Background:
The molecular and histologic characteristics associated with the development of post-traumatic capsular fibrosis of the shoulder joint following open reduction and internal fixation (ORIF) of proximal humerus fractures (PHF) remain incompletely understood. To help elucidate these mechanisms, this hypothesis-generating pilot study aimed to examine gene expression and histological changes in the joint capsule of patients undergoing implant removal and adhesiolysis for post-traumatic stiffness after surgical fixation of PHF.

Methods:
For this pilot study, tissue samples were collected from the joint capsule of patients with a clinical diagnosis of post-traumatic shoulder stiffness during implant removal and adhesiolysis surgery (n = 6; median 349 days post-fracture). For comparison, equivalent samples were obtained from patients undergoing fixation of acute proximal humeral fractures (n = 5; median 9 days post-fracture). Gene expression analysis of 24 selected genes was performed using RNA extraction, reverse transcription, and quantitative PCR. In addition, histological evaluation was conducted using H&E staining and type V collagen immunohistochemistry.

Results:
Exploratory RNA analysis revealed significant upregulation of COL14A1, THBS4, EPHA4, and SIX1 in the post-traumatic stiffness group. Histological assessments demonstrated features of capsular fibrosis, including increased extracellular matrix deposition and disorganized collagen architecture.

Conclusions:
RNA analysis on capsular tissue harvested during implant removal in patients with PTSS could be reliably performed. Histology found mature fibrosis while molecular analysis could identify candidate genes as potential target for further research.

Trial Registration:
Trial was retrospectively registered at the German Clinical Trials Register (DRKS) on Feb 2nd, 2026 (Registration-ID DRKS00039180.)