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Technical Note: A Casting and Modification Method for the M.L. Socket

Vernon Rothschild, C.P.

This article presents a casting method and cast modification technique for the fabrica- tion of a Narrow M.L. socket. My colleagues and I were introduced to the concept at Duke University¹ and at Northwestern University² by Ivan Long. Like manv of our colleagues, we were simply astounded that the M.L. socket, after almost 20 years of fabri cating above-knee limbs as an alternative to the quadrilateral socket, suddenly appears. Developers made bold and lofty claims about its superiority, and attestations of the patients fitted at the demonstrations supported these claims. Ambitious efforts were made to implement the new concepts. but unfortunately the results did not duplicate those of the instructors. We read and reread articles on the socket; attended seminars ad-dressing the NSNA,³ Cat-Cam.⁴ or Narrow M.L. socket, and we produced check sockets at a maddening pace. Despite these efforts, we did not approximate those results previously witnessed. The decision was made at this point to approach the problem with the same skills we have always used in fabricating other types of sockets.

For the past 10 years we have measured our above-knee amputees for quadrilateral sockets with a hand casting method. This produces a socket configuration that is essentially more square than it is quadrilateral. Such sockets are comparable, if not superior in comfort and function, to those fabricated with any of the commercially available cast- ing brims. Presently, we are using a technique that consistentlv and without extraordinary effort provides a Narrow M.L. socket that is more comfortable, functional. and "normal." Our results. However, are not as dramatic as we had hoped. It is felt that our Narrow M.L. results are directly proportion-ate to our quadrilateral results. In other words, if one can fabricate a satisfactory quadrilateral socket then it should be possible to fabricate a Narrow M.L. design socket which is better in manv ways. but not dramatically so.

Taking the Cast: Materials Needed

The casting method requires:

1. Two practitioners.
2. Four to six 4' non-elastic plaster of Paris bandages. (Elastic bandages are not used when converting quad socket wearers. Reduction values normally used produce a socket which is too large.)
3. Panty hose for both male and female. The stocking leg is cut 2' distal to the residual limb and not sewn, but wrapped into the cast.

The cast is taken with the patient standing. The prosthetist taking the cast is positioned in front of the patient, while the assistant is behind the patient ensuring that the wrap is high enough to capture the ischial tuberosity. Once the wrap is applied, the practition ers change positions. The assistant's sole responsibility is to place his hands on the medial and lateral aspects of the residual limb. His medial hand is at a level approximatelv 1-1/2" distal to the perineum; his lateral hand is at the same level, and both hands are parallel to the line of progression. Considerable pressure must be exerted to create the Narrow M.L., as well as the flaring of the medial wall (Figure 1) . When the cast is removed this measurement should be within 1/2" of Long's chart,⁵ which we have found to be consistently accurate.

The senior practitioner, who is behind the patient, cradles the posterior and medial aspects of the ischial tuberosity with his left first and second finger DIP joints (assuming the patient is a right above-knee amputee). His hand parallels the floor, and his thumb reaches around and exerts considerable pressure in the "wallet hollow" - the space posterior to the greater trochanter. His right hand thumb grasps the trochanter which also accentuates the "wallet hollow" area (Figure 2) . These depressions will be used to reduce the cast to an exact contour of the whole proximal brim. Utilizing the medial and lateral depressions, as well as the proximal brim depressions, facilitates the modification of the positive cast (Figure 3) .

The reduction values will vary according to the individual prosthetist and the many variables involved, i.e., elastic versus non-elastic plaster bandages; suction versus partial suction, etc.

Two methods are used for cast modification depending on the cast: (1) modify the positive, and (2) modify the negative. Modifying the positive is done in the conventional manner. Modifying the negative is accomplished by placing plaster of Paris inside the negative and molding it to shape while using the depressions as a guide. While the technique of modifying the negative was used frequently when fabricating quadrilateral sockets, it is rare to do it with a Narrow M.L. socket. The method is used when inspecting the contours of the finished socket before pouring the model. The casting method identifies the anatomical points necessary for fabricating a Narrow M.L. socket.

The casts of a converted quadrilateral socket wearer are different from the cast of a new patient. They are usually a more boxlike compromise of both types. However, patients still preferred the newer type socket to their previously worn quadrilateral sockets.

Long's Line⁵ is used for bench alignment and has been found to be reliable for 95% of our patients. The "bony lock,"⁶ or locking effect, occurs when the musculature is tensed in the area of the tuberosity and along the entire medial wall. When the patients are fully weight-bearing, the socket cannot be pulled laterally and the patient is comfortable in the groin area. Many of the sockets have two angles to the medial wall. The first angle, measured distally at the wall's narrowest point, should be parallel to the line of progression, or may be externally rotated up to approximately 7°. The second angle measured at the most proximal portion of the brim follows the contour of the leg at the groin (Figure 4) .

Vernon R. Rothschild, C.P., is the President of Rothschild's Orthopedic Appliances. Inc.. 7532 Partson Drive. Forrestville, Maryland 20745 (301) 736-9350. Certified since 1972. Rothschild received his training in prosthetics at V.A.P.C.

References:

1. Long. Ivan, "Norrnal Shape-Normal Alignment (NS-NA)," Duke University, October, 1985.
2. Long, Ivan, "Normal Shape-Normal Align-ment (NS-NA)," Duke University. November. 1985.
3. Long. Ivan, "Normal Shape-Normal Alignment (NS-NA).' Clinical Prosthetics and Orthotics, 9(4), Fall, 1985.
4. Sabolich, John, "Clinical Adducted Trochanteric-Controlled Alignment Method," Clinical Prosthestics and Orthotics 9(4). Fall. 1985.
5. Long. Ivan, "Normal Shape-Normal Alignment (NS-NA),' Clinical Prosthetics and Orthotics 9(4), Fall 1985.
6. Sabolich, John, "Contoured Adducted Trochanteric-Controlled Alignment Method." Clinical Prosthetics and Orthotics 9(4), Fall. 1955.