National Science Foundation, General and Age Related Disabilities Engineering under CBET Grant: 1034251

This NSF sponsored project involves the development of a statistical shape model of the knee and a computational methodology for performing population-based evaluations of natural and implanted joint mechanics considering inter-subject geometric, property and surgical variability. The statistical shape model and educational training components are publicly available; please contact us for more information.

Project Summary

Osteoarthritis affects 17.5 million people in the United States and results in more than 300,000 total knee replacements (TKR) each year. In the natural knee, joint mechanics provide insight into musculoskeletal function and are used to assess pathologies. In the implanted knee, resultant post-operative joint mechanics contribute to the success of TKR, which is influenced by implant design and component alignment. The most significant source of uncertainty in prior knee mechanics studies is patient-to-patient variability, yet it remains the least understood. Subject-specific models developed from imaging data provide the fidelity required to accurately represent anatomical structures. To then account for variability within a population, statistical shape models have been created to characterize the modes of variation across subjects. Current state-of-the-art analyses have developed shape models of individual bones, but have not considered shape models with multiple structures like a joint, nor integrated shape models into joint mechanics prediction.

Accordingly, the objective of this proposal is to develop a computational methodology for the population-based evaluation of natural and implanted joint mechanics considering inter-subject geometric, property and surgical variability. The specific aims are:

• To characterize geometric variability in a population of subjects with a statistical shape model of the structures of the knee.
• To evaluate variability in natural and TKR-implanted knee mechanics in a population using a probabilistic platform combining the statistical shape model with uncertainty in mechanical properties, e.g. ligament stiffness, and surgical parameters, e.g. alignment of a TKR component.
• To identify relationships between geometric variability and natural joint mechanics and between component alignment, geometric variability, and implanted joint mechanics using a novel combined probabilistic and principal component analysis technique.

The project will develop the statistical shape model of the knee from magnetic resonance image datasets from the Osteoarthritis Initiative and perform probabilistic finite element assessments of joint mechanics in natural and implanted knees under a simulated deep flexion loading condition.

Intellectual merit of the proposed activity
The intellectual merit of the proposed activity is in the novel development of a statistical shape modeling of the knee joint and the creation of a novel integrated framework where the effects of geometric and other uncertainties on joint mechanics can be holistically assessed. The population-based predictions can address concepts of form and function in the natural knee and provide insight into interdependencies between patient geometry and TKR component alignment and their impact on knee mechanics.

Broader impacts resulting from the proposed activity
The broader impacts are related to the dissemination of the findings by creating a website to allow the statistical shape model of the knee to be downloaded by other researchers and the publication of the newly developed methodologies. Additionally, based on the highly visual and relevant nature of the project, interactive curricular components will be developed, implemented in the Making of an Engineer outreach program which promotes engineering and the sciences to underrepresented minorities, and made publicly available to assist teachers in creating and carrying out science, technology, engineering and mathematics (STEM) curricula.

Current Research Results

The project has developed statistical shape and alignment model to characterize the anatomic variability present in the training sets of subjects. To account for alignment variability, the training set consisted of specimens at a known, loaded position from testing in an experimental simulator. The intersubject variability is represented by a series of modes of variation with images shown at +/- 2 standard deviations to highlight the geometric differences. The bone-only model has application to surgical instrumentation related to implant placement, while the bone and cartilage model represents the articular geometry for evaluations of natural joint mechanics. The statistical models are created in a finite element environment to facilitate investigations between shape and alignment and joint mechanics. The addition of soft tissue representations to the model is ongoing.

Most significant modes of variation from the bone-only statistical shape and alignment model.

Statistical shape and alignment model with bone and cartilage. First three modes of variation shown.

Publications

1. Fitzpatrick, C.K., Clary, C.W., Laz, P.J., and Rullkoetter, P.J., "Relative Contributions of Design, Alignment, and Loading Variability in Knee Replacement Mechanics". Journal of Orthopaedic Research, in press.
2. Rao, C., Fitzpatrick, C.K., Rullkoetter, P.J., Kim, R., Maletsky, L.P., Laz, P.J., "A statistical model accounting for intersubject shape and alignment variability in the knee". Medical Engineering and Physics, in review.
3. Rao, C., Deacy, J.S., Kaschinske, S., Fitzpatrick, C.K., Maletsky, L.P., Rullkoetter, P.J., Laz, P.J., 2012. Representing Intersubject Variability with a Statistical Shape and Alignment Model of the Knee. Transactions of the Annual Meeting of the Orthopaedic Research Society, 0859.

Outreach

Biomechanics learning modules were developed and taught in the Making of an Engineer – Engineering of Extreme Sports Summer Camp in July 2011. The Understanding Forces in the Body module included a lesson on forces and moments, experiments on muscle forces in the arm and leg, and an exercise using the Orthoload database to learn about joint forces. The Biomechanics: Materials, Imaging and Failure module included lessons on stress and strain and injury mechanisms, an exercise where students measure a ligament from a magnetic resonance image and compute strain, and discussion of total joint replacement, including current research directions.

Please contact us for the learning module materials or for information about participating in the program either for teacher training or as a student.

Making of an Engineer – Engineering of Extreme Sports Camp, 2011

High school students performing image based calculation of ACL length