**Purpose.**:
To evaluate fluid dynamics and fluid shear stress on the retinal wall in a model eye after vitrectomy and gas tamponade in relation to saccadic eye movements and sudden head movements and to correlate the results with gas fill fraction (GF).

**Methods.**:
Analyses was undertaken using high-resolution computational fluid dynamic software. The fluid volume within the eye was discretized using 6 × 10^{5} elements and solved with a volume-of-fluid method. The eye was abstracted to a sphere. Vertical and horizontal saccades and sudden rectilinear displacement of the head were examined. GF was varied from 20% to 80% of the eye height filled with gas.

**Results.**:
Maximum shear stress during horizontal and vertical saccades was 1.0 Pa (Pascal) and 2.5 Pa, respectively, and was dependent on GF. Rapid rectilinear acceleration of the head caused a maximum shear stress of 16 Pa, largely independent of GF. Fluid sloshing within the eye decayed within 0.1 second. Stresses were maximum at the contact line and equator of the eye and were parallel to the direction of motion.

**Conclusions.**:
This study predicts that saccadic eye movements and normal head movements after vitrectomy and gas tamponade generate only small fluid shear stresses on the retina that are below published norms for retinal adhesion strength. Sudden, jerking head movements generate fluid shear forces similar to retinal adhesion strength that localize to the area of gas–fluid interface. Fluid sloshing occurs after movement, but rapidly decays on cessation of movement. These results suggest that restrictive posturing after vitrectomy and gas tamponade may be unnecessary. Patients should avoid sudden head movements.

^{ 1 }Despite this, there is still considerable debate and insufficient clinical (no randomized control trials) and scientific evidence surrounding the role of postoperative posturing in terms of the final functional and anatomic outcomes after surgery. The issue of patient compliance with posturing after surgery is a significant factor when considering available evidence and in determining best practice.

^{ 2 }

^{ 3 }In this context posturing may be beneficial in terms of maintaining contact of the tamponade with the retinal break and theoretically by reducing intraocular fluid movements (as the patient would theoretically remain still during this period).

^{ 4 }It is common practice to ask patients with inferior retinal detachments to posture. Uncertainty about patient compliance with posturing leads some surgeons to use supplementary scleral buckling procedures in addition to vitrectomy. Despite these measures, there is no conclusive evidence that posturing and/or supplementary buckling improves outcomes in this group of patients, and recent publications have reported surgical success with vitrectomy and gas without posturing.

^{ 5 –7 }These reports suggest that factors other than the barrier function of the tamponading agent is responsible for surgical success in these sorts of cases.

^{ 8 }In 1934, Lindner

^{ 9 }demonstrated the contribution of intraocular fluid currents in a simple experiment when he coated the inner surface of a glass flask with an alginate gel, and made a nick in the gel, lifting up a small flap to simulate a horseshoe retinal tear. The flask was then filled with water. While the fluid remained static, the gel remained attached to the flask. When the fluid was agitated by spinning the flask, the gel gradually detached from the wall of the flask in a manner similar to a retinal detachment. Spinning the fluid without a simulated tear did not lead to gel separation. In this context, saccadic and pursuit movements of the eyes and movements of the head may cause intraocular fluid currents that exert shear stresses on the retina.

*a*= 12.5 mm. The sphere was partially filled with water to model partial filling of the eye after vitreoretinal surgery. CFD codes were applied to accurately calculate the unsteady flow within the partially filled sphere, which underwent rotational or rectilinear displacement to mimic respectively saccadic eye movements and head movement.

- Rotation around a vertical axis (horizontal saccadic movements)
- Rotation around a horizontal axis (vertical saccadic movements)
- Movement in the horizontal plane of the whole eye (rectilinear head movement)

^{ 10 –12 }The angular displacement (for rotation around a vertical and horizontal axis) was fixed at Δθ = 30°, whereas the maximum angular velocity was θ̇

_{m}= 500 deg/s.

*X*∼0.1 m, with a maximum velocity of

*Ẋ*

_{m}= 1 m/s, over a time period of Δ

*X/Ẋ*

_{m}∼0.05 seconds. This expression represents a sudden exaggerated jerking or whiplash motion of the head.

^{ 13 }

*u*

_{E}

*a*/ν ∼1000 where

*u*

_{E}is the speed at the eye surface and ν is the kinematic viscosity of water. The effect of the GF on the eye was examined by varying the water depth 2

*af*from 0.4

*a*to 1.6

*a*. In the simulation, water properties were viscosity μ = 8.9 × 10

^{−4}kg m

^{−1}s

^{−1}and density 997 kg m

^{−3}and air properties were dynamic viscosity of 1.83 × 10

^{−5}kg m

^{−1}s

^{−1}and density of 1.185 kg m

^{−3}. The air–water interface was characterized by a surface tension of σ = 75 × 10

^{−3}mN/m. The temperature was 37°C.

_{S}= |

**n̂**× τ ·

**n̂**| where τ is the stress tensor and

**n̂**is the unit vector normal to the surface of the retina. For each GF, the maximum tangential shear stress as a factor of time was calculated in relation to: The maximum stress corresponded, in all cases, to the period of maximum velocity. The location of the maximum shear stress was obtained from shear stress contour plots.

*t*= 0.2 seconds), the maximum shear stress suddenly drops to 0. This sudden decline occurs because, during the deceleration phase, the relative velocity between the eye wall and interior fluid is momentarily small. After

*t*= 0.3 seconds, the eye has stopped moving, but the residual motion decays over ∼0.1 seconds. The angular velocity of the fluid dragged by the sphere is largest at the extreme positions (for

*f*< 0.5) and at the widest width of the sphere (for

*f*> 0.5). The maximum shear stress occurs at

*t*= 0.2 seconds. To interpret the location, the shear stress contours on the sphere are plotted at

*t*= 0.2 seconds (Fig. 2a). Beyond the contact line, the shear stress increases from 0, up to a maximum, where the sphere is widest, and then decreases. The maximum shear stress is estimated (from the analysis in the Appendix) to be 0.6 Pa and is weakly dependent on

*f*for

*f*> 0.2 (range, 0.6–1.0 Pa; Fig. 3a). The calculations confirm values of similar magnitude, which vary between 0.6 and 1.0 Pa (Fig. 3a).

*t*= 0.2 seconds shows maximum stress at the contact line and a force distribution that was focused as a vertical equatorial band (Fig. 2b). The maximum shear stress is of a magnitude similar to that for horizontal saccades but is slightly larger in value, ranging from 1 to 2.5 Pa (Fig. 3b).

*t*= 0.2 seconds shows that the maximum shear stress occurs either at the front of the sphere (where splashing can occur) or in a band at the equator, where the surface of the sphere is parallel to the direction of motion (which gives the largest slip; Fig. 2b). The thicker band is asymmetrically displaced to the back of the sphere as a consequence of inertia in the fluid. The shear stress is estimated (in the Appendix) to be ∼3 Pa over most of the sphere surface, it is ∼8 Pa at the moving contact line, and the numerical results indicate a higher value of 10 to 16 Pa. The variation of the maximum shear stress with GF is plotted in Figure 3c.

^{ 13 }Hence, actual shear stresses on the retina are likely to be considerably lower during normal activities. Halving the maximum speed used here reduces the shear stress by 75%.

*a*

^{2}/

*v*∼100 seconds after the eye movement has stopped. When the gas–liquid interface is disturbed, it generates waves and sloshing, which oscillates with a period

*a*/ $ \sigma / \rho g $ ∼0.15 seconds, which is consistent with the numerical results.

^{ 14 }

^{ 15 –18 }After cryotherapy and photocoagulation, there is an initial decrease in retinal adhesive strength for 8 to 48 hours after treatment (3.9 Pa). This decline is followed by an increase to supranormal levels in the following weeks (13–39 Pa).

^{ 3,19,20 }As retinal adhesive strength is known to decrease rapidly after death in all animals, accurate measurements ex vivo in human eyes are difficult to obtain.

*f*< 0.5) or at

*r*=

*a*for

*f*> 0.5. Thus, the maximum speed of the fluid at the edge is

*u*

_{max}=

*a*θ̇

_{m}max(1, $4f(1\u2212f)$. The maximum shear stress corresponding to this is where δ is a typical boundary layer thickness and μ is the dynamic viscosity of the fluid. Since the eye rotates over a period ∼ Δθ/2θ̇

_{m}, the boundary layer thickness is estimated to be δ ∼ $\nu \Delta \theta /\theta \u02d9m$ where ν = μ/

*s*.

^{21}When these estimates are combined, the maximum shear stress is The effect of eye fill indicates that the maximum shear stress increases with fill fraction until

*f*= 0.5 and then does not increase further.

*u*

_{max}=

*Ẋ*

_{m}and δ ∼ $ \nu \Delta X / X \u02d9 m $ . Combined together,