1. Introduction
Pterygium is defined as a fibrovascular formation of triangular morphology that extends from the conjunctiva to the cornea [
1]. This neoformation is more frequent in the nasal sector [
1], and it is characterized by inflammation and fibrosis, leading to tissue remodeling [
2]. Histopathologically, it affects the conjunctival collagen, leading to elastotic degeneration; Bowman´s membrane and corneal surface destruction are observed along with stem cell alterations [
2]. A higher prevalence has been found in regions located thirty-seven degrees above and below the equator, with a higher ultraviolet (UV) intensity [
1]. Di Girolamo et al. [
3] showed that UV radiation stimulated the expression of matrix metalloproteinase (MMP)-1 in human ocular epithelial cells. Moreover, Nolan et al. [
4] found overexpression of heparin-binding epidermal growth factor (HB-EGF) in pterygial tissue caused by UV radiation, which is considered as a driving force in the development of pterygium as it is a potent mitogen. Tsai et al. [
5] highlighted in their study the importance of UV-mediated oxidative DNA damage in the formation of pterygium.
The prevalence of pterygium in the worldwide population is 12%, while in Spain, it is 5.9% (95% CI: 4.3–7.9) [
6,
7]. The most frequent symptoms are nonspecific due to tear film alteration (irritation, burning, photophobia, tearing, and foreign body sensation). Other less frequent and more specific symptoms are pain due to ulceration or decreased visual acuity because of corneal invasion [
8]. Excision surgery is the only effective procedure in the treatment of pterygium. The usual procedures, according to complexity, are simple excision [
8], excision with conjunctival autograft (CAG) [
8,
9,
10], excision with an amniotic membrane (AM) graft [
11], excision with mitomycin C [
9,
12,
13,
14], excision with limbal autograft [
8], and lamellar sclerokeratoplasty [
8]. Pterygium surgery with CAG remains the gold standard procedure, and involves placing the donor tissue using either suture or with a biological adhesive [
15]. Autograft suturing requires surgical experience and technical skills. Suzuki et al. [
16] reported that the use of silk or nylon sutures causes conjunctival inflammation and the migration of Langerhans cells in the cornea. Other drawbacks are increased surgical time, patient discomfort, Dellen, symblepharon, or graft rupture [
17,
18].
AM grafting is a widely used technique in ocular surface surgery [
19]. Preserved human AM can be used as a substrate to replace damaged mucosal surfaces and successfully reconstruct the cornea [
20] or conjunctival tissues after ocular surface neoplasia excision [
21], as well as to repair scleral and corneal melting and perforations [
22,
23] and has been successfully included in pterygium surgery [
24]. AM transplantation (AMT) improves ocular surface epithelialization, reducing inflammation, vascularization, and scarring [
2,
19]. However, some complications, including granulomas, superior anterior scars, symblepharon, and recurrences, have been found after using these surgical techniques [
25,
26]. Recently, numerous investigations have shown the relevance of platelets in regeneration processes by releasing biological mediators such as growth factors [
27,
28]. Plasma Rich in Growth Factors (PRGF) is a standardized type of platelet-rich plasma (PRP) with specific characteristics that differentiate it from other blood-derived products [
29,
30]. PRGF has been used in different medical fields [
31,
32,
33]. Several properties of PRGF, in addition to its autologous origin and the absence of preservatives and stabilizers offers broad applicability in the ophthalmic field by using several formulations (eye drops, injectables, fibrin membranes or fibrin clots) [
34,
35,
36].
Several studies have evaluated the features of PRGF in its eye drop formulation (ePRGF); it has been well-tolerated, demonstrating proliferative, cell migration, anti-inflammatory, antibacterial and antifibrotic capabilities [
34,
36,
37,
38]. Several studies have been carried out to evaluate the stability and safety of ePRGF during its storage. These studies showed that ePRGF maintains its biological activity after 12 months of storage under frozen conditions, for 7 days of daily, and use even stored at room temperature. No contamination was observed in any of the different storage conditions and temperatures analyzed [
39]. On the other hand, the tolerance and usefulness of autologous PRGF fibrin membrane (mPRGF) in ophthalmology have been evaluated as an adjuvant to nonpenetrating deep sclerectomy [
40] or ocular surface disorders [
41] with positive results. This study aims to provide information about the safety and efficacy of mPRGF as a graft for pterygium surgery, compared with CAG and AM grafts.
4. Discussion
Surgical techniques for pterygium treatment have been improved over the years; nowadays, it is necessary to achieve the closure of the tissue defect, avoid recurrence, improve symptoms of the ocular surface, and increase life quality of patients [
8,
13,
14]. To treat ocular defects and reduce the risk of ocular perforation, many techniques have been used in the past, including AMT, tissue adhesives (collagen, fibrin), animal-based tissue patches, limbal stem cell transplants, conjunctival autograft transplants or keratoplasty surgery [
46,
47]. The recurrence rate is the main result obtained in most clinical studies; meanwhile, the efficacy and safety results are evaluated using different surgical techniques. In our study, the primary outcome was pterygium recurrence, a fact that is consistent with the interests of current research [
15].
In recent years, the field of ocular surface tissue regeneration has experienced significant progress. Some examples include the use of tissue replacements and auto-, allo- and xeno-grafts for limbal cell therapy, or pterygium surgery, either alone or in combination with a temporary graft such as an AM [
48,
49,
50]. These grafts are not always useful, mainly due to the imbalance between demand and tissue availability and the immunological response between the donor tissue and the host [
49,
50]. Moreover, the use of allogeneic fibrin glues may potentially present certain biosafety risks, in the case of the AM, these risks will be enhanced due to its also allogeneic origin as one of its main disadvantages, along with the requirement of a tissue bank. Accordingly, using a safe and effective autologous tissue as a graft would be highly desirable, avoiding the risk of viral or prion transmission. In this sense, mPRGF provides a fibrin scaffold used as a regenerative and physical support membrane in many ocular defects. PRGF technology has a standardized protocol that guarantees the reproducibility of the treatment, the availability of direct costs related to its preparation and use, and immediate availability in the surgery room. Furthermore, it is also important to highlight that ePRGF is obtained during the same mPRGF preparation process and can be used as a postsurgical treatment, thus increasing the periodical availability of growth factors [
34].
The main PRGF feature responsible for most of its biological effects is the sustained release of growth factors. However, the absence of leukocytes and antibacterial, anti-inflammatory, and anti-fibrotic activity are also essential characteristics of PRGF [
35,
51]. The growth factor release from the platelet´s alpha granules is mediated by calcium chloride, which activates fibrinogen and is converted to fibrin, and then begins to develop a three-dimensional acellular matrix with high stability [
35,
51]. Moreover, being mPRGF a leukocyte-free formulation potentially avoids faster fibrin degradation kinetics and a more significant proinflammatory response. It has been demonstrated that this mPRGF fibrin matrix retains trapped in the fibrin clot, almost 30% of the amount of growth factors remained trapped after eight days of incubation, for sustained release [
35,
51].
Nonetheless, this sustained release has shown an increment of the proliferation and migration activity of corneal keratocytes and conjunctival fibroblasts and the reduction of the TGF β1–induced myofibroblast differentiation reducing the number of α-SMA positive cells. This inhibition limits the fibrosis pathways, which is especially relevant in the pathogenesis of pterygium and its tissue remodeling [
35,
38,
51,
52]. Several studies have evaluated the potential benefits of mPRGF alone or in combination with other membranes like AM [
34,
41], showing a stable closure of the corneal defect in all patients treated with PRGF with no evidence of infection, inflammation, or pain [
34,
41].
In this study, no differences in BCVA and IOP were observed in the intergroup and intragroup analysis. In the anatomical evaluation, a progressive and sustained decrease in the size (horizontal and vertical) and thickness of the conjunctiva was observed in group B. In a study carried out by Zhang et al. [
53] in 771 healthy subjects, a full conjunctiva thickness of 240.1 ± 29.8 μm was shown. In another study, the progression of the graft thickness in 40 pterygium surgery patients showed a graft thickness of 430 ± 127 μm in the primary pterygium group and 461 ± 178 μm in the recurrent group at one week after surgery and a graft thickness of 109 ± 15 μm and 107 ± 18 μm at month three, postoperatively [
54]. The results obtained in the present study showed similar initial postsurgical thicknesses among the three groups for the graft placement area, most likely due to similar iatrogenic reasons in all procedures. All groups underwent a gradual size and thickness decrease, group B was the first to show graft thickness outcomes similar to those reported in healthy subjects at month 1 [
53]. Moreover, there were significant differences between group B and the other two groups in month 3, with the mPRGF group achieving the lowest graft thickness outcomes. These findings might have been caused by a combined effect of autologous fibrin degradation and conjunctival tissue remodeling. When a fibrin graft is applied for wound healing purposes, it is invaded by surrounding cells, which will produce a new extracellular matrix to replace the fibrin meshwork, and the new tissue formation will be regulated by the gradual degradation of the fibrin clot (fibrinolytic process) [
55]. The use of AS-OCT as a diagnostic and follow-up aid in ocular surface diseases such as pterygium or conjunctival tumors is increasingly common [
56,
57].
During the healing process in the mPRGF and AM groups, it is suggested that the degradation of the fibrin meshwork occurs, leading to a graft tissue replacement. A study carried out by Oscar Gris et al. [
58] established that AM degradation may take a mean of 12.5 days (3 to 34 days). These results are similar to the use of mPRGF for the surgery of ocular surface disorders, in which complete mPRGF reabsorption occurred after a mean of 12.67 days [
41]. Moreover, part of the fibroblast cells will be transformed into myofibroblasts during the wound healing process, favoring epithelial and endothelial cell migration through the graft and promoting wound contraction [
59]. However, the persistence of myofibroblastic cells after wound healing could lead to the development of scarring tissue. Interestingly, it has been demonstrated that PRGF formulations reduce the number of myofibroblasts and modulate their action during wound healing, improving tissue regeneration and avoiding fibrosis formation [
60,
61,
62]. Further studies are needed to determine the optimal graft size and degradation kinetics to avoid the risk of fast degradation that could compromise the pterygium surgery results.
The gold standard for pterygium surgery is excision with conjunctiva autograft, observing a recurrence between 1.9–8%. On the other hand, in a meta-analysis it was found that the graft with an amniotic membrane has greater recurrence (3.7–40.9%) than surgery with a conjunctival autograft (2.6–17.7%) [
63]. In our study, for the Solomon scale, the overall results showed no statistical differences among the three groups at 12 months of follow-up (
p > 0.05), showing that the three surgical techniques are similar in pterygium recurrence rates.
A clinical study with 108 patients comparing the use of platelet-rich fibrin (PRF) grafts and limbal conjunctival autografts (LCA) in pterygium surgery has recently been published, and it was observed that the surgery time was shorter in the PRF group (25.0 ± 4.2 min) than in the LCA group (36.5 ± 6.3 min) (
p < 0.001) [
64]. The use of mPRGF could decrease the surgical time compared to the conjunctival autograft group, since conjunctival dissection is not necessary; we believe that the surgical time would be similar to that of the amniotic membrane group.
In terms of ocular surface symptoms, the mPRGF group showed a higher percentage of improvement in VAS frequency (86.2%) and VAS severity (79.5%) compared to the other treatment groups. Similar results were observed in other studies treating several ocular surface diseases with PRGF, in which improvement of the VAS was demonstrated [
34,
36,
37,
38,
41]. For the OSDI questionnaire, significant improvement was observed in the AM group than in the CAG group. However, no significant differences were showed between AM and mPRGF groups. Regarding the categories of symptoms, the mPRGF group obtained significant improvement (
p < 0.05) in 6 of the twelve categories. However, the AM group only improved in 2 categories (
p < 0.05), and the CAG group did not improve in any. One of the categories that improved with mPRGF treatment was eye pain. Several studies in different medical areas reported pain improvement after using PRGF [
34,
36]. The absence of leukocytes and endocannabinoid-mediated analgesic effects may be two of the main reasons for the pain reduction scores after PRGF treatment [
35,
65].
This study has some limitations, such as the fact that it was carried out at a single center, with a small cohort, and lacks inflammation biomarker measurements. Further studies are needed to determine the optimal surgical approach of mPRGF in graft placement and thickness. The results show that mPRGF is a safe and effective treatment for primary pterygium surgery, which produces an autologous graft in an agile way and contributes to preserve the patient´s healthy conjunctiva.