The branch point and course of the motor branch of the nerve to vastus medialis
ELMANSOURY, AMR MOHAMED ALY
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STUDY OBJECTIVE: To investigate the exit point of the motor branch of the nerve to vastus medialis (mNVM) from the posterior division of the femoral nerve. An enhanced understanding of the origin and course of the mNVM will minimize the risk of anesthetizing it when performing a local anesthetic adductor canal nerve block for postoperative analgesia following total knee arthroplasty. METHODS: Anatomical donors, obtained through generous donation to the Anatomical Gift Program at Boston University School of Medicine (BUSM), were used in this study. Dissection of the mNVM was performed in 22 body donors (44 lower limbs). Before the dissection, the thigh length, from the anterior superior iliac spine (ASIS) to the base of the patella was measured. Additionally, following the dissection, the distance from the exit point of the mNVM to the base of the patella was measured. Then, the ratio between the two distances was calculated to provide a straightforward and efficient estimate of the mNVM branch point. SUMMARY OF RESULTS: The mean ± SD thigh length in the 44 lower limbs was 43 cm ± 2.9cm. The mNVM exited the posterior division of the femoral nerve 19.2 cm ± 1.90 cm superior to the base of the patella. The exit point of the mNVM from the posterior division of the femoral nerve was 0.56 ± 0.04 of the distance from the ASIS to the patella base. Pearson correlation was performed in order to examine the relationship between the distance of mNVM from the base of the patella to the total length of the thigh in the 44 limbs. This relationship was significant with P = 0.003. The One-Way Anova revealed no significant difference between male and female body donors in the ratio of the exit point of the mNVM relative to the length of the thigh. CONCLUSION: The mNVM emerges from the posterior division of the femoral nerve at 0.56 ± 0.04 of the thigh lengths as measured from the ASIS to the base of the patella. This finding provides clinicians with a practical and useful guide for determining the injection point for an adductor canal nerve block, a procedure utilized for postoperative analgesia following total knee arthroplasty. Protecting the mNVM during the block will avoid weakening the vastus medialis muscle and may result in greater patient participation in postoperative physical therapy, safer gait and may reduce the risk of falls.