Definition
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Custom/patient-specific 3D-printed lesser metatarsal bone replacement is based on measurements generated by a CT scan of the ipsilateral and/or contralateral lesser metatarsal bone.
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It is used for the 2nd through 5th lesser metatarsal/metatarsophalangeal joint (MTPJ) to return normal biomechanical function and range of motion.
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It is indicated in:
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Avascular necrosis of the metatarsal head
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Inflammatory arthritis affecting the MTPJ
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Metatarsophalangeal joint osteoarthritis
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Primary tumors affecting the lesser metatarsal bones
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Failed lesser metatarsal osteotomy with severe bone shortening.
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It is contraindicated in active infection, severe peripheral neuropathy, poor skin and soft tissue coverage, and ischemic foot.
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In this chapter, custom 3D-printed lesser metatarsal replacement and polyvinyl alcohol (PVA) implants will be discussed.
Anatomy of the 2nd to 5th lesser metatarsophalangeal joints
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While standing in an upright position, all metatarsal heads weight bear at the same level and the metatarsophalangeal joints are slightly extended.
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The lesser MTPJs are crossed dorsally by the extensor digitorum longus and brevis tendons, which both dorsiflex the digits during the swing phase of gait, and ventrally by the flexor digitorum longus and brevis tendons, which both apply passive forces across the joint during plantarflexion.
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The 2nd MTPJ has two dorsal interosseous muscles, while the other lesser MTPJs have both dorsal and plantar interosseous muscles for joint stabilization in the transverse plane.
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The plantar aspect of the metatarsal head has a rounded contour while the medial and lateral aspects have notched contours. The plantar plate is a fibrocartilaginous capsular thickening made of type 1 collagen observed from the metatarsal neck to the proximal phalanx base. Each MTPJ has a medial and lateral collateral ligament.
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MTPJs can move in a biaxial direction, including greater sagittal plane movement and a little transverse plane movement. The joints demonstrate hyperextension to 90 degrees and flexion of 30 to 50 degrees.
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The metatarsal heads have two main arterial sources, the dorsal metatarsal arteries from the dorsalis pedis artery and the plantar metatarsal arteries from the posterior tibial artery. Small branches of the arterial network enter the metatarsal head distally. These branches run close to the metaphyseal capsular and ligamentous insertion.
Pathogenesis
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There are many etiologies for metatarsophalangeal pain which include trauma, capsulitis (predislocation syndrome), overuse injury, stress fracture, avascular necrosis (Freiberg disease), osteoarthritis, and inflammatory diseases.
Predislocation syndrome
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Capsulitis is a slowly progressing condition where structures that create stability in lesser MTPJs begin to degenerate. It represents a continuum of pathological processes from acute capsulitis to plantar rupture.
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The most probable etiologies include congenital predisposition, aging, female sex, and inappropriate fitting shoes.
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As the disease progresses, the digit may become elevated and the 2nd MTPJ may deviate dorsomedially over the hallux. Chronic dislocation can potentially lead to arthritic changes in the metatarsal head.
Freiberg disease
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Osteonecrosis of the 2nd metatarsal head is most common in female adolescents.
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The etiology is multifactorial, but the main causes include vascular compromise, trauma due to an elongated 2nd metatarsal length, and hypercoagulable inflammatory conditions.
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Staging is described by the Smillie classification, with five stages outlined based on the structural changes of the metatarsal head.
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A vascular pattern with collateral branches supplying the 2nd metatarsal and absent 2nd metatarsal arteries could be a predisposing factor for Freiberg disease.
Rheumatoid arthritis
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During the first 3 years of the disease, 65% of patients will have involvement of the lesser MTPJs.
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Among patients with chronic polyarthritis, 67% will have subluxation and dislocation of the lesser MTPJs.
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The local immunological response causes synovial hyperplasia and angiogenesis in the periarticular recesses. Later, this reaction extends into the articular cartilage and subchondral bone. The synovial inflammation disrupts the MTPJ collateral ligaments, capsule, and intrinsic muscles, which leads to classic rheumatoid forefoot deformities.
Osteoarthritis
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Primary degenerative osteoarthritis of the MTPJ is rare.
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Trauma, MTPJ instability, increased bodyweight, age, and genetics are more common causes of osteoarthritis.
Patient history and physical exam findings
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The clinical presentation for MTPJ pathology depends on the etiology.
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Pain with weight bearing, limited range of motion, tenderness, and palpable grinding are common symptoms in late-stage osteoarthritis of the MTPJ (second toe rigidus).
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In complex hammer toe deformity, patients present with progressively worsening toe deviation, plantar callosities over the metatarsal head, and severe pain when walking barefoot.
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Early-morning joint stiffness is common in patients with rheumatoid arthritis.
In Freiberg disease, effusion and tenderness are common findings on physical exam. Palpable grinding with a passive range of motion and toe deformity become more prominent as the disease progresses.
Imaging and other diagnostic testing
Radiographs
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Three weight-bearing views of the foot are required to properly assess the MTPJ ( Fig. 21.1 ).
• Fig. 21.1
Anteroposterior and lateral views of the right foot showing an initial presentation of the patient 6 months after a 2nd metatarsal head resection and proximal interphalangeal joint resection. He was experiencing severe pain on the plantar aspect of his second ray.
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Subluxation or dislocation of the MTPJ are seen on oblique views.
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During progression of Freiberg disease, the first sign on imaging is widening of the joint space. Late in the disease process, central joint depression followed by an area of rarefaction surrounded by a sclerotic rim is seen proximal to the subchondral bone. Metatarsal shaft thickening can be a late finding as well. Flattening of the metatarsal head may appear only on the oblique view.
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Early radiological findings of rheumatoid arthritis include bone erosions. Symmetrical or concentric joint space narrowing may be seen later on.
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Typical radiological findings of osteoarthritis include joint space narrowing, the presence of osteophytes and subchondral cysts, and an increase in the metatarsal head size early in the disease.
Magnetic resonance imaging
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When standard radiographs appear normal, MRI can assist in detecting Freiberg disease as bone marrow changes are visualized early in the disease.
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In rheumatoid arthritis early signs of bone edema, synovitis, and erosions are seen on MRI.
Other imaging modalities
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Nuclear medicine, CT scan, and ultrasound have been reported to be helpful in rheumatoid arthritis diagnosis.
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A preoperative CT scan can help determine the diameter of the metatarsal head for surgical planning.
Laboratory testing
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Rheumatoid arthritis blood testing detects the presence of rheumatoid factor, anticyclic citrullinated peptide antibody (anti-CCP), and erythrocyte sedimentation rate (ESR). Anti-IL1 alpha is associated with less severe joint disease.
Nonoperative management
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A conservative treatment modality depends on the etiology causing the MTPJ disease. Nonoperative treatment options include protected weight bearing, oral antiinflammatory medications, and lifestyle modifications that include avoidance of high-impact sports and shoe modifications.
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Shoe orthotics with metatarsal pads are effective for unloading the metatarsal heads.
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Hammer toe with MTPJ dislocation can be treated with splinting (Budin splint).
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In the early stages of Freiberg disease, a period of non–weight bearing could prevent the disease progression.
Traditional surgical management
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Recommended surgical interventions differ based on the etiology of MTPJ disease. In mild-to-moderate Freiberg disease, joint-sparing procedures include joint debridement with cheilectomy and arthroscopic or open removal of loose bodies and osteophytes. A few studies have reported promising results with core decompression of the metatarsal head using 1.1-mm Kirschner wire. Bone grafting and transfer of the femoral condyle osteochondral plug have also been successfully reported. Intraarticular osteotomies have been used to remove the affected cartilage dorsally and realign the plantar cartilage. Extraarticular osteotomies have been reported either at the metatarsal neck or the shaft to allow better fixation and controlled shortening.
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In more advanced MTPJ disease, a spectrum of procedures has been described. Interpositional soft tissue arthroplasty using an autograft (tendons and joint capsule) have shown successful outcomes. An allograft (acellular dermal matrix) has also been studied with no long-term follow-up. An osteochondral distal metatarsal allograft replacement has demonstrated satisfactory outcomes in a cases series.
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Multiple implants have been used to replace the lesser MTPJ with sufficient pain relief, but inadequate clinical outcomes owing to a high associated complication rate including transfer metatarsalgia and implant failure.
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When compared to previous studies, recent studies have shown improved surgical outcomes on lesser MTPJ arthrodesis for MTPJ instability in the setting of a complex hammer toe deformity. Potential adverse outcomes include difficulties with running and altered gait mechanics.
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Resection arthroplasty of the metatarsal head has been described for rheumatoid forefoot with promising outcomes, but the procedure may be associated with iatrogenic metatarsalgia.
Implant design specifications and considerations for lesser MTPJ replacement
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3D-printed lesser metatarsal replacement:
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Cobalt chromium implants.
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CT scan images are used to assemble a 3D model and then build the final implant.
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The base of the implant has a porous surface for bone growth.
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The implant is to be fixed to the remaining metatarsal shaft using a long intramedullary peg and a dorsal plate with the screws interconnecting the peg.
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4 to 6 weeks are required to have the final implant available.
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PVA implant:
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Cartiva is a synthetic cartilage implant (Cartiva, Alpharetta, GA) made from PVA and saline, which has been used for the treatment of 1st MTP arthritis.
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This implant is much softer than metal spacers and has similar levels of water content and tensile strength (17 MPa) as native human cartilage.
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After first being used in Europe in 2002, the Cartiva implant was utilized in Canada and Brazil. Osteochondral defects in the knee, talus, and 1st MTPJ have been treated with the Cartiva implant.
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The advantages of a PVA implant compared with other hemiarthroplasty modalities include reduced friction under start-up conditions, decreased inflammation compared with ultra-high–molecular weight polyethylene used in standard prosthesis, and limited debris in articulation with cartilage compared with stainless steel.
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The implant is available in both an 8- and 10-mm diameter.
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Bone support of at least a 2-mm bone rim is necessary to provide adequate stability for the implant.
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Surgical technique
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3D-printed lesser metatarsal replacement:
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Positioning: supine.
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Anesthesia: general and regional.
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Surgical approach: a direct dorsal approach to the metatarsal shaft extending to the MTPJ is used as follows:
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A midline skin incision is made 6 cm proximal to the MTPJ and extended for 2 cm distal to the joint.
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A sharp dissection down to the extensor tendon sheath is made with a scalpel.
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The extensor tendon sheath is opened and the tendon is retracted laterally.
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The joint capsule is opened longitudinally with a scalpel.
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The capsular flaps are raised medially and laterally.
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The metatarsal shaft is exposed. A metatarsal osteotomy to be performed based on the preoperative planning. The implant can be used to mark the level of osteotomy intraoperatively ( Fig. 21.2 ).
Related posts:
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Material science for 3D printing in medicine
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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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