Experimental and Numerical Determination of the Local Temperature Distribution during Phacoemulsification and Comparison of Different Surgery Situations within Enucleated Porcine Eyes.

Phacoemulsification, a common treatment for cataract, is accompanied by cell damage at the corneal endothelium. Thermal exposure during treatment has been considered a reason for this damage, but a thorough experimental and theoretical assessment of the local temperature distribution inside the eye had not yet been conducted. Measurements in porcine eyes and a finite-element simulation enabled such an assessment and localized the highest temperature rise very close to the probe. The results described in this study indicate that a distance of 1 mm between the probe and the endothelium should be maintained during treatment as a safety margin, especially when fluid flow is blocked. The highest measured temperature rise with surgically reasonable system settings and unblocked fluid flow was 1.11°C. The finite-element simulation described here is able to calculate the temperature rise at the endothelium and could serve as a tool for comparing arbitrary surgical situations with respect to thermal exposure of the endothelium.

Computation of the temperature rise at the corneal endothelium during cataract surgery by modeling of heat generation inside the anterior chamber.

The corneal endothelium sustaining the transparency of the cornea is a vulnerable cell layer. Thermal exposure during phacoemulsification is considered to be a potential cause of post-operative cell loss. Knowledge of the temperature rise and particularly of its dependence on region and system settings could deliver useful information about a potential correlation with cell damage. However, there exists a lack of understanding of the process and location of heat generation. Analytical calculations and experiments enabled the quantification of different mechanisms acting as heat sources during phacoemulsification. Heat generation caused by viscous friction was estimated using both an analytical approach and a numerical simulation. In contrast to absorption of sound and self-heating of the probe, this effect was ascertained to be the main heat source. Calorimetric measurement of the power input verified this modeling. On the basis of these results, the local temperature distribution inside a porcine eye was computed time dependently using the finite-element method. Two different amplitude settings were compared with respect to the temperature increase at the corneal endothelium. Various conclusions on the mitigation of thermal exposure during treatment can be drawn from this finite-element simulation.

[Frequency of reoperations after amniotic membrane transplantation]. / Reoperationsrate nach Amnionmembrantransplantation.

BACKGROUND: Retrospective evaluation of the frequency of reoperations after amniotic membrane transplantation for different pathologic entities. MATERIAL AND METHODS: We included 81 cases of amniotic membrane transplantation (AMT), which were operated on at the university eye clinic Tübingen since 1997 and which had been followed up for at least 12 months. Patient diagnoses were subdivided as follows: conjunctival defects after tumour excision (N = 2), bullous keratopathy (N = 5), external fistula after glaucoma filtering surgery (N = 3), recurrent pterygium (N = 5), symblepharon (N = 6), corneal ulcer with descemetocele (N = 3), non-perforated corneal ulcer (N = 51), perforated corneal ulcer (N = 1), other (n = 5). RESULTS: The overall reoperation rate was determined to be 42 % after a follow-up period of one year. Most frequently, a second AMT (N = 16) and a perforating keratoplasty (N = 16) were performed after the initial AMT. The following reoperation rates were determined for the subgroups: Conjunctival defects after tumour excision (0 %), Bullous keratopathy (60 %), External fistula after glaucoma filtering surgery (67 %), Recurrent pterygium (60 %), Symblepharon (67 %), Corneal ulcer with descemetocele (67 %), Non-perforated corneal ulcer (49 %), Perforated corneal ulcer (100 %). CONCLUSIONS: Particularly in patients with conjunctival defects after tumour excision and with non-perforated corneal ulcers, stabilisation of the ocular surface homeostasis can be achieved with a single amniotic membrane transplantation for at least one year.