8 Socket Impression Techniques

1. Before patient arrives

• Create action plan
• 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. Patient evaluation

Review ICF factors from Fall quarter. For this socket design, specifically evaluate the shape and movement of the epicondyles through elbow flexion and extension.

3. Preparation

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 bandage material to allow room at the cubital fold
• Cut out material above the olecranon
• Open the splint to full length and wrap it around the limb to ensure it captures the
  • epicondyles, supracondylar fossa anatomy, triceps bar, and olecranon

• 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.

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

• Check
  •  Alignment
    • Forearm neutral pronation/supination
    • Elbow 90’ flexion
  • Suspension
  • Donning and doffing

• 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

• 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

• 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

• 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)

2. 2. 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.

Encompass the epicondyle at the level of the P-M-L with enough of a brim to flare out of the soft tissue.

Lateral: relief for the humerus in the ROM (“fan relief”)

Standard: 25mm proximal

Encompass the olecranon

Compress the triceps tendon

Relief for bony pressure on the posterior humeral epicondyles

•       short limbs closer to cubital fold

•       longer limbs ok to allow room (distal to cubital fold)

Clearance for biceps tendon.

4. Capture the impression

1. 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. Saturate the rigid plaster splint.
      • It should be positioned exactly in the cubital fold.
      • Ensure that the splint covers the epicondyles, supracondylar fossa anatomy, triceps bar, and olecranon
      • Fold the edge of the wet plaster over to match the trimlines. This fold will increase rigidity of the trimlines.
      • Avoid extending above the cubital fossa or too high above the olecranon.
   3. Saturate the elastic bandage and place it on the arm.
      • The first layer should be exactly in the cubital fold and across the olecranon.
      • Follow your trimlines and palpate as you smooth the plaster to ensure accurate placement.
   4. 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.)
   5. Saturate the rigid bandage roll and wrap proximally to anchor down the splint. Continue wrapping distally to cover the distal end.
      • 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.

5. Review

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

Inspect the cast:

• Is the cast in the correct alignment?
• Did you cover all areas with enough plaster?
  • Do you see the triceps bar?
  • Is it a finger breadth above the epiconcyles?
• Do the inside caliper measurements of the cast match your outside caliper measurements on the patient at epicondyles and supra-epicondyles?
• Do your markings match the areas you wanted to highlight, such as the bony prominences?

Review functional goals and purpose with the patient.