CORNEAL GP contact lenses are often used to rehabilitate vision for patients who have irregular corneas, including for keratoconus management (Zadnik et al, 1998). However, intolerance to GPs may occur secondary to epithelial insult and/or suboptimal fit due to high corneal irregularity.

Piggyback or tandem contact lens systems, typically composed of a soft contact lens (SCL) carrier acting as a “bandage” under the GP lens, have been used to improve comfort by protecting the cornea and stabilizing the GP fit (Kok and van Mil, 1993; Smith and Carrell, 2008). Modern applications include tandem use of soft lenses with scleral contact lenses (SLs) or “reverse” piggyback systems with the application of the soft lens on top of GPs to prevent dislodging during sports (Ensley, 2013) or over SLs to improve surface wettability (Bliss et al, 2023).

POWER CONSIDERATIONS

One study evaluated the impact of various soft lens powers on a traditional piggyback system (soft spherical carrier under a corneal GP lens) in normal corneas in vivo (Michaud et al, 2013). In addition to the optics, they assessed soft lens power effects on GP lens centration. The researchers found that the soft lens power contribution to the optics in a piggyback system were 21.3% with the –6.00D lens and 20.6% with +6.00D lens.

This agreed with previous theoretical reports that an average of 20% of the soft lens power is transferred to the piggyback system when using low- to moderate-power carriers, ranging from –3.00D to +3.00D (Woo and Weissman, 2011). The +0.50D carrier did not contribute any power to the system. Plus-powered lenses (+0.50 and +6.00D) were shown to enhance initial GP lens centration compared to the negative-powered lens (–6.00D), which required GP lens refitting to achieve acceptable centration in 30% of subjects compared to 7% for plus-powered lenses.

Regarding soft lens power considerations in piggyback systems fit on patients who had keratoconus, Romero-Jiménez and colleagues (2013 and 2015) conducted two studies. In the first, they looked at 30 eyes diagnosed with keratoconus and obtained corneal topography and higher-order aberration (HOA) data on the naked eye and with variable soft lens (senofilcon A) powers.

They found that minus- and plano-powered soft lenses created a flattening effect on mean central keratometry values as compared to the naked eye, whereas plus-powered lenses did not result in significant changes. There were no significant differences in HOA root-mean-square (RMS) and spherical-like aberration, but statistical differences were found in coma-like and secondary astigmatism.

The authors concluded that for patients who have keratoconus, a minus-powered lens carrier may theoretically be more suitable for piggyback systems by providing a flatter anterior surface to fit the GP. This conclusion contradicts commonly accepted practice of using low-plus soft lens carriers.

In their second study, Romero-Jiménez and colleagues fit 30 eyes diagnosed with keratoconus in soft lenses (senofilcon A) of varied powers with corneal GP lenses over. No clinically significant differences were found on GP centration or movement between all soft lens powers fit. Like their previous study, mean central keratometry flattened with both minus-powered lenses, did not change with the +3.00D, and steepened with the +6.00D soft lens, as compared to the naked eye.

With the GP lens over the soft lenses of various powers, over-refraction did not change significantly. However, the back optic zone radii (BOZR) of the GPs that provided optimal fits were steeper with both plus-powered lenses compared to both minus-powered soft lenses.

The estimated GP final power was significantly increased with both plus-powered lenses compared to both minus-powered lenses. Best-corrected visual acuity (VA) did not change significantly between the soft lens powers used. The use of minus-powered soft lenses in piggyback systems for keratoconus provides a flatter anterior surface and reduces GP lens powers without affecting VA.

OXYGEN TRANSMISSION CONCERNS

Because piggyback systems comprise of two separate contact lenses on the eye, there have been concerns about whether oxygen transmissibility (Dk/t) through tandem lenses are sufficient to avoid hypoxic stress to the cornea. Theoretical calculations predicting Dk/t of piggyback systems show that oxygen supply is adequate in open-eye conditions (daily wear) when both the soft and GP lenses have an oxygen permeability of more than 60 Fatt Dk units (Weissman and Ye, 2006).

One group measured Dk/t of 66 piggyback systems in vitro including combinations of hydroxyethyl methacrylate (HEMA) and silicone hydrogel soft lens carriers with polymethyl methacrylate (PMMA) and GP lenses (López-Alemany, 2006). They concluded that PMMA lenses should not be used in piggyback systems, and if using low-Dk GP lenses, the soft lens carrier should be a silicone hydrogel material.

In scleral lenses, oxygen to the cornea is compromised due to increased thickness from the fluid reservoir, increased lens thickness, and reduced tear exchange. In a study looking at a reverse piggyback system, a standard non-fenestrated scleral lens (hexafocon A, Dk 100) with a silicone hydrogel lens over (delefilcon A, Dk 140) was fit on 10 subjects with normal corneas (Bliss et al, 2023). After 90 minutes of wear time, central corneal thickness and fluid reservoir thickness were measured with optical coherence tomography. The reverse piggyback system did not induce a clinically significant difference in corneal edema when compared to the SL alone.

COMFORT AND VISUAL ACUITY

Piggyback systems comprised of a silicone hydrogel lens (Dk/t 110) with corneal GPs of hyper-Dk (Dk/t 95.9) and high-Dk (Dk/t 58.8) were fit on 22 patients (28 eyes) with moderate and severe keratoconus (Xie et al, 2011). Compared to the corneal GP alone, VA with the piggyback significantly improved and 90% of cases had increased comfort. Few cases showed slight conjunctival hyperemia and corneal staining.

In a different study, 16 patients (29 eyes) who had keratoconus, who were unable to wear corneal GP lenses alone, were fit with a tandem system of hyper-Dk silicone hydrogel (lotrafilcon A, Dk/t 150) and corneal GP lens (Dk/t 100) over (Sengor et al, 2011). Improvement in VA occurred in 89.7% of eyes with the piggyback system compared to the corneal GP alone; the rest (10.3% of eyes) had no change in VA. After limited wear time of the piggyback system (mean six months), all but two patients were able to tolerate corneal GP lenses alone.

WHAT CAN BE LEARNED

Several publications describe novel applications of these systems. Cromelin and colleagues (2017) presented a case series of four children fit with piggyback lenses after penetrating corneal trauma. All patients were fit with GPs first, but the tandem use of a soft lens under the GP improved lens tolerance and VA.

A soft lens can also be used to improve comfort in scleral lens patients in both the pediatric and adult population (Murphy et al, 2021; Porcar et al, 2017). In addition to vision rehabilitation, piggyback lenses can serve a functional purpose.

One case report outlines using a cosmetic soft lens as occlusion therapy to prevent amblyopia in a patient with familial dysautonomia who wears scleral lenses (Michaud and Carrasquillo, 2010). Another describes the application of a photochromic soft lens and GP tandem system to manage traumatic aniridia (Miyoshi et al, 2021). Finally, most piggyback systems use a spherical soft lens, but toric soft lenses have also been employed in tandem with GPs to correct for residual astigmatism (Lindsay et al, 2013).

CONCLUDING REMARKS

Piggyback lens systems can be used to manage a variety of conditions by improving comfort, VA, and function. The most common approach uses a low-plus-powered soft silicone hydrogel lens with a GP lens over.

However, for patients who have keratoconus, a minus-powered soft lens carrier may be considered. When using higher-Dk materials for both lenses in the tandem system, oxygen transmissibility should be sufficient to avoid hypoxic changes to the cornea, even when using scleral lenses rather than GPs. CLS

References

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