Wei-Ting Wu; Ke-Vin Chang; Levent Özçakar

doi:10.12701/jyms.2025.42.74

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Dynamic ultrasound examination for postoperative intermittent irritation of the infrapatellar branch of the saphenous nerve: Wei-Ting Wu^1,2, Ke-Vin Chang^1,2,3, Levent Özçakar⁴; Journal of Yeungnam Medical Science 2025;42:74.
DOI: https://doi.org/10.12701/jyms.2025.42.74
Published online: November 15, 2025

¹Department of Physical Medicine and Rehabilitation, National Taiwan University Hospital, Bei-Hu Branch, Taipei, Taiwan

²Department of Physical Medicine and Rehabilitation, National Taiwan University College of Medicine, Taipei, Taiwan

³Center for Regional Anesthesia and Pain Medicine, Wang-Fang Hospital, Taipei Medical University, Taipei, Taiwan

⁴Department of Physical and Rehabilitation Medicine, Hacettepe University Medical School, Ankara, Türkiye

Corresponding author: Ke-Vin Chang, MD, PhD Department of Physical Medicine and Rehabilitation, National Taiwan University Hospital, Bei-Hu Branch, No. 87, Nei-Jiang Rd., Wan-Hwa District, Taipei 108, Taiwan Tel: +886-2-2371-7101 (ext.) 5309 • E-mail: kvchang011@gmail.com

• Received: October 23, 2025 • Revised: November 1, 2025 • Accepted: November 10, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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A 72-year-old man presented with persistent pain in the right knee localized to the infrapatellar region. Three months earlier, he had sustained a fall, causing a fracture of the inferior pole of the patella, which was treated with open reduction and internal fixation using two screws. Although postoperative pain gradually improved, he continued to experience intermittent electric shock-like pain while climbing stairs. Follow-up radiographs confirmed good fracture union (Fig. 1A), and the patient was referred for ultrasound evaluation to determine the pain source.

Static ultrasonography showed that the fracture fragment was well united, and the infrapatellar tendon was thickened and hyperechoic without hypervascularity (Fig. 1B). Sonopalpation indicated that the tendon was not the source of the pain. As the patient reported pain during knee flexion, further ultrasound examination revealed a small mobile bony fragment adjacent to the inferior patella (Fig. 1C), which was not visible on plain radiography. Dynamic imaging revealed that this fragment compressed the infrapatellar branch of the saphenous nerve (IPBSN) during flexion but not during extension (Fig. 1D, Supplementary Video 1). Owing to intermittent nerve irritation, the patient was referred for surgical removal of the fragment. During postoperative telephone follow-up, the patient reported complete pain relief.

This case illustrates a rare cause of postoperative knee pain, irritation of the IPBSN by a mobile bony fragment (Fig. 1E). The saphenous nerve, the terminal branch of the femoral nerve, bisects at the knee into the infrapatellar and sartorial branches [1]. The former travels through subcutaneous tissue superficial to the patellar tendon, making it vulnerable to injury or compression. Possible involvement of the IPBSN should be considered in patients with anterior knee pain localized near the tendon after knee surgery [2,3]. Notably, when nerve compression is intermittent, static ultrasound may fail to reveal abnormalities, emphasizing the importance of dynamic evaluation. In our case, owing to the relative movements of the nerve and patellar tendon, the IPBSN was separated from the fragment in extension but in contact with it during flexion. Without dynamic assessment, the diagnosis could have been easily overlooked. As such, in addition to its high spatial resolution, which permits direct visualization of small peripheral nerves [4], real-time assessment of soft tissue (e.g., nerve–tendon) interactions collectively makes ultrasound an invaluable tool for the interventional management of postoperative knee pain.

Supplementary materials

Supplementary Video 1 can be found at https://doi.org/10.12701/jyms.2025.42.74.

Supplementary Video 1.

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Ethics statement

Written informed consent was obtained from the patient for publication of this report.

Conflicts of interest

Ke-Vin Chang and Wei-Ting Wu have been editorial board members of Journal of Yeungnam Medical Science since 2021. They were not involved in the review process of this manuscript. There are no other conflicts of interest to declare.

Funding

This study was funded by the National Taiwan University Hospital, Bei-Hu Branch; the Ministry of Science and Technology, Taiwan (MOST 106-2314-B-002-180-MY3 and MOST 109-2314-B-002-114-MY3); and National Science and Technology, Taiwan (NSTC 112-2314-B-002-134, NSTC 113-2314-B-002-208-MY2, NSTC 113-2314-B-002-209-MY2 and NSTC 114-2923-B-002 -002 -MY3).

Author contributions

Conceptualization, Funding acquisition: KVC, WTW; Investigation: KVC; Validation: WTW, LÖ; Writing-original draft: WTW; Writing-review & editing: KVC, LÖ.

Fig. 1.

(A) Sagittal knee radiograph showing the patellar fixation screws. (B) Ultrasound imaging of the inferior pole of the patella and the proximal portion of the patellar tendon demonstrates near-complete approximation of the fracture ends (black arrow). Dynamic ultrasound imaging of the infrapatellar branch of the saphenous nerve (white arrowheads) during (C) knee extension and (D) flexion reveals irritation of the nerve caused by an unfixed bony fragment adjacent to the patella (black arrowheads). (E) Schematic illustration depicts the anatomical relationship between the infrapatellar branch of the saphenous nerve and the unfixed bony fragment. The dashed squares indicate the transducer’s position. P, patella; F, femur; PT, patellar tendon; T, tibia.

References

1. Chang KV, Mezian K, Naňka O, Wu WT, Lou YM, Wang JC, et al. Ultrasound imaging for the cutaneous nerves of the extremities and relevant entrapment syndromes: from anatomy to clinical implications. J Clin Med 2018;7:457.Article PubMed
2. Hung CY, Chang KV. Letter to the editor: Infrapatellar saphenous neuralgia after TKA can be improved with ultrasound-guided local treatments. Clin Orthop Relat Res 2015;473:1843–4.Article PubMed PMC
3. Saif DS, Eltabl MA. Frequency of infrapatellar neuropathy post-total knee replacement and arthroscopic surgery in Egyptian patients. Egypt Rheumatol Rehabil 2020;47:19.Article
4. Hung CY, Hsiao MY, Özçakar L, Chang KV, Wu CH, Wang TG, et al. Sonographic tracking of the lower limb peripheral nerves: a pictorial essay and video demonstration. Am J Phys Med Rehabil 2016;95:698–708.Article PubMed

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Figure

Dynamic ultrasound examination for postoperative intermittent irritation of the infrapatellar branch of the saphenous nerve

Fig. 1. (A) Sagittal knee radiograph showing the patellar fixation screws. (B) Ultrasound imaging of the inferior pole of the patella and the proximal portion of the patellar tendon demonstrates near-complete approximation of the fracture ends (black arrow). Dynamic ultrasound imaging of the infrapatellar branch of the saphenous nerve (white arrowheads) during (C) knee extension and (D) flexion reveals irritation of the nerve caused by an unfixed bony fragment adjacent to the patella (black arrowheads). (E) Schematic illustration depicts the anatomical relationship between the infrapatellar branch of the saphenous nerve and the unfixed bony fragment. The dashed squares indicate the transducer’s position. P, patella; F, femur; PT, patellar tendon; T, tibia.

Fig. 1.

Dynamic ultrasound examination for postoperative intermittent irritation of the infrapatellar branch of the saphenous nerve