A CAD/CAM Digitizing Adapter for Spinal Casts


CAD/CAM systems (Computer Aided Design / Computer Aided Manufacture) are used in many prosthetic clinics as a manufacturing, research, and/or educational tool (Houston, et al., 1992; Krouskop, et al., 1989; Lemaire, 1994). While CAD/CAM benefits have been shown for prosthetics (Engsberg et al., 1992; Oberg, et al., 1993), few orthotic CAD/CAM applications have been reported in the literature.

One reason for the lack of orthotic CAD/CAM applications could be the difficulty in obtaining the necessary surface topography to produce an orthosis. This is especially true for lower extremity orthotics since leg/ankle/foot topography deviates from the conical residual limb shape used in prosthetics. Spinal orthoses, however, have a cylindrical shape which can be accommodated by most prosthetic CAD software packages. Previous literature on CAD/CAM and spinal orthosis design involved dedicated spinal cast digitizers to convert a spinal cast shape into a computer data file (Rashke, 1989; Rashke, et al., 1990) or used physical measurements to mathematically create an orthosis shape (Ramos et al., 1994).

Physical measurement based systems required custom software to produce the computerized orthotic shape and, while they are currently used as service tools, do not accommodate severe spinal deformities. The cast digitization approach may require a larger custom digitizer for large models; however, existing prosthetic cast digitizers can be adapted to accommodate the majority of spinal orthotic shapes.

By using existing prosthetic CAD/CAM technology to create the positive model, clinics which service both prosthetic and orthotic clients can benefit from the advantages of CAD/CAM without having to purchase two separate systems. Orthotists at The Rehabilitation Centre (Ottawa) have found that the use of CAD/CAM has reduced the time required to create a positive model by eliminating manual plaster work and reducing modification time. Since a positive model can be carved out of medium density foam (IPOS carver), orthotic technicians benefit by not having to work with the heavy, plaster spinal cast models during fabrication (foam blanks are approximately 90% lighter than plaster blanks). One contraindication of the foam blanks is that lining materials cannot be stapled onto the foam; therefore, liners cannot be molded into the orthosis.

To permit spinal cast digitization on a standard prosthetic cast digitizer (Seattle Digitizer, M+IND), an adapter has been developed to hold a spinal cast in position without requiring special modifications to the cast. This document will describe the design criteria, fabrication procedure, and operation of this device.


Design Criteria

The CAD/CAM spinal cast adapter was designed using the following criteria:


The spinal cast adapter has five main components: interface, round table, vertical bars, and attachment arms. The interface section was machined out of aluminum stock to the prosthetic cast holder's dimensions and secured to the bottom of the table with four screws. Since screws were used to attach the interface section to the table, different interface sections can be attached to accommodate different digitizing machines.

Six tracks, 1.3 cm wide, were cut in the circular (51 cm diameter) table to allow the vertical bars to move radially from the edge of the table. Although all six tracks are not needed at the same time, the ability to move the vertical bars to a better angular location could help with extremely asymmetrical shapes. The table dimensions were chosen to provide the largest possible diameter without interfering with digitizer operation.

Four 55 cm by 1.6 cm aluminum bars were used to either support the cast or attach the horizontal bars. The vertical bars were threaded at the bottom so that the threaded end could be inserted through a track and secured underneath by a 5.0 cm threaded disk.

To prevent axial rotation of the bar within the track, a 1.3 cm by 3.0 cm rectangular key was pinned to the mid-point of the threaded region. A 3.2 cm long, threaded cylindrical piece (threaded sleeve) and a 5.0 cm, unthreaded disk were inserted into the top side of the threaded region. After the bar had been placed in the track and the lower disk screwed snugly against the key, the cylindrical piece can be tightened against the table by turning the threaded sleeve (thereby securing the vertical bar's position). This "top tightening/loosening" action allows an orthotist to move the vertical bars to the correct position without having to reach under the table to tighten a nut or screw.

The attachment arms were used to hold the cast in a vertical position. The arms consisted of a 15.0 cm by 1.0 cm aluminum rod with a 2.5 by 1.0 cm knurled piece screwed into the distal end. This smaller piece was oriented perpendicular to the horizontal bars and was free to rotate about its mid point to accommodate the cast's contoured surface. An aluminum threaded sleeve was attached to the proximal end of the 15.0 cm rod to secure the attachment arm to the vertical bar.

The locking mechanism is shown in figure 3. This mechanism simplifies the process of holding the cast in position, moving the attachment arm to the cast's exterior surface, and tightening the arm in position (i.e., hand tightening only, no tools are required to tighten the vertical or horizontal bars). Since the screw section is attached to the bar and the collar, rotating the threaded sleeve will push the locking piece tight against the horizontal bar.

Installation - Operation

Before installing the cast adapter in the digitizer, the orthotist should ensure that the horizontal bars are in the desired tracks and secured to the table. Orthotists at our centre have found that having three horizontal bars on the back tracks and one on the centre front track were appropriate for the majority of spinal casts. The table is installed in the digitizer by placing the interface piece into the standard digitizer socket and tightening the digitizer's locking screws (i.e., same procedure as with the prosthetic cast holder).

To install the cast in the adapter, place the cast on the table such that the cast's cross-section centre is in line with the centre of the table and the cast's longitudinal axis is perpendicular to the table. It is important that the cast be aligned correctly when digitizing to facilitate future CAD modifications. Push the centre-back vertical bar forward until it touches the back of the cast and tighten the bar's locking sleeve. An attachment arm is not required for this vertical bar since the bar acts as a positioning support for the cast. Move the front vertical bar to approximately 5 cm in front of the cast. Two attachment arms are used on this bar to hold the front of the cast. The two attachment arms are rotated to the left and the right of the bar respectively, pushed tight against the cast, and tightened in place. The remaining two vertical bars are placed 5-8 cm from the sides of the cast. The single attachment arms on these bars are rotated to the side of the cast at a level just above the iliac crests. When these arms are tightened in position, the cast cannot move laterally. Tests with this digitizer adapter have shown that, with the cast securely installed, the adapter can be turned upside down without changing the cast position.

The digitizing procedure for the spinal orthosis is the same as for a prosthetic cast or socket. However, the clinician or technician may have to slow down the table's angular velocity to ensure that the tracking arm does not skip over high contour areas.


Maintenance of the spinal cast adapter is minimal. If the adapter is kept clean (i.e., free of plaster build-up that inhibits free movement of the components), no lubrication or other regular maintenance is required.


The spinal cast digitizing adapter is an easy to use, low maintenance, cost effective option for adapting prosthetic digitizing hardware to orthotic CAD/CAM applications. By changing the adapter's interface unit, this device could be used with a variety of cast digitizers.


The authors wish to acknowledge the technical assistance of Paul Washer and Harold Gay and the administrative assistance of Guy Martel.


Engsberg Jr, Clynch Gs, Lee Ag, Allan Js, Harder Ja (1992) A CAD/CAM Method for Custom Below-Knee Sockets. Prosthet Orthot Int 16, 183-188.

Houston Vl, Burgess Em, Childress Ds, Lehneis Hr, Mason Cp, Garbarini Ma, Lablanc Kp, Boone Da, Chan Rb, Harlan Jh, Brncick Md (1992) Automated Fabrication of Mobility Aids (AFMA): Below-knee CASD/CAM Testing and Evaluation Program Results. J Rehabil Res Dev, 29(4), 78-124.

Krouskop Ta, Malinauskas M, Williams J, Barry Pa, Muilenburg Al, Winningham Dj (1989) A Computerized Method for the Design of Above-Knee Prosthetic Sockets. J Prosthet Orthot, 3(1), 131-138.

Oberg T, Lilja M, Johannsson T, Karsznia A (1993) Clinical Evaluation of Trans-Tibial Prosthesis Sockets: A Comparison Between CAD CAM and Conventionally Produced Sockets. Prosthet Orthot Int, 17:164-171.

Ramos Jv, Pellas F, Vitton Jm, Abelard P, Conil Jl, Delarque A, Bardot A (1994) Computer Aided Conception of Cheneau-toulouse-munster Scoliosis Braces: Quantitative Valuation by Comparison with Classical Craftsman Devices. Clin Rehabil, 6(suppl), 76.

Rashke Su (1989) Applications for Spinal Orthotics. Proc ISPO Sixth Word Cong, Kobe, Japan, 268.

Rashke Su, Bannon Ma, Saunders Cg, Mcguiness Wj (1990) CAD-CAM Applications for Spinal Orthotics - A Preliminary Investigation. J Prosthet Orthot, 2(2):115-118.