Injury to the infrapatellar branch of the saphenous nerve (IBSN) can be triggered by many factors, including arthroscopic portals, incision for drilling of the tibial tunnel or dissection, and graft harvest. Most studies report that IBSN injury IBSN leads to sensory comorbidities in most cases. Problems can include loss of sensation, paresthesia, neuralgia, or hypersensitivity in the medial infrapatellar area of the lower extremity (1). Persistent pain after knee arthroscopy is rarely reported. We recently admitted a patient with pain on the anteromedial infrapatellar side of the right knee due to knee arthroscopy.

The saphenous nerve is the longest cutaneous branch of the femoral nerve. It divides into the IBSN, which serves the proximal tibia inferior medial to the patella (2, 3). The infrapatellar branch of the saphenous nerve splits off from the saphenous nerve in a highly variable way. It penetrates the sartorius muscle, after which it runs on a superficial course and generally forms two branches. Both branches cross the patellar tendon in the transverse direction to form the infrapatellar plexus. The infrapatellar branch of the saphenous nerve innervates the anteromedial aspect of the knee, the anterolateral aspect of the proximal part of the lower leg, and the anteroinferior part of the knee joint capsule (4–6). There are many reports of IBSN injury caused by surgery, especially knee arthroplasty (7), tendon extraction incision for anterior cruciate ligament reconstruction (8), and incisions for anteromedial knee surgery (9). Among these reports, the proportion of IBSN injuries caused by total knee replacement is 50–100% (10). Because the IBSN is highly variable, nerve damage in this area is considered inevitable (11). The most common symptoms associated with IBSN injury are hypoesthesia, numbness, or allergy in the anteromedial knee joint (12). Usually, these symptoms trouble the patient only for a short time, but in this case, the symptoms continued over a long period of time. We believe that the patient's IBSN, which was not completely disconnected, was the reason for the continued presence of sensory conduction; therefore, the patient's symptoms were primarily pain rather than numbness. The presence of scarring around the nerve, trapping the nerve, was what caused the symptoms to persist.

Prior to surgery, we had suspected subpatellar fat pad inflammation to be the cause of the patient's pain, but it was not until we observed positive Tinel's sign that the diagnosis was biased toward nerve damage or neuroma formation. However, this patient only underwent arthroscopic meniscus formation, and no large surgical incision was found; this made our diagnosis questionable, because the incision made by arthroscopic portal is small, and IBSN injury due to arthroscopic portal is rarely reported (13). Therefore, we used ultrasound to examine the affected area before the surgery (14). Unfortunately, ultrasound did not detect the formation of neuroma. Eventually, we had to make an enlarged incision along the original incision, and finally discovered the trapped nerve.

In general, symptoms of nerve entrapment do not progress rapidly, and there are few reports of neuroma formation leading to persistent pain 1 week after surgery. However, this patient felt pain 1 day after surgery, with a VAS score of up to 5. We do not believe that direct injury to the nerve was caused by the arthroscopic portal, but rather that the cause was unintended irritation of the nerve by the suture used to close the portal. Over the course of a few weeks, the sutures were absorbed and caused scarring around the nerve, which further perpetuated the symptoms. Therefore, we believe that the arthroscopic approach does not need to pursue aesthetics too strongly, and the epidermis can be directly sutured with less irritating sutures, without the need to apply too much suturing to subcutaneous tissue.

Krijgh et al. dissected 18 cadavers from adult donors and found a total of 23 infrapatellar branches. Based on this, they argued that the origin of the infrapatellar branch from the saphenous nerve is highly variable, as well as the position at which the infrapatellar branch passes the knee joint (15). This makes IBSN injury difficult to avoid. Gousopoulos et al. dissected the space between the subcutaneous and the capsule; this dissection is performed by grasping and knotting the sutures through the anteromedial portal to avoid iatrogenic saphenous nerve injury (16). Although there are many ways to avoid nerve injury, diagnosis and early treatment after injury are more important.

There are many ways to diagnose IBSN. Riegler et al. have reported that ultrasound successfully pinpoints the variable course of the IBSN from the origin to the most distal point and may therefore enable the correct identification of (iatrogenic) nerve damage in any location (17). We also initially used ultrasound to examine the patient's nerves after suspecting IBSN injury. Unfortunately, no positive results were observed in our examination.

After knee arthroplasty and other procedures that can lead to IBSN injury, denervation is usually the choice of most surgeons if IBSN injury occurs (18). The current conservative treatment for IBSN neuritis is traditionally performed with nerve blocks containing a steroid and a local anesthetic (19). Reports of cryoneurablation have also been published (20). Existing nonsurgical treatments can provide symptomatic relief but with variable long-term outcomes.

This case report still has several shortcomings. First, our follow-up time for this patient was short, and the long-term outcome remains unclear. Second, we have reported on only one case, so we are unable to present more useful information, such as Guidance on how to avoid IBSN injury and the incidence of IBSN injury.

In conclusion, we should always be vigilant in relation to the possibility of IBSN injury during surgery, since there is a risk of saphenous nerve injury even if only a 1-cm long incision is made during arthroscopic surgery. The symptoms after an IBSN injury can easily confuse clinicians and make treatment difficult.

The authors have read the CARE Checklist (2016) and the manuscript was prepared and revised according to the CARE Checklist (2016).

The datasets presented in this article are not readily available because of ethical and privacy restrictions. Requests to access the datasets should be directed to the corresponding author.

The studies involving human participants were reviewed and approved by Wuxi 9th People's Hospital. The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

JT and XW reviewed the literature and contributed to drafting of the manuscript. FX and QY followed up the patients. JQ was the patient's first doctor. JM was responsible for revision of the manuscript for important intellectual content. All authors issued final approval for the version to be submitted.