Three-dimensional kinematics of the flexor pollicis longus tendon in relation to the position of the FPL plate and distal radius width

The standard therapy of intra-articular and extra-articular distal radius fractures consists of open reduction and stabilization using palmar osteosynthesis with an angularly stable plate. The integrity of the flexor pollicis longus tendon (FPLT) may be mechanically affected by the plate, with rupture rates between 1 and 12% reported in the literature, occurring during a postoperative time period from 4 to 120 months. The aim of this study was to investigate the position of the tendon in relation to the distal edge of the plate using high-resolution ultrasonic imaging.

Nineteen patients undergoing osteosynthesis for distal radius fracture in 2015 with the Medartis^® APTUS^® FPL plate were included in this study. Of these, seven dropped out for various reasons. Therefore,  twelve patients with a median age of 52 years (range 24–82 years) were included in the final analysis. High-frequency ultrasound was performed within a median of 28 (range 10–52) weeks by an experienced radiology specialist to locate the FPLT position in two separate wrist positions: (1) wrist held in 0° position and fingers extended and (2) wrist held in 45° of dorsal extension and actively flexed fingers II to V (functional position). For analysis, we used the axial ultrasound videos. Postoperative X-rays and CT scans were included for the analysis, especially the soft-tissue CT scan window for the exact localization of the FPLT.  Dynamic ultrasound scanning was used to localize the FPLT in relation to the plate in 0° and functional position of the hand. Using CT scanning, the position of the plate relative to the bone was determined. In this way, we were able to correlate the functional FPLT position with the osseous structures of the distal radius.

In all cases, the FPLT was positioned closer to the volar distal edge of the FPL plate in functional position than in 0° position. In four cases, the FPLT did not touch the plate at all and was shown to shift diagonally from radio-volar in ulno-dorsal direction during wrist movement from 0° to functional position, similarly to the sliding of the tendon in the assumed physiological motion sequence. In these cases, in the functional position the center of the FPLT was positioned slightly ulnarly of the center of the distal radius (i.e., less than 50% of the distal radius width measured from the radial border of DRUJ), and positioned more ulnarly than in all other cases (i.e., in which the FPLT came into contact with the plate). In the remaining two-thirds of the cases (eight patients), the FPLT touched the plate during wrist movement from 0° to functional position, shifted in dorsal direction and slid into the plate indentation, irrespective of whether the tendon entered the indentation from the radial or the ulnar side, and independent of the ulnoradial position of the plate. No signs of tendinopathy of the FPLT were found in any of the cases.

The results show that the indentation of the Medartis^® APTUS^® FPL plate reduces the tendon-plate contact and ideally even prevents it entirely. In particular, ulnar positioning of the plate lowers the risk of tendon-plate contact. If the FPLT touches the plate, the tendon pulls into the plate indentation, thus lowering the contact. Consequently, the Soong criteria are not applicable when a FPL plate is used.