Background: The midline skin incision for total knee arthroplasty may be an important generator of chronic neuropathic pain. The incision is innervated by the medial femoral cutaneous nerve (MFCN), the intermediate femoral cutaneous nerves (IFCN) and the infrapatellar branch from the saphenous nerve. The MFCN divides into an anterior (MFCN-A) and a posterior branch (MFCN-P). The primary aim was to compare the areas anesthesized by MFCN-A versus MFCN-P block for coverage of the incision.
Methods: Nineteen healthy volunteers had IFCN and saphenous nerve blocks. The subgroup of volunteers with a non-anesthetized gap between the areas anesthetized by the saphenous and the IFCN blocks was defined as the study group for the primary outcome. Subsequently selective MFCN-A block and MFCN block (MFCN-A + MFCN-P) were performed to investigate the contributions from MFCN-A and MFCN-P to the innervation of the midline incision. All assessments were performed blinded.
Results: Ten out of 19 volunteers had a non-anesthetized gap. Nine out of these 10 volunteers had coverage of the non-anesthetized gap after selective anesthesia of the MFCN-A, whereas anesthesia of the MFCN-P did not contribute to coverage of the gap in any of the 10 volunteers.
Conclusions: In half of the cases, a gap of non-anesthetized skin was present on the surgical midline incision after anesthesia of the saphenous nerve and the IFCN. This gap was covered by selective anesthesia of the MFCN-A without contribution from MFCN-P. The selective MFCN-A block may be relevant for diagnosis and interventional management of neuropathic pain due to injury of MFCN-A.