The Effect of Femoral Nerve Block on Strength and Patient-reported Outcomes Following ACL Reconstruction

Objectives: Femoral nerve block (FNB) has been proposed for pain control following anterior cruciate ligament (ACL) reconstruction. Although numerous high level studies have assessed the efficacy of FNB’s, there has been little to no research into the effect of such blocks on post-operative strength and patient-reported outcomes. Exacerbation of post-operative quadriceps weakness by a FNB could negatively impact recovery, particularly in the early post-operative period. We hypothesized that performance of a FNB would result in decreased quadriceps strength and poorer patient-reported outcome score in the first three months following ACL reconstruction. Methods: Thirty patients scheduled to undergo hamstring autograft ACL reconstruction following an acute ACL injury were randomized to a single shot FNB group or a control group. The surgical procedure and post-operative rehabilitation were performed identically in both groups. Pre-operatively, all patients completed a KOOS and isokinetic quadriceps strength testing at 60 deg/s. At 6 weeks post-operative, all patients completed a KOOS and isometric quadriceps strength testing at 90 deg. At 12 weeks post-operative, all patients but three completed a KOOS and isokinetic strength testing. Quadriceps strength limb symmetry indices (LSI) were calculated at all time points by dividing the strength of the involved limb by the strength of the uninvolved limb. LSI and KOOS subscales for Activities of Daily Living (ADL), Pain, and Symptoms at 6 weeks and 12 weeks were compared with pre-operative values. Results: Patients who underwent nerve block demonstrated a significant decrease in their LSI from pre-operative (0.80 ± 0.14) to 6 weeks post-operative (0.67 ± 0.29) (p = 0.05). Patients who did not undergo nerve block demonstrated no significant difference in the pre-operative (0.77 ± 0.23) and 6-week post-operative (0.78±0.28) LSI (p = 0.99). At 12 weeks post-operative, both groups demonstrated LSI values that were not statistically different from pre-operative. Patients who had a FNB demonstrated no change in their KOOS subscales from pre-op to 6 weeks post-op, while those patients who did not undergo FNB demonstrated a statistically significant increase in KOOS ADL, pain, and symptoms subscales from pre-operative to 6 weeks post-operative (Figure 1). All patients, regardless of group, demonstrated significant increases in KOOS subscales at 12 weeks relative to pre-operative (Figure 1). Conclusion: Femoral nerve block is associated with decreased quadriceps strength at 6 weeks post-operative, slowing the post-operative improvement in patient-reported outcome scores. While strength returns to pre-operative levels at 12 weeks post-operative, the effect of early quadriceps weakness on movement patterns and functional outcome at later time points has yet to be determined.