Definition
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Utilizing patient-specific custom 3D-printed metallic navicular cages for the treatment of chronic navicular pathology in sensate patients without Charcot neuroarthropathy.
Anatomy and biomechanics
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The navicular is an irregular-shaped tarsal bone which has a concave surface that articulates with the talus and a convex surface that articulates with the cuneiforms.
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There are ligamentous and capsular attachments on all four sides of the navicular, while there is a solitary tendon attachment. The posterior tibial tendon attaches to the navicular tuberosity and acts as a dynamic stabilizer of the rearfoot during pronation while providing supination during push-off or propulsion.
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The navicular is a vital part of the medial column, which also includes the 1st metatarsal, medial cuneiform, and the talus. These bones and corresponding articulations are important for maintaining foot structure and function. It is essential for the medial column to operate as a rigid lever for normal biomechanical function during propulsion.
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The alignment of the medial column radiographically is assessed on:
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Lateral projections via Meary’s angle, which may be defined as normal where the axis between the midline of the talus and 1st metatarsal is 0 degrees. Abnormal values may be considered when the axis is >4 degrees. Increased angles downward may be described as medial column sag, which infers inability of the medial column to act as a rigid lever during propulsion.
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Anteroposterior (AP) projections via talar-navicular coverage, which compares the articular surface of the talar head and that of the proximal navicular. An axis of >7 degrees is considered abnormal and consistent with transverse plane deformity that occurs in pes planus.
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Vascular supply to the navicular primarily comes from the dorsalis pedis artery, which supplies dorsal arterial supply via three to five terminal perforators, and the medial plantar artery, which provides plantar vascularization. Abundant vascular supply from these two networks is supplied predominantly to the medial and lateral superficial of the navicular. The navicular however lacks substantial vascular supply to the deep and central portions. This anatomic discrepancy leads to higher risk of stress fracture and avascular necrosis (AVN) within the central navicular.
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Nervous innervation is provided from the medial plantar and deep peroneal nerves.
Pathogenesis
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An array of etiologies exists for navicular pathology which include AVN, acute fractures, and stress fractures, all of which have a poor prognosis.
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Köhler disease, a rare idiopathic AVN of the navicular, most commonly occurs in children aged 4 to 7 years. The poor blood supply to the central third of the talus paired with the increasing weight of the child is thought to increase the chance of Köhler disease. Fortunately, the process is self-limiting and surgical intervention is rarely necessary.
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Mueller-Weiss syndrome is also a spontaneous, idiopathic form of navicular AVN; however, it occurs in adults most commonly between the fifth and seventh decade of life. Theories regarding the development of Mueller-Weiss syndrome include congenital etiologies and chronic microtrauma secondary to poor biomechanics, which lead to overload of the lateral third of the navicular which tends to be less ossified. Radiographically, the lateral collapse of the navicular appears as a comma-shaped navicular.
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Navicular stress fractures compromise 10% to 35% of all stress fractures and are most commonly seen in young, active individuals.
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Regardless of etiology, AVN is secondary to disruption of the vascular supply to the navicular.
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The progression of AVN, regardless of etiology, can be monitored radiographically. Ficat and Arlet classified femoral head AVN, and later Mont et al. modified the classification for the talus. Despite no classification specific for navicular AVN, descriptions are often similar to other bones.
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Medial column shortening, transverse forefoot deformity, and loss of arch height may be observed in late-stage navicular AVN due to progressive collapse of the navicular.
Patient history and physical exam findings
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Navicular pathology is associated with pain and swelling due to a localized inflammatory process.
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Dysfunction and pain are worse with activity.
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Vague midfoot pain, often with normal subtalar and ankle range of motion, is observed.
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Navicular fractures may occur secondary to a known acute injury, whereas chronic overuse with high functional demands often presents with navicular stress fractures.
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Deformity of the forefoot, midfoot, and hindfoot occurs in late-stage navicular AVN ( Fig. 14.1 ).
• Fig. 14.1
Anteroposterior (AP) and lateral radiographs of a patient with late-stage avascular necrosis with resultant deformity in the transverse and sagittal planes.
Imaging and other diagnostic testing
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There are four types of navicular fractures: cortical avulsion, tuberosity avulsion, body, and stress fractures. Sangeorzan et al. classified acute navicular fractures based on orientation of the fracture line, adjacent joint involvement, and the presence of foot deformity.
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Stress fractures most often occur in the central third of the navicular body. If a high index of suspicion for navicular stress fracture exists despite unremarkable initial X-rays, then consideration is given to MRI or CT evaluation.
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Navicular AVN progression ( Fig. 14.2 ):
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Initially plain radiographs may appear normal but alterations are visible on MRI, thus MRI remains the most reliable diagnostic test for stage 1 AVN.
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Osteosclerotic and cystic changes without navicular collapse are observed at stage 2. Often contralateral views can be helpful to determine if early collapse is present.
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Subchondral lucency may be indicative of subchondral collapse, which is characteristic of stage 3 AVN. During this stage the adjacent joints are spared and subtle deformity may be present.
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Stage 4 characteristically displays collapse, deformity of the forefoot with or without hindfoot involvement, and adjacent joint degeneration.
• Fig. 14.2
The progression of navicular avascular necrosis on CT images. (A) Osteosclerotic changes without collapse is indicative of stage 2. (B) Subchondral lucency and early collapse is noted in stage 3. (C) Significant collapse with adjacent joint changes is evident in stage 4. Stage 1 may appear normal on radiographs and CT.
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For surgical planning purposes, the authors recommend CT and MRI evaluation to determine the extent of AVN.
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If there is a possibility of infection, further work-up may include laboratory assessment, including inflammatory markers and bone biopsy.
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If a previous procedure resulted in a nonunion, then further work-up should be considered and possible systemic causes such as osteoporosis and endocrine etiologies may need to be assessed. The patient should be optimized appropriately. This may include smoking cessation and nutritional optimization.
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Noninvasive vascular studies are recommended for primary and revision cases. Pending results, the patient may benefit from further endovascular optimization.
Nonoperative management
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Nondisplaced acute or stress fractures are often initially treated nonsurgically; however, considerable debate remains due to a high risk of nonunion, refracture, and AVN.
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Nonoperative management may be successful in early-stage AVN. This may involve immobilization with a period of non-weight bearing followed by protective weight bearing with noninvasive bone stimulation and/or extracorporeal shockwave therapy.
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Close follow-up with serial imaging is recommended.
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In the authors’ experience, nonoperative management tends to be less successful with symptomatic late-stage AVN, especially in younger and physically active patients.
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Only low-level evidence exists for operative and nonoperative management of AVN in the foot and ankle.
Traditional surgical management
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When nonoperative treatment fails to improve symptoms or there is continued progression of AVN, operative treatment may be considered.
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Operative intervention for nondisplaced fractures has been shown to lower rates of delayed union, nonunion, and AVN; however, potential risks and complications are possible. ,
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Core decompression may be considered for early-stage (without collapse) navicular AVN; however, poor efficacy due to the tenuous blood supply may exist.
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Vascularized bone grafting for several donor sites has been described with some success; however, this may be limited to early-stage AVN if preservation of joint function is desired. If arthritis is present, fusion should be strongly considered.
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When fusion is indicated, necrotic bone must be thoroughly debrided, which may be approximated on CT and MRI. In the authors’ experience this leads to a considerable osseous deficit, in which case in situ fusion (talar-cuneiform) is a poor option, as this would lead to considerable deformity.
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Prior to the advent of 3D printing, the authors would employ bridge plating with bone grafting to the medial column to preserve length. Often the osseous deficit being grafted is >2 to 3 cm, therefore the concern for nonunion is high and these cases remain at high risk for failure and need for multiple surgeries.
3D-printed implant considerations
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In the United States, titanium (Ti) alloys and cobalt chromium (CoCr) are currently the ideal biocompatible materials for 3D printing. Typically, CoCr is used when the implant articulates with native cartilage or polyethylene components used in joint replacement. Ti has a higher coefficient of friction, therefore making it inferior to CoCr for articular components.
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When osseous integration into the implant is desired such as during an arthrodesis procedure, Ti tends to be used more often. Research continues to explain osseous integration abilities of CoCr compared to Ti.
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A major limitation in the use of CoCr implants is the lack of complementary fixation methods and mixing different types of metals is to be avoided. That is, screws, plates, and intramedullary fixation is not available in CoCr. On the contrary, Ti fixation is widely available, therefore if supplemental fixation is necessary then Ti 3D-printed implants are preferred.
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If corrective osteotomies are planned, the surgeon may consider using a patient-specific cut guide to improve accuracy and reduce surgical time.
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Basic implant design considerations:
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If osseous integration is desired, the authors recommend porous Ti implants to preserve the length and alignment of the medial column while promoting biologic stability through the bony ingrowth into the implant.
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Ti implants with a rough-surface finish provide additional advantages. The rough surface increases friction at the bone-implant interface, thus making the implant more stable.
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Ideal porosity has yet to be determined. Ultimately, the porosity should provide a network of structural strength and permit the packing and retention of the bone graft or biologics of choice ( Fig. 14.3 ).
• Fig. 14.3
3D-printed titanium implant designed for medial column fusion which is packed with bone graft and biologics. The porosity allows for bone graft retention and osseous integration. This implant was designed to be fixated with multiple 3.5-mm screws.
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The implant should be smooth and polished to decrease friction where the implant is to articulate with native, adjacent joint cartilage. Although CoCr would be preferable at the articulation, there is no CoCr off-the-shelf fixation available. This may represent an area of future advancement ( Fig. 14.4 ).
• Fig. 14.4
3D-printed titanium navicular implant with porous areas with surface roughness in locations of desired osseous integration, while smooth areas are close in proximity to soft tissues that could be affected by the implant. This implant has a 9-mm hole to accommodate a midfoot beam.
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The implant shape is patient specific and should replicate the osseous deficit created after aggressive bone debridement.
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Sizing options should mimic the minimum and maximum amount of bone to be excised. In the authors’ experience, for navicular AVN the implant will vary based on proximal to distal length alone. In other words, the medial-to-lateral width remains constant while the implant length may vary based on osseous resection and resultant deficit as well as adjunctive procedures. Medial column length, in absolute measurement as well as compared to lateral column length, may be determined based on contralateral CT scans preoperatively ( Fig. 14.5 ).
• Fig. 14.5
Computer-aided design images depicting three sizes of a navicular implant. This implant’s width (medial to lateral) remains constant while it varies in length (anterior to posterior).
(Reprinted with permission from restor3d, Inc, Durham, NC.)
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The authors recommend requesting a minimum of three sizes to ensure ideal fit and alignment restoration.
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Further implant specifications may be dependent on the osseous involvement and adjacent joint arthritis. In the authors’ experience, the need for 3D-printed navicular implants most often presents in one of three patterns. The basic implant design and procedure selection is often dictated on the pattern of pathology ( Table 14.1 ).
TABLE 14.1
Indications for Navicular Implants Commonly Present in Three Different Patterns, Which May Aid in Implant Design and Procedure Selection
CAD images depicting implant examples reprinted with permission from restor3d, Inc, Durham, NC.
AVN , Avascular necrosis; TNJ , talonavicular joint.
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Pattern 1: An osseous deficit limited to the navicular without degeneration to the talar head with or without cuneiform degeneration:
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This situation may be seen in Ficat/Mont stage 2 or 3 navicular AVN.
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Implant design features include a smooth, concave surface of the proximal navicular, which articulates with the talar head and porous distal surface to promote osseous integration from the cuneiforms.
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Fixation may include holes within the implant body as well as flanges to allow passage of screws into the cuneiforms. Osseous integration serves to stabilize the implant but is inherently limited to one surface, therefore the soft tissues are vital to implant stability.
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Attachment of the posterior tibial tendon to the implant is vital for function. Adams and Danilkowicz recommended a tunnel within the implant from the medial aspect to the dorsal aspect of the implant to allow passage of the posterior tibial tendon, which can be anchored to the cuneiforms. Additional eyelets can be incorporated into the implant to allow attachment of adjacent ligaments and capsular tissue. The footprint of the ligaments may be roughened to promote soft tissue ingrowth and scar formation.
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Pattern 2: An osseous deficit limited to the navicular with degeneration of the talar head and cuneiforms:
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This situation may be seen with stage 4 navicular AVN, nonunion of navicular fracture, nonunion of previous talonavicular joint (TNJ) fusion, or osteomyelitis limited to the navicular.
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The navicular implant is designed to recreate medial column length and alignment. Unlike Pattern 1 implants, soft tissue attachments are much less important given the much larger surface area available for osseous integration.
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Porosity for osseous integration should be designed on all surfaces except where scar contracture and vital soft tissues are adjacent to the implant.
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Obtaining adequate fixation in the talus can be challenging but is paramount to providing stability of the overall construct. The surgeon should consider intramedullary beams or external fixation for Pattern 2 implants.
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From the authors’ experience, spanning the entire medial column from talus to 1st metatarsal is preferable, even in sensate patients. Attempts to preserve the first tarsometatarsal joint have led to inadequate fixation, hardware breakage, and the need for revision.
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Pattern 3: Osseous deficit involving multiple bones including the navicular with adjacent joint arthritis:
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This situation may be seen subsequent to severe rearfoot/ankle trauma, nonunion of previous fusions, AVN, and osteomyelitis involving multiple rearfoot/ankle bones.
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Pattern 3 cases may require vastly more bone debridement and excision, which may be staged especially if the pathology is secondary to chronic infection.
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The implant size and shape should reflect the deficient created. These implants tend to span a much larger area. Fixation should therefore be robust.
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Similar to Pattern 1 and 2 implant designs, abundant porosity should exist at areas of osseous integration, and smooth surfaces should exist where articulating with native cartilage or areas of concern for undesirable soft tissue ingrowth and contracture.
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Fixation methods are surgeon and pathology dependent:
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The authors recommend using a 3D-printed cage-type implant to accommodate the osseous deficit, which may have holes for screw fixation. Screw fixation from the implant to the adjacent bone provides compression at the bone-implant interface and enhanced stability of the implant; however, it decreases the surface area available for osseous integration into the implant. Screw trajectory is determined in the preoperative planning stage and additional guides may be created to ensure the desired placement is achieved.
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Solid screws may provide higher fatigue strength compared to cannulated screws. Using a cannulated technique is recommended when using solid screws to confirm the trajectory fluoroscopically and compared to preoperative computer-aided design (CAD) images.
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The authors typically use a combination of 3.0, 3.5, and 4.0 mm screws.
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Additional supplemental fixation is recommended. Off-the-shelf traditional Ti internal fixation such as spanning locking plates, intramedullary beams, and external fixation may be considered.
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Surgical management with 3D-printed devices
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The authors recommend general anesthesia with regional nerve block for this procedure.
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The patient is placed supine with an ipsilateral hip bump and thigh tourniquet.
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Treatment of navicular AVN without TNJ arthritis (Pattern 1) with TNJ sparing navicular replacement :
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A dorsomedial approach is recommended to allow access to the posterior tibial tendon. If visualization of the lateral navicular is difficult, then a dorsal lateral/sinus tarsi incision should be considered.
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Once the navicular is exposed, it is sharply released of soft tissue attachments including the posterior tibial tendon, which is tagged.
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The authors recommend using a patient-specific cutting guide and sagittal saw to remove the cuneiform bases, which aids in implant fit and determining the fixation trajectory ( Fig. 14.6 ).
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