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Page 2 of 8 Aman et al. BMC Surg (2021) 21:310 options, but also come with the burden of a certain donor site morbidity. The pronator quadratus (PQ) muscle flap was first described by Dellon et al. [5] as a small muscle flap with minimal donor site morbidity. Since it’s first descrip- tion in 1983, it was only reported twice as a free func- tional flap case report of a single thumb reconstruction by Lee and Idler [6] and for facial reconstruction by Tzou and Aszmann [7], indicating the broad indication spec- trum of this flap. With many potential benefits for the- nar reconstruction, such as minimal donor site morbidity and excellent muscle properties it is a viable option for reconstruction [8, 9]. Therefore, we present a reconstructive concept for thenar reconstruction of thumb opposition using a free functional pronator quadratus flap (FFPQ). Furthermore, we present the outcome of a two-year follow up of our patients who underwent this functional reconstruction. Methods Three patients (one female, two males) with a mean age of 38.6 years (range from 16 to 53 years) all suffering from a loss of thumb opposition underwent FFMT reconstruc- tion using the ipsilateral PQ. All patients had a severe combined traumatic injury of the motor branch and the- nar muscle mass and were transferred to our clinic after primary reconstruction efforts failed. All patients were evaluated in a long-term follow up during routine examination for pre- and postopera- tive force as well as functional thumb opposition using Kapandji’s score and the angle of Bourrel [10–12]. This angle refers to the angle between the thumb and the fourth digit during opposition. Force was measured on both hands using a JAMAR dynamometer pre and post- operatively. Furthermore, neurological diagnosis was performed to assess the conduction velocity of the nerve as well as EMG of the thenar muscle to verify nerve dam- age and consecutive regeneration. Local institutional ethic consent was obtained and the principles according to the declaration of Helsinki were fulfilled. Reconstructive approach‑ preparation and patient selection Admitted patients were examined for patient history fol- lowed by a detailed clinical examination to verify damage of the thenar branch and thenar muscles and to docu- ment preoperative hand function. Patient one was a young (16y) student with a trau- matic thenar laceration wound. Exploration of the wound showed severe loss of thenar muscle mass especially at the motor entry point of the thenar branch. Therefore, primary nerve repair was not feasible and a FFMT was performed immediately as this young patient and his parents demanded for optimum functional outcome. The second patient was a 46-year-old lawyer with a deep cut at the thenar, who received primary wound closure with- out further exploration in an emergency department. She was then referred with delay of almost a year after the injury to our center due to severe thenar atrophy and functional impairment. Intraoperative exploration then demonstrated complete injury to the thenar branch with- out potential for nerve transfer surgery due to prolonged and severe thenar muscle damage. Our third patient (53y) had a work-related injury to the hand with combined muscle and nerve damage of the thenar region. A primary attempt of direct nerve repair was performed but was found to be insufficient in a one year follow up. After intraoperative exploration and preoperative discussion with the patient he wanted to undergo FFMT and denied tendon transfers due to potential donor site morbidity. We performed electro- physiological testing of nerve and muscle function of median nerve and thenar muscle electromyography in all patients. Neuroimaging with ultrasound further sup- ported preoperative planning and helped with decision making for use of the PQ-flap. However, the preopera- tive workup in some cases cannot give the full informa- tion of the real quality of the functional structures. In these cases, the surgery should start with exploration of the situs and the surgeon has to be prepared for per- forming any other technique of the armamentarium of thenar reconstruction, such as tendon transfers or nerve transfers. Surgical procedure of the free functional PQ A typical skin incision for open carpal tunnel release is made and extended to the forearm in line of the modi- fied Henry approach, usually up to 10–12 cm proximal to the wrist crease. First step is carpal tunnel reopen- ing and delicate exploration of the thenar branch as the donor branch for the flap by neurolysis of the median nerve from proximal to distal. If the careful assessment shows this branch not to be viable as donor branch (hid- den in scarring, not identifiable from the main median nerve) the first ulnar nerve branch to the abductor digiti minimi (ADM) muscle can be chosen and carefully pre- pared via opening of the Guyon’s canal. For later muscle insertion the aponeurosis of the abductor pollicis muscle at the base of the thumb proximal phalanx is dissected. As counter-insertion, the dissected ulnar portion of the retinaculum flexorum is freed and prepared. For free flap anastomosis the radial artery on tabatière level or the ulnar artery on wrist level as well as their concomitant veins are dissected. Afterwards the pronator quadratus muscle is prepared underneath the flexor tendons. This allows for maximum length of the neurovascular pedicle | The Surgical Technologist | OCTOBER 2021 444