To characterize the 3-D morphology of lateral malleolar fractures of Danis-Weber type B, especially the distribution of fracture apexes, for formulation of 3-D maps of fracture lines which may facilitate the treatment of the fractures.

A retrospective study was conducted to analyze the CT tomography data of 114 patients who had been operatively treated for lateral malleolar fractures of Danis-Weber type B at Department of Orthopaedic Surgery, Yangpu Hospital from January 2017 to May 2022. There were 46 males and 68 females with a mean age of 61.5 (51.8, 68.0) years. The morphology of distal bone blocks was measured and positions of fracture apexes were observed on 3-D CT reconstruction. According to the positions of apex, the fractures were divided into 4 types: type Ⅰ with the apex located on the lateral ridge, type Ⅱ with the apex located on the posterolateral side, type Ⅲ with the apex located on the posterior ridge, and type Ⅳ with the apex located on the medial side. CT images of all fractures were superimposed on a standard template to create 3-D fracture line maps.

In this cohort, there were 7 cases of type Ⅰ (6.1%, 7/114), 65 cases of type Ⅱ(57.0%, 65/114), 39 cases of type Ⅲ (34.2%, 39/114) and 3 cases of type Ⅳ (2.6%, 3/114). In 49 cases (43.0%, 49/114), the fracture apex was not located on the posterolateral fibula (so that a conventional posterolateral steel plate cannot compress the apex). In fracture morphology of the 114 patients, the anterior fracture height, posterior fracture height, vertical fracture length, inclination angle, spiral angle, and apex spiral angle were, respectively, (-6.22±4.62) mm, (27.23±12.32) mm, (33.45±11.89) mm, 56.9°±9.6°, 269.8°±37.1°, and 156.2°±24.0°. The anterior fracture height, posterior fracture height, vertical fracture length, inclination angle, spiral angle, and apex spiral angle in type Ⅲ were all significantly larger than those in type Ⅱ (P<0.05). The 3-D fracture line maps indicated that the lines of type Ⅲ fracture were steeper than those of type Ⅱ fracture.

Since the apexes are not located on the posterolateral fibula in nearly half of the Danis-Weber type B lateral malleolar fractures, a conventional posterolateral steel plate cannot provide an effective anti-glide role. The higher a fracture line, the closer the fracture apex is to the posteriomedial fibula (posterior ridge and medial side). Fractures of type Ⅲ and type Ⅲ often indicate greater injury violence and more accompanying injuries.

Ankle joint; Lateral malleolar fractures; Type B fracture; Morphology; Fracture maps