After a brief overview on the evolution of hardware and techniques for total ankle replacement devices, these authors discuss essential factors such as patient activity demands, fixation, vascular status and ankle malalignment in determining appropriate surgical candidates for fourth-generation total ankle replacements and fostering strong outcomes.
Recent advances and further understanding in total ankle replacement (TAR) stem from a history of patients requiring a mobile, pain-free solution to end-stage ankle arthritis. The success of hip and knee arthroplasty naturally led to the expansion of arthroplasty as it relates to other joints, including the ankle.
The first reported series of total ankle implants were by Lord and Marotte in 1970.1 The original implant involved a long tibial stem and a polyethylene talar component. The surgery, modeled after hip implants at that time, required a concomitant subtalar joint fusion. Regrettably, as time would tell, the mechanics necessary for hip implant design translated poorly to total ankle replacement and subsequently led to poor patient outcomes. Despite early failures, attempts to improve TAR outcomes moved forward.
First-generation TARs began to demonstrate a more conventional design tailored to match the native ankle joint. The older or first-generation products mostly consisted of a two-component design varying from a “constrained” to an “unconstrained” implant, meaning the tibia or talar component would have an attached polyethylene spacer without a separate independent bearing.1-3 These early designs included a variety of implants with both European and North American origins.4
Unfortunately, published results for these early first-generation total ankle replacements showed high failure rates in the short- to mid-term, thus leading to abandonment. Overall, many first-generation prostheses were eventually withdrawn from the market due to high failure rates with subsidence, continued patient pain and progressing deformity.1
However, advances in total ankle implant technology continued to emerge and eventually led to newer second- and third-generation designs that included the use of metallic tibial and talar components in combination with ultra-high molecular weight polyethylene (UHMWPE).
One could further categorize the second-generation designs by their number of components (two versus three) and device type (fixed- versus mobile-bearing). The two-component system (fixed-bearing without a mobile piece) has the UHMWPE spacer retained within the tibial component of the implant. A three-component system (mobile-bearing with a mobile spacer) allows for independent mobility of the spacer as it articulates with the tibial and talar components. It soon became apparent that constrained implants led to higher impact forces and stress across the implant components, leading to prosthetic loosening.5
Furthermore, the robust use of polymethylmethacrylate cement on both the tibial and talar components during implantation became more popular. We observed surgeons gradually abandoning this practice during the use of second-generation implants as research began to focus on component surfaces that could promote bone ingrowth with the metal implants.
Additionally, the foot and ankle surgical community began to evaluate the importance of bone preservation during resection. Second-generation implants frequently required significant resection of bone from both the tibia and talus, but Gougoulias and colleagues showed this negatively impacted the fixation and stability of ankle replacement.6 Therefore, in the evolution of TAR, a transition to minimal resection, especially as it pertained to the tibial side of the implant, was desirable.
Over the years, new instrumentation facilitated more accurate implant placement, reduced bone resection and preserved bone stock. The challenges encountered with ankle replacement also led to further changes in prosthesis design, biomaterials, surface coatings and instrumentation during a 30-year period. Study of outcomes, overall failures and the move toward evidence-based medicine ultimately sparked changes in the progression of total ankle replacement.7
Moving Forward With Newer Generations Of Total Ankle Replacement
Currently, we now have mid- to long-term literature on TAR. This is in large part due to the research into the development and understanding of the pitfalls of previous generations of total ankle implants. Implants, surgical techniques and clinical outcomes continue to improve, leading to the currently available fourth-generation implants. Taking previous knowledge of TAR into account, we feel the developers of the fourth iteration of TAR implants have focused their efforts on improving outcomes, minimal bone resection, biomaterials, surface coatings and preoperative navigation systems.
Additionally, continued improvement in surgical techniques such as ankle implant staging and soft tissue balancing aid in the improvement of total ankle replacement outcomes.
We now have 10 approved total ankle replacement systems in the United States. These include a combination of second-, third- and fourth-generation implants. The implants by name include the Scandanavian Total Ankle Replacement (STAR™) (DJO), Inbone II™ Total Ankle System (Wright Medical), Infinity™ Total Ankle System (Wright Medical), Invision™ Total Ankle Revision System (Wright Medical), Cadence® Total Ankle System (Integra LifeSciences), Vantage® Total Ankle System (Exactech), Salto Talaris® Ankle (Integra LifeSciences), Apex 3D™ Total Ankle Replacement System (Paragon28), Hintermann Series H3™ Total Ankle Replacement System (DT MedTech) and the Kinos Axiom Total Ankle System (Kinos Medical). Specifically, the Infinity, Cadence, Vantage, Apex, Hintermann H3 and Kinos Axiom devices are fourth-generation systems.
Addressing Patient Age, Activity Demands And Other Criteria
The need for patients to understand the risks and benefits of the surgery remains paramount. Managing patient expectations as they pertain to the discussion of ankle replacement surgery should include a thorough understanding of individual patient demands and lifestyle.7
The foot and ankle community still lacks uniformity on TAR indications and they may vary somewhat from surgeon to surgeon based partly on experience and learning curve among other criteria. However, the American Academy of Orthopaedic Surgeons (AAOS) have noted up to 14 factors that one should consider when maximizing outcomes with surgical treatment of ankle arthritis.8 These factors include type of device, patient age, patient weight, preoperative infection, fracture, surgical side, sex of patient, deformity, comorbidities, previous ankle surgery, presence of hindfoot arthritis, surgeon experience, year of surgery and hospital surgical volume.8 Accordingly, surgeons should consider a variety of factors when assessing whether patients are good candidates for fourth-generation total ankle replacement procedures.
Patient-related criteria. Patient selection should spotlight the overall health of the patient, health of the extremity and, specifically, the status of the patient’s ankle joint. Age, weight and activity level are also relevant considerations for patient selection, but ultimately it comes down to surgeon preference on a case-by-case basis. A systematic approach to patient selection remains essential in improving outcomes in total ankle replacement.
Patient age. Currently, there is no definitive data evaluating age as a contraindication for fourth-generation TAR. While most researchers agree that the minimum age for TAR ranges between 50 to 55 years old, there are studies that both support and refute age as a consideration.9 We believe that a decreased age during primary implantation of a TAR will increase the risk for revision due to the patient outliving the implant. However, we are beginning to see an increase in implant longevity, which is likely due to improving biomaterials and designs.
Activity demands. While this may seem intuitive, unlike knee and hip arthritis, the primary cause of ankle arthritis is not primary osteoarthritis, but rather arthritis due to chronic ligamentous instability and post-traumatic etiologies.7 This tends to indicate the patients electing TAR are younger and more active. The general recommendation post-ankle replacement is for the patient to seek low-impact activities such as swimming, biking and hiking as benchmarks in matching activity demand with good outcomes in the short- and medium-term.
What About Fixation?
Prosthetic fixation. Today, almost all available TARs are FDA 510K cleared with the use of polymethyl methacrylate (PMMA) cement. The STAR total ankle implant is the only one not requiring cement for fixation. Polymethyl methacrylate cement facilitates the implant to bone interface, enhancing structural support. Some physicians have transitioned to a combination of injectable bone graft substitute, biologics and PMMA to encourage implant to bone integration, limit the surface area of PMMA to bone interface (providing initial fixation and stability) and increase the surface area for bony ingrowth to the prosthetic component itself (providing durable long-term surface bonding). Additionally, one should consider and evaluate bone stock as a risk factor for insufficiency fracture during implant placement.10
Why Vascular Status Is A Key Factor
Soft tissue envelope and blood supply. The soft tissue envelope and vascular status are also important to address with fourth-generation ankle implant outcomes. Studies prove that tobacco smoking greater than 12 pack-years, peripheral vascular disease and cardiovascular disease are statistically significant in predicting an increased rate of incision breakdown following TAR.9
Bibbo has also discussed the vascular status of the anterior tibial artery as being a significant risk factor for soft tissue failure.9 A thorough understanding of blood supply to this distribution of the ankle is advisable prior to the procedure.9
When There Is Ankle Malalignment
Ankle malalignment. In the presence of sufficient bone quality and vascular supply to both soft tissues and bone, the alignment of the tibiotalar joint can dictate whether TAR is appropriate.7 The malalignment of the ankle joint may be the result of a more proximal or distal deformity. In such instances, one must address this prior to ankle replacement. The goal is to return the ankle joint to a plantigrade state.
The major problem found with misaligned ankle joints following TAR is a phenomenon referred to as “edge loading.” This leads to asymmetric forces across the implant affecting the polyethylene component.7 Ultimately, the malalignment produces uneven and increased wear of the polyethylene, increasing the risk of implant loosening.
Most studies on TAR have noted that the predominant deformity is a varus malalignment, which the surgeon can address at the index TAR procedure via standard osseous control and soft tissue reconstruction.7 Occasionally, the deformity at the ankle joint may be severe enough that a staged approach to ankle replacement is necessary in order to first correct the relevant bone deformity prior to subsequent implant placement. One often encounters this scenario in patients with a history of significant ankle trauma.
Candidacy for TAR is less restricted than in the past with respect to the severity of tibiotalar deformity. Studies now show clear reduction of malalignment in deformities greater than 15 degrees, reducing the concern that greater than 15 degrees of deformity is a contraindication for TAR.7 In our experience, valgus deformity more frequently requires a staged approach to TAR. In these cases, it is essential to identify and correct deformities via soft tissue and osseous procedures during the first stage in anticipation for the subsequent ankle replacement.
Total ankle replacement using fourth-generation ankle implant systems will hopefully aid in bridging the gap between ankle arthroplasty and hip/knee arthroplasty outcomes. Total ankle replacement prostheses still fall behind in meeting the successes seen with hip and knee replacement secondary to the complex mechanics of the foot and ankle, the causes of end-stage ankle arthritis and the anatomic restrictions regarding bone resection involving the tibiotalar joint.10,12 Emerging fourth-generation ankle replacement systems are among the first to confront these challenges with the end goals of improved implant survivorship and lasting patient outcomes.
Total ankle replacement is a challenging procedure with a well-known learning curve that varies by implant. Currently, with fourth-generation TAR systems, we have witnessed a transition to simplicity in implant placement and assisted guidance through preoperative navigation systems. Current implants and surgical techniques continue to improve on past successes, but TAR remains a non-universal procedure. The indications for TAR have remained essentially static over the years with painful end-stage osteoarthritis, post-traumatic osteoarthritis and inflammatory arthridities being the vast majority of presenting conditions.10
Fourth-generation ankle implant advancements in implant design, simplicity and techniques as well as surgeon comfort have begun to push the boundaries of what is possible with total ankle arthroplasty. The physician should no longer just focus on the condition of the ankle joint prior to replacement but on the patient as whole when determining whether TAR is appropriate. Physicians must continue to do their due diligence in addressing and correcting key factors in TAR. It is important to recognize that implants continue to transform year to year, but the patients and surgeons change as well. Future generations of implants will continue to evolve with technology and set the limits for what is possible in total ankle replacement.
Dr. Consul is a currently a Fellow at the Orthopedic Foot and Ankle Center in Worthington, OH. He is an Associate of the American College of Foot and Ankle Surgeons.
Dr. Thompson is a currently a Fellow at the Orthopedic Foot and Ankle Center in Worthington, OH. He is an Associate of the American College of Foot and Ankle Surgeons.
Dr. Prissel is the Fellowship Director at the Orthopedic Foot and Ankle Center in Worthington, OH and is a Fellow of the American College of Foot and Ankle Surgeons.
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8. AAOS: The Surgical Treatment of Ankle Arthritis. Available at: https://orthoinfo.aaos.org/ en/diseases--condition/arthritis-of-the-foot-and-ankle . Accessed December 21, 2020.
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