The static stability of the DRUJ is achieved by the bony congruity between the sigmoid notch of the radius and the ulnar head and by the ligaments which hold the joint together. Part of the ligaments constitute the main stabilizer of DRUJ, which run from the fovea of the ulnar head to the dorsal and palmar edges of the sigmoid notch on the distal radius. [23–25]. The distal interosseous membrane (DIOM) of the forearm acts as a secondary soft tissue stabilizer of DRUJ. DIOM originates from the distal ulna 54 mm (on average) proximal to the ulnar head and runs distally to insert on the dorsal inferior rim of the sigmoid notch of the radius, which is at the terminal of the central band of the interosseous membrane [26–28]. Therefore, when the ulnar head breaks, the ligament will lose its stable attachment point, resulting in the instability of DRUJ. Distal ulnar metaphyseal fracture can be deemed as a fracture ranging from the ulnar neck to within 5 cm of the distal dome of the ulnar head and the high incidence of it is related to osteoporosis. Since 2000, with the development of internal fixation technology and increasing aging population, people's requirements for the recovery of wrist joint function are gradually improved. More and more surgeons choose open reduction and internal fixation to treat unstable distal ulnar fractures[30, 31]. Palmer and Werner showed up to 42% of force passing through the ulna, in which axial force passing down the ulnar head fracture end was closer to 20%. The above studies indicated that the loss of the ulna head would disrupt the biomechanics and load-bearing capacity of the DRUJ. Therefore, the demand for internal fixation treatment become higher owing to the biomechanical characteristics of ulna head fracture.
However, the number of reported cases and literatures is rather sparse, which is mostly limited by the low incidence, merely 5–6%, of distal radius fractures accompanied by a distal ulnar metaphyseal fracture.. At present, it remains plenty of controversies around the treatment of distal ulnar head fracture. It is challenging to perform an internal fixation of distal ulnar metaphyseal fractures because the distal fracture fragment is small, comminuted, osteoporotic, covered with articular surface over a 270° arc and surgical exposure of the distal ulna for hardware placement raises the possibility to damage the dorsal sensory branch of the ulnar nerve. The most widely used fixation methods are dorsal micro-locking plate and anatomical hook plate, but it remains unclear about their merits and drawbacks and mechanical properties. Although the hook plate conforms to the ulnar anatomical structure of the distal ulna, there are few screw holes in the head which are arranged vertically, and the screw placement is limited during operation. On the other hand, the locking plate has more screw holes and the characteristics of pre bending. Considering that the horizontal arrangement of screws has higher anti rotation ability, we propose a method of placing the micro-locking plate on the ulnar side. Nevertheless, limited by the number of clinical cases, retrospective study is difficult to carry out. Therefore, a new way of analysis is urgently demanded.
Nowadays, thanks to the latest development of finite element model generation, such as improved quality of CT imaging, segmentation algorithm and computing rate, the accuracy of finite element modeling has been greatly elevated. With the maturity of technology, 3D finite element analysis (FEA) can simulate the biomechanical analysis of complex orthopedic diseases and get rid of the limitation of the lack of cases. In this study, we chose to use FEA to figure out whether placing the ulnar side locking plate had better biomechanical properties than the current choice of the dorsal side locking plate. We hope the mechanical results of this study provide experimental guidance to its application in clinical surgeries.
As shown in Table 2, the ulnar-side locking plate models provided more stable fixation than the dorsal-side models and the stability increased with the augment of head screws. Figures 5 and 6 illustrated that the stress of the four fixation systems was concentrated at the fracture line. Both the stress concentration zone and the maximum displacement were decreased in ulnar-side locking plate fixation. As shown in Table 1, under torsion moments, the peak VMS of the ulnar-side fixation models are lower than the dorsal-side one, and it reduced as adding the additional ulnar head screw, which evidently indicated the anti-torsion function of the horizontally arranged ulnar head screw. Under the axial loading, the peak VMS increased on the dorsal-side fixation models and concentrated at the middle additional screw, while it decreased on the ulnar-side fixation models. The results mentioned above indicated that ulnar-side locking plate fixation provided better stability, resulting in the lower stress distribution on the plate and greater security of the fixation system. Ulnar-side plate fixation could generate a rigid, stable mechanism and provide a strong resistance ability to counter compression and torsion effect. Adding the additional screw enabled the fixation models to generate a better stability, but concentrated the stress on the middle screw, which will guide the design of subsequent plate improvement. This study is the first FEA comparing the mechanical efficiency of dorsal-locking plate and ulnar-side locking plate in the fixation of ulnar head fracture. However, with the limitation of no experimental validation was conducted and no soft tissue structure was built in the models, the application of these fixation plates still requires more research.
This finite element simulation may facilitate the further mechanical research and give guidance to the treatment of the ulnar head fracture clinically.