Definition
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Total talus replacement (TTR) is a surgical procedure which involves the removal of a pathologic talus and replacement with a patient-specific 3D-printed titanium (Ti) prosthesis coated with titanium nitride (TiN).
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DICOM data from a CT scan is used with specialized software to determine the size and dimensions of a TTR prosthesis.
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CT scans of bilateral ankles are preferable and recommended; however, data only from a CT scan of the ipsilateral (pathologic) talus may be used if there is no collapse, deformity, or bony deficit.
Indications and contraindications
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Diffuse talus avascular necrosis (AVN) is a primary indication for TTR; however, other causes of bony deficit within the talus may be an indication for TTR, such as high-level trauma or malignancy.
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Isolated TTR may be considered in patients with talar pathology but without pathology of the adjacent joints.
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TTR may be combined with a total ankle replacement in the setting of talar pathology with secondary changes to the distal tibia.
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Contraindications for TTR include active infection, neuropathy, gross deformity in the sagittal or coronal planes, and AVN of the calcaneus, distal tibia, or navicular bones.
Anatomy
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The talus ( Taxillus , referring to the ankle bone of a horse) is the second largest bone in the hindfoot with an irregular saddle-shaped architecture. It is composed of a head that forms the talonavicular joint with the navicular bone anteriorly and the anterior talocalcaneal joint inferiorly; a neck that connects the head and the body. The latter has three processes (medial, lateral, posterior), two facets (middle and posterior), two tubercles, and one talar dome ( Figs. 5.1 to 5.3 ).
• Fig. 5.1
Superior view of the talus showing the body of the talus, the talar neck, and head.
• Fig. 5.2
Medial view of the talus showing the medial side of the talar head and the articular surface with the medial malleolus.
• Fig. 5.3
Lateral view of the talus showing the lateral side of the talar head, the talar neck, the articular surface with the lateral malleolus, and the posterior facet.
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The middle facet articulates with the sustentaculum tali. The posterior facet forms with the calcaneus the posterior talocalcaneal joint.
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The talar dome or trochlea located superiorly forms the tibiotalar joint with the tibia and fibula.
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The talus is covered by more than 60% of articular cartilage.
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There are no muscle attachments, but the talus does possess multiple ligament attachments, including the deltoid complex and spring ligaments medially, and the anterior talofibular and posterior talofibular ligaments laterally.
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The modified Boyan Classification is used to describe the variable morphology of the subtalar joint facets based on the number of facets present and the distance between those facets.
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The tenuous blood supply is provided by three arterial sources ( Fig. 5.4 ):
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The posterior tibial artery breaks into the tarsal canal artery that supplies most of the talar body except the medial third, which is supplied by the deltoid branch of the tarsal canal artery.
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The anterior tibial artery (becoming the dorsalis pedis artery) gives off the lateral tarsal artery, which anastomoses with the peroneal artery to form the tarsal sinus artery.
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The tarsal sinus and tarsal canal arteries anastomose in the sinus tarsi.
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The medial branches of the dorsalis pedis artery supply the superomedial talar neck.
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The inferior talar neck branches of the tarsal sinus artery or tarsal canal artery supply the inferolateral talar neck.
• Fig. 5.4
Medial view of the talus showing the posterior tibial artery giving off the artery of tarsal canal, which supplies the talar body and branches into the deltoid artery. The anterior tibial artery becomes the dorsalis pedis artery, which gives off the lateral tarsal artery that anastomoses with the perforating branch of the peroneal artery to form the artery of tarsal sinus. The tarsal canal and tarsal sinus arteries anastomose in the sinus tarsi.
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Pathogenesis
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The tenuous vascularity combined with a lack of periosteal blood supply increases the risk of talar AVN.
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AVN may be secondary to fractures and trauma, prolonged steroid use, alcoholism, or vasopressors.
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The extent of necrosis along with the severity of bone compromise dictates treatment management.
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Extensive AVN has a higher risk of talar dome collapse, which can lead to degenerative changes in the ankle and subtalar joints.
Patient history and physical exam findings
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The clinical presentation of AVN may be quite variable.
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A high level of suspicion is the key to establish an early diagnosis in nontraumatic cases.
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Deep, aching, or sharp ankle pain is a common initial complaint.
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Locking and catching might be felt with more advanced disease.
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A thorough history must be undertaken to look for systemic diseases, substance abuse, or corticosteroid use.
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The physical exam may be unremarkable in the early stages. Effusion and talar tenderness may be present in later stages.
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Limited ankle and subtalar range of motion with malalignment is seen in severe cases.
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In posttraumatic cases, several signs and symptoms may be noted, including ankle and hindfoot pain, effusion, limited range of motion, and crepitus.
Imaging
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Plain radiographs:
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Multiple weight-bearing views of the ankle to assess for talar sclerosis, collapse, fragmentation, malunion, or nonunion. Early AVN is commonly missed on plain radiographs.
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Specific views, such as the Canale view, could be obtained to assess talar neck varus malunion.
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Hawkin’s sign might be present at 6 to 8 weeks on an anteroposterior ankle view. It is a sign of subchondral bone resorption in the setting of revascularization.
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CT scan:
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This is routinely obtained to assess the extent of the necrosis, the quality of the talar bone, and articular changes.
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Weight-bearing CT scans provide a better assessment of the ankle/hindfoot alignment and arthritic changes.
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A contralateral talus CT scan is to be requested for a 3D-printed, custom-made TTR, especially in the setting of talar collapse or anatomic abnormalities of the talus.
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MRI:
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MRI is the most sensitive modality to assess bone edema, loss of cartilage, quality, and viability of the talar bone.
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Gadolinium contrast might be considered to differentiate bone edema from an early stage of osteonecrosis.
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Nonoperative management
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Etiology, severity of pathology, and patient comorbidities may guide decision making for nonoperative (conservative) versus operative treatment. In cases of talar AVN, nonoperative treatment may include:
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Protected weight bearing
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Patellar tendon-bearing bracing
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Extracorporeal shock wave therapy.
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Extracorporeal shock wave therapy has been shown to be superior to physical therapy.
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In cases of talar neck fracture complicated by AVN, Hawkins reported that only 12.5% had good or excellent outcomes.
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Patients without talar dome collapse who were treated nonoperatively for a greater period of time (more than 6 months) were not found to have a poorer outcome when an intervention was needed.
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Therapeutic strategies have been suggested to manage talar AVN, but there is no consensus with regard to the ideal treatment algorithm.
Traditional surgical management
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Traditional surgical treatment may be divided into joint-sparing and joint-destructive procedures. Traditional joint-sparing procedures include vascularized and nonvascularized bone grafting, core decompression, intraosseous injections, and bone stimulation.
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Vascularized and nonvascularized bone grafting:
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Different types of vascularized bone graft (VBG) have been described.
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Nunley et al. successfully used a vascularized cuboid bone graft in 13 patients with AVN involving <60% of the talus.
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Other techniques using VBG from the medial femoral condyle graft, first cuneiform, and distal tibia have been described with satisfactory outcomes in the treatment of talar AVN.
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VBG is limited to talar AVN without collapse as these techniques will not alter the morphology of the talar bone.
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Core decompression (CD):
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CD is used to reduce intraosseous pressure and promote neovascularization of the necrotic zone.
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CD is limited to treatment of early-stage, nontraumatic talar AVN without talar collapse.
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Intraosseous stem cell and platelet-rich plasma (PRP) injection therapy :
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This treatment may be considered in early-stage talar AVN.
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The treatment should be done under fluoroscopic guidance.
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May be combined with CD.
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Bone stimulation:
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Bone stimulators create an exogenous electric current across the osteonecrotic bone that would cause differentiation of the progenitor cells into osteoblasts and enhancement of bone healing.
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Internal implantation of a bone stimulator into the talus with bone grafting has been described.
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Joint-destructive procedures—arthrodesis:
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Arthrodesis may be considered as a last-line salvage treatment.
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Arthrodesis is often used in advanced talar AVN with collapse and/or adjacent joint arthritis.
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Arthrodesis leads to gait abnormalities and adjacent joint arthritis.
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A variety of techniques and considerations may be considered:
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Rarely a limited subtalar or talonavicular joint arthrodesis is indicated.
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Blair’s technique for tibiotalar arthrodesis has a reported high rate of pseudoarthrosis but modifications of this surgical technique have improved fusion rates. These modifications include arthroscopic joint preparation and fixation with a retrograde intramedullary nail to enhance compression and stability. The use of allograft such as femoral head or metal cages to maintain limb length and alignment have shown variable fusion rates.
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Subtalar arthrodesis following an ipsilateral ankle arthrodesis has a markedly high nonunion rate (40%).
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3D-printed implant design specifications and considerations
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First-generation implants:
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In 1997, the first stainless steel talar prosthesis was reported. The implant was designed to replace the talar body with a peg for fixation into the talar head and neck with bone cement.
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The authors reported only five failures among their 33 patients with a 10- to 36-year follow-up.
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Second-generation implants:
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The design was similar to the first generation in replacing the talar body but did not include a peg fixation.
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Taniguchi et al. reported satisfactory results on 12 patients with a 7-year follow-up.
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Third-generation implants:
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TTR with 3D-printed alumina ceramic prosthesis was first described by Taniguchi in 2015 :
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The dimensions of the prosthesis were determined from bilateral CT scans. A 3D-printed stereolithographic model was created as a negative cast which was then filled with alumina ceramic, creating the prosthesis. The manufacturing process took 4 weeks.
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Alumina ceramic has been shown to have less wear than stainless steel.
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The use of 3D-printed cobalt chromium molybdenum (CoCrMo) and Ti implants for TTR has been described. Manufacturing of CoCrMo and Ti implants incorporates use of electron beam melting (EBM), which uses a high vacuum setting to produce metallic materials with a high affinity to oxygen.
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In 2019, Ti implants with TiN coating were made available. The coating showed improved wear properties in Ti implants and minimized friction on adjacent joint cartilage.
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Third-generation implants can be used in isolation or combined with a total ankle prosthesis and may be considered constrained or unconstrained:
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Constrained implants utilize additional fixation into the calcaneus or through ligamentous reconstruction. In 2017, Regauer et al. described design modifications that allow attachment of ligaments to the implant, which may enhance stability.
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Unconstrained implants are press fitted into the mortise and have no supplemental attachment to adjacent bones or ligaments.
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Surgical management with 3D-printed devices
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Positioning: supine
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Anesthesia: general and regional
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A direct anterior approach to the ankle is utilized ( Fig. 5.5 ):
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A midline skin incision is made 4 cm proximal to the tibiotalar joint and extended to the level of the navicular tuberosity.
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A sharp dissection is made down to the extensor hallucis longus (EHL) sheath, which is opened and retracted laterally.
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Identify and protect the superficial peroneal nerve branches in the distal part of the incision and the anterior neurovascular bundle underneath the extensor tendons. The vascular branches travelling medially need to be cauterized and the neurovascular bundle mobilized, protected, and retracted laterally.
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A large Gelpi retractor can be used.
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Sharp dissection down to the tibiotalar joint and the capsular flap should be elevated with caution to allow for repair later.
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All capsuloligamentous attachments of the talus need to be freed from the talonavicular joint.
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The medial and lateral gutters are debrided.
Related posts:
Historical perspectives on 3D printing
Material science for 3D printing in medicine
Custom constrained 3D total talus/navicular replacement
3D-printed solutions for avascular necrosis of the talonavicular joint
3D-printed total cuboid replacement for lateral column pathology
3D-printed custom hemi-talus replacement
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