Abstract
This is the first study to quantify the measurement error due to the physical thickness of Fujifilm for several material combinations relevant to orthopaedics. Theoretical and experimental analyses were conducted for cylinder-on-flat indentation over a series of forces (750 and 3000 N), cylinder diameters (0 to 80 mm), and material combinations (metal-on-metal, MOM; metal-on-polymer, MOP; metal-on-bone, MOB). For the scenario without Fujifilm, classic Hertzian theory predicted the true line-type contact width as WO = {(8FDcyl)/(πLcyl)[(1 − νcyl2)/Ecyl + (1 − νflat2)/Eflat]}1/2, where F is compressive force, Dcyl is cylinder diameter, Lcyl is cylinder length, νcyl and νflat are cylinder and flat Poisson's ratios, and Ecyl and Eflat are cylinder and flat elastic moduli. For the scenario with Fujifilm, experimental measurements resulted in contact widths of WF = 0.1778 × F0.2273 × D0.2936 for MOM tests, WF = 0.0449 × F0.4664 × D0.4201 for MOP tests, and WF = 0.1647 × F0.2397 × D0.3394 for MOB tests, where F is compressive force and D is cylinder diameter. Fujifilm thickness error ratio WF/WO showed a nonlinear decrease versus cylinder diameter, whilst error graphs shifted down as force increased. Computational finite element analysis for several test cases agreed with theoretical and experimental data, respectively, to within 3.3% and 1.4%. Despite its wide use, Fujifilm's measurement errors must be kept in mind when employed in orthopaedic biomechanics research.
Highlights
Various experimental methods exist in orthopaedic biomechanics research for measuring interfacial contact areas of human and artificial joints [1]
Bachus et al [13] showed that Fujifilm contact area measurement error for a 405 mm2 circular area under a 1250 to 4250 N load could range from a 1% overestimate to a 27% underestimate, depending on the image processing technique used
Fujifilm contact area underestimations for total knee replacements have been reported as 35% by Szivek et al [7] and 11 to 36% by Harris et al [11], which were attributed to the lower pressure threshold limit of the film
Summary
Various experimental methods exist in orthopaedic biomechanics research for measuring interfacial contact areas of human and artificial joints [1] These techniques can be characterized as direct contact substances (e.g., castings, dyes), electrical resistivity sensors (e.g., piezoelectric transducers, resistive ink sensors, and radiotelemetry), mechanically deformable films (e.g., microindentation pads, chemically sensitive pads), and nonintrusive techniques (e.g., radiography, ultrasound) [1]. It is limited to two-dimensional quasistatic in vitro use, there is a minimum pressure threshold to detect contact, spatial resolution is restricted, it is sensitive to shear, and there is difficulty in accurately detecting pressure gradients near the edges of the contact area. Despite this and the availability of K-scan real-time thin film technology
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