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The WILMER Appealing Prehensor

Plettenburg, Dick H. PhD, MSME

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JPO Journal of Prosthetics and Orthotics: April 2006 - Volume 18 - Issue 2 - p 43-45
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The standard split-hook prosthesis (Figure 1), despite its functionality, is most often rejected by parents of a child with an upper limb defect because of its deterring outward appearance. The more cosmetically appealing, but less functional, hand prosthesis is usually preferred. The WILMER group developed a new prosthetic prehensor for children, with the objective of preserving the functionality of the standard split-hook prosthesis, while improving its outward appearance.

Figure 1.:
An example of the standard split-hook prosthesis.


A shape study was performed1 to determine the outline of the new prehensor (Figure 2). The preferred shape is the hook-like prehensor depicted at the right in Figure 2. Its volume and outline are derived from the contour of a hand of a 4- to 6-year-old child. The length of the fingertips and the position of the rotating finger approximate those of a healthy hand. The connection to the forearm is harmonic and smooth. All mechanical parts, including the operating cable, can be placed out of sight in the interior of the prehensor.

Figure 2.:
A shape study for the new prehensor design was performed with three different shapes. Left: a split-hook design. Center: an “anthropomorphic” hook design. Right: a “styled anthropomorphic” hook design.

The mechanism design underwent several iteration steps.1–3 After each step, a successful laboratory test was followed by a clinical trial with four to five children. The initial mechanisms required many repairs, resulting in a third redesign (Figure 3).4 A four-bar linkage mechanism is used to reduce the input forces. A slightly inclining input characteristic ensures controllability for both shoulder harness and elbow control. The resulting grip force is nearly constant over the full range of opening.

Figure 3.:
Mechanical aspects of the WILMER appealing prehensor. Top: a cross-sectional drawing of the mechanism. Bottom right: several parts, partly assembled. Bottom left: the assembled four-bar mechanism.

Integrated into the frame of the prehensor is a lightweight friction wrist prosthesis. The frame is enclosed by a cosmetic cover made of flexible polyurethane resin, allowing for several unique features: the outside of the prehensor is rugged and easy to maintain; the cover can be easily removed to access the mechanism; and the cover can be colored (Figure 4). Giving the cover a bright primary color emphasizes the toy-like nature of the prehensor, thus advancing the acceptance and use of the prehensor by the child. It is even possible to supply several covers in different colors, which can be exchanged by the child according to daily moods.

Figure 4.:
The WILMER appealing prehensor. The cover of the new prehensor can be made in many different colors.

After successful laboratory testing, the latest version of the WILMER appealing prehensor was issued for clinical trials. It has been in daily use by four children for more than 200 months of total testing time (Table 1).

Table 1:
Users of clinical prototype WILMER appealing prehensor


Each of the four children highly appreciated his or her device. Because of the smooth outline of the prehensor and the integration of the control cable, damage to clothing is reduced considerably. The children are delighted by the brightly colored appearance of the prehensor. All children chose a color out of our standard palette, with one mixing colors between the housing and the thumb.

During the entire testing period, only a few repairs were necessary. On two occasions an axle of the four-bar mechanism was lost because of corrosion of the stainless steel retaining rings. After replacement of the axle and the retaining rings from a second delivery batch, the problem did not reoccur. On three occasions the operation cable broke at its connection to the mechanism. This could be repaired by renewal of the soldered connection. Finally, on two occasions the rubber grip surface on the fingers of the hook partly detached from the fingers.

One child (JG) experienced several medical problems, not related to the arm defect, a few months after the onset of the clinical trial. Her parents decided to discontinue use of any arm prosthesis. The other participants continue to use the WILMER appealing prehensor. AS changed the color of the hook on two occasions but still uses the original mechanism. NB and YK each received a second prototype because of excessive wear of the fingers of the hook after 27 and 40 months of use, respectively.

The overall results of the clinical testing have been very satisfying. The children and their parents highly appreciate the prehensor because of its looks, functionality, and reliability. The reliability is also appreciated by the designers and by the prosthetists.

Encouraged by this success, a second larger size has been developed (Figure 5), intended for children 7 to 14 years of age. This medium-size prehensor has been in clinical use by NB since April 2004 (Figure 6). Again, the colorful appearance of the hook is highly appreciated, and the mechanism proves to be very reliable. No repairs have been necessary despite continuous daily use.

Figure 5.:
The small- and medium-size designs. The black line denotes the distal front plane of the wrist, which acts as a reference for size comparison.
Figure 6.:
NB wearing her WILMER appealing prehensor.

The appealing prehensor has drawn the attention of practitioners who deal with body image and personal identity in relation to bodily differences.5,6 Wearing a brightly colored artificial prehensor can be noted as a clear statement of personal uniqueness.

The development of this new appealing prehensor provides a solution to the classic prehensor dilemma: either cosmetics or function. This prehensor combines the two.


The author gratefully acknowledges the contribution of the current and former members of the WILMER research group at Delft University of Technology. The author also thanks his clinical partners of the rehabilitation centers De Hoogstraat and Sint Maartenskliniek for their cooperation.

The research is co-funded by the Phoenix Foundation, the Netherlands, and the Delft University of Technology.


1. Walta JA. Design of a cosmetic prosthetic prehensor (in Dutch: Esthetische haakprothese ontwerp). DUT-report N-193, ISBN 90-370-0034-7. Delft, 1989.
2. Blom TJ. Philosophic background of the redesigned cosmetic prosthetic prehensor (in Dutch: Filosofie achter het herontwerp van de esthetische haakprothese). DUT-report N-362. Delft, 1991.
    3. Plettenburg DH, Cool JC. The WILMER cosmetic prosthetic prehensor for children. In: Zupko JK, ed. Proceedings of the 7th ISPO World Congress, Chicago, June 28–July 3, 1992, p62.
      4. Sol AAM, Plettenburg DH. The WILMER appealing prehensor. In: Congress Book IX ISPO World Congress, Amsterdam, the Netherlands, June 28–July 3, 1998, pp 646–648.
      5. Bolt I, Wijsbek H. Beauty and the doctor. Moral issues in health care with regard to appearance. Report to the European Commission, Directorate-General XII, Biomedical and Health Research Program, Rotterdam, EUR PL 963164, 2002.
      6. Hilhorst M. Prosthetic fit: on personal identity and the value of bodily difference. Med Health Care Philos 2004;7:303–310.


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      arm; hand; prehensor; prosthetics; upper extremity

      © 2006 American Academy of Orthotists & Prosthetists