The Relationship between Contact Pressure, Insert Thickness, and Mild Wear in Total Knee Replacements

• Published in: Computer modeling in engineering & sciences 2004-01, Vol.6 (2), p.145
• Main Authors: Bei, Y, Fregly, B J, Sawyer, W G, Banks, S A, Kim, N H
• Format: Article
• Language: English
• Subjects: Contact pressure; Fluoroscopy; Gait; Inserts; Knee; Mathematical analysis; Multibody systems; Polyethylene; Simulation; Slip velocity; Surgical implants; Thickness; Ultra high molecular weight polyethylene; Wear
• ISSN: 1526-1492; 1526-1506
• DOI: 10.3970/cmes.2004.006.145

Mild wear of ultra-high molecular weight polyethylene tibial inserts continues to affect the longevity of total knee replacements (TKRs). Using static finite element and elasticity analyses, previous studies have hypothesized that polyethylene wear can be reduced by using a thicker tibial insert to decrease contact pressures. To date, no study has taken this hypothesis to the next step by performing dynamic analyses under in vivo functional conditions to quantify the relationship between contact pressures, insert thickness, and mild wear. This study utilizes multibody dynamic simulations incorporating elastic contact to perform such analyses. \textit {In vivo} fluoroscopic gait data from two patients with different implant designs were used to drive dynamic contact simulations. The first design was coronally flat-on-flat while the second was coronally curved-on-curved. Variations in minimum plastic thickness (6, 8, 10, 12, and 14 mm) and applied load profile (corresponding to body masses of 50, 62.5, 75, 87.5. and 100 kg) were used to modify the contact pressures in each of 25 simulations performed with each implant design. Mild wear following five million cycles of gait was calculated from the contact pressure and slip velocity time histories of elements on the tibial insert surfaces. The maximum values of peak and average contact pressure during the gait cycle were found to be poor predictors of wear depth. In contrast, contact pressures were good predictors of wear volume when the pressures were varied by changing the applied load profile. However, when the applied load profile was fixed and the contact pressures varied by changing the insert thickness, no changes in wear volume were predicted. Decreases in contact pressure due to a thicker insert were offset by increases in contact area subjected to sliding in the wear calculations. These findings suggest that use of a thicker tibial insert may not necessarily lead to decreased mild wear in total knee replacements and that further investigation of this issue is warranted.