Conformation of Pressure-Sensitive Film to Complex Three-Dimensional Interfaces
Surry, K.J.M.*, Liggins, A.B.+ and Finlay, J.B.++
* Imaging Research Laboratories, Robarts Research Institute, London, Ontario, Canada.
+ Department of Orthopaedic Surgery, Temple University, Philadelphia, Pennsylvania, USA.
++ Trudell Medical International, London, Ontario, Canada.
Contact areas and interface pressures have been used to investigate normal joint function, examine the effects of pathological changes, assess the function of prosthetic replacements and study post-surgical function.
The most popular pressure/contact area transducers feature a thin sheet (or sheets), placed between the contacting surfaces: their output may be a single image of maximum pressures/contact areas, or a dynamic image of pressure/contact areas during loading.
Fuji Prescale pressure-sensitive film (Fuji Photo Film Co. Ltd, Tokyo, Japan) is arguably the transducer of choice in biomechanics. This sheet-based medium uses a chemicalrelease mechanism to produce a pink stain: a greater pressure generates a darker stain. A relationship between applied pressure and stain-density can be determined, allowing full field pressure-maps to be generated.
Biomechanical joint-interfaces (physiological, pathological and prosthetic) can be small, and can contain complex three-dimensional surfaces. Hence, a sheet-based transducer may “crinkle”, due to an inability to conform to the surfaces; this effect can create both pressure and contact area artifacts.
An image processing technique for removing crinkle artifact from a digital Fuji film stainimage has been described. However, it has been shown that crinkling may cause the film to interfere with the joint interface, such that true contact areas may fail to register. Such findings cast doubt on the ability of image processing techniques to provide adequate artifact-compensation.
A mechanical solution is to cut the film into a shape, or shapes, that will conform to the surface. An early method was to cut small circles of Fuji film and place them over a joint surface; however, this approach results in a loss of coverage across the surface and the potential loss of significant data.
Petal-shaped cut-outs have been used on the spherical femoral head. Figure 1a shows such a cut-out, on a simulated prosthetic femoral head. Here, the petals connected at a single focal point; their shape was developed from simple geometry. Figure 1b shows the subsequent Fuji film stain.
For more geometrically-complex, surfaces, a simple cut-out has been shown to regain the true contact areas that were previously lost to crinkle artifact.
Figure 1. Fuji film cut-out (a) and resulting pressure-stain (b) for a spherical surface.
The objective of this work was to develop a standardized, optimal, technique for producing cut-out shapes for complex surfaces and to examine the practical considerations necessary for its implementation.
The trial-surface for this study would be that of the patella component of a total knee replacement.
Patellar Component Surface Contours
The ultra high molecular weight polyethylene (UHMWPE) patellar component (Figure 2a) was designed to articulate against a metallic femoral component. The “saddle-shaped” articulating surfaces consisted of a uni-planar central section with concave medial and lateral sections, as indicated.
A three-dimensional model of the articulating surface was created in the AutoCad environment (AutoDesk Inc., Sausalito, CA). AutoCad’s three-dimensional modeling routines were then used to sequentially cut slices (at 0.5mm spacing) through the surface (Figure 2b): the slice-outlines, projected onto a single plane, produced a contour map of the surface (Figure 2c).
Two distinct groups of contours (1 and 2) were clearly identified. Between these groups were a pair of delineating “border-contours” (3).
Figure 2. Patellar component (a) and surface contours (b,c).
For any cut-out, there are four main considerations: determination of an attachment point(s) for the petals; location of the focal point(s) for petal-cutting; petal-shape development; and petal-overlap elimination.
A strip of film lying over the uniplanar section of the patellar component will not crinkle (as it will bend in one dimension only). Therefore, an area of the cut-out corresponding to the uniplanar section (Figure 2a) provided a suitable base for the cut-out and an attachment point for the petals.
For other three-dimensional surfaces, such uniplanar sections can be determined by the presence of sets of straight, parallel, contour-lines.
Focal Point Location
Unlike sphere-geometry (Figure 1), the patellar component contours did not indicate an obvious single focal point, from which cut-lines could be radiated.
Considering the contours in groups 1 and 2 to be quasi-concentric, perpendicular lines were drawn from the endpoints of the outermost curve for each group (Figure 3a): this curve corresponded to the border-contour (3), defined above. The intersection of these perpendiculars provided four focal points (FS and FD, see explanation below) from which to radiate cut-lines, and hence develop the cut-out petals.
Figure 3. Determination of focal points (a), cut-lines (b), medians (c) and contour extensions (d).
The following development was conducted in the AutoCad environment. Symmetry was used to reduce surface analysis.
Contour-group 2 (Figure 2c) was designated as the “dominant” group. That is, it dominated the surface and would be the main determinant for decisions regarding petal-shape development.
Cut-lines were radiated from the dominant focal point, FD, (Figure 3b) to the bordercontour. These lines were such that the resulting segments were regularly spaced. Similarly, cut-lines were radiated from the secondary focal point, FS, to intersect the bordercontour at the same points as those from FD.
The cut-lines crossing the secondary contour-group (group 1 in Figure 2c) were truncated at the border of the uniplanar section, thereby creating attachment points between this section and the subsequent petals.
A median line for each petal was radiated from FD (bisecting the cut-lines) and across both sets of contour-lines (Figure 3c), to the border of the uniplanar section. If a contour-line did not cross the median, the AutoCad “Extend” command was used to extend the contour to the median (Figure 3d).
Along each median (Figure 4a), the distance, dn, between each contour and the lengths of the contour-segments (lnL and LnR) between the median and the cut-lines were measured.
Using Pythagoras for the lengths of a right-angled triangle, the two-dimensional distance between contours (dn) and the height between contour-slices (0.5mm) provided the three-dimensional distance, Dn, between contours, along the surface. Hence, the positions of the contours along the median could be corrected for the three-dimensional surface (the lengths of the contours remain the same) and the petal-edges determined (Figure 4b).
Each petal-shape was determined separately and then attached to the uniplanar section, at an angle determined by the three-dimensional geometry at this point.
Figure 4. Petal-shape development.
Due to surface concavity, petal-overlap was observed (Figure 5). In these cases, an alternative cutting line was determined, mid-way between the two overlapping petal-edges.
Figure 5. Petal-overlap elimination.
Output and Cutting
A flat-bed plotter, with felt-tip attachment, was used to transfer the cut-out outline to the reverse (non-active) surfaces of the Fuji film. The resulting lines were cut with a sharp scalpel. Here, care must be taken to avoid undue surface pressure, which could prematurely activate the film.
The final cut-out shape is shown in Figure 6a. Here, tabs have been added to each end of the uniplanar surface, to aid in handling and orientation. Figure 6b shows the cut-out applied to the patellar component. Figure 6c shows a sample pressure-stain from articulation with the prosthetic femoral component. This result can be compared with Figure 6b: here uncut Fuji film was placed in the same interface, resulting in extensive crinkle-artefact. This technique was clearly developed for one particular application; however, the steps described here could be easily adapted to other complex interfaces.
Figure 6. The cut-out shape (a), its application on the patellar component (b), the resulting pressure-stain (c) and the crinkle-artefact produced by uncut film.
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