11 Socket Impression Technique

Transhumeral Socket Design (Type II – 50-90% residual limb length)

The transhumeral level amputation presents new challenges to prosthetists. The socket no longer has inherent stability and suspension. The cylindrical shape of the limb provides no rotational control of the socket on the limb making it challenging to securely capture the movement of the residual limb.

The length of the humerus affects the potential leverage capability as well as the potential for generation of forces. The images (A and B) illustrate potential fitting problems that may occur with the transhumeral socket design.

Consider the anatomy and the way that the socket will balance triplanar forces and counter-forces during both static and dynamic activities

• Anterior wall
  • Provides rotational and extension and protraction stability
  • Deepest portion of this wall is located in deltopectoral groove
  • Flare at edges

  Posterior wall

  • Provides rotational and flexion stability
  • In same plane as the scapula

  Lateral wall

  • If sensitive, pressure relief at distal lateral

  Medial wall – humerus and axilla

  • Depression control of elbow lock
  • Total contact between medial and lateral walls
  • Gentle contact with tendons

• The deepest point of deltopectoral groove is 2.5-5cm distal to clavicle and medial to head of humerus
• Posterior compression aligns with plane of scapula

Cast Impression Process

Follow the following steps to capture the impression.

1) Perform a thorough evaluation of the patient’s limb including ROM, strength, tissue compressibility, muscle bunching, and areas of potential skin irritation.

2) Measurements Lengths: Residual limb: acromion to distal end acromion to distal end, slightly compressed Contralateral side: acromion to lateral humeral epicondyle lateral humeral epicondyle to distal tip of thumb measure old prosthesis too
Circumferences: Residual limb Measure circumferences at and 3-4 levels below axilla level Snug – tape is snuggly wrapped around limb Snugger – skin is compressed somewhat tightly Contralateral limb
Measure the A-P dimension: Mark a location for your measurement that will be transferred to the mold, such as the deepest spot in the deltopectoral groove and just below the spine of the scapula. Use calipers with rounded tips to measure the dimension, compressing slightly. Measure M-L dimension: Place a Ritz stick in the axilla and measure from the pectoralis tendon to the latissimus tendon. Do not compress.
3) Cast Prep Cut one splint for the proximal section using 5” or 6” plaster.  The length of the splint should extend from the medial border of the scapula to 2-3 inches past the deepest portion of the deltopectoral groove.  Make this splint 6-10 layers and cut so that it is not a continuous length of plaster. Cut the second splint about 5-6″ long using 5″ plaster. Make this 4 layers. Optional: cut splint to cover distal end.
Use a gown and towels/sheet to cover the patient’s torso, clothing, and shoes. Don stockinette or nylon.  Cut one leg off the nylon, then place the nylon on the pts limb.  Twist the bottom of the nylon, tie off and reflect back up onto the limb. Apply a liberal amount of Vaseline/separator to hair and place plastic wrap over the area.
4) Communicate with the patient about the process and what they might expect.  Ask if they have any questions, so they can relax.   5) Practice your hand placement on the patient and verify that it is comfortable for the patient.  For this two part cast: (1) Medially:  One hand is placed in the axilla, while the opposite hand compresses softly from the lateral aspect of the residual limb. The hand in the axilla should avoid compression into the neurovascular bundle. If the fingers are too thick in the axilla, use a small dowel or 10mm thick sheet of soft foam. (2)  Anteriorly:  Place your thenar or hypothenar eminence in the deltopectoral groove, avoiding pressure on the clavicle or the humeral head. Posterior aspect:  The fingers lie on the scapula in the same plane as the scapula. UCLA pages 244-249

Hand position for casting.

Notice:

• Locations of the fingers/palm relative to the bony landmarks, and the right thumb marking acromion
• Angles of the hands matching the anatomy

6) Mark your trimlines

7) Capture the impression of the humerus

Begin by wrapping circumferentially at midpoint of the limb and moving distally.

Use about 3 layers of elastic, 3 layers of rigid plaster.

After wrapping the limb with plaster circumferentially, place a splint over your inside hand. The palm of your hand rests against the chest wall, the dorsal side of your palm touches the plaster wrap, and the top of your fingers press comfortably in the axialla.

ML pressure (1st part)

Place the inside hand comfortably in the pts axilla. Its only purpose is to locate the height of the axilla and location of tendons. Be sure to have plaster over the tips of your fingers. Do not flex the fingers of the medial hand – this may create undo pressure at the neurovascular bundle.

The opposite hand presses medially parallel to the other hand. The palm of the hand is moves proximally starting from just above the distal-lateral end moving towards the acromion to create a smooth lateral wall.

AP Pressure (2nd part)

After the circumferential wrap has set, reposition the plinth and practice your AP hand placement again.

Recheck your trimlines and the length of the splint.

• Extend from 1” distal to level axilla to clavicle and spine of scapula.
• Extend ~3” medial from the axilla or middle of clavicle.

Wet the splint material and lay on pt so that the top of the splint is at the clavicle and the spine of the scapula. You want to be able to identify the clavicle and the spine of the scapula in your cast.  Place the winkles of the plaster bandage laterally.

Apply AP compression: Significant force. Posteriorly:  your fingers are below the spine of the scapula and in the plane of the scapula. This hand is stabilizing the anterior force. Anteriorly:  Your thenar or hypothenar eminence is pressing posteriorly in the deltopectoral groove. Do not press on the coracoid or the head of the humerus or clavicle.
Check Alignment Pt should be seated comfortably with arm relaxed at his/her side. Neutral frontal plane and sagittal plane alignment If casting with patient in too much abduction, the anterior/posterior stabilizers will not be positioned accurately.

These are the trimlines you might expect.

8) Mark landmarks and alignment lines

• Measure the A-P dimension while the plaster is on the limb. Mark a location anteriorly and one posteriorly. Use the outside calipers to measure the A-P dimension. This dimension will likely change as the cast is removed and you can use this measurement to reset the cast to the proper dimension.
• Mark the carrying angle in the frontal plane, 1” lateral to hip.
• Mark the handing angle in the sagittal plane.

9) Gently remove the cast and check:

• That the anatomical landmarks are captured: clavicle, spine of the scapula and axilla.
• It’s shape relative to the shape of the residual limb.
  • o Is the deltopectoral groove in the “right” location?
  • o Is the plane of the scapula in the right plane?
• The AP dimension from deltopectoral groove to scapula.