Dr Elinor Jones

Introduction

Cataract surgery is the most commonly performed procedure within the National Health Service (1). In most cases, the intended post-operative visual outcome is emmetropia, which is significantly influenced by the intraocular lens (IOL) power calculated and selected by the surgeon (2). The capsular bag is the preferred position for IOL implantation due to its stability and proximity to the position of the native lens (3). The most common intraoperative complication of cataract surgery is posterior capsular (PC) rupture (1). This is one instance in which placement of the IOL in the ciliary sulcus, rather than the capsular bag, may be necessary, in addition to other situations such as cases of zonular laxity (3). The Royal College of Ophthalmologists state in their cataract guidance that back-up lenses should be available in theatre in case non-capsular bag fixation is required (4).

Anterior displacement of the lens increases its effective power and therefore its refractive effect (5). This means the power needs to be reduced when inserting an IOL into the sulcus, rather than the capsular bag, to result in the same refractive outcome for the patient and to avoid a myopic shift from the predicted post-operative refraction (6).

Adjusting IOL Power to Account for Placement in the Sulcus

A lack of consensus on appropriate reduction in power has led various authors over time to publish suggested guidance on how to adjust lens power to account for its placement in the sulcus, rather than the capsular bag.

Dubey and colleagues did a retrospective review of patients who had a sulcus lens inserted following a PC rupture, where the lens power was reduced between 0.5-1.0D, at the surgeon’s discretion, to account for sulcus placement. They found that the degree of post-operative myopia was lower when the reduction was 1.0D rather than 0.5D; this postoperative myopic shift was greater for eyes with a shorter axial length and stronger IOL powers. Their work concluded with guidance to adjust IOL power between 0.5-2.0D depending on the capsular-bag power and axial length of the eye (7).

Bayramlar et al suggested a power reduction between 1.25 and 1.5D after ascertaining that implantation of a sulcus IOL with the same power as planned for the capsular bag gave, on average, a 1.0D myopic shift in their patients (6). Suto and colleagues’ work suggested a 1.0D reduction (8) although this author later adjusted their approach, recognising that a greater reduction in power is needed for eyes with a shorter axial length and stronger IOL, versus eyes with a longer axial length and weaker IOL (9). Other authors have suggested a 5% reduction in power (10).

Theoretical guidance from Doctor Hill (11), a United States surgeon, is available and referenced in the literature (7, 12) and refers to IOL power as the main factor to consider in the calculation. It is unclear whether this guidance has been studied through clinical trials.

The “Rule of 9s”

An approximation method for adjusting lens power for the sulcus is the “Rule of 9s”, put forward by the ‘Cataract Coach’ (Figure 1) (13). The first step is to calculate what the power would be if the sulcus model lens was placed in the capsular bag. This may be different to the originally chosen IOL power, if the A-constant is different between the two lens models. A further reduction in power may then be necessary to account for the lens placement in the sulcus, the extent of which depends on the original in-the-bag power. This is in keeping with literature referenced above where eyes with a shorter axial length and stronger lens requirement need a greater reduction in power than in eyes of a longer axial length and lower lens power.

Conclusion

Whilst the “Rule of 9s” approach is referenced in the literature (3), it is unclear whether it has been compared to alternative approaches in clinical research. It may be a suitable and systematic approach to sulcus lens power adjustment. Despite this, confusion can exist regarding the sufficient power reduction necessary for sulcus lens placement (7). Current UK guidance states that on average, power should be reduced by 0.5-1.0D relative to capsular-bag power, when placing a sulcus lens (4). Clinical data comparing different methods would be useful to determine consensus approaches that can be published within national guidelines.

References

1. Royal College of Ophthalmologists – National Ophthalmology Database Audit. Year 6 annual report – the fifth prospective report of the national ophthalmology database audit [Internet]. 2022 [cited 2022 Oct 26]. Available from: https://www.nodaudit.org.uk/u/docs/20/pxawiazimp/NOD%20Audit%20Full%20Annual%20Report%202022..pdf
2. Abdelghany AA, Alio JL. Surgical options for correction of refractive error following cataract surgery. Eye and Vision. 2014 Dec;1(1):1-7.
3. Mehta R, Aref AA. Intraocular lens implantation in the ciliary sulcus: challenges and risks. Clinical ophthalmology (Auckland, NZ). 2019;13:2317.
4. Royal College of Ophthalmologists. Quality Standard – Correct IOL implantation in cataract surgery. 2018 [cited 2022 Oct 26]. Available from: Correct-IOL-implantation-in-cataract-surgery-quality-standard-1.pdf (rcophth.ac.uk)
5. Olsen T. Calculation of intraocular lens power: a review. Acta Ophthalmologica Scandinavica. 2007 Aug;85(5):472-85.
6. Bayramlar H, Hepsen IF, Yilmaz H. Myopic shift from the predicted refraction after sulcus fixation of PMMA posterior chamber intraocular lenses. Canadian journal of ophthalmology. 2006 Jan 1;41(1):78-82.
7. Dubey R, Birchall W, Grigg J. Improved refractive outcome for ciliary sulcus-implanted intraocular lenses. Ophthalmology. 2012 Feb 1;119(2):261-5.
8. Suto C, Hori S, Fukuyama E, Akura J. Adjusting intraocular lens power for sulcus fixation. Journal of Cataract & Refractive Surgery. 2003 Oct 1;29(10):1913-7.
9. Suto C. Sliding scale of IOL power for sulcus fixation using computer simulation. Journal of Cataract & Refractive Surgery. 2004 Nov 1;30(11):2452-4.
10. Cartwright NE, Aristodemou P, Sparrow JM, Johnston RL. Adjustment of intraocular lens power for sulcus implantation. Journal of Cataract & Refractive Surgery. 2011 Apr 1;37(4):798-9.
11. East Valley Ophthalmology. Calculating bag vs. sulcus IOL power [Internet]. [cited 2022 Oct 26]. Available from: https://doctor-hill.com/iol-power-calculations/bag-vs-sulcus/
12. Spokes DM, Norris JH, Ball JL. Refinement of lens power selection for sulcus placement of intraocular lens. Journal of Cataract & Refractive Surgery. 2010 Aug 1;36(8):1436-7.
13. Devgan U. The rule of nines for sulcus IOL power [Internet]. 2018 June 25 [cited 2022 Oct 26]. Available from: https://cataractcoach.com/2018/06/25/how-to-determine-sulcus-iol-power-and-ac-iol-power/