Conservative treatment of ulnar styloid fractures following volar-plate fixation of distal radius fractures: incidence of nonunion evaluated by computed tomography

Purpose The first aim of this study was to investigate the incidence of ulnar styloid fractures (USFs) accompanied by distal radius fractures (DRFs), treated with volar locking plates. The fracture type of DRFs was evaluated by the classifications, based on computed tomography (CT) scan findings. The second aim was to investigate the bone union rate of USFs, depending on the fracture type of DRFs, by comparing union and nonunion groups in the USFs groups. Methods Between May 2012 and December 2019, 239 consecutive patients with DRFs were treated. Of these patients, 177 DRFs met inclusion criteria. The fracture patterns of the DRFs, based on the classification, using CT scans, which included the AO classification, sagittal angulation, and axial fracture patterns of the articular surface of the distal radius in two-part intra-articular fractures were evaluated. The size of USFs, classified as a tip or base fracture was also investigated. Results The incidence of USFs was significantly higher for AO types A and C than for type B. Analysis of the sagittal angulation of DRFs showed that the incidence of USFs was higher for the extension type than for the flexion type. Axial CT classification of two-part fractures revealed that DRFs with a dorsal fracture line was more frequent than the volar type of DRFs. These results suggested that dorsal displacement of DRFs was associated with a higher incidence of USFs. Finally, the analysis of the bone union rate of USFs revealed that AO classification and sagittal angulation were not correlated with bone union in USFs. However, it was found that a fracture line on the radial side of the radius had a significantly low rate of bone union, compared to a fracture line on the dorsal side. The size of USFs was also not correlated with the bone union rate. Conclusions The incidence and the bone union rate of USFs have different patterns. The incidence of USFs was higher in the dorsal displacement type of DRFs. However, the bone union rate of USFs was lower for a fracture line on the radial side. Therefore, USFs with DRFs that have a fracture line on the radial side is a candidate for fixation to prevent nonunion.