Effect of twisting of intravitreal injections on ocular bio-mechanics: a novel insight to ocular surgery

Abstract

Although intravitreal (IVT) injections provide several advantages in treating posterior segment eye diseases, several associated challenges remain. The current study uses the finite element method (FEM) to highlight the effect of IVT needle rotation along the insertion axis on the reaction forces and deformation inside the eye. A comparison of the reaction forces at the eye’s key locations has been made with and without rotation. In addition, a sensitivity analysis of various parameters, such as the needle’s angular speed, insertion location, angle, gauge, shape, and intraocular pressure (IOP), has been carried out to delineate the individual parameter’s effect on reaction forces during rotation. Results demonstrate that twisting the needle significantly reduces the reaction forces at the penetration location and throughout the needle travel length, resulting in quicker penetration. Moreover, ocular biomechanics are influenced by needle insertion location, angle, shape, size, and IOP. The reaction forces incurred by the patient may be reduced by using a bevel needle of the higher gauge when inserted close to the normal of the local scleral surface toward the orra serrata within the Pars Plana region. Results obtained from the current study can deepen the understanding of the twisting needle’s interaction with the ocular tissue.

Appendices

Appendix A

1.1 Effect of different posterior layers on reaction forces

Fig. 15

Variation of reaction forces due to combined effect of translation and rotation for different layers at Pars Plana using $23-G$ bevel shaped needle

Injections through the highly vascular choroid are significantly prone to choroidal hemorrhages if they lie in the needle insertion path (Meyer et al. 2010; Javey et al. 2009). However, A mock-up simulation is performed in which two posterior layers, choroid and sclera, are geometrically separated, having a thickness of 0.3 $\textrm{mm}$ and 0.16 $\textrm{mm}$, respectively (Gökmen et al. 2017). The effect of different posterior layers on the reaction forces is compared with the scenario in which the choroid and sclera are considered a single layer for both cases with and without needle rotation. Apart from different layer thicknesses, the rest of the parameters are kept the same as in the baseline study (Table 3). Figure 15 shows an insignificant change in the reaction force along the needle penetration length when incorporating different posterior layers compared to a single layer.

Table 16 Comparison of maximum reaction forces for different layers with translation (3 $\textrm{mm}/\textrm{sec}$), and translation (3 $\textrm{mm}/sec$) clubbed with rotation (10 $\textrm{RPM}$) for $23-G$ bevel needle at the penetration point

Table 17 Comparison of averaged reaction forces for different layers with translation (3 $\textrm{mm}/\textrm{sec}$), and translation (3 $\textrm{mm}/\textrm{sec}$) clubbed with rotation (10 $\textrm{RPM}$) for $23-G$ bevel needle along the needle travel length

A quantitative comparison of the reaction force shows a reduction in values due to the needle rotation when considering different posterior layers compared to that with no needle rotation. Further, a minor change can be observed in the reaction force values at the penetration point (Table 16) when the two layers are considered separate in contrast to combined, irrespective of rotation. A similar pattern is observed along the needle travel length (Table 17). This implies that future modeling studies can consider the sclera and choroid as a combined layer while performing insertion simulations to study ocular biomechanics, thereby saving computational time and resources. The authors would like to mention that the total computational time involved in simulating the problem considering two different layers is approximately (84 hours), which is $91.6\%$ more than a single combined layer (7 hours), with insignificant change in the reaction force values.

Appendix B

1.1 Comparison of kinetic energy with total energy

Figure 16 compares the total energy and kinetic energy induced on the sclera due to the penetration of the needle. It is observed that the ratio of kinetic energy to total energy of the system is less than 5% on average. This suggests that the dynamics of the simulation are not being affected and can be considered a quasi-static (Srivastava and Basu 2020) system using an explicit method. Therefore, the needle insertion dynamics may be modeled by considering the sclera linearly elastic.

Fig. 16

Comparison of kinetic energy with total energy for the system