Diagnosing and Managing Chronic Ankle Instability
The ankle joint consists of many bones, ligaments and tendons that all play an integral part in maintaining its stability. All these function to allow a multitude of movements within the ankle joint. Consequently, ankle instability can result if any of the related bones, tendons, or ligaments are compromised. The tibia and fibula are intimately joined by the interosseous membranes. These two bones also share connections with the ligamentous complexes of the foot so that any sprain on the ligaments can also potentially cause a distal fracture in the bones of the lower leg. Additionally, the tendons from the muscles of the lower leg—mainly the Achilles tendon and the peroneus longus and brevis, are closely associated with the bones of the foot. Injury to these tendons will likely result in ankle instability.
Anatomy and Mechanics of Ankle Motion
The ankle joint has no distinct boundary due to its intimate connections with the lower leg and foot. The ankle is a synovial, hinge joint that is comprised of three articulating bones, three groups of ligaments and three supporting tendons. The bones of the lower leg are the tibia and the fibula. Their distal ends form a mortise that encompasses the talus, which is a bone in the foot (see Figure 1). The tips of the tibia and fibula surrounding the talus form the medial and lateral malleoli, respectively. An imaginary line connecting the malleoli approximates the axis around which the ankle moves during motion.
This axis acquires an oblique angle that points posteriorly and inferiorly. Because of this, plantarflexion of the ankle also produces internal rotation or inversion, while dorsiflexion causes external rotation or eversion.2
The ligaments of the ankle are best described by categorizing them into the interosseous ligamentous complex, the medial ligamentous complex and the lateral ligamentous complex (see Fig-ure 2).
Together, these ligaments work to provide stability to the ankle joint while in motion. The interosseous complex tightly binds the tibia and fibula and, at the same time, allows slight rotation of the tibia during ankle movement. The medial complex functions to limit excessive external rotation of the talus within the joint space. These ligaments are collectively referred to as the deltoid ligament. Lastly, the lateral complex, consisting of the posterior talofibular ligament (PFL), calcaneofibular ligament (CFL), and anterior talofibular ligament (AFL) serves to prevent extreme internal rotation of the talus.
The ankle joint is further supported by tendons from muscles of the lower leg. The Achilles tendon, which connects the gastrocnemius and soleus muscles to the calcaneus bone in the foot, is the most important tendon for walking, running, jumping, and standing on tiptoes. The peroneus longus and brevis tendons course just posterior to the lateral malleolus and inserts on the lateral first metatarsal and cuneiform bone of the foot. These tendons allow eversion of the sole of the foot and plantarflexion of the ankle (see Figure 3).
Mechanism of Ankle Instability
Due to the complexity of the ankle joint and surrounding structures, any injury to the bones, tendons, or ligaments can result in instability. Fractures or dislocations of the tibia or fibula can cause ankle unsteadiness. For example, axial impact on the heel of the foot—that can occur with simple actions like jumping from a higher to a lower level—will produce a vector of force directed into the distal tibia. This, in turn, can cause a pilon fracture, or a fracture of the horizontal articular surface of the tibia.3 Such an injury to the tibia causes an increase in contact stress between the tibia and talus and is an important pathomechanical factor in unstable joints.4 Additionally, the fibula may also have a profound effect on ankle stability.
Although stable ambulation can occur with a section of the fibula removed, it was found that resection of the fibula segment close to the tibiofibular joint resulted in significant ankle instability. Consequently, by resecting parts of the distal fibula, a mobile remnant is produced which causes an inability to withstand loading pressure. This instability is partially due to the compromised integrity of the interosseous membrane between the tibia and fibula.5 In a study done by Thordarson et al, it was found that displacement of the fibula by a shortening or lateral shift of two millimeters, together with five degrees or more of external rotation, increases the contact pressures in the ankle joint and causes ankle instability.6
Damage to the tendons supporting the ankle joint can also cause ankle instability. Even a simple tendonitis due to overuse of the respective muscle can produce significant unsteadiness. For example, there has been an association between a split in the peroneus brevis tendon and chronic ankle instability. Lateral ankle instability can cause laxity of the superior retinaculum which is a ligament that wraps around the proximal ankle joint. This laxity can cause a peroneus brevis split and, in turn, this split results in chronic ankle instability.7 Accordingly, in a study done by Kim et al, the researchers concluded that people with chronic ankle instability also possessed decreased spinal reflexes of the peroneal tendon and soleus muscle. They theorized that this altered reflex may be a potential mechanism of sensorimotor deficits that are associated with chronic ankle instability.8 Further, as the peroneal tendons are not the only tendons that traverse the ankle joint, patients with Achilles tendinopathy should also be examined for ankle instability. With a chronic Achilles tendinopathy, the area of swelling and pain moves with plantar or dorsiflexion of the ankle. This pain and swelling will further limit ankle joint mobility and increase the instability.9
Diagnosis/Etiology of Ankle Instability
When a person presents with ankle instability, the first step is to determine whether or not the injury demands emergency care. A cost-effective way to determine whether the etiology of the ankle sprain is purely ligamentous or may include bone fractures is by using the Ottawa Ankle Rules (OAR) since it has a sensitivity of over 98%.11 The OAR state that there is an increased likelihood that the ankle injury is accompanied by a fracture if the person experiences pain in the malleolar or midfoot region and meets at least one of the two criteria: