Incisura fibularis (IF) is a clinically useful anatomic landmark used to assess syndesmotic stability radiologically post-injury. However, it is subject to wide anatomical variability, primarily influenced by gender.
A shallow IF depth may decrease bony stability and increase the rate of anterior talofibular ligament injury during an ankle sprain. Therefore, it is necessary to accurately evaluate IF morphometry and its relevance to syndesmotic reduction.
Definition
The incisura fibularis (Latin for “fibular notch”) is a triangular space on the postero-lateral surface of the distal end of the tibia in which the fibula is lodged. It is a key landmark used clinically to assess syndesmotic stability after an ankle injury. Recently, it has been proposed that variations of the shape of the incisura may be associated with an increased risk of a posterior malleolus fracture and recurrent lateral ankle sprains.
This study evaluates the morphometry of the incisura fibularis on preoperative bilateral weightbearing CT scans in patients with HAS and a control group, with particular emphasis on the depth, width, version, and level of fibular engagement of the incisura. Measurements were made at a level 1 cm proximal to the tibial plafond, on axial images of the tibiofibular joint mortise view and the AP view of the foot.
The results of the study suggest that a shallow, retroverted and disengaged incisura fibularis is more likely to be associated with ligamentous syndesmotic injury, particularly in HAS patients. However, the underlying mechanisms of this relationship are yet to be determined. Further investigation is required into the morphological characteristics of the incisura fibularis and its relationship to syndesmotic stability and injury. Moreover, the morphology of the incisura fibularis in those who have not sustained an injury of the ankle is also important.
Morphology
The morphology of the incisura fibularis (IF) has been investigated in various studies. Most of the studies were retrospective comparative, whereas others were prospective cohort or observational in design. In these studies, the IF depth was measured and compared between groups of individuals with and without ligamentous ankle injuries. In general, a shallower IF was found to be associated with recurrent ankle sprains. The IF height and angle were also studied in these studies. A lower IF height and angle was found to be associated with increased incidence of posterior malleolus fractures.
A number of studies have evaluated the morphology of the IF using CT scans and 3D models. These studies have been able to identify distinct shapes of the DTS, including three types: a “C” shape with giving the syndesmosis a crescent shape, a “1” shape with the incisura fibularis being shallow, and a G shape with a deeper incisura and a smaller protrusion from the anterior or posterior tibial tubercle.
In general, a shallower IF is associated with recurrent ankle sprains, possibly due to the lower osseous resistance against tibiofibular displacement. Several studies have also reported a correlation between a shallow IF and the frequency of anterior talofibular ligament rupture. However, these studies are of moderate methodological quality and further prospective investigations are needed to confirm these findings.
Diagnosis
The tibiofibular syndesmosis is an important but challenging anatomy to assess radiographically. There are many different methods to evaluate the stability of the syndesmosis, ranging from simple plane radiographs to comparisons with the uninjured contralateral ankle. However, these techniques are time consuming and expose the patient to excessive radiation exposure. In addition, there are limitations to their use in identifying injury to the posterior malleolus and posteroinferior tibiofibular ligament (PTFL/PMA).
The Incisura fibularis is an area of the distal fibula that connects the tibial plafond and the lateral malleolus. It is involved in several types of injuries, including Mason-Malloy type 1 posterior malleolar fracture and recurrent lateral ankle sprains caused by supination external rotation.
In a retrospective study, the authors examined preoperative bilateral weightbearing ankle computed tomography scans of patients with PM fractures and controls. They measured the depth, width and engagement of the incisura fibularis at a location 1 cm proximal to the tibial plafond. They also compared the morphometry of the incisura fibularis to the tibial plafond and tibial tubercle.
They found that the incisura had a pyramidal shape with a shallow depth and an average width of 1 cm. They also found that the incisura had varying degrees of engagement with the tibial plafond and the anterior tubercle. There was a significant correlation between the incisura width and the pattern of PM fractures.
Treatment
The Incisura fibularis (IF) is a landmark that is used to assess syndesmotic reduction radiologically after ankle injury. However, the anatomic variability of the IF has made this assessment difficult, and has led to inconsistent results in clinical practice. The aim of this study is to investigate the anatomy and morphometrics of the IF, in order to develop a more accurate method of intraoperative evaluation of the distal tibiofibular syndesmosis.
Under anesthesia, a tourniquet is placed on the proximal leg, and the flexor hallucis longus muscle is divided by a longitudinal incision. With this technique, the posterior interval that is broader than that of the anterolateral (AL) and posterolateral (PL) approaches can be obtained, with access to the entire plafond articular surface. This allows direct manipulation of the IF, with good visualization of its contents. Typically, the IF is held in lateral rotation, and the Tillaux fragment, die-punch fracture, and Volkmann fragment are reduced with several temporary K-wires.
The mean sagittal IF engagement difference between healthy and injured sides was 0.22 mm, while the anterior translation was -0.52 mm, both of which are non-statistically significant. Furthermore, the IF angulation was 1.3 deg, which is similar to the normal position. This finding suggests that a precise IF angulation is not required for adequate syndesmotic reduction. However, it may be beneficial to measure the IF depth at a location slightly more proximal than 5 mm from the plafond, in order to ensure that all the syndesmotic structures are properly exposed.