Comparison of CAD-CAM and hand made sockets for PTB prostheses
P. Köhler *
L. Lindh *
P. Netz *
The aim of the present study was to compare sockets for below-knee (BK) prostheses made by Computer Aided Design-Computer Aided Manufacture (CAD-CAM) to those made by hand. The patients in the study were provided with two prostheses each, which apart from the sockets, were identical. One socket was made by the CAD-CAM technique developed at the Bioengineering Centre, Roehampton, University College London and one was made by hand at the OT-Centre, Stockholm, Sweden. The results were based on investigation of eight unilateral below-knee amputees evaluating their own sockets by Visual Analogous Scale with respect to comfort, pressure, and pain. The sockets were evaluated on seven occasions, at two tests, on delivery, after use every second day for six days and every second week for two weeks. All CAD-CAM sockets except one had to be changed once as compared to the hand made of which only two had to be changed. As to comfort it could not be demonstrated that there was any significant difference between the two types of sockets and both types were well accepted by all patients. Differences in pressure and pain were rarely reported. There were obvious differences between the two types of socket with respect to height, width, and inner surface configuration.
The authors feel that CAD-CAM will in the near future be an excellent tool for design and manufacture of prosthetic sockets.
About fifteen years ago industry began to use computers for design and manufacturing i.e. Compter Aided Design and Computer Aided Manufacturing (CAD-CAM). This technique is now being applied to the manufacture of sockets for BK amputees. The form of the stump is digitized and transferred into a computer, which makes standard changes. In addition the form may be changed by the prosthetist on the screen. When the prosthetist is satisfied with the form, a numerically controlled (NC) carver makes a model over which the socket is made.
In the mid 1970's Jim Foort and Carl Saunders began to develop a CAD-CAM system at MERU (Medical Engineering Resource Unit) in the University of British Columbia, in Vancouver. The socket shape is defined by the computer on the basis of manually made measurements of the stump taken at certain predetermined points. Changes can be made subsequently to any area by the prosthetist, via the computer, before carving.
At the Bioengineering Centre, University College London another CAD-CAM system has been developed using a different principle A plaster wrap is taken of the undeformed stump and placed in a measuring device. The wrap is rotated while a probe follows its inner surface and transfers the form of the stump digitally into the computer. At standard areas the average rectifications of a skilled prosthetist are made by the computer. In addition the prosthetist can change the standard areas as well as the length and width of the socket.
It has been reported that both systems have provided several BK amputees with well tolerated sockets. However, the inevitable question arises: Is a CAD-CAM-produced socket as comfortable as a manually designed one?
The aim of the present study was to compare the function of sockets made by CAD-CAM to those made by hand.
Materials and methods
Ten unilateral BK amputees, two with heavy, four with rather heavy and two with light employment, who use their prostheses all day were provided with two prostheses each. Apart from the sockets the prostheses were identical. One socket was made by using the UCL CAD-CAM technique. The undeformed stump form was obtained from a plaster wrap taken in Sweden and sent to London. The changes, based on a rectification pattern, produced by a research prosthetist at the Bioengineering Centre were made and a model was carved in London. The sockets were made in Sweden directly on the model, leaving the ridges made by the NC-carver. As a control a second socket was made by hand according to Swedish principles at the OT Centre in Stockholm. Five prosthetists with work experience ranging from 1 to 10 years made control sockets for two patients each. The five different prosthetists were chosen to obtain an average of expected hand made quality. For each patient both types of sockets were adjusted as many times as needed i.e. until both the patients and the prosthetists were satisfied. All prostheses were aligned by the same prosthetist. It was not indicated to the patients which of the sockets was made by CAD-CAM and which was made by hand. The patients evaluated their sockets by a 100mm Visual Analogous Scale (VAS) with respect to comfort, pressure, and pain (Fig. 1 ). Both sockets were evaluated seven times i.e. at two tests, on delivery, after use every second day for six days and every second week for two weeks. The patients were taught how to fill in the forms at the two tests and the delivery and carried out the evaluation themselves at home on the other occasions.
Calculations and statistics
For each socket the value of comfort was measured from the VAS in mm. The ratios between the two measurements for the two types of sockets for each patient at each of the seven evaluations were calculated. The mean and standard deviation of the seven evaluations were calculated and used for statistical analysis.
Student's T-test was used for statistical analysis and a significance level of 99% was chosen.
Two patients had to be excluded since they had not understood how to fill in the forms properly. The results are based on the remaining eight patients.
All CAD-CAM-sockets but one had to be changed once as compared to the manual sockets of which only two had to be changed (Table 1 ).
With respect to comfort considerable variations were found in the absolute values (Fig. 2 ). Analysis of the ratios of comfort evaluation between the two types of sockets, however, showed no significant difference at any time (Fig. 3 ). Both types of sockets were well accepted by all patients. As can be seen in Table 2 and Table 3 differences in pressure and pain rarely occurred.
In spite of the few differences observed by the patients it was obvious that the two types of socket differed with respect to trim-line, width, and inner surface configuration (Fig. 4 ).
It was found that sockets made by the CAD-CAM technique were equally comfortable when compared to those made by hand.
The degree to which feelings of comfort, pressure and pain are experienced cannot be measured objectively. The only known way to transform such subjective parameters is to use the Visual Analogous Scale (VAS). The relevance of using the VAS for evaluation of socket comfort can be derived from Fig. 3. In the interval between test 2 and delivery no changes of the socket or prosthetic alignment were made. Since mean values and standard deviation were almost identical at both occasions there are reasons to assume that the evaluation by VAS is relevant for comparative studies on socket comfort. Whether the absolute values also can be used in a non-comparative situation is a matter for reasonable doubt. In patient no. 5 for instance the ratio varied very little whereas the absolute values varied from 96 to 45 mm. The absolute values of VAS probably reflect how well the patients have accepted their handicap. There are reasons to assume that patients no. 1 and 4 have accepted their handicap much better than no. 8.
The study was aimed solely at comparing sockets made by CAD-CAM to those made by hand. Since there was no possibility of using the computer system with a Swedish socket design the English one had to be used. This made it necessary to compare not only sockets made by CAD-CAM to those made by hand but also the skill of an English prosthetist with that of a Swedish one. In spite of this difference in socket production technique the patients could not detect any differences in comfort, indicating that the skills are quite compatible.
The material is small but the patients were used as their own controls and the result showed no tendency in favour of any one of the two types of sockets. This fact may justify the conclusion that probably there are no differences in comfort between the two techniques. If there is a difference it seems to be small and to prove this it would be necessary to make an investigation which comprises a very much larger material. This might reach the same conclusion. The amputees were not told which socket was made by hand and which was CAD-CAM made. The ridges made it obvious to those who deal with sockets every day which was which, but none of the patients made any remark on the subject. Moreover, there are no reasons to assume that the patients should be prejudiced against any type of socket thereby biasing the study even if they realised the difference.
The intention was to verify using the VAS the differences between the two types of sockets with respect to pressure and pain. However, the few differences reported by the patients made it irrelevant to make any calculations on the subjects.
None of the patients had any complaints about ridges in the CAD-CAM sockets. Some of the patients even remarked that these sockets were more airy. None of the patients could detect ridges that were 2 cm wide and 0.5 cm high. The reason is probably that two point discrimination of the lower leg is approximately 2.5-3cm making the ridges impossible to detect.
Making the stump form available for the computer from the plaster wrap is a rather complicated process. However an improved tool is now being evaluated. The undeformed stump form is obtained from silhouette pictures as a tv-camera swings around the stump over a three second period. This new technique will simplify the procedure and minimize the time needed for stump measurement.
Despite the CAD-CAM technique being newly applied to patients the authors could not find any differences in socket comfort. Their opinion is that the UCL technique will in the near future be an excellent tool for improved design and manufacture of prosthetic sockets. The technique would also give other advantages such as uniformly high quality, easy storage of stump-forms and improvement in the teaching of prosthetists.
The authors wish to express their sincere deeply felt gratitude to Steven J Cousin, Mike Dewar, Joseph Wilkinson and all staff members of the Bioengineering Centre, Roehampton, University College London for their cooperation making the study possible to perform. This study was supported by grants from the Swedish Institute for the Handicapped.
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