7 Socket Impression Techniques

Cast Impression Process

Notice that capturing the impression is step #10 of 11.

1. Prepare and gather materials

• Measurement sheet
• ML gauge and tape measure
• Towels
• Cast sock or nylon (double of the nylon to ease doffing)
• Yates clamps and elastic webbing
• Indelible pencil
• Cast scissors and plaster scissors
• Container for water
• Plaster bandage
• Gown

2. Evaluate the condition of the residual limb Shape (conical, bulbous, cylindrical, elliptical, edematous) Soft tissue coverage (bony, redundant tissue, atrophic, firm muscle tone) Muscle bellies (firmness) Skin (dermatitis, folliculitis, scars, open areas) and mobility of skin especially the cubital fold Bony edges/protuberances – especially the epicondyles and head of the radius Ligaments, tendons – mark location of the biceps tendon Temperature (warm, cool, moist dry, cyanotic) Neuroma Scar: location ________________ Dermatological issues Sensation Light Touch: intact, impaired or absent Position Sense: intact, impaired or absent Palpation and tolerance to force couples Evaluate the movement of the epicondyles through flexion and extension
3. Take Measurements with elbow at 90’ Length and circumferences ML at epicondyles Proximal epicondyle ML (PML) Circumference through the cubital fold and across the apex of the olecranon. Width of the biceps muscle belly in the cubital fold (3cm)
4. Prepare rigid plaster splint Make the splint using six (6) layers of 5” rigid plaster Splint length = circumference around apex of olecranon and cubital fold with elbow at 90 + 8-10cm (3-4”) Fold the splint in half and draw the trimlines on the splint. Height of socket brim: ML trimlines must encompass the epicondyle and flare into the soft tissue. In general: Children: 2-2.5cm (3/4” to 1”) Average adults: 3-4.5cm (1 1/4” to 1 3/4”) Large adults: 5cm (1 3/4” to 2”) Anterior trimline = cubital fold: about 1.5cm (when splint is folded) Posterior trimline should encompass the epicondyles and the olecranon Cut out the bandage over the olecranon and cubital fold. Open the splint to full length and wrap it around the limb to ensure you are capturing the epicondyles and olecranon. The splint needs to cover the PML and the supracondylar fossa anatomy.

5. Prepare the residual limb for casting

• Moisten the casting sock and wring out excess water. Apply the cast sock snugly and use the elastic strap in figure 8 fashion to hold it wrinkle free while the elbow is at 90 degrees.
  • Avoid bridging in the cubital fossa
• Use Vaseline on residual limb if hairy.

6. Practice hand position

There are multiple options for hand placement depending on the size of your hands, the size of the patient’s limb and the socket design. The next few slides describe three options to position your hands when casting for a transradial self-suspension prosthesis.

Regardless of the hand position you choose, practice it multiple times before laying the plaster until you feel comfortable and confident that you can reliably capture the anatomy. Assess pressure tolerant areas and potential for suspension.

• • You want pressure in the supracondylar fossae, on either side of the biceps tendon and on the triceps bar.
    • A-P Compression between the ante-cubital fold and the triceps bar
    • M-L Compression at supracondylar fossae: anterior and proximal to the humeral epicondyles
  • You do NOT want pressure over the bony areas

Hand position 1

Place your index and middle fingers on each side of the biceps tendon. THE FINGER TIPS MUST BE ON THE EDGE OF THE PLASTER WITH THE PADS OF THE FINGERS ON THE CAST SOCK Place your other hand around the back (or front) of the elbow to maintain snug medio-lateral stabilizers. Hold the stabilizers and “U-shape” of the mold to prevent their spreading. Place your other thumb and middle finger in the supracondylar fossae (anterior and proximal to the humeral epicondyles) Practice hand position with patient positioned appropriately Patient position: 1) Elbow at 90 degrees, Neutral pronation/supination and 2) After hand positioned, have patient flex, then extend Hand position: 1) Index and middle fingers on each side of biceps tendon and 2) Other hand around the back of the elbow to maintain snug mediolateral stabilizers

Hand Position 2

Place the hand in front to grasp the supracondylar fossae and the biceps tendon, alternating positions to capture a balanced A-P and M-L grasp. Place the other hand around the back of the elbow to maintain snug anterior-posterior grasp. Note For shorter residual limbs, the Anterior force is directed toward the olecranon to capture more residual limb For longer residual limbs, the anterior force is directed toward the radius to allow more ROM

Hand position 3
Modified Muenster Thumbs on either side of biceps tendon Index fingers in supracondylar fossae Middle fingers (very light pressure) over supracondylar ridge Ring fingers or small fingers on the triceps bar

7) Consider the following

• Locations of the pressure distribution
  • Forces and counterforces in various positions
• Alignment
  • Forearm neutral pronation/supination
  • Elbow 90’ flexion
• Suspension
• Donning and doffing

8) Mark the landmarks and draw trimlines

With elbow at 90 degrees, palpate the following:

2. • Anatomy: cubital fold, epicondyles, biceps tendon and lateral humeral supracondylar ridge, olecranon, and the electrode sites (as relevant).
   • Place your finger and thumb around the residual limb just above the epicondyles into the supracondylar area. Mark the stockinette just above the level of your finger and thumb. Depending on the anatomy, the line is likely 3 cm (1¼”) proximal to apex of epicondyles.
   • Place your finger along the triceps bar and mark the stockinette just above the level of your finger. This trimline varies based on the residual limb length but is likely about 2.5cm (1”) proximal to proximal edge of olecranon.
   • Put your fingers on the posterior aspect of each epicondyle. Draw the shape of the brim between your fingers.
   • Connect the cubital fold line with the medial/lateral marks to complete the brim shape. Be sure to include the supracondylar fossae in the trimlines.

9) Reassess reference anatomy, trimlines, and alignment

10) Capture the impression

2. 2. 1. Check
         • Landmarks and trimlines in appropriate locations and comfort with hand position.
         • Supplies: Elastic bandage, splint, rigid bandage, distal end splint, scissors, water located under the arm.
         • Pt knows what to expect.
         • Alignment
      2. Moisten the elastic splint and place it on the arm. The first layer should be exactly in the cubital fold and across the olecranon.
      3. Lay the splint around the limb – cover the olecranon and triceps bar.
         • If using the distal end splint, place this on the limb. (Note, that splints are always rigid plaster and they most often need to be secured with a circumferential layer of plaster.)
      4. Saturate the rigid bandage and wrap proximally to anchor down the splint. Continue wrapping distally to cover the distal end. Wrap once more around the olecranon to secure the rigid splint in place. As you wrap, massage the plaster over the residual limb.
         • As you massage, feel the underlying anatomy and the thickness of the plaster.
         • Massage the plaster to ensure it is not above or below the pre-determined trimlines.
         • Cover the sides of the arm to the height of the trimlines only. Fold the edge of the wet plaster over to match the lines, if necessary. This will add strength to the negative mold.

11) Remove the cast

• To ease removing the negative, pull tissue out of the undercut created by the flexion action.
• Re-check the condition of the residual limb
• Assess the cast impression
• Compare ML measurements at epicondyles and just proximal to epicondyles