distal radius distal radius fracture VAP osteosynthesis barton bone splinter cancellous impacted fragment cortical impacted fragment diastasis displaced dorsal lunate fragment displaced fracture dorsal lunate fragment fracture fracture pattern fracture type impacted fracture internal fixation intra-articular fracture jesse jupiter operative treatment orthopaedics patients age scapholunate stable fixation surgery surgical treatment traumatology volar barton volar barton's volar cortex volar plating volar shearing wrist wrist fracture wrist joint