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Issues and themes in computer aided design for external prosthetics and orthotics
Issues and themes in computer aided design for external prosthetics and orthotics M. Lord and D. Jones* Department of Mechanical Engineering, University College London, UK *National Centre for Training and Education in Prosthetics and Orthotics, Strathclyde, Glasgow, UK
University
of
ABSTRACT (,‘omputer match
aIded
de.Ggn .ry.\tem.r are ,jindiny
to and support
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and mantfacbre
pre.,,\ureji~r
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geometq
use in pro.,thetiw
and
Such systems consist essentially
body segments, representation
and manufacturing oJ:Julurr
body segments.
surj&e ilsperts
of customized
of shape measurement,
themes of design philosopy,
of shape injkmation
of e.Gting
in the production
at present
or its mould. General
andpreJentation
are considered.
orthotk
by computer,
sy.rtems arepresented
surjhre
to highlight
computer relationship
adjustments,
issues which
components
to
manipulation of-shape
and
measurements
may be the .rubject
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Keywords: (ND/CAM,
rehabilitation
cnginccring,
INTRODUCTION The past decade has seen the introduction of computers to aid in the design of prosthetic and orthotic devices where previously these were produced by craft methods. The primary target applications are presently the design of lower limb prosthetic sockets’,‘,” and customized orthopaedic footwear’.‘. Special seating is also suggested as a potential area of application. In each of these applications there is arguably a need to design a customized component to support the uniquelyshaped body segment. The support surface shape required, be it a prosthetic socket, a shoe or a seat, is not simply a model of the body contours. These must transmit gravitational or other supporting forces through specific areas of skin that can tolerate pressure, and in such a way as to maintain stability between the bodv and external hardware. The need to retain the dev&c in place impinges on the shape design, as ma); cosmetic factors. To achieve all of this, the container shape is made as a distortion of the body shape carefully selected to yield pressure in certain areas and relief in others. In prosthetics, this process is known as rechjication; both knowledge of skin and skeletal structure, and consideration of the tissue deformability under loading are incorporated. The design for a shoe or seat additionally takes into account possible articulations at the joints within the contained body segment. Thus the surface geometry is also dependent on body orientation. A computer aided design/computer aided manufacture (CAD/CAM) system for prosthetic or orthotic use has three stages: 1. measurement whereby body shape and other pertinent information are converted to digital data; generation by integration of individual 2. shape measurements, a knowledge base and interactive adjustment via the computer; and
0 1988 Butterworth & Co (Publishers) 0141-.5425/88/060491~08 SO3.00
prosthrtirs
and
orthotics
3. manufacture of this physical computer-controlled machine.
component
via
a
Potential advantages of using advanced technology to replace craft are flexibility of design, speed of production, consistency of quality and standardization. These aspects, together with other factors of relevance to the impact of advanced manufacturing technology on prosthetic and orthotic practice, have been revlewed recently in this
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Stereoscanning
of footshape
lnteractive’design .
of last
+
NC machining of last Figure
1 Schematic shoe last design
Computer pattern generation
of a CAD/CAM
system for customized
contours do not facilitate stabilization of the trunk. However this need not necessarily restrict CAD/ CAM design to these few cases, since CAD designs may not mimic exactly the moulded seat shapes. Much of the early development work has concentrated on preliminary development of measurement and machining hardware. As these facilities become operational there are a number of common issues and thcmcs that have become apparent as important to the continued development of such systems in which we expect to see an expansion of thought and experience over the coming decade. In related fields, CAD systems for computer representation and manipulation of head shape are being used to assist in facial reconstruction surgery both abroad and in the UK”,‘2,, (“Ed CAD techmques are described in dental design ‘1 . Applications are also found in the customized design of internal prosthetics, where the contours of the ‘intermedullary shaft ofa hip prosthesis have been successfuily shaped to match the individual bone shape’“.
PHILOSOPHIES IMPLEMENTATION
OF CAD
Already schools of philosophies are emerging in the approach to computer aided design in prosthetics and orthotics. The approach adopted at the onset largely dictates the specification of the measurement system, the complexity of the computer hardware, and software, and the potential range of cases which can be accommodated. Thus the decision needs to be considered in the light of the overall concept for
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the target patient the system operation - namely population, the timescale for development, and the operational and financial constraints. The Vancouver team2 have pioneered what has become known as the ‘reference shape’ approach to lower limb prosthetic socket design. As outlined in the introduction, the principle underlying this philosophy is that all limbs have similar structure and therefore a geometric similarity, and thus the basic socket shape to fit one person must be a simply scaled version of the socket to fit another. Foort had already successfully applied this principle to aboveknee socket design, proposing that the brim be a standard quadrilateral shape sized to the individual’“,“. In the early version of the Vancouver system the scaling was based on linear diametral measurements of the residuum in the anteroposterior or mediolateral directions only, together with residuum length. One factor apparent in our early trials of this system at University College London was that the measurements are not taken with reference to any fixed axes, and are all relative. Thus a patient with marked bowing or axial twist of the tibia may produce the same diametral measures as another with a straighter tibia of quite different shape. Consequently the socket may not fit all cases well. This does not invalidate the approach; more detailed study of the critical description of residuum shape may reveal what measurements are required to scale and distort the reference. The UCL socket system commences with the representation of the residuum, which is then converted to the appropriate socket shape by the overlay of a rectification ‘map’ generated from previous research of the average rectification changes done manually by a prosthetist. Similarly, in the CadShoe system from LIC Orthopaedic a range of shoe last shapes are stored digitally for different common deformities; the patient’s foot size can be matched by appropriate scaling4,‘s. Another similar philosophy would be to best-match a normal last shape, and then to rectify this by on-screen sculpting or by the addition of a number of set patches appropriate to the foot condition. This is analogous to what the last-maker frequently does in British orthopaedic shoe-making, attaching leather pieces onto a wooden last to produce relief for, say, a bunion. Other issues of importance are the extent to which the system uses automation in its rectification, and the ability to develop an expert system; these are interrelated. The expert computer system is one which is capable oflearning during its own use, which implies that it must build a knowledge base of successful actions. If the philosophy of the system is to replace a hand crafting method by an equivalent computer based method with the ability to sculpt the surface extensively, say with a lightpen on the graphics screen, then it is difficult to see how a record of these modifications can be kept in such a way as to generalize and learn from the experience. If on the other hand most of the routine rectification is achieved by more automated means, as in the UCL system where the prosthetist specifies numerically the severity of the rectification in various predetermined
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map areas, then this information can be organized into a knowledge base. There are many other ways in which an expert system could be developed, and this is an exciting growth area which could pay great dividends in speed and consistency of design, as well perhaps as clarifying the differences between the multitude of shape design philosophies which have their own adherents.
SHAPE LOAD AND TISSUE DEFORMABILITY Underpinning much of the controversy about the relative merits of existing craft techniques is the problem that any body segment does not have a unique shape, even in a fixed joint orientation. Its shape is heavily dependent on the tissue loading at the time of measurement. This is sometimes further complicated over periods of time by fluctations in tissue bulk due to oedematous changes. In craft methods, since only one shape record can be made there is an attempt to incorporate some degree of tissue deformability into the data. This reduces the amount of correction which needs to be done subsequently in the shape rectification process. In conventional measurement of the foot for last making, there are those who prefer the cast to be taken with partial or full load bearingI and those who use unloaded casting. Similarly for below-knee sockets, most casting techniques require some loading of the tissues which may take the form of the all-over support afforded by a suspended stocking (the Northern Western casting ring*‘) or pressure by placement of the prosthetist’s hands. Even for the same notional design, the balance between rectification built in at the casting stage and that performed subsequently on the cast may vary. Both must complement each other for a consistent result. Among the advantages claimed for CAD systems is the potential to reduce variability in the rectification process, but to do this would require an objective shape measurement with as little manual intervention as possible. Deformability information may then be obtained separately if necessary, to be mapped onto the shape in the computer. Attempts are being made to measure and model the mechanical properties of residual limb tissue for this purpose. Using a surface ultrasonic probe, characterize the tissue in Krouskop et al. ” directly selected localities. Finite element models of the bony and soft tissue are being developed and verified to in shape under specified predict the changes College London we are loading **s*~. At University performing experiments by indenting limb tissue to compare with finite element predictions of socket interface pressures*“. The geometry of tissue distribution for any individual can be obtained from radiographic or magnetic resonance imaging, and the possibility of deducing an accurate representation of bony shape itself by CAD from scaled reference models has been explored25. An ultrasonic system developed for detecting surface contours could be developed to image bony shape’“.
SURFACE REPRESENTATION, AND ADJUSTMENT
MODELLING
After surface shape measurement, the raw data held on file will usually consist of coordinates of a number of points on the surface or of dimensional measurements which are used then to generate a set of coordinates. The surface can either be assumed to consist of facets with the data points at their vertices, or a ‘surface model’ can be generated to interpolate or approximate the data points. Modelling for our applications has many attractions. Not least it allows for efficient storage and transmission of the surface representation, reducing the storage space by at least an order of magnitude. The other desirable properties are continuity, which guarantees the smoothness of the surface to a given order dependent on the mathematical model selected, and the ability to make controlled adjustments. The UCL socket system retains the surface data in the form of an ordered set of radii of points on a cylindrical mesh, with the long axes of cylinder and residual limb aligned. Automatic adjustments to the surface are performed largely by simply adding or subtracting from the radii of the points in the vicinity of the area to be rectified. Whereas this works satisfactorily for the majority of the areas, where the surface normal is tangential to the long axis then radial displacements can cause unexpected distortion of the shape rather than a simple radial expansion. Modelling techniques already used in this field include bi-beta patche?, Bezier patches’ and Bsplines”. We have previously indicated the benefits of using a parametric representation of B-splines in a previous paper*“; the possibility exists in this system to describe a consistent set of anatomically anchored surface changes to any individual shape. This has implications on the ability to quantify changes irrespective of individual contours and thus develop a knowledge base to refine the design procedure. One of the primary problems m many of the parametric techniques is the need to establish a rectilinear grid of points for the model - essentially a flat sheet with a rectangular mesh needs to be distorted and wrapped over the surface. This works well on cylindrical topology but the lower limb residuum only approximates to a cylinder; its distal end is in fact hemispherical. Likewise the ankle and foot approximate to a cylinder with a hemispherical end. The problems resulting from attempting to circumvent the problem and pull the cylinder in at one end are poor continuity and adjustment potential in this region. Recent work in computer aided graphic design is aimed at relaxing the grid mesh requirements 9, and may provide solutions in the future. From the point of view of comparison of residual limb and socket shapes for research, trial or teaching purposes3’,s2, a modelled surface may be more amenable than a data-point defined surface to mathematical treatment. On the whole, trials of CAD designs still rely on the subjective reports of the wearers to evaluate the system, and whilst this is the acid test, it does not give the detailed design
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information necessary to refine stages of the entire process.
GRAPHICAL INTERACTIVE SURFACES
the various
REPRESENTATION ADJUSTMENT
interim
AND OF
Not all of the surface adjustments are anticipated to be made by automatic algorithms; some may need to be implemented by interactive graphical editing. The prosthetist or orthotist used to holding a three dimensional object in his hand may have difficulty in visualizing the 3D shape from a screen representation in two dimensions. With realistic costs it is now feasible to present the surface as a grey-scale shaded image, and the addition of cross-sectional information can assist in interpretation (Figure 2). However note both that the cross-section is with reference to an arbitrary centre-line, and it is often confused by the inexperienced with the silhouette of the equivalent solid object, and thus must be approached warily. Wireframe drawings traditionally used in engineering can be extremely misleading on nongeometric surfaces, since the parameterization of mesh can easily give a visual distortion. Animation helps with visual perception of shape. Visualization of surface geometry is useful to check for data dropout at the measurement stage, and is also necessary for interactive adjustment. This is an area where we may welcome and anticipate advances in the techniques. For surfaces stored only as coordinate sets, the ‘belt and braces’ way of recontouring surfaces might be to allow the user to move individual points, working from profile displays; this should be regarded as a last resort, since it is inelegant, inevitably slow and likely to produce discontinuities of slope and curvature working from a 2-D display. algorithms can Alternatively, be developed to automatically raise or lower the surface in an area, extent and patch profile specified graphically by the user. This technique is encorporated in the Vancouver CASD system, and in collaborative work the visual editing facility from this system has been
Figure 2 The UCL computer aided socket design system showing limb shape capture and a shaded graphics representation of the resultant socket design
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successfully used for other socket applications in the laboratory at the National Centre. In the automobile industry, the conceptual design of car body shape has made extensive use of parametric B-spline models referred to earlier because of their facility for Q . These allow the user to push graphical adjustment and pull control points which in turn remodel the surface in that local vicinity, and is also available in microcomputer design packages.
SHAPE MEASUREMENT AND FUNDAMENTAL GEOMETRIC CONSTRAINTS Even a modest review of shape measurement systems developed or under development for non-contacting body shape scanning would form a complete paper in itself. What is of general interest to consider, though, are the fundamental geometric constraints which dictate the possibilities for successful scanning. This includes the characteristics of the particular segment shape, the visual access required by the measurement system, and arrangements for scanning where required. One of the oldest methods proposed to extract shape contours is surely silhouetting. For this technique to give faithful reproduction, geometrically the surface must not have reentrants, which is largely true of residual lower-limbs at both above and below knee levels. Silhouetting has recently been investigated for CAD measurement both at the National Centre33 and at University College London34, using automated video analysis to give rapid results. A very positive advantage is that the system can be arranged so that only 180” access around the limb is required to image the entire surface. Thus the subject can stand normally with the residual limb vertical during measurement. An optical triangulation principle exploited for obtaining a number of horizontal cross-sections of the trunk35, has also been used for scanning of below-knee casts and limbs3. According to this method, a fixed vertical line of light is projected onto the surface and viewed by a video camera from an oblique angle; distortion of the line as seen by the camera is used to calculate the coordinates along the line. Scanning can be conveniently arranged either by rotating the camera around the object, or by rotating the patient/cast on a turntable, both requiring a 360” clear view. This method can only define the surface entirely if two constraints are met; the vertical rotation axis must lie entirely within the body throughout its length, and there must not be any undercuts. The latter is less severe than the reentrant requirement for silhouetting, and generally all residual limb, socket and foot shapes could accomodate this. The requirement for the axis to lie within the body does cause some problems in live limb scanning where it is probable that some of the areas around the distal tip will be misrepresented because of misalignment. Alternatively linear relative movement of subject/camera can be used. Other optical triangulation methods use a moving beam of light with fixed camera to scan over the
CAD
These have been successfully stationary surface. developed for capturing the contours in a single camera field-of-view as required for analysis of back this type ofequipment could shape 36.37 . Theoretically be used to reconstruct a full 360” scan from several images converted to the same frame of axes. The limb segment need only be maintained in the field-of-view without a strict orientation, and views can be arranged to miss, say, chair legs. Similar planar views can be obtained from Moire photography and stereophotography, both of which are rather time-consuming techniques, or from more recent methods of projecting grids or dots onto the work, we scan subject ‘so In our own experimental foot casts hy rotating these in a number of equal steps about a single axis in front of a fixed camera/light unit (Figure 3), and reconstructing the surface from combination of all the scans. No single method has yet emerged as acceptable as a standard, taking into account not only geometric constraints but also issues of spatial accuracy, cost and transportability.
DEVICE
MANUFACTURE
The computer controlled manufacture ofa prosthetic or orthotic device is a rather more difficult problem than it appears at first slance. Current research and developement efforts m the field have tended to emphasize the sensing and design aspects of the CAD/CAM process but a great deal of effort is still required before existing practical problems of manufacture are overcome. Manufacturing techniques in prosthetics and orthotics have for a number of years taken advantage of mass production technology and new materials to create those parts of these devices that can be standardized. Even the prosthetic sockets, seats and other devices which interface directly with the tissues are increasingly preformed with the aid of some mechanization. Nevertheless, these interfaces are still typically fashioned, as was described above, about an intermediate model of plaster of Paris, polyurethane foam or other rigid, workable material created by the hands of the craftsman. stage is time This intermediate ‘modelling;’ c
Figure 3 ‘I‘hrec views optical scan of the foot
of the points
captured
during
a 1.5 s
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consuming and so much of the current research and development has tried to utilize CNC milling machines to produce the plaster or foam model ready for final manufacture directly from the designed shape data. There is a great deal of interest in trying to eliminate this intermediate manufacturing stage completely; going right from the design data to the finished socket for example. A number of issues have to be addressed before the approach can meet its promise. The productivity issue: balancing the need for skilled custom design with the desire for rapid production A major obstacle to the creation of prosthetic and orthotic devices with one computer controlled manufacturing stage is the necessity to have a customized interface with a patient. Each interface between device and body will have a different shape and the quality of this interface is critical to the success of the whole process. What is needed in the application of manufacturing technology to the CAD/CAM process is a means to have the flexibility to produce a batch size of one without sacrificing most of the speed and efficiency we might expect from mass production technology. In prosthetics and orthotics we now would like to have productivity benefits anticipated through the use of such technology without losing the control and the flexibility needed and enjoyed in current practice. In industry g.enerally it has been the trend in recent years to substitute manufacturing technology for manpower. Technological advances in so called flexible manufacturing systems (FMS) have allowed a shift towards the economic production of small batches of goods. In industries such as shipbuilding, aerospace, tool and die making and other precision industries, technological trends have allowed the requirement for size-one batches to become common. For these industries it is the skills of the craftsman that have been of utmost importance in guiding the machines to produce the required result. Therefore there is a conflict of trends; the advent of new manufacturing technology has tended to reduce the number of skilled craftsmen available and yet this same technology, by making small batch sizes more economic, puts the skills of the craftsman at a premium. Experience shows that a learning curve effect in engineering part manufactypically applies ture40. Consider the manufacture of a batch of identical items. In such cases the first five parts might require close observation and frequent adjustments, the next ten parts are likely to exhibit problems in 20% and the next fifteen parts are likely to exhibit problems in 10 %. This makes it very difficult to achieve totally hands-off precision manufacture with a batch size of one. The system designer therefore needs to provide facilities which remove as many sources ofcostly error as possible. We can learn from these experiences in other fields and anticipate that in manufacturing prosthetic and orthotic devices all aspects of the process should be sensitive to the problems of small batch sizes. This can be achieved
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via inspection of data in graphical form and by interactive simulation of parts of the manufacturing process. Both of these techniques allow the operator to preview the manufacturing process before commiting the design to physical reality. The methodology of socket shape and shoe last manufacture being developed by the National Centre and by University College will be described to illustrate the key problems and issues.
Manufacturing
process
Ideally we would just like to think of the manufacturing process as a system with an input and an output. At the input we have the shape description as a set of surface point coordinates and at the output we have the physical manifestation of that shape. Unfortunately between the input and the output we have many processes to consider. Most research by ourselves and others has depended on the use of CNC type of milling machines to produce the intermediate models for socket, seat or last manufacture and with these systems the following stages are ideal. Inpul dala. This is a set of several thousand surface points represented by x, y, z coordinates in measurement reference frame. The volumes of data encountered with the complex shapes of prosthetic and orthotic practice mean that it becomes impossible to inspect visually the set of coordinate values and spot any aberrations by merely inspecting the numerical values. Verz$y data inlegrily. The data representing the surface should be checked for integrity. This is most efficiently carried out on a high performance interactive graphics workstation which allows the operator to identify visually any surface irregularities. The authors have employed high resolution interactive graphics which in mesh types of display very quickly allow an operator to identify any errors at this stage. It is also necessary to remember that the numerical precision of the original coordinate data should be sufficient to achieve the desired precision in manufacture. The surface data points may require Converl dala. conversion to a new coordinate system suitable for the manufacturing process. Most work in this field has tended to rely on conversion to a form suitable for the helical tool path which will be used. Selecl machining parameters. The tool diameter, tool length, cutting speed, spindle speed, tool end profile, raw material and other factors must be considered. These parameters interact and their choice requires some compromises to be made. For example, a small tool diameter and fine pitch for the tool path produce a fine surface finish at the expense of machining time; but too fine a tool diameter can also introduce tool vibration and therefore machining errors and tool wear.
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Generale machine loo1 instruck’on codes. This must incorporate so-called interference correction for the shape in question. One unavoidable factor concerns the need for interference correction. This consists of a need to adjust the path of the machine tool to prevent the removal of the desired surface adjacent to each of the surface coordinate points during the manufacturing process. Just as in Shannon’s sampling theory for time series signals it is absolutely necessary to have sufficient samples to represent accurately the shape; intuitively it is necessary to have more samples when the shape changes rapidly. Methods have been described4’,@’ which will cope reasonably well with most portions of prosthetic shapes. Each of these techniques should be recognized and described as a form of digital filter with a frequency response, a transient and a steady state response behaviour. An examination of interference checking from the digital filtering perspective can be beneficial as it becomes necessary to have techniques which can adapt to arbitrarily complex shapes such as those found with severely deformed feet and in custom seating. This will be the subject of a future paper. There is no doubt that interference checking is essential in device manufacture and that existing techniques do not cope well with the variety ofshapes with which we wish to deal.
Simulale manufaclure. Here we verify that the generated machine tool instructions are going to create the desired shape. Simulation of manufacture using high performance interactive three dimensional graphics is a powerful technique which will prove to be a significant aid in the future. During our early work manufacturing below-knee socket shapes, there was no way of verifying that a particular shape could be carefully manufactured from a workpiece blank. An hour could be spent waiting for completion of manufacture only to find that the shape did not fit within the workpiece. This fact proved the stimulus for the development of software for the simulation of manufacture. With this process all the manufacturing parameters are held in a specification file which can be edited at will. Similarly the shape of the workpiece from which the shape will be manufactured is held in a file and both are picked up by the graphics simulator which initially allows the operator to verify that the shape to be manufactured will in fact fit within the workpiece. Performed on an Apollo 590T workstation the relative positions of the shape and the workpiece can be adjusted graphically. This interactive process can also accommodate alignment adjustments to the socket and allow inspection of the clearance between the support components and bracket and the tool path (Figure 4). Successful positioning is followed by simulated manufacture on the computer display allowing visual inspection of the results of the choice of manufacturing parameters. As Figure 5 shows, the shaded surface view of the socket shape appears to be ‘carved ’ from the workpiece and the simulation utilizes the actual machine tool instructions so that the manufacturing process can be truly previewed.
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\ i Figure within
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41. 42.
AR, Harris JD. Isis-an automated shape measurement and analysis system. In: JD Harris, AR Turner-Smith, eds, Surface Topography and Spinal Deformity, Stuttgart: Gustav-Fischer, 1986; 31-8. Livingstone F, Tulai A. Surface measurements using the HYSCAN 3-D camera. In: IAF Stokes, JR Pekelsky, MS Moreland, eds. Surface Topography and Spinal Deformity, IV, Stuttgart: Gustav Fischer, 1987; 411-15. Yoshizawa T, Ohtsuka Y, Okamoto Y, Shinoto A, Kimoto K. Automatic three-dimensional measurement by protection of a grating pattern. In: IAF Stokes, JR Pekelsky, MS Moreland, eds. Surface Topography and Spinal Deformity IV, Stuttgart: Gustav Fischer, 1987; 403-10. D’Ambramo G, Lorini G, Merolli A. Computer assisted dot array analysis in back surface topography. In: IAF Stokes, JR Pekelsky, MS Moreland, eds. Surface Topography and Spinal Deformity ZV, Stuttgart: Gustav Fischer, 1987: 385-94. Wright PK, Bourne DA. Manufacturing intelligence. New York: Addison Wesley, 1988. Duncan JP, Mair S. Smtt#&red surfaes in engineering and medicine. Cambridge: Cambridge University Press, 1983. Saunders CG, Vickers GW. A generalised approach to the replication of cylindrical bodies with compound curvature. Trans ASME 1984; 106: 7&6.
University
of
ABSTRACT (,‘omputer match
aIded
de.Ggn .ry.\tem.r are ,jindiny
to and support
o/‘.rhapr.
and mantfacbre
pre.,,\ureji~r
oJf‘ the Jupport
geometq
use in pro.,thetiw
and
Such systems consist essentially
body segments, representation
and manufacturing oJ:Julurr
body segments.
surj&e ilsperts
of customized
of shape measurement,
themes of design philosopy,
of shape injkmation
of e.Gting
in the production
at present
or its mould. General
andpreJentation
are considered.
orthotk
by computer,
sy.rtems arepresented
surjhre
to highlight
computer relationship
adjustments,
issues which
components
to
manipulation of-shape
and
measurements
may be the .rubject
/~~rrrlc~f)rnenl.~.
Keywords: (ND/CAM,
rehabilitation
cnginccring,
INTRODUCTION The past decade has seen the introduction of computers to aid in the design of prosthetic and orthotic devices where previously these were produced by craft methods. The primary target applications are presently the design of lower limb prosthetic sockets’,‘,” and customized orthopaedic footwear’.‘. Special seating is also suggested as a potential area of application. In each of these applications there is arguably a need to design a customized component to support the uniquelyshaped body segment. The support surface shape required, be it a prosthetic socket, a shoe or a seat, is not simply a model of the body contours. These must transmit gravitational or other supporting forces through specific areas of skin that can tolerate pressure, and in such a way as to maintain stability between the bodv and external hardware. The need to retain the dev&c in place impinges on the shape design, as ma); cosmetic factors. To achieve all of this, the container shape is made as a distortion of the body shape carefully selected to yield pressure in certain areas and relief in others. In prosthetics, this process is known as rechjication; both knowledge of skin and skeletal structure, and consideration of the tissue deformability under loading are incorporated. The design for a shoe or seat additionally takes into account possible articulations at the joints within the contained body segment. Thus the surface geometry is also dependent on body orientation. A computer aided design/computer aided manufacture (CAD/CAM) system for prosthetic or orthotic use has three stages: 1. measurement whereby body shape and other pertinent information are converted to digital data; generation by integration of individual 2. shape measurements, a knowledge base and interactive adjustment via the computer; and
0 1988 Butterworth & Co (Publishers) 0141-.5425/88/060491~08 SO3.00
prosthrtirs
and
orthotics
3. manufacture of this physical computer-controlled machine.
component
via
a
Potential advantages of using advanced technology to replace craft are flexibility of design, speed of production, consistency of quality and standardization. These aspects, together with other factors of relevance to the impact of advanced manufacturing technology on prosthetic and orthotic practice, have been revlewed recently in this
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C.4D for prosthetics and orlhotics: M. Lord and D. Jones
Stereoscanning
of footshape
lnteractive’design .
of last
+
NC machining of last Figure
1 Schematic shoe last design
Computer pattern generation
of a CAD/CAM
system for customized
contours do not facilitate stabilization of the trunk. However this need not necessarily restrict CAD/ CAM design to these few cases, since CAD designs may not mimic exactly the moulded seat shapes. Much of the early development work has concentrated on preliminary development of measurement and machining hardware. As these facilities become operational there are a number of common issues and thcmcs that have become apparent as important to the continued development of such systems in which we expect to see an expansion of thought and experience over the coming decade. In related fields, CAD systems for computer representation and manipulation of head shape are being used to assist in facial reconstruction surgery both abroad and in the UK”,‘2,, (“Ed CAD techmques are described in dental design ‘1 . Applications are also found in the customized design of internal prosthetics, where the contours of the ‘intermedullary shaft ofa hip prosthesis have been successfuily shaped to match the individual bone shape’“.
PHILOSOPHIES IMPLEMENTATION
OF CAD
Already schools of philosophies are emerging in the approach to computer aided design in prosthetics and orthotics. The approach adopted at the onset largely dictates the specification of the measurement system, the complexity of the computer hardware, and software, and the potential range of cases which can be accommodated. Thus the decision needs to be considered in the light of the overall concept for
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the target patient the system operation - namely population, the timescale for development, and the operational and financial constraints. The Vancouver team2 have pioneered what has become known as the ‘reference shape’ approach to lower limb prosthetic socket design. As outlined in the introduction, the principle underlying this philosophy is that all limbs have similar structure and therefore a geometric similarity, and thus the basic socket shape to fit one person must be a simply scaled version of the socket to fit another. Foort had already successfully applied this principle to aboveknee socket design, proposing that the brim be a standard quadrilateral shape sized to the individual’“,“. In the early version of the Vancouver system the scaling was based on linear diametral measurements of the residuum in the anteroposterior or mediolateral directions only, together with residuum length. One factor apparent in our early trials of this system at University College London was that the measurements are not taken with reference to any fixed axes, and are all relative. Thus a patient with marked bowing or axial twist of the tibia may produce the same diametral measures as another with a straighter tibia of quite different shape. Consequently the socket may not fit all cases well. This does not invalidate the approach; more detailed study of the critical description of residuum shape may reveal what measurements are required to scale and distort the reference. The UCL socket system commences with the representation of the residuum, which is then converted to the appropriate socket shape by the overlay of a rectification ‘map’ generated from previous research of the average rectification changes done manually by a prosthetist. Similarly, in the CadShoe system from LIC Orthopaedic a range of shoe last shapes are stored digitally for different common deformities; the patient’s foot size can be matched by appropriate scaling4,‘s. Another similar philosophy would be to best-match a normal last shape, and then to rectify this by on-screen sculpting or by the addition of a number of set patches appropriate to the foot condition. This is analogous to what the last-maker frequently does in British orthopaedic shoe-making, attaching leather pieces onto a wooden last to produce relief for, say, a bunion. Other issues of importance are the extent to which the system uses automation in its rectification, and the ability to develop an expert system; these are interrelated. The expert computer system is one which is capable oflearning during its own use, which implies that it must build a knowledge base of successful actions. If the philosophy of the system is to replace a hand crafting method by an equivalent computer based method with the ability to sculpt the surface extensively, say with a lightpen on the graphics screen, then it is difficult to see how a record of these modifications can be kept in such a way as to generalize and learn from the experience. If on the other hand most of the routine rectification is achieved by more automated means, as in the UCL system where the prosthetist specifies numerically the severity of the rectification in various predetermined
G'ADfor jmsthetics and orthotics: M. Lord and D. Jones
map areas, then this information can be organized into a knowledge base. There are many other ways in which an expert system could be developed, and this is an exciting growth area which could pay great dividends in speed and consistency of design, as well perhaps as clarifying the differences between the multitude of shape design philosophies which have their own adherents.
SHAPE LOAD AND TISSUE DEFORMABILITY Underpinning much of the controversy about the relative merits of existing craft techniques is the problem that any body segment does not have a unique shape, even in a fixed joint orientation. Its shape is heavily dependent on the tissue loading at the time of measurement. This is sometimes further complicated over periods of time by fluctations in tissue bulk due to oedematous changes. In craft methods, since only one shape record can be made there is an attempt to incorporate some degree of tissue deformability into the data. This reduces the amount of correction which needs to be done subsequently in the shape rectification process. In conventional measurement of the foot for last making, there are those who prefer the cast to be taken with partial or full load bearingI and those who use unloaded casting. Similarly for below-knee sockets, most casting techniques require some loading of the tissues which may take the form of the all-over support afforded by a suspended stocking (the Northern Western casting ring*‘) or pressure by placement of the prosthetist’s hands. Even for the same notional design, the balance between rectification built in at the casting stage and that performed subsequently on the cast may vary. Both must complement each other for a consistent result. Among the advantages claimed for CAD systems is the potential to reduce variability in the rectification process, but to do this would require an objective shape measurement with as little manual intervention as possible. Deformability information may then be obtained separately if necessary, to be mapped onto the shape in the computer. Attempts are being made to measure and model the mechanical properties of residual limb tissue for this purpose. Using a surface ultrasonic probe, characterize the tissue in Krouskop et al. ” directly selected localities. Finite element models of the bony and soft tissue are being developed and verified to in shape under specified predict the changes College London we are loading **s*~. At University performing experiments by indenting limb tissue to compare with finite element predictions of socket interface pressures*“. The geometry of tissue distribution for any individual can be obtained from radiographic or magnetic resonance imaging, and the possibility of deducing an accurate representation of bony shape itself by CAD from scaled reference models has been explored25. An ultrasonic system developed for detecting surface contours could be developed to image bony shape’“.
SURFACE REPRESENTATION, AND ADJUSTMENT
MODELLING
After surface shape measurement, the raw data held on file will usually consist of coordinates of a number of points on the surface or of dimensional measurements which are used then to generate a set of coordinates. The surface can either be assumed to consist of facets with the data points at their vertices, or a ‘surface model’ can be generated to interpolate or approximate the data points. Modelling for our applications has many attractions. Not least it allows for efficient storage and transmission of the surface representation, reducing the storage space by at least an order of magnitude. The other desirable properties are continuity, which guarantees the smoothness of the surface to a given order dependent on the mathematical model selected, and the ability to make controlled adjustments. The UCL socket system retains the surface data in the form of an ordered set of radii of points on a cylindrical mesh, with the long axes of cylinder and residual limb aligned. Automatic adjustments to the surface are performed largely by simply adding or subtracting from the radii of the points in the vicinity of the area to be rectified. Whereas this works satisfactorily for the majority of the areas, where the surface normal is tangential to the long axis then radial displacements can cause unexpected distortion of the shape rather than a simple radial expansion. Modelling techniques already used in this field include bi-beta patche?, Bezier patches’ and Bsplines”. We have previously indicated the benefits of using a parametric representation of B-splines in a previous paper*“; the possibility exists in this system to describe a consistent set of anatomically anchored surface changes to any individual shape. This has implications on the ability to quantify changes irrespective of individual contours and thus develop a knowledge base to refine the design procedure. One of the primary problems m many of the parametric techniques is the need to establish a rectilinear grid of points for the model - essentially a flat sheet with a rectangular mesh needs to be distorted and wrapped over the surface. This works well on cylindrical topology but the lower limb residuum only approximates to a cylinder; its distal end is in fact hemispherical. Likewise the ankle and foot approximate to a cylinder with a hemispherical end. The problems resulting from attempting to circumvent the problem and pull the cylinder in at one end are poor continuity and adjustment potential in this region. Recent work in computer aided graphic design is aimed at relaxing the grid mesh requirements 9, and may provide solutions in the future. From the point of view of comparison of residual limb and socket shapes for research, trial or teaching purposes3’,s2, a modelled surface may be more amenable than a data-point defined surface to mathematical treatment. On the whole, trials of CAD designs still rely on the subjective reports of the wearers to evaluate the system, and whilst this is the acid test, it does not give the detailed design
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for prosthetics and orthotics:
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information necessary to refine stages of the entire process.
GRAPHICAL INTERACTIVE SURFACES
the various
REPRESENTATION ADJUSTMENT
interim
AND OF
Not all of the surface adjustments are anticipated to be made by automatic algorithms; some may need to be implemented by interactive graphical editing. The prosthetist or orthotist used to holding a three dimensional object in his hand may have difficulty in visualizing the 3D shape from a screen representation in two dimensions. With realistic costs it is now feasible to present the surface as a grey-scale shaded image, and the addition of cross-sectional information can assist in interpretation (Figure 2). However note both that the cross-section is with reference to an arbitrary centre-line, and it is often confused by the inexperienced with the silhouette of the equivalent solid object, and thus must be approached warily. Wireframe drawings traditionally used in engineering can be extremely misleading on nongeometric surfaces, since the parameterization of mesh can easily give a visual distortion. Animation helps with visual perception of shape. Visualization of surface geometry is useful to check for data dropout at the measurement stage, and is also necessary for interactive adjustment. This is an area where we may welcome and anticipate advances in the techniques. For surfaces stored only as coordinate sets, the ‘belt and braces’ way of recontouring surfaces might be to allow the user to move individual points, working from profile displays; this should be regarded as a last resort, since it is inelegant, inevitably slow and likely to produce discontinuities of slope and curvature working from a 2-D display. algorithms can Alternatively, be developed to automatically raise or lower the surface in an area, extent and patch profile specified graphically by the user. This technique is encorporated in the Vancouver CASD system, and in collaborative work the visual editing facility from this system has been
Figure 2 The UCL computer aided socket design system showing limb shape capture and a shaded graphics representation of the resultant socket design
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successfully used for other socket applications in the laboratory at the National Centre. In the automobile industry, the conceptual design of car body shape has made extensive use of parametric B-spline models referred to earlier because of their facility for Q . These allow the user to push graphical adjustment and pull control points which in turn remodel the surface in that local vicinity, and is also available in microcomputer design packages.
SHAPE MEASUREMENT AND FUNDAMENTAL GEOMETRIC CONSTRAINTS Even a modest review of shape measurement systems developed or under development for non-contacting body shape scanning would form a complete paper in itself. What is of general interest to consider, though, are the fundamental geometric constraints which dictate the possibilities for successful scanning. This includes the characteristics of the particular segment shape, the visual access required by the measurement system, and arrangements for scanning where required. One of the oldest methods proposed to extract shape contours is surely silhouetting. For this technique to give faithful reproduction, geometrically the surface must not have reentrants, which is largely true of residual lower-limbs at both above and below knee levels. Silhouetting has recently been investigated for CAD measurement both at the National Centre33 and at University College London34, using automated video analysis to give rapid results. A very positive advantage is that the system can be arranged so that only 180” access around the limb is required to image the entire surface. Thus the subject can stand normally with the residual limb vertical during measurement. An optical triangulation principle exploited for obtaining a number of horizontal cross-sections of the trunk35, has also been used for scanning of below-knee casts and limbs3. According to this method, a fixed vertical line of light is projected onto the surface and viewed by a video camera from an oblique angle; distortion of the line as seen by the camera is used to calculate the coordinates along the line. Scanning can be conveniently arranged either by rotating the camera around the object, or by rotating the patient/cast on a turntable, both requiring a 360” clear view. This method can only define the surface entirely if two constraints are met; the vertical rotation axis must lie entirely within the body throughout its length, and there must not be any undercuts. The latter is less severe than the reentrant requirement for silhouetting, and generally all residual limb, socket and foot shapes could accomodate this. The requirement for the axis to lie within the body does cause some problems in live limb scanning where it is probable that some of the areas around the distal tip will be misrepresented because of misalignment. Alternatively linear relative movement of subject/camera can be used. Other optical triangulation methods use a moving beam of light with fixed camera to scan over the
CAD
These have been successfully stationary surface. developed for capturing the contours in a single camera field-of-view as required for analysis of back this type ofequipment could shape 36.37 . Theoretically be used to reconstruct a full 360” scan from several images converted to the same frame of axes. The limb segment need only be maintained in the field-of-view without a strict orientation, and views can be arranged to miss, say, chair legs. Similar planar views can be obtained from Moire photography and stereophotography, both of which are rather time-consuming techniques, or from more recent methods of projecting grids or dots onto the work, we scan subject ‘so In our own experimental foot casts hy rotating these in a number of equal steps about a single axis in front of a fixed camera/light unit (Figure 3), and reconstructing the surface from combination of all the scans. No single method has yet emerged as acceptable as a standard, taking into account not only geometric constraints but also issues of spatial accuracy, cost and transportability.
DEVICE
MANUFACTURE
The computer controlled manufacture ofa prosthetic or orthotic device is a rather more difficult problem than it appears at first slance. Current research and developement efforts m the field have tended to emphasize the sensing and design aspects of the CAD/CAM process but a great deal of effort is still required before existing practical problems of manufacture are overcome. Manufacturing techniques in prosthetics and orthotics have for a number of years taken advantage of mass production technology and new materials to create those parts of these devices that can be standardized. Even the prosthetic sockets, seats and other devices which interface directly with the tissues are increasingly preformed with the aid of some mechanization. Nevertheless, these interfaces are still typically fashioned, as was described above, about an intermediate model of plaster of Paris, polyurethane foam or other rigid, workable material created by the hands of the craftsman. stage is time This intermediate ‘modelling;’ c
Figure 3 ‘I‘hrec views optical scan of the foot
of the points
captured
during
a 1.5 s
forprosthetics and
orthotics: M. Lord and D. .Jones
consuming and so much of the current research and development has tried to utilize CNC milling machines to produce the plaster or foam model ready for final manufacture directly from the designed shape data. There is a great deal of interest in trying to eliminate this intermediate manufacturing stage completely; going right from the design data to the finished socket for example. A number of issues have to be addressed before the approach can meet its promise. The productivity issue: balancing the need for skilled custom design with the desire for rapid production A major obstacle to the creation of prosthetic and orthotic devices with one computer controlled manufacturing stage is the necessity to have a customized interface with a patient. Each interface between device and body will have a different shape and the quality of this interface is critical to the success of the whole process. What is needed in the application of manufacturing technology to the CAD/CAM process is a means to have the flexibility to produce a batch size of one without sacrificing most of the speed and efficiency we might expect from mass production technology. In prosthetics and orthotics we now would like to have productivity benefits anticipated through the use of such technology without losing the control and the flexibility needed and enjoyed in current practice. In industry g.enerally it has been the trend in recent years to substitute manufacturing technology for manpower. Technological advances in so called flexible manufacturing systems (FMS) have allowed a shift towards the economic production of small batches of goods. In industries such as shipbuilding, aerospace, tool and die making and other precision industries, technological trends have allowed the requirement for size-one batches to become common. For these industries it is the skills of the craftsman that have been of utmost importance in guiding the machines to produce the required result. Therefore there is a conflict of trends; the advent of new manufacturing technology has tended to reduce the number of skilled craftsmen available and yet this same technology, by making small batch sizes more economic, puts the skills of the craftsman at a premium. Experience shows that a learning curve effect in engineering part manufactypically applies ture40. Consider the manufacture of a batch of identical items. In such cases the first five parts might require close observation and frequent adjustments, the next ten parts are likely to exhibit problems in 20% and the next fifteen parts are likely to exhibit problems in 10 %. This makes it very difficult to achieve totally hands-off precision manufacture with a batch size of one. The system designer therefore needs to provide facilities which remove as many sources ofcostly error as possible. We can learn from these experiences in other fields and anticipate that in manufacturing prosthetic and orthotic devices all aspects of the process should be sensitive to the problems of small batch sizes. This can be achieved
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via inspection of data in graphical form and by interactive simulation of parts of the manufacturing process. Both of these techniques allow the operator to preview the manufacturing process before commiting the design to physical reality. The methodology of socket shape and shoe last manufacture being developed by the National Centre and by University College will be described to illustrate the key problems and issues.
Manufacturing
process
Ideally we would just like to think of the manufacturing process as a system with an input and an output. At the input we have the shape description as a set of surface point coordinates and at the output we have the physical manifestation of that shape. Unfortunately between the input and the output we have many processes to consider. Most research by ourselves and others has depended on the use of CNC type of milling machines to produce the intermediate models for socket, seat or last manufacture and with these systems the following stages are ideal. Inpul dala. This is a set of several thousand surface points represented by x, y, z coordinates in measurement reference frame. The volumes of data encountered with the complex shapes of prosthetic and orthotic practice mean that it becomes impossible to inspect visually the set of coordinate values and spot any aberrations by merely inspecting the numerical values. Verz$y data inlegrily. The data representing the surface should be checked for integrity. This is most efficiently carried out on a high performance interactive graphics workstation which allows the operator to identify visually any surface irregularities. The authors have employed high resolution interactive graphics which in mesh types of display very quickly allow an operator to identify any errors at this stage. It is also necessary to remember that the numerical precision of the original coordinate data should be sufficient to achieve the desired precision in manufacture. The surface data points may require Converl dala. conversion to a new coordinate system suitable for the manufacturing process. Most work in this field has tended to rely on conversion to a form suitable for the helical tool path which will be used. Selecl machining parameters. The tool diameter, tool length, cutting speed, spindle speed, tool end profile, raw material and other factors must be considered. These parameters interact and their choice requires some compromises to be made. For example, a small tool diameter and fine pitch for the tool path produce a fine surface finish at the expense of machining time; but too fine a tool diameter can also introduce tool vibration and therefore machining errors and tool wear.
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Generale machine loo1 instruck’on codes. This must incorporate so-called interference correction for the shape in question. One unavoidable factor concerns the need for interference correction. This consists of a need to adjust the path of the machine tool to prevent the removal of the desired surface adjacent to each of the surface coordinate points during the manufacturing process. Just as in Shannon’s sampling theory for time series signals it is absolutely necessary to have sufficient samples to represent accurately the shape; intuitively it is necessary to have more samples when the shape changes rapidly. Methods have been described4’,@’ which will cope reasonably well with most portions of prosthetic shapes. Each of these techniques should be recognized and described as a form of digital filter with a frequency response, a transient and a steady state response behaviour. An examination of interference checking from the digital filtering perspective can be beneficial as it becomes necessary to have techniques which can adapt to arbitrarily complex shapes such as those found with severely deformed feet and in custom seating. This will be the subject of a future paper. There is no doubt that interference checking is essential in device manufacture and that existing techniques do not cope well with the variety ofshapes with which we wish to deal.
Simulale manufaclure. Here we verify that the generated machine tool instructions are going to create the desired shape. Simulation of manufacture using high performance interactive three dimensional graphics is a powerful technique which will prove to be a significant aid in the future. During our early work manufacturing below-knee socket shapes, there was no way of verifying that a particular shape could be carefully manufactured from a workpiece blank. An hour could be spent waiting for completion of manufacture only to find that the shape did not fit within the workpiece. This fact proved the stimulus for the development of software for the simulation of manufacture. With this process all the manufacturing parameters are held in a specification file which can be edited at will. Similarly the shape of the workpiece from which the shape will be manufactured is held in a file and both are picked up by the graphics simulator which initially allows the operator to verify that the shape to be manufactured will in fact fit within the workpiece. Performed on an Apollo 590T workstation the relative positions of the shape and the workpiece can be adjusted graphically. This interactive process can also accommodate alignment adjustments to the socket and allow inspection of the clearance between the support components and bracket and the tool path (Figure 4). Successful positioning is followed by simulated manufacture on the computer display allowing visual inspection of the results of the choice of manufacturing parameters. As Figure 5 shows, the shaded surface view of the socket shape appears to be ‘carved ’ from the workpiece and the simulation utilizes the actual machine tool instructions so that the manufacturing process can be truly previewed.
C’AD
for prosthetics and orthotics:
M. Lord and D. Jones
REFERENCES 1. Nakajima
2.
3.
4.
5.
\ i Figure within
4 A wireframe the workpiece
view
showing
alignment
6.
adjustment 7.
8.
9.
10.
11.
12.
Figure
5
Simulation
of the ‘carving’
of a socket
shape 13.
14.
This is done to provide the Perform manujizcture. output shape. Most of the work reported so far has gone on to manufacture a device from the carved intermediate model, either by using a system such as the Rapidform to produce a polypropylene draped model or by a lamination technique. Neither of these approaches is ideal and most groups world wide are direct manufacture thereby seeking to perform avoiding the need for the intermediate model.
15.
16.
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CONCLUSION
19.
From the initial forays into computer aided design common central issues are emerging. Whereas in the past we have seen individual centres attempt to develop a complete system necessary to gain this first in future there may be more clinical exposure, concentration on fundamental aspects which by the nature of advanced technology will each take major coordinated effort to produce durable and generally accepted solutions.
20. 21.
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N, Suzuki M, Inatomi Y. Application of CAD/CAM technology for socket design of artificial legs. Proc Conf on CADjCAM technology in Mechanical Engineering 1982; MIT, USA, 153-161. Saunders CG, Foort J, Bannon M, Dean D, Panych L. Computer aided design of prosthetic sockets for below-knee amputees. Prosthetic and Orthotics Int 1985; 9: 17-22. Frrnic GR, Griggs G, Bartlett S, Lunau K. Shape sensing for computer aided below-knee prosthetic design. Prosthetics and Orthotics Int 1985 ; 9 : 12216. Obcrg E’I‘K. Swedish Attempts in using CAD/CAM principles for Prosthetics and Orthotics. Clinica/ Prosthetics and Orthotics 1985; 9: 19-23. Oshima Y, Saito Y. CAD/CAM systems applied to the foot shape for the prosthetic dcvicc. Proc: oJ*RESNA 8th Annual ConjPrence 1987; 222-4. Jones D. Impact of advanced manufacturing technology on prosthetic and orthotic practice. .I Biomed Eng 1988; 10: 179-83. Dcwar ME, Reynolds DP. ‘The UCL system for computeraided socket design-Principles. Abstracts of Proc. C’ World Congress, Int. SOL. Prosthetics and Orthotics London, 1986; 3 15. Rouse DJ. hilodern orthopaedic footwear practice: report of a workshop to dejine clinical criteriafor a computer-aided design and manufacturing system for orthopaedic footwear. Sponsored by NIHR Dept of Education Contract No. 300-83-0236, 1984, Washington DC. Nclham RL. Paediatric adaptive seating in the UK, present scrvicc and future developments. Adaptive Seating. Int Symp on Adaptive Seating, Univ. of Manitoba, Winnipeg, Canada, 1985 l-32. Nelham RL, Mulcahy CM, Pountncy ‘I’E, Green EM, Billington GD. Clinical Aspects of the Chailey Adaptaseat. J Riomed Eng 1988; 10: 175-8. Arridge S, Moss JP, Linncy AD, James DR. Threedimensional digitisation of the face and skull. J h4axillofacial Surg 1985; 13: 136-3. Moss JP, Linncy AD, Grindrod SR, Arridge SR, Clifton J. Three-dimensional visualisation of the face and skull using computerised tomography and laser scanning technology. Europ J Orthodontics 1987; 8: 247-53. Duret F, Blouin JL. De L’empreinte optique a la conception et la fabrication assist&es par ordinateur d’unc couronne dentaire. .J Den&ire du Quebec 1986; XXIII: 177-80. Durct F. Computcrised Dentistry. Dental Practice Management 1986; 10-13. Robertson DD, Walker PS, Granholm JW. cl al. Design of custom hip stem prostheses using 3-D CT modelling. .I Comput Ass.i.rted Tomogr 1987 ; 11: 804-9. Radcliffe CM’, Johnson NC, Foort J. Somr experiences with prosthetic problems of above-knee amputees. Artzf Limbs, Spring 1957; 41-75. Foort J. Socket design for the above-knee amputee. Prosthetics and Orthotics Int 1979; 3: 73-81. Forss J. CAD/CAM of lasts for orthopaedic shoes. Abstracts oJ_IT Cb’orld Congress. Int Socfor Prosth and Orth London, 1986; 312. BS 5943 : 1980 Mcasuremrnt and recording for orthopaedic fhotwcar, British Standards Institution, Milton Keynes. Hampton F. Suspension casting for below-knee, above-knee and Symes amputations. Art;f Limbs Autumn 1966; 5526. Krouskop ‘I’A, Dougherty DR, Vinson FS. A pulsed Doppler ultrasonic system for making noninvasive mcasurements of the mechanical propertics of soft tissue. .I Rehab Res Dev 1987; 24: 1-8. Krouskop ‘I’A, Muilenberg AL, Dougherty DR, Winningham DJ. Computer-aided design of a prosthetic socket for an abovr-knrc amputee. J Rehab Res Dev 1987; 24: 31-8. (:hildrcss DS, Schur MS. Computer-aided analysis of
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24.
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