Femoral and tibial insert downsizing increases the

Authors: John Kyle P Mueller Fred A Wentorf Richard E Moore

Publish Date: 2014/10/02

Volume: 22, Issue: 12, Pages: 3003-3011

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Abstract

A robotic testing system was utilized to measure laxity envelopes in the implanted knee by in the anterior–posterior AP medial–lateral ML internal–external IE and varus–valgus VV directions Five freshfrozen cadavers were tested with a modern cruciate retaining TKA implantation a 1mm thinner polyethylene insert and a femoral component 2 mm smaller in the AP dimensionThe downsized tibial insert was more lax throughout the flexion arc with up to 20 mm more laxity in the AP direction at full extension a 438  increase over the original implantation A thinner insert consistently increased laxity throughout the arc of flexion in all degrees of freedom Downsizing the femoral component resulted in 85 mm increase in AP laxity at 90° a 739  increase In midflexion downsizing the femur produced similar laxity values to the downsized insert in AP ML IE and VV directionsDownsizing the TKA components had significant effects on laxity throughout flexion Downsizing a femoral component 2 mm had an equivalent increase in laxity in midflexion as downsizing the tibial insert 1 mm This study quantifies the importance of choosing the appropriate implant component size having the appropriate size available and the effect of downsizing The laxity of the implanted knee contributes to how the implant feels to the patient and ultimately the patient’s satisfaction with their new kneeTotal knee arthroplasty TKA is a successful treatment for knee pain due to osteoarthritis However 25  of patients are dissatisfied after their TKA surgery 18 Common complaints include less than desired range of motion ROM an unnatural feeling knee instability and difficulty performing daily activities like ascending or descending stairs 4 Reduced function has been attributed to laxity in the knee 1 22 and a recent review of US and Norwegian registries found that between 16 and 20  of revisions are due to instability 20Laxity and functional kinematics have been published in both the intact and implanted knee 1 5 8 9 12 14 15 24 25 35 however the effect of downsizing the femoral component and polyethylene insert on the knee’s laxity envelope is not well reported Surgeons strive to restore function and feel of the unimplanted healthy knee by matching native boney and soft tissue anatomy with the TKA component options available 19 26 The challenge for the surgeon is to create a stable and functional joint that is not too tight or loose 21 33 There are several techniques to tune the knee to the desired laxity including adjusting tibial slope femoral orientation and soft tissue releases 1 3 Choosing the correct component size is part of this process and offers another variable for the surgeon to adjustIf the knee is tight in extension the surgeon may downsize the tibial insert 21 If the native femoral anterior–posterior AP dimension lies somewhere between available sizes or the knee is tight in flexion the surgeon may downsize to a smaller femoral component 13 21 Although it is understood that both of these adjustments increase the laxity the magnitude of this increase has not been investigated in detail and the clinical consequences of this increased laxity are still not well understood 1 30 Is there a way to make this balancing act of tuning joint laxity easier for the surgeon and better for the patientsA traditional TKA system has 3 or 4 mm increments in the AP dimension between femoral sizes and 2 mm between tibial insert thicknesses To better match the native anatomy and offer more options to better balance the knee TKA manufacturers have recently gone as far as offering custom implants Others have increased the number of component sizes available decreasing the increments between sizes Some may question whether the costs associated with the extra instrumentation manufacturing and inventory of these designs outweigh the benefits This current study quantifies the changes in knee laxity as component sizes are changed

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