A "Hard Socket" Patellar Tendon Bearing Below-Knee Prosthesis

Frank W. Clippinger, M.D. Bert R. Titus, C P & O

The patellar tendon bearing below-knee prosthesis, fabricated without a socket insert, has been utilized to a limited degree by several centers, but it has never been widely adopted.

In a humid climate, the socket insert made of Kemblo and either leather or Naugahyde, has been a source of trouble. It is hot and becomes wet, is difficult to clean, and wears out in a few months due to the acid action of accumulated perspiration. In addition, the stump sock tends to stick to the internal surface of the insert, resulting in abrasion of the stump within the wet sock. In the past, we have recommended that two inserts be furnished with each patellar tendon bearing prosthesis so that when one became wet it could be replaced.

Interest in Temporary Prostheses

During the past ten years, we have been experimenting with various types of temporary prostheses in the Orthopaedic Amputee Clinics at Duke Medical Center and the Durham Veterans Administration Hospital. The goals of this endeavor have been to: 1) obtain rapid shrinkage of the stump, 2) expedite the patient's return to relatively normal activity, and 3) accurately determine the patient's prosthetic requirements and tolerances.

The present model below-knee temporary prosthesis is a patellar tendon bearing limb, the socket made without a Kemblo insert and worn with one five-ply wool stump sock (Fig. 1 ). If needed, a second sock is added. In almost all cases, this type of socket has been carried over to the permanent prosthesis.

Socket Construction

The prosthesis is made in the usual way, utilizing any of the casting methods now being practiced viz.: hand molding, Northwestern ring suspension, or Veterans Administration Prosthetics Center molding plates. Modifications of the positive mold should be in accordance with the directions developed for the individual molding technique, except that the patellar tendon bar must be more prominent, and all reliefs must be feathered into the mold to provide a smooth transition from pressure bearing to pressure relief areas.

Following lamination, all edges are carefully rounded and the entire inside of the socket is made as smooth as possible.

The socket is fitted to an adjustable leg or pylon with a SACH foot attached and is applied to the patient's stump using one heavy cast sock, a three-ply cotton sock, or a five-ply wool stump sock. After the patient has walked for several hours and maximum alignment has been determined, X-rays are obtained in the anteroposterior and lateral projections, with the patient in weight bearing position. The X-ray technician is instructed to include the distal end of the stump on the film.

The X-ray of the leg-sock-socket unit should show even contact between the socket and the stump (Fig. 2 ). If air space is present at the distal end, a hole is drilled in the socket and the space is filled with silicone foam. If circumferential pressures are not even, or if the placement of the patellar tendon bar is not exact, a new socket is fabricated over a more carefully prepared positive mold.

The Sample

To date, 63 below knee amputees, including 11 children, have been fitted with "hard socket" patellar tendon bearing prostheses. Causes of amputation are listed in Table 1 .

The age range of the adult group is from 21 to 84 years. Four patients have bilateral below-knee amputations, including one with bilateral below-elbow loss also; three are bilateral below-knee/above-knee amputees.

The children range in age from nine months to 19 years. One was fitted bilaterally and ten unilaterally.


The present status of these patients is outlined in Table 2 . Two adult patients have remained on temporary prostheses because of limited finances.

The temporary prosthesis was well accepted by all children and the transition to a permanent limb was made without difficulty. Usually the prostheses had become worn out by the time the socket was outgrown.

In the fitting of children, antever-sion of the hip, and rotational problems at the knee, which are commonly associated with terminal hemimelias and complicate prosthetic use, should be corrected, and useless digits should be removed.

For this group, the conventional suprapatellar strap suspension is not always appropriate. In the smaller children a tod-

dler harness with waist belt is furnished. In two teenaged boys, knee joints and thigh lacer were added to the patellar tendon bearing socket to provide additional stability and to prevent loss of the prosthesis during participation in competitive athletics.


In our experience, the problems associated with conventional patellar tendon bearing prostheses are diminished when the socket insert is eliminated and a plastic socket is made directly over the plaster positive mold. Friction, particularly, is diminished; any piston action present occurs between the smooth surface of the socket and the stump sock and skin abrasion is decreased

The major disadvantage of the patellar tendon bearing socket without an insert is its limited adjustability. However, this limitation is compensated for by a more intimate fit. Also some provision for growth can be made by manufacturing the socket over two stump socks.

No contraindication has been found to the use of the "hard socket" in any age group where a patellar tendon bearing prosthesis is indicated. Total contact is necessary, and X-rays should be taken to verify the fit. Air spaces are filled in with silicone foam and the patellar tendon bar must be accurately placed.

Frank W. Clippinger, M.D. is Associate Professor Orthopaedic Surgery and Associate Chief, Duke Orthopaedic Amputee Clinics

Bert R. Titus, C P & O is Assistant Professor of Prosthetics and Orthotics Director, Department of Prosthetic and Orthotic Appliances Duke University Medical Center Durham, North Carolina