Implant Analysis of a Distally Positioned Volar Locking Plate—A Microscopic Proof of Soong's Theory?

Case Report Open reduction and internal fixation of distal radius fractures can be complicated by the placement of the volar locking plate distal to the watershed line, as described by Soong et al.[ 1 ] Plate placement 2 mm or more above the volar critical line (Soong grade 1) or within 3 mm of the volar rim (Soong grade 2) is hypothesized to cause the tendon to abrade against the plate, leading to tendon irritation and possible rupture. The flexor pollicis longus (FPL) and deep flexor of the index finger (FDP2) are the tendons at risk due to their proximity to plate prominence. Consequently, a common presentation of this complication is wrist pain during thumb and index motion.[ 2 ] A 21-year-old woman presented 1 year after open reduction and internal fixation (ORIF) of a left distal radius fracture using an ACUMED® titanium alloy plate and screws. She reported a good functional outcome yet complained of persistent wrist pain triggered by index finger and thumb movements. Physical examination showed tenderness at the volar radial side, with palpable hardware in that region. Radiographs confirmed the anatomical alignment of the fracture with a plate positioned distal to the watershed line with volar prominence (Soong grade 1). The patient's hardware was removed and sent for microscopic and corrosion inspection analysis. The combination of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) was utilized for surface and chemical characterization. The analysis was conducted by a Hitachi SU3900 large chamber variable pressure SEM coupled with an Oxford ULTIM MAX 65 SDD X-ray analyzer. SEM images and EDS analysis were obtained by applying a 15-kV accelerating voltage at a 10-mm working distance. No corrosion of the plate or screws was observed. Microscopic analysis revealed three deposits over the distal radial area of the plate ([ Fig. 1 ]), corresponding to the anatomical location of the FPL and FDP of the index. The deposition underwent compositional analysis and elements typical for biological origins (carbon, oxygen, phosphorus, calcium) were found in these deposits, while the metallic plate elements (titanium, Ti; aluminium, Al, and vanadium) were absent. Moreover, a change in relative alloy composition (Al to Ti) was not detected, which supports that plate corrosion did not take place.[ 3 ] Fig. 1 Radiographs (top left) of a distally placed distal radius plate, with the corresponding image (top, second from right) of the plate after extraction from the patient. Higher-resolution scanning electron microscopy images of the deposits found on the plate are shown in the middle row, with the locations marked on the top right image. The chemical composition maps of all main alloying elements of the plate (titanium, aluminium, and vanadium) and elements corresponding to organic deposits (carbon, oxygen, phosphorus, calcium) are shown for location 1 in the bottom row. A more intense color corresponds to a higher detected concentration of the element, suggesting spots of organic deposition on the plate. The significance of these findings is that discrete friction spots exist between the volar locking plate and deep tendons. The friction causes damage to the tendon, as evidenced by the shedding of organic material on the plate.[ 4 ] Conversely, the tendon–plate friction spot had no corrosive effect on the plate. From our analysis, we cannot determine the type or extent of the friction and irritation on the tendons but can determine the location of the points of contact between the tendon and the plate. The microscopically thick deposition of organic-origin elements suggests that this tendon–plate friction has been extensive. We believe that this novel analysis of a volar locking plate placed distal to the watershed line is a microscopic verification of the Soong theory regarding pain with distal plate position. We found friction spots of the plate composed of organic deposits in the anatomical location of the deep flexors of the thumb and index. These friction spots might be the pain generator in motion of the index finger and thumb that often leads to symptomatic hardware and plate removal. Confirmation of Authorship We, the undersigned, confirm that we are the joint authors of the above paper. We confirm that all the authors have had material input into the submission. We confirm that, to our knowledge, all the claims, statements, and conclusions are true and are our jointly held opinions. Ethical Approval This study is exempted from the laws of the local institutional review board. Patient Consent Informed consent was obtained from the patient. Authors' Contributions A.K.: Editing and finalization of the report, implant analysis interpretation. I.M.M.S.: Writing the first draft. S.N.: Implant analysis and interpretation. Y.S.H.: Implant analysis interpretation, editing, funding acquisition. Publication History Received: 13 December 2024 Accepted: 07 January 2025 Article published online: 21 February 2025 © 2025. Thieme. All rights reserved. Thieme Medical Publishers, Inc. 333 Seventh Avenue, 18th Floor, New York, NY 10001, USA

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