Incidence and Characterization of Injury to the Infrapatellar Branch of the Saphenous Nerve after ACL Reconstruction: A Prospective Study

Abstract

Objectives:The infrapatellar branch of the saphenous nerve is commonly injured in anterior cruciate ligament reconstruction (ACLR) causing sensory deficits around the knee. The primary purpose of this prospective study was to determine the incidence of patient reported sensory deficits around the knee following ACLR. The secondary purpose was to determine if sensory deficits caused by intraoperative injury present at 6 weeks changed in severity and total area after 6 months and 1 year postoperatively.Methods:Two-hundred and fifty patients that underwent ACLR with or without meniscal repair were prospectively enrolled. Variables for each patient included: type of graft, direction of tibial incision, number of portals, and length of surgical incision. The grafts used were categorized into three types: Allograft (allo), hamstring autograft (HS), or patella tendon autograft (BTB). At 6 weeks, patients completed a questionnaire to ascertain any sensory deficits over their knee. Patients rated their sensory deficit on a scale from 0-10 (“0” = (no deficit) to “10” (complete lack of sensation) and shaded areas on a picture of a knee split into nine rectangular segments (3 by 3 grid) to determine the location of any numbness. Patients completed the same questionnaire at 6 months and 1 year. Any patient that was noted to have no stated numbness at 6 weeks or 6 months was noted to have completed the study. A mixed effects linear regression model was used to identify variables which were predictors for the patient-reported severity of numbness.Results:Overall, 67/221 (30.3%) patients who underwent ACLR stated that they had no numbness at 6 weeks. Of those patients who reported numbness at 6 weeks, 16.6% (25/151) considered their numbness completely resolved by six months. At 1 year, 73.2% (90/123) reported their numbness had gotten better and 14.2% (18/123) considered their numbness resolved. The most common location of numbness was along the inferolateral aspect of the knee. The mean numbness rating for allografts was 2.73 +/- 0.32 (mean +/- standard error) at 6 weeks, decreasing to 1.04 +/- 0.26 at 6 months and 0.64 +/- 0.26 at 1 year for oblique and vertical incisions combined. A statistical model, controlling for time and incision direction, indicated that HS patients were 1.94 +/- 0.52 points higher than allograft patients across all time points, and BTB patients were 1.57 +/- 0.51 points higher than allo. However, there were no significant difference in mean numbness score between BTB and HS patients (p=0.521). Time had a negative impact on the patient reported severity of numbness score for all graft types. At 6 months this effect was -0.95 +/- 0.17 and at 1 year, -1.21 +/- 0.18. The use of BTB increased the mean numbness of affected segments by 0.67 +/- 0.23, while the use of a HS increased the mean numbness of segments by 0.39 +/- 0.21. The mean number of segments decreased slightly with time, down by 0.20 +/- 0.08 at 6 months (p=0.008) and 0.28+- 0.08 at 1 year (p=<0.001).Conclusion:Sensory deficits after ACLR follow the direction of the infrapatellar branch of the saphenous nerve. Patients who underwent ACLR with allo were less likely to develop sensory deficits compared to BTB or HS. Sensory deficits in allo patients were on average, less severe. Surprisingly, there was no significant difference in numbness between HS and BTB grafts. Surgeons should counsel their patients that sensory deficits are common postoperatively after ACLR, but that this sensory disturbance is likely to dissipate with time.