The Slip-Socket Prosthesis For Juveniles: A New Clinical Application

Dietrich Bochmann, C.P.O.(C) Hugh G. Thomson, M.D., F.R.C.S.(C)


The slip-socket below-knee prosthesis for the patient with a short below-knee stump has been used for many years 1,2,3. However, its specific application to the below-knee patient with a skin-grafted stump has not been widely recognized. Slocum and others 4,5 have recorded the impracticability of skin grafts in stump areas which are to bear weight in the prosthesis. It has been the accepted practice to excise these skin grafts secondarily, apply skin traction, and close the stump with normal skin, prior to the fabrication of a prosthesis.

The purpose of this paper is to describe the use of a slip-socket prosthesis for children during the maturation phase of the skin-grafted stump. In this series, the patients' stumps had been skin grafted primarily in an attempt to preserve normal knee joint function rather than converting the patient to a knee-disarticulation or above-knee amputee.

Usually, the immediate skin-graft area on a stump is fragile and susceptible to breakdown if subjected to friction. The same susceptibility exists with the skin pedicle flap but to a lesser degree. This tendency toward breakdown gradually diminishes as the vascular supply becomes well established and the skin graft becomes soft, mobile, and mature. The process of maturation requires a period of six to eight months in such a dependent area. It is during this period that extreme care must be exercised with prosthetic fabrication in order to protect the compromised stump. The slip-socket principle to be described has been employed in three patients with this type of stump closure.

Method

The condition of the early skin graft on these types of below-knee stumps (Fig. 1A and Fig. 1B ) prohibits the use of a stationary, weight-bearing below-knee socket. Nevertheless, it is necessary that the stump be used, with good knee movement, as a lever to activate the shank of the prosthesis. Therefore, the prosthesis must have a free-floating below-knee socket, and require that only a minimal amount of weight be supported by the stump. With this free-floating socket the friction associated with stationary sockets can be eliminated almost entirely. The slip socket must be connected to the lower shank in such a way that it will rotate, and piston up and down, as well as activate the shank of the prosthesis.

This type of prosthesis can be fabricated either as an early postoperative unit that can be adjusted to accommodate stump shrinkage (Fig. 2 A/B ), or as a permanent unit which requires no major modifications (Fig. 2 A/B ).

Specific Fabrication Procedures

Early Postoperative Unit

Aside from differences in materials, the design and components of the early postoperative and permanent versions of the slip-socket prosthesis are essentially the same. Fig. 3 diagrams the configuration of components in the permanent prosthesis.

Detailed procedures for the fabrication of the temporary prosthesis are presented below, followed by a consideration of factors involved in the preparation of the permanent limb.

An adjustable quadrilateral socket, modeled after the units developed by Jim Foort in Winnipeg, is selected for the patient. This socket is trimmed at its distal end to provide clearances of two inches proximal to the superior edge of the patella, one inch proximal to the knee center both on the medial and lateral aspects; and three inches posteriorly, to allow for clearance of the hamstrings on knee flexion.

To initiate fabrication of the slip socket, a six-ply wool sock is applied to the stump. The sock must be pulled smoothly onto the stump and held firmly in place by an assistant. Orthoflex® elastic plaster bandage is applied to the stump. This material will provide a socket with a smooth and accurate inner surface. No pressure must be applied in the region of the patellar tendon or the popliteal fossa. Four layers of elastic plaster bandage are sufficient to provide the foundation for the receptacle. The socket is trimmed on its proximal edge to clear the patella and at the knee center level medially, laterally, and posteriorly. Knee center and ischial tuberosity heights are measured at this stage.

A SACH foot with the appropriate shoe is bolted to a wood ankle block trimmed to match the foot base. The height of the ankle block is governed by the length of the stump and the length of the guide tube (see below). The ankle block must be short enough to provide sufficient clearance for the distal end of the tube during stance phase (Fig. 4 A ).

Side joints, hinged at the knee, are bolted medially and laterally to the ankle block in a position related to the measurements obtained earlier. The position of the knee joints must allow enough medial and lateral clearance to permit passage of the slip socket during knee flexion and extension. The knee hinges must be aligned so that they will bend in the same plane. The thigh portions of the joints are shaped to match the contour of the quadrilateral socket.

The quadrilateral socket is set into place, with its ischial shelf at the level determined by the earlier measurements. The knee hinges are aligned posterior to the TKA (Trochanter-Knee-Ankle) line. The isolated prosthesis must be stable on heel contact without benefit of the extending force of the stump. After this stage of the fabrication has been completed, the patient must support his full weight on the quadrilateral socket with the below-

knee stump hanging free in the frame of the lower shank. The plaster slip socket is then applied to the stump. The socket is held firmly against the stump, the wool sock is pulled downwards and trimmed one-half inch below the outside rim. The sock is fastened with adhesive tape, thus retaining its position for subsequent plaster coverage.

A guide tube is split and spread at its upper end to permit adequate fixation to the plaster socket. This fixation is obtained by use of a standard plaster bandage to hold the guide tube in a perpendicular plane. A one-quarter inch thick nylon washer, with its hole chamfered at a 45-degree angle on the upper side, is mounted on a metal strap and serves as a sleeve through which the guide tube slides. The metal strap is riveted to the medial and lateral side hinges. Sufficient space is provided between the distal end of the socket and the sleeve so that the socket does not "bottom" on the washer or strap during stance phase; while the tube must be long enough so that it does not pull completely through the sleeve during swing phase (Fig. 4 B ). A split pin, inserted in the distal end of the tube, prevents the tube from slipping out of the guide when the prosthesis is not in use. Cross-over elastic straps are riveted to the top section of the slip socket and the mid-thigh section of the quadrilateral socket to hold the socket snugly on the stump. The elastic straps are tightened as the stump skin graft matures and an increase in stump pressure can be tolerated.

Permanent Prosthesis

Similar steps should be followed if a permanent prosthesis is prepared, except that a standard quadrilateral socket will be used in lieu of the adjustable socket and a plastic-laminate socket with an inner liner of Kemblo in place of the plaster slip socket. The plastic slip socket remains in place and the shank section around the slip socket is built up with wax prior to the lamination of the lower shank. The wax is moulded to give the desired cosmetic shape to the shank and a standard lamination is then completed. The strong shank frame can support the patient's weight and, therefore, the cosmetic shell is of light construction. The wax is melted out after lamination of the shank, the slip socket is removed, and the wax cleaned off. The socket is reinstalled and the prosthesis is ready for the patient's use. The elastic straps, which cross proximal to the patella on the early postoperative prosthesis, would be placed along the medial and lateral aspects of the permanent prosthesis. This placement will improve the total cosmetic appearance of the finished appliance.

Results

Each patient initially fitted with this type of prosthesis experienced an eventual skin graft maturation. This technique permitted early prosthetic ambulation and training, and reduced morbidity because no revisionary surgery was necessary.

Bibliography

  1. American Academy of Orthopaedic Surgeons, Orthopaedic Appliances Atlas: Artif. Limbs, Ann Arbor: J.W. Edwards, 1960, Vol. 2,p. 219.

  2. Klopsteg, P.E., Wilson, P.D. et al: Human Limbs and Their Substitutes, New York: McGraw Hill, 1954, p. 754.

  3. McLaurin, C.A.: "Prosthetic Research and Training Unit," Inter-Clin. Information Bull., 6:13-22, Nov. 1966.

  4. Slocum, D.B.: An Atlas of Amputations , St. Louis: C.V. Mosby Co., 1949, p. 32.

  5. Speed, J.S., Knight, R.A. (eds.): Campbell's Operative Orthopaedics, ed. 3, St. Louis; C.V. Mosby Co., 1956, p. 1024.

® Johnson & Johnson.

Dietrich Bochmann, C.P.O.(C) and Hugh G. Thomson, M.D., F.R.C.S.(C) are associated with the Ontario Crippled Children's Prosthetic Clinic Toronto, Ontario, Canada