Abstract
Numerous studies have reported on the effects of modular insert design on stress at the tibial/femoral articular surface. However, while the insert / tibial component surface (“backside”) wear and motion have been investigated, backside stress is not well delineated. Because stress may be related to observed backside damage, this study addressed the backside stress response to insert thickness, material, and articular geometry.
Twelve Natural Knee II tibial inserts (Sulzer Orthopedics Inc.) with three thicknesses (6, 12.5, and 18.5 mm), two materials (Durasul and 4150 UHMWPE), and two types of condylar geometry (congruent and ultra-congruent) were tested. Fuji film was placed between the baseplate and insert. A femoral component was loaded onto the insert in axial compression at four times Body Weight. The film was scanned into Adobe Photoshop to measure mean and peak luminosity, which was converted into stress. Analysis of Variance was performed with main effects and all two-way interactions to determine significance.
The mean stress ranged from 0.61 to 3.92 MPa and the peak stress ranged from 2.17 to 10.4 MPa. Insert thickness significantly influenced both mean (p=0.001) and peak (p=0.001) backside stress. Stress for the 6 mm inserts (7.17 MPa mean, 9.91 MPa peak) were approximately 2.1 times the 12.5 mm inserts (3.47 MPa mean, 4.66 MPa peak), and were approximately 2.6 times the 18.5 mm inserts (2.74 MPa mean, 3.71 MPa peak). There was not a significant effect on mean or peak stress from material or condylar geometry. None of the interactions were significant.
This study provides two important contributions. First, it establishes the backside stress magnitude during simple loading. Second, the relationship between backside stress and the insert thickness is experimentally quantified. Understanding this stress magnitude and response may be important to controlling observed in-vivo backside damage.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
