Abstract
Purpose: Lateral hypoaesthesia of the knee following peroperative section of an infrapatellar sensorial branch during anteromedial access for knee surgery is a well-known phenomenon. Development of a painful neurinoma at this level is much more exceptional but can be the cause of persistent pain, often becoming violent in the anteromedial region of the proximal tibia which may often evoke the diagnosis of mechanical failure of the prosthesis due to superficial conflict or loosening. There is relatively little in the French orthopaedic literature on this topic. We wanted to draw attention to this often missed diagnosis.
Material and methods: We report the cases of three patients with degenerative knee disease who underwent total knee arthroplasty using a posterior stablised prosthesis implanted via an anteromedial approach. Persistent pain which developed over more than six months led these patients to consult. Several surgeons evoked a “classical” cause of postoperative pain resulting from postoperative loosening but the x-ray and scintigraphy findings were inconclusive. Physical examination demonstrated dysaesthesia in the lateral infra-pateller region with a positive Tinel sign on the anteromedial aspect of the knee immediately below the tibial implant suggestive of a neurinoma of the infrapatellar branch of the saphenous nerve. An anatomic study conducted on ten knees enabled us to identify the anatomic bases necessary to achieve local anaesthesic blocks providing the pretherapeutic diagnosis. The diagnostic tests were performed with injection of 5 ml xylocaine in the subcutaneous tissue over the medial aspect of the knee upstream from the suspected neurinoma. Complete resolution of pain evaluated 10 minutes after injection led to proposing neurotomy of the infra-patellar nerve via a separate medial incision.
Results: Denervation provided immediate relief in three patients. For two, the pain and dysaesthesia regressed completely. Substantial improvement in hyperaesthesia was noted in one patient.
Discussion: The anatomic studies demonstrated that one or more branches of the infra-patellar nerve cross the mid line from the apex of the patella to the anterior tibial tuberosity in 98% of the cases. In ongoing work during revision procedures for total knee arthroplasty, we have noted hypoaesthesia or anaesthesia in the infra-patellar nerve territory in 15% of the patients. Dell reported a series of 70 patients with postoperative neurinomas of the knee and obtained 86% good results after denervation.
Conclusion: Hyperalgic iatrogenic neurinoma of an infra-patellar branch of the saphenous nerve is a certain but rare cause of pain after knee surgery, generally for prosthesis. Misdiagnosis of a mechanical complication is not uncommon. The clinically suggested diagnosis is easy to confirm by a subcutaneous local anaesthesia test. Neurotomy of the infra-patellar nerve can be proposed after failure of drug and physical treatments.
The abstracts were prepared by Docteur Jean Barthas. Correspondence should be addressed to him at Secrétariat de la Société S.O.F.C.O.T., 56 rue Boissonade, 75014 Paris.
