Ankle fractures involving the syndesmosis complex generally have a worse prognosis than comparable fractures without tibiofibular ligament injuries. In 347 examined patients, Egol et al. were able to demonstrate that after 12 months the functional outcome and the pain level were significantly worse in the group with syndesmosis injuries . Chissel et al. already reported poor clinical results in 1995 when syndesmosis width after surgical treatment exceeded the radiologically measured value of > 1.5 mm . Andersen noted a difference of more than 2 mm of the sagitall anterior tibiofibular distance as a predictor for poorer clinical outcome . Moreover, Leeds and Ehrlich proved a significant correlation between arthrosis development and accompanying syndesmosis injury . Current medium-term study results obtained by Veen et al. confirm a significantly higher arthrosis rate associated with ankle fractures with syndesmosis injury . In addition, Ovaska et al. were able to show that, at 59%, malrepositioned syndesmosis is the most frequent cause of revision surgery of ankle fractures .
The intraoperative malposition rate of the distal tibiofibular syndesmosis in closed reduction is up to 52% and can be reduced to 15% by open reduction of the fibula with direct visualisation of the syndesmosis region [3,18]. However, also malposition rates after open reduction are still high and require a reliable position control of the distal syndesmosis region. All conventional X-ray parameters (tibiofibular clear space, tibiofibular overlap, etc.) do not allow a sufficient assessment of the fibula position to the tibia [5,6,19].
This applies to all syndesmosis injuries since Franke et al. could not identify risk factors such as injury type or fracture morphology after analysing 251 patients with syndesmosis injuries, which are associated with a lower rate of syndesmosis malposition .
Relevant evaluation criteria are the position of the fibula in the tibial incisura and the rotation of the fibula considering correct length reconstruction . CT-Measurements 10 mm above the tibial plafond taking into consideration the diastasis and anterior-posterior translation of the fibula were found to be parameters with high interobserver and intraobserver reliability .
The position of the foot, according to the studies by Levack and Vetter, has no relevant influence on the tibiofibular distance nor on the tbiofibular angle. Therefore, the intraoperative scan can be performed in any position of the ankle and foot[21, 22].
Multidimensional intraoperative imaging of the syndesmosis region is also possible using 3D image intensifiers. A small case series of 10 patients with syndesmosis injuries was presented by Ruan et al. in 2011 . An intraoperative 3D scan was performed before positioning the adjusting screw with the joint being temporarily adjusted by means of reduction forceps. The measurement parameter was the distance to the anterior and posterior facets of the tibia. The aim was to achieve equal measuring distances. Once fine correction and adjusting screw application had been completed, a final second 3D scan was performed. In all cases, this scan showed a central and symmetrical positioning of the fibula in the tibial incisura.
Summers et al. reported a lower rate of 5.5% (1/18 patients) malreduction in syndesmotic injuries shown by intraoperative 3D scan . They used conventional X-ray settings of the uninjured side as a template to assess the reduction before the intraoperative 3D scan. They concluded that intraoperative CT is only necessary in cases where conventional radiologic signs didn`t indicate an accurate restoration The results of the small case study do not match the results of many other studies in which a significantly higher rate of malreduction was detected.
Moon et al. reported an significant higher intraoperative revision rate of 23,1% using a 3D image intensifier for ankle fractures with syndesmosis injuries .
Franke et al. performed intraoperative 3D scans in 251 consecutive patients with syndesmosis injuries after adjusting screw placement, which resulted in direct intraoperative correction of osteosynthesis in 32.7% of patients . The main reason was a malposition of the fibula in the tibial incisura in 25.5% and a necessary correction of the fracture reduction in 5.2% of the patients. Corrections due to implant misalignments were necessary in 2% of patients.
Davidovitch et al. compared the conventional versus 3D scan controlled intraoperative reduction of the ankle joint in 36 patients . In the relevant measuring range of 2 mm difference, significantly more postoperative malpositions in the control CT were found in the conventionally radiologically controlled group. Our own data showed a lower malpositioning and correction rate of 7% which may be explained by the generally direct visualization of the syndesmosis stabilisation. Another surgical parameter that significantly influences the reduction result is the positioning of the reduction forceps in the anterior third of the tibia .
A problem that has not yet been finally resolved is the correct assessment of the syndesmosis region. The common parameters used are the fibular length, fibular position and rotation in the incisur . However, the tibial incision has a large anatomical variance in shape. Seventy-five percent of the incisions have a concave shape, 16% a convex shape and 8% are not typable . Elgafy et al. found 67% convex and 33% flat angled incisures 9-12 mm above the tibial plafonds . This makes reliable rotation measurement difficult in the absence of normal values and uncertainty about the best measurement method at the level of the syndesmosis 10 mm above the ankle joint. Knops, for example, compared the reliability and accuracy of 4 measurement methods using a 3D rotational C intensifier . Two of these measurement methods were difficult to carry out and even the best method, measuring the angle between the tangent of the anterior tibia surface and the bisection of the vertical midline, was only fairly reliable and accurate.
Comparison of the healthy opposite side is therefore seen as the gold standard for assessment in the CT. Schon summarized the results of 16 CT studies that carried out a total of 35 different measurement methods . The study demonstrated low native side-to-side symmetry. Furthermore, there is no single measurement method that adequately captures the complexity of the possible misalignments. At least 3 different measurement methods are necessary to record the relevant criteria of translation medial / lateral, anterior / posterior and fibular rotation. In particular, the sole qualitative side-by-side comparison without measurement data collection shows a very low level of intra- and interobserver reliability and should not be used as an assessment parameter .
The healthy opposite side for comparison is not available with the intraoperative cone beam ct because the imaging volume is too small. A second scan of the healthy opposite side would have to be performed. In terms of radiation protection and the additional expenditure of time, it must be assessed critically and this approach has not yet been investigated in studies. Complex intraoperative measurements of the rotation and translation of the fibula in the tibial incision are also not expedient because of the lack of normal values, the large anatomical variance and the lack of comparison with the contralateral side [29,30,32].
It is therefore important to define comprehensible criteria in order to be able to assess the intraoperative cone beam ct examinations. The criteria we have described correspond to the assessment parameters defined by Franke and, more recently, by Vetter [26,34]. The fibula should be symmetrical in the incisure 10 mm above the tibial joint line, and the arch between the anterior margin of the fibula and the tibia should be harmoniously elliptoid. There must be an equal fibulotalar and tibiotalar clear space in coronal and sagitall view. The fibular length needs also be assessed.
The intraoperative measurement of the fibula rotation has proven to be impractical at the level of the syndesmosis. The rotation measurement below the ankle joint line seems to be easier. The joint-side corticales of the malleoli are used as reference. Vetter et al. found the area 4-6 mm below the talar joint line to be the ideal measurement point for fibular rotation in 100 healthy joints . The mean angle was 8.4 ° +/- 4.9 . However, the absolute values varied between 0-26 .
Compared to CT examinations, intraoperative radiation exposure resulting from the 3D scan can be classified as very low in total. Beerekamp et al. reported a maximum dose of 17 µSV for a 3D extremities scan compared to a 200 µSV dose for a postoperative CT examination .