To propose a new classification of posterior malleolus fracture to further clarify its pathoanatomy.

Twenty fresh frozen cadaver specimens of normal morphology of lower limbs were selected and dissected and the extent of the tibial insertion of posterior malleolus associated ligaments was measured. At the same time, a retrospective case series analysis was made on the clinical and CT information of 296 patients with posterior malleolus fracture treated at Tongji Hospital of Tongji University from January 2012 to July 2020 or at Karamay Central Hospital from January 2018 to July 2020. The percentage of articular involvement of the fracture, proximal displacement of the posterior malleolus fracture and extent of posterior talar subluxation were measured. A clinically practical new classification system for posterior malleolus fracture was created by correlating posterior malleolus associated ligaments with CT images of posterior malleolus fracture. The new classification included three types: type I was posterior malleolus fracture with only the tibial insertion of inferior transverse tibiofibular ligament involved; type II was posterior malleolus fracture with both the tibial insertions of inferior transverse tibiofibular ligament and posterior inferior tibiofibular ligament involved, which was divided into subtypes IIA and IIB based on the presence of articular cartilage and subchondral bone damage, compression or Die-Punch fragments; type III was posterior malleolus fracture that involved all the tibial insertions of inferior transverse tibiofibular ligament, posterior inferior tibiofibular ligament and posterior tibiotalar ligament, which was sub-classified into subtypes III A and III B according to number of fracture fragments. Anatomic characteristics of the extent of the tibial insertion of posterior malleolus associated ligament, CT imaging parameters for posterior malleolus fracture and corresponding fracture typing were determined. In addition, the new classification system for posterior malleolus fracture was compared with Haraguci classification and Mason classification.

Posterior malleolus associated ligaments included the posterior inferior tibiofibular ligament, inferior transverse tibiofibular ligament and posterior tibiotalar ligament from posterolateral to posteromedial tibia. The posterior inferior tibiofibular ligament was attached to the posterolateral tibia and the distance between the highest point of its tibial insertion and the joint line was (45.2±5.6)mm. The inferior transverse tibiofibular ligament was attached to the posterior distal tibia and the distance between the highest point of its tibial insertion and the joint line was (5.5±1.0)mm. The posterior tibiotalar ligament was attached to the posterior colliculus and intercollicular groove of the medial malleolus and the distance between the center of its tibial insertion and the intercollicular groove was (2.5±0.6)mm. Among 296 patients with posterior malleolus fracture, there were 36 patients with type I, 229 with type II (150 type IIA, 79 type IIB) and 31 with type III (11 type IIIA, 20 type IIIB). The percentage of articular involvement of the fracture, proximal displacement of posterior malleolus fracture and extent of posterior talar subluxation in type IIB fracture were significantly greater than those in type II A fracture [23.7(18.6, 28.8)% vs. 18.4(12.7, 21.7)%, 4.1(2.1, 6.0)mm vs. 1.9(0.2, 3.0)mm, 4.7(1.5, 6.2)mm vs. 2.3(1.1, 3.0)mm] (all P<0.01). The proximal displacement of posterior malleolus fracture and extent of posterior talar subluxation in type III fracture were significantly greater than those in type II fracture [7.2(6.0, 8.2)mm vs. 2.7(0.4, 4.0)mm, 10.1(6.0, 15.0)mm vs. 3.1(1.1, 5.0)mm] (all P<0.01). The new classification for posterior malleolus fracture combined the posterior malleolus ligament and injury mechanism of posterior malleolus fracture as compared with Haraguchi classification, which not only further detailed the classification, but also was more practical in clinic for increased the severity of injury was elevated with higher classification level. The new classification was more comprehensive as compared with Mason classification for it mainly added the type of simple-rotation-type posterior malleolus fracture (type IIA of the new classification).

In combination with posterior malleolus associated ligaments, injury mechanism and fracture morphology, posterior malleolus fracture is divided into three types. The new classification system more comprehensively describes pathoanatomy of posterior malleolus fracture that contributes to related basic research and clinical diagnosis and treatment.

Ankle fractures; Ligaments; Classification