1. Ankle Biomechanics and Gait Analysis
Dr. Tim Daniels
MD FRCSC
Assistant Professor
University of Toronto

2. Ankle Biomechanics and Gait Analysis
Anatomy and Biomechanics of the ankle joint.
Histology of ankle cartilage.
Ankle function during gait.

3. The Ankle Joint
Primary motion at the ankle joint is in the sagittal plane as dorsiflexion and plantarflexion.
There has been a misconception that biomechanically the ankle functions as a hinged joint
Not a hinged joint.
The ankle joint was considered a hinge joint for many years. This assumption was based on the observation that the primary motion at the ankle joint is in the sagittal plane – dorsiflexion and plantarflexion.
Subsequent studies have clearly demonstrated that the ankle is a biaxial articulation – meaning that there is a significant amount of rotation that is occurring through its’ arc of motion.
The axis of a hinged joint would be run within one of the orthogonal planes – in the ankle joint this would be the transverse plane.
The true axis of the ankle joint – as described by Inman – runs obliquely, corresponding to the tips of the malleoli. This biplanar axis reflects the fact that the ankle is a biplanar joint. The primary motions are flexion- extension in the sagittal plane and internal – external rotation in the transverse plane.

4. The Ankle Joint 15° Biplanar motion
external rotation with dorsiflexion
internal rotation with plantarflexion.
15°
The ankle externally rotates in dorsiflexion and internally rotates in plantarflexion. This is clearly demonstrated by the changing position of the transverse axis of the foot in this diagram.
Sterophotogrammetric analysis has clearly demonstrated that the amount of rotation at the ankle joint level can be as much as 15 degrees through the arc of dorsiflexion and plantarflexion.

5. Ankle Rotation A B A > B Apical Angle Cone shaped articulation.
Rotation of the ankle joint occurs around an axis that corresponds to the position of the deltoid ligament – also referred to as the A B
When considering the rotation at the ankle joint – understanding the cone shaped articulation of the ankle joint is important.
The axis of rotation at the ankle joint is medially based near to the insertion of the deltoid ligament. This is referred to as the Apical Angle.
With rotation, the distance that the talus must move is shorter on the medial vs.. the lateral side. This is one reason why a single fan shaped ligament such as the deltoid is capable of supporting the ankle joint on the medial side.
Apical Angle
A > B

6. Ankle Stability
A single fan shaped deltoid ligament is adequate for medial stabilization.
Posterior tibiotalar ligament
Tibiocalcaneal ligament
Anterior tibiotalar ligament
Tibionavicular ligament
The ability of the ankle to rotate is dependent not only on its unique geometric shape but by the soft tissue that stabilize the joint.
The deltoid ligament is located on the medial side of the joint. It is a fan shaped ligament consisting of two deep arms – the anterior and posterior tibiotalar ligaments – and tow superficial arms – the tibionavicular and the tibiocalcaneal ligaments.

7. Ankle Stability
Larger surface area laterally – stability provided by three separate bands.
Anterior talofibular ligament
Posterior talofibular ligament
The fibula extends more distally than the medial malleolus thus providing greater osseous support on the lateral side.
With the larger surface area laterally and a greater amount of rotation necessary the lateral side is supported by three separate bands. This geometric configuration of the ligaments allows for both stability and the increased motion necessary for normal ankle kinematics.
The 3 lateral bands consist of the anterior talofibular ligament, the posterior talofibular ligament and the calcaneofibular ligament.
Because the axis of rotation is medially based with increased surface area and rotation on the lateral side – the lateral side of the ankle is inherently less stable than the medial side. This is probably why the lateral malleolus extends more distally than the medial malleolus – thus in parting more stability.
Calcaneofibular ligament

8. Ankle Stability
Larger surface area laterally – stability provided by three separate bands.
Anterior talofibular ligament
Posterior talofibular ligament
The fibula extends more distally than the medial malleolus thus providing greater osseous support on the lateral side.
With the larger surface area laterally and a greater amount of rotation necessary the lateral side is supported by three separate bands. This geometric configuration of the ligaments allows for both stability and the increased motion necessary for normal ankle kinematics.
The 3 lateral bands consist of the anterior talofibular ligament, the posterior talofibular ligament and the calcaneofibular ligament.
Because the axis of rotation is medially based with increased surface area and rotation on the lateral side – the lateral side of the ankle is inherently less stable than the medial side. This is probably why the lateral malleolus extends more distally than the medial malleolus – thus in parting more stability.
Calcaneofibular ligament

9. The foot minus the talus is considered as one fibro-osseous unit.
MacConaill and Basmajian 1945
MacConail and Basmajian introduced the concept of the twisted plate as early as 1945.
(MacConail M, Basmajian J. Muscles and Movement: A Basis for Human Kinesiology. Baltimore, Williams & Wilkins, 1969.)
The foot minus the talus is considered as one fibro-osseous unit. The joints in this unit can be congruent in which the ligaments are under maximal tension, a situation achieved by stretching the ligaments and “screwing” the osteofibrous mass about its long axis until it screws home.
In certain motions and with certain loads, the position of the joints can be incongruent and therefore less stable. Lamina Pedis can be seen as a twisted plate.
By controlling the amount of twist in the laminar plate, its flexibility and rigidity can be controlled. During push-off, a stiff foot is needed to attain maximal propulsion force, whereas flexibility is necessary to adapt the foot to an uneven surface.

10. Inherent stability of the ankle is important:
It is the primary articulation responsible for the transmission of forces from the ground to the remainder of the lower extremity.
It is subjected to greater forces per square cm than any other joint of the body:
FORCES
CONTACT AREA
When one considers the foot minus the talus as a single fibro-osseus unit, then it becomes apparent that the talus is the primary articulation responsible for the transmit ion of forces from the ground to the remainder of the lower extremity.
This is also reflected by the fact that the talus is subjected to greater forces than any other joint in the lower extremity. This is in spite of the fact that it has the lowest contact area when compared to the knee and the hip.
Hip = X body weight - Knee = X body weight - Ankle = X body weight
1100 mm² mm² mm²

11. Ankle Stability
The actual surface area of the Ankle is larger than the Knee and Hip - 12 cm² (Stouffer)
The surface area available to distribute the forces is smaller
The actual surface area of the ankle joint is greater than the hip and the knee. However, this figure takes into consideration the medial, lateral and posterior articular surfaces that are in contact with the malleoli. The actual surface area available for the distribution of the ground reactive forces is a fraction of the total surface area.
The increase surface area of the ankle helps in providing stability to the articulation but not in the distribution of ground reactive forces.
= contact area
The increased surface area aids in stability of the ankle joint but not in the distribution of forces.

12. Ankle Stability The ankle joint is a stable articulation.
It is supported by three bony prominences:
Medial malleolus
Considering the forces placed through the ankle joint at unpredictable directions, there is an inherent need for this articulation to be geometrically stable.
This stability is provided by three bony prominences:
- medial malleolus
- lateral malleolus
- posterior malleolus
Lateral malleolus
Posterior malleolus

13. There is a common misconception that the ankle is much more stable in dorsiflexion than it is in plantarflexion
There is a common misconception that the ankle is much more stable in dorsiflexion than it is in plantarflexion.
Although the ankle may be slightly more stable in dorsiflexion, the degree of stability is not as great as once thought.

14. The Ankle Joint Foot Fulcrum Ankle Joint 5 X body weight A >B A B
This concept of instability in plantarflexion began with the observation that the width of the anterior aspect of the talar body was greater than the posterior width.
If the ankle joint were considered as purely a hinged articulation than this geometric design would impart instability when in the position of plantarflexion. However, from a functional prospective this would not make much sense since the ankle joint is subjected to the greatest forces when in a plantarflexed position.
In the planatarflexed position at the latter portion of the stance phase the ankle joint is subjected to 5 X the body weight. This is due to the lever arm effect of the foot with the fulcrum being at the ankle joint. If the joint were unstable in this position then there would be considerable medial and lateral translation of the talus within the ankle mortise. This would create unacceptable shear forces leading to cartilage damage and early OA.
Fulcrum
A >B

15. The Ankle Joint A > B A > B A B
Using the apical angle as a reference point, the discrepancy in the functional width of the anterior and posterior portions of the talus is not as significant as the discrepancy in the anatomic width.
A > B
A > B

16. The Ankle Joint
This concept is more clearly represented in this diagram.

17. There is a common misconception that the ankle is much more stable in dorsiflexion than it is in plantarflexion
Lever Arm
A >B
The ankle is more commonly injured in the position of plantarflexion and the lateral structures are more commonly torn than the medial. This has more to do with the lever action of the foot than inherent instability of the ankle joint in the position of plantarflexion.
So, although there may be some inherent instability of the ankle in plantarflexion, the extent of instability is not as great as initially thought. B A

18. The Ankle Joint
The ankle joint is subjected to the highest forces per square centimetre.
It is more commonly injured than any other joint in the body.
Yet, the incidence of symptomatic ankle arthritis is approximately nine times lower than that of the knee and hip.
There is a paradox when one considers the high forces that the ankle joint is subjected to and the low incidence of ankle osteoarthritis.

19. The Ankle Joint (Histology)
Articular cartilage thickness: • Knee - 1 to 6 mm • Ankle - 1 to 1.7 mm
Ankle cartilage is more uniform in nature when compared to the knee and hip.
Ankle cartilage is stiffer than the knee and hip.
Talar cartilage is more resistant to physiologic stresses created by cyclical loading.
Ankle cartilage more resistant to the actions of MMP (matrix mealloprotinases) than the knee and hip.
Though the anatomical and biomechanical differences exist between the joints of the lower limb, these differences alone do not entirely account for the differences in susceptibility to arthritic change. The destructive process accompanying the development of osteoarthritis is mediated by matrix metalloprotinases (MMP). This family of substances are proteolytic to all known cartilage matrix components including proteoglycans, collagens, noncollagenous proteins and matrix glycoproteins. In studies of human tissues, it has been observed that MMP-8 is expressed by chondrocytes and is elevated in OA cartilages [46, 47].
Chubinskaya et al [48] demonstrated that expression of MMP-8 was seen in knee cartilage but not in ankle cartilage, unless the latter showed degenerative change. The MMP-8 expression was seen following the action of the catabolic cytokine, interleukin-1 (IL-1) beta on normal ankle cartilage. Recent study has suggested that some MMPs may play a role in normal cartilage; only some members (MMP-3, 8) are expressed as being related to articular cartilage damage [49].
Kang et al [50] demonstrated a significant difference in chondrocyte response to stimulation by fibronectin fragments. Knee chondrocytes showed greater proteoglycan (PG) loss and increased aggrecanase activity when compared to the ankle. Ankle cartilage was much more refractory to damage than knee cartilage from the same donor.
Interleukin-1 (IL-1) beta is effective in inhibiting PG synthesis from chondrocytes of both the knee and the ankle [51]. The inhibitory response was much greater in the knee than in ankle chondrocytes and appears to be related to the numbers or types of receptors present on the chondrocytes. The effect of IL-1 was more effectively overcome by IL-1 receptor antagonist protein in the ankle than in the knee.
The pathogenesis of ankle osteoarthritis is a complex event to which anatomical, biomechanical and metabolic factors may all contribute. All the joints of the lower limbs are subjected to similar loads during weightbearing [52], yet the incidence of osteoarthritis in these joints is quite variable. The differences in the incidence of osteoarthritis and etiology of arthritis suggest that the ankle is more resistant to primary osteoarthritis but more susceptible to post-traumatic degeneration. The ankle may be protected from the development of primary osteoarthritis by its metabolic properties, tensile characteristics, uniformity, congruency and restrained movement. The thinness of ankle articular cartilage as well as the high peak contact stresses to which it is submitted may make it less adaptable to the incongruity, decreased stability or increased stresses which may follow a traumatic event.

20. Ankle Joint Function during the Gait Cycle

21. The Foot functions as a Rocker
During the stance phase, three rockers are described, which contribute to the controlled forward fall of the body during gait.
1/ The rocker heel occurs during the loading response.
2/ The ankle rocker (ankle dorsiflexion) during midstance.
3/ The forefoot rocker during terminal stance.
The ankle allows for the smooth transfer of body weight from the hindfoot through to the forefoot while the foot is firmly planted on the ground.
Maximum dorsiflexion occurs during the stance phase. A common misconception is that maximum dorsiflexion is during the swing phase as the ankle dorsiflexes so that the foot can clear the ground.

23. Remarkably resilient to the development of Osteoarthritis
The Ankle Joint
Remarkably resilient to the development of Osteoarthritis
Subjected to the greatest forces.
Lowest contact area.
Most commonly injured joint.
The paradox of the ankle joint is that in spite of the fact that it is subjected to the highest forces, it has the lowest contact area and is the most commonly injured joint of the lower extremity it has a remarkable resilience to primary arthritic changes. Most patients afflicted with ankle arthritis have either had previous fractures or inflammatory arthropathies.

24. The ankle appears to be protected from the development of OA by its:
Metabolic properties
Uniform cartilage mantle
Congruency
Stability
Restrained movement
The ankle may be protected from the development of primary osteoarthritis by its metabolic properties, tensile characteristics, uniformity, congruency and restrained movement. The thinness of ankle articular cartilage as well as the high peak contact stresses to which it is submitted may make it less adaptable to the incongruity, decreased stability or increased stresses which may follow a traumatic event.