Biomechanical Consequences of Patellar Component Medialization in Total Knee Arthroplasty Carolyn Anglin, PhD,*yz§ Jill M. Brimacombe, MASc,* David R. Wilson, DPhil, O b # Bassam A. Masri, MD, FRCSC,# Nelson V. Greidanus, MD, FRCSC,# Jérôme Tonetti, MD,** and Antony J. Hodgson, PhD* Abstract: The optimal amount of patellar component medialization in knee arthroplasty is unknown. We measured the impact, on patellofemoral kinematics and contact force distribution, of 0.0-, 2.5-, and 5.0-mm patellar component medialization in 7 cadaveric specimens implanted with knee arthroplasty components. The knees were flexed dynamically in a weight-bearing rig. Medialization led to lateral shift of the patellar bone, slight medial shift of the patellar component in the femoral groove, lateral tilt of the patella, reduced patellofemoral contact force in later flexion, and lateral shift of the center of pressure in early flexion. Effects on shift and tilt were proportional to the amount of medialization. As a result of this investigation, we recommend medializing the patellar component slightly—on the order of 2.5 mm. Keywords: total knee arthroplasty, patella, kinematics, tracking, contact force, center of pressure, knee mechanics. © 2010 Elsevier Inc. All rights reserved. In total knee arthroplasty, opinion is divided regarding whether or not to medialize the patellar component in an effort to reduce the incidence of patellar subluxation and the need for lateral retinacular release. Clinical and biomechanical investigators have identified some of the consequences of medialization, but previous studies have each had their limitations. The limitations of clinical studies to date [1-4] are that: they are unable to test different component positions for the same individual; they cannot invasively measure contact forces or center of pressure (COP); and they are normally limited to static measurements at a single flexion angle, using radiographs. Even when surgeons aimed clinically for a specific amount of medialization, to compare the results to central placement, there was a wide range of overlapping values between the centralized and medialized groups [1]. It is therefore difficult to isolate the effects of medialization in clinical studies. Biomechanical studies to date [5-9] have not, to our knowledge, investigated the effects of medialization on the COP, and have only tested one level of medialization, thus leaving it unclear whether the observed effects grow in proportion to the degree of medialization. The COP takes into account both the magnitude of the contact force and its location, providing an overall indication of patellar component loading. Given the range of patellar options [10] and the range of clinical practice, we examined the impact of medializa- tion on patellar kinematic and kinetic parameters to determine the most suitable amount of medialization. A cadaveric study was designed to answer the following research questions: (1) what impact do 2.5- and 5.0-mm medialization have on patellar shift, tilt, contact force, and COP relative to central positioning during dynamic flexion, and (2) are these effects proportional to the degree of medialization? Materials and Methods We recorded patellofemoral kinematics and contact force distributions throughout weight-bearing flexion and extension in 7 cadaveric specimens implanted with an adjustable patellar component that allowed 2 levels of patellar medialization in addition to the central position. From the *Department of Mechanical Engineering, University of British Columbia, Vancouver, Canada; yCentre for Bioengineering Research and Education, University of Calgary, Calgary, Canada; zDepartment of Civil Engineering, University of Calgary, Calgary, Canada; §McCaig Institute for Bone and Joint Health, University of Calgary, Calgary, Canada; O Division of Orthopaedic Engineering Research, University of British Columbia, Vancouver, Canada; bVancouver Coastal Health Research Institute, University of British Columbia, Vancouver, Canada; #Department of Orthopaedics, University of British Columbia, Vancouver, Canada; and **Department of Orthopaedics and Traumatology, Hôpital Michallon, Grenoble, France. Submitted June 23, 2008; accepted April 20, 2009. Benefits or funds were received in partial or total support of the research material described in this article. These benefits or support were received from the following sources: The Canadian Arthritis Network, the Michael Smith Foundation for Health Research (Canada), the Natural Sciences and Engineering Research Council of Canada, Praxim (Grenoble, France), and Zimmer Canada. Reprint requests: Carolyn Anglin, PhD, Department of Civil En- gineering, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4. © 2010 Elsevier Inc. All rights reserved. 0883-5403/2505-0020$36.00/0 doi:10.1016/j.arth.2009.04.023 793 The Journal of Arthroplasty Vol. 25 No. 5 2010