Graft choice for isolated MPFL reconstruction: gracilis versus semitendinosus

Introduction After the first patellar dislocation, most patients report damage of the medio-patellofemoral ligament (MPFL) and surgical reconstruction is required. The purpose of this study is to systematically review current evidence and to clarify the role of the gracilis and semitendinosus tendons as graft for isolated MPFL reconstruction. Materials and methods The present systematic review was conducted according to the PRISMA guidelines. The literature search was conducted in October 2019. All clinical trials using the semitendinosus and/or gracilis tendon grafts for isolated MPFL reconstruction in patients with patellofemoral instability were considered for inclusion. Only articles reporting a minimum of 12-month follow-up were considered. The PEDro score was used for the methodological quality assessment. Results Data from 1491 procedures were collected. The mean follow-up was 36.12 months. There was comparability among the patient baseline. All the scores of interests (Kujala, Tegner, Lysholm) and range of motion scored better in the semitendinosus group. Moreover, in favour of the semitendinosus group, a statistically significant reduction of the revision surgeries and re-dislocations were evidenced. Apprehension test and persistent instability sensation found any statistical correlations. Conclusion Isolated MPFL reconstruction through semitendinosus tendon graft performed better than the gracilis in selected patients suffering from recurrent patellofemoral instability.


Introduction
Patellofemoral instability is a common cause of complaint in active young patients [1]. After the first dislocation, in about 96% of patients, the medial patellofemoral ligament (MPFL) is significantly damaged and surgical reconstruction can be necessary [2]. The reconstruction of the MPFL reports excellent results and patient satisfaction and is related to a low rate of complications and post-operative failures [3]. In conjunction, the centres performing MPFL reconstruction have doubled in the last decades [4]. For an optimal MPFL reconstruction, the graft choice is of fundamental importance. The graft can be harvested from several tendons, auto-versus allograft or even synthetic graft. However, the most used grafts are the gracilis or semitendinosus tendons [5,6]. Semitendinosus and gracilis tendons are often preferred grafts for ligament reconstruction because of their intrinsic biomechanical proprieties [7], geometric proprieties [8], availability and low donor-site morbidity [9].
However, there is a lack of clinical studies comparing directly the two tendons, and the best graft for MPFL reconstruction is still unclear. The purpose of this study was to systematically review the current evidence and to investigate which is the best graft between gracilis and semitendinosus tendons for MPFL reconstruction. We focused on the clinical scores, physical examination, further revision surgeries and failures.

Search strategy
This systematic review of the literature was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [10]. The authors drafted a preliminary protocol to guide the search: • (P) Population: recurrent patellofemoral instability; • (I) Intervention: isolated MPFL reconstruction; • (C) Comparison: semitendinosus versus gracilis tendon graft; • (O) Outcomes: clinical score and examination, re-operations, failure.

Literature search
Two independent authors (FM, JE) performed the literature search. In October 2019, the following databases were accessed: Pubmed, Embase, Google Scholar, Scopus. The following keywords were used isolated or in combination: patellofemoral and/or patellar combined with instability, dislocation, luxation, syndrome combined with MPFL and/ or rupture, tear, reconstruction combined with semitendinosus, gracilis, hamstring, tendon, graft, combined with bundle, doubled, single. The same authors screened the resulting articles. If the title and related abstract matched the topic, the full text was accessed. Furthermore, the bibliographies were screened to find additional articles.

Eligibility criteria
All studies reporting the outcomes of MPFL reconstruction using the semitendinosus and/or gracilis tendon graft for recurrent patellofemoral instability were considered for inclusion. According to the Oxford Centre of Evidenced-Based Medicine [11], levels of evidence I to IV were included. According to the authors' language capabilities, articles in English, German, Spanish, Italian and French were included. Only articles reporting data concerning isolated MPFL reconstruction that were included with a minimum of 12 months of follow-up were considered. Only articles reporting quantitative data under the outcomes of interest were considered for inclusion. Techniques, comments, letters, editorials, protocols and guidelines were excluded. Biomechanical, animal and cadaveric studies were also excluded. Articles reporting data on patellofemoral instability after total knee arthroplasty were excluded. Articles reporting data of revision surgeries were also rejected. Articles combining MPFL reconstruction with other proximal or distal alignment were excluded. Disagreements between the authors were debated and solved by a third author (AD).

Outcomes of interest
Two independent authors (FM, JE) extracted the following data: generalities (author, year, type study), patient demographics (number of knees, mean age), follow-up duration, onset (recurrent or acute), presence of risk factors, patellar and femoral graft fixation. The following outcomes of interest were collected: Kujala Anterior Knee Pain Scale [12], Lysholm Knee Scoring Scale [13], Tegner Activity Scale [14]. In addition, apprehension test, range of motion (ROM), persistent sensation of instability, revisions and redislocations were recorded.

Methodological quality assessment
The PEDro score was used for the methodological quality assessment. Two authors (FM, JE) independently performed the score. This score analysed the papers under several items: clear statement of inclusion and exclusion criteria, allocation, randomization, blinding methods, follow-up duration, point estimates and variability. The final result was a value from 0 (poor quality) to 10 (excellent quality). Values > 6 points were considered acceptable (high quality = 10-8; good quality: 8-6; fair quality = 6-4; poor quality ≤ 3).

Statistical analysis
For the statistical analysis, we referred to the IBM SPSS Software. The arithmetic mean and standard deviation (SD) for continuous variables were adopted. For binary variables, the odd ratio (OR) effect measure was adopted. The confidence interval was set at 95% in all the binary comparisons. The unpaired T test was performed in all the comparisons, with values of P < 0.05 considered statistically significant.

Search result
The initial literature search resulted in 894 articles. Of them, 379 were rejected because of duplications. Another 360 were rejected as they did not match the eligibility criteria. A further 107 articles were rejected because no quantitative data under the endpoints of interest were reported. Other 11 articles were rejected due to unreliable data. Ultimately, a total of 37 papers were included in this work, 11 using the gracilis tendon graft and 26 using the semitendinosus tendon graft (Fig. 1).

Methodological quality assessment
The methodology quality assessment showed several limitations. First, only 8% of the enrolled studies provided a randomization of the samples. Furthermore, no study provided a blinding method of the samples, limiting the quality of the overall results. Strength points included the length of followup and the well-designed analysis performed by most of the included papers. Moreover, most articles included a large number of patients in their studies. Consequently, the PEDro score resulted in 6.03 points, attesting to this work a good methodological quality assessment. The results of the PEDro score assigned for each study are shown in Tables 1 and 2.

Patient demographics
In the present study, a total of 1491 MPFL reconstructions, with a mean of 36.12 ± 17.1 months of follow-up, were enrolled. In the gracilis graft group, a total of 574 knees were analysed, with a mean age of 22.39 ± 4.8 years. In the semitendinosus graft group, a total of 917 knees were analysed, with a mean age of 22.68 ± 7.4 years. Between the two groups, there were no significant differences among the samples ages (P = 0.5), attesting a good baseline comparability. Patient demographics concerning the gracilis tendon graft group are shown in Table 1. Patient demographics concerning the semitendinosus tendon graft group are shown in Table 2.

Discussion
According to the main findings of this systematic review, we found that the semitendinosus tendon graft performed better overall. Worthy of note was the statistically significant reduction of re-dislocations and revisions rate observed in the semitendinosus group. The endpoints Kujala and Lysholm and range of motion were both statistically significant in favour of the semitendinosus graft group. They showed homogenous values, with poor data variance and small confidence intervals, yielding trustworthy results. Concerning the other analysed endpoints, apprehension test and persistent instability sensation, no statistical differences between the two groups were found.
The MPFL is the most important restraint to patellar lateralization during the first 30° of flexion [52,53]. Current literature reported no clinical study comparing directly the two grafts. However, the biomechanical proprieties of the grafts have been investigated. The study of Mountney et al. [54] stated that the MPFL ruptured at a mean of 26 ± 7 mm, and the patella dislocated at approximately 50 mm, ensuring an MPFL rupture [54]. Graft choice is of fundamental importance for MPFL reconstruction. Tendon tensile strength and viscoelastic properties are some of the most important mechanical parameters to respect when choosing a graft for a successful ligament reconstruction. The MPFL is a ligament of tissue connecting the tubercle of the adductor on the femur epicondyle to the proximal medial edge of the patella [55]. Although a small structure, this ligament shows a remarkable tensile strength and viscoelasticity. As the native tensile strength of the MPFL is approximately 208 N [54,56], both gracilis and semitendinosus tendon grafts are far more resistant [5,6]. In fact, the estimated tensile strength of the semitendinosus and gracilis tendon is 1216 N and 838 N, respectively [57]. The semitendinosus tendon graft has more resistance to traction than the gracilis [58]. Therefore, the semitendinosus represents the most commonly used graft for MPFL reconstruction [59,60]. However, interest concerning gracilis tendon grafts has    reconstruction via the semitendinosus tendon graft, being more resistant than MPFL and gracilis, can therefore explain the reduced tendency to re-dislocations and revisions. Therefore, according to the present results, the usage of the semitendinosus tendon graft should be encouraged. The present systematic review evidenced important limitations. The overall poor quality of the included studies represented an important point of weakness. No study took advantage of blinding methods, and only 8% provided randomization of the samples. Therefore, the data must be interpreted with caution. There is a lack of randomized clinical trials in the current literature, and further high-quality studies are strongly required. The present study performed the analyses regardless of the type of onset of instability, type of patellar and femoral fixation. This represents another limitation of the present work, and further studies are required. Points of strength of this systematic review were the comprehensive nature of the literature search, along with the strict eligibility criteria and rigorous quality assessment. Another point of strength, the methodological assessment of this work, which according to the PEDro score, resulted in a good quality assessment. Furthermore, as confirmed by the Student's t-test, the study presents an optimal baseline comparability of the samples. All these observations provide an overall reduction of the risk of publication bias, generating feasible results.

Conclusion
For isolated MPFL reconstruction, the semitendinosus performed better overall. The Kujala and Lysholm scores were both statistically significant in favour of the semitendinosus graft group. The ROM was statistically significant and greater in favour of the semitendinosus graft group. Furthermore, a statistically significant reduction in the failure rate was observed in the semitendinosus group. The gracilis tendon graft group reported a reduction in complication rate, but without statistical significance.
Funding No external source of funding was used.

Compliance with ethical standards
Conflict of interest The authors declare that they have no conflicts of interest.
Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors.
Informed consent For this type of study, informed consent is not required.
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