The Impact of Leg Length on the Path of the Infrapatellar Branch of the Saphenous Nerve
Authors: Johnson Kenneth , Rowe Joanna , Gordon Victoria , Lewis Adam , Hans Kanwalgeet , Marolt Clayton , McCumber Travis , Willett Gilbert M. , Janssen Sarah Keim 
Published: April 2020

Objective

Iatrogenic injury to the infrapatellar branch of the saphenous nerve (IPBSN) is a common sequela of anteromedial incisions during knee procedures. To more accurately predict a “safe‐zone” for incisions, we investigated the correlation between the path of the nerve and the length of the leg. The origin of the IPBSN relative to the sartorius muscle (SM) was identified to compare to previous studies.

Methods

103 knees from 53 formalin preserved cadavers were dissected at Kansas City University of Medicine and Biosciences, Creighton University, and the University of Nebraska Medical Center. The IPBSN was located as it approached the medial aspect of the patellar ligament. The location was documented by measuring from the intersection of the patella with the medial (Point A) and lateral (Point B) borders of the patellar ligament at angles from 0°, 30°, 45°, 60° and 90° from horizontal. Leg length was determined by the distance from the tibial plateau at the intersection of the medial collateral ligament to the distal aspect of the medial malleolus. The nerve was exposed proximally to identify its origin relative to the SM.

Results

From Point A, at angles from 0°, 30°, 45°, 60° and 90° from horizontal, we calculated the mean minimum and mean maximum distances to the nerve. These measurements allowed the investigators to identify a “danger zone” for expected injury to the IPBSN.

No correlation existed between leg length and the mean minimum distance to the nerve. However, length had a significant, but low degree of correlation to the mean maximum distance to the IPBSN that increased as the nerve approached its most distal reaches. At angles of 45°, 60°, and 90° from Point A as well as 90° from Point B, the distance to the IPBSN increased as leg length increased.

We also identified variations not previously reported. In some individuals, we identified multiple nerves serving the infrapatellar area. Many had innervation originating directly from the femoral nerve in the femoral triangle. Further, the emergence of the IPBSN relative to SM differed from previous studies, with the majority of individuals having at least one nerve penetrating through rather than emerging posterior to the SM.

Conclusion

We defined safe areas for incisions as −1 SD from the mean minimum distance and +1 SD from the mean maximum distance from point A. Given there was no correlation between leg length and the minimum mean distance from point A, the safe‐zone superior to the nerve is unaffected by the length of the leg. While there was a correlation between leg length and the maximum mean distance from point A at 45°, 60°, and 90°, those are outside the area of possible incisions. Therefore, the safe‐zone inferior to the nerve is also unaffected. Existing literature has not identified infrapatellar branches arising directly from the femoral nerve. Individuals with this variation would not respond well to saphenous nerve blocks, requiring a more proximal, femoral nerve block that would temporarily compromise motor function of the anterior thigh.