Chapter 12
FUTURE DIRECTIONS
The ideal total ankle prosthesis has yet to be deter- mined, but much has been learned from early ex- periences in total ankle arthroplasty. Modern implants are typically more respectful of anatomic concerns, and have found new approaches to decrease interface stress. Biologic fixation has improved upon cemented results. Surgical techniques and understanding of associated ligamentous deficiency, malalignment, and deformity have advanced. Current series still have varied results, and longer follow-up is needed. Despite this, some modern ankle replacements represent significant progress, with improved results and sur- vival rates challenging those of arthrodesis. Further, benefits of preserved motion and avoidance of foot osteoarthritis outstrip the “gold standard.”
12.1 Current Concerns to be Addressed
Currently, many patients with painful ankle osteo- arthrosis and arthritis can be offered a total ankle replacement as a viable alternative to ankle arthro- desis. Although great progress has been made in recent years, total ankle replacement is still plagued by a relatively high rate of perioperative complications and revisions, and some concerns remain with respect to its long-term success.
12.1.1 Prospective Studies
A major concern is the paucity of well-documented prospective studies, in spite of the numerous pros- thetic designs introduced on the market. To learn from the successes and failures by meticulous analy- sis of each case would help to increase current knowledge, and would allow further improvement of total ankle replacement.
Another issue is the selection of patients, which differs markedly among surgeons. Consequently,
obtained results can only be compared with cau- tion.
There is, therefore, a need for extensive, well- documented prospective studies that use standard- ized protocols with comparable parameters.
12.1.2 Prosthetic Design
Each prosthetic design introduced on the market has been an attempt to address the specific demands of the ankle joint. The fact that each new prosthesis has included several new features and/or changes compared to previous designs, however, has made it extremely difficult to evaluate new ideas and changes with respect to their success. Never- theless, the current concepts have supported the belief that anatomy should be respected as much as possible.
As the mechanical demands on an ankle pros- thesis completely change in the hindfoot after ex- tensive fusion, there is probably a need for special designs to address these cases.
Custom-made ankle prostheses based on CT scan data could also have potential advantages for severely traumatized ankles, and particularly for total ankle revisions.
12.1.3 Preoperative Planning and Implantation Technique
The techniques for implantation can still be im-
proved by more accurate instrumentation, and by
better pre- and intraoperative planning tools to help
in achieving optimal component positioning. Com-
puter-assisted preoperative planning may be parti-
cularly sought after to enhance surgical implanta-
tion and to improve the clinical outcomes of total
ankle replacement. Such planning may be based on
minimizing changes in ligament length and the
amount of bone stock to be removed. The main out- put report might be a detailed picture of the re- placed ankle, with indications for component loca- tions and quantitative measures for bone cuts. In all likelihood, future design improvements may help to reduce ligament strain, restore normal axes of rota- tion, and maintain mobile bearing stability.
12.1.4 Polyethylene Wear
As experience with total ankle replacement broa- dens, ankle component wear and related long-term durability will become the next major concern. In a recent study, polyethylene wear particles retrieved in joint fluid did not show differences in particle number and size between well-functioning total ankle replacements and total knee replacements, however, total ankle replacement generated rounder particles than total knee replacement. The authors concluded that the long-term result of total ankle replacement might be expected equal to total knee replacement in terms of polyethylene wear and the prevalence of osteolysis. Further studies are needed to elucidate polyethylene wear in more detail.
12.1.5 Stability of Bone-Implant Interface
Another issue to be addressed is the long-term stability of the bone-component interface. It is not yet clear whether double-coated surfaces (with or with- out hydroxyapatite coating) can create stable bony in- growth and thereby long-term interface stability, as has been the case in total hip and knee replacements.
Recently, osteoblast culture coating has been tested in a clinical trial (Dr. Yoshinuri Takakura, Nara, Japan, personal communication 2002), however, longer follow-up is needed to assess this method.
Although early results with semi- and noncon- strained ankle designs are encouraging, it is not yet clear to what extent the current designs can dissi- pate rotational forces while maintaining the stability of the joint. Careful assessment of long-term follow- up will determine how closely the present designs are mimicking the unique requirements of the ar- thritic foot and ankle. Further work in biomechanics is necessary to better understand the kinematic changes of the arthritic ankle joint and to estimate the forces acting on the implant-bone interface.
12.2 Further Success will Increase Patient Demand
Successful total ankle replacement may relieve pain and restore some motion, allowing patients to return to certain physical activities that they were not able to perform prior to implantation. Conse- quently, the physical demands of patients may increase, and successful primary replacements may fail because the replaced ankle is subjected to too much stress.
Analogously, increased success may lead to indi- cations for total ankle replacement being extended to pathologic conditions that are probably not suit- able for replacement surgery.
On the other hand, total ankle replacement may be, in some instances, a viable intermediate solution prior to fusion. If, for example, a severe osteoarthri- tis has developed in a young patient after a trauma, total ankle arthroplasty may allow him or her to regain some motion and to properly bear the affect- ed ankle, which, in turn, may protect the foot from further degenerative joint disease. When the re- placed ankle wears out and revision of the prosthe- sis is no longer possible, ankle arthrodesis remains an option for salvage. The patient, nevertheless, may have benefited greatly from this intermediate solution, which allowed continued participation in some activities (including professional activities) during a significant period of his or her life that would not have been possible with a primary ankle arthrodesis. The intermediate total ankle arthro- plasty may additionally have preserved his or her foot from the development of secondary osteoarthri- tis. Further clinical research is needed, however, to understand the pathologic changes taking place in the neighboring joints of a replaced and fused ankle, respectively, and how they can be influenced.
12.3 Further Research
More accurate research is necessary for a better understanding of the mechanics of the intact and replaced ankle joint complex. First, this should help to further improve the implants, and second, it should help to elaborate guidelines for better plan- ning and more accurate implantation techniques,
186 Chapter 12: Future Directions
which would result in more reliable and effective total ankle replacement. Future studies with an emphasis on the objective analysis of long-term clinical results are also necessary to define and delin- eate the role of total ankle arthroplasty.
12.4 Conclusions
As total ankle arthroplasty continues to evolve as a viable treatment option for end-stage ankle osteo- arthritis, the adverse clinical and biomechanical consequences of ankle arthrodesis are far more apparent. Proper patient selection is a critical aspect of promoting successful results. Acceptable results have been reported in older, low-demand patients who have osteoarthritis or rheumatoid arthritis. A significant percentage of patients with end-stage ankle osteoarthritis, however, are younger patients with post-traumatic osteoarthritis. The use
of ankle replacement in younger, more physically active patients, and in those with significant defor- mity in the ankle or hindfoot remains a question to debate. More studies must be completed and further developments must be made to maximize the longev- ity and functional results of total ankle arthroplasty in future designs and applications.
Along with improved implants that are typically more respectful of anatomic concerns, proper posi- tioning of the implants (particularly of the talar component with respect to the center of rotation of the talus), accurate balancing of the soft tissues, and appropriate correction of malalignment are far more important for the success of total ankle arthroplasty than previously believed. Careful clinical investiga- tion and reliable diagnostic tools should thus be used to identify all of the associated problems so that they can be properly addressed during ankle replacement.
12.4 Conclusions 187
