There are several therapeutic options for treating LSCD. Restoring of the corneal epithelium depends on the presence of remaining limbal epithelial stem cells (LESC). In cases with no remaining stem cell reserves the cornea must be reseeded with new LECS.
In recent years, the possibility of ex vivo cultivation of limbal stem cells and subsequent transplantation has been established, which is known as cultivated limbal epithelial transplantation (CLET). This process was first described in 1997 . In CLET, small limbal biopsy (2 x 2 mm) is performed either allogenously or autologously. This way, the limbal epithelial stem cells are cultivated and expanded on human cryopreserved amniotic membrane or on a fibrin-based substrate. Two different cultivation techniques have so far been developed. In the first technique, the explant technique, the removed cells are cultured in vitro on human amniotic membrane for 2–3 weeks. After this, the cells are re-transplanted onto the diseased eye . In the second technique, the suspension method, the limbal stem cells are enzymatically isolated from the tissue. These cells are cultured on fibrin substrate carriers or on an amniotic membrane. After confluence, cells are retransplanted onto the diseased eye . The CLET procedure is particularly suitable for LSCD caused by burns (including chemical burns), which are the most common cause of the disease . In 2015, the CLET technique was approved for use in the manufacture of a stem cell-based drug in the European Union named Holoclar. The drug contains autologous human corneal epithelial cells with about 3.5% limbal stem cells on a fibrin membrane and is only approved for the therapy of unilateral and bilateral LSCD caused by burns or chemical burns. This therapy requires an intact limbal area of 1-2 mm2 from which a biopsy is taken, and the cells obtained in this way are expanded ex vivo. The cells are expanded until the optimal cell count of 79 000 to 315 000 cells per cm2 is reached. The membrane with the expanded cells is applied to the defect area. This method minimizes the risk of transplant rejection and iatrogenic LSCD. In a multicenter, uncontrolled, retrospective case series cohort study with 106 patients, a positive result was found in 72.1% of the cases. People's visual acuity has been improved and the number of people with symptoms has been decreased .
In 2012, Sangwan et al. developed a procedure called simple limbal epithelial transplantation (SLET) . In this surgical technique, a 2 x 2-mm tissue sample is taken from the limbal region of a healthy eye and divided into many small pieces. These pieces are applied by fibrin glue onto a fresh amniotic membrane, which is attached to the diseased eye in advance. The small pieces are expanded in vivo. The ideal patient for this procedure has unilateral LSCD. Postoperatively, a contact lens is placed onto the eye to protect the transplanted grafts and antibiotic and steroid-containing eye drops are used. Proliferation of the corneal epithelium starting from the transplants becomes visible after the second postoperative day and epithelialization of the cornea is complete after 4-12 days . A 2020 review summarized 404 cases of SLET. A success rate of 83 % (stable corneal epithelium and absence of vascularization) was obtained as well as an improvement in visual acuity in 69% of cases. The most common postoperative complications were focal recurrence of LSCD, progressive conjunctivalization, progressive symblepharon, and keratitis. Also, a risk of iatrogenic LSCD was described .
All these described methods require one eye with a healthy limbal region or at least an undamaged part of the limbal region to obtain a sample of the limbal stem cells. So, these methods are not suitable for patients with both-sided LSCD. One important advantage of the PALT procedure is that no separate donor eye for the transplantation of the limbal region is necessary. The transplantation is allogeneic and the limbal stem cells are residual tissue of the keratoplasty from the same donor. So PALT can be carried out in all patients with LSCD. Even patients with a totally damaged limbal region in both eyes can be included in this procedure. Because of this, there is no risk of iatrogenic LSCD. Due to the iatrogenic damage of the limbus during the biopsy when SLET or CLET is performed, there is always a risk of iatrogenic LSCD in the biopted eye.
Another advantage of PALT is that there are no special requirements necessary for cultivation. To perform CLET, a specialized laboratory and corresponding laboratory staff are crucial. Also, the Holoclar procedure, which is quite similar to CLET, needs a specialized team for culturing and transportation of the obtained limbal stem cells.
In contrast to the already mentioned procedures, PALT includes keratoplasty. Of course perforating keratoplasty is more invasive than a biopsy or stem cell transplantation alone. Nevertheless, patients who undergo CLET/Holoclar® or SLET often need more surgeries than a surgery for implantation of limbal tissue. Therefore, additional keratoplasty is also often necessary after the biopsy has been performed to get increased BCVA. The reason for is to remove the stromal-located corneal scars or reduce clouding, which often remain even if there are limbal stem cells again. So to get increased BCVA, perforating keratoplasty is performed.
Some other surgical techniques also include keratoplasty. One method is the penetrating limbo-keratoplasty. Here a graft with a diameter of 7.7-10.0 mm is transplanted with a proportion of 40% limbal tissue at the graft´s circumference [10,11]. The long-term result over five years showed that 14% of the untyped grafts remained clear. This shows that there is probably a higher risk of rejection, because a high immunogenic tissue is transplanted. To reduce this risk, higher-dosed immunosuppression is perhaps necessary. The outcome is much better when HLA-typed transplants are used. Here, 41–65 % of the grafts are transparent after five years depending on the number of mismatches of the HLA status . This leads to the finding that the immunogenic reaction when using a HLA-typed graft is lower. The disadvantage of course is that the waiting period for a suitable transplant is much longer than for an untyped graft.
Viestenz et al. reported on a patient with bilateral corneal burn and corneal perforation. They placed a 15-mm corneoscleral graft over the anterior segment without removing the central cornea. After 2–3 weeks, a 23-gauge vitrectomy was performed to remove the collagenolytic central recipient cornea. The corneal graft remained transparent for 3–5 years. The allogeneic stem cells were adjacent to the cornea  and possibly created a new limbal region. Nevertheless, rejection happened after 3–5 years.
These two surgical techniques show that the limbal region is highly immunogenic and rejection often cannot be avoided. When using the PALT technique, the explanted transplants are placed not in the limbal region but further to the center of the cornea. This distance to the immunogenic limbal region of the patients may reduce the risk of rejection. Of course, there are no long-term results for the PALT technique in terms of the rejection rate, but the first results have so far shown no rejection.
Nevertheless, immunosuppression is necessary because allogenic tissue is used for keratoplasty and PALT. For this, prednisolone was used as well as topic and systemic therapy. Prednisolone has proved effective in preventing rejection , although there are well-known side effects of immunosuppression with corticosteroids like development of cataract, glaucoma, arterial hypertension, increased blood sugar, weight gain, mental abnormalities, and much more. So the indication for long-term therapy with corticosteroids has to be evaluated carefully.
Another surgical method that includes PK for LSCD treatment is the so-called SCET (simple conjunctival epithelial transplantation), which was developed in 2020 by Sakimoto et al. They suggest that transplantation of autologous conjunctival cells is a promising therapeutic procedure in patients with LSCD. The idea of this procedure is based on the SLET technique. To perform a SCET, the pannus is removed and, afterwards, PK or lamellar keratoplasty is carried out. Then AMT is placed over the ocular surface. Once a 4 x 3-mm piece of the temporal superior bulbar conjunctiva has been separated, 10–15 pieces of this conjunctival tissue are fixed with fibrin glue on the amniotic membrane. The amniotic membrane is then covered with a contact lens. Four patients with LSCD originating from different causes underwent this surgical procedure and showed increased visual acuity and stable ocular surfaces .
In contrast to this procedure, PALT uses allogenic limbal tissue. It is likely that PALT pieces also contain conjunctival tissue because of the localization of the limbus. In the SCET procedure, conjunctival cells can accept corneal epithelial properties, resulting in a transparent cornea and a stable ocular surface. This has already been shown in two other publications [16,17]. It leads to the assumption that the conjunctival part of the PALT also has an impact on the transparency and stability of the transplanted corneal tissue. However, the mechanism behind how the conjunctival tissue suddenly changes to the corneal epithelial cells is still unknown.
Another surgical method whereby the conjunctival tissue is transplanted is the conjunctival limbal autografting (CLAU), which was developed in 1989. Here, two conjunctival-limbal autotransplants (120 degrees of the corneal circumference) are transferred to the affected eye . The improvement in visual acuity is 25–100% [19,20]. This leads to the assumption that the transplanted conjunctival tissue gives a positive effect to the outcome. A disadvantage of this method is, as already mentioned above, a high risk of iatrogenic LSCD because a large quite part of the limbus of the unaffected eye is explanted.
Patients who undergo PALT require just one surgery if no postoperative complications develop. The exact functionality of how the PALT pieces integrate into the tissue and how the limbal stem cells from the transplant reach the limbus has not yet been clarified. Our study shows that patients who underwent PALT surgery seem to create a new intact limbal region. This is indicated by the lack of corneal vascularization and the intact postoperative epithelium. It is not clear whether the limbal stem cells migrate from the PALT transplant in the direction of the damaged limbus and implant there or whether a new limbal region is created in the area of the PALT. This study only includes 14 patients and the median follow up period was 12 months. Therefore, further investigations are necessary to prove the long-term results of PK combined with PALT.