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Biomechanical Considerations With Total Ankle Arthroplasty
Given the increased demand for total ankle arthroplasty procedures, these authors discuss essential biomechanical factors, including pre- and post-op assessment of gait, ensuring appropriate bone support and prosthesis sizing, and key steps in post-op physical therapy protocols.
The ankle joint is subject to the highest forces per surface area in the entire body yet the rate of arthritis is nine times less than that of the knee and hip.1 During the gait cycle, greater than five times one’s total body weight is transmitted vertically through the ankle joint.2,3 If there is so much force transmitted through one joint, why is the rate of arthritis significantly less in the ankle than in other joints and what abnormality or abnormalities will result in ankle arthritis?
With these points in mind, let us take a closer look at the biomechanical pathology that results in end-stage ankle arthritis, evaluation of the preoperative patient for consideration of total ankle replacement (TAR) as well as biomechanical considerations for the postoperative TAR patient.
Total ankle arthroplasty is now widely accepted as an alternative to ankle arthrodesis for the treatment of end-stage ankle arthritis. Although this procedure was once plagued by high rates of aseptic loosening and subsidence, newer prosthetic designs and refined surgical techniques have since improved total ankle implant survivorship to 95 percent at five years.4-6 Clinical outcome data also shows equivalent pain relief, better improvements in function and higher patient-reported outcome measure (PROM) scores in comparison to patients who have ankle arthrodesis procedures.4-6
Since 1998, there has been a sixfold increase in the number of total ankle arthroplasty procedures in the United States while utilization of ankle arthrodesis has remained constant.7,8 As demand for the procedure continues to increase among patients and surgeons, it is important to highlight that successful outcomes for total ankle arthroplasty depend on a number of factors including biomechanical considerations that we will detail in this article.
Understanding Biomechanical Factors And Differences In Normal And Arthritic Ankles
The talocrural joint functions as a complex interaction between the tibia, fibula and dorsal articular surface of the talus. There is three-dimensional motion throughout the joint, resulting in motion in multiple planes during the gait cycle. The movement of the ankle allows for concomitant external rotation and abduction with dorsiflexion, and the exact opposite motions with plantarflexion. The normal sagittal arc of the ankle joint is a total of 70 degrees of motion.3 Leardini and colleagues evaluated the axis of rotation of the normal ankle, and found the instantaneous axis of rotation translates and rotates during passive flexion motion in the saggital plane.9
When evaluating the patient both pre- and postoperatively, gait analysis is typically divided into three general categories: temporal-spatial parameters, kinematic parameters and kinetic parameters. Temporal-spatial parameters evaluate the speed of walking, including cadence, stride length and support time for each limb. Kinematic parameters evaluate measures of movement, analyzing the separate forces driving locomotion. These individual kinematic parameters include linear or angular displacement, velocity and acceleration. Finally, kinetic parameters measure the forces generated during gait. These include assessments of power and moment (force plus direction), ground reactive forces, joint moments and joint mechanical power.3,10
Patients with rheumatoid arthritis have been well-described as presenting with a uniquely characteristic gait and biomechanical pathology. Excessive eversion at the ankle and internal rotation of the leg are associated with painful hindfoot valgus deformity in the rheumatoid patient.11
In patients that have a stiff ankle joint and have greater than 10 degrees of equinus, this results in a vaulting gait as well as increased knee extension and recurvatum. Patients with an equinus deformity have abnormal cadence of gait and are predisposed to developing knee laxity at the medial collateral ligament and posterior capsule.12
When there is pain associated with the ankle joint, patients develop a shortened stance phase of gait and insufficient use of the midstance and terminal rocker aspects of the gait cycle. When evaluating the kinetic parameters of this antalgic gait, one will note these patients are particularly energy- and mechanically inefficient.13
In addition to rheumatoid arthritis and other systemic inflammatory arthropathies, alternate etiologies for ankle arthritis include prior trauma and ligamentous instability. Conversely, hip and knee arthritis are predominately a result of primary osteoarthritis.
What The Literature Reveals About The Role Of Bone Support In Total Ankle Arthroplasty
Sufficient bone support (density and contact area) is an essential tenet to successful total ankle arthroplasty and directly affects stress distribution. Although modes of failure for total ankle arthroplasty may vary somewhat by implant, failure from aseptic loosening and subsidence are common etiologies. Historically, these failures have manifested more commonly on the tibial versus the talar side of the prosthesis, in part, owing to the stiffness characteristics of the distal tibia.
With regard to the distal tibia, Aitken and colleagues found that with an intact subchondral plate, the distal tibia’s elastic modulus was 300 to 450 mPa.14 With removal of the subchondral cortical plate, however, one could observe a 50 percent reduction in compressive resistance. With removal of three cm or more from the subchondral cortical plate, there was virtually no compressive resistance. The study authors concluded that the stiffness characteristics in the distal tibia parallel the radiographic appearance of the trabeculae with the strongest cancellous bone of the tibia being that nearest to the subchondral cortical plate. The study authors went on to recommend that surgeons minimize resections of the distal tibia, if possible, when performing total ankle arthroplasty.
Bischoff and coworkers evaluated the influence of geometry and resection depth on bone support for total ankle arthroplasty with finite element analysis (FEA).15 With bone support serving as a surrogate for primary implant stability, the researchers determined bone support by the bone density (and therefore stiffness) and contact area for the prostheses. Overall, the authors observed reductions in bone support ranging from eight to 19 percent and eight to 46 percent for the distal tibia and talus respectively. The reductions were greater when the researchers compared flat versus round resections at all the resection depths of the tibia and talus. At a six to eight mm resection depth for the tibia, the authors observed bone density and contact area reductions simultaneously. This suggested that round resections may afford improved primary implant stability over flat resections, particularly in the tibia, by minimizing reductions in bone density and maximizing the coverage area.
It is known that for a larger contact area, the resultant contact stresses are lower. Thus, given that loads transmit vertically first through the prosthesis, then to the prosthesis-bone interface and finally the trabecular bone of the distal tibia, sufficient bone support (i.e. primary implant stability) is essential in mitigating eccentric edge loading, polyethylene wear, aseptic loosening, and failure.
What The Surgeon Should Know About Prosthesis Sizing
Prosthesis size is another important consideration for a successful total ankle arthroplasty and also affects stress distribution. For reasons we discussed previously, surgeons must ensure adequate anterior to posterior and medial to lateral bone coverage for the prosthesis during the index procedure. Smaller prostheses require less bone resection during implantation and reduced joint lever arms and moments, hence reducing shear forces.16
Additionally, minimal resections preserve ligamentous origins and insertions. Along with osteoligamentous balancing, this is essential in counteracting torsional forces.17 Although “overstuffing the poly” can afford additional coronal deformity correction on the table, surgeons should avoid this in order to mitigate polyethylene wear and failure.5
A Closer Look At Gait Mechanics After Total Ankle Arthroplasty
It is important to consider the difference in gait and functionality when comparing total ankle replacements to ankle arthrodesis. Piriou and colleagues evaluated postoperative gait following total ankle replacement in comparison to those who had ankle arthrodesis and a control group.18 The results showed the total ankle arthroplasty group had greater movement of the ankle, more symmetrical timing of gait and restored ground reactive forces. In contrast, the ankle arthrodesis group exhibited a faster gait with longer step stride length in comparison to the total ankle arthroplasty group.
Brodsky and coworkers evaluated the changes in gait following a single mobile-bearing total ankle replacement.19 The study simply compared the preoperative parameters to the postoperative parameters in a cohort of 50 patients. When evaluating the temporal-spatial parameters, researchers found the walking velocity increased postoperatively as a result of increased cadence and stride length. The average ankle range of motion in the sagittal plane increased from 14.2 degrees to 17.9 degrees with the increase coming from plantarflexion. The study authors also reported a significant increase in ankle power and plantarflexion moment.
Queen and colleagues performed a randomized trial in order to determine if there was a difference in the gait mechanics with a fixed-bearing implant in comparison to a mobile-bearing implant at one year of follow up.21 While the authors did not report any difference between the two implant types, this was an underpowered study.20 When evaluating postoperative range of motion with utilization of a fixed-bearing implant, Choi and team reported an average increased motion of 3.7 degrees.21 The biggest difference between these results in comparison to studies evaluating mobile-bearing implants was increased motion with both plantarflexion and dorsiflexion postoperatively with the fixed-bearing implants.21 The significance of this difference is not well explored and further evaluation is necessary.
Pertinent Pearls On Rehabilitation Protocols After Total Ankle Arthroplasty
Patients with end-stage ankle arthritis show a significant deficiency in their gait pattern.9 We tend to see a reduction in triplanar ankle movement, a decrease of the second active maximal vertical and maximal medial ground reaction force, reduction of the sagittal and transverse ankle joint moments, and a reduction of ankle joint power.22 These deficiencies are all factors that one must address through a proper postoperative physical therapy protocol.
Currently, our practice protocol for total ankle arthroplasty involves four weeks of protected non-weightbearing. Once the cast is removed, the first two weeks of therapy movement are restricted to the sagittal plane. The focus is restoring active and passive dorsiflexion, and plantarflexion. Open kinetic chain exercises are initiated at therapy week four (post-op week eight) with closed kinetic chain exercises initiated at six weeks (post-op week 10).23 At this time, the physical therapist would introduce proprioception and balance activities. The main goal of these activities is to reestablish the patient’s ability to balance on one leg, which is pivotal for bipedal ambulation.
Through previous research, we know that following a total ankle arthroplasty, patients show improvements in inversion/eversion stability, axial rotation and greater dorsiflexion and plantarflexion.24 There is also a possible reduction of frontal plane stability.24 The aim of skilled rehabilitative services is to, first and foremost, restore functional range of motion post-surgery. Once proper levels of dorsiflexion and plantarflexion have returned, we can begin to focus on supination and pronation.25 It is important to restore functional levels of supination and pronation when retraining the patient’s gait cycle.
After total ankle arthroplasty, De la Fuente and coworkers demonstrated that there can be increased coactivation of the tibialis anterior and the medial head of the gastrocnemius.24 This increased coactivation can lead to excessive hip extensor activity secondary to excessive hip rotation.
One method that physical therapists use to combat this is targeted neuromuscular electrical stimulation (NMES). Researchers have shown that selective targeting of the tibialis anterior with neuromuscular electrical stimulation can control plantar pressure via modulation of the medial head of the gastrocnemius’ activity during gait.26 This works by creating a reciprocal inhibition of the gastrocnemius when the tibialis anterior is contracted to full force. In other words, from a clinical standpoint, if we can retrain the gait cycle through the use of neuromuscular electrical stimulation, we can begin to restore optimal gait mechanics sooner in patients after they have had total ankle arthroplasty.
Other postoperative treatments to consider include addressing hip and core stability. We know these play a role not only in gait but in overall function of the lower extremity. The current research is limited on best practices for specific postoperative protocols. There is a need for further research focused on a more clear and accurate protocol for initiating rehabilitation for patients following total ankle arthroplasty.
In Conclusion
The ankle joint is a complex joint that has intricate interactions with surrounding joints, ligaments and tendons. It is critical to consider the biomechanical implications of end-stage ankle arthritis and the effect it has on the entire gait cycle and surrounding structures. While it has been demonstrated that total ankle replacements have minimal to slight improvement of ankle motion postoperatively, total ankle arthroplasty does increase the patient’s gait cadence, increase ground reactive forces and improve the patient’s function in daily activities.
The biomechanical implications of a total ankle replacement are complex and require a strong understanding by the foot and ankle surgeon so he or she can set appropriate expectations for patients having this procedure.
Dr. McKenna is a Fellow at the Orthopedic Foot & Ankle Center in Westerville and Dublin, Ohio.
Dr. Rushing is a Fellow at the Orthopedic Foot & Ankle Center in Westerville and Dublin, Ohio.
Dr. Banks is the Director of Physical Therapy at the Orthopedic Foot & Ankle Center in Westerville and Dublin, Ohio.
Dr. Prissel is a Partner Physician at the Orthopedic Foot & Ankle Center in Westerville and Dublin, Ohio.
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