Syndesmotic Disruption in Low Fibular Fractures Associated With Deltoid Ligament Injury

Abstract

Low fibular fractures that were associated with deltoid ligament disruption and inferior tibiofibular syndesmotic disruption were studied. All of the patients had a Type B Weber fibular fracture associated with a deltoid ligament injury. It was difficult to detect the syndesmosis disruption on the initial assessment of the anteroposterior and mortise radiographs obtained preoperatively because there was no obvious talar shift on the plain radiograph. Careful evaluation of the plain radiograph and determination of all the recommended measurements were necessary to diagnose the syndesmotic disruption. However, the syndesmotic disruption was easily recognizable on axial computed tomography scans when comparing the injured and the noninjured sides. Axial computed tomography scans also showed a shallow incisura fibularis in all patients and in three cases it revealed anterior fibular subluxation that was not appreciated on the plain radiographs obtained preoperatively. On the basis of the current study using the level of the fibular fracture as a guideline for application of the syndesmotic screw as suggested by some authors may not be accurate. There are several factors that should be considered including the depth of the incisura fibularis, posterior malleolus fractures, deltoid ligament injury, and subluxation of the fibula. The surgeon’s impression in the operating room of syndesmosis stability should be considered as the best guideline in the application of syndesmosis fixation rather than depending on the level of the fibular fracture.

Although ankle fractures traditionally have been considered noncontroversial with respect to the indications for operative intervention, recent advances in the understanding of the biomechanics of the ankle have given rise to particular areas of clinical uncertainty, including treatment of syndesmotic injuries and reliability of radiographic assessment of ankle fractures. ¹⁴ Accurate reduction of the syndesmosis and maintenance of this reduced position until the ligaments heal is necessary to ensure good outcome and to avoid long-term arthritic changes in the distal tibiofibular joint. ^11,20,26 Several authors have supported routine use of a syndesmotic screw for most Weber Type C fractures to permit ligamentous healing in the reduced position. ^{4,5,8,10,11,13,17–19} Other authors have suggested a limited role for the syndesmotic screw. ^2,12,22

Boden et al, ¹ in a study using cadavers, suggested that syndesmotic screw fixation in Weber Type C injuries associated with a deltoid tear was unnecessary if the fibular fracture was less than 3 to 4.5 cm above the tibiotalar joint.

Colton ³ reported on seven patients with low oblique fibular fractures, with a lower limit at the level of the tibial articular surface, which were associated with diastasis of the inferior tibiofibular joint. Three patients had injury of the deltoid ligament and four had fracture of the medial malleolus. He stated that the diagnosis of inferior tibiofibular diastasis could be made only after surgical exposure of the fibular fracture and concluded that fixation of the fibular fracture alone in such a case would leave the injury of the inferior tibiofibular joint untreated. He recommended syndesmotic screw fixation.

The purpose of the current study is to report on patients with low fibular fractures associated with deltoid ligament injury and syndesmotic disruption. Axial computed tomography (CT) scanning of the inferior tibiofibular syndesmosis was important for evaluation of the diastasis and the associated factors that might have contributed to the syndesmosis instability.

MATERIALS AND METHODS

The charts and radiographs of seven patients with a low fibular fracture that was associated with deltoid ligament disruption and diastasis of the inferior tibiofibular joint were reviewed retrospectively. There were four men and three women. The average age of the patients was 45 years (range, 29–70 years). The mechanism of injury was a fall in five patients and a motor vehicle accident in two. The average followup was 24.7 months (range, 18–32 months).

Anteroposterior (AP), mortise, and lateral plain radiographs were reviewed to identify diastasis of the distal tibiofibular syndesmosis and involvement of the posterior malleolus. The following criteria have been used as the normal limits; talocrural angle (83° ± 4°) with as much as a 5° normal difference between both sides; medial clear space (4 mm); talar tilt (2 mm); tibiofibular clear space (< 5 mm); tibiofibular overlap (10 mm); and talar subluxation, which is a subjective assessment of congruity of the tibial articular surface and the talar dome. Any incongruity is abnormal. All the measurements were taken 1 cm above the tibial plafond. The first three measurements were taken on the mortise view and the other three were taken on the AP view ^16,17,23 (Fig 1A–B).

Fig 1A–E.: (A) The mortise view of the ankle shows the medial clear space measured 5 mm that might indicate a deltoid ligament disruption. The talocrural angle was within the normal limit indicating absence of fibular shortening. There was no talar tilt. (B) The AP view shows widening of the tibiofibular clear space and reduction of the tibiofibular overlap that indicate diastasis of the distal tibiofibular joint. There was no talar subluxation. (C) The lateral view shows a fracture of the posterior malleolus with no obvious anterior displacement of the fibula. (D) The CT scan, coronal view of both ankles, shows widening of the medial clear space of the right ankle in comparison with the left ankle, which indicates a deltoid ligament disruption. There was an old injury of the talus on the left side. (E) The axial CT section 9 to 12 mm proximal to the tibial plafond shows a shallow incisura fibularis measuring 3 mm, posterior malleolus fracture, and anterior subluxation of the fibula that was not appreciated on the plain radiograph.

Axial CT scans were reviewed for distal tibiofibular diastasis, subluxation of the fibula, associated posterior malleolus fracture, and to identify the depth of the incisura fibularis to detect any possible relationship between a shallow incisura fibularis and distal tibiofibular diastasis. ⁷ The width of the tibiofibular syndesmosis and the width of the incisura fibularis were measured in the third section proximal to the tibial plafond. This section was 3 mm thick and 9 to 12 mm proximal to the tibial plafond. This was done to provide measurements comparable with those previously described for evaluation of the tibiofibular syndesmosis on plain radiographs. ²⁵ The depth of incisura fibularis was measured from the deepest point of the incisura fibularis to a line drawn between the anterior and posterior tibial tubercles. ⁷

All the patients had Type B Weber fibular fracture that started at the level of the syndesmosis. Two patients had a posterior malleolus fracture that involved less than 25% of the articular surface and did not require fixation. The deltoid ligament injury was confirmed clinically by swelling and tenderness on the medial side of the ankle and radiologically by widening of the medial clear space more than 4 mm (Fig 1A–D).

Open reduction and internal fixation of the fibular fracture was done according to standard AO guidelines with a ⅓ tubular plate combined with an interfragmentary screw. ²⁴ The deltoid ligament injury was not repaired. The syndesmosis was inspected to confirm the finding from the CT scans obtained preoperatively. Intraoperative stress tests after fixation of the fibular fracture and before application of the syndesmotic screw were done to assess the diastasis. These stress tests included a lateral force applied to the fibula via a hook and observing the lateral fibular displacement and Cotton test ²³ that was done by stabilizing the distal leg with one hand and grasping each side of the foot at the talus with the thumb and forefinger of the other hand. By applying a mediolateral force, crepitus and instability could be assessed from mortise widening. Intraoperative stress radiographs also were obtained to assess syndesmotic instability.

Syndesmotic screw fixation was used in five patients. The other two patients initially were treated at another institution and did not have syndesmotic fixation.

Postoperatively a short-leg cast was applied and the patients were instructed to not bear weight for 8 weeks. The syndesmotic screw was removed after 8 weeks.

RESULTS

In all the patients the axial CT section 9 to 12 mm proximal to the tibial plafond showed that the syndesmosis was disrupted and the incisura fibularis was shallow (Fig 1E). The average depth of the incisura fibularis in the patients included in this study was 2.7 mm (range, 2–4 mm). In three patients the axial CT scan showed anterior fibular subluxation that was not appreciated on the plain radiographs obtained preoperatively (Fig 1C, E).

It was difficult to detect the syndesmosis disruption on the initial assessment of the AP and mortise radiographs obtained preoperatively because there was no obvious talar shift on the plain radiograph. Careful evaluation of the plain radiograph and determination of all the recommended measurements were necessary to diagnose the syndesmotic disruption. However, the syndesmotic disruption was easily recognizable on the axial CT scan. Intraoperative stress tests and stress plain radiographs before application of the syndesmotic screw confirmed the diastasis. Intraoperative inspection showed a tear of the interosseous membrane that extended above the level of the lateral malleolus fracture. This might have contributed to the instability of the syndesmosis in such low fibular fractures.

Syndesmotic screw fixation was used in five patients. Plain radiographs obtained postoperatively confirmed reduction of the syndesmosis. There was no talar subluxation or talar tilt, the average medial clear space was 3.4 mm (range, 3–4 mm), the average tibiofibular clear space was 3.8 mm (range, 3–5 mm), and the average tibiofibular overlap was 8.2 mm (range, 7–10 mm). No patients in this group had any complaints and all were satisfied with the outcome results. Two patients initially were treated at another institution and did not have syndesmotic fixation. At 18 and 20 months after fixation of the fibular fracture, the two patients had diffuse pain around the ankle that increased in severity as the day progressed. They also had swelling around the ankle and difficulty walking up and down stairs. Plain radiographs and CT scans showed persistent diastasis of the distal tibiofibular syndesmosis (Fig 2).

Fig 2A–C.: (A) The AP plain radiograph obtained preoperatively of the right ankle shows a Weber Type B fibular fracture associated with widening of the tibiofibular clear space and reduction of the tibiofibular overlap that indicate diastasis of the distal tibiofibular joint. (B) The plain AP radiograph was obtained 15 months after treatment of the initial fracture in which a syndesmotic screw was not applied. Widening of the tibiofibular clear space and reduction of the tibiofibular overlap indicate persistent diastasis of the distal tibiofibular joint. (C) The axial CT section 9 to 12 mm proximal to the tibial plafond obtained 18 months after treatment of the initial fracture shows widening of the syndesmosis on the right ankle in comparison with the left side. Persistent diastasis of the distal tibiofibular joint is confirmed.

DISCUSSION

Danis Weber classification, referred to as the AO classification, is based on the level of the fracture of the fibula. ¹⁵ There are three types: A, fibula fracture below the syndesmosis (infrasyndesmotic); B, fibula fracture at the level of the syndesmosis (transsyndesmotic); and C, fibula fracture above the syndesmosis (suprasyndesmotic). In this classification the more proximal the injury, the greater the risk of syndesmosis disruption and ankle instability. ¹⁵ All the patients in the current study had a Type B Weber fibular fracture that extended proximally for less than 3.5 cm above the ankle. The fibular fracture was associated with deltoid ligament injury that was confirmed clinically by swelling and tenderness on the medial side of the ankle and radiologically by widening of the medial clear space more than 4 mm.

Boden et al ¹ did a study on cadavers and reported that ankle fractures associated with deltoid tear showed syndesmotic diastasis only when the fibular fracture was more than 3 to 4.5 cm above the tibiofibular joint. Their study suggested that syndesmotic screw fixation in Weber Type C injuries associated with a deltoid tear was unnecessary if the fibular fracture was less than 3 to 4.5 cm above the tibiotalar joint. In their study only a mortise view was obtained and only one measurement was used to determine the width of the syndesmosis, which was measured from the incisura to the medial part of the fibular cortex. This measurement could detect only overt diastasis and they did not consider occult diastasis.

Yamaguchi et al ²⁷ did a prospective study to validate the indications for syndesmotic screw fixation. In their series they reported on eight patients with Weber Type C fibular fractures associated with a deltoid ligament injury. Three of these patients had fibular fracture greater than 4.5 cm proximal to the joint and were treated with a syndesmotic screw in addition to the fibular plating. The remaining five patients had fibular fracture less than 4.5 cm proximal to the joint and did not have syndesmotic fixation. After 1 to 3 years followup, they concluded that syndesmotic fixation only is necessary for fibular fracture greater than 4.5 above the ankle when associated with deltoid ligament injury. However, in their study detection of syndesmotic diastasis also depended only on plain radiographic measurement, which has been shown to be inaccurate because of the wide anatomic variability in the depth of the incisura fibularis and the shape of the tibial tubercles. ^6,9,21 Furthermore, Yablon and Leach ²⁶ showed in 26 patients who had a reconstructive surgical procedure for treatment of malunited fibula, that most of the patients were relatively asymptomatic for an average of 5 years after treatment of the initial fracture, which means that problems with the syndesmosis after ankle fractures may take a long time to develop.

Yablon and Leach ²⁶ also described occult malunion of the fibula in which the talus remained in a normal position, but the lateral malleolus showed residual displacement characterized by external rotation that could be seen on the CT scan by comparing the involved side with the uninvolved side.

Wuest ²⁵ stated that placement of the syndesmosis screw alters normal ankle biomechanics and fibular motion. However, syndesmosis fixation has not yet been proven to cause any obvious detrimental clinical sequelae. Wuest ²⁵ also stated that it is preferred to err on the side of overuse of syndesmosis fixation. The subtle changes in ankle biomechanics and limitation of motion in the short term are not as crucial as obtaining and maintaining anatomic reduction of the syndesmosis and the mortise. In the current series, the syndesmotic screw was removed at 8 weeks. None of the patients had any problem related to the syndesmotic screw.

Solari et al ²² stated that routine plain radiographs were not sensitive enough to detect significant rotational displacement of the talus within the ankle mortise. They also showed that although reduction of the lateral malleolus was critical in reconstructing the ankle mortise, it was the medial side composed of the deltoid ligament and the medial malleolus that provided mortise stability. Harper ⁹ suggested that comparison of CT scans of the injured ankle and the noninjured ankle in similar orientations was a more precise technique for detecting subtle rotational abnormalities of the fibula when the plain radiographs of the patient with a suspected syndesmotic injury were normal or equivocal. In a previous study, it was shown that the CT scan was more sensitive than the plain radiograph in detecting syndesmotic diastasis of 3 mm or less. ⁶ There are variations in the depth of the incisura fibularis and in the position of the fibula in relation to the medial malleolus. In a previous cadaveric study, it was shown that in 60% of the specimens the incisura fibularis was significantly concave, greater than 4 mm depth, and in 40% the incisura fibularis was shallow concave, less than 4 mm depth. ⁷ Perhaps the depth of the incisura fibularis is one of the important factors that play a role in syndesmosis stability and subluxation of the fibula. In the current study it was difficult to detect the syndesmosis disruption on the initial assessment of the AP and mortise radiographs obtained preoperatively because there was no obvious talar shift on the plain radiograph. However, the syndesmotic disruption was easily recognizable on the axial CT scan when the injured and the noninjured sides were compared. Furthermore, in three patients the axial CT scan obtained preoperatively showed anterior fibular subluxation that was not appreciated on the plain radiographs obtained preoperatively. The CT scan also showed that in all the patients the incisura fibularis was shallow. The average depth of the incisura fibularis in the patients included in this study was 2.7 mm (range, 2–4 mm).

The current authors suggest that shallow incisura fibularis might have contributed to the instability of the distal tibiofibular syndesmosis. Additional studies are required to establish this association.

Tile discussed the key role of the posterior syndesmotic ligament complex (Volkmann triangle fracture) to the stability of the ankle mortise. ²⁴ In the current series, two patients had posterior malleolus fracture that involved less than 25% of the articular surface and did not require fixation.

On the basis of the current study it would seem that using the level of the fibular fracture as a guideline for application of the syndesmotic screw as suggested by some authors may not be accurate. ^1,27 There are several factors that should be considered including the depth of the incisura fibularis, posterior malleolus fracture, deltoid ligament injury, and subluxation of the fibula that is difficult to detect on the plain radiographs and only can be seen on CT scans. Careful evaluation of the plain radiograph and determination of all the recommended measurements are necessary to diagnose the syndesmotic disruption in patients with low fibular fractures. It would be more appropriate to depend on intraoperative stress tests (Cotton test and a lateral force applied to the fibula via a hook and observing the lateral fibular displacement) and intraoperative stress plain radiographs in deciding whether the application of the syndesmotic screw is necessary. Furthermore, each case should be taken on its merit and the surgeon’s impression in the operating room of syndesmosis stability should be used as the guideline in the application of syndesmosis fixation rather than setting guidelines that depend on the level of the fibular fracture.