by James K. DeOrio, MD
Many ankle replacement systems are best used when the deformity does not exceed varus or valgus greater than 10 degrees and when there is minimal bone loss. However, that would exclude many ankles from being replaced which would leave ankle fusion as the only option. I have chosen to pursue one ankle replacement sytem whenever there is significant deformity. This modular intramedullary total ankle replacement is a fixed-bearing two-component design with a modular stem system for both tibia and talar components. It is indicated for resurfacing of the ankle in severe inflammatory, traumatic or osteoarthritis. Contraindications include poor skin quality over the anterior ankle, peripheral vascular disease, paralysis and ongoing infection. The tibia is inserted into the intramedulary tibia, but does not resurface the malleoli. The talar component entirely replaces the superior aspect of the natural talus, after a flat dome resection. Multiple modular segments may be added to the tibial stem, depending on the surgeon’s determination of how much stability is needed or how much the stem should pass beyond a simultaneous supramalleolar osteotomy performed for tibial malunion. The talar component’s stem may be limited to the body of the talus or can be can be extended across the subtalar joint into the calcaneus if greater support for the talar component is required or when a simultaneous subtalar arthrodesis is warranted. The longer talar component calcaneal stem is not currently FDA approved and is only available after approval of compassionate use.
Unique to the modular intramedullary total ankle system is the alignment guide system. The ankle is opened identical to the other ankles between the tibialis anterior and the EHL. The leg is then placed in the leg holder and the rotation of the leg holder aligned parallel to the medial mortise. The calcaneus is fixed with two pins and the foot and lower leg secured to the leg holder with elastic wrap. The large fluoroscopic C-arm is guided into place and the anterior-posterior aiming sites are aligned confirming center location of the guide over the talus and the tibia. Then the lateral centering is accomplished with the C-arm in the lateral view. The AP view is then reobtained with proper centering and the plantar calcaneal heel pad is opened. This routine technique requires simultaneous alignment of the talus with the tibia. (For more severe cases I have aligned the talus only and then rotated the talus with the drill bit inserted to obtain tibial alignment.) Once that is achieved, the drill is passed from the plantar foot through the calcaneus, just anterior to the posterior facet, through the center of the talar body into the center of the tibial metaphysis, much like the guide pin for a retrograde ankle arthrodesis nail. While many argue that it is undesirable to violate the subtalar joint when performing TAA, the designers of the alignment guide maintain that if the device is applied appropriately, the drill safely negotiates the subtalar joint between the arterial anastamosis on the inferior talar neck and the posterior facet’s articulation with the inferior talus. No detriment has been observed thus far for this 6 mm hole.
A cannula is locked into position through the soft tissue and the calcaneus, talus and tibia drilled. The cutting guide is now applied (its size predetermined on templated x-rays and confirmed intraoperatively) and verified with the C-arm. Alignment of the cutting guide on the drill is accomplished under fluoroscopy and the guide pinned into position. The antirotation drill is used to create a hole in the tibia. Then the tibia and talus are cut through the saw guide. The saw guide is removed and the bone extracted. The tibia is reamed by applying the reamer onto the reaming rod inserted up through hole previously drilled in the calcaneus and talus. The ankle is then plantar flexed and the hole for the talar stem drilled. Then the cone portion of the prosthesis with one attached cylinder is inserted into the tibia followed by one additional cylinder, then the cylindrical base. The Morse taper tibial component is then tamped into place and the whole prosthesis driven into the tibia. Next, the talar component is slid into place with the 10mm stem attached. If the longer 14 mm stem is chosen, it is inserted first (same for even longer stems, not yet FDA approved). Then the talar component is inserted over the top of this stem and locked onto the Morse taper design. Finally, the polyethylene component is inserted and impacted into place. The wound is closed in layers.
Primary modular intramedullary ankle replacement is relatively straightforward. However, malalignment in the form of varus or valgus makes it more difficult to insert the INBONE when it exceeds 10 degrees in either direction and is especially problematic when it is over 15 degrees. However, newer techniques make this possible. For example, with varus malalignment, the use of a complete medial deltoid ligament “peel” combined with the use of lamina spreaders medially to tension the remaining lateral ligaments had led to expanding use of the modular intramedullary TAA for these deformities. Similarly, lamina spreaders may also be used to align valgus deformities by placing tension laterally and distracting and realigning the ankle before making bone cuts. The surgeon must be prepared in the end to achieve bony alignment with calcaneal and sliding osteotomies, subtalar and TN fusions, Achilles tendon releases or gastrocnemius recessions, ligament reconstructions and even tendon transfers.
Significant bone loss has previously been a contraindication for ankle replacements. However, the modular intramedullary ankle, by allowing an extended intramedulary stem gives the surgeon the ability to get good stability even with significant bone loss. Once the stem is in, the remaining defects can be bone grafted. Similarly, a flat top cut on the talus with the use of stems which vary in length, can be used to gain as much purchase on the talus as necessary. This is particularly valuable in cases of avascular necrosis where you want living bone to be in contact with the prosthesis.
Previously for tibial malunions, it has been recommended that realignment procedures be done as a staged procedure. However, modularity of the intramedullary tibial stem allows the surgeon to do a simultaneous supramalleolar osteotomy, temporarily hold it with K-wires and/or a plate and then use the intramedulary portion of the tibial stem to fixate it.
For many of these same reasons, the modular intramedullary ankle system is ideally suited to revise failed ankle replacements. After prophylactic screw fixation has been inserted in the malleoli, existing loose ankles can be removed and well fixed ankle components can be sawed away from ongrowth bone. Then, by resecting minimal bone, again with the use of the lamina spreaders tensioning the soft tissue, a revision ankle can be inserted much like a primary ankle.
Finally, painful ankle fusions can also be taken down and replaced with the modular prosthesis. Of course it helps to have the fibula retained but takedowns have also involved those cases in which the fibula has been removed. Once more, prophylactic screws are recommended in the malleoli because this unstressed bone is weak and could lead to fracture. Placing the cutting jigs on the ankle without recutting the joint line has worked well if the ankle has been fused in a correct position. Afterwards the gutters are opened to once more allow freedom of motion. If the ankle was fused in malposition, it is first necessary to recreate the ankle joint to allow orthogonal bone cuts.
These newer ankle systems will potentially allow all patients, regardless of deformity, to have an ankle replacement if no other contraindications exist.
Dr. DeOrio is an Associate Professor of Surgery specializing in Orthopaedic Surgery at the Duke University School of Medicine. His special interests are lower extremity reconstruction, especially total ankle replacements and all other procedures involving the hind foot, midfoot, and forefoot deformities.