In a CT-based analysis of 100 patients with Maisonneuve fractures (MF) we found associated posterior malleolus (PM) fractures in almost three quarters of cases (74%). The incidence of PM fractures in MF has been mentioned in several previous studies with considerable variations. Some of them presented low incidence rates in a range of 35–37% [12,13,14], while others published substantially higher incidences in the range of 77–83% [2, 5, 15,16,17]. Only 4 fractures (5%) in our study group were not detected on plain radiographs, all of them being small fragments of type 1 or 2 of the B-R classification. In contrast, He et al. [5] presented 17% of PM fractures seen only on CT scans. Good quality radiographs with exact projections are a prerequisite for detecting PM fractures. These may be difficult to obtain in the setting of an acute fracture with a painful ankle that is immobilized in a splint or cast. The indication to CT scanning should therefore be made generously if a PM fracture is suspected [3].

The share of PM fractures in individual patterns of ankle fracture-dislocations varies considerably. Jedlička et al. [1], in a radiographic study of 232 patients with ankle fracture-dislocations, found a PM fracture in 4% of Weber type A and in 46% of Weber type B and C fractures. Kostlivý et al. [18] identified a PM fracture in 70% of 110 cases of Bosworth fracture-dislocation (BF). These findings point to a higher severity of MF and BF as compared to other types of ankle fracture-dislocations.

Comparison of individual types of PM fractures with previous studies [2, 5] showed certain differences. Bartoníček et al. [6] when analyzing 141 consecutive cases of a PM fractures found a ratio of types 1 and 2 (milder types resulting merely from rotation) to types 3 and 4 (more severe types with a compression component) of 1.6:1, while in the present series of MF it was 3.2:1 (Table 3). Similar to our results, the most frequent PM fragment morphology was type 2 of the B–R classification.

Besides the size of PM fragment, in terms of involvement of the articular surface, involvement of FN and the presence and dislocation of ICFs as a criterion for operative treatment has been introduced only recently [3, 10].

As the percentage of the articular surface carried by the PM fragment increases from medial to lateral on the sagittal scans, measurements have to be standardized (Fig. 6). The same applies to assessment of FN involvement which is based on axial scans, because the values increase from proximal to distal direction [6, 9]. Based on the anatomical study by Fojtík et al. [19], we measured the values 5 mm proximal to the tibio-talar joint line, where FN is the deepest. The best way to measure the size of PM fragment including involvement of FN are 3D CT reconstruction views of FN with subtraction of the fibula (Fig. 4) and in mortise view with subtraction of the talus (Fig. 1).

The size of PM fragment and its articular surface depending on location sagittal CT scans. a 3D CT posterior view; b axial CT scans; c medial (blue line) sagittal CT scan; d central (yellow line) sagittal scan; e lateral (red line) sagittal scan

In our series, ≥ 33% of FN was involved in 14% of all PM fractures and 25–33% of FN in 27% of all PM fractures. Reduction and fixation of a displaced PM from the posterior approach were performed in 15 patients with ≥ 25% and in eight patients with ≤ 25% of FN involvement.

Displacement of the PM fragment is an indication criterion for direct internal fixation from the posterior approach [3, 4, 6, 20,21,22,23,24,25]. However, the direction of displacement has not yet been described in the literature. In a majority of cases (58%), fragments showed complex displacement in several directions.

The most important from the clinical viewpoint is, in our view, proximal or lateral displacement. A proximally displaced fragment as seen in 47% of cases in the present study reduces the size of the tibiotalar articular surface. A laterally displaced fragment found in 25% of cases leads to additional malalignment in FN and, as a result, prevents reduction of the distal fibula resulting in an incongruent ankle mortise.

We recorded solid intraarticular ICF in 43% of cases, similarly as Mueller et al. [10] who found ICF in 41% of PM fractures in tri- and quadrimalleolar fractures, most frequently in types 2 and 3 of the B–R classification. These fragments were most often observed in the region of the fracture line, in zones 7 and 8. In another 8 cases we found minor bone chips extruded into FN or the tibio-talar joint space, where, similarly as ICF, they may act as a mechanical obstacle.

The Tillaux–Chaput tubercle which carries the anterior syndesmosis shares many features with the posterior malleolus and may be considered a 4th or anterior malleolus [11]. In the present study, 17 of 100 MF had a concomitant anterior and posterior malleolar fracture which would be considered a quadrimalleolar fracture or pronation external rotation stage 4 according to the Lauge–Hansen classification. Taken together with the more severe types of PM fractures, this shows the relatively high energy producing a MF.

Our results point to a high variability of the shape, size and displacement of PM fragments associated with MF, as well as to their clinical importance from the viewpoint of impairment of FN integrity. Restoration of FN integrity is one of the basic prerequisites of anatomical reduction of the distal fibula into FN. Such reduction is also one of the basic factors influencing clinical and radiological results of MF treatment.

For this reason, all MFs should be examined by CT which will show pathoanatomy of a PM fragment, that is essential for the choice of treatment method, and may also reveal additional injuries not detected by radiographs [3]. Further research evaluating individual MF subtypes is required to establish clear criteria for operative treatment of individual lesions within MF.

One of the benefits of our study is the high number of prospectively included patients and their standardized CT evaluation, while a relative disadvantage may be absence of MRI examination providing important details about injuries to ligamentous structures.