Complex Ankle and Hindfoot Arthrodesis Using Circular External Fixation
TAKE-HOME POINTS
- Ankle and hindfoot fusion using circular external fixation is a useful surgical technique in patients with diabetes, Charcot, osteomyelitis, deformity, and/or bone and soft tissue compromise in order to obtain solid bony fusion, stable limb alignment, and eradication of infection in cases of complex pathology.
- Deformity correction with osteotomies and meticulous joint preparation is required in order to obtain broad, cancellous bony surfaces for fusion with neutral alignment. Autograft from the distal fibula and/or medial malleolus can be combined with bone allograft to assist with joint fusion.
- The ankle and hindfoot are provisionally pinned into neutral coronal and sagittal alignment through the plantar surface of the foot using large K-wires prior to placement of the lower leg in the center of a circular 3-ring compression frame. Typically, 2 to 3 points of fixation are used per ring with a combination of half-pins and smooth wires.
- Ring attachments are built up or down to the level of the half-pins and wires in order to prevent pins and wires from bending, breaking, or causing iatrogenic deformity during tensioning. Crossing olive wires are used in the midfoot and calcaneus with dual tensioning devices to ensure an even pull on both sides of the foot.
- Dynamic or static compression struts are used to obtain 8 to 10 mm of compression across the ankle and hindfoot, followed by addition of an anterior foot ring to increase construct rigidity. Daily pin care is started 3 to 4 days after surgery and patients are kept non-weight-bearing for approximately 2 months in the frame with a total frame period of 3 to 8 months depending on bony healing on X-ray.
X-rays should be carefully checked to ensure proper alignment. Wounds are gently irrigated, and vancomycin powder (2 g) can be placed within wounds for local antibiotic delivery. Lateral tissues are sharply debulked to allow for decreased tension on the incision, and small ulcers can be excised in their entirety. Wounds are closed in a layered fashion using 0-polydioxanone (PDS, Ethicon) suture for deep tissue, 2-0 PDS for subcutaneous tissue, and 2-0 nylon for skin closure. The tourniquet is deflated for the remainder of the case to reduce limb ischemia during frame placement.
CIRCULAR FRAME CONCEPTS AND PLACEMENT
The majority of circular frames for both ankle and hindfoot fusion have multiple ring sizes available in aluminum and radiolucent carbon fiber reinforced polymer (Hoffmann LRF, Stryker). Rings are available in full, open, segment, and both short- and long-foot options. Frames can be sterilized in a prebuilt 3 to 4 ring construct with 4 static or dynamic (telescopic) struts (100-277 mm). The most commonly used tibia and foot ring sizes are 155 cm, 180 cm, and 210 cm. Ring size should be able to accommodate posterior soft tissue swelling and avoid circumferential soft tissue abrasion against the rings. Anterior foot arches are used for increased construct stability and can be locked to the distal tibia ring for weight-bearing support. Wire and half-pin bolts, adaptors, and nuts are used to join each ring of the frame to the patient’s bone.
For TTC arthrodesis, 2 rings are typically used in the tibia, and 1 ring is used in the foot. For isolated ankle arthrodesis, an additional ring can be added with olive wires in the talus to permit compression only across the ankle joint. Multiple points of fixation are used in each ring in different planes to achieve both maximal stability and rotational control. If a single wire or half-pin becomes infected and requires removal, there are still multiple other points of fixation in the ring to maintain stability. Fixation within each ring should be off axis compared with the adjacent ring to both avoid stress risers and increase construct rigidity.
The prebuilt frame is checked on the back table to ensure proper orientation and component alignment. The frame is then placed over both the foot and ankle, and multiple stacks of towels are placed behind the heel, ankle, and calf to center the foot and ankle in the frame (Figures 4A-4F). At least 4 to 6 cm of space is needed in between the posterior soft tissues and each ring to accommodate postoperative swelling. On the lateral view, the foot ring should be in the mid-portion of the calcaneus. If there is a concern, particularly in Charcot patients, regarding early weight-bearing noncompliance, the foot ring can be placed flush with the plantar aspect of the foot, and olive wires can be inserted using longer adaptors. The frame should be checked from multiple viewpoints to ensure that both the foot and ankle are centered and in neutral rotation.
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