Currently, vitrectomy is commonly combined with cataract surgery in clinical practice for the treatment of both RRD and cataracts. The refractive changes following surgery play an important role in the quality of vision. The size of vitrectomy incision, suturing of vitrectomy ports, intraocular fillers (type, nature, density, and volume), and the size and suturing of cataract incision influences the postoperative refractive status of the eyeball to varying degrees.
Pentacam is an accurate modality for measuring corneal changes . In the past, the A-scan ultrasound was used to measure the distance between the anterior surface of the cornea and the inner limiting membrane of the retina to measure AL. The AL Scan is an optical biometer that measures the distance between the anterior surface of the cornea and the retinal pigment epithelium with a superior accuracy compared to the A-scan ultrasound. Since the AL Scan is based on the principle of partial coherence interferometry, the results were not affected by silicone oil, and there was good agreement between the AL Scan and IOLMaster in measuring the AL [3-5]. Thus, we justify the use of Pentacam for measuring corneal astigmatism and ACD and AL Scan for measuring AL.
Mechanisms in post-surgical corneal astigmatism
Postoperative corneal astigmatism that appears following vitrectomy arises mainly from the scleral three-port incision and the physical traction of suturing. These factors inconsistently change the corneal refractive power in every diameter, resulting in the formation of corneal astigmatism [6-8]. With the development of minimally invasive vitrectomy techniques in recent years, the diameter of the vitreous-cutting head has become thinner, making the surgery “sutureless.” Some groups reported that sutureless 23G and 25G vitrectomy eliminated the inconsistent changes of corneal refractive power in every diameter . Park DH et al. found that during 23G TSV with silicone oil filling, the cornea could still undergo a transient change in refraction resulting mainly from the tension and filling volume of silicone oil .
In combined surgery (phacovitrectomy), the main reason for corneal astigmatism was cataract surgery, especially the thermal injury caused by phacoemulsification and the sutures after operation . Electrocoagulation of bleeding vessels could cause shrinkage of scleral collagen fibers, resulting in wound contraction. This not only changed the corneal curvature but also led to a steeper cornea at the corresponding site and thus corneal astigmatism . Sayed KM et al. described that a transscleral incision for phacoemulsification was associated with no significant effect on corneal astigmatism, especially in the absence of suturing, and only the corneal surface irregularity index increased in the early postoperative period, which usually returned to baseline after 6 months .
A larger and longer cataract surgical incision destroyed the structure of the corneal dome more severely, leading to a more significant postoperative corneal astigmatism . Translimbal incision more likely causes corneal astigmatism than transscleral tunnel incision , and tighter sutures caused noticeable changes in corneal astigmatism .
In the current study, we eliminated the inconsistent changes of corneal refractive power in every diameter during 23G TSV and 25G TSV by making the procedure sutureless. This explains why both the anterior and posterior corneal astigmatism did not change significantly following surgery. In the phacovitrectomy group, the anterior and posterior astigmatism increased most obviously at post-surgical week 1 and returned to baseline by 3 months. The possible reasons being the thermal injury of electrocoagulation, and physical traction exerted by the sutures. Our findings were similar to previous studies that reported cataract surgery as the main cause of corneal surface astigmatism in combined phacovitrectomy [11, 12].
Mechanisms influencing the post-surgical axial length
During phacovitrectomy, the change in AL is mainly caused by vitrectomy since cataract surgery had no significant effect on AL . The suturing of the three-port incision shortens the scleral length in the axial direction, which in turn results in a shortening of the AL, with no apparent relationship to the properties of the fillers . Federman JL et al. believed that the reason for the postoperative increase in AL might be measurement errors. They used an A-scan ultrasound that measured the distance between the anterior surface of the cornea and the inner limiting membrane of the retina. Thus, the measurement was smaller than the real length before the recovery of the retina. But when the retina was reset, the AL measurement tended to be accurate, but longer . Huang C et al. used the IOL master and similarly found that the AL increased after combined phacovitrectomy. However, they speculated that the physical property of fillers was different from that of the vitreous and could not provide sufficient support for the eyeball. With simultaneous lens removal, the eyeball deformed and tended to elongate [20-22]. Furthermore, the gas-liquid exchange and the transient high intraocular pressure (IOP) during vitrectomy could also contribute to axial elongation . Silicone oil, a commonly used filler, is a viscous substance, which could redistribute the liquid density of the vitreous cavity, producing a certain tension that increases the eyeball’s AL . Certain groups postulated that the combination of low preoperative IOP along with postoperative normal or high IOP could result in a long eyeball .
The AL changes were positively correlated with the viscosity and filling amount of silicone oil [23, 25]. Also, the IOL does not provide enough brace to the eye compared to the original lens, so it was necessary to increase the amount of silicone oil to support the eyewall to some extent. This effect was transient and usually returns to baseline between a week to a month following surgery [20-22], after which period, the AL tended to be stable .
In our study, the AL of three groups increased the most in week 1, and these changes lasted until 3 months after surgery. We believe the main reasons for this AL increase were due to the tension exerted by silicone oil on the eyewall, as well as the transient high IOP of gas-liquid exchange. We eliminated the influence of suturing and measurement error by making the surgery sutureless and by using the AL Scan for AL measurements. There was a minimal increase in AL at 3 months compared to 1 month, possibly due to head position, inflammation, and IOP fluctuations. The phacovitrectomy group showed the most AL change at week 1, probably due to the temporary effect of silicon oil tension and lens removal.
Mechanisms influencing post-surgical anterior chamber depth
During vitrectomy, the ACD decreases due to the tension and viscosity of silicone oil and prone positioning, which displaces the iris-lens septum forwards [20, 27]. Surgical trauma and scleral suturing could lead to ciliary body edema and effusion, resulting in a shallow anterior chamber . ACD is also positively correlated with IOP, which means that a low postoperative IOP might result in a shallow anterior chamber .
In contrast, the ACD tends to increase during phacovitrectomy. The crystalline lens is disc-shaped with an anteroposterior diameter of about 5 mm, whereas the thickness of the IOL is approximately 1 mm, resulting in insufficient anterior segment support following cataract surgery. Also, the anterior segment pressure is eliminated after lens removal, which increases ACD. The effect of vitrectomy-related factors is relatively small compared to cataract surgery-related factors, and so the ACD tends to increase in combined phacovitrectomy .
When using silicone oil, a more substantial filling volume has a greater influence on ACD . Scleral suturing is more likely to cause a shallower anterior chamber than a sutureless incision . A large scleral incision that is sutured tight causes a more pronounced ciliary body edema and has a more considerable influence on ACD [20, 31]. ACD showed the most obvious change during the first post-surgical week. With wound healing and the stabilization of the filling and IOL, the ACD gradually returned to baseline by 2 to 4 months after surgery [1, 32].
In our study, the ACD decreased the most during the first week following 23G TSV and 25G TSV. The tension and viscosity of silicone oil and the prone patient positioning that promotes anterior displacement of the iris-lens septum could have caused the observed effect. Some degree of ciliary body edema and effusion might have influenced the final ACD. We found that with phacovitrectomy, the ACD increased due to IOL implantation and vitrectomy, as discussed previously. This change was most obvious at post-surgical week 1 and lasted until 3 months. ACD changes were more obvious in the phacovitrectomy group than the 23G TSV and 25G TSV groups. Besides, statistically significant differences between 23G TSV and 25G TSV groups were more pronounced in the 25G TSV group. This might be due to the small sample size of the 23G TSV group.
This study has a few limitations. First, this was a retrospective study with a small sample size per group. Second, results were obtained for only 3 months of follow up after the surgery. Third, since all measurements were performed before the removal of oil and sutures, the influence of these factors on the refractive changes cannot be ruled out. Nevertheless, this study compared the refractive changes between some of the widely practiced new surgical techniques. We recommend large-sample, prospective studies with longer follow-up to accurately determine the changes in refractive error following vitrectomy versus phacovitrectomy.