Biomechanics of the foot and ankle

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Axis of Rotation of the Ankle

The complex morphology of the ankle joint brings about a complex axis of rotation of the ankle. Since the 1950s, it has been recognized that the ankle joint axis during dorsiflexion is different than that of plantar flexion.1,2 Barnett and Napier measured trochlear surfaces of 152 human tali to discover that the medial and lateral curvatures of the talus are different, indicating that the axis of rotation of the ankles changes its position during the arc of motion.2 Hicks defined these different axes as the “dorsiflexion axis” and “plantarflexion axis,” stating that movement cannot occur about these two axes simultaneously.1

Lundberg et al used roentgen stereophotogrammetry to analyze the axis of rotation in eight healthy volunteers. Measurements taken at 10° increments between 30° plantar flexion and 30° dorsiflexion showed that not only does the axis of rotation change continuously, but large inter-individual differences also exist.3

Range of Motion of the Ankle

Ankle range of motion (ROM) in humans has been shown to have significant variations due to geographical and cultural differences in the activities of daily life.4,5 Nevertheless, all movements of the ankle during normal gait – dorsiflexion, plantar flexion, supination, and pronation – occur at multiple levels and involve several hindfoot and midfoot joints.6 For example, movement of the hindfoot results from a combination of movements about the tibiotalar and subtalar joints.7-11 The motions of plantar flexion and dorsiflexion are the major contributors to overall ankle motion are of particular import during clinical evaluation. The average plantar flexion and dorsiflexion of the ankle has been measured at 40°-56° and 13°-33°, respectively.4,5,10,12-15

During most activities of daily living, only a partial ROM is required: walking on an even surface (10°-15° plantar flexion and 10° dorsiflexion), walking upstairs (37° total ROM), walking downstairs (56° total ROM).13 A study of elderly patients found that a total ROM of 15°-20° is normally sufficient to complete their daily routines.13

Several studies have shown that the ROM may change as a function of age and gender.16-18 Nigg et al measured ankle ROM in subjects between the ages of 20 and 80 years.18 The authors demonstrated that in younger persons, the ROM is usually greater in women than in men. However, among older persons, female subjects had 8° less dorsiflexion and 8° more plantar flexion than males. In general, the ROM in all persons – male and female – decreased between 20 and 80 years of age.18

The same working group measured ROM in children (9-13 years), adolescents (14-16 years), and young adults (17-20 years) and compared the data with those from the aforementioned study.17,18 Within each gender, there was a consistent trend for a decrease in ROM, from a maximum at 14 to 16 or 17 to 20 years to a minimum after age 60 years.17 These results were not confirmed by Macedo and Magee.16 They measured ankle ROM in 90 women from 18 to 59 years of age. Both, plantar flexion and dorsiflexion increased with age.16 However, the highest age in this patient group (59 years) was lower that in the study by Nigg et al; therefore, results may not be wholly comparable^16,18^

Limb dominance and asymmetry in the actions of the lower extremities during walking and running is a common phenomenon.2 Macedo and Magee measured differences in ROM between dominant and nondominant sides of upper and lower extremities in 90 healthy females aged between ages 18 and 59 years.20 The following significant differences between dominant and nondominant sides were found for the ankle:

  • Passive dorsiflexion,  2.1°; 95% CI 0.5° to 3.7°
  • Passive plantar flexion, -2.5°; 95% CI -4.6 to -0.4°
  • Active plantarflexion, -3.4°; 95% CI -5.6° to -1.2°
  • Active inversion, 1.5°; 95% CI -0.2° to 3.2°

Ferrario et al measured the asymmetry of the active ROM of the ankle and its coupled movements in 65 young adults.21 In total, 20% of female and 34% of male subjects had more than a 5° difference in total ROM (dorsiflexion and plantar flexion).21 The measured differences may not have high clinical significance; therefore, the opposite side of the body may be used as reference value of pre-injury or normal ROM.20,21

Mitchell et al addressed the effect of pelvic, hip, and knee position on ROM of the ankle.22 In their biomechanical study, they found that pelvic posture may not significantly influence ankle ROM regardless of hip and knee joint position. However, in persons with hip flexion and knee extension, a significantly reduced ROM of the ankle was measured. This should be considered in patients after long periods of sitting.22 Thoms and Rome measured active ankle joint dorsiflexion in 26 healthy subjects in three different positions: supine, prone, and sitting.23 The following values were obtained: 6.59° ± 4.96°, 6.45° ± 4.64°, and 11.67° ± 5.96°, respectively. These results should be considered for evaluation of data collected on different occasions where hip and knee position may have varied.23

The ROM of the ankle may be reduced in patients with systemic diseases (eg, diabetes mellitus).24,25 The subjects with diabetes mellitus have reduced passive ROM of the ankle and increased stiffness compared to control group without diabetes. This may explain the pathological changes in plantar loading and high rate of ulcer formation in patients with diabetes mellitus.24,25

Load Transmission in the Ankle Joint

Numerous studies have been performed addressing measurements of surface contact area of the ankle joint.26-31 Kura et al measured contact surfaces in various joint positions in nine cadaveric ankles.27 The total available articular area was divided into central, medial, and lateral zones, which were measured in unloaded ankles as 922 ± 120 mm2, 178 ± 66 mm2, and 308 ± 60 mm2, respectively. After load application of 667 N in neutral position, the measured contact surfaces showed no significant changes. In plantar flexion, there was a decrease in contact area. In an in-vitro created instability of the medial arch, a decrease was also seen in the central and lateral zone contact areas.27

Wan et al performed in vivo measurement of articular cartilage contact areas in nine healthy persons using three-dimensional models created from MRI.30 The average cartilage coverage area was 964.9 ± 156.1 mm2 and 1304.8 ± 208.4 mm2 on the tibial and talar surfaces, respectively. The cartilage contact areas were measured at different phases of the gait: heel strike position, mid-stance position, and toe off position. Wan et al measured the following contact areas for these three positions for tibial and talar cartilage surface areas: 28.2 ± 4.2% and 20.9 ± 3.6%, 43.7 ± 5.5% and 31.0 ± 5.0%, and 34.8 ± 4.1% and 25.8 ± 3.9%, respectively.30

Millington et al measured the ankle joint contact area under physiological load (1000 N) in 10 cadaveric specimens in neutral and 20° dorsiflexion, supination, pronation, and plantar flexion using a stereophotography technique.29 The greatest tibiotalar contact area was measured in dorsiflexion, with 7.34 ± 1.69 cm2, and the smallest in plantar flexionm with 4.39 ± 1.41 cm2. The greatest talofibular contact area was measured in dorsiflexion, with 2.02 ± 0.78 cm2, and smallest in pronation, with 0.77 ± 0.49 cm2.29

Calhoun et al analyzed pressure distribution in the ankle joint under different axial loads (490, 686, and 980 N) in different joint positions using pressure-sensitive Fuji film.26 The authors showed that inversion or eversion of the ankle in the neutral or dorsiflexion position leads to a decrease in total contact areas and an increase in average pressures. In general, the total contact area was higher and the average pressure was lower in dorsiflexion. The patterns of pressure distribution were different in dorsiflexion and plantar flexion: the main force transmission area moved anteriorly to posteriorly on the talus during the movement from dorsiflexion to plantar flexion. The talar dome contributed the most to the load transfer with 77% to 90% of the load.26 These findings were confirmed later by the study from Michelson et al.28

During gait, a significant cartilage contact deformation occurs in the ankle joint according to Wan et al, who used a combined dual fluoroscopic and MRI technique.32 The average cartilage thickness in the ankles without weight bearing was found to be 1.43 ± 0.15 mm and 1.42 ± 0.18 mm in the distal tibial and proximal talar cartilage layers, respectively. After application of full loading, contact strains higher than 15% were observed in more than 40% of the contact area, showing that the ankle cartilage on both sites, tibial and talar, experienced large deformation during the load transmission in the ankle joint.32

Haraguchi et al performed discrete element analysis to assess the three-dimensional contact characteristics and ligament tension in the ankle joint during the stance phase of gait.33 They showed that most ankle joint loading during the stance phase occurred across the articular surfaces, while the stabilization due to ligament tension played an inferior role. The medial ligaments have a more important part at the ligamental stabilization than the lateral ligaments: the deep deltoid ligament transferred the most force during the stance phase (57.2%), followed by the superficial part of the deltoid ligament (26.1%).33 Tochigi et al also identified in their biomechanical study that the integrity of articular surface geometry plays a key role in the passive ankle stability.34

In patients with post-traumatic tibiotalar osteoarthritis (OA), significant changes in kinematics of the ankle joint have been observed.35,36 Kozanek et al measured ankle kinematics in six patients with unilateral post-traumatic ankle OA using MRI and dual fluoroscopic imaging techniques.36 The obtained data were compared with data from a normal cohort of a previous study.37 In general, a significant decrease of the ankle ROM was observed in patients with post-traumatic OA during the gait. A breakdown in the normal motion coupling of the hindfoot also occurred in this patient group.36

Horisberger et al measured plantar pressure distribution in 120 patients with post-traumatic OA using dynamic pedobarography.35 The obtained results were compared with those from unaffected contralateral feet. In ankles with post-traumatic OA, the maximum force and contact area were significantly lower, as were pressures in the hindfoot and toes areas. No correlation was observed between plantar pressure distribution and functional outcome as assessed by AOFAS hindfoot score. However, significant differences in plantar pressure distribution patterns occurred in patients with valgus and varus malalignment.35

The dynamic function of the fibula has been shown to play an important role at load transmission during gait.38,39 Lambert indirectly measured that the fibula received approximately one-sixth of the load applied to the knee joint.40 These results were confirmed by direct measurements performed by Takebe et al, who addressed the role of the fibula in six cadaveric specimens under loads of 60 kg.41 In neutral position, an average of 6.4% of the load was transmitted by fibula. Positioned at 15° dorsiflexion, the fibula transmitted a higher load (10.4%), whereas ankles at 15° plantar flexion transmitted lower loads (2.3% of load) than at neutral position.41

Goh et al performed a biomechanical study to address load transmission characteristics of the fibula and the effect of fibular resection.42 An average of 7.12% of the load was transferred by the fibula, with increasing load transmission in dorsiflexion and eversion. Resection of the proximal fibula led to a radical decrease of fibular load transmission, down to between 0.62% and 0.81%.42 Wang et al addressed the role of the fibula in weight bearing and its contribution to ankle joint stability.43 Under an axial load of 1500 N, an average of 17% of the load was transmitted by the fibula. The position of the ankle joint also significantly influenced the fibular load transmission (eg, higher values were measured in dorsiflexion). The artificially created distal tibiofibular instability changes the fibular load transmission as well.43

The effect of fibulectomy on the load transmission and kinematics of the ankle joint has been addressed in the current literature.43,44 Bozkurt et al analyzed gait in three patients with proximal, middle, and distal fibulectomy.44 Their study revealed that the distal fibulectomy significantly alters ankle kinematics, while the proximal fibulectomy impairs knee joint stability. The middle fibulectomy with in-situ remaining proximal and distal fragments has only a slight influence on the biomechanical properties of the gait.44 Similar results were observed in a biomechanical study by Pacelli et al addressing the ankle instability following free fibular graft harvest.45 The authors showed that only 10% of the fibular length was needed distally to maintain the intrinsic ankle stability.45

Kinematics of a Total Arthroplasty of the Ankle

With recent advances in biomechanics and surgical techniques, the new generation of total ankle replacement (TAR) may better reproduce the physiological function and kinematics of the ankle. Valderrabano et al assessed kinematic changes after ankle fusion and TAR with regard to ROM,12 movement transfer,46 and talar movement.47 All measurements were performed in native ankles, then in ankles with two-component (Agility) and three-component (HINTEGRA and STAR) prostheses and in fused ankles. The results showed near normal kinematics after TAR and significantly impaired results in fused ankles.12,46,47 Komistek et al used fluoroscopy to evaluate translational and rotational motions of the hindfoot in the sagittal and frontal planes in 10 subjects having a normal ankle and TAR on the opposite side (Buechel-Pappas prosthesis).48 The average ROM for normal versus replaced ankles was 37.4° and 32.3°, respectively. In general, comparable kinematic patterns of motion were observed for normal and replaced ankles.48

Detrembleur and Leemrijse addressed the effects of TAR on gait disability by analysis of energetic and mechanical variables.49 The study included patients who underwent TAR using three-component implant designs: AES prosthesis (n=16), Mobility prosthesis (n=3), and HINTEGRA prosthesis (n=1). All patients were analyzed before and approximately 7 months after surgery using instrumented motion analysis to assess spatiotemporal parameters, ankle kinematics, mechanical work, and electromyographic activity. In addition, energy expenditure was analyzed using an ergospirometer. The authors showed that TAR has a beneficial effect on locomotor function, resulting in improvement of AOFAS score, speed, spatiotemporal parameters, ankle amplitude instance, and vertical center of mass displacement.49

One of the demanding steps of the TAR procedure is the correct positioning of the talar component. First, the original center of rotation of the tibiotalar joint may have changed due to joint degeneration and/or concomitant deformities. Second, even normal ankles have been shown to have different axes of rotation, which may change during the arc of motion.3 Consequently, sagittal malposition of the talar component is a common intraoperative complication of TAR.50,51 Furthermore, Lee et al have reported that the number of malpositions may not decrease with increased surgeon experience.50

Positioning of prosthesis components, especially the talus, has been shown to strongly affect resulting kinematics. Tochigi et al investigated the effect of talar component sagittal positioning on ROM using a specially-modified STAR prosthesis.52 They found that anterior talar component displacement significantly decreased the plantar flexion, which was associated with bearing lift-off, while posterior displacement of the talar component significantly decreased the dorsiflexion.52 Saltzman et al addressed the effect of ankle prosthesis misalignment on the peri-ankle ligament using an in-vitro Agility prosthesis model.53 The anterior talofibular ligament was sensitive to transverse plane displacements, while the tibiocalcaneal ligament was sensitive to coronal plane displacements.53 Furthermore it has been shown that sagittal misalignment has a negative influence not only on biomechanics of the ankle, but also on other factors, such as malrotation.

Recently, Espinosa et al reported on two finite-element models of Agility and Mobility ankles.54 They modeled potential misalignments with respect to version of the tibial component, version of the talar component, and relative component rotation of the two-component design. Measured pressures consistently increased with misalignment, suggesting that accurate positioning of prosthesis components is one of the most important requirements for success of the procedure.54 Fukuda et al reported the results of an in vitro study on the effect of talar component malrotation of Agility prosthesis.55 They found that the talar component malrotation leads to increased peak pressure, decreased contact area, and increased rotational torque, further supporting the need for accurate component placement.55


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