Lateral ankle instability is often found with associated conditions such as frequent ankle inversion sprains, peroneal tendonitis and peroneal overuse injury.
To understand the nature of this condition, what it is and how and why it occurs, one needs to understand the anatomy and function of the peroneal muscles and tendons.
There are three peroneal muscles. Peroneus longus, peroneus brevis and peroneus tertius.
Peroneus brevis arises from the lower one third antero-lateral surface of the fibula and is innervated by the common peroneal nerve. The muscle becomes a tendon superior to the lateral malleolus, passing inferiorly and posterior to the fibula beneath the superior retinaculum. Curving around the distal end of the fibula the tendon passes beneath the inferior retinaculum and inserting into the postero-lateral aspect of the base of the 5th metatarsal. The function of the peroneus brevis is primarily to evert the foot. It also stabilises the 5th metatarsal posteriorly against the cuboid and the cuboid against the calcaneus during midstance and early propulsion. Peroneus brevis also provides an antagonistic pronation force at the subtalar joint to resist supination of the foot and the external leg rotation by the calf muscles.
Peroneus longus arises from the upper two thirds of the antero-lateral surface of the fibula and is also innervated by the common peroneal nerve. The muscle becomes a tendon superficial to peroneus brevis muscle and passes posterior and lateral to the lateral malleolus, superficial to the tendon of peroneus brevis. It is held in place by a superficial retinaculum above the malleolus and an inferior retinaculum, anterior and distal to the ankle joint. Together with the tendon of peroneus brevis, the tendon follows a groove in the distal fibula curving to pass under the trochlear process of the calcaneus, then along the lateral aspect of the calcaneus to curve medially in the groove in the plantar aspect of the cuboid to attach on the plantar surface of the medial cuneiform and base of the 1ST metatarsal. The function of the peroneus longus is to evert the foot and plantar flex the 1ST ray. Although it is a pronator of the subtalar joint, it has a longer lever arm to plantar flex the 1ST ray than to pronate the subtalar joint. So, spasm of peroneus longus will plantar flex the 1ST ray and supinate the subtalar joint. Therefore ‘spasm of peroneus longus’ does not cause a ‘peroneal spastic flatfoot’. In normal locomotion, the peroneal tendons provide an abduction, stabilising force on the lesser tarsus during the midstance and propulsive phases of gait. They also lift the lateral side of the foot to transfer weight medially to the opposite foot as it contacts the ground.
Peroneus tertius is contained within the anterior muscle compartment of the lower leg and lies inferior to extensor digitorum longus. It is innervated by the superficial peroneal nerve. It passes anterior to the ankle joint, inferior to the anterior retinaculum, and inserts into the dorsal aspect of the base of the 5th metatarsal. Its function is to evert and dorsiflex the foot.
Peroneus brevis is a stance phase muscle that contracts halfway through the midstance period into the first half of the propulsive phase. The lateral malleolus and the peroneal retinaculum act like a pulley for the tendon. It has a long lever arm to the subtalar joint axis and is therefore a strong pronator. However, it has a short lever arm to the oblique axis of the midtarsal joint and so pronates that joint with less strength. It passes through the axis of motion of the 5TH ray and therefore has no influence on the function of the 5TH metatarsal to which it attaches. It also passes close to the axis of the ankle joint and therefore plantar flexion force is minimal. Peroneus brevis produces extensive pronation of the foot when the abduction force is unopposed by tibialis posterior or when it is in spasm, causing a peroneal spastic flatfoot. Peroneus brevis spasm is not restricted by ground reaction force, so the subtalar joint pronates, the midtarsal joint supinates and the foot becomes flat.
Peroneus longus spasm or contracture, on the other hand, causes supination of the foot, not pronation. Both are pronators of the subtalar joint, but peroneus longus has a stronger lever arm to plantar flex the 1ST ray than to pronate the subtalar joint and spasm or strong contraction, therefore, causes supination of the subtalar joint, not pronation.
In normal gait, the antagonistic effect of peroneus brevis on the supinating muscles allows for smooth decelerating pronation and internal leg rotation at the contact phase of gait and, likewise, smooth resupination and external leg rotation during propulsion. Both peroneus longus and brevis act to lift the lateral side of the foot to transfer weight medially to the opposite foot.
Lateral ankle instability commonly presents as frequently occurring inversion ankle sprains.
Patients recount a history of often falling over onto the outside of the foot and ankle. This occurs mainly during sporting activity with sudden changes of direction, however, this can occur with walking or running, but most often on uneven surfaces. If this rolling motion causes the ankle, subtalar or midtarsal joints to move beyond their normal range of motion or in an abnormal direction, then the joint capsules and ligaments that control and restrict the range of motion can be compromised and torn. This damage is classified into Stages, one, two and three depending on the severity. Stage three can involve an avulsion fracture of the bone at the site of the ligament attachment. Ligaments have poor vascular blood supply and are therefore slower to heal than other soft tissues.
The most commonly injured ligament is the anterior talo-fibular ligament. The next is the inferior calcaneo-fibular ligament and the least common the posterior talo-fibular ligament. Other structures that can be damaged are the peroneal retinaculae and the anterior inferior tibio-fibular ligament and interosseus membrane. This membrane stabilises the distal tibia against the distal fibula, stabilising the ankle joint. Tears to this membrane increase the space between the two bones, creating gross ankle instability. The trochlear surface of the talus can be damaged by excessive frontal plane motion through the ankle joint. A talar-dome lesion or bone cyst can develop which is notorious for non-healing and will require surgery. In a similar way, the articulating surfaces of the subtalar and midtarsal joints can be damaged leading to arthritic degeneration.
The tendon of peroneus brevis can also be damaged through an inversion sprain with longitudinal tears or splits and inflammation within the tendon sheath. This makes normal function painful. There can be an avulsion fracture where the tendon attaches at the base of the 5TH metatarsal. This can be problematic as blood circulation to the base of the metatarsal comes from the shaft of the bone. Therefore, the avulsed portion may not heal and the efficiency of the tendon of peroneus brevis may be compromised. This can cause chronic pain and instability.
The cause of this lateral instability is poor lower limb and foot alignment. Any movement, alignment, or compensation for that alignment that places the centre of gravity of the body and lower limb lateral to the foot, or, sometimes outside the foot, creats lateral instability through an increase in the inversion force moment at the ankle and subtalar joints. Most commonly this is an external tibial torsion, creating an inverted subtalar joint position together with a forefoot valgus alignment and/or a plantar flexed 1ST ray to balance the forefoot on the ground. Forefoot valgus alignment is compensated by longitudinal midtarsal joint axis supination to a total of 8 degrees. A forefoot valgus alignment greater than 8 degrees requires subtalar joint inversion for compensation. In a similar way, a plantar flexed 1ST ray that cannot be compensated by 1ST metatarsal dorsiflexion, is compensated by a retrograde force from the ground that inverts the subtalar joint. This compensation increases the inversion moment creating lateral instability. Any sudden change of direction playing sport, or an uneven surface increases this force which requires excessive peroneal muscle activity to control. If control of this force exceeds the ability of the muscles, then an ankle inversion sprain will occur. This alignment leads to chronic overuse of the peroneal muscles with lateral leg and foot pain as well as tendon tears. Other abnormal alignments or functional positions that can create lateral ankle instability are an externally rotated or abducted hip position, genu varum, excessive tibial varum and an uncompensated rearfoot varus alignment with limited subtalar joint pronation. Hypermobile joints can also contribute to this instability.
Examination of the patient to diagnose the extent of such an injury is often difficult because of pain and inflammation. However, gentle movement of the ankle, subtalar and midtarsal joints to the available end range of motion will often enable diagnosis of the involved structures and the degree of damage. Lateral compression of the middle portion of the lower leg will determine whether the interosseous membrane is affected. Xrays may be required to diagnose avulsion fractures or talar-dome lesions, though the latter most often develop chronic, intermittent, symptoms at a later date.
Treatment of the acute condition requires ‘RICE’ but not total immobilisation as it has been found that some protected movement improves the rate of healing. Although compression bandaging is necessary to reduce swelling as well as provide support, this must be adjusted at regular intervals to allow adequate circulation to the foot. A supportive ‘air-walker boot’ is preferred to Plaster of Paris casting as this prevents damaging movement but allows supportive weight bearing and mobility.
Gradual return to pain free movement will allow physiotherapy to regain a normal range of motion and strength in the soft tissues. Research has shown that rehabilitation therapy following ankle inversion sprains, to strengthen the lateral structures, has no effect in preventing further ankle sprains. This indicates that effective treatment must control the underlying pathology and forces acting on the ankle.
Therefore, orthotic therapy with appropriate valgus forefoot and rearfoot posting to reduce the inversion forces is necessary to reduce symptoms, both acute and chronic, but also prevent long term muscle and tendon overuse and further inversion ankle sprains.