Phacoemulsification versus extracapsular cataract extraction: a comparative study of cell survival and growth on the human capsular bag in vitro

Posterior capsule opacification: What's in the bag?

Cataract, a clouding of the lens, is the most common cause of blindness in the world. It has a marked impact on the wellbeing and productivity of individuals and has a major economic impact on healthcare providers. The only means of treating cataract is by surgical intervention. A modern cataract operation generates a capsular bag, which comprises a proportion of the anterior capsule and the entire posterior capsule. The bag remains in situ, partitions the aqueous and vitreous humours, and in the majority of cases, houses an intraocular lens (IOL). The production of a capsular bag following surgery permits a free passage of light along the visual axis through the transparent intraocular lens and thin acellular posterior capsule. Lens epithelial cells, however, remain attached to the anterior capsule, and in response to surgical trauma initiate a wound-healing response that ultimately leads to light scatter and a reduction in visual quality known as posterior capsule opacification (PCO). There are two commonly-described forms of PCO: fibrotic and regenerative. Fibrotic PCO follows classically defined fibrotic processes, namely hyperproliferation, matrix contraction, matrix deposition and epithelial cell trans-differentiation to a myofibroblast phenotype. Regenerative PCO is defined by lens fibre cell differentiation events that give rise to Soemmerring's ring and Elschnig's pearls and becomes evident at a later stage than the fibrotic form. Both fibrotic and regenerative forms of PCO contribute to a reduction in visual quality in patients. This review will highlight the wealth of tools available for PCO research, provide insight into our current knowledge of PCO and discuss putative management of PCO from IOL design to pharmacological interventions.

Evaluation of posterior capsule opacification of the Alcon Clareon IOL vs the Alcon Acrysof IOL using a human capsular bag model

BACKGROUND

Posterior capsule opacification (PCO) after cataract surgery is influenced by intraocular lens (IOL) design and material. The following is an ex vivo comparison of PCO between the Clareon vs. the AcrySof IOL in human capsular bags.

METHODS

Twenty cadaver capsular bags from 10 human donors were used, with the novel hydrophobic IOL (Clareon, CNA0T0) being implanted in one eye and the other eye of the same donor receiving the AcrySof IOL (SN60WF) following phacoemulsification cataract surgery. Five capsular bags of 3 donors served as controls without IOL. Cellular growth of lens epithelial cells was photo-documented daily. The primary endpoint was the time until full coverage of the posterior capsule by cells. Furthermore, immunofluorescence staining of capsular bags for the fibrotic markers f-actin, fibronectin, alpha smooth muscle actin, and collagen type 1 were performed.

RESULTS

The new Clareon IOL did not show any disadvantages in terms of days until full cell coverage of the posterior capsule in comparison to the AcrySof (p > 0.99). Both, the Clareon (p = 0.01, 14.8 days) and the AcrySof IOL (p = 0.005, 15.7 days) showed a slower PCO development in comparison to the control (8.6 days). The fibrotic markers f-actin, fibronectin, alpha smooth muscle actin, and collagen type 1 were equally distributed between the two IOLs and differed from the control.

CONCLUSIONS

A comparable performance has been found in the ex vivo formation of PCO between the two IOLs. Long-term clinical studies are necessary to reach final conclusions.

The human capsular bag model of posterior capsule opacification

Posterior capsule opacification (PCO) is the most common complication following cataract surgery and affects millions of patients. PCO is a consequence of surgical injury promoting a wound-healing response. Following surgery, residual lens epithelial cells grow on acellular regions of the lens capsule, including the central posterior capsule. These cells can undergo fibrotic changes, such that cell transdifferentiation to myofibroblasts, matrix deposition and matrix contraction can occur, which contribute to light scatter and the need for further corrective Nd:YAG laser capsulotomy in many patients. It is therefore of great importance to better understand how PCO develops and determine better approaches to manage the condition. To achieve this, experimental systems are required, and many are available to study PCO. While there may be a number of common features associated with PCO in different species, the mechanisms governing the condition can differ. Consequently, where possible, human systems should be employed. The human capsular bag model was established in a laboratory setting on donor eyes. A capsulorhexis is performed to create an opening in the anterior capsule followed by removal of the lens fibre mass. Residual fibre cells can be removed by irrigation/aspiration and if required, an intraocular lens can be implanted. The capsular bag is isolated from the eye and transferred to a dish for culture. The human capsular bag model has played an important role in understanding the biological processes driving PCO and enables evaluation of surgical approaches, IOLs and putative therapeutic agents to better manage PCO.

Dual function of TGFβ in lens epithelial cell fate: implications for secondary cataract

The most common vision-disrupting complication of cataract surgery is posterior capsule opacification (PCO; secondary cataract). PCO is caused by residual lens cells undergoing one of two very different cell fates: either transdifferentiating into myofibroblasts or maturing into lens fiber cells. Although TGFβ has been strongly implicated in lens cell fibrosis, the factors responsible for the latter process have not been identified. We show here for the first time that TGFβ can induce purified primary lens epithelial cells within the same culture to undergo differentiation into either lens fiber cells or myofibroblasts. Marker analysis confirmed that the two cell phenotypes were mutually exclusive. Blocking the p38 kinase pathway, either with direct inhibitors of the p38 MAP kinase or a small-molecule therapeutic that also inhibits the activation of p38, prevented TGFβ from inducing epithelial-myofibroblast transition and cell migration but did not prevent fiber cell differentiation. Rapamycin had the converse effect, linking MTOR signaling to induction of fiber cell differentiation by TGFβ. In addition to providing novel potential therapeutic strategies for PCO, our findings extend the so-called TGFβ paradox, in which TGFβ can induce two disparate cell fates, to a new epithelial disease state.

Effect of femtosecond laser-assisted lens surgery on posterior capsule opacification in the human capsular bag in vitro

PURPOSE

To compare posterior capsule opacification (PCO) by observing lens epithelial cell growth in the human capsular bag in vitro between conventional lens surgery using phacoemulsification (Phaco) technique and femtosecond laser-assisted lens surgery (FLACS).

METHODS

For the in vitro human capsular bag model, 18 cadaver eyes from nine human donors underwent three types of lens surgery. Three groups consisting of six capsular bags were established, that is FLACS, Phaco and extracapsular lens extraction (ECCE). The capsular bag was transferred into equal cell culture conditions after using one of the defined surgical approaches. Cellular growth of lens epithelial cells was observed and photo-documented. The time until full cell-coverage of the capsular bag was measured.

RESULTS

The human capsular bag model can be successfully prepared using FLACS. There was no statistically significant difference in time until cell-coverage of the human donor capsular bag in vitro in all three surgical settings (ECCE versus Phaco p = 0.6; ECCE versus FLACS p = 1.0; Phaco versus FLACS p = 1.0).

CONCLUSIONS

In our in vitro human capsular bag model, we could not observe a statistically significant difference in PCO formation using different surgical approaches of lens extraction. Therefore, PCO formation might not be attributed to the type of surgery. Furthermore, this study shows that FLACS can be used for the human capsular bag model preparation and validates the human capsular bag model for future research.

Experimental models for posterior capsule opacification research

Millions of people worldwide are blinded due to cataract formation. At present the only means of treating a cataract is through surgical intervention. A modern cataract operation involves the creation of an opening in the anterior lens capsule to allow access to the fibre cells, which are then removed. This leaves in place a capsular bag that comprises the remaining anterior capsule and the entire posterior capsule. In most cases, an intraocular lens is implanted into the capsular bag during surgery. This procedure initially generates good visual restoration, but unfortunately, residual lens epithelial cells undergo a wound-healing response invoked by surgery, which in time commonly results in a secondary loss of vision. This condition is known as posterior capsule opacification (PCO) and exhibits classical features of fibrosis, including hyperproliferation, migration, matrix deposition, matrix contraction and transdifferentiation into myofibroblasts. These changes alone can cause visual deterioration, but in a significant number of cases, fibre differentiation is also observed, which gives rise to Soemmering's ring and Elschnig's pearl formation. Elucidating the regulatory factors that govern these events is fundamental in the drive to develop future strategies to prevent or delay visual deterioration resulting from PCO. A range of experimental platforms are available for the study of PCO that range from in vivo animal models to in vitro human cell and tissue culture models. In the current review, we will highlight some of the experimental models used in PCO research and provide examples of key findings that have resulted from these approaches.

Matrix metalloproteinase-9-null mice are resistant to TGF-β-induced anterior subcapsular cataract formation

Epithelial-mesenchymal transition (EMT) is associated with fibrotic diseases in the lens, such as anterior subcapsular cataract (ASC) formation. Often mediated by transforming growth factor (TGF)-β, EMT in the lens involves the transformation of lens epithelial cells into a multilayering of myofibroblasts, which manifest as plaques beneath the lens capsule. TGF-β-induced EMT and ASC have been associated with the up-regulation of two matrix metalloproteinases (MMPs): MMP-2 and MMP-9. The current study used MMP-2 and MMP-9 knockout (KO) mice to further determine their unique roles in TGF-β-induced ASC formation. Adenoviral injection of active TGF-β1 into the anterior chamber of all wild-type and MMP-2 KO mice led to the formation of distinct ASC plaques that were positive for α-smooth muscle actin, a marker of EMT. In contrast, only a small proportion of the MMP-9 KO eyes injected with adenovirus-expressing TGF-β1 exhibited ASC plaques. Isolated lens epithelial explants from wild-type and MMP-2 KO mice that were treated with TGF-β exhibited features indicative of EMT, whereas those from MMP-9 KO mice did not acquire a mesenchymal phenotype. MMP-9 KO mice were further bred onto a TGF-β1 transgenic mouse line that exhibits severe ASC formation, but shows a resistance to ASC formation in the absence of MMP-9. These findings suggest that MMP-9 expression is more critical than MMP-2 in mediating TGF-β-induced ASC formation.

Incidence and risk factors of Nd:YAG capsulotomy after phacoemulsification in non-diabetic and diabetic patients

PURPOSE

To estimate the cumulative incidence and identify the risk factors of posterior capsule opacification (PCO) that required Nd:YAG capsulotomy in non-diabetic and diabetic patients.

METHODS

Retrospective case-note review of 806 consecutive patients that underwent phacoemulsification and intraocular lens (IOL) implantation, 327 (40.6%) of whom were diabetic.

RESULTS

The cumulative incidence of Nd:YAG capsulotomy were 10.6%, 14.8%, 21.2% and 28.6% in non-diabetic patients; and 9%, 9.4%, 15.3% and 5.3% in diabetic patients after 1, 2, 3 and 4 years, respectively. A multivariate Cox regression analysis showed that, over the follow-up period, diabetes mellitus was associated with a decreased risk of Nd:YAG capsulotomy (hazard ratio [HR]=0.69; 95% confidence interval [CI] 0.47-0.99; P=0.047), whereas age of 65 years or younger (HR=1.58; 95% CI 1.09-2.27; P=0.02), polymethylmethacrylate (PMMA) (HR=3.98; 95% CI 1.60-9.95; P=0.003) or plate-haptic silicone IOLs (HR=3.75; 95% CI 1.60-8.80; P=0.002) in comparison with three-piece silicone IOLs, postoperative inflammation (HR=2.62; 95% CI 1.56-4.42; P<0.001) and pars plana vitrectomy (HR=1.85; 95% CI 1.20-2.83; P=0.005) were associated with an increased risk. Subgroup analysis showed that in non-diabetic patients, male gender (HR=1.63; 95% CI 1.04-2.57; P=0.03) was an additional risk factor and in diabetic patients there was no significant association between diabetes type, duration or retinopathy grade and the risk of Nd:YAG capsulotomy.

CONCLUSION

Although diabetes mellitus appears to be associated with a lower long-term incidence and a decreased risk of Nd:YAG capsulotomy, younger age, pars plana vitrectomy, postoperative inflammation, plate-haptic silicone and PMMA IOLs in addition to male gender in non-diabetic patients appear to be associated with a greater risk. Estimation of the incidence and risk factors of PCO should help in patient counselling and to design methods to reduce or prevent its development.

A focus on the human lens in vitro

The lens is a unique organ in that it is avascular and non-innervated, obtaining all nutrients from the aqueous and vitreous humours that bathe the lens. All lenses attempt to achieve the same goal, namely to maintain transparency and focus light on to the retina. However, the mechanisms by which these processes are maintained, or disrupted leading to a loss of transparency, are likely to differ in some cases between animals and humans. To allow comparison to take place, human in vitro models have been developed, ranging from whole organ culture to the generation of human lens cell lines. All have their merits and limitations, but as a whole, they permit extensive studies of lens cell behaviour and function to be carried out. Together, these in vitro methods allow the biological events of the lens to be further understood. Moreover, they could help identify the mechanisms that give rise to cataract and posterior capsule opacification, a problem that occurs following surgery, providing therapeutic targets for their prevention.

Posterior capsule opacification-like changes in rat lens explants cultured with TGFbeta and FGF: effects of cell coverage and regional differences

Following cataract surgery, many patients suffer secondary loss of vision because of posterior capsule opacification (PCO), which arises when residual lens epithelial cells become aberrant and migrate into the light path. Transforming growth factor-beta (TGFbeta)-induced transdifferentiation of lens cells appears to play a key role in this process. Fibroblast growth factor (FGF) may also play a role by promoting the survival of TGFbeta-affected cells and influencing their subsequent behaviour. In the present study, the effects of two different TGFbeta and FGF treatment regimes were compared in rat lens epithelial explants with either low or high initial cell coverage. Explants treated with 50 pg ml(-1) TGFbeta2 and 20 ng ml(-1) FGF-2 sequentially (day 0, day 1) or simultaneously (day 0), then cultured for up to 30 days with FGF, were assessed by light microscopy and immunolocalisation of markers for transdifferentiation (alpha-smooth muscle actin (alphaSMA) and type I collagen) or lens epithelial phenotype (Pax6) and fibre differentiation (beta-crystallin). By day 4, most cells had lost Pax6 reactivity, alphaSMA reactivity was evident, and there were differences between growth factor treatment groups, low and high initial cell coverage explants, and peripheral and central regions of explants. On day 30 of culture, all explants were well populated with cells, irrespective of treatment and initial cell coverage, and exhibited diverse PCO-like morphological changes, with expression of transdifferentiation markers and beta-crystallin in virtually all cells. Such overall resilience to variations in conditions may contribute to the insidious nature of PCO, while factors related to observed early differences between groups may contribute to PCO pleiomorphism.

Effect of surgical technique on in vitro posterior capsule opacification

PURPOSE

To compare the effect of different cataract extraction surgical techniques on residual lens epithelial cell (LEC) density and cell regrowth rates using an in vitro model of posterior capsule opacification (PCO).

SETTING

Comparative Ophthalmology Research Laboratories, North Carolina State University, Raleigh, North Carolina, USA.

METHODS

Lens capsule explants were prepared from freshly enucleated canine globes after extracapsular cataract extraction (ECCE), phacoemulsification, or phacoemulsification followed by capsule vacuuming. Initial cell density on the capsule and cell proliferation were determined by phase contrast microscopy. The effects of the surgical technique on time to confluent growth of the cells across the posterior lens capsule were determined.

RESULTS

Residual cell density on the remaining anterior capsule immediately after lens removal was 31.6% +/- 19.3%, 16.1% +/- 8.9%, and 7.7% +/- 5.7% in the ECCE, phacoemulsification, and phacoemulsification/capsule-vacuuming groups, respectively. Time to confluence (range 5.0 to 6.3 days) was not significantly different among the 3 groups when the lens capsules were cultured in serum-supplemented media. The confluence rate was significantly longer (by approximately 5 to 7 days) in the phacoemulsification/capsule-vacuuming group than in the other 2 groups when the capsules were cultured in serum-free media.

CONCLUSIONS

Phacoemulsification with and without anterior and equatorial capsular vacuuming led to less initial LEC density in the capsular bag than ECCE. However, because cell proliferation rates among the 3 groups were only marginally affected, near 100% removal of LEC at the time of cataract extraction may be necessary to prevent PCO.

Capsular opacification after cataract surgery

Posterior-capsule opacification, by far the most common complication of primary cataract surgery, continues to stimulate important work toward understanding its causes, preventing it, and effectively treating it. Of special note here are a report by Koch and Kohnen that a combination of vitrectomy and posterior capsulorhexis is required to inhibit posterior-capsule opacification in pediatric patients; work by Nishi et al. toward the dream of replacing the cataractous lens with a flexible artificial lens, supported by the natural capsular bag; and methods by Tetz et al. and Pande et al. for precise quantification of posterior-capsule opacification.