Fuji Film and Ultrasound Measurement of Total Knee Arthroplasty Contact Areas R. Zdero, PhD,*,† P. V. Fenton, MD, FRCP(C),‡ J. Rudan, MD, FRCS(C),§ and J. T. Bryant, PhD, PEng*,† Abstract: This article describes tibiofemoral contact area measurement results from tests on 1 commercial total knee arthroplasty (TKA) using 2 experimental methods— fuji film and diagnostic ultrasound. The study presents a novel diagnostic ultrasound technique developed specifically for measuring TKA contact areas. Because most experimental investigations have been concerned with interimplant comparison, this article is one of few parametric TKA studies in the literature. Fuji film and ultrasound provide lower and upper bound contact area measurements based on their physical operating principles; this implies that no single measurement method can be relied on exclusively to glean contact area data. Designers should be cautious in using contact area and contact stress as the exclusive predictors of TKA failure. Key words: contact area, contact stress, total knee arthroplasty, ultrasound, fuji film Fixed-bearing total knee arthroplasties (TKAs) are composed mainly of noncongruent metal-on-plas- tic mating components used in resurfacing worn cartilage in patients with arthritis. Ultra-high mo- lecular weight polyethylene (UHMWPE) compo- nents of TKAs wear under degradation mechanisms often associated with polyethylene fatigue under high contact stresses [1–5]. The corresponding in- terfacial tibiofemoral contact areas can be catego- rized as area, line, point, or quasiline in shape [6] and are related to tibiofemoral geometric congru- ency. To assess indirectly the average contact stress during TKA articulation, measuring contact areas in vitro commonly has been done with pressure-sen- sitive fuji film [2,4,5,7]. Although fuji film is ap- pealing because of its simplicity, low cost, and ra- pidity of use, its disadvantages are disruption of natural mating at a TKA interface, its limitation to 2-dimensional static in vitro use, and minimum pressure threshold required for detection of contact. A new diagnostic ultrasound technique previously was developed to eliminate these drawbacks; the main advantage of ultrasound is its nonintrusivity [8]. The main disadvantages of ultrasound are the time costs associated with calibration and image analysis. It is hypothesized that surface deformation in- curred in the contact region by polymer tibial com- ponents results in an outer ring of plastically deformed material, which would be detected differ- ently by ultrasound and fuji film, due to the phys- ical operating principles of each method (Fig. 1). Specifically, ultrasound would overestimate contact area because of proximity artifact, whereas fuji film From the *Mechanical Engineering Department, Queen’s University, and †Human Mobility Research Centre, Apps Medical Research Centre, and Departments of ‡Radiology and §Orthopaedic Surgery, Kingston General Hospital, Kingston, Ontario, Canada. Submitted March 31, 2000; accepted November 8, 2000. Benefits or funds were received from Osteonics Corporation, Allendale, NJ, and the School of Graduate Studies, Queen’s University, in partial or total support of the research material described in this article. Reprint requests: R. Zdero, PhD, Human Mobility Research Centre (Angada 1), Apps Medical Research Centre, Kingston General Hospital, Kingston, ON, Canada K7L-2V7. E-mail: zdero@me.queensu.ca. Copyright © 2001 by Churchill Livingstone 0883-5403/01/1603-0015$35.00/0 doi:10.1054/arth.2001.21501 The Journal of Arthroplasty Vol. 16 No. 3 2001 367