Timothy R. Daniels, M.D., FRCSC
Life is motion - Motion is life is a motto adopted by the Canadian Orthopaedic Association to promote the activities of its members and to emphasize the importance of motion in the management of joint disease. Hip and knee arthroplasties are two of the most successful operations of the past century. For management of end-stage ankle arthritis, however, arthrodesis continues to be the mainstay of an orthopaedic practice. This is in part due to the early catastrophic failures of the 'first generation' ankle arthroplasties and the high patient satisfaction following an ankle arthrodesis. Despite the initial failures of ankle arthroplasties, several individuals remained committed to the possibilities of replacing the ankle joint and their persistence has resulted in the introduction of 'second generation' ankle implants1,2. Preliminary clinical results are promising, giving rise to total ankle arthroplasty (TAR) as an option for managing end-stage arthritis. The recent developments have prompted the current debate in the COA Bulletin and my role is to defend arthrodesis of the ankle joint. No doubt, preservation of ankle motion is important; however, there are several functional and mechanical peculiarities of the foot that continue to make ankle arthrodesis a viable surgical alternative in the management of ankle arthritis and/or deformity.
The ankle joint is a unique articulation, unlike any in the lower extremity. A highly congruent joint with a very thin cartilage mantle, the average thickness of its articular cartilage is approximately 1.6 mm as compared to 6-8 mm of the knee3. The ankle accepts more forces than any other articulation: 5-7 times the body weight is placed through it at the latter portion of the stance phase as compared to 3-4 times the body weight in the knee and 2-3 times in the hip4. The surface area of the ankle joint is similar to that of the knee and hip, but the contact area during load is only one-third that of the knee or hip (350 mm2 vs. 1100 mm2)5-7. The primary motion of the ankle joint occurs in the sagittal plane. The average arc of ankle flexion and extension is 43°-63° and only 30° of this motion is required for steady state walking (10° dorsiflexion and 20° plantarflexion)8,9. Rotation of the talus within the ankle mortise also occurs and averages 10° 10. The presence of rotation makes the ankle joint a biplanar articulation.
The foot and its articulations allow for a smooth transition of forces from the hindfoot to the forefoot with minimal energy expenditure. The ankle joint and the motion it provides in the sagittal plane plays a pivotal role in the mechanics of gait. During steady state walking, the foot functions as a three-rocker mechanism. (1) The heel functions as the first rocker during initial heel strike and the first 30% of the stance phase. (2) The ankle joint functions as the second rocker during the mid-portion of the stance phase and allows the body weight to be effectively transferred to the forefoot. This is the point in the gait cycle where maximum ankle dorsiflexion is attained. (3) The forefoot break functions as the third rocker during the latter portion of the stance phase11. If ankle motion is limited either by disease or an arthrodesis, the second of the three rockers is eliminated. If the sagittal position of the ankle is neutral, the hindfoot and forefoot rockers can compensate for the loss of the second rocker. At heel strike, the Chopart joints plantarflex to promote foot contact with the ground. During midstance, the heel rises earlier and the weight is transferred more rapidly to the forefoot. Because heel rise occurs earlier in the stance phase, greater shearing forces are placed through the midtarsal joints. In the presence of an ankle arthrodesis and a normally functioning subtalar joint complex, the mechanics of gait are minimally affected - there is a slight decrease in stride length and cadence. The oxygen consumption during steady state walking is increased by 3% and gait efficiency is 90%. Arthrodesis of the hip has a greater deleterious effect on gait with oxygen consumption increasing by 32% and gait efficiency decreasing to 53% of normal12. With an arthrodesed ankle, walking fast or running is difficult because the full functional capacity of all three rockers is required during these activities.
The three most important functions of the foot during gait are shock absorption, maintenance of hindfoot stability and provision of a rocking mechanism. Shock absorption is provided by spring effect of the medial arch and transverse mobility of the subtalar joint complex. Stability is obtained statically by ligament and bony support, and dynamically by the strong lower limb musculature. The rocking mechanism is provided by three components described above. Ankle arthrodesis has minimal effect on shock absorption because most of this is provided by the spring in the medial arch. Ankle arthrodesis allows for maintenance of stability. Ankle arthrodesis removes only one of the three rockers that are necessary for the smooth forward progression of the body weight from the hindfoot to the forefoot during steady state walking.
The primary goals of an ankle arthrodesis are pain relief, maintenance of stability and correction of deformity. These goals are readily achieved if the ankle is positioned properly - i.e. neutral position in the sagittal plane, slight hindfoot valgus and external rotation equal to the opposite side - and the ipsilateral joints of the foot are free of disease. The adverse effects on steady state gait are minimal and the majority of patients are satisfied (most published series reviewing the intermediate and long-term outcomes of ankle arthrodesis indicate they would undergo the surgery again). Over time, deterioration of the early results can be expected since the development of ipsilateral hindfoot arthritis is common, particularly of the subtalar joint13. The predisposition of the ipsilateral hindfoot to the development of arthritis has been the primary impetus for continued exploration of alternatives to ankle arthrodesis. In addition, recent gait analysis and validated functional outcome evaluations of patients with an isolated ankle arthrodesis demonstrate that even among satisfied patients, physical limitations continue to be significant when compared to healthy controls14.
The 'second generation' ankle arthroplasties are substantially improved and the intermediate results hold promise. However, long-term outcomes are yet to be determined and there is a paucity of data comparing the functional results of ankle arthroplasty vs. ankle arthrodesis. Gait analysis of the 'second generation' TARs reveal substantial improvement but the gait is not normalized15. Patients continue to protect the ankle during the terminal portion of the stance phase as evidenced by a decrease in the vertical ground reaction forces at toe-off. In addition, the full range of ankle motion provided by the implant is not necessarily utilized during steady state walking16. If 10° dorsiflexion is not obtained during the mid-portion of the stance phase then, from a functional perspective, the gait adopted is similar to what is observed following an ankle arthrodesis. The problems related to salvaging a failed ankle arthroplasty remain a challenge. Most orthopaedic surgeons continue to counsel their patients preoperatively of a potential below knee amputation for a failed TAR.
Both the clinician and scientist face a daunting task when considering treatment alternatives other than an arthrodesis for end-stage ankle arthritis and/or deformity. The bone stock available for implantation of the prosthesis is far less in the ankle than at the hip or knee. Stability of an ankle implant is paramount for its success; however, given the large forces distributed through the ankle joint during gait, it is difficult to obtain and maintain ligamentous support once it has been lost. Salvaging a failed ankle arthroplasty will continue to be difficult due to the sparse soft tissue envelope and limited bone stock. Therefore, it is my opinion that an arthrodesis of the ankle will continue to be and/or must remain a viable treatment alternative in the management of hindfoot deformity and/or end-stage arthritis. Improvements in the 'second generation' ankle arthroplasties have re-introduced TAR as a treatment option for end-stage arthritis of the ankle and I am one of a few orthopaedic surgeons in Canada who has performed ankle arthroplasties for the past six years. I am, thus far, pleased with the functional outcomes and the intermediate survival rates. Nonetheless, clinical scenarios will exist where an ankle arthrodesis provides a more predictable outcome or where an ankle arthroplasty is contraindicated. Questions yet to be answered are: 1) Will the functional outcomes of ankle arthroplasty surpass that of ankle arthrodesis?; 2) Will the motion attained following ankle arthroplasty protect the ipsilateral subtalar joint complex from arthritis?; 3) What will be the survival rates of the newer ankle arthroplasties? The challenge in the future for TAR will be to clearly define its indications and contraindications.
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- Daniels, T.R., R.H. Thomas, and K. Parker. Gait Analysis and Functional Outcomes of Isolated Ankle Arthrodesis. in International Federation of Foot and Ankle Societies Triennial Scientific Meeting. 2002. San Fransisco.
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