Muenster

The UCLA Prosthetics-Orthotics Program conducted a case study project on theMuenster type prosthesis from 1968 to 1969. Results of the two-year proj ectgenerally concur with the results of a study by New York University on theMuenster.* This study states:

1. Amputees reacted positively to the comfort and security of the socket.

2. The decrease in flexion range had no appreciable effect on the prostheticfunctions of unilateral amputees. However, for bilateral subjects, mod-ification of the anterior trim line and the provision of a wrist flexiondevice were necessary for performance of tasks close to the body.

3. Lifting and holding forces available to the amputee were generally superior.

A prescription for a Muenster prosthesis is determined by the patient? s t u m plength.

Minimum Length

Stumps that are categorized as very short, from l-l/2 - 3 inches long, dis-appear into the cubital fold during elbow flexion at angles exceeding 90’. Onthe Muenster prosthesis, the maximum elbow flexion angle is limited accordingly,usually less than 90°. Thus, a limited amount of elbow flexion is needed forthe Muenster prosthesis.

Maximum Length

Below elbow stumps which retain forearm rotation (pronation-supination) to asubstantial degree can benefit most from the below elbow flexible hinge pros-thesis. Active pronation-supination of the terminal device is eliminated onthe Muenster.

Bilateral Amputees

The fitting of a bilateral amputee is complicated by the difficulty he hasdonning two closely fitted prostheses without assistance and the limitationimposed by restricted elbow flexion, particularly on the dominant side. Theseproblems can be resolved by fitting the sockets less snugly to facilitatedonning and lowering the anterior trim line to provide additional elbow flexion.A wrist flexion unit on the dominant side for activities close to the body.Some juvenile bilateral amputees could be successfully fitted with similarmodifications.

* The “Muenster-Type” Fabrication Technique for Below-Elbow Prostheses,Adult Prosthetic Studies, Research Division, College of EngineeringNew York Univeristy, New York, New York, June 1964.

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Measurements

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The stump length measurement from theolecranon, as illustrated, and theolecranon to thumb tip measurement onthe sound arm are for reference duringfabrication of the prosthesis.

Wrap Cast

The wrap cast serves as the check or trial fitting socket. The procedure must befollowed exactly for the desired results. Proper application of the molding pro-cess is also essential for the success of the &rap. -

Materials:

cotton stockinette or casting sock

scissors

elastic webbing-

Yates clamp

bucket of water-

sheet (to drape patient)

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skin marking pencil

elastic plaster bandage

towel

straight edge ruler

Show the patient how the molding process will be done after the wrap cast isapplied. Practice the molding grip on the patient so that the characteristics ofhis stump are familiar.

Place the index and long fingers intoLthe proximal cubital fold and locatethe biceps tendon. (Have the patientcontract his biceps muscle if the ten-don is diff icult to locate.)

With the fingers extended, one oneither side of the tendon, gentlyforce them towards the humerus andthen downward onto the cubital foldand the anterior surface of the stump.Avoid any contact by the fingers onthe distal stump.

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The above procedure is more difficulton a very short stump. If the pros-thetist’s and amputee’s arms are onthe same level, the molding can beapplied more conveniently. Childrenshould therefore sit on a table orstand on a raised platform.

The dorsal aspect of the elbow iswedge shaped. Shape the left hand forright amputations. Shape the righthand for left amputations. The thenarand hypothenar eminences should form achannel into which the elbow will fit.

Position the hand as described above tothe proximal ulna with as little con-tact on the olecranon as possible.This part of the hand application isonly for support without distortion ofthe tissues.

Place the index, middle, and ringfingers on the posterior aspect ofthe humerus proximal to the olecranon.With the finger pads, as illustrated,gently apply pressure against thehumerus down toward the olecranon.

The proper application during themolding process is illustrated. Thereis no pressure between the palm of thehand and the olecranon.

In this manner a relief is automati-cally provided for the olecranon. The .directional forces of the molding gripor the fingers against the posteriorhumerus are balanced by the pressureof the fingers on the cubital fold.

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Mark the proximal line of the wrap.Draw a line beginning at the poster-ior , connecting two points, l-inchabove the condyles of the humerus tothe anterior. Include all of thecubital t issues. Outline the olecran-on and the epicondyles.

Spiral the wraps to cover the distalend.

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Begin the wrap by placing a layer ofplaster across the anterior elbowspanning the cubital fold. Use 6 or 8CM elastic plaster bandage. Continuethe wrap with figure of eight patternsto cover the elbow and stump.

Continue the wrap at least 1/4 inchabove the reference marks. Add atleast four layers of plaster, strongenough to be used as the check socket.

Locate the biceps tendon and smooththe wrap. Mold over the distal endand along the shaft of the ulna. Donot mold over the co ndY les or f l arZFof the humerus, or o ver the prox imaledge of the wrap on the medial orlateral aspect of the arm.

Apply the molding grip. Press thefingers until all loose tissues aredisplaced and firm resistance isattained.

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The Muenster type socket encapsulatesthe elbow. It cups up over the ole-cranon, fits snugly over the cubitalfold and contains a channel for thebiceps tendon.

To take the cast apply a casting sockor stockinette sewn across the end onthe stump. *

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Maintain the molding grip until the plaster sets. Now support the cast and askthe amputee to push slightly against resistance to elbow flexion, extension androtation. (Although the stump cannot rotate the cast, unfortunately it may ro-tate within the cast.) If gross movements of the stump occur within the wrap,it should be rejected.

When the cast is applied to longerstumps, the patient should extend hiselbow approximately 10’ below that ofthe 90’ position.

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Do this when the plaster begins to getfirm but has not set. Remove the fin-gers from the cubital fold and supportthe cast until the plaster thoroughlysets.

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The procedure outlined in the previousstep will give the amputee with alonger stump a greaterrange of elbowmotion. If this procedure is used ona prosthesis for a short stump, somestability will be sacrificed.

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Remove the cast by pulling the stock-inette down over the cast. Thread thetissues away from the proximal brim tobreak suction and work the cast offthe stump.

When this technique is used, the casts are remarkably similar, as shown. Castsfrom three amputees were used in this section.

Preparation of the Check Socket JJsing the Wrap Cast._

Materials:

knife

scissors

fresh plaster

water

Mark the trim lines. The posteriortrim line is at the upper margin of theindents, made from the three fingersproximal to the olecranon. Continuethe line up, proximal to the humeralcondyles at least 1 inch. Mark a crossat the apex of the olecranon bulge asshown. Transfer the marks for epicon-dyles from the inside of the cast.

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Continue the line on the medial sideincluding all of the indent. It wil lform a flare and permit greater flex-ion of the elbow without pinching.

Repeat the process on the lateral-anterior side of the cast.

At the distal end mark an outline onlylarge enough to allow for the passageof a pulling sock. Do not shorten thecast.

Cut the hole on the end.

Cut the cross marked over theolecranon.

Smooth the inside of the cast withplaster siurr>r.

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Muenster Check Socket Fit

Materials:

sharp knife

plaster cutting shears

marking pencil

cotton stockinette (stump pulling sock)

powder

Powder the stump and socket. APPlYthe pulling sock. Insert the end ofthe pulling sock through the distalopening of the socket.

Thread the sock through the distalopening by pulling it gently on oneside and then the other. Note theamount of tissues at the anteriorbrim. A typical illustration isshown.

If excessive tissues gather and thestump does not enter the socket evenwith much effort, trim small amountsof plaster from the area above theolecranon at the posterior area ofthe cast.

Draw a reference line on the cast par-allel to the shaft of the ulna for therange of motion tests.

If the cubital tissues pass freelythrough the proximal opening of thesocket without assistance from thepulling sock, the wrap is too loose.The molding grip did not have enoughpressure. A new wrap should be made.For all Muenster fittings, the cubitaltissues should be threaded into thesocket with a stump pulling sock.This forms a seal at the proximal brim,

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For longer stumps, careful trimmingthe check socket as outlined in thetwo previous steps will surpass thegoals. In the illustration the ini-t ia l flexion angle is 30’.

On this very short stump, 36’ of ini-tial flexion began to adversely affectsocket retention on the stump.

If the anterior brim is the correctheight and is properly flared with agroove or channel for the biceps ten-don, the flexion angle can be in-creased by relief over the olecranon.If the anterior brim is too high, theupper arm will be impinged. In theillustration almost 120’ of elbowflexion was obtained.

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On this short stump only 103’ elbowflexion could be obtained withoutsacrifice of socket retention.

To create enough clearance for theolecranon, expand the cuts by forcingthem from the inside of the socket.This will also help increase the flex-ion angle. If excessive pressureexists , repeat this over theepicondyles.

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Identify the reference line by makinga hole at either end.

Seal the opening over the olecranonarea with plaster bandage. Carefullymaintain the enlargement madepreviously .

Form a cone extension over the distalend for the stump sock channel.

Seal over the cone. The laminationmodel is now ready to be made.

Forearm Extension Preflexion Angle

In the illustration below, the term preflexion or prepositioning is demonstrated.Prepositioning is the medical term used when the hand is positioned in relation tothe upper extremity. In the illustration, the stump to the upper arm approximatesthe 105’ angle which is the goal for the check socket. The forearm extension ispositioned an additional amount of flexion which prepositions the terminal deviceto a range of motion most frequently used by below elbow amputees.

For the Muenster technique the stump motion is limited and the extended position ofthe stump (elbow extension) is also limit.ed. The normal angle of the humerus tothe ulna approximates Woo, in some cases 180’ or’ more (hyperextension) may bereached. .The attitude of the stump in the Muenster socket will approximate only a145’ angle without sacrifice of the retension characteristics of thesocket (illus-

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trated below). The reduced amount of extension from the normal position requiredby the Muenster socket is termed initial flexion.

I n i t i a l Flexion 35’

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The two illustrations below may help the prosthetist or clinic team and ache amputeeto more fully understand the biomechanics of the Muenster prosthesis.

The active elbow joint flexion is 105Q, the forearm extension, 20’ a The patientcan reach his mouth with a slight flexing of his neck.

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-- In the natural hanging position the prosthesis is similar in appearance to thenormal arm.

The prosthetist should now be able to establish the desired preflexion angle inconsultation with the patient. The amputee must accept the appearance of theprosthesis. A common complaint about the preflexed forearm extension and initialflexion angle is that the arm does not hang down the side. Instead, it gives theappearance of being bent at right angles all the time. Another complaint is thebanana like shape. This is generally limited to the cosmetically conscious patientfor whom the prosthesis is constructed with passive hand. This type of patientrequires little preflexion of the forearm extension.

Mold Preparation For Socket Lamination

Apply a parting agent to the inner surface of the check socket and fill the socketwith liquid plaster of Paris. Before the plaster hardens, insert a hollow pipeinto the plaster. Make a recess in the plaster around the pipe at the proximalend of the mold approximately 1 - l-l/2 inches in diameter and l-l/2 inches deep.To facilitate vacuum lamination, drill (approximately 1/4-inch in diameter) in thepipe towards the bottom of the recess. After the mold hardens, puncture the wrapwith an awl marked earlier on the reference line. Be sure to penetrate into the-mold. Project a round head screw or BB from the puncture for future alignmentreference.

Remove the plaster wrap and smooth themold. Do not change the proximal brimline.

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Insert tl-x BE’s or round head screwsinto the reference marks.

Buildups are not ccmmonly made in thistechnique. If, however, the patientdoes have a very boney stump, build upthe boney or sensitive areas previous-ly established during the check socketf i t t ing .ing agent.

Cover the model with a part-

The model is now ready for socketlamination”

Plastic Socket and Forearm Extension

Materials:

dacron f e l t

nylon stockinette (approximately 2 inches wide)

string

PVA

scissors

heat pad (for PVA)

polyester resin, catalyst, promoter and color pigment

sander

cardboard

wrist unit

masking tape

straight edge ruler

goniometer

polyurethane foam

paint brush (cheap)

Standard procedure is recommended for socket lamination. The use of vacuum willmake a light but strong socket (see page 1 .

When the lamination has cured, roughenthe socket so that it will bond withthe final lamination. Scribe or marka reference line from the projectionson the lateral wall of the socket. Ifpossible, this line should correspondto the long shaft of the ulna.

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To determine the amount of preflexionof the forearm extension> use a pro-tractor. Draw a line from the refer-ence just established toward theanterior side of the model.

In the illustration at right, the lineis drawn at a 20’ angle.

--The forearm extension should projectdistally from the 20’ line. On shortstumps, as much as 35’ has been used.--

Forearm Extension Alignment

To establish the length of the below elbow prosthesis, a filler between the end ofthe stump and the wrist unit must be provided. This filler is the holding recep-tacle for the wrist unit. If the amputation is unilateral, the overall length ofthe prosthesis including the terminal device is equal to the length of the elbowto thumb tip on the sound arm. This is a standard procedure; variations do occur,however, for congenital anomalies or special occupational requirements. If theamputations are bilateral, consult the anthropometrical chart on page .

Materials used to form the extension vary according to stump length, and the natureof the prosthesis, exoskeletal or endoskeletal. Place the socket in a holdingdevice. In the pictures below, the Milmo alignment jig is used. However, anysuitable holding fixture may be used.

Adjust the jig to theMeasure from the flatto the mark (olecranonthe joint spacer, whitbeing used). At the sthe jig to the preflexence l i n e , as i l lustra

Using the reference drline as a guide, alignfollows: Viewed fromaspect. To bring theinward towards the bodTilt the socket so thatoward the body at theA right arm prosthesisChange directions forthesis. Caution : Retwith a protractor. Threct. An error at thiruin the prosthesis.ing device so that it

awn on the socketthe socket as

the posteriorterminal devicey (ulnar flexion).t it leansproximal end.is i l lustrated.

a left arm pros--heck both angles.ey must be cor-s juncture wouldSecure the hold-will not move.

correct length.level surface up

epicondyle orhever method isame time alignion angle refer-ted.

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Using the prescribed wrist unit, maskthe threaded hole and seal the vitalparts with clay or playdough. Coverthe knurled portion with masking tapeto keep it clean for subsequent lamina-tion. Make a cone for the forearm ex-tension, (see Chapter IV, Materials).Attach the cone securely to the wristunit. Temporarily place the wrist unitand cone into position. Adjust themfor size and length desired in thebuildup.

Center the wrist unit on the preflexionangle reference line on the lateral and

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posterior aspect of the socket. If an-

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oval wrist unit is used, rotate it in-ternally about 20’ until it appears tohang at a natural angle and can be usedon flat surfaces with a terminal device.Fasten the wrist unit and cone; theyshould not move. Clay is satisfactoryfor fastening if additional support isprovided during the foaming action. Amore elaborate holding device than clayis i l lustrated.

prepare the foaming mixture (see page9 Chapter IV, Materials). Pour the

mixture into the cone. Safety precau-tions should be taken for polyurethane.

The build-up should contain amplematerial for shaping.

Shape and contour the foam to match thesound side as much as the wrist unitwill allow. Remove all foam at theproximal trim line of the socket.Paint all of the model except the wristunit with mixed polyester resin. Thisseals the foam and eliminates air bub-bles during lamination. Sand the modeluntil it is smooth; the final lamina-tion will then bond.

Mask off the end of the wrist unit.

Final Lamination: Make a* lay-up of twolayers of nylon stockinette over the.mode 1; double back one piece and tie itat the groove in the wrist unit. Gor aheaIry duty prosthesis increase .:he jay-ers of stockinette. Make a PVA bzg.Mix the resin, and laminate. [See page

9 Chapter IV, Materials .)