Definition
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Custom/patient-specific 3D-printed metallic porous spacer for revision of first tarsometatarsal (TMT) joint arthrodesis nonunion.
Anatomy and pathogenesis
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The first TMT joint is a trapezoid-shaped joint with a complex anatomic configuration.
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It is well constrained by soft tissue structures but allows a certain amount of rotational and sagittal plane movement.
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The dorsalis pedis artery and deep peroneal nerve are encountered in the interval between the extensor hallucis longus and the extensor hallucis brevis.
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The dorsoplantar extent of the first TMT joint is approximately 30 mm.
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The medial cuneiform is approximately 1.5 cm wide.
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The neurovascular bundle traverses from dorsal to plantar within the first intermetatarsal 18 mm distal to the second TMT joint.
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The nonunion rate has been reported to range from 4% to 12%.
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Transfer metatarsalgia, hardware pain, and infection may also occur after Lapidus bunionectomy.
Patient history and physical exam findings
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Clinical nonunion is defined as a painful, swollen arthrodesis site 6 months postoperatively.
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Assess for location of pain.
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Degree of recurrent deformity.
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Consider overall foot alignment.
Imaging and other diagnostic testing
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Radiographs:
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Nonunion may be represented by a lack of callus and/or trabeculation along the osteotomy or fusion site ( Fig. 18.1 ), bony sclerosis, change in alignment, subluxation/dislocation of the capital fragment, and hardware failure.
• Fig. 18.1
Preoperative anteroposterior X-ray demonstrating a nonunion of the first tarsometatarsal joint.
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Evidence of osteomyelitis: osteopenia, radiolucency, lytic changes/cortical erosion, trabecular destruction, bone necrosis, brodie abscess, sclerosis, and periosteal reaction.
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CT scan:
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CT is often more helpful in staging avascular necrosis (AVN) as well as deciphering between fusion and nonunion, and implant loosening.
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CT is often more 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 and/or metatarsal pads
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Immobilization
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Rest, ice, compression, elevation (RICE) therapy and nonsteroidal antiinflammatory drugs (NSAIDs)
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A bone stimulator for the treatment of aseptic nonunion.
Traditional surgical management
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In situ fusion:
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This results in acute shortening and consequential biomechanics.
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First TMT joint fusion with bone block:
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Graft incorporation and fusion on both sides of the graft is required. This may increase the time to union and nonunion rate. An autograft may be used in most cases, but depending on the size of the bone defect, allograft may be considered.
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Autograft or allograft may be used in a staged manner using the Masquelet technique.
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Distraction osteogenesis:
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This requires an extended time with an external fixator, which is associated with pin site infection and poor patient satisfaction. Bone regenerate formation may be compromised in patients with comorbidities such as tobacco use or diabetes.
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Amputation:
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Partial foot amputation may be considered in situations in which there is severe infection, trauma, or failure of multiple revisions.
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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/oblong-shaped implant is recommended ( Fig. 18.2 ), which promotes a larger surface area of fusion and enhanced interposition of the implant to bone.
• Fig. 18.2
Image of a 3D-printed custom implant. Note the porous design to promote osseous ingrowth as well as the truss design to serve as a lattice for osseous incorporation through the implant.
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Inherent stability is provided by the rough texture, which assists in resisting torsional forces.
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The porous surface is in direct contact with the adjacent bone and areas of smooth/polished surfaces to prevent soft tissue ingrowth and contracture.
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Diameter, width, length, and height are patient specific and are based on the size at the proposed resection level ( Fig. 18.3 ):
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The implant can be designed with screw holes to allow direct implant-to-bone fixation; however, the more screws used, the less surface area is available for bone ingrowth and fusion.
• Fig. 18.3
Preoperative computer-aided design images demonstrating the dimensions of the implant and screw holes that should be measured during the preoperative planning stage.
(Reprinted with permission from 4WEB Medical, Frisco, TX.)
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A preoperative CT scan is obtained to guide surgical planning:
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Images as scanned are used to gain an appreciation for the amount of bone resection required ( Fig. 18.4 ).
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A 3D-printed solution for evans calcaneal osteotomy nonunion
3D-printed total cuboid replacement for lateral column pathology
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