Introduction

The meniscus, a fibrocartilaginous structure essential for stabilizing the knee joint, absorbing shocks, distributes forces and protects the articular cartilage [1,2,3,4,5]. Acute tears of the meniscus may be symptomatic, impacting negatively quality of life and sport participation, and may lead to early onset osteoarthritis [6,7,8,9]. Rotational and shear forces on the menisci, especially during kneeling, carrying heavy loads and movements with acceleration, deceleration, jumping, and change of direction, are the main causes of acute tears of the meniscus [10,11,12,13]. Direct traumas to the knee might also cause meniscal damage and are often associated with damage to adjacent bone and ligaments [14, 15]. In adults with meniscal degeneration, meniscal tears develop from relatively minor forces or trauma [16, 17].

Meniscal repair is associated with reduced chondral damage compared to meniscectomy [18,19,20,21,22]. In this context, by stabilising the knee joint, meniscal repair prevents cartilage damage, thus preventing early-onset osteoarthritis [23,24,25]. Repair of the damaged meniscal tissue was introduced in the 1980s [26, 27]. Arthroscopic repair of meniscal injuries has become popular [28,29,30]. Inside-out and all-inside are two well-established methodologies to repair the damaged meniscus during arthroscopy. Though these techniques are widely performed and validated in several clinical settings, it remains unclear which method promotes greater clinical outcomes. This study compared inside-out versus all-inside arthroscopic meniscal repair in terms of patient-reported outcome measures (PROMs), failures, return to play, and symptoms.

Methods

Search strategy

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [31]. The PICO algorithm was established:

  • P (population): meniscal tears in the active population;

  • I (intervention): arthroscopic meniscal repair;

  • C (comparison): All-inside, inside-out;

  • O (outcomes): PROMs, clinical examination, complications.

Literature search

Two authors (**;**) independently performed the literature search by accessing the following database PubMed, Google Scholar, Scopus in February 2023. The following keywords were used for the search in combination using the Boolean operator AND/OR: meniscal, injury, trauma, acute, defects, tear, rupture, sport, arthroscopy, repair, refixation, all-inside, inside-out. If the title matched the topic, the abstract was read and the full text of the article was accessed. The bibliographies of the articles of interest were screened by hand. Disagreements between the authors were debated and solved by a third author (**).

Eligibility criteria

All clinical studies which investigated the outcomes of all-inside and/or inside-out meniscal repairs were considered. Articles with levels of evidence I to III, according to the Oxford Centre of Evidenced-Based Medicine [32], were considered. Given the authors language capabilities, articles in English, Italian, French, Spanish, and German were considered. Technical notes, editorials, protocols, comments, guidelines, and reviews were excluded. Biomechanical, animal, and cadaveric studies were also not eligible. Studies that reported data on meniscal procedures augmented with mesenchymal stem cells were not considered. Only articles reporting quantitative data under the outcomes of interest were included.

Data extraction

Two authors (**,**) independently performed data extraction and collection. The generalities of the studies were retrieved. The length of follow-up, sample size, and percentage of women in each study were collected. The outcomes of interest were the average age of patients at the time of injury, the incidence between male and female sex, the type of meniscal lesion, and the degree of effectiveness of each technique based on the percentage of patients who returned to play and of re-injured. Specifically, the rate of return to play was also assessed, also considering the patients who managed to return to play at a pre-injury level. The following PROMs were evaluated: International Knee Documentation Committee (IKDC) [33], Lysholm Knee Scoring Scale [34], Tegner Activity Scale [35]. Data on the following complication were collected: rate of re-injury, failures, chronic pain, and reoperation. Data concerning the rate of return to play at a pre-injury level, return to play and daily activities were also retrieved.

Methodological quality assessment

For the methodological quality assessment, the Coleman Methodology Score (CMS) was used [36]. The CMS is a reliable tool to evaluate the methodological quality of systematic reviews and meta-analyses. This score analyses each included study with several endpoints: study size, follow-up duration, surgical approach, type of study, description of the diagnosis, surgical technique, and rehabilitation. The procedures for assessing outcomes and the subject selection process are also evaluated. The CMS rates articles with values between 0 (poor) and 100 (excellent). Articles with values greater than 60 are considered satisfactory.

Statistical analysis

The statistical analyses were conducted by the first author (**) using the IBM SPSS software (version 25). For descriptive statistics, mean and standard deviation were evaluated. The t-test was performed to assess baseline comparability, with values of P > 0.05 considered satisfactory. For the comparisons of continuous data, the mean difference (MD) effect measure and the unpaired t-test were performed. For binary data, the Odd Ratio (OR) effect measure was evaluated. The confidence interval (CI) was set at 0.95. Values of P < 0.05 were considered statistically significant.

Results

Study selection

The initial literature search resulted in 12,843 articles. Of them, only 663 articles matched the topic. Duplicate records (N = 209) were excluded. Of these, a further 390 articles were excluded for reason: not matching the topic (N = 271), study design inappropriate (N = 112), language limitation (N = 4), not available full-text (N = 3). A further 25 articles did not report quantitative data under the outcome of interest, and thus, excluded. This left 39 articles for inclusion. Of them, 22 are prospective and 17 are retrospective studies. The literature search results are shown in Fig. 1.

Fig. 1
figure 1

Flow chart of the literature search

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Study risk of bias assessment

The CMS identified some limitations and strengths in the present study. The size of the study and the duration of follow-up of the included articles were acceptable. The surgical approach, diagnosis and rehabilitation were well described in most of the articles. The outcome measures and timing of the evaluation were often defined, providing moderate assurance. General health measures were rarely reported. Procedures for outcome evaluation and subject selection were often biased and unsatisfactorily described. The CMS for the articles was 66, testifying to this study a good quality of the methodologies for the articles included. The CMS is reported in Fig. 2 (Table 1).

Table 1 Coleman Methodology Scores for the included articles (mean ± standard deviation)
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Study characteristics and results of individual studies

Data from 1848 patients were retrieved. The mean follow-up was 36.8 (9–120) months. The mean age of the patients was 25.8 ± 7.9 years. 28% (521 of 1848 patients) were women. Generalities of the included studies are shown in Table 2.

Table 2 Eneralities of the included studies (CMS: Coleman Methodology Score)
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Results of syntheses

Comparability was found in mean age, rate of women, time span from injury to surgery, Tegner scale, Lysholm and IKDC scores (P > 0.05). Comparability of the demographic baseline is shown in detail in Table 3

Table 3 Demographic of the included studies
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No difference was found in PROMs: Tegner Activity Scale (P = 0.4), Lysholm score (P = 0.2), IKDC score (P = 0.4) among patients undergoing meniscal repair with all inside or inside-out technique. These results are shown in greater detail in Table 4.

Table 4 Results of PROMs (MD: mean difference)
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The all-inside repair resulted in a greater rate of re-injury (OR 2.7; 95% CI 1.29–5.74; P = 0.009), but also a greater rate of return to play at pre-injury level (OR 2.2; 95% CI 1.48–3.22; P = 0.0001). No difference was found in failures (P = 0.7), chronic pain (P = 0.05), and reoperation (P = 0.1) between the two techniques. No difference was found in the rate of return to play (P = 0.5), and to daily activities (P = 0.1) between the two techniques. These results are shown in greater detail in Table 5.

Table 5 Results of binary comparisons (MD: mean difference, CI: confidence interval)
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Discussion

According to the main findings of the present meta-analysis, arthroscopic all-inside meniscal repair demonstrated a greater rate of re-injury and return to play at the pre-injury level compared to the inside-out meniscal repair technique. Arthroscopic all-inside meniscal repair may be of special interest in patients with a particular interest in a fast return to sport, while, for less demanding patients, the inside-out suture technique may be recommended.

Many surgeons advocate the inside-out technique to repair the meniscus, as it allows a more secure and perpendicular suture at the side of the lesion [76, 77]. Moreover, the inside-out meniscal repair is versatile and can be performed in all types of meniscal tears of the posterior horn or body [78,79,80]. However, during arthroscopy accessory posteromedial or posterolateral skin incisions are required for the execution of the suture [81, 82]. Using the inside-out technique, sutures are introduced intra-articularly and are knotted on the capsule. In recent times, the all-inside technique has become increasingly popular [83, 84]. Devices have been introduced to allow all-inside meniscal suture [2, 85]. These devices consist of an anchoring component to the meniscal wall with a sliding and self-locking knot, which allows compression of the injured meniscal fragments [73]. These tools make the meniscal suture surgical technique much easier and simpler, reducing surgical time and the risk of neurovascular complications. Regardless of the repair technique, the present study demonstrated an improvement in PROMs in patients undergoing meniscal sutures. However, whether inside-out performs better than the all-inside meniscal repair technique is debated, and no consensus has been reached. According to our findings, patients undergoing all inside meniscal repair demonstrated a greater risk of re-injury but also a greater rate of return to play at the pre-injury level. Previous clinical investigations included in the present study inferred the same conclusions [37, 43, 48, 49, 54, 65, 74, 86]. No further differences in symptoms, failures, and return to normal activities have been evidenced. In the present study, no difference was found in PROMs and rates of surgical failure, chronic pain, and reoperation. No difference was found in the rate of patients unable to return to play and in the rate of return to daily activities. Hence, all inside meniscal repair may be of special interest to patients who desire a fast return to sport, while, for less demanding patients, the inside-out suture technique may be recommended.

We were able to identify only two clinical studies which compared inside-out versus all inside techniques for meniscal repair [41, 69]. Choi et al. [41]conducted a comparative clinical study on 48 consecutive patients who underwent meniscal repairs of longitudinal tears of the posterior horn of the medial meniscus combined with anterior cruciate ligament reconstructions [41]. At approximately three years of follow-up, no difference was found in ROM and meniscal healing at MRI [41]. Lachman test, KT-1000 arthrometer side-to-side differences, Lysholm scores, and Tegner activity scales were also similar between the two groups [41]. One patient in the inside-out group required manipulation, and two patients had limited ROM [41]. Two transient saphenous nerve injuries were observed in the inside-out group [41]. Spindler et al. [69] comparatively assessed 125 arthroscopic meniscal repairs [69]. The rate of failures (meniscal re-operation) was similar between the groups [69]. Both Kaplan–Meier curves and the Cox proportional hazards model evidenced no difference in time to reoperation between techniques [69].

This study has some limitations. The sample size and length of the follow-up were not adequate in some studies. Moreover, 43% (17 of 40) of the included investigations were retrospective, which increases the risk of selection bias in the present study. Only two comparative clinical trials were included, and all other studies were observational studies. The studies which reported the outcomes of meniscal repair were included irrespective of the type and location of the lesion. However; most studies did not report information on these endpoints or did not conduct the analyses of the patients separately. No information was given in relation to the previous conservative management, for example, platelet-rich plasma injection. Rehabilitation protocols were often biased and general health measures were not reported. Procedures for outcome evaluation and subject selection were often biased and unsatisfactorily described. Most authors did not report information on the injury onset (acute or chronic); therefore, no further subgroup analyses were possible. Many authors performed other procedures (e.g. anterior cruciate ligament) in association with the meniscal repair; therefore, results might be not fully generalizable. Further high-quality comparative studies are required to validate the results of the present study in a clinical setting.

Conclusion

Arthroscopic all-inside meniscal repair demonstrated a greater rate of re-injury and return to play at the pre-injury level compared to the inside-out meniscal repair technique. Arthroscopic all-inside meniscal repair may be of special interest in patients who wish for a fast return to sport, while, for less demanding patients, the inside-out suture technique may be recommended. High-quality comparative trials are required to validate these results in a clinical setting and to evaluate the potential of these techniques according to the type and place of the lesion.