Diagnostics of infrapatellar saphenous neuralgia-a reversible cause of chronic anteromedial pain following knee surgery

Abstract

Objectives: To evaluate the impact of diagnostic nerve block and ultrasound findings on therapeutic choices and predict the outcome after concomitant surgery in patients with suspected neuropathy of the infrapatellar branch of the saphenous nerve (IPBSN).

Methods: Fifty-five patients following knee surgery with suspicion of IPBSN neuralgia were retrospectively included. Ultrasound reports were assessed for neuroma and postsurgical scarring (yes/no). Responders and non-responders were assigned following anesthetic injection of the IPBSN. The type of procedure (neurectomy/interventional pain procedure/other than nerve-associated therapy) and pain score at initial follow-up were recorded and patients were assigned as positive (full pain relief) or negative (partial/no pain relief) to therapeutic nerve treatment. Factors associated with a relevant visual analog scale (VAS) reduction were assessed using uni- and multivariate logistic regression models and chi-square for quantitative and qualitative variables (p ≤ 0.05).

Results: Responders (37/55) more often had an entrapment or an evident neuroma of the IPBSN (97% vs. 6%). A positive Hoffmann-Tinel sign (p = 0.002) and the absence of knee joint instability (p = 0.029) predicted a positive response of the diagnostic nerve block (90%; 26/29). In the follow-up after therapeutic nerve treatment, all patients with full pain relief showed neuromas or entrapment of the IPBSN. Patients negatively responding to therapeutic nerve treatment more frequently showed an additional knee joint instability (25% vs. 4%).

Conclusion: Selective denervation for neuropathic knee pain is beneficial in selected patients with significant VAS reduction after diagnostic nerve block. Non-responders following diagnostic nerve block but sonographic evidence of IPBSN pathologies need to be evaluated for other causes such as knee joint instability.

Key points: • Sonographic diagnosis of neuroma or entrapment of the IPBSN is frequently seen in patients with anteromedial knee pain and leads to a good response to diagnostic nerve block following knee surgery. • The vast majority of patients with clinical signs of IPBSN neuropathy and response to a diagnostic nerve block sustained full pain relief following therapeutic nerve treatment. • Patients not responding to therapeutic IPBSN treatment have to be evaluated for other causes of anteromedial knee pain such as knee joint instability.

Keywords: Knee surgery; Nerve block; Saphenous nerve; Total knee arthroplasty; Ultrasound.

Fig. 1

a Illustration of the infrapatellar branch of the saphenous nerve (IPBSN) bifurcating into two terminal branches, the superior and inferior IPBSN, innervating the anteromedial skin of the knee, the anterolateral aspect of the proximal lower limb, and the anteroinferior joint capsule. The IPBSN is a pure sensory nerve arising from the saphenous nerve in the subsartorial canal (arrowhead) piercing the fascia anterior or through the sartorius muscle (asterisk). The sartorius muscle (asterisk) is reflected to better demonstrate both the saphenous nerve and the IPBSN. b Transverse ultrasound image at the adductor canal shows the IPBSN (arrow) and the saphenous nerve (arrowhead) in between the sartorius and vastus medialis muscles. c Longitudinal ultrasound image at the level of the medial femoral condyle shows the IPBSN (arrow) adjacent to the medial collateral ligament (asterisk) in the deep subcutaneous tissue

Fig. 2

a Neuroma-in-continuity of the infrapatellar branch of the saphenous nerve (IPBSN). Longitudinal sonogram shows diffuse hypoechoic enlargement of the inferior branch of the IPBSN (arrowheads) with loss of fascicular architecture. b Intraoperative photography of the corresponding neuroma-in-continuity of the inferior branch of the IPBSN (arrow). The IPBSN (wavy arrow) and the superior branch of the IPBSN (arrowhead) are unremarkable. c, d Entrapment of the inferior branch of the IPBSN (arrows) due to scar tissue (arrowheads) at the level of the tibial tuberosity on transverse (c) and longitudinal (d) transducer positions

Fig. 3

Ultrasound-guided perineural injection of the infrapatellar branch of the saphenous nerve (IPBSN; arrows) using a 25-gauge 60-mm cannula (arrowheads) before (a) and after (b) injection of 1 mL lidocaine 1% distributing around the IPBSN (asterisk)

Fig. 4

Flowchart demonstrates therapeutic procedures of the infrapatellar branch of the saphenous nerve (IPBSN) and their clinical response following nerve treatment in responding and non-responding patients following diagnostic nerve block. Data are expressed as raw numbers and percentages. Other therapeutics included revision total knee arthroplasty (n = 3), arthrolysis (n = 2), and anterior cruciate ligament reconstruction (n = 1). Mean time interval between IPP and neurectomy in four patients was 2.8 months. IPP interventional pain procedure

Fig. 5

Flowchart demonstrates therapeutic procedures of the infrapatellar branch of the saphenous nerve (IPBSN), additional knee joint instability, and clinical response following nerve treatment in responding and non-responding patients following diagnostic nerve block. Data are expressed as raw numbers and percentages. IPP interventional pain procedure