1. Corneal Biomechanics after Penetrating Keratoplasty and Descemet’s Stripping Automated Endothelial Keratoplasty
Samer Hamada MRCOphth FRCS(Ed)1,3
Louai Wehbeh MRCOphth3
Ben Royal Ophthalmic Physiologist1,2
Mohammed Laiquzzaman MBBS, PhD1,2
Sunil Shah FRCS(Ed) FRCOphth1,2,3
1. Birmingham Heartlands and Solihull Trust, Solihull UK
2. Anterior Eye Group (AEG), Neurosciences Research Institute, Aston University, Birmingham, UK
3. Birmingham and Midland Eye Centre, Birmingham, UK
Financial Interest Disclosure: The authors have no financial interest in the subject matter of this poster

2. Corneal Hysteresis
When air pulse is released it causes the cornea to move inwards past applanation and its shape becomes slightly concave but when the air puff shuts off the pressure decreases and the corneal shape tries to gain its normal shape, during this process the cornea again passes through an applanation phase - ‘outward applanation’.
Theoretically these two pressures should be the same but this is not the case and this is described as Dynamic corneal Response (DCR), which is the resistance to applanation manifested by the corneal tissue due to its viscoelastic properties. The difference between these two pressures is termed as corneal hysteresis
Corneal Hysteresis (CH) is believed to reflect the aggregate effects of thickness, rigidity, hydration and perhaps other factors yet to be determined.
Corneal Resistance Factor (CRF) is believed to be a measure of the elastic properties of the cornea.

3. Measuring corneal hysteresis
Various investigators have in the past tried to measure the ocular rigidity (elasticity) of the eye in order to assess the pathological processes affecting the corneal tissue
Studies showed a reduced rigidity in keratoconic eyes, however there is not enough studies to describe corneal hysteresis post penetrating keratoplasty or posterior lamellar keratoplasty

4. Purpose
To compare corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT), and intraocular pressure (IOP) following penetrating keratoplasty (PK) and Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) and compare results with normal eyes using Ocular Response Analyser(ORA)

5. Methods Prospective comparative masked study of
34 consecutive post unilateral PKP eyes
28 consecutive post unilateral DSAEK eyes
166 normal eyes (as control)
CH, CRF, and IOP of each eye was measured by ORA (Reichert’s non- contact tonometer with additional analysis software; the Ocular Response Analyser (ORA) [Reichert Ophthalmic Instruments, Buffalo, USA]
CCT was measured with a hand held ultrasonic pachymeter (SP-2000, Tomey Corp, Aichi, Japan) in the mid-pupillary axis. Three readings were taken and the lowest value was used as the CCT.

6. Results Mean CH Normal eyes: 10.6 ± 2.0mmHg Post-PK eyes: 8.9±3.3mmHg
Post-DSAEK eyes: 6.4 ± 2.9mmHg
Mean CRF
Normal eyes: 10.2 ± 2.0mmHg
Post-PK eyes: 8.1±3.3mmHg
Post-DSAEK eyes: 7.5 ± 2.7mmHg

7. CCT and IOP Mean CCT Normal eyes: 541.8±36.1µm
Post-PK eyes: 556.0±69.2µm
Post-DSAEK eyes: 632.5±57µm
Mean IOP
Normal eyes: 16.1± 3.1mmHg
Post-PK eyes: 12.4± 2.9mmHg
Post-DSAEK eyes: 17.2± 3.1mmHg

8. Results
CCT was higher in post-DSAEK eyes compared with normal and post-PK eyes but not statistically significant (p>0.4, and>0.38)
Correlation between CH/CCT, CRF/CCT, and IOP/CCT were statistically significant in normal eyes (p<0.0001) but not significant in the post-PK group (p>0.4, p>0.1 and p>0.2 respectively) and post-DSAEK group ( p>0.2, p>0.2, and p>0.1 respectively)

9. Discussion (1)
Our results agree with previous findings that increased CCT can affect corneal biomechanical properties in normal eye, however this is not the case in grafted corneas (post-PKP, post DSAEK).
CCT was higher after PKP/DSAEK but corneal hysteresis and resistance factor were reduced after corneal grafting

10. Discussion (2)
Higher CCT did not mean higher levels of rigidity or elasticity. Mean higher CH and CRF in the normal eyes may be related to an intact cornea while the reduced CH and CRF, weak relationship between CH and CCT and CRF and CCT in post-PK/DSAEK eyes is postulated to be due to the weakened cornea because of the graft-host interface (i.e. Post-PK/DSAEK cornea may not achieve the same tensile strength after surgical intervention and thus its rigidity is compromised)
We postulate that corneal structure is altered in post- PK/DSAEK eyes and that CH/CRF are moderately independent corneal properties.

11. Conclusion Biomechanical parameters were reduced post
PK and DSAEK, possibly a result of altered
corneal structure
A larger study is underway to correlate corneal biomechanics to clinical outcomes after corneal transplantation