Premodified casting for the Patellar-Tendon-Bearing prosthesis.

1 This work was supported in part by Veterans Administration Contract V663P-505. 2 Director, Prosthetics Research Study, Seattle, Wash. 3 Consultant, Rehabilitation Engineering, Social and Rehabilitation Service, Department of Health, Education, and Welfare, Washington, D.C. Methods for producing a functional, comfortable, and well-fitting patellartendon-bearing prosthesis have been the subject of considerable discussion, and in fact some controversy, since the prosthesis was first introduced several years ago. Prosthetists use a variety of techniques to cast below-knee stumps, and there is an extensive literature on the subject, not excluding the technicians' differing viewpoints. There is agreement, however, that the effectiveness of the prosthesis depends to a great extent upon how well the wrapcast (negative) was taken and, subsequently, how precisely the male plaster mold (positive) was modified. The positive mold is modified in order to relieve pressure-sensitive areas by the addition of build-ups, and to increase the pressure to the pressure-tolerant (or natural weight-bearing) areas of the stump by the judicious removal of small amounts of plaster. These alterations prevent vertical displacement during stance and provide for comfortable accommodation of the stump during full weight-bearing. The precise amount of plaster removed varies with the individual patient, depending upon the muscle tone and the amount and resilience of the subcutaneous tissue. The procedure is by no means a difficult one, but timing is a complicating factor. Authorities on the subject encourage immediate rather than later modification of the positive cast in order to prevent improper interpretation of the individual stump characteristics. Consequently, the well-qualified prosthetist who finds himself with a large number of plaster positives to be modified, or the less experienced prosthetist who is just developing a keen sense of technical judgment, is at a disadvantage because, even with the best memory and with detailed prosthetic information, he is limited by techniques which involve nothing more than intelligent guesswork and which are conducive to at least an occasional error, regardless of the individual's experience and skill. This difficulty can be overcome by modifying the cast on the patient's stump when the negative-cast impression for the permanent prosthesis is taken. This paper describes such a procedure, essentially initial socket fitting during casting, which provides a plaster negative-positive that requires only a final smoothing to be ready for socket lamination. The method includes the application of felt pads to strategic areas of the stump. Elastic plaster bandage is used for the negative plaster wrap because it effectively conforms to the irregular stump surfaces, controls tissue compression and displacement, and yields a precise stump impression. The resulting positive plaster mold resembles the stump contours accurately, thus providing the basis for a comfortable, well-fitting, and functionally acceptable PTB prothesis. Provision of a total-contact, hard PTB socket, without a soft end or the customary insert, is the standard procedure at the Artificial Limbs. Vol 1 5, No l .pp 1-14, Spring 1971