Transcript Corneal Biomechanics after Penetrating Keratoplasty and

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
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.
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
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)
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 noncontact 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.
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
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
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)
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
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 postPK/DSAEK eyes and that CH/CRF are moderately
independent corneal properties.
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