Supracondylar Wedge Suspension of The P.T.B. Prosthesis
Carlton Fillauer, C.P.O.
Background
The relatively brief history of the P.T.B. Prosthesis has been one of dramatic success and perhaps as no other U.S. development, influenced prosthetic practice throughout the world.
There is no single feature of the P.T.B. that has made this possible. So much information was introduced by this new technique that only a detailed study of it would reveal the full scope of its innovations. Then one can appreciate that this was the product of a comprehensive biomechanical study  of all aspects of the BK amputee and the prosthesis and not just a novel socket design.
During recent years a variety of modifications have appeared here and in Europe. Some disappeared rapidly while others and persist for they have merit in specific applications. It is noteworthy and a credit to the investigators at U.C.B.L. that the basic principles not only stand today but they have been reinforced.
If there has been a persistent need for a change it lies in the method of suspension. The original cuff suspension strap as described provides adequate retention for a good percentage of patients. Properly fitted the "holding on" effect is obtained over the patella and not circumferencially around the distal thigh. Some difficulties have developed which relate to restricted circulation, piston action and to lateral instability with short stumps. In these areas the P.T.S. technique offered substantial improvement over the cuff suspension.
Last fall while visiting the Orthopedic University Clinic in Muenster, I had the opportunity to become acquainted with a new approach. This involved the use of a rigid wedge inserted inside the socket over the medial femoral condyle, after the prosthesis is donned. (Fig. 1 ) It was not considered an experimental item since they were employing it routinely on all BK amputees including their immediate post operative plaster sockets. My interest was so aroused that I remained in the area another day for a closer look.
Of particular interest was their rather complicated procedure for obtaining a plaster wrap under pressure. It involved a three step operation of partially covering the stump with a few layers of plaster bandage then applying a negative pressure bag. First the area of the knee was cast, followed by the proximal half of the stump and then finally the remaining distal segment.
Prior to the application of the plaster bandages the height of the medial flair of the socket was measured and marked off on the stump (Fig. 2 ) so that it would transfer to the plaster wrap. The determinant factor was the top of the knee when flexed to 90°. The location of the wedge would be determined by the marks and the impression of the condyle in the plaster model.
Immediately upon my return to Chattanooga we proceeded to fit our next BK with a supracondylar wedge for suspension. You can be sure that this was preceeded by a considerable amount of planning, more mental than physical. Good success with this first patient spurred us on to more fittings.
Our initial experience was with a tapered wedge similar to that seen at Muenster, first of neoprene crepe then of hard felt. We tried several measuring techniques to give us accurate wedge location, as this is very critical. Soon after completing a few fittings we began casting the stump with a flexible wedge held in its proper location by masking tape. This we feel has proven best since it was more reliable and it provided, by the pressure of the elastic plaster bandage on the wedge, adequate depth of the impression into the thigh.
The Wedge, now made of plastisol, does not present a sizing problem. Three sizes* have been standardized with the medium size in two thicknesses (Fig. 3 ). We have found that most adults can be fitted with the medium size (Fig. 4 ). It covers the medial area of the knee from just posterior to the patella to and including the posterior medial face of the femoral condyle (Fig. 5 ). Its centerline cross section is comma shaped and as fitted the thick lip area is placed just proximal to the medial condyle (Fig. 6 ). When held in place by a rigid socket wall it effectively prevents distal displacement of the prosthesis.
The proximal brim of the medial wall has a medially projecting lip which retains the wedge in position (Fig. 7 ). A loop of 1/2" dacron webbing, extending from the proximal margin of the wedge and impregnated into the plastisol, provides a "handle" which facilitates removal from the socket.
When used as described below it will be seen how its shape is individualized. The plastisol selected is of approximately 40°-50° shore durometer. This does not permit much compressibility yet it is friendly to the stump and provides a feel of security to the patient. At the same time its flexibility is quite adequate to permit if to adapt in the plaster wrap to the slight variations in knee shapes.
Procedure
These steps cover only those that we consider departures from the routine in the standard P.T.B. sequence of measuring, stump marking, and hand modeling of the plaster wrap.
(1) If the wedge and the tape is located over the first layer of tube gauze** it will not interfere later with removal of the plaster wrap (Fig. 8 & Fig. 9 ). Care must be taken that the lip of the wedge is actually above the condyle (as in Fig. 6 & Fig. 10 ) and of sufficient width to encompass the A.P. dimension of the condyle (as in Fig. 5 ). Adults with a large bony structure will require use of the large wedge.
(2) Since the wedge remains a part of the wrap until it is filled with plaster, the wrap cannot be removed from the stump unless it is cut open to the level of the tibial tubercle.
With the elastic webbing inserted as shown (Fig. 11 ) running from laterally above the patella down medially to the tibial tubercle a short diagonal cut (Fig. 12 ) over the strap will permit opening of the proximal aspect of the wrap sufficiently to allow removal. If the wrap can be removed without cutting, something is wrong and a new wrap and recheck on wedge size and thickness is in order.
(3) Wrap stump with 2 to 4 elastic plaster bandages (8-10 cm width) beginning proximally above the wedge (a small dab of plaster mix rubbed in above and below the wedge helps lock the wedge to the wrap). In the area of the condyles it is important that the bandage be wrapped tightly to pull the wedge firmly against the condyle (Fig. 13 ) and in this area it should be 3-4 layers heavier than usual. Continue wrapping distally in the standard procedure.
(4) When the model is stripped of the wrap the outline of the wedge should be clearly defined (Fig. 14 & Fig. 15 ). No modification in this area is necessary except to remove plaster from above the wedge area so that a medial lip will form over the proximal edge of the wedge during the plastic lamination (Fig. 16 ). Preparation of the model is routine below the level of the wedge. The lateral proximal extension area should be brought medial by removal of l/4 " to 1/2" plaster to assure close contact and counter support to the wedge. The proximal edge of the lateral wall should be at approximately the same height as the medial side and be flaired away from the thigh (Fig. 17 ) so as not to leave a sharp edge which would increase the difficulty in donning. The usual preparations for lamination should be followed though the use of double vacuum is superior in holding the lamination close to the wedge and around its margins. Extra thickness of the laminate in the area above the mid patella tendon is desirable to increase the rigidity of the extensions. For the average size (13" circumference at M.T.P.) the following lay up is recommended: 2 dacron, 3-4 nylon stockinettes overall with extra dacron filler around the M.T.P. and popliteal area and an additional 2-3 layers of nylon above M.T.P. level. The usual 80-20 rigid to flexible resin mixture is recommended. When the resin has gelled sufficiently to permit cutting the laminate without fraying, the area above the trim line should be cut away. Lift the medial extension with screw driver and remove wedge. The socket can then be pulled off the model.
Fitting Suggestions
The proximal opening (M-L) should be just wide enough to permit entrance of stump (Fig. 18 ). Only a narrow 1/8-3/16 lip is required to retain the wedge in the socket. After the stump is well seated in the socket check for correct height of wedge, if too low (distal) pressure on condyle may be excessive, constant and soon painful. When atrophy makes the stump loose in the proximal area, a thicker wedge may be needed.
Advantages
Once familarity with this technique has been acquired, few extra steps and little extra time, or expense is consumed.
- Shorter stumps can be fitted than with the conventional cuff suspension.
- Short stumps are much more secure with regard to medial-lateral stability.
- The wedge can be used on practically all fittings without a soft liner, though a soft liner may be used.
- Suspension is usually excellent, better than with any strap combination.
- There is less constriction in knee flexion, especially in comparison when the cuff circumferential strap is worn tightly.
- The stump sock does not tend to wrinkle around patella as with the suspension cuff.
- Donning prosthesis is accomplished easily without pull on socks, etc.
- Unlike the P.T.S., suspension is as good in flexion as in extension. For those prosthetists who are routinely fitting P.T.B.s with success and want to change over to the wedge method of suspension they do not have to learn a new technique of socket making. The standard U.C.B.L. methods have proved adequate with minor changes. One need only adopt a few steps to incorporate the higher medial and lateral flanges and casting in the condylar wedge. Many of the above "claimed" advantages are duplications of the claims for the P.T.S. From our experience and in the experience of the group in Muenster, the crux of the matter is that the wedge accomplishes these in a less complicated technique, more effectively.
Summary
We have learned that much can be benefited from the higher medial and lateral extensions not only in terms of suspension but in medial-lateral stability so important for the short stump.
Cosmesis cannot be overlooked and this innovation does not create a new problem (Fig. 19 & Fig. 20 ). Nor has fitting been appreciably complicated, except that this system does require intimate fitting of the medial-lateral aspects of the socket. New and serious attention must be directed to the areas above the M.T.P. level as essential parts of the socket. Use of the supracondylar wedge will provide a new tool to the prosthetist. It should enable him to improve suspension to the majority of BK amputees and as a result further established the P.T.B. as the standard in modern prosthetics. This new system might be known by the designation S.T.P., Supracondylar Tibial Prosthesis.
* The four wedges are commercially available from Fillauer Surgical Supplies.
** Tube gauze is superior to a cast sock because it impregnates well with the plaster bandage. Size #78 (3 5/8" width) is suitable for most below knee stumps but for smaller circumferences we recommend size #5 (2" width).
References:
- University of California, San Francisco, Biomechanics Laboratory, "Manual of Below-Knee Prosthetics," November 1959.
- Air Cushion Socket, U.C.B.L.
- The P.T.S. Prosthesis (Complete enclosure of patella and femoral condyles in below knee fittings), Kurt Marshall, C.P. and Robert Nitschke, C.P., June 1966.
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