Aman et al. BMC Surg (2021) 21:310 https://doi.org/10.1186/s12893-021-01308-x RESEARCH Single incision thenar muscle reconstruction using the free functional pronator quadratus flap Martin Aman * , Arne H. Boecker, Mirjam Thielen, Camillo T. Mueller, Amir K. Bigdeli, Ulrich Kneser and Leila Harhaus Abstract Background: Injuries to the thenar muscle mass or the thenar branch of the median nerve and resulting loss of thumb opposition lead to a massive impairment of hand function. For decades, reconstructive approaches were based on tendon transfers. To broaden the reconstructive repertoire, we present the free functional pronator quad- ratus flap as a viable alternative for functional reconstruction and provide a specification for its indication. We dem- onstrate our surgical technique to a single incision reconstruction using the free functional pronator quadratus flap. Based on a series of three patients, which were analyzed for hand function using Kapandji’s score and the angle of Bourrel, grip strength and nerve conduction velocity in a two year follow up, we present an indication algorithm. Results: After successful reinnervation of all flaps, we found an improvement of Kapandji’s score from 4.3 ± 0.94 preoperatively, to 8.7 ± 0.47 after two years. Accordingly, the angle of Bourrel decreased from 75.75 ± 3.45 degrees to 36.96 ± 3.68 degree. Grip strength also improved from 14 ± 2.2 kg to 26.2 ± 1.2 kg. No impairment of wrist pronation was observed. Conclusion: We found excellent functional recovery of thumb opposition and strength, showing similar or even superior results compared to results from tendon transfers. With the benefit of a single incision surgery and therefore minimal donor site morbidity, this free functional muscle transfer is a viable alternative to classic tendon transfers. Keywords: Reconstructive surgery, Reconstructive microsurgery, Flap surgery, Plastic surgery, Free functional muscle transfer, Thenar muscle loss, Thenar muscle reconstruction, Thumb opposition ©The Author(s) 2021.This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/.The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Background Loss of thenar muscle function is a rare, but consider- able injury. Thus, this rather small trauma often leads to severe impairment of hand function considering range of motion, grasp function, and grip strength during occupa- tional and daily life activities. Since their first description in 1917, reconstructive approaches in hand surgery to restore thumb opposition are based on tendon transfers, accompanied with all their concomitant factors such as donor site morbidity and limited axes of movement [1, 2]. With the improvement of microsurgery, reconstructive approaches such as nerve transfers and free functional muscle transfers (FFMT) have become a viable option for functional reconstruction. Hereby, several case reports exist, describing FFMT for thenar reconstruction, includ- ing the gracilis flap, the serratus flap or the minor pecto- ral muscle flap [3, 4]. These flaps are all viable yet bulky Open Access *Correspondence: [email protected] Department of Hand-, Plastic and Reconstructive Surgery, Burn Center, BG Trauma Center Ludwigshafen, Department of Hand- and Plastic Surgery, University of Heidelberg, Ludwig-Guttmann-Str. 13, 67071 Ludwigshafen, Germany OCTOBER 2021 | The Surgical Technologist | 443