The loss of function associated with facial muscle movement is termed facial paralysis or facial nerve palsy. Facial nerve damage arising from various etiologies can cause facial paralysis. The most common cause of facial paralysis is Bell's palsy, an acute idiopathic unilateral lower motor facial nerve lesion [1]. Other common causes of facial paralysis are cerebrovascular events, facial trauma, viral infections (notably herpes zoster), bacterial infections, neoplasms, and congenital causes, including birth trauma and syndromic conditions such as Moebius syndrome. Bilateral facial paralysis is rare, often caused by systemic manifestations of a disease, and it occurs in up to 2% of all cases [2]. Facial paralysis is associated with a poor quality of life, likely because of physical and psychosocial sequelae. Altered facial appearance, difficulty with swallowing and speech, and ophthalmic complications may occur [3-6]. Patients with facial paralysis may experience altered self-perception, social anxiety, and depression [7]. The etiology and onset of facial paralysis determine the treatment. In acute-onset Bell's palsy, the urgent administration of high-dose corticosteroids is indicated [8]. Other medical adjuncts for acute facial palsy include antivirals, such as acyclovir, especially if the facial palsy is associated with herpes zoster infection (Ramsay Hunt syndrome). Surgical intervention is indicated if medical therapy is unsuccessful or facial paralysis has been present for many months [9]. Within a year of onset, nerve reconstruction to stimulate native muscles may be effective for patients. In facial palsy lasting over a year, there is irreversible fibrosis of the neuromuscular junction, and therefore, nerve reconstructive surgery is unlikely to lead to improved facial nerve function [10].

Facial reanimation surgery using free functional muscle flaps, such as the gracilis muscle flap, provides smile potential in facial paralysis. These free muscle flaps have been reported to provide effective long-term outcomes even in long-standing facial paralysis [11-13]. Since its conception in the 1970s, 2-stage facial reanimation using free muscle flaps has been widely accepted in the surgical management of long-standing unilateral facial palsy [14,15]. The first stage of this approach involves establishing a cross-facial nerve graft (CFNG) by connecting the sural nerve to the proximal facial nerve on the unaffected (contralateral) side. The second stage involves a separate operation for free muscle transfer and coaptation to the CFNG [16]. This is the single innervation technique. In the dual innervation technique, a CFNG and nerve to the masseter are used to restore facial muscle function. The immediate drawbacks of 2-stage facial reanimation are the need for 2 separate operations and the risk of complications secondary to nerve grafting, such as paraesthesia.

Single-stage facial reanimation was developed in 1998, with researchers reporting faster reinnervation times and impressive cosmetic outcomes [17,18]. In single-stage facial reanimation, the nerve of the free muscle flap may be connected to the contralateral facial nerve using a CFNG, connected to the ipsilateral nerve to the masseter (a branch of the trigeminal facial nerve), or connected to both (dual innervation) within a single operation. This avoids multiple operations, and if a CFNG is not used, this approach avoids nerve grafting. Recent studies have shown the key benefits of single-stage facial reanimation, namely stronger smile excursion and faster muscle neurotization and recovery [18-20]. Textbox 1 summarizes the benefits and risks of the single-stage and 2-stage facial reanimation techniques according to the current literature.

There are limited randomized controlled trials and systematic reviews reporting on facial reanimation surgery [21-35]. Most studies discuss the optimal types of free muscle flaps, nerve reconstructive techniques, or patient-reported outcome measures (PROMs). There is a paucity in the literature of comparisons between the single-stage and 2-stage reanimation techniques, with only 1 systematic review published a few years ago [30]. Understanding the broad outcomes and patient experiences of these surgical techniques is important to provide patients with up-to-date and accurate information when consenting to facial reanimation. Moreover, it is vital for clinicians to better understand the indications, effectiveness, and complications of the single-stage and 2-stage facial reanimation techniques. Consequently, this proposed systematic review aims to explore how the outcomes of single-stage facial reanimation compare against those of 2-stage facial reanimation. Ideally, the results of this review will further inform clinical practice and provide collated evidence for future research and guidelines.

This systematic review formally began in February 2025, although it has been planned since July 2024. The final systematic review will be performed and reported according to this protocol and the PRISMA guidelines [37,39]. A PRISMA flow diagram and checklist have been included to summarize the search and selection strategy up to this date (Multimedia Appendices 3 and 4). Initially, the search returned 2120 records, and 56 records are currently being screened for eligibility. Data from the final included studies will be extracted, collated, and analyzed. The risk of bias and the quality of evidence will be ascertained. A narrative synthesis will be used to summarize the results of the systematic review. Where possible, statistical analyses will be presented using graphs and figures. The final systematic review is expected to be published in December 2025.

The authors hypothesize that the included studies, particularly those that are more recent, will show that patients who undergo single-stage facial reanimation using free functional muscle flaps have quicker reinnervation and recovery times than patients who undergo 2-stage facial reanimation. As mentioned, there is an expected heterogeneity in the outcomes used by studies, which may mean that the hypothesis is difficult to objectively display. For example, it is unlikely that the data extracted and analyzed from the included studies will allow for a forest plot that would summarize the results of this review’s outcomes. This systematic review, however, will still be clinically useful as the most up-to-date review comparing the single-stage and 2-stage facial reanimation techniques. The information collated and discussed in this review may be used to aid clinicians and patients in shared decision-making. Moreover, future research questions may be raised, allowing for the conception of further studies on this topic.

To the best of our knowledge, Natghian et al [30] published the only systematic review comparing single-stage and 2-stage facial reanimation using free functional muscle flaps. The study specifically examined smile excursion as an outcome and concluded that single-stage facial reanimation may produce superior smile excursion and symmetry when compared with the 2-stage technique; however, the study lacked data on the spontaneity of smiling after surgery, which is a key indicator of smile quality. Previous research has shown that 2-stage facial reanimation produces a greater spontaneous smile [11]. Since the study by Natghian et al [30], further articles discussing facial reanimation with free functional flaps have been published; however, a contemporary systematic review that discusses the most recent literature is yet to be performed. The proposed systematic review will be a necessary addition to the current literature because it will provide an updated comparison of the single-stage and 2-stage facial reanimation techniques using a broader range of outcomes than that in the study by Natghian et al [30], such as recovery time, nerve regeneration, and smile spontaneity. Therefore, the authors argue that this proposed systematic review’s strengths are its recency and wide scope.

A limitation of the proposed review is the heterogeneity of the outcomes used in the topic, which limits the scope for statistical analyses such as meta-analysis. There are likely to be differences even within the single-stage and 2-stage techniques, such as different free functional muscle flaps and different innervations. While the proposed review will extract these data where possible, the scope for subgroup analysis is likely limited due to the limited number of studies. Many included studies will be prone to selection bias because clinicians may prefer one technique due to psychosocial factors, such as appearance, and clinical characteristics, such as comorbidities, which may impact recovery [58,59]. Therefore, the findings may be undermined. Hence, tools, such as ROBINS-I and GRADE, will be used to provide the risk of bias and quality of evidence of each included study for transparency. To disseminate the results, the authors will aim to present this review at a relevant conference and to publish it in a relevant journal.