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ANKLE: LATERAL SPRAINS

Arie M. Rijke1

Frank C. McCue III

Andrew M. Schuett2

1 Department of Radiology

2Department of Orthopedics

University of Virginia Health Sciences Center

Charlottesville, VA 22908

Ankle injuries make up a large proportion of injuries in sports. Primary care and emergency departments report as many as 10% of presentations to involve the ankle joint. The vast majority of these are sprains. Eighty - five percent of ankle sprains occur on the lateral side. These sprains are caused by inversion of the foot with external rotation of the lower leg on the fixed foot. As such they are most frequently seen in sports that involve running and jumping such as basketball, volleyball and football. There is no predisposition for age or sex, but systematically trained and supervised athletes are less likely to sustain a lateral sprain than amateur or weekend athletes.

The most important ligaments involved in a lateral ankle sprain are the anterior talo-fibular ligament and the calcaneofibular ligament. Less important is the posterior talofibular ligament that stabilizes against posterior displacement of the talus and is usually spared in the milder forms of lateral sprains. However, isolated ruptures of this ligament have been occasionally found at surgery and on magnetic resonance imaging (MRI) scanning.

It is preferable to grade lateral ankle sprains according to the ligaments involved in the trauma rather than the severity of their clinical presentation:

¥ Grade 1, partial tear of the anterior talofibular ligament with intact calcaneofibular ligament.

¥ Grade 2, complete rupture of the anterior talofibular ligament with intact calcaneofibular ligament.

¥ Grade 3, complete rupture of the anterior talofibular ligament and partially torn calcaneofibular ligament.

¥ Grade 4, both ligaments are completely torn.

Isolated tears of the calcaneofibular ligament have been reported to occur in 3% of sprains and are specifically associated with hyperdorsiflexion, but no isolated tears were encountered in studies of several hundred patients with lateral ligament injury.

There is no consensus on how to treat acute sprains in spite of their common occurrence and the large number of comparative studies. Many surgeons manage these patients conservatively with elastic bandages, a cast, a pneumatic brace, or simple immobilization of the foot, with satisfactory results in most cases. Early mobilization has been beneficial for early return to activity, because range-of-joint motion may stimulate healing of the torn ligaments. Nevertheless, 20% to 40% of conservatively treated patients seek further medical attention for residual symptoms. These may include pain and swelling, or a sense of instability when walking on uneven ground. When conservative management fails, reconstruction of the lateral ligaments eliminates disabling symptoms and restores good function in a high percentage of ankles. However, there is reason to believe that surgical repair of acutely injured ligaments has a higher success rate than any reconstructive method for old ruptures.

There is no question about the success of surgical treatment of acute lateral ligament tears. Various authorities have reported excellent results in as many as 97% of sprains and, for this reason, surgery is the treatment of choice in some centers, particularly in Europe. However, these results have to be balanced against the 70% satisfactory results with casting, bracing, and taping, as originally reported by Brostršm in 1966. Since then, the pneumatic brace has become available, and aggressive rehabilitation programs have proved their value. We must consider, therefore, whether a surgical procedure is justified to achieve an additional 27% (or less) increase in satisfactory results. This is why some surgeons only operate on severe sprains in highly competitive athletes who are eager to return to sports participation, or on a patient's specific request. Although such a selection of patients seems reasonable, the significant number of conservatively treated patients with persisting symptoms suggests that many lateral ligament lesions are either incompletely diagnosed or inadequately managed or both.

Diagnosis

History and physical examination relate to an experience of a sudden, sharp pain on impact followed by inability to support weight and the rapid development of an egg - shaped soft tissue swelling over the lateral anterior aspect of the ankle joint. Frequently the patient cannot detail the exact course of events that led to the sprain. Icing and an ace bandage provide some relief and support. Later, discoloration of the skin occurs in the more severe cases.

A careful physical examination is indispensable in the diagnosis of lateral sprains, but it cannot assess the separate involvement of individual ligaments. Because the prognosis for conservatively treated ankle sprains is largely determined by the extent of ligament injury, an early and complete analysis is needed to decide on the treatment of choice. Plain x-rays of the ankle-including anteroposterior, lateral, and oblique views to rule out fractures and dislocations-should be searched for avulsion fractures of the tip of the distal fibula, the talus, and the medial malleolus. These x-rays should also be examined for evidence of joint fluid and soft-tissue swelling over the lateral malleolus, which may indicate lateral ligament injury. Physical examination should also include an anterior drawer test. If there is any question about the translation in that compartment, further work-up is indicated.

Ideally, an examination of the sprained ankle would provide information on which ligaments are torn and correlate the resulting anatomic derangement with loss of ankle stability. A dynamic test that quantifies the loss of functional support in terms of ligament injury is required. Follow-up examinations should use the same parameters to monitor therapeutic results. In actual practice, few controlled studies have been performed that fulfill these needs. In a typical study, the efficacy of the various modes of treatment is evaluated by relating history and physical examination scores at follow-up to the diagnostic findings based only on indirect signs at the time of trauma.

Arthrography

Ankle arthrography has been used by many investigators to determine the extent of ligament damage. Here, capsular extravasation of contrast and leakage anterior to, around, and lateral to the distal fibula are read as indications of rupture of the anterior talofibular ligament. If simultaneous filling of the peroneal tendon sheath occurs, the calcaneofibular ligament is assumed to be ruptured as well. The validity of ankle arthrography is based on the intimate relationship between the anterior talofibular ligament and the ankle capsule. In fact, the ligament is often seen at surgery as a thickening of the anterolateral aspect of the capsule. Parallel tracts of contrast medium into the peroneal tendon sheath indicate calcaneofibular ligament injury and are a result of the close anatomic relationship between this ligament and the tendons of the peroneus longus and brevis. Apparently, rupture of the extracapsular calcaneofibular ligament produces a communication between the joint space the peroneal tendon sheath. Not unexpectedly, occasional false-negative results have been obtained because the ligament ruptured while leaving the peroneal sheath intact. Also, naturally occurring communications between the peroneal sheath and the joint space in the presence of an intact calcaneofibular ligament have been reported in both cadaver and clinical studies, leading to false-positive results.

Despite these shortcomings, arthrography is extremely accurate in diagnosing ligament rupture in the acute rupture-that is, when performed within 72 hours of trauma; it is unreliable if the examination is further delayed. This is due to the formation of blood clots and fibrin that eventually close off the communication between the joint space and the peroneal space. Arthrography has no role in follow-up examinations and in the evaluation of chronic ankle instability.

Stress Radiography

Stress examinations are regarded as standard noninvasive procedures in the diagnostic analysis of acute ankle sprains. The examination procedure is simple and straightforward. An inversion force is applied to the lateral dorsal part of the foot while keeping the lower leg fixed. This separates the articular surfaces of the tibia and talus with an angle opening laterally on the anterior posterior view; this is referred to as the talar tilt angle. Alternatively, a force can be applied to the anterior aspect of the distal tibia with the heel remaining fixed. Viewed in the lateral projection a resulting displacement called the anterior drawer, can be observed. The stress can be applied manually or mechanically. Mechanical devices have the obvious advantage that the applied force can be quantified and examiner-to-examiner variability can be eliminated. Local anesthesia may be needed to ensure the patient's cooperation and to prevent muscle splinting.

Because the measured talar tilt angle and anterior drawer result from a discrete force, instrumented stress examination provides, at least in principle, a basis for making distinction among different grades of ligament injury. However, the ranges of talar tilt angles and anterior drawers that represent low or high grade lesions have been the subject of much controversy, largely as a result of the extensive overlap of ranges for normal and abnormal ankles. This overlap can be significantly reduced if patients with a history of ligament trauma to the opposite ankle (which routinely serves as normal comparison) can be excluded. In some studies, as many as 35% of patients had a positive history on the opposite ankle. Nonetheless, there remain serious difficulties when trying to differentiate low from high grade lesions and selection of patients for conservative or surgical treatment on the basis of these findings alone remains controversial.

It has also been shown that the average talar tilt angle of the opposite ankle increases with the extent of ligament damage to the injured ankle. A predisposition for ankle sprains in patients with preexisting ligament instability probably explains this finding. Other studies also seem to point this out. Sanders has shown that the average talar tilt angle of normal volunteer subjects is lower than that of the opposite ankles of patients with ankle sprains. It is unlikely, however, that preexisting ligament laxity is the single most important factor in causing ligament injury. Activities that involve running and jumping, such as basket ball and football, predispose individuals to repeated sprains far beyond the relatively low prevalence of true laxity disease, such as Marfan syndrome and Ehlers-Danlos syndrome. This is evidenced by the high percentage of often-forgotten sprains of the opposite ankle.

Graded Stress Radiography

Recently, a graded stress technique has been developed that makes it possible to distinguish the different grades of ligament lesions. Any stress device that combines proper positioning of the foot with the capability to monitor the applied pressure, thus permitting consistent and reproducible measurements of the talar tilt angle and the anterior drawer, can be used for this purpose (Fig. 1). The stress is increased gradually and anterior posterior x-rays are taken after application of increasing pressures (Fig. 2 a, b).

To assess the extent of injury to the individual ligaments the applied pressure (or force) is plotted against ( -1/2), where  represents the ratio between the lengths of the stretched and unstretched lateral ligaments. According to viscoelastic theory the pressure P = G( -1/2), where the proportionality factor G is related to the cross-sectional area of the unstretched ligaments and to their shear modulus. Figures 3a and b show such plots for grade 2 and grade 4 lesions, respectively. Note that the applied pressure in kiloPascals is plotted against linear increments of (-1/2) on the lower x-axis. The corresponding talar tilt angles from which (-1/2) has been calculated, are plotted on the upper x-axis in non-linear increments. Using this type of preprinted graphical arrangement makes it unnecessary to perform multiple calculations from the observed talar tilt angles.

Using the above method of graphical plotting, intact ligaments show as a straight line going through the origin, with a slope in the range of 16 to approximately 40 kPa reflecting the anatomic variation among patients. An isolated anterior talofibular injury will reduce the slope proportional to the extent of injury and allow an estimate, at least in principle, of the percentage of rupture of the ligament by comparing it with the findings on the intact ligaments of the opposite ankle. A grade 2 lesion shows the slope as 50% of normal, whereas a grade 3 lesion reduces the slope even further. A grade 4 lesion, on the other hand, is represented by a straight or curved relationship that does not go through the origin.

The validity of these concepts have been tested on nine cadaver ankles and one freshly amputated lower leg. The accuracy of this method has been further verified in 24 surgically treated patients. Similar results are obtained when evaluating the anterior drawer, but we prefer the talar tilt examination because this procedure is simpler and requires no correction for photographic magnification.

By accurately assessing the extent of ligament involvement, graded stress radiography has proved to be a powerful tool in grading lateral ligament injury and has therefore helped in deciding between conservative or surgical treatment. The examination, including the graph evaluation, can be completed in 15 minutes. Unlike arthrography, graded stress radiography can supply diagnostic information independent of the time of injury and is therefore particularly well suited for follow-up studies and for the evaluation of chronic ankle instability. Because the same parameters are measured at the time of injury and at follow-up, the data can be directly compared.

We have applied this technique to 36 athletes with acute and nonacute lateral ankle injuries who were reexamined 11/2 to 5 years after treatment. Of the surgically treated patients with grade 4 lesions, 83% showed 70% to 100% recovery of ligament function compared with 21% of patients treated conservatively with a short leg case, elastic bandages, a brace, crutches, high-top shoes, or a combination of these. If we assume that optimal management of conservatively treated grade 1,2 or 3 lesions results in a recovery rate of nearly 100%, we can calculate (based on the above figures and the observed 2:1 prevalence of low-grade lesions over high-grade lesions) that conservative treatment for all patients, regardless of their grade of injury, would result in a 73% success rate. This figure is close to Brostršm's estimate of 70% and correlates well with the 20% to 40% of conservatively treated patients who seek further medical attention for persisting symptoms.

These results underscore the importance of the diagnostic distinction between grade 1,2 and 3 lesions on one hand, and a grade 4 lesion on the other. They also emphasize the importance of modern, aggressive methods of nonoperative treatment and rehabilitation. Patients with grade 4 lesion, particularly competitive athletes, should be further screened for possible surgery.

MRI

MRI scanners are now fairly widely available in the larger institutions. Because of its capability of demonstrating soft tissue detail, the potential of MRI in detecting lateral ankle ligament injury has been recently explored. The anterior and posterior talofibular ligaments can be adequately demonstrated on axial scans with the foot in neutral position. The calcaneofibular ligament can be best seen in plantar flexed position. Acute tears are usually not directly visualized in the presence of hemorrhage and edema, but in the subacute or chronic stage disruption and fraying of the injured ligaments can be identified. Occasionally, a wavy course of the ruptured calcaneofibular ligament can be seen associated with a grade 3 or 4 injury (Fig. 4). Atrophy of the ligament may show as a nubbin at its site of attachment.

The role of MRI in the detection of lateral ankle sprains is limited. Assessment of anatomical damage is restricted to the subacute and chronic stages only, and unlike the cost-effective graded stress radiography, does not correlate with the observed ankle instability.

Treatment

The ultimate goals in the treatment of ligament injuries are to obtain rapid and complete rehabilitation with minimal morbidity and cost. The method chosen should also insure the lowest possible risks for chronic late instability patterns. Basically, the choice is between some form of conservative treatment and some type of surgical procedure. The current trend favors individualized activity and sports-specific proprioceptive training program as part of a non-surgical rehabilitation, whereas surgery is reserved for severe sprains in highly competitive athletes eager to return to sports participation, and for those athletes who have failed to respond to individualized conservative treatment. Late reconstructive attempts are, however, no more successful than acute repair. The need for these late reconstructions will decrease with accurate diagnosis at the time of injury and proper choice of management.

Conservative Approach

Acute, conservative management of the lateral ankle ligament injury should include ice, elevation, compression and stabilization. Initial goals include minimizing the secondary hypoxic injury caused by decreased oxygen delivery to the region. Many authors have stated, however, that optimal non-operative management of lateral ankle sprains has not been established.