Definition
- •
Patient-specific 3D custom-printed metallic constrained total talus replacement includes modifications to implant design and technique in order to incorporate subtalar arthrodesis.
Anatomy
- •
The talus, through its articulations, is the link between foot and ankle motion.
- •
The talus is covered by approximately 60% cartilage, with little area remaining for soft tissue attachments or penetration of blood supply. ,
- •
The neck of the talus allows for ligament attachment such as the deltoid and the anterior talofibular, which aid in ankle stability. It also contains the interosseous talocalcaneal ligament on its undersurface for portions of subtalar joint stability. A majority of the blood supply to the talus enters through this region, making it vulnerable during injury and surgery.
- •
The posterior tubercles of the talus allow posterior ligaments such as the posterior talofibular and components of the deep deltoid to aid in ankle stability, as well as the posterior talocalcaneal ligament of the subtalar joint.
- •
Medial support of the subtalar joint comes from the corresponding talocalcaneal ligament, while the lateral talocalcaneal ligament is aided in its support by the calcaneal fibular ligament.
- •
The blood supply to the talus is fragile and easily compromised by trauma and surgery. ,
- •
The main blood supply to the body of the talus is from the posterior tibial artery and its branches through the medial deltoid, in addition to its supply to the artery of the tarsal canal. ,
- •
The dorsalis pedis artery supplies the dorsal neck and sinus taris region of the talus through the artery of the tarsal sinus.
- •
The perforating peroneal artery supplies blood to the posterior body as well as to the tarsal sinus plexus.
- •
The inferior surface of the talus contains facets for articulation with the calcaneus to form the subtalar joint.
- •
The subtalar joint has an oblique axis that allows for supination and pronation, working in conjunction with the ankle, talonavicular, and calcaneal cuboid joints.
- •
The subtalar joint axis runs anterior and superomedial from the posterolateral tubercle toward the neck of the talus and is dynamic, changing as the joint progresses through motion.
- •
The average axis of the subtalar joint is 16 degrees from the sagittal plane, 42 degrees from the transverse plane.
- •
The accepted range of motion of the subtalar joint is approximately 30 to 40 degrees total, with it divided one-third eversion and two-thirds inversion from neutral.
- •
Subtalar joint fusion can be a useful procedure and is performed in deformity corrections, to decrease pain in arthritis, to aid in rearfoot stability, and to provide a stable platform for the talus in complicated reconstructions.
- •
Altered mechanics are still debated after subtalar fusion, with conflicting data. Long-term arthritic changes may not be as definite as once thought but pressure studies show loading shifts after fusion, which may have implication on the ankle joint or implant arthroplasty following fusion. The effects on adjacent joints need to be considered when incorporating subtalar joint fusion into constrained total talus replacement.
Pathogenesis
- •
Large talar deficits that may necessitate the need to incorporate subtalar arthrodesis include:
- •
Severe osteoarthritis including both the ankle and subtalar joints.
- •
Avascular necrosis of the talus with concomitant subtalar arthritis or deformity.
- •
Other destructive pathology with extensive cystic changes to adjacent bone, such as other arthritides, hemophilic joint pathology, and neoplastic processes.
- •
Failed arthrodesis with subsequent bone loss.
- •
Failed total ankle arthroplasty with component subsidence violating the subtalar joint.
- •
- •
Trauma is the most common cause of talar avascular necrosis (AVN), occurring in approximately 75% of cases, with medication and idiopathic trauma also being described. ,
- •
Arthritides such as rheumatoid arthritis and osteoarthritis can affect both the ankle and subtalar joints leading to cystic changes and joint destruction, which in some cases involves both the ankle and subtalar joints concurrently.
- •
With the increasing number of total ankle joint arthroplasties being performed annually, more complications are being reported. The development of revision techniques is essential and, for certain cases, total talar replacement is a possibility. For implants that have violated or subsided into the calcaneus and subtalar joint, incorporating subtalar arthrodesis has been a valuable tool to add fixation and provide a stable platform for the revision.
- •
Talar deficiency is a challenging dilemma, with surgical options including talectomy, tibiotalocalcaneal arthrodesis with or without bone grafting, tibiocalcaneal arthrodesis, or even major amputation.
- •
Arthrodesis of the ankle and subtalar joints has been the standard of treatment for large talar deficits and patients with concomitant ankle and subtalar pathology.
- •
Local bone quality and quantity can make fixation for arthrodesis around the talus challenging. Without adequate fixation, fusion is more likely to fail.
- •
Unreliable blood supply adds to the challenges of talar deficiency and surgery to address these deformities may disrupt blood supply, further compromising results.
- •
Long-term consequences of arthrodesis include the potential for limb shortening, stress on adjacent structures, and chronic pain.
- •
Arthrodesis can lead to increased disability, decreasing overall dorsiflexion by 63% and plantarflexion by 82%.
- •
Fusion for large talar deficits can have high failure rates including nonunion of 16% to 52%, increased infection rates up to 21.8%, and hardware complications of 14%.
- •
Fusion for large talar deficits can have an overall high rate of reoperation up to 39.6%, or even conversion to major amputation of up to 16%.
Patient history and physical exam findings
- •
A complete history must be obtained including history of trauma, arthritis, prior surgeries, and any complications encountered.
- •
Patients will have localized pain to the ankle and subtalar joints. Typically there is loss of motion, but attempting to isolate the ankle from the subtalar joint can be useful.
- •
Any deformity of the ankle and subtalar joint is noted. Differentiating soft tissue from bone deformity, or rigid from flexible, can assist with surgical planning.
- •
Equinus is one of the most common associated deformities and should be addressed surgically.
- •
Instability of the ankle and subtalar joints should be evaluated, which may be caused by soft tissue, loose implant components, or bone loss.
- •
The range of motion of adjacent joints should be assessed to understand the patient’s ability to compensate.
- •
Joint effusion or crepitation may be noted.
- •
Consideration of old incisions is important in preoperative planning. Scars from prior fracture care, fusion attempts, or implant surgery may influence your subsequent approach.
- •
Understanding the patient’s activity level and discussing their expectations in particular without long-term data available for patient-specific implants is an important element of the informed decision.
Imaging and other diagnostic testing
- •
Radiographs:
- •
Standard radiographs of the foot and ankle will allow initial assessment of the problem. These images give a sense of the overall mechanics as well as basic deformity planning measurements.
- •
Evaluate any angular deformities of the ankle and foot, as these may need to be addressed with additional procedures.
- •
The need for refinement with osteotomies and soft tissue work is common when trying to achieve the best outcome in complicated revisions.
- •
Take note of shortening or loss of height due to AVN collapse, failed total ankle replacement with component subsidence or similar.
- •
Note all hardware as it may require removal for revision. Having an understanding of the current hardware can aid in its removal. Obtaining old reports or notes can benefit the planning process.
- •
Understanding the bone quality and quantity is paramount. This can have a bearing on implant design if voids need to be accommodated. Bone may be entirely missing or severely compromised, requiring different configurations of implant surfaces. Violation and variations to the calcaneus, in particular for the purpose of this chapter, require the custom total talus implant to be constrained. Being fixed to the calcaneus and having the appropriate choice of implant surface will promote fusion and stability to the implant, encouraging better outcomes.
- •
- •
Advanced imaging:
- •
MRI may be useful in the diagnosis of AVN. This can be challenging, however, in cases of talus fracture with history of open reduction and internal fixation (ORIF). Hardware can limit the usefulness of MRI and due to interference, the true extent of bone involvement may be disguised by poor image quality.
- •
MRI may be useful to evaluate the extent as well as monitor the progression of treatment in reconstructions where infection is an element of the pathology.
- •
CT scan is the standard of care for evaluating bone, making it useful for peritalar pathology. CT scans give more accurate information regarding bone quality in the presence of large cysts and will often reveal larger cysts or an increased number from what is seen on conventional radiographs.
- •
CT scan can be more useful in determining nonunion and aids in establishing bone quality with regard to loosening around implants.
- •
CT scan is the modality of choice for planning and manufacturing patient-specific 3D implants. It is utilized to model the abnormal anatomy and then calculate the more “normal” desired position and size implant to achieve the surgeon’s goals.
- •
- •
Other testing:
- •
Infection work-up for any nonunion or failed total ankle arthroplasty is warranted. This includes appropriate laboratory values and inflammatory markers such as complete blood count (CBC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and others at the surgeon’s discretion.
- •
In cases of infection or concern for infection, infectious disease consultation should be coordinated.
- •
If desired, joint aspiration can be performed and analyzed.
- •
Likewise if there is concern, surgery may be staged so that hardware can removed and appropriate cultures taken. This should include frozen sections from the surrounding soft tissue for more complete information.
- •
Hardware removed should also be sent for sonication and subsequent culture.
- •
In cases that are staged, external fixation is considered. This allows for stability and maintenance of the correction. Stability will limit inflammation, and since the deformity has already been reduced and held by the external fixator, the definitive surgery is easier to finalize.
- •
In cases of concern, vascular testing may be judicious and an appropriate vascular consultation scheduled where necessary.
- •
Nutrition can be monitored with appropriate laboratories including total protein, albumin, and prealbumin.
- •
Decisions can be made by the surgeon to monitor and address any smoking history. In particular, in cases of arthrodesis smoking may have a role in failure to fuse.
- •
Nonoperative management
- •
Upright ankle bracing, hinged or solid.
- •
Custom bracing such as Arizona bracing or similar.
- •
Assistive devices to aid in ambulation, including a cane or walker.
- •
Pain-management modalities.
Traditional surgical management
- •
For large talar voids and simultaneous subtalar pathology, the standard has been arthrodesis of the ankle and subtalar joints with internal fixation.
- •
External fixation has been utilized but requires knowledge and comfort with this form of fixation. Depending on the construct, external fixation can add significant cost.
- •
In situ tibiotalocalcaneal fusion may require a bone graft, which may be from local bone if a lateral incision is utilized. The fibula can be harvested and then utilized to aid in height loss and to fill voids, but this option has limits.
- •
For larger deficits including talectomy, allograft bone has been described.
- •
A large allograft utilized for arthrodesis has limitations as far as the overall size of graft that can be incorporated. In addition, the bone has to heal at two locations, increasing the possibility for one end to fail union.
- •
Tibiocalcaneal fusion is an alternative but leads to significant loss of length to the affected limb. This may be balanced by shoe lifts, but has a mechanical cost and may not be tolerated well.
- •
Distraction osteogenesis is a possibility but requires a lengthening osteotomy and a procedure more proximal. It also requires advanced knowledge of deformity correction. Generating length proximally while performing a distal arthrodesis is more challenging. Complications exist with external fixation, including pin site infection, and in the case of distraction osteogenesis, there is now potential for complications at the proximal surgical site. The distraction performed proximal must regenerate bone with appropriate quality and quantity to have a successful outcome.
- •
Below-knee amputation is an option but is generally reserved for severe infection or nonreconstructable cases, such as those with bone loss that cannot be overcome or failed revisions. Major amputation can lead to a slower gait, a shorter stride length, and increased energy expenditure.
3D-printed implant design specifications and considerations
- •
Titanium and cobalt chrome are the materials of choice for 3D-printed implants ( Fig. 6.1 ).
• Fig. 6.1
Clinical image of a (A) cobalt chrome and (B) titanium total talus implant.
- •
In the case of an implant surface being utilized for arthrodesis, the surface to interface with the native bone must be able to accept bone ingrowth. There have been many materials applied to implants over the years, but the exposed porous surfaces of metal seem to be the most popular and studied.
- •
Because most of the total talus implant requires articulation, cobalt chrome is the best option as the surfaces can be polished to meet medical standards.
- •
For constrained total talus applications, the talus articulates with the navicular distal and the tibia or total ankle component dorsally; however, the undersurface requires a porous exposed surface for integration with the calcaneus ( Fig. 6.2 ).
• Fig. 6.2
Preoperative planning and (A) CAD drawing of a planned implant with a gyroid surface covering areas for projected fusion. (B) Clinical image of a custom implant undersurface with a gyroid surface for bone integration. CAD , Computer-aided design.
(Reprinted with permission from Restor3d, Inc, Durham, NC.)
- •
The porous style preferred by the author is a “gyroid” design that allows for maximum bone ingrowth to the implant. The amount and location of the porous structure is defined by the anatomy and aided in construction by a CT scan.
- •
Most constrained total talus cases have three sizes of implant for trial and subsequent placement. This allows flexibility for the surgeon and it is the surgeon’s decision regarding what is to be made available. Based on CT scans, the implant can be made “nominal” or what is thought to be the actual size of the native talus. One size larger and one size smaller are typical alternatives that can be produced and made available, typically 5% smaller and 5% larger ( Fig. 6.3 ).
• Fig. 6.3
Preoperative CAD planning with suggested implant sizes (A) along with proposed trials (B). CAD , Computer-aided design.
(Reprinted with permission from Additive Orthopaedics, LLC, Little Silver, NJ).
- •
At the time of surgery there are typically three sizes of implants available and the surgeon decides on the best “fit” after trialing them intraoperatively ( Fig. 6.4 ).
Related posts:
Historical perspectives on 3D printing
Material science for 3D printing in medicine
Distraction subtalar joint fusion using a custom 3D-printed implant
Custom constrained 3D total talus/navicular replacement
A 3D-printed solution for evans calcaneal osteotomy nonunion
3D-printed total cuboid replacement for lateral column pathology
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
