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Wednesday, February 28, 2001

All mobile bearing designs don't cut stresses

The damage observed in Ultrahigh Molecular Weight Polyethylene (UHMWPE) total knee arthroplasty (TKA) component retrievals is the result of high cycle fatigue loads, which act on the tibial insert during daily ambulation. A study located in scientific exhibit 24 reveals the contact areas and stresses that are associated with tibial insert damage in a variety of mobile bearing knee designs during three highly loaded points in the walking gait cycle; heelstrike, midstance and toeoff. While all of the designs studied offered large contact areas and low stresses during the heelstrike portion of walking gait, several designs displayed less contact area and higher stresses as they moved into flexion during midstance and toeoff.

The researchers created a three-dimensional, finite element model for each mobile bearing design in the study by employing a process of reverse engineering where direct measurements are taken from manufactured and sterilized TKA components. The average loading conditions for the heelstrike (0 degrees of flexion, 2.5 times body weight), midstance (20 degrees, 2 x BW) and toeoff (15 degrees, 3 x BW) portions of the stance phase of the walking cycle were simulated. Contact areas and stresses on the tibial insert were calculated and their magnitudes and locations were then imaged photorealistically to allow visual comparisons of the different TKA designs for each loading condition.

While some designs maintained consistent contact areas and stress patterns during all three portions of walking gait, others changed from broad regions of contact with significant stress attenuation to narrow bands where the peak stresses indicated a greater potential for UHMWPE damage.

The researchers report that not all mobile bearing designs maintain conformity as they move through the stance phase. This decreases their effectiveness at maintaining large contact areas, lowering stresses and reducing UHMWPE tibial insert damage potential. A design conflict between the significance of maintained conformity over the range of flexion and that limited by the J-curve of a particular femoral component will need clinical reporting to establish efficacy. Both clinicians and regulatory agencies should monitor the increasing international use of these devices carefully, as their potential for reducing UHMWPE damage is seen to be design dependent.

Coauthors of the study are Edward A. Morra, MSME; Paul D. Postak, BSc; and A. Seth Greenwald, D. Phil. (Oxon), all of Orthopaedic Research Laboratories, Lutheran Hospital, Cleveland Clinic Health System, Cleveland, Ohio.

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2001 Academy News February 28 Index B

Last modified 20/February/2001 by IS