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Category: Ankle; Hindfoot Introduction/Purpose: Flexor Hallucis Longus (FHL) tendon transfer to the calcaneus is a common adjuvant procedure in the treatment of Achilles pathology. The FHL tendon can be harvested using a posterior incision where the tendon is dissected along its course into the fibroosseous tunnel. Alternatively, the FHL may be harvested through a separate plantar medial incision as it crosses the flexor digitorium longus at the Knot of Henry. This study aims to quantify FHL tendon lengths achieved through the two common approaches utilizing pair matched cadavers. Methods: Seven pair matched fresh-frozen cadaver legs without signs of musculoskeletal abnormalities were used for this assessment. One leg in each pairing underwent a single incision harvest while the contralateral leg underwent an accessory medial plantar harvest. After dissecting the tendon, a calcaneus tunnel was prepared from dorsal to plantar in both calcanei in standard fashion. Two measurements were obtained. The first measurement was taken from the distal aspect of the muscle belly to the distal end of the tendon. The tendon was then pulled through the calcaneus, and the foot was held in tension at 20 degrees of plantar flexion. The second measurement was taken from site where tendon entered the calcaneous to the distal end of the tendon graft. The measurements were analyzed using Wilcoxon Signed Ranks Test and Fischer Exact Test. Results: Using a posterior incision, the mean tendon measurement from calcaneous tunnel to the distal end of the tendon was 4.0 cm. Using an accessory plantar medial incision, the mean tendon measurement from the calcaneous tunnel to the distal end of the tendon was 7.2 cm. The average tunnel length obtained using an accessory medial incision was significantly greater than the length obtained using the single incision approach (p= 0.0003, p=0.0022, and p=0.0016). The accessory plantar medial incision obtained an FHL tendon tunnel length that was an average of 2.9 cm greater than the posterior incision. Conclusion: The single incision approach provided sufficient length to safely anchor the FHL into the calcaneus which suggests that the accessory plantar medial approach is not necessary for routine FHL transfers to the calcaneus with interference screw fixation. However, if additional length is needed for other applications such as posterior tibialis tendon dysfunction or peritoneal tendon tears, the accessory incision does provide an average of 2.9 cm of additional length.