Medical Polymer Group

Novel Research in Orthopaedic and Biomaterials

 

 

 

Wear Testing

The figure below shows our custom-built, multi-directional tribological system. This system, nicknamed "Elvis", was constructed in our lab to perform wear testing of metal-on-polymer bearing system used in orthopedic devices. This system utilizes a ball-on-flat setup to simulate three basic joint kinematic motions - translation, rotation, and rolling - to evaluate the amount of potential wear a material will have in vivo. Previous work in our lab has demonstrated a positive correlation between ultrahigh molecular weight polyethylene (UHMWPE) wear and cross-shear that results from multidirectional motion. Current projects involve testing a polycarbonate-urethane material (Bionate, DSM Biomedical) that could potentially be used as a counter-bearing material in the shoulder joint.

Aside from testing new materials, Elvis is currently being adapted to replicate shoulder kinematics to develop a clinically-relevant wear gait model of the joint to better understand the relationship between shoulder designs and joint kinematics on wear. Ongoing work is focused on understanding the influence of fixation and conformity on wear and surface damage on UHMWPE. With our extensive library of retrieved shoulder devices, we are currently taking advantage of our damage assessment protocol and profilometry work to directly correlate damage reproduced in our laboratory simulator with in vivo damage seen on devices obtained from patients.

References

Patten, E. W., Van Citters, D., Ries, M. D., & Pruitt, L. A. (2013). Wear of UHMWPE from Sliding, Rolling, and Rotation in a Multidirectional Tribo-system. Wear. 304 (1-2), 60-66.

Fatigue Crack Propagation of UHMWPE

In recent years, efforts to improve the wear resistance of UHMWPE has led to the development of improved microstructural toughness. One such method includes radiation crosslinking of the UHMWPE chains, which greatly improves wear rate but at the cost of oxidative and fracture resistance. Studies in our lab have explored the role that radiation crosslinking, heat treatments, and antioxidants have played in the fatigue crack propagation resistance of UHMWPE utilizing a linear elastic fracture mechanics approach. We particularly focus on developing structure-property relationships that dictate crack growth through a combination of both mechanical testing and microstructural evaluation (SEM, TEM, DSC, etc.).

Recent efforts are currently focusing on how design features, such as notches included in hip and knee implants to improve fixation and kinematic stability, affect crack growth. This work derives from several retrieval studies recently published by our lab and collaborators in which stress concentrations played a key role in the failure of UHMWPE components. Ultimately this work seeks to further our knowledge of the role that microstructure, environment, and design plays on crack growth in UHMWPE.

References

Atwood, S. A., Van Citters, D. W., Patten, E. W., Furmanski, J., Ries, M. D., & Pruitt, L. A. (2011). Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene. Journal of the Mechanical Behavior of Biomedical Materials4(7), 1033-1045.

Pruitt, L. A., Ansari, F., Kury, M., Mehdizah, A., Patten, E. W., Huddlestein, J., ... & Ries, M. D. (2013). Clinical trade-offs in cross-linked ultrahigh-molecular-weight polyethylene used in total joint arthroplasty. Journal of Biomedical Materials Research Part B: Applied Biomaterials. 101B (3), 476-484.

Ansari, F., Chang, J., Huddleston III, J., Van Citters, D., Ries, M., & Pruitt, L. (2013). Fractography and oxidative analysis of gamma inert sterilized posterior-stabilized tibial insert post fractures: Report of two cases. The Knee, 20 (6), 609-613.

Retrievals Analysis

Our lab currently works with three orthopedic surgeons to collect and analyze joint replacements that have been retrieved from patients due to failure. Our analysis spreads across both metallic and polymeric components, with a primary focus on total shoulder arthroplasty.

References

Pruitt, L. A., Ansari, F., Kury, M., Mehdizah, A., Patten, E. W., Huddlestein, J., ... & Ries, M. D. (2013). Clinical trade-offs in cross-linked ultrahigh-molecular-weight polyethylene used in total joint arthroplasty. Journal of Biomedical Materials Research Part B: Applied Biomaterials. 101B (3), 476-484.

F. Ansari, C. Major, T. R. Norris, S. B. Gunther, M. Reis, L. Pruitt. (Accepted for publication Nov 2013) “Unscrewing Instability of Modular Reverse Shoulder Prosthesis Increases Propensity for In Vivo Fracture: A Report of Two Cases.” The Journal of Shoulder and Elbow Surgery.

Ansari, F., Chang, J., Huddleston III, J., Van Citters, D., Ries, M., & Pruitt, L. (2013). Fractography and oxidative analysis of gamma inert sterilized posterior-stabilized tibial insert post fractures: Report of two cases. The Knee, 20 (6), 609-613.

Atwood, S. A., Patten, E. W., Bozic, K. J., Pruitt, L. A., & Ries, M. D. (2010). Corrosion-Induced Fracture of a Double-Modular Hip ProsthesisA Case Report. The Journal of Bone & Joint Surgery, 92(6), 1522-1525.

Furmanski, J., Anderson, M., Bal, S., Greenwald, A. S., Halley, D., Penenberg, B., ... & Pruitt, L. (2009). Clinical fracture of cross-linked UHMWPE acetabular liners. Biomaterials, 30(29), 5572-5582.

UHMWPE Glenoid Retrievals

Our lab has an extensive retrieval database of UHMWPE glenoids with various fixation designs. Many of these

implanted glenoids have varying forms of damage after implantation in the body. Our lab has developed a scoring methodology for assessing macroscopic wear on retrieved UHMWPE glenoids. Information gathered from ongoing damage assessment of these implants is currently being statistically correlated with patient data, design variables, and surgical techniques to elucidate mechanisms of device failure in vivo.

References

Malito, L., Ansari, F., Mehdizadeh, A., Koller, J., Gunther, S., Norris, T., Ries, M., Pruitt, L. “Bearing Surface Damange Analysis of Coupled Total Shoulder Replacement Retrievals.” Poster #657 at the Society for Biomaterials Meeting, Boston, Massachusettes, April 10-13, 2013.

Cobalt Chrome Humeral Head Retrievals

Our lab has developed a scoring methodology for assessing  macroscopic wear on cobalt chrome (CoCr) humeral head retrievals establish a more thorough understanding of the tribological effects of Total Shoulder Replacements (TSRs) within the joint space. Current work involves the continued collection of bearing surface damage for over 100 implants in our database to perform a robust statistical analysis that correlates damage types and clinical/patient data.

References

Ansari, F., Patten, E., Cruz, C., Beitel E., Swan, A., Gunther, S. B., Norris, T. R., Ries, M., Pruitt, L.  “Profilometry analysis and improvements of a novel damage scoring method for metal bearing surfaces of shoulder replacements.” Presented at the ASME 2012 Summer Bioengineering Conference, Farjardo, Puerto Rico, June 20-23, 2012.

3D Profilometry Analysis of Surface Damage on CoCr Humeral Head Retrievals

To complement macroscale analysis performed on CoCr humeral heads, microscale analysis using 3D profilometry is currently being used to assess individual damage types (e.g. scratching, abrasion, pitting. Microscale roughness measurements has revealed that scratches exhibit higher peaks and deeper valleys, which abrasive modes may have the a greater degree of material pileup. Ongoing analysis seeks to further correlate these roughness trends on a larger sampling of devices and verify such trends have implications for counterbearing wear.

References

Ansari, F., Beitel, E., Swan, A., Gunther, S., Norris, T., Ries, M., Pruitt, L. “Analysis of Scratching Damage on Cobalt Chrome Humeral Head Retrievals Using 3D Profilometry.” Poster #1857 at the Orthopedic Research Society Meeting, San Antonio, Texas, January 26-29, 2013

Modular Junctions in Reverse Shoulder Arthroplasty

Reverse Total Shoulder Arthroplasty has been introduced in recent years to overcome instabilities presented by hemi- and total arthroplasty. This device design reverses the ball and socket design of the natural shoulder, moving the cup ("socket") to the upper arm and the ball to the scapula. Many Reverse Shoulder Replacements (RSRs) are designed as modular components that either screw or press fit together in an assembly to facilitate future revisions and enable customized fitting to different patients. However, modular RSRs are often implanted in patients with proximal humeral fractures who thus exhibit little to no bone support around the upper half of the implant. With lack of bone support and implant modularity, the stem has a tendency to unscrew from the cup portion of the implant. As a result of unscrewing, the RSR can potentially fret at the screw threads, leading to metallosis or even device fracture. The goal of the project is to reconstruct different RSR modular assemblies and assess their propensity for fracture as a result of unscrewing.