Ankle sprains are one of the most common soft tissue injuriesaccounting for nearly 40% of sports injuries [1–7]. Sports requiringregular change of direction activities, such as basketball, football/soccer, running, and volleyball, have a higher incidence of ankleinjuries [1,8–12]. Conservative management is effective in treatingthese injuries in the majority of cases. Nevertheless, the incidence of chronic ankle instability after an acute ankle sprain has beenreported as being between 5% and 70% [4,13–16]. There are anumber of procedures described for the surgical management oflateral ankle instability with both anatomical and nonanatomicalprocedures showing an 81–97.2% good to excellent result .
Instability can recur early or late, with early being aconsequence of an acute injury, while late is more difficult todefine with a more subtle pathogenesis. Several factors have beenshown to predispose patients to operative failure. The Brostro¨manatomic repair with its modifications has limitations particularly in patients with ligamentous laxity, longstanding instability and high functional demand. A specific subset of late recurrentinstability occurs as a consequence of hindfoot malalignment.Fuller  proposed the cause of lateral ankle sprain as an increased supination moment around the subtalar joint axis. With a rigid supinated hindfoot (calcaneal varus), a laterally placed subtalar axis relative to the ground reaction force increases the supination moment and causes excessive inversion and injury.
Ankle and hindfoot varus has been reported to be present in 8% of those with primary lateral instability and in 28% of those undergoing revision surgical procedures for such instability .
Acknowledging the varying anatomy, aetiologies and tissue structure presented under the guise of ankle instability, it is difficult to ascertain in the literature if the results reported on its treatment is truly comparable. We present a new classification for lateral hindfoot instability. There are two intents of this classification.
Firstly, the classification demonstrates an assessment and treatment guideline for the many causes of peritalar lateral instability. The second use of the classification is for research purposes so that cohorts of patients can be accurately described and the efficacy of different operations in different groups can be properly assessed.
2.1. Type 1: axis deviations
For successful surgical treatment of ankle and hindfoot varus deformity, it is imperative that the anatomical level of the deformity is determined. The level of deformity is calculated using the CORA as described by Paley . It is therefore possible to subdivide this group into which joint has the predominant instability and where the level of the deformity occurs.
2.1.1. The ankle
An alteration in the axis of the ankle joint leads to an asymmetric distribution of the load vectors across this joint. A varus deformity secondary to an increased lateral distal tibial angle will obviously lead to higher compressive forces on the medial side of the joint. It will also put increased strain on the lateral structures. The mechanical axis will now pass laterally to the centre of the talus and to the normal point of initial contact on the heel. Muscles close to the midline may have their secondary function of inversion potentiated (e.g. Achilles tendon and tibialis anterior) . The overall effect of these changes will be an increased chance of lateral instability along with an increasing chance of medial ankle osteoarthritis.
The risk of lateral instability is high with subtalar axis deviation . This is due to the hindfoot being locked in supination, thus any varus deforming forces can only be compensated for at the ankle joint. If there is no compensatory ankle deformity, the full deforming force will be transferred to the lateral ligaments and, initially, the medial gutter of the ankle joint. As with the ankle axis deviation, medialisation of tendons will potentiate their inversion force of action. If the hindfoot supination is secondary to a neurological disease (e.g. Hereditary Motor Sensory Neuropathy) tibialis anterior is weakened and is therefore unable to compensate for the plantarflexion effect of peroneus longus on the first ray. This will cause a secondary pronation deformity of the forefoot, causing furter forefoot-driven hindfoot varus and thereby instability . Anatomic geometry can also be implicated. Kato  described chronic subtalar instability occurring in young females with chronic stress on the foot and a poor bony block. These patients had a shallower angle of the posterior facet compared to other patients with subtalar instability and a control group. This was thought to be a predisposing factor to them developing subtalar instability in the anteroposterior plane.
In a small number of patients there is multilevel deformity. In this group careful assessment will need to be made on the proportion of deformity that is determined by each level. These will either be in patients with congenital or developmental deformities or in those where secondary arthritic changes have taken place.
2.2. Type 2: ligamentous instability
There is a significant ligamentous injury in 10–15% of ankle inversion injuries. The anterior talofibular ligament alone is injured in 50–75% cases. There is associated calcaneofibular ligament injury in approximately 15–25% cases. Isolated calcaneofibular ligament injuries occur in just 1% cases . Medial ankle ligament injuries are rare in isolation and are more commonly combined with either a lateral ligament complex injury or fractures .
188.8.131.52. Subtalar instability secondary to ankle instability. The typical mode of injury for this type of lesion is an inversion of the ankle joint with the foot in plantarflexion. This causes progressive rupture of the anterior talofibular ligament followed by the calcaneofibular ligament then the cervical ligament .
184.108.40.206. Isolated subtalar instability. The mode of injury for this subgroup is sudden deceleration of the talus compared to the calcaneus with the foot in maximal plantarflexion. This causes a supination moment at the subtalar level. Mayer’s arthographical and Pisani’s MRI studies demonstrated that this force causes a rupture of the interosseous and cervical ligaments, followed by the calcaneofibular ligament with the anterior talofibular ligament being spared [9,24]. At lower level of force, this mode of injury causes a sprain/partial tear of the first two ligaments with secondary inflammation. This is a common cause of sinus tarsi syndrome.
2.3. Type 3: functional instability
Freeman  introduced the notion of functional instability in 1965. It is thought to occur due to proprioceptive changes in the ligamentous and muscular structures following injury. Weakness of the peronei has been reported amongst individuals with chronic ankle instability, along with impaired reflexive response times. Tropp  defined functional instability as motion beyond voluntary control but within the normal physiological range. Most patients presenting with chronic ankle instability will invariably have an element of functional instability and it is often the predominant problem.
3. Pre-operation evaluation
In order to correctly diagnose the cause of a patient’s instability, a thorough pre-operative evaluation is necessary.
3.1. Clinic evaluation
3.1.1. Acute lesions
Initial evaluation in the emergency or clinic room is frequently challenging. The mode and force of injury as well as the patient’s ability to keep on playing or even weight bearing is often a useful triage tool. The presence of pain inhibition can often produce false negative results on clinical examination. However, an examination’s reliability in distinguishing between minor soft tissue injuries and ligamentous disruption can be improved by repeating the evaluation a few days post injury. Tenderness alone is ineffective in determining ligamentous injury. In contrast, tenderness together with significant ecchymosis or haematoma formation has a sensitivity of 90% in determining ligamentous disruption. The anterior draw test alone has a sensitivity of 73% and a specificity of 97%. If all three tests are used in conjunction, then there is a sensitivity of 100% and a specificity of 73% . On the basis of this data, the ISAKOS (International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine) has recently defined the optimum time for clinical evaluation in suspected cases of ligamentous disruption as being on day of injury and after 48 h rest, ice, compression and elevation [27,28].
Malliapoulos et al.  used this method as the basis for a clinical classification of lateral ligament injuries. The patients were initially assessed, treated for 48 h with rise, ice, compression and elevation whilst allowing early passive range of movement exercises. They were then re-assessed and graded as set out in Table 1 .
symptoms, and how much force is typically required. This may, in subtle cases, be simply a lack of confidence in the ankle. Initial physical examination should be assessment of the alignment of the lower leg in all planes. Any axis deviation should be noted in the entire limb. Assessment of the foot should include inspection for cavus or planus deformities, and whether these deformities are flexible of fixed. Tenderness is usually maximal over the lateral gutter (anterior talofibular ligament) and Molloy’s impingement test may reveal impingement syndrome due to post-traumatic synovitis . Hindfoot examination must also exclude a gastrocnemius contracture (Silfverskiold test), as a plantar flexed foot would accentuate the instability. The two most important tests for evaluation of ankle instability are the anterior draw test and inversion stress test (talar tilt test).
The anterior draw test is a combination of internal rotation and anterior thrust of the foot with the tibia held. This results in anterior subluxation of the ankle joint .
The characteristic sign seen with the anterior draw test is the ‘suction sign’ as the skin is sucked inwards over the lateral gutter . The talar tilt test is usually positive if there is a complete disruption of the calcaneofibular ligament. Although it is an indication of instability, the absence of an abnormal test does not preclude instability. Both ankles should be tested simultaneously as it is useful to identify asymmetry.
Standard anteroposterior and lateral weightbearing radiographs of the ankle should be performed. The lateral radiographs are used for assessing Meary’s angle and calcaneal pitch (Fig. 1).
The anteroposterior ankle radiograph is for assessing the lateral distal tibial angle as well as assessing for any osteoarthritis and possibly osteochondral defects. The authors find Saltzman’s view  invaluable in determining the level, or multiple levels, of the CORA, providing more information than Coby’s view alone (Fig. 2) . If pes cavus is present, anteroposterior and lateral weightbearing plus oblique foot radiographs should be performed. Full length standing radiographs should be performed if there is any suspicion of a proximal deformity.
The efficacy and reproducibility of ankle stress views are controversial. One of the main limitations of stress radiographs is their reproducibility and in determining normal values [30,33].
Subtalar stress views are very difficult to reproduce accurately. The most useful views are those proposed by Broden and Kato [8,13].
Heilman et al.  stated that the positive result was shown by a 7 mm separation of the posterior facet whereas Yamamoto et al.
 preference was for an angular measurement of the separation of the posterior facet.
3.2.1. Advanced imaging
MRI is a useful adjunct in determining additional pathologies such as osteochondral lesions, which can be associated with instability. We would recommend its use if there are painful
symptoms in addition to those of instability. It is however of limited benefit in ligamentous assessment. Although extremely specific (100% for ATFL and 83% for CFL) it has a relatively low
sensitivity (56% for ATFL and 50% for CFL) . In addition,
although it may show acute or chronic ligamentous injury, it gives no information on the function of the aforementioned ligaments. MRI can also be useful in the investigation of subtalar instability, demonstrating changes of (acute on) chronic inflammation within the supportive ligamentous structures. In cases of severe pes cavus or if previous fusion surgery has been performed, a CT scan with 3D reconstruction may be of use.
4. Surgical treatment
If conservative measures fail then surgery will be indicated.
Treatment based on our classification will provide a logical framework to base surgical strategies on for the treatment of
4.1. Ankle axis deviations
This group of patients’ predisposition to instability, because of abnormal force vectors across the joint, means that soft tissue procedures alone are more likely to result in failure. It is therefore essential that the mechanical axis and joint congruence be restored
to within normal limits. The osteotomy should be performed as close as possible to the CORA. This prevents altering both the anatomical, and thereby mechanical axis, as the further the
osteotomy is made from the CORA the greater the degree of compensatory translation will be. As previously stated, there are two forms of deformity that can cause angular deformity and/or
joint incongruence of the ankle joint; i.e. extra-articular and intraarticular.
Extra-articular deformities are the most common type.
Standard treatment is with a supra-malleoli osteotomy. This may be performed by an opening wedge, closing wedge or dome osteotomy. It can be stabilized with internal fixation or with
external fixation. With the latter, the correction can be gradual, for the larger deformities, rather than be performed acutely. Knupp and Hintermann  set out a robust technique for its use using a medial based approach with opening wedge for varus ankle and closing wedge for valgus deformities. Anterior extrusion of the talus can also be corrected with a biplanar wedge. Care should be taken in assessing whether the fibula should be osteotomised as well, especially if the fibula length is incorrect after the tibial osteotomy or the talus does not follow the medial malleolus on correction. Failure to do so will cause increase loads across the ankle joint .
Intra-articular deformities are less common. They may be caused traumatically (e.g. a plafond crush injury associated with a severe ankle fracture with gross initial displacement) or be due to localized primary osteoarthritic erosions. This second group is far more challenging to treat, and with greater controversy, with an osteotomy due to the almost inevitable severe osteoarthritis that will be present around the lesion.
4.2. Subtalar axis deviations
Pes cavus has a spectrum of severity with many described surgical strategies, the description of which is outside the remit of this paper. However one needs to determine whether it is
forefoot or hindfoot driven and address the predominant pathology. Isolated subtalar axis deviations are far more common than ankle and can be successfully treated by a lateral displacement calcaneal osteotomy, of which there are many described variations in the literature (Fig. 3). Mann et al.  advocated the use of subtalar fusion with good effect in recalcitrant cases. These results are discussed in more detail in Section 4.5. lateral peritalar instability as the algorithm is centred upon the aetiology of the instability
The surgical approach for these complex patients will be defined on the level and severity of deformity for which there may be multiple CORAs. The appropriate technique for each deformity will be used with the difficult decision being on how many different levels need to be addressed.
4.4. Ankleligamentous instability
There are over 20 different described surgical reconstruction procedures available for chronic lateral ankle instability . All of which report good results. Broadly speaking there are two main types of operative stabilization, anatomical and non-anatomical.
The majority of surgeons prefer anatomical although the techniques have to be individualized according to the patient and their anatomy. Bell et al.  reported on the 26 year follow up
of Brostro¨m procedures performed for ankle instability with 91% of patients reporting good to excellent results. The authors’ standard practice is to carry out an anatomical repair in primary or simple revision cases and a non-anatomical procedure in the most complex cases.
4.5. Isolated subtalar instability
Subtalar instability is well recognized as part of a combined peritalar instability, as previously mentioned. Anatomic repairs, Chrisman–Snook procedure and allograft/autograft procedures will deal with both ankle and subtalar instability. Larsen , Karlsson and Andreasson , Schon  and Mann  presented surgical reconstruction techniques for the calcaneofibular and cervical ligaments in case of subtalar instability in conjunction with ankle instability.
Two authors have focused on reconstruction of subtalar instability alone using tendon tunnel techniques. Kato  proposed the use of a 4 mm autograft taken from the Achilles’ tendon, reinforced with an artificial Leeds-Keio ligament. The combination is then inserted in the tunnel in the calcaneus from the lateral-plantar angle to the medial talus trochlea, and from
there to the dorsal surface of the talus. Fixation was provided with 2 staples. Kato  presented the results of this fixation in 14 patients of which, 2 patients had isolated subtalar instability. All patients had excellent results. Pisani  proposed the use of hemitendon of the peroneus brevis. He outlined one and twostrand techniques using one-half the peroneus brevis tendon. He created a V-shaped talar tunnel and two parallel calcaneal tunnels,
entering lateral plantar on the neck and directed towards the sinus tarsi. The tendon was then weaved from plantar to dorsal through the proximal calcaneal tunnel into the talar tunnel. There were 47 patients with a follow-up of at least two years in 38 patients. It refers to 29 patients being satisfied with the procedures, although 6 required subtalar arthrodesis. Mann  also advocated the use of subtalar arthrodesis for recalcitrant cases, citing excellent
results in 86% at 6 years.
4.6. Functional instability
This is a multi-factorial pathology that should be treated conservatively. Surgery should be reserved for the cases where a mechanical problem can be definitely objectively demonstrated.
Lateral hindfoot instability is a complex pathology with a multifactorial aetiology. We believe that our classification represents a solid clinical framework for the assessment and treatment of this condition. Only by correction of all pathology causing the instability the best results will be achieved. This classification presents a logical approach for clinical and radiological evaluation as well the relevant surgical strategies that can be used in their
We also believe that it essential that greater scientific vigour is applied in assessing our success in treating this complex condition.
This classification will enable the results of treatment in similar groups of patients with their respective pathology accurately described.
Conflict of interest statement
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