Abstract: BACKGROUND: Poor incision healing and infection often occur in elderly patients with lateral malleolar fractures after traditional lateral plate fixation. With the application of engineering software in medicine, a new type of plate placed posterolateral can be designed to solve the above-described problems.
OBJECTIVE: To design a new type of posterior lateral low-profile steel plate with the aid of medical bioengineering software, based on the 3D CT data of the distance between the top of the lateral ankle fracture line to the anterior starting point (ACD), the distance between the top of the fracture line to the tip of the lateral ankle (CTD), the distance between the top of the fracture line to the posterior edge of the fracture line (PCD) and the angle between the anterior and posterior lateral sides of the distal fibula (CA). 
METHODS: Thirty cases of unstable lateral malleolar fracture and normal ankle were taken for CT scanning and three-dimensional reconstruction. The ACD, CTD, and PCD values in patients with lateral malleolar fracture were measured by 3-matic software, and the characteristics of lateral malleolar fracture line were plotted and described. The mimics software was used to measure the value of CA in the normal ankle joint. Based on the data measured above, 3-matic software and solidworks software were used to design the low-profile steel plate and the thickness of the steel plate and the direction of the nail path were constructed. In Geomagic Studio software, fine surface, automatic surface, and fitting surface were used to generate the prototype of the low-profile steel plate, and then 3D printing was performed. After making a posterolateral incision of the lateral malleolus, the peroneus longus and brevis tendons were removed, and the prototype of the 3D-printed steel plate was placed behind the fibula to test its size and fit to the bone surface.
RESULTS AND CONCLUSION: (1) The mean of ACD was (2.97±0.03) cm, and the variation was 5.23. The mean of PCD was (3.17±0.11) cm, and the variation was 17.60. The mean of CTD was (4.52±0.07) cm, and the variation was 8.60. (2) The fracture line of the lateral malleolus was drawn with an inverted “V” shape. The mean of CA between anterior and posterior lateral surfaces of the distal fibula was (103.20±1.94)°. At the midpoint section of the upper and lower vertices of the anterior edge of the distal fibula, the angle of the anterior and posterior lateral sides (CA) of the distal fibula was (78.50±1.78)°. (3) By using 3-matic, Solidworks, and Geomagic Studio software, a new type of posterior lateral low-profile steel could be successfully designed. Three to four holes were reserved for the screw holes at the proximal end of the plate with screw directions from back to front, and three screw holes were reserved on the inner and outer sides at the far end. The direction of the inner three holes could be from back to front, and the outer three screw holes needed to be biased towards the inner side, with an angle of 9.72°-13.28°. (4) It is indicated that the variability of the ACD position on the anterior lateral fracture line of the lateral malleolus is relatively small, while the variability of the posterior lateral PCD position is relatively large. The angle between the anterolateral and posterolateral sides of the lateral ankle fracture block shows a decreasing trend, with a smaller variation in the proximal angle and a larger variation in the distal angle. Based on three-dimensional CT reconstruction data of the external ankle, with the help of computer bioengineering software and the use of reverse design concept, a new type of low-profile lateral malleolus steel plate with a good fit can be quickly and conveniently designed to provide a valuable reference for the design of internal fixation devices.

Key words: external ankle fracture, posterolateral fibula, low-profile steel plate, reverse design, medical bioengineering