Astigamtism Clinical Trial
Official title:
Prospective Comparative Study of the Efficacy, Predictability, Safety and Stability of the STAAR and Alcon Toric Intraocular Lenses Implanted in Asian Eyes During Phacoemulsification Surgery
Several methods have been developed to address and reduce preexisting astigmatism. The refractive results associated with the use of toric IOLs are likely to be more predictable. Other advantages of toric IOL include absence of need for any additional astigmatic correction procedure during cataract surgery, faster visual recovery as well as the reversibility of the procedure. There has not been any trial carried out to evaluate the results of toric IOL in either the investigators local population or in East Asia. Hence, the investigators set out to evaluate and compare the efficacy, safety, predictability and stability of the STAAR and Alcon toric IOL in Asian eyes.
The improvements in surgical technique as well as advances in technology employed during
modern cataract surgery have made it a safe procedure in routine cases. Moving forwards, the
single most pressing challenge facing cataract surgeons in the developed countries today is
the ability to achieve predictable and accurate refractive outcomes. Increasingly, surgeons
and patients alike have come to measure the success of their surgery by the refractive
outcome.
Using partial coherence interferometry has led to more accurate biometry and consequently
more predictable spherical results. This also helped paved the way for satisfactory
correction of presbyopia with multifocal intraocular lenses (IOL).
Astigmatism is also an important component of the refractive equation. Estimates of the
incidence of significant, naturally occurring astigmatism vary widely from 7.5% to 75%1. It
is thought that 3% to 15% of eyes may have 2 or more diopters (D) of astigmatism2. Many
surgeons would agree that astigmatism of greater than 0.5 D will have visual consequences
such as symptoms of ghosting and shadows and they now aim for both spherical and astigmatic
outcomes of +/ - 0.5 D3.
Several methods have been developed to address and reduce preexisting astigmatism. While
corneal relaxing incisions for astigmatic correction can be unpredictable and highly surgeon
dependent, the opposite is true for toric IOLs. The refractive results associated with the
use of toric IOLs are likely to be more predictable as it is independent of corneal wound
healing. The implantation of an IOL is also less surgeon dependent. Other advantages
associated with toric IOLs include the absence of need for any additional astigmatic
correction procedure during cataract surgery, faster visual recovery as well as the
reversibility of the procedure.
Studies have been carried out to evaluate the efficacy and rotational stability of the STAAR
AA4203 TF/TL toric IOL since its FDA approval in November 1998. These trials have only been
carried out on Caucasian eyes and have shown a significant reduction in the pre-existing
refractive astigmatism, improvement of postoperative unaided visual acuity as well as
excellent refractive as well as rotational stability (latter especially so with the TL
model). Ruhswurm et al.6 reviewed 37 eyes with STAAR toric IOL over a mean duration of 20.3
months and found that 78.4% of eyes had residual refractive astigmatism within +/- 1.00 D
and 48.6% within +/- 0.50D. 83.4% of these eyes achieved best-corrected visual acuity of
20/40 or better. Sun et al.7 demonstrated improved unaided visual outcome between eyes with
toric (130 eyes; 86.4% with visual acuity 20/40 or better) and spherical IOL (51 eyes; 76%
with visual acuity 20/40 or better). Similar results have also been obtained with the Alcon
toric IOL. In Horn et al.9, the clinical trial results of 250 patients with Alcon toric IOL
have been described, demonstrating good unaided visual acuity of 20/25 or better at the
6-month postoperative visit (66% of patients with unilateral toric implant and 97% with
bilateral implants). 97% of these toric patients achieved spectacle freedom for distance
vision, compared to 50% of control patients. It is well documented that the rotational
stability of these toric IOL is crucial to the efficacy of astigmatic reduction. Theoretical
calculations show that an estimated 1/3 of astigmatic correction is lost if the IOL is
rotated by 10 degrees off axis, as well as potential worsening of preoperative astigmatism
in the presence of more than 30 degrees of off-axis rotation. Toric IOL stability is
dependent on multiple factors, one of which is the capsular bag size. Vass et al.11 showed a
good correlation between axial length and capsular bag diameter (CBD), especially so in eyes
with axial lengths of less than 25.0mm. On the other hand, highly myopic eyes are not shown
to have a greater risk of having a larger CBD than those with normal axial lengths. A
formula derived to calculate a predicted CBD (CBDp) showed that values greater than 10.3mm
are likely to have existing larger CBD and hence benefit from implantation of longer toric
IOL due to better centration.
To date, there has not been any trial carried out to evaluate the results of toric IOL in
either our local population or in East Asia. There has also not been any prospective
clinical trial conducted to compare the difference (if any) between a plate-haptic and
loop-haptic toric IOL. Hence, we set out to evaluate and compare the efficacy, safety,
predictability and stability of the STAAR and Alcon toric IOL in Asian eyes.
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Allocation: Randomized, Endpoint Classification: Safety/Efficacy Study, Intervention Model: Parallel Assignment, Masking: Single Blind, Primary Purpose: Treatment