Definition
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A custom/patient-specific 3D-printed porous metallic spacer for revision of Evans calcaneal osteotomy nonunion.
Anatomy and pathogenesis
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With Evans calcaneal osteotomy the foot follows the displacement of the calcaneocuboid joint, suggesting that the long plantar ligament acts as a pivot, the talonavicular joint as the axis of rotation, and the spring ligament as a hinge.
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This 3D movement allows the peroneus longus tendon to recover its plantarflexory effect on the first metatarsal. These changes and the recovery of the windlass mechanism of the plantar fascia increase the bowstring effect, which restores the longitudinal arch and increases the stability of the osteotomy.
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The rich vascularization of the calcaneus and press-fit nature of the graft allows for a relatively low rate of nonunion.
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In a systematic review of 73 feet with a mean age of 22 years and an average follow-up of 3.6 years, Prissel and Roukis found that the incidence of nonunion in unfixated Evans osteotomies was 1.4%. Other studies have shown similar incidences, as Haeseker et al. reported a nonunion rate of 5.3%.
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Other complications of the Evans calcaneal osteotomy include , :
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Dorsal displacement of the anterior calcaneal tuberosity
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Lateral column pain
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Loss of calcaneal lengthening
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Overcorrection
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Undercorrection.
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Structures at risk :
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The sural nerve
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Peroneal tendons
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Anterior and middle facets of the subtalar joint.
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Patient history and physical exam findings
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Assess for location of pain.
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Assess the function of the posterior tibial tendon.
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Assess for undercorrected or recurrent forefoot varus, hindfoot valgus, medial ankle instability, subfibular impingement, or forefoot abduction.
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Assess the range of motion to the subtalar, talonavicular, and ankle joints.
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Assess for contractures to the gastrocsoleus and peroneal tendons.
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Obtain prior operative records to understand the size of graft used and type of hardware implanted.
Imaging
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Radiographs: nonunion may be represented by :
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lack of callus and/or trabeculation along osteotomy site ( Fig. 16.1 )
• Fig. 16.1
Preoperative radiographs of a nonunion of an Evans calcaneal osteotomy.
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bony sclerosis
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loss of correction of the hindfoot joints
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subluxation/dislocation of the capital fragment
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hardware failure.
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CT is useful in assessing for the presence of nonunion, implant loosening, and overall alignment of the hindfoot:
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CT is helpful in surgical planning and procedure selection.
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Currently, CT is the imaging modality of choice if 3D printing technology is being utilized.
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Nonoperative management
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Shoe modification: a wide, rigid toe box
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Orthotics/shoe modifications
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Immobilization
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Rest, ice, compression, and elevation (RICE) therapy and nonsteroidal antiinflammatory drugs
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Bone stimulator for the treatment of aseptic nonunion.
Traditional surgical management
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Revision:
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Results in retained motion through the talonavicular and subtalar joints.
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Must consider the size of the prior graft.
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Must consider local anatomic constraints such as the calcaneocuboid joint.
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Autograft may be obtained from the iliac crest; however, donor site morbidity is a significant concern.
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Cancellous autograft from the distal or proximal tibia.
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Allograft and other biologics may be considered.
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Revision with calcaneocuboid joint arthrodesis:
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Due to the location of the Evans calcaneal osteotomy, a revision may require inclusion of the calcaneocuboid joint for adequate bone stock and fixation purchase.
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This results in limited range of motion through the talonavicular and subtalar joints by 48% and 30%, respectively.
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Triple arthrodesis:
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Allows for correction of any residual hindfoot deformity, particularly if there is degenerative arthritic disease or if the deformity has become rigid.
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Allows a stable, plantigrade hindfoot during weight bearing.
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Sacrifices motion through the hindfoot.
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May be performed in combination with revision of the osteotomy.
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Ancillary procedures:
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Consider the need to perform a gastrocnemius recession, Cotton osteotomy, posterior tibial tendon debridement and repair, FDL tendon transfer, peroneal brevis lengthening, or spring or deltoid ligament reconstruction to address any residual deformity.
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3D-printed implant and instrumentation considerations
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Titanium has been shown to have osteoconductive properties and is biologically inert, therefore titanium is the material of choice , :
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Pore size has been described in the literature from 300 to 1000 μm.
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Truss configurations provide the greatest strength with the least mass. These allow for a mechanically robust construct that resists the load-bearing forces of the foot and ankle. The truss configuration acts as a lattice for bone grafting and other orthobiologic material.
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The 3D configuration of the implant must be designed to allow it to be successfully implanted through the planned surgical exposure, cognizant of the constraints imposed by the local anatomy and previous implants.
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The length of the implant is case specific and based on the proposed resection level. The length of osseous deficit after resection of nonviable bone and preparation for fusion determines the nominal length. Custom cutting guides may be created and used to ensure the resection is accurate.
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Degree of porosity: the percentage of porosity is inversely proportional to implant strength. This should be considered during implant design.
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A rectangular/trapezoidal-shaped implant is recommended ( Fig. 16.2 ), which promotes a larger surface area for fusion and enhanced interposition of the implant to bone.
Related posts:
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Material science for 3D printing in medicine
Distraction subtalar joint fusion using a custom 3D-printed implant
Custom constrained 3D total talus/navicular replacement
3D-printed total cuboid replacement for lateral column pathology
3D-printed custom hemi-talus replacement
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