Definition
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A patient-specific custom 3D–printed porous titanium implant for subtalar joint fusion to restore height, length, and alignment of the hindfoot.
Indications
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Calcaneus malunion/nonunion
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Subtalar fusion malunion.
Anatomy and pathogenesis
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The subtalar joint is comprised of the talocalcaneal joint and the talocalcaneonavicular joints.
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The subtalar joint allows for inversion and eversion of the hindfoot but also has a rotational component, allowing the talar head to internally rotate and drop down during eversion.
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Calcaneal malunion after a calcaneus fracture typically treated nonoperatively can lead to loss of height, length, varus malposition, widening of the heel, and joint incongruity.
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Posterior facet and articular surface malreduction can lead to subtalar arthritis.
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Lateral wall blowout leads to increased heel width.
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Loss of talar declination due to decreased calcaneal height leads to anterior ankle impingement.
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Varus malposition of the calcaneus leads to lateral column overload.
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Calcaneal malunion after calcaneus open reduction and internal fixation (ORIF) can cause similar issues as nonoperative treatment of calcaneus fractures. Although calcaneus fractures can be very challenging cases, not restoring appropriate heel alignment (out of varus) and calcaneal height and width, and restoring the articular surface can result in similar clinical findings as above.
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Peroneal dislocation is also commonly missed after calcaneus fractures.
Patient history and physical exam findings
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A history of calcaneus fracture treated nonoperatively or operatively.
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Pain and instability walking on uneven surfaces; pain along the sinus tarsi.
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Anterior ankle impingement pain.
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Pain/subluxation of peroneal tendons and with resisted eversion.
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Issues fitting into shoes due to heel width as well as subfibular impingement.
Imaging and other diagnostic testing
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Radiographs in the setting of malunion will demonstrate decreased talar declination, decreased hindfoot height, subtalar arthrosis, and varus heel position with increased calcaneus width on axial views, as seen in Fig. 12.1 .
• Fig. 12.1
Lateral radiograph and CT reconstruction demonstrating loss of calcaneal height, length, varus alignment, as well as anterior ankle impingement.
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Nonunion will demonstrate lack of callus or bony consolidation across the fracture site as well as subluxation of the subtalar joint or change in alignment.
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MRI can be useful to look at the soft tissue etiology of the pain, including peroneal tendonitis/dislocation as seen in Fig. 12.2 , cartilage injury in ankle and subtalar arthrosis, as well as bony edema resulting from impingement.
• Fig. 12.2
Axial CT slice with soft tissue window demonstrates dislocated peroneal tendons (white arrow) from the fibular groove
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CT is typically more useful in analyzing arthrosis and union/malunion. Weight-bearing CT allows for additional assessment of more accurate hindfoot alignment as well as impingement. There is even some, although limited, information about the soft tissue that can be learned from the CT scan such as dislocated peroneals with a soft tissue window.
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CT also is the imaging modality of choice for 3D-printing technology.
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Bony metabolic workup should be initiated in cases of nonunion including vitamin D, parathyroid hormone (PTH), and thyroid stimulating hormone (TSH).
Nonoperative management
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Shoe modification with a wide heel
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Orthotics, such as a solid ankle cushion heel (SACH)
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Bracing
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Oral and topical antiinflammatories
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Corticosteroid injections
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Physical therapy.
Traditional surgical management
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Malunion/nonunion revision ORIF: typically not useful, likely due to already rapidly accelerated subtalar arthritis as well as difficulty in restoring alignment.
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In situ subtalar arthrodesis: fusion can relieve pain from subtalar arthritis but does not restore alignment and height, resulting in similar problems as calcaneal malunion such as anterior ankle impingement, subfibular impingement, and lateral column overload. , ,
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Subtalar distraction osteogenesis: useful in restoring alignment but inconvenient for patients due to prolonged use of the external fixator.
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Subtalar distraction bone block arthrodesis with autograft/allograft: improves anterior ankle impingement by regaining height, and restores hindfoot alignment out of varus. Overall good radiographic and clinical outcomes but very technically difficult, with increased nonunion rates.
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Subtalar distraction arthrodesis with trabecular metal augment: a titanium wedge has equivalent outcomes but possibly stronger and better bony incorporation compared to autograft/allograft.
3D-printed implant and/or instrumentation considerations
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The main advantage in using a 3D-printed implant is the ability to preoperatively plan an already very challenging and technically difficult procedure. The hindfoot alignment correction can be dialed in using appropriate cuts and patient-specific custom wedges.
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Surgical time is likely to be considerably decreased due to cut guide and predesigned implants allowing for fewer intraoperative measurements/errors.
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A variety of proprietary porous structures are available. The senior author currently prefers a gyroid structure.
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The porous architecture allows for bony ingrowth.
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The wedge is designed larger medially to allow for correction out of varus.
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3D-printed implants allow for intended fixation designed into the implant, which also decreases surgical time and the ideal implant position. The senior author prefers to use a 7-mm–diameter MedShape DynaMini and a 4.0-mm Medline screw for subtalar fixation through the implant, as seen in Fig. 12.3 .
• Fig. 12.3
Proposed custom subtalar wedge featuring a porous gyroid structure as well as planned fixation with a 7 × 70-mm DynaMini hindfoot nail and a 4.0-mm Medline small cannulated screw.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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