Introduction

Patellar dislocation is the most common injury of the patellofemoral joint in young patients [1,2,3], with a high annual incidence at 147.7 per 100,000 in patients between 14 and 18 years, and recurrence rates reaching up to 70% after a primary dislocation [1, 2, 4,5,6]. Patients report giving away, joint effusion, anterior knee pain, limited range of motion, restricted sports activities participation and are at higher risk of developing osteoarthritis [2, 7]. The etiology of recurrent patellar dislocation is complex [1, 3, 8]. Trochlear or patellar dysplasia, patella alta, genu valgus or recurvatum, and increased femoral anteversion, and lateral tibial torsion have all been associated with an increased risk of patellofemoral dislocation [3, 9]. Therefore, several techniques, including proximal and distal realignment procedures, ligament reconstruction, or a combination of them, have been proposed for its management [2].

The medial patellofemoral ligament (MPFL) is the primary patellar restraint between 0° and 30° of knee flexion [3, 10,11,12]. Its anatomic reconstruction has shown satisfactory clinical outcomes, and it is considered a milestone in the management of recurrent patellofemoral instability [2, 3, 10, 13, 14]. Although many MPFL reconstruction techniques have been described, the ideal graft or fixation method are still debated [8, 15, 16]. Hardware-free fixation techniques, also called implantless, soft tissue, elastic, or dynamic fixation techniques, were initially developed to preserve the distal femoral physis in skeletally immature patients [17,18,19,20,21]. However, given their potential advantages, such as no implant-related costs, no need for hardware removal, and no implant-related complications, they are becoming increasingly popular [13, 22,23,24,25,26,27,28]. These advantages are particularly relevant in cost-sensitive populations [29, 30].

This systematic review evaluated the clinical outcomes of hardware-free MPFL reconstruction techniques in patients with recurrent patellofemoral instability. The focus was on patient-reported outcome measures (PROMs), redislocation rate, and complications. The hypothesis was that hardware-free MPFL reconstruction with or without associated soft-tissue or bony realignment procedures is safe and effective in patients with recurrent patellofemoral instability.

Material and methods

Search strategy

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [31]. Two independent reviewers (TMF, GK) searched PubMed, Scopus, and Virtual Health Library databases in October 2021. The following terms, "medial patellofemoral ligament", "MPFL", "reconstruction", and "outcomes", were used alone and in combination with Boolean operators AND and OR. Inclusion and exclusion criteria were established before the search and were used to identify potentially eligible studies by title and abstract screening. Disagreements between reviewers were resolved by a third author (EP). The bibliographies of the included studies were also screened to identify additional studies.

Eligibility criteria

All the clinical studies which investigated the efficacy and feasibility of hardware-free MPFL reconstruction were screened for inclusion. Given the linguistic abilities of the authors, only studies in English or Spanish were considered. Only studies with a minimum 24-month follow-up were considered eligible. Only studies that used the Kujala Anterior Knee Pain Scale as PROM. Reviews, commentaries, editorials, and opinions were excluded as were biomechanic and animal studies. Studies that did not properly describe the surgical procedure were also excluded. Missing data on the outcomes of interests warranted the exclusion from the present study.

Data extraction

Two independent investigators (TMF, GK) reviewed the resulting articles and performed data extraction. For each included study, the following data were extracted: author, year, study design, patients demographic at baseline, length of the follow-up, surgical technique. Data concerning the Kujala Anterior Knee Pain Scale at baseline and at last follow-up were retrieved. The rate of complications was also collected.

Outcomes of interest

The improvement in the Kujala Anterior Knee Pain Scale and redislocation rate after surgical treatment were evaluated as primary outcomes. The Kujala Anterior Knee Pain Scale is a 0–100 thirteen-question patient-reported outcome assessment tool widely used to evaluate the outcomes following surgical procedures in patients with patellofemoral instability [7]. A score of 95 points or greater was considered excellent, 94 to 85 as good, 84 to 65 as fair, and 64 or less as poor [32]. The rate of postoperative complications was evaluated as a secondary outcome.

Methodological quality assessment

The quality of the methodological assessment was assessed using the Modified Coleman Methodology Score (mCMS) (Table 1) [38].

Table 1 Modified Coleman methodology scores of the included studies
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Statistical analysis

The statistical analysis was performed using SPSS V.19 and Microsoft Excel 2016 (Microsoft®, USA). Continuous data were presented as mean values, standard deviations. Dichotomic data were presented as percentages. The t-test was used for continuous data, and the chi-square test for binary variables. P values < 0.05 were considered significant.

Results

The initial literature search yielded 932 potentially relevant records after the removal of duplicates (N = 411). Titles and abstracts were independently screened, and 27 articles were selected for full-text evaluation. Seven studies were excluded because of insufficient follow-up [19, 39,40,41,42,43,44] and seven more because Kujala Anterior Knee Pain Scale was not used or data were insufficient to evaluate post-surgical improvement [45,46,47,48,49,50,51]. Finally, eight studies met the predetermined eligibility criteria, and no additional studies were included after citation screening in the systematic review (Fig. 1). There were six case series [27, 33,34,35,36,37], one multicenter longitudinal prospective comparative study [21], and one randomized controlled trial [18].

Fig. 1
figure 1

Flow-chart of the literature search

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The descriptions of diagnosis and surgical techniques were consistent and accurate in most studies. The rehabilitation process was poorly described in some studies. All studies adequately reported outcome measures, the timing of outcome assessment, and the unbiased selection criteria of the subjects involved. Of the mCMS items, 'study size' and 'mean follow-up' scored the lowest because five out of eight studies had included less than 30 patients [18, 21, 27, 36, 37], and the follow-up was within 12–36 months in six of them [18, 21, 27, 33, 34, 37]. Furthermore, among these studies, six were case series [27, 33,34,35,36,37]. The lack of general health measures and the procedures for outcomes assessment were the most important limitations. It was unclear whether investigators were independent of surgeons, and completion of assessment by patients with minimal investigator assistance was not explicit in most studies. Recruitment rate was lower than 90% in five studies [18, 27, 33, 36, 37]. Concluding, the average mCMS value was 62.88 (range 57–70), demonstrating moderate methodological quality.

Narrative analysis of the collected data was conducted and summarized in Table 2.

Table 2 Outcomes of hardware-free medial patellofemoral ligament reconstruction techniques
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Recurrent patellar dislocation was the main indication for hardware-free MPFL reconstruction in all the included studies [18, 21, 27, 33,34,35,36,37]. Three studies reported data from patients with physiological limb alignment and bone morphology [21, 33, 37]. Patients with increased tibial tubercle-trochlear groove (TT-TG) distance were included in three studies [18, 35], patella alta in two [35, 36], severe trochlear dysplasia in one [18], increased Q angle in one [34], concomitant general ligament laxity in two studies [27, 34]. Double bundle MPFL reconstruction using a free autograft [18, 21, 35, 36] was the most common technique, followed by single-bundle MPFL reconstruction with pedicled autograft [27, 34], and combined MPFL and MPTL reconstruction with pedicled autograft [33, 37]. Concomitant procedures included debridement [18, 33], microfractures [33], fixation of osteochondral lesions [36], osteochondral transplantation [36], distal realignment procedures [18, 34, 35], Insall's proximal realignment procedure [36], medial retinaculum plication [27], and lateral retinacular release [34].

The preferred method for patellar graft fixation was bone tunnels [18, 21, 33,34,35,36,37], except for Abouelsoud et al. [27] technique, in which the patellar tendon quadriceps attachment was preserved as a pedicled autograft. The most commonly used method for femoral fixation was looping the tendon graft around the adductor magnus tendon [18, 21, 33, 35]. Femoral fixation was also achieved by (1) suturing the graft to the periosteum and bone in the MPFL femoral footprint and the adductor magnus tendon [27], (2) a bone tunnel in the MPFL footprint [37], (3) looping it through a slit in the medial collateral ligament [36], and (4) preserving the adductor magnus tendon distal attachment when prepared as a pedicled autograft [34]. In combined MPFL and MPTL reconstruction, a gracilis tendon pedicled autograft was prepared to preserve its distal attachment [33, 37]. Graft tensioning and fixation at 30° of knee flexion was the favored method [18, 27, 34, 36], followed by 5–10 mm manual patellar lateralization [33], or a combination of both [21, 35, 37].

The gracilis tendon was the most commonly used autograft in the included studies [18, 21, 33, 35], followed by semitendinosus tendon [36, 37], quadriceps tendon [27], and adductor magnus tendon [34].

Short- to long-term improvement of Kujala score was observed in all included studies comprising patients from both sexes with mean ages ranging from 11.5 to 26.5 years [18, 21, 27, 33,34,35,36,37]. Mean score improvement ranged from + 13.2/100 to + 54/100, with mean postoperative scores ranging from 82/100 to 94/100. The final outcome was graded as good in seven studies [18, 21, 27, 34,35,36,37] and fair in one [33]. In two comparative studies, hardware-free MPFL reconstruction showed no statistical difference in Kujala score compared to femoral fixation using interference screws [18] or suture anchors [21]. Similarly, there were no statistical differences when comparing Kujala scores in patients with or without osteochondral injuries [33]. After surgery, patellar redislocation was observed in three of eight included studies [21, 33, 34]. Malecki et al. [34] reported four cases (10.26%), Maffulli et al. [33] three cases (8.82%), and Marot et al. [21] only one case (3.45%). All but one redislocations occurred during sports activities.

A positive apprehension test [34,35,36,37] and flexion deficit [27, 35,36,37] were the most commonly reported complications, ranging respectively from 2.86 to 25% and 3.4 to 6.25% overall. Other complications included osteoarthritis [33, 36], sensation of joint instability [18, 21], patella drill hole-related problems [33], hypoesthesia [33], anterior knee pain [33], pain at the medial femoral condyle [18], and hypertrophic wound scarring [35].

Discussion

Hardware-free MPFL reconstruction with or without associated soft-tissue or bony realignment procedures provided short- to long-term improvement and a low redislocation rate in patients with recurrent patellofemoral instability, as initially hypothesized.

The number of bundles, type of fixation, and graft tensioning for MPFL reconstruction in patients with patellofemoral instability is still debated [2, 8, 10, 37, 52,53,54,55,56,57,58]. Thus, several variations and combinations of procedures have been described. Double-bundle MPFL reconstruction using a free gracilis autograft was the preferred method. Likewise, the most frequently implemented hardware-free fixation methods were patellar bone tunnels and looping the autograft around the adductor magnus tendon at 30° of knee flexion.

At least half of the world’s population lives in poverty and lacks access to quality essential health services [30]. Thus, investigations aiming to reduce the surgical-related burden represent a significant breakthrough for developing countries. Zhang et al. [59] exposed the contrasting cost differences of a pair of suture anchors and three high-strength sutures (US$800 vs. US$100, respectively) when comparing two different patellar fixation techniques. In fact, various authors have remarked on the high costs of suture anchors and interference screws [25, 26, 29]. Biomechanical studies have found no significant differences among fixation methods in MPFL reconstruction, and all provide higher failure loads than the native ligament [24, 60, 61]. Therefore, populations at economic disadvantage may benefit from hardware-free fixation techniques, being safe [18, 35, 37], cost-effective [3, 21, 35, 41, 42, 47, 59, 62]. Also, an effective hardware-free MPFL reconstruction can be performed in skeletally immature patients [3, 21, 27, 35, 36, 41, 42] and avoids implant-related complications or further surgery for implant removal [41, 42, 44].

Graft femoral fixation in hardware-free techniques is a debated technical point. Implant-based fixation techniques have shown similar pullout strength to hardware-free fixation techniques but higher stiffness [18, 60]. However, it has been suggested that the elastic behavior and lower stiffness of hardware-free fixation can result in a more compliant graft physiometry, lowering the risk of joint overconstrain and early-onset osteoarthritis [3, 18, 19, 21, 41, 46, 47]. Additionally, many hardware-free fixation techniques do not require intraoperative fluoroscopy, lowering associated costs and radiation exposure [21, 42]. It is still unknown whether higher fixation stiffness results in clinically relevant improvement or higher expenses.

In a recent systematic review, the clinical outcomes of patients with recurrent patellofemoral instability undergoing MPFL reconstruction using interference screws or anchors for autograft femoral fixation were compared [55]. The analysis of 19 clinical trials revealed no significant differences in Kujala Anterior Knee Pain Scale, Lysholm Knee Scoring Scale, and Tegner Activity Scale scores outcomes. The mean Kujala score improvement for anchor and interference screw fixation was 30.35 versus 35.75, respectively. The last follow-up scores were 86.23 ± 7.71 versus 88.37 ± 3.71 at a 46.5 ± 20.9 months follow-up, respectively. These results agree with the findings of the present systematic review and are further supported by additional studies which have not been included because, though published in peer-reviewed journals, they did not meet our strict inclusion criteria [19, 39,40,41,42,43,44,45,46,47,48,49,50,51]. On the other hand, the complication profile of hardware-free fixation shares similarities with implant-based fixation techniques, including subjective instability, positive apprehension test, and redislocation [55].

Hardware-free MPFL reconstruction was initially developed for skeletally immature patients to avoid growth damage to the distal femur physis [17, 51, 61, 63]. However, this technique has also been extended to the adult population [18, 33, 35, 36]. Indeed, among the studies considered in the present systematic review, only two included studies included solely patients younger than 18 years [23, 44], are adult population was the most commonly investigated [18, 21, 33, 35,36,37]. These findings confirmed a trend towards hardware-free techniques implementation regardless of the patient's age.

The present study certainly has some limitations. Only three studies reported information on isolated hardware-free MPFL reconstruction. Five studies combined MPFL reconstruction with additional soft-tissue or bony realignment procedures [18, 27, 34,35,36], and three studies with other treatments addressing to osteochondral injuries [18, 33, 36]. The combination of such procedures limits the extent of the findings of the present systematic review. Nevertheless, more than two-thirds of the patients presenting recurrent patellar dislocations demonstrate two or more pathoanatomical predisposing factors, which may synergistically predispose them to joint instability [64,65,66]. The association of additional procedures is still debated and should be evaluated at an individual level [1, 67]. Six of eight studies were case series, thus negatively impacting the overall quality of the results. Future comparative studies should follow a cost-effectiveness analysis methodology to find the most efficient MPFL reconstruction technique. The clinical relevance of the present systematic review is that the use of hardware-free MPFL reconstruction fixation techniques may represent an effective alternative for the surgical treatment of recurrent patellofemoral instability in cost-sensitive environments. Orthopaedic surgeons may benefit their patients with lower costs, no need for implants, lack of implant-related complications, and further surgery for implant removal.

Conclusion

Hardware-free MPFL reconstruction provided clinical improvement and was associated with a low redislocation rate in patients with recurrent patellofemoral instability. Advantages such as safety, femoral physis preservation, and comparable complication profiles with implant-based techniques endorse their implementation. Orthopaedic surgeons in cost-sensitive environments may also benefit their patients with lower costs, no need for implants, lack of implant-related complications, and surgery for implant removal.