A practitioner can spend hours fine-tuning the alignment of a lower-limb prosthesis. A technician transferring this alignment can lose it in seconds.
Alignment transfer is a common task with ample failure potential. Think of it as a 3D puzzle with only one correct solution—every knob on a vertical-alignment jig plus every setscrew in the components offers you a chance to make a wrong turn.
We've developed a strict procedure at Otto Bock, making lost alignments a thing of the past. Today, there's just one way to do an alignment transfer here. Because we all live by the same standard, one technician can hand off a project midway through an alignment transfer and any other technician can confidently finish the project.
The secret is consistency. We have two rules for the jig arms on routine alignments:
- If it rotates in a vertical plane, turn it to the far left (counterclockwise) and tighten.
- If it swivels horizontally, move it to the far right and tighten.
We have two additional rules to cover special circumstances:
- If you must lift a locking key out of its slot to move an arm outside the normal range, mark that arm's position. Do not rotate or swivel the arm before tightening.
- If the practitioner specifies any adjustments, make changes after the original alignment has been transferred.
Here's how we do the whole process:
Set up to capture the alignment of the original socket. Start by making sure the alignment jig is mounted on a horizontal surface and remove any tape or markings from a previous transfer.
Remove two of the screws that attach the foot, and use the same two screws to secure the prosthesis to the jig's ankle plate. After adjusting the distance on the ankle-plate arm to accommodate the size of the socket, lift the locking key into the slot. This arm rotates in a vertical plane. When the key is in the slot, the arm can rotate a few degrees in either direction. This "play," which occurs on every arm while its key is in the slot, can be a major factor in lost alignments if not returned to the exact position at the transfer. To eliminate any confusion, rotate the ankle plate to the far left (counterclockwise) and tighten the lock knob.
This also applies to the entire arm and plate assembly, which swivels horizontally and has "play" from left to right. Hold the entire arm to the right and tighten the lock knob.
For a transfemoral prosthesis, position the yoke assembly around the knee center. Tighten the yoke screws and the arm's lock knobs to hold the knee in place. Follow both rules for locking the arm in place.
Prepare a pipe for the plaster mold. Because they are round and smooth, pipes are prone to turn with a little torque. The following two strategies will help lock the pipe in place.
- Flare the bottom. Saw two one-inch cuts, roughly perpendicular to each other, into the end of the pipe. With a bending iron, pry the four arcs outward. Those four flanges will keep the pipe from rotating once the plaster hardens.
- Drill an indent near the top of the pipe. Using a bit the same size as the setscrew in the mandrel, drill about 1/16-inch deep. Slide the pipe into the mandrel and tighten the setscrew to immobilize the pipe in the mandrel's grip.
Position the pipe. Secure the mandrel in the clamp on the jig's top arm. A notch restricts the mandrel to one consistent position in the clamp. While holding the arm, loosen the knobs near the post. Roughly center the pipe over the socket and tighten the horizontal lock knob, turning it counterclockwise. Lower the pipe until the flared end is an inch or so from the bottom of the socket. Remember to move the arm to the far right and tighten the lock knob. Slide the lock collar flush with the bottom of the arm assembly and tighten the collar's lock knob.
If having all locking keys in their slots prevents you from centering the pipe in the socket, the alignment probably falls outside the normal range. Determine which arm needs to go beyond its normal range. Lift the appropriate key out of its slot, allowing the arm to move freely in its plane. Roughly center the pipe over the socket. The key is out of the channel, so don't move the arm right or counterclockwise before tightening the lock knobs. Follow the same procedure to lower and center the pipe. Slide the lock collar flush with the bottom of the arm assembly and tighten the collar's lock knob.
Double-check that all settings are tight.
Mark the settings. Use tape or a marker to record the position of the top arm. We use a marker, tracing the bottom of the collar on the post for the height. If you had to swing the key out of the channel to swivel the arm, you'll need to mark the exact placement of the arm in relationship to the post. We make a hash mark where the slot on the clamp assembly meets the lock collar and draw two lines crossing the clamp assembly and the lock collar.
Make a plaster mold. Create a dam that extends at least one inch beyond the top of the socket. Masking tape does this job nicely. Fill the dam with plaster. Once the plaster has cured, remove the socket and the plaster mold. We loosen the clamp on the mandrel and on the swivel arm, and then move the arm up and to the side.
Transfer the alignment. Because of our standard precautions, we can take the mandrel off the pipe with confidence. (Yes, we take the mandrel off.) Fabricate an initial lamination from the plaster mold. Then place the new socket on the alignment jig. Slide the mandrel onto the pipe. Align the screw hole with the indent on the pipe, then replace and tighten the setscrew. Insert the mandrel into the clamp and tighten.
If you use an adapter with prongs, do not contour the prongs while the adapter is attached to components in the alignment jig. Remove the adapter, and anchor it in a vise to bend the prongs. Warning: Bending metal on an alignment jig can bend the ankle plate, altering the alignment.
Secure the attachment component to the components on the alignment jig. Next, reposition the socket over the components, following the same procedure as the initial set-up, and tighten the lock knobs.
If you're not making any adjustments to an existing alignment, you only need to verify that the height is correct. Is there a gap between the socket and the attachment component? If there is, you can either fill the gap with a bonding material or install new components with the proper length. Is there a gap between the top arm assembly and the lock collar? This gap is a precise measurement of how much you have increased the original height of the alignment. Remove this extra height from the components. You are now ready to bond the attachment component to the socket.
Make minor adjustments. The new socket is now in the same alignment as the original. If the practitioner requests an adjustment, now is the time to make a small change.
Height: To add height, raise the top arm to create a gap for the prescribed addition between the assembly and the collar. To subtract height, measure the prescribed reduction down from the bottom of the collar. Mark the new height on the vertical post. Lower and tighten the collar at the lower height, then lower and tighten the top arm. Whether adding or subtracting height, you probably will have to adjust components.
Foot position: Adjust screws on the ankle plate to inset or outset the foot.
Zero-out components: The practitioner may have refined the alignment by adjusting setscrews in the adapter. To maximize the ability to make modifications in the future, you can adjust the screws to a neutral position.
Bond the attachment component. Raise the top arm several inches and tighten the lock knob to give yourself working space. Prepare your preferred bonding material. Apply a mound of the bonding material to the top of the attachment component; use more if you need to fill a small gap. Lower and tighten the top arm, and wipe away excess bonding material.
Do the final lamination. Once the bonding material has cured, loosen two adjacent setscrews at the adapter, unclamp the mandrel, and remove the socket with the bonded attachment component. Immediately replace the attachment component with a plug, and tighten the two screws to keep the four setscrews in that precise setup. If you don't, and the wrong setscrews are moved, the alignment will be lost.
Now you can do the final lamination and permanently adhere the adapter to the socket. Once the socket has been completed, reassemble the components.
Alignment transfer is a complex project that can easily go wrong. To ensure success, never stray from a consistent process. Do it the same way every time, and you can duplicate existing alignments every time.
Jason Kimmel is lead fabrication prosthetic technician for Otto Bock US HealthCare. He oversees all prosthetic fabrication and is the main prosthetic contact at the Otto Bock Technical Center in Minneapolis.