Vacuum Dilatancy Casting for the Construction of Individually Molded Seats



The merits of the vacuum dilatancv casting (VDC) technique for the production of accurate body impressions of patients suffering from sitting problems are discussed. Impressions are taken with the patients seated in the desired posture.

A summary of experiences with the first 15 individual seats constructed according to body casts made with the VDC technique is presented. The range of potential applications for this technique includes, among others, prevention of decubitus ulcers, postural correction, postural support, and improvement of stability.


If the air is evacuated from an impermeable flexible bag filled with some grainy material, the grain-filled bag becomes a solid body which resists deformation. Using this principle of vacuum dilatancy casting, very strong structures, which have even been considered for the construction of dams 7 , can be made.

In medicine, the VDC technique has been used to take body impressions accurately. A patent for such a procedure was granted to Mead10 in 1950. Its use in the production of molds of leg stumps and of body impressions for the construction of individually adapted seats has also been reported1, 3,4,8 . the applications concern the fixation of body parts such as those of patients during orthopaedic operations", the fixation of animals during laboratory experiments 12 , and the bracing of fractured limbs 5 . In the present paper, the application of VDC for the production of individually adapted seats is discussed.

Several clinicians had asked us to find solutions for patients with severe sitting problems. The most important problem to be solved in the construction of an individual seat is the production of a proper body cast of the seated person. The use of standard techniques is often difficult or impossible for the following reasons:

  • The geometry of the patient is so complicated that making a suitable body impression by carving is extremely difficult.
  • Taking a plaster-cast impression with the patient sitting in the correct posture and under normal loading conditions is also difficult or impossible.
  • The cycle of successive evaluations and corrections of individual seats is very time-consuming and expensive.

From experience gained in other applications of VDC, it was felt that a better method of taking body impressions could be developed with this technique. At present, we are developing individually adapted seats for two major groups of patients:

  • Patients with decubitus problems. Applications of VDC to this group have also been described by McCluer 9 and by Hassard6. The basic idea is to obtain an optimal pressure distribution over the seat of the patient, thereby avoiding any excessive peak pressures which could cause decubiti. For this purpose, we are experimenting with buttock seats shaped for the individual patient.
  • Patients with sitting problems due to anatomical deviations. For this group of patients, we are developing individually formed bucket seats which incorporate support for the back and, where necessary, the head of the patient.

Although the developments are not yet completed, our preliminary results are very encouraging 4 . The present status of the project is reported in this paper.


Typically the patient is seated on one or more flexible impermeable bags which are filled with a grainy material. His correct sitting posture is indicated by the responsible medical team. If necessary, the patient may be given additional support to correct his posture. His buttocks must sink into the bag to the proper depth in order to get a close contact and a proper distribution of pressure. If necessary, this sinking in can be facilitated by a light vibration of the bag by the use of a vibrator or vibrating table. It is also possible to control the depth to which the buttocks sink by stopping the vibration when the proper depth has been reached.

The above procedure suffices to produce impressions for buttock seats. To produce an impression for a bucket seat, another bag, or a part of the same one, is placed around the patient's back and pulled forward under his arms.

After a check as to whether the correct posture has been maintained and sufficient support provided, the bag (or bags) is (are) evacuated. The atmospheric pressure compresses the bag and its contents, increasing the frictional forces between the grains and solidifying the grain-filled bag in the shape it was just given. If the procedure has been successful, then the patient is properly supported in this solidified structure ( Fig. 1 ).

Now the patient can be removed from the bag, leaving the impression of his buttocks ( Fig. 2 ). The form of the impression can be judged by its quality and its functionality can be evaluated by having the patient sit in it again for some time. If it is accepted, a plaster cast of the impression is made; otherwise, the procedure is repeated until it is successful. The inclination of some reference surface of the cast is recorded for the future adjustment of the finished seat. The plaster cast obtained is finished by smoothing and represents an accurate copy of the patient's shape in the desired posture under normal loading conditions ( Fig. 3 ).

In practice, a more elaborate two-step procedure is often followed to improve the quality of the impression. The first time the impression is made, the air evacuation is not complete, leaving the bag in a somewhat plastic condition so folds can be eliminated. Then the final impression is taken with the preformed bag. The remaining folds are smoothed out later in finishing the plaster cast.

Once the proper positive plaster cast is obtained, a seat molded around it takes the shape of the VDC impression ( Fig. 4 ). Various molding techniques have been used and these are discussed under Materials. This seat is then finished and adapted to the purpose for which it was prepared, such as attachment to a wheelchair, a working chair, a mobility aid for a child, a bicycle seat, etc. Special attachments, which often must be adjustable, are added for this purpose. It is important to obtain the correct inclination of the seat and the proper adjustment with respect to other mechanical structures such as foot supports, tables, wheelchair controls, etc.


In order that this technique may be applied clinically in the orthopaedic workshop, a proper choice of materials is essential. The critical materials are the sheet for the bag, the grains to fill it, and the material for the seat. Conflicting demands imposed upon these materials often call for a compromise solution, depending on the specific application. For none of the requirements has a totally satisfactory solution been found, and we are continually searching for better materials and techniques.

Initially, the bags were made of polyvinyl-chloride (PVC) foil. More recently, thin rubber envelopes which have a greater flexibility and allow the making of impressions with fewer folds have been used. Bags of various sizes and shapes have been used for different applications. For buttock seats, we have also worked with a metal box covered with a thin sheet of rubber. In the construction of bucket seats we have worked with the following configurations:

  • Two separate bags for the seat and back.
  • One large bag supported by an adjustable wooden structure ( Fig. 5 and Fig. 6 ).
  • A bag with a canvas-reinforced back to improve handling.
  • Preformed chairs with a back of reinforced latex and an inner lining made of flexible latex ( Fig. 1 and Fig. 2 ).

In the latter case, versions with one compartment or with two separate compartments for seat and back have been used. These latter allow filling with different types of beads and independent evacuation of seat and back. Although in this manner bags which are less vulnerable have been obtained, careful handling is still necessary in order to avoid tearing the inner lining. We are still looking for a less vulnerable but still flexible and compliant material.

The material used for the grains varies according to the application. For body casts, we have worked with solid polystyrene beads of approximately 1 mm diameter, or with expanded ones of about 3 mm. The solid ones give a good impression of the seat, but are too heavy to be used for the back. The expanded ones are light enough to be used for the back, but are slightly compressible under atmospheric pressure; therefore, some harder material would be desirable for this purpose. We are still looking for a lighter and harder but unbreakable material without sharp edges which could cut into the envelope. However, among the materials known to us, the polystyrene beads are still the best choice for our purpose.

The next problem is the choice of the material for the final seat. Both the expected use of the seat by the patient and ease of handling during its construction determine the requirements imposed upon the material. In collaboration with various rehabilitation centres and industries, we have been experimenting with the following techniques:

  • Glass fibre-reinforced epoxy with an inner layer of Plastazote*.
  • Resur**, hot molded by hand over the plaster cast.
  • Hard polyurethane foam, poured or sprayed (in collaboration with Shell Plastics Laboratories, Delft, The Netherlands) on the plaster cast ( Fig. 7 ).
  • Acrylonitrile butadiene styrene (ABS) sheet, vacuum-formed over the plaster cast ( Fig. 4 ).
  • Plastazote, hot molded over the cast and supplemented with a hard polyurethane foam base.

Where necessary, various stabilizing structures have been added, often consisting of a base of hard polyurethane foam. Also, a highly pliable replaceable Ventapad (Uncoated Ventapad: brand name for a simulated sheepskin with an uncoated back) lining with an uncoated back is often used. This skin can be stretched to follow the shape of the seat without the formation of folds which could cause pressure spots. This flexibility is essential, particularly for the prevention of decubiti 4 .


Up to now, we have contributed to the construction of 15 seats, the body casts of which have been made by the VDC method. Data on these cases are presented in the Table on page 8. Preliminary results with the first cases (patients 1 through 4) have recently been reported4. The others, except for patient number 12, are still being evaluated clinically. Although some of the seats have required minor corrections, none of them has been rejected by either the patient or the medical staff thus far, in spite of the fact that several of these cases reached us only after other methods had failed.

Particularly worthy of mention is the highly successful solution of the severe recurrent decubitus problems of patient number 3, by means of our adapted buttock seat, in combination with the uncoated Ventapad simulated skin, and an adapted wheelchair ( Fig. 8 ). With this combination, the patient has now been free of decubiti for a period of 14 months, interrupted by one recurrence when the uncoated Ventapad skin was replaced by some coated skin after the uncoated one had worn out. After renewed application of the uncoated material, no new problems have developed so far.


The first results of this research have shown that adapted seats can be produced with the VDC technique, even in those cases where conventional techniques have failed. The problems encountered with the latter have been largely circumvented, and the following advantages are obtained with the VDC:

  • The impression can be taken with the patient in his normal or corrected sitting posture.
  • The impression can be made with the patient wearing his normal clothing and, where necessary, his orthopaedic appliances, thus producing the shape required during actual use.
  • This technique is less disturbing to the patient than one in which a plaster cast is made directly on his body. This aspect is of particular importance where children are concerned.
  • The impression obtained is less dependent on the ability of the particular orthopaedic technician taking it.

It should be noted that, in general, the production of an adapted seat only partially resolves the sitting problems of a particular patient. Although a seat of good shape can be obtained, the following aspects also require special attention:

  • The feet of the patient should be properly supported at a height which allows an optimal distribution of pressure over the legs and buttocks 6 .
  • The seat should be correctly inclined in order to obtain an optimal functional posture.
  • The patient should be examined at regular intervals for the occurrence of skin lesions due to the possible development of pressure spots after a changed configuration due to growth, change in weight, etc.

At present, we cannot yet foresee the total range of cases where a VDC seat would be indicated, but we are convinced that many categories of patients suffering from sitting problems could benefit from it. Some of these categories are patients with spina bifida, cerebral palsy 2 , spinal-cord lesions, muscular dystrophy, and dysmelia. In general, it is probable that patients with anatomical deformations who are already suffering from decubiti, or are in danger of doing so, will qualify.

Thus far we have only considered applications for patients, but individually adapted seats may possibly also be of value for healthy persons whose professions require prolonged sitting. One could think of professional car drivers, etc., who could benefit from a light, individual seat which could be easily mounted on the adjustable driver seat. In this way, each driver could have his personal seat insert.

* Plastazote: brand name of a polyethylene foam with closed-cell structure.
** Resur: brand name of a polyethylene sheet which can be easily molded after heating.

Institute of Medical Physics TNO, Utrecht, The Netherlands

Institute of Medical Physics TNO, Utrecht, The Netherlands

Institute of Medical Physics TNO, Utrecht, The Netherlands

Institute of Medical Physics TNO, Utrecht, The Netherlands

Institute of Medical Physics TNO, Utrecht, The Netherlands

Institute of Medical Physics TNO, Utrecht, The Netherlands

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