# Painful Intra-articular Collagen Fibroma of the Shoulder: A Case Report

## TL;DR

A rare case of collagen fibroma in the shoulder joint caused severe pain and was misdiagnosed for years before successful surgical removal.

## Contribution

Highlights the rare intra-articular occurrence and painful presentation of collagen fibroma, emphasizing diagnostic challenges.

## Key findings

- Collagen fibroma can present as a painful intra-articular tumor in the shoulder.
- The tumor was misdiagnosed for six years due to imaging similarities with surrounding tissues.
- Surgical excision resolved pain and showed no recurrence after five years.

## Abstract

Collagen fibroma is a rare benign fibroblastic tumor. It typically arises in the subcutaneous tissue or fascia and is usually asymptomatic; intra-articular occurrence and painful presentation are extremely rare. We report a case of collagen fibroma arising within the shoulder joint and associated with severe pain. A 62-year-old man presented with resting and motion-related pain in the left shoulder. Magnetic resonance imaging performed at another institution demonstrated an intra-articular lesion; however, it was overlooked due to its signal intensity similarity to that of the surrounding subscapularis muscle and joint capsule. When the patient was referred to our institution six years later, the shoulder pain had markedly worsened, with a visual analog scale score of 100 mm. The tumor enlarged between the scapula and the subscapularis muscle, markedly compressing the anterior joint capsule. We performed surgical excision, and histopathological examination confirmed the diagnosis of collagen fibroma. Both resting and motion-related pain resolved rapidly after surgery, and the patient remained recurrence-free during a five-year follow-up period. This case demonstrates that collagen fibroma can arise intra-articularly and cause severe pain, which may lead to diagnostic difficulty. For patients with unexplained shoulder pain, it is essential to perform careful imaging evaluation with consideration of intra-articular tumors.