Documentation:FIB book/Kinematics of Ankle Injury

Overview

Introduction

The ankle is a joint comprising the lower leg and the foot, allowing interaction between the leg and the ground and acting as a kinetic linkage, fulfilling the key requirements of walking, running, and other gait activities. Ankle injuries are among the most common musculoskeletal injuries, affecting individuals of all ages and activity levels. Proper classification is essential for accurate diagnosis, treatment planning, and rehabilitation.

Due to the range of severity and frequency of occurrence, there is no reliable measure of the percentage of the population which undergoes ankle injuries. Studies estimate the occurrence of ankle sprains in anywhere from 10% to 44% of all injuries in physically active populations. Because of the role of the ankle in sports and daily activities, injuries to the ankle can often lead to a deterioration in the quality of life of individuals who experience them.

This review aims to cover the anatomy and physiology of the ankle, relevant ankle injury classification, cadaver, and finite element testing methodologies, as well as future work.

Ankle Anatomy and Physiology

Connection to the Leg

The ankle and foot is made up of 33 joints comprising the 26 bones of the foot and the long bones of the leg. These 33 joints are frequently referred to as the “ankle joint”, and facilitate the motion of the foot.

Bones

Figure 1: The main bones of the AJC (superior view)

Rigid structures and joints are needed for muscles and ligaments to convert force into movement. There are three bones that play an important role in the functionality of the ankle joint. The tibia and fibula are the two bones of the lower leg. Between the tibia and fibula, the talus is connected to the fibula on the lateral side (further away from the midline of the body) and to the tibia on the medial side (closer to the midline of the body), acting as a hinge joint.

There is synovial fluid between the bony structures, which serves to prevent direct bone-to-bone contact during movement. To allow for flexion of the foot, the talus is anteriorly (towards the front) wider and more narrow on the posterior (towards the rear). This is important to allow for plantarflexion (as shown in figure 2) when pushing off the foot during gait and for achieving a stability during dorsiflexion.

On the posterior side of the foot, the talus is connected by a joint to the calcaneus and on the anterior side to the navicular bone, as shown in figure 1. These, and more than 25 other joints are also important for other movements of the foot relative to the leg and for balancing. To facilitate discussions about ankle fractures and other injuries, we often refer to all of these joints (including ligaments) as the ankle joint complex (AJC).

Range of Motion

Figure 2: Movement of the AJC

The AJC has three degrees of freedom when it is perpendicular to the lower leg. As shown in figure 2, the foot can undergo plantarflexion and dorsiflexion, inversion and eversion, and abduction and adduction. To improve stability, movements are limited in all directions, either passively by ligaments and bones or actively by muscles. Therefore, the AJC can only move within a certain range, defined as the range of motion (ROM). The ROM of each motion is listed in table 1.

Table 1: Range of Motion in all planes.

Dorsiflexion	18-28°
Plantarflexion	40-48°
Inversion	15-26°
Eversion	11-17°
Abduction (external axial rotation)	33-49°
Adduction (internal axial rotation)	25-46°

The current position of the foot also has an impact on how the foot can be moved. For example, the maximal dorsiflexion is constrained by the shape of the talus.

The range of motion varies between sexes and depends on age. Females typically show a greater ROM due to higher estrogen levels compared to males, rendering ligaments and tendons less stiff resulting which greater flexibility. Highest ROM is also generally reached before the age of 20. The decrease in ROM after reaching adulthood is due to the loss of elasticity of cartilage and tendons and the decrease in skeletal strength. Physical activity and muscle strength also influence ROM. ROM depends on the current position of the foot, and it is therefore important to take this into account when studying AJC injury mechanisms. If a force is applied at the limits of the ROM, this generally places a large load on the limiting structures, possibly exceeding these limits and resulting in plastic deformation and injury of these structures.

Ligaments

Figure 3: Ligaments on the Lateral Side of the Ankle

The ligaments of the ankle comprise of three groups:

1. Lateral ligaments
2. Deltoid ligament on the medial side, and
3. Ligaments of the tibiofibular (which join the tibia and fibula bones of the leg).

Damage to these three groups of ligaments are commonly associated with different types of injuries. These ligaments on the lateral side of the ankle are illustrated in figure 3, and the ligaments on the medial side of the ankle are illustrated in figure 4.

The anterior talofibular ligament is the most commonly injured ligament of the ankle, and plays a role in plantar flexion of the ankle and limiting movement of the talus. Talofibular ligaments are often the first or only ligaments to sustain injury in the case of an inversion to the foot.

In the case of eversions, damage will be caused primarily to the deltoid ligaments on the medial side of the ankle joint.

Figure 4: Ligaments on the Medial Side of the Ankle

With hyperdorsiflexion (excessive flexing upwards of the foot), the tibiofibular ligaments sustain the bulk of the damage. These injuries are further discussed in the Ankle Sprain section.

Muscles

Most of the muscles associated with the ankle joint are responsible for plantarflexion and dorsiflexion, as well as inversion and eversion. Muscles on the anterior side of the leg are responsible for dorsiflexion whereas posterior muscles produce plantarflexion. Similarly, contraction of muscles that are connected to the medial bones of the foot cause inversion and contraction of muscles connected to the lateral side cause eversion.

The activation of all these different muscles results in the positioning and movement of the foot. However, the muscles are also crucial for joint stability, reflexes and pretension, which can influence the mechanism and severity of the injury.

Ankle Injury Types and Classifications

This section will categorize ankle injuries into three key areas: different kinds of ankle fractures, various types of ankle sprains, and the Abbreviated Injury Scale (AIS) standards used to assess the severity of ankle trauma. By understanding these classifications, healthcare providers and researchers can ensure standardized evaluation and improved patient outcomes.

Figure 5: Lauge-Hansen Classification

Classification Systems of Ankle Fractures

Figure 6: Danis-Weber Classification

Lauge-Hansen (figure 5) classifies ankle fractures based on the position of the foot and the direction of the force experienced by the foot at the time of injury. Type A and B show supination of the ankle in an eversion and abduction case, respectively, whereas Type C and D shows the same cases but in pronation. The supine-eversion pattern is the most frequent in emergency departments.

The syndesmosis is a complex fibrous joint between two bones connected by ligaments and a strong membrane. The Danis-Weber classification system (figure 6) highlights the localization at the fibula, dividing the fractures into three groups: type A (below the syndesmosis level), type B (at the syndesmosis level), and type C (above the syndesmosis).

Figure 7: AO Classification for Ankle Fracture

The Arbeitsgemeinschaft für Osteosynthesefragen (AO) system as an expanded form of Danis-Weber. Not only does it classify based on the location of the fracture but also the severity of the injury. Despite the AO system being beneficial in the in-depth specification of the injury, it is substantially more difficult to diagnose when covered with skin. The AO classification (illustrated in figure 7) follows the same letter pattern as Danis-Weber but with corresponding numbers to the severity of the injury (1-mild, 2-moderate, 3 severe) as listed below:

• Type A: A1 (isolated infrasyndesmotic fibular fracture), A2 (infrasyndesmotic fibular fracture with fractured medial malleolus), A3 (infrasyndesmotic fibular fracture with posteromedial fracture)
• Type B: B1 (isolated transsyndesmotic fibular fracture), B2 (transsyndesmotic fibular fracture with medial lesion, fracture of the medial malleolus or of the deltoid ligament), B3 (transsyndesmotic fibular fracture with medial lesion and posterior malleolus lesion)
• Type C: C1 (simple suprasyndesmotic fibular fracture), C2 (multifragmentary suprasyndesmotic fibular fracture), C3 (proximal fibular fracture – Maisonneuve fracture).

There is little agreement between the Lauge-Hansen, Danis-Webber, and AO classifications, indicating inconsistencies in how different evaluators classify ankle fractures . However, findings suggest that the Danis-Weber classification is more reliable compared to the Lauge-Hansen and AO systems. This suggests that while variability exists among observers of the injury, the Danis-Weber system may provide a more consistent and reproducible method for categorizing ankle fractures. Further research may be needed to enhance the reliability of existing classification systems or develop new, standardized approaches for improved fracture assessment.

Classification of Ankle Sprain based on Ligaments

Figure 8: Types of Ankle Sprain

An ankle sprain is also a common type of injury often resulting from sudden twisting or rolling motions. These injuries are typically categorized based on both the severity of ligament damage and the specific ligaments involved. The severity classification ranges from mild (Grade 1) to severe (Grade 3), depending on the extent of ligament tearing and joint instability. The grading system for ankle sprains is as follows:

• Grade 1 (Mild Sprain): This involves minor stretching and microscopic tearing of the ligament fibers. Swelling and tenderness are mild, and there is little to no joint instability.
• Grade 2 (Moderate Sprain): A moderate sprain includes a partial tear of the ligament, leading to noticeable swelling, bruising, and pain. Some instability may be present, along with reduced ankle strength.
• Grade 3 (Severe Sprain): This is the most severe type, involving a complete rupture of the ligament. It results in significant swelling, bruising, and intense pain. The ankle becomes highly unstable and may feel weak or unable to support weight.

Additionally, the classification of the ankle sprains is based on which of the three lower leg ligaments are affected as shown in figure 8. Depending on the location of ligament injury, ankle sprains are classified as follows:

• Lateral Ankle Sprain is the most common injury that involves the lateral ligaments (outside of the ankle) which is caused by inversion motion (rolling the ankle inward).
• Medial Ankle Sprain is considered less common and involves the deltoid ligament (inside of the ankle) which is caused by eversion motion (rolling the ankle outward).
• High Ankle Sprain or also known as Syndesmotic Sprain involves the syndesmosis ligaments (between the tibia and fibula) which is caused by excessive twisting or forceful dorsiflexion (toes pushed up). Ankle injuries that involve these ligaments tend to be more serious and require a longer healing period.

Abbreviated Injury Scale (AIS) and Foot and Ankle Severity Scale (FASS)

The Abbreviated Injury Scale (AIS) is a standardized system that classifies injuries across various body regions, including the ankle, on a scale from 1 to 6, with higher scores indicating more severe injuries. However, the AIS lacks the sensitivity to capture the full range of severity in foot and ankle injuries, as the majority are categorized as AIS 1 or 2.

To address this limitation, the Trauma Committee of the American Orthopaedic Foot and Ankle Society developed the Foot and Ankle Severity Scale (FASS). This scale ranks 91 common foot and ankle injuries based on severity (FASS-S) and estimates long-term impairment for each injury (FASS-I). Since this study focuses on acute trauma, FASS-S is the most relevant. It ranks 91 common foot and ankle injuries on a 1 to 6 scale, where:

• 1 = Minimal injury
• 2 = Mild injury
• 3 = Moderate injury
• 4 = Severe injury
• 5 = Very severe injury
• 6 = Currently untreatable

While these categories help in general stratification, the specific assignment of FASS-S values can be nuanced and context-dependent. For detailed examples of how injuries are classified within this system, readers are referred to the original paper.

Typical AIS classifications for ankle injuries are as follows:

• AIS 1: Minor
• AIS 2: Moderate
• AIS 3: Serious
• AIS 4: Severe
• AIS 5: Critical
• AIS 6: Maximum
• AIS 9: Unknown

As with facial injuries, ankle injuries are rarely life-threatening, with the AIS rating seldom exceeding AIS 4. The use of FASS-S provides greater specificity and clinical relevance in assessing foot and ankle trauma for research and practice.

Testing and Methodologies

Researchers have worked towards analyzing ankle loading conditions for diagnosis and treatment purposes, mainly through cadaver and finite element testing. Their objectives are generally focused on understanding how different ankle loads and directions affect ankle injury and failure.

Cadaver Testing

Ankle cadaver testing in the past has sometimes been associated with load applications that cannot be measured by ATDs such as footplate force.

Funk et al. 2002 explores axial loading of the foot/ankle complex by conducting blunt axial impact tests on isolated lower extremities. This test aimed to determine the effects of active muscle tension, and simulated an active triceps surae by applying tension to the Achilles tendon. Using acoustics emission (AE), a method where fracture sound is measured, they were able to determine the exact time of fracture during tests. For this paper, acoustic sensors were mounted onto the bone itself, and sampled at a rate of 10000 Hz. The amplitude of the sampled sound indicated how loud the fracture was, which was then used to determine if the fracture was macroscopic or microscopic.

A standout aspect to the tests in this paper is that the load cell was implanted midway in the tibia shaft. This method allowed for preservation of the knee joint, but the implantation process may have caused weaknesses in the tibia. However, qualitative comparison of the ankle and tibia was still found, as the ankle was always fractured when tibial plateau fracture occurred, indicating that the ankle is likely a weak link in the lower extremities.

Finite Element Testing

Contrary to other critical parts of the body (head, neck, abdomen, etc.), the ankle has very few, if any, official criteria to evaluate injury. Ankle injuries are the most common among all lower limb injuries in car crashes, and quality of life can be impacted by the loss of their function. Attempts to define precise loading conditions resulting in ankle injury in literature exist, mostly exploring pedal-ankle interactions in car accidents. These loading factors can eventually be generalized to be relevant in contexts outside of car crashes. Similarly, studies have shown how finite element analysis is used to evaluate the probability of injury in some of these loading conditions .

The simplest analysed load is axial, where a certain force applied by a flat plate (measured through the tibia) causes failure of the ankle. The maximum axial value found in the study with an ankle model is evaluated around 5.3kN, which corresponds to typical cadaver response. This value could be considered as the fracture load threshold when evaluating results from an ATD.

Other relevant loads analysed are inversion and eversion, which were applied through a rotating plate on the foot creating a moment around a center point in the ankle. The same study found a moment of 41.2 Nm obtained at 31.6 degree plate inclination would cause failure through eversion (rotation of the ankle outwards) and a moment of 23.1 Nm at 35.1 degrees through inversion (rotation inwards). The availability of these values implies that general injury criteria can be developed.

Future Work

Figure 9: Example Kite Plot for Ankle Injuries (Disclaimer: this is from the group´s own creation and idea)

figure 9 shows an idea of what an injury criteria might look like. Ankle injuries are sometimes a combination of multiple load types and a thorough study of the limits before failure could be a promising way to develop a generalized criteria. This is purely a proposition inspired from the neck injury criteria, Nij, To combine possible loads in the form of a kite plot. Moment limits and the shape of the kite plot depend on the amount of the preload (muscle bracing or weight resting on the ankle).

Conclusion

This review described the ankle as a complex, vulnerable part of our body, sometimes overlooked in the context of injury prevention. Through the analysis of basic anatomy and physiology, it is possible to evaluate patterns and weaknesses which stand out in the different resulting injuries to introduce a generalized ankle injury criteria. This would allow for more informed diagnosis and treatment depending on the scale of the injury. In a time where quality of life is a priority, further study would make a considerable difference in maintaining the general well-being of an active population.