Technical Note: RMB Reinforcement

Robert O. Gooch, CP. *

Description

https://www.oandplibrary.org/cpo/pdf/1986_03_119.pdf
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Title:

Technical Note: RMB Reinforcement

Creator:

Robert O. Gooch, CP. *

Page Number(s):

119 - 121

Volume:

3

Issue:

3

Text:

View as PDF

with original layout

Technical Note: RMB Reinforcement

Robert O. Gooch, CP. *

Because of the humid climate, the Department of Prosthetics and Orthotics at Duke University Medical Center receives many prescriptions for hard socket below-knee prostheses. The great majority are supracondylar wedge suspension, utilizing the Removable Medial Brim (RMB) concept. For the past several years, we have designed and fitted approximately 150 such prostheses annually.

Based on this experience, we have developed a method to reinforce the RMB structure and prevent gradual loss of alignment under the constant pressure of the femoral condyles. We now use this technique routinely, and find it greatly enhances the stability of the removable brim.

Method

Fabricate the socket in the conventional manner, following the instructions supplied by the hardware manufacturer. Rather than packing the mechanism with clay, we prefer to substitute Johnson's Stik-Wax, which is easier to work with and lubricates the assembly, allowing easier removal. Once the lamination is fully cured, break out the positive model.

At this point, the medial brim is cut away from the socket. Although a variety of tools can be used for this operation, we prefer a simple modification of an ordinary hacksaw blade.

Grind the fine-tooth hacksaw blade into the contour shown in (Fig. 1). This is preferable to a commercial sabre saw blade, because its wide, thin shape creates a smoother, less irregular cut.


Figure 1. Fine-toothed hacksaw blade, modified to fit sabre saw.


Using the sabre saw, cut the anterior and posterior portion of the brim free, being careful not to nick the metal upright. Cut the area adjacent to and over the metal upright with a cast saw or sharp knife. Carefully pry the medial brim free with a thin-bladed screwdriver.

Grind the distal end of the upright an amount equal to the saw kerf, to insure the wedge will seat fully (Fig. 2). Place the brim back onto the socket to be certain it fits properly, with minimal gapping along the cut edge.


Figure 2. Grind distal upright to insure the wedge fits without gapping.


Reinforcement

Remove the brim and apply PVC tape to the lateral surface and distal trimline. This serves as a parting agent, and prevents the resin used in subsequent steps from bonding the wedge back onto the socket.

Roughen the socket immediately beneath the cut-line, to insure good adhesion for the reinforcement lip (Fig. 3). Lubricate the cut edge with petroleum jelly and reapply the wedge carefully to avoid gapping.


Figure 3. Tape wedge and roughen socket prior to lamination of lip.


Cut three 1 1/2" wide strips of Xynole-polyester fabric long enough to cover the saw cut. This material saturates readily when used with polyester resin and forms a thin, strong, and rigid reinforcement.

Promote a small amount of pigmented polyester 4110 (rigid) resin. Paint the roughened area of the socket with resin, and apply one layer of Xynole reinforcement extending at least 1/2" onto the wedge (Fig. 4). Brush additional resin onto the Xynole until it is fully saturated, and apply the second layer. Fully saturate this layer and apply the final layer. Saturate this in a similar manner.


Figure 4. Saturate Xynole layers individually with the polyester resin.


When the resin has gelled, but not fully set, remove the wedge. This insures that the wedge will insert smoothly, without binding, in the finished prosthesis.

Once fully cured, trim the reinforcement to form a 3/16" lip (Fig. 5). Using a felt arbor, bevel the inside edge of the lip and the outside edge of the wedge (Fig. 6). This unobtrusive lip will significantly reinforce the wedge, particularly against malrotation.


Figure 5. Trim lip to 3/16" above socket edge.



Figure 6. Posterior view of lip with wedge in place. Note bevel on inner edge of lip and outer edge of wedge.


Finishing

Once dynamic alignment and transferring are completed, the prosthesis is ready for the finish lamination. We typically set the wedge aside and relaminate the prosthesis without the proximal brim in place.

An old RMB upright can be inserted into the channel and clamped in a vise. This prevents resin from filling the channel and provides a mandrel to secure the prosthesis during the lamination procedure. Lubricate the upright with Stik-Wax to fully seal the channel.

Summary

Fabrication of a Xynole reinforcing lip significantly improves the stability of the supracondylar wedge when using the Removable Medial Brim procedure. Based on the Duke experience with hundreds of RMB prostheses, we recommend this be done routinely.

References:

  1. Durr-Fillauer Medical, Inc. 
    P.O. Box 5189 
    Chattanooga, TN 37406 
    RMB Hardware Kit 
    Catalog #127019 (Heavy Duty) 
    Catalog #127001 (Standard Duty)
  2. S.C. Johnson & Sons, Inc. 
    Racine, WI 53403 
    #140 Stik-Wax-15 oz. container
  3. Otto Bock Industries 
    4130 Highway 55 
    Minneapolis, MN 55422 
    Coroplast PVC tape 
    Catalog #616F8
  4. Durr-Fillauer Medical, Inc. 
    P.O. Box 5189 
    Chattanooga, TN 37406 
    Xynole-Polyester cloth 
    Catalog #211094

Reference
2. S.C. Johnson &Sons, Inc. Racine, WI 53403 #140 Stik-Wax-15 oz. container
Reference
4. Durr-Fillauer Medical, Inc. P.O. Box 5189 Chattanooga, TN 37406 Xynole-Polyester cloth Catalog #211094
Reference
3. Otto Bock Industries 4130 Highway 55 Minneapolis, MN 55422 Coroplast PVC tape Catalog #616F8
Reference
2. S.C. Johnson &Sons, Inc. Racine, WI 53403 #140 Stik-Wax-15 oz. container
Reference
1. Durr-Fillauer Medical, Inc. P.O. Box 5189 Chattanooga, TN 37406 RMB Hardware Kit Catalog #127019 (Heavy Duty) Catalog #127001 (Standard Duty)
Robert O. Gooch, CP.
Robert O. Gooch, CP., is with the Department of Prosthetics and Orthotics at the Duke University Medical Center.

Full Text PDF:

https://www.oandplibrary.org/cpo/pdf/1986_03_119.pdf

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Figure 2:

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Figure 3:

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Figure 4:

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Figure 5:

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Citation

Robert O. Gooch, CP. * , “Technical Note: RMB Reinforcement,” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 1, 2024, https://library.drfop.org/items/show/179381.