[Show abstract][Hide abstract] ABSTRACT: The potential causes of the optic nerve injury as a result of blunt object trauma, were investigated using a computer model.
A finite element model of the eye, the optic nerve, and the orbit with its content was constructed to simulate blunt object trauma. We used a model of the first phalanx of the index finger to represent the blunt body. The trauma was simulated by impacting the blunt body at the surface between the globe and the orbital wall at velocities between 2-5 m/s, and allowing it to penetrate 4-10 mm below the orbital rim.
The impact caused rotations of the globe of up to 5000 degrees /s, lateral velocities of up to 1 m/s, and intraocular pressures (IOP) of over 300 mm Hg. The main stress concentration was observed at the insertion of the nerve into the sclera, at the side opposite to the impact.
The results suggest that the most likely mechanisms of injury are rapid rotation and lateral translation of the globe, as well as a dramatic rise in the IOP. The strains calculated in the study should be sufficiently high to cause axonal damage and even the avulsion of the nerve. Finite element computer modelling has therefore provided important insights into a clinical scenario that cannot be replicated in human or animal experiments.
Full-text · Article · Jul 2006 · British Journal of Ophthalmology
[Show abstract][Hide abstract] ABSTRACT: A finite element model of the eye and the orbit was used to examine the hypothesis that the orbital fat provides an important mechanism of eye stability during head trauma. The model includes the globe, the orbital fat, the extra-ocular muscles, and the optic nerve. MRI images of an adult human orbit were used to generate an idealized geometry of the orbital space. The globe was approximated as a sphere 12 mm in radius. The optic nerve and the sclera were represented as thin shells, whereas the vitreous and the orbital fat were represented as nearly incompressible solids of low stiffness. The orbital bone was modelled as a rigid shell. Frontal head impact resulting from a fall onto a hard floor was simulated by prescribing to the orbital bone a triangular acceleration pulse of 200 g (1962 m/s(2)) peak for a duration of 4.5 ms. The results show that the fat provides the crucial passive mechanism of eye restraint. The mechanism is a consequence of the fact that the fat is incompressible and that its motion is restricted by the rigidity of the orbital walls. Thus, the acceleration loads of short duration cannot generate significant distortion of the fat. In contrast, the passive muscles provide little support to the globe. When the connection between the orbital fat and the eye is absent the eye is held mainly by the optic nerve. We discuss the possible role that this loss of contact may have in some cases of the evulsion of the eye and the optic nerve.
No preview · Article · Mar 2005 · Computer Methods in Biomechanics and Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: The terms abusive head injury and shaken baby syndrome are used to describe a unique pattern of nonaccidental traumatic injuries occurring in children that many clinicians and
researchers have good reason to believe is caused by violent shaking. Typical injuries include severe brain injury, with intracranial
and retinal hemorrhages, but the pathogenesis of injuries is poorly understood. A major paradox in head trauma in infants
is that the injuries induced by a shaking event are much more severe than those caused by even very violent single-impact
head trauma, despite the relatively low accelerations in shaking.
We have developed a finite element computer model of the eye, orbit, and orbital bone and used it to simulate the effects
of single-impact and oscillatory motion inputs. The model was informed by data from semiquantitative in vitro anatomical traction
experiments on in situ rabbit eyes. The new results reported here strongly suggest that suction between the eye and its surrounding fat dominates
the dynamical stability of the system composed of the eye, its socket, and the components and material supporting the eye.
Computer simulations incorporating this functional anatomical relationship show that deceleration of the head generates pressure
gradients inside and outside the eye; these could cause damaging shear stresses in structures such as the retina and blood
vessels. Simulations also show that oscillating the bone of the orbit causes the eye to move anteriorly and posteriorly with
an increasing amplitude, building up the stresses within the eye over time. This is the first time that any biomechanical
mechanism has been identified that might explain the disproportionally severe injuries caused by an oscillatory mechanism
such as violent shaking of an abused infant. However, further study is required and this conclusion is therefore preliminary
No preview · Article · Mar 2005 · Forensic Science Medicine and Pathology
[Show abstract][Hide abstract] ABSTRACT: Optic nerve evulsion is a traumatic lesion of the optic nerve which is usually caused by high-speed traffic accidents, or
a blunt object injury. The direct causes leading to nerve damage are not clear. We used a finite element model of the eye
and the orbit to investigate the possible mechanisms of injury. The model, which involved the globe, optic nerve, recti muscles,
and the orbital fat, was subjected to triangular acceleration pulses of peak intensity between 100 and 450g. Collision with
a foreign object of cylindrical shape 6mm in diameter was used to simulate blunt object injury. The results point to rapid
rotation of the globe, and a steep pressure gradient between the orbital fat and the soft tissue within the optic nerve as
possible factors contributing to the optic nerve damage.
[Show abstract][Hide abstract] ABSTRACT: To compare the Sbisa bar (Bagolini filter bar) with neutral density filters (NDF) in quantifying relative afferent pupillary defects (RAPD).
11 patients with a RAPD were graded and a neutral density filter bar was used to quantify the RAPD. This was repeated using the Sbisa bar. The Sbisa bar (Bagolini filter bar) is used by orthoptists to quantify density of suppression in amblyopia and is of a similar construct to NDFs. Before this clinical part of the study the luminance for each filter was measured, which enabled a direct comparison to be made.
In the analysis of patients with RAPD a high correlation was found when comparing the Sbisa and NDF bars r = 0.95. This was statistically significant (p = <0.001). Correlation was also high when the luminance values for the filters were substituted for the clinical readings (r = 0.92; p=<0.001).
The Sbisa bar is a comparable instrument to the NDF bar in measuring RAPD. Its availability in the clinical situation makes it a practical choice.
Preview · Article · Sep 2002 · British Journal of Ophthalmology
[Show abstract][Hide abstract] ABSTRACT: To determine if uncomplicated phacoemulsification cataract surgery is associated with an accelerated rate of progression of diabetic retinopathy or maculopathy postoperatively.
A prospective trial of 50 type 2 diabetics undergoing monocular phacoemulsification cataract surgery by a single consultant surgeon. The grade of diabetic retinopathy and diabetic maculopathy in the operated and non-operated fellow eye was assessed preoperatively and for 12 months postoperatively.
Overall, retinopathy progression was observed in 11 patients. In seven the retinopathy progressed in both eyes, in three it progressed in the operated eye alone, and in one it progressed in the fellow eye alone. Macular oedema was observed in 13 eyes postoperatively. Four had transient pseudophakic cystoid macular oedema and nine true diabetic maculopathy. Where maculopathy progressed it did so symmetrically in five patients, it progressed in the operated eye alone in four patients, and the fellow eye alone in two patients. There was no significant difference in the number of operated and fellow eyes whose retinopathy or maculopathy progressed postoperatively. In both the operated (OE) and non-operated (NoE) eyes retinopathy progression was associated with a higher mean HbA(1)C (OE p=0.003; NoE p=0.001) and insulin treatment (OE p=0.008, NoE p=0.04).
Uncomplicated phacoemulsification cataract surgery does not cause acceleration of diabetic retinopathy postoperatively and any progression that is observed probably represents the natural history of the disease. Although macular oedema is common after cataract surgery it may follow a benign course and in many patients the development of clinically significant macular oedema postoperatively probably represents natural disease progression rather than being a direct effect of surgery.
Preview · Article · Jun 2002 · British Journal of Ophthalmology
[Show abstract][Hide abstract] ABSTRACT: We describe a mother and all her offspring with congenital superior oblique palsy (CSOP), and a father and all his sons with unilateral CSOP. We discuss the inheritance pattern in our pedigrees and compare it with previous reports.
All available family members were examined. The prism cover test was performed. Ocular movements were examined in all positions of gaze and where possible a Hess chart plotted. Lang and TNO stereotests were used to determine the stereo-acuity. The results of these tests combined with the Bielschowsky head tilt test (BHTT) were used to confirm the diagnosis of superior oblique palsy. The condition was classified as congenital if it presented early based on history or the observation of old photographs and in the absence of a causative factor.
The affected members of family A consist of a father and his three sons with unilateral CSOP. His daughter had a mild weakness of her left inferior and superior rectus muscle. One of his sons was asymptomatic and only recognised on screening of the family for the study. The affected members of family B consist of a mother and her younger daughter with unilateral CSOP and her older daughter with bilateral CSOP. She had no other children.
Our families demonstrate what is probably an autosomal dominant form of CSOP. It is possible that hereditary CSOP is more common than previously reported.
[Show abstract][Hide abstract] ABSTRACT: To search for the cause of scleral thinning and pupillary distortion following trans-scleral contact diode laser cyclophotocoagulation (TCDLC).
We reviewed the records of 3 patients in whom there were complications of scleral thinning and pupillary distortion following TCDLC. One of the eyes was later enucleated, and we present the histopathological findings. Using the histopathological features in this patient, we discuss the possible pathogenesis of the scleral thinning and pupillary distortion.
Case 1 is a 46-year-old white woman who following TCDLC in an area of clinically normal sclera developed a staphyloma. Case 2 is a 52-year-old white woman who following TCDLC in an area of scarred sclera developed mild thinning. Case 3 is an 85-year-old white man who following TCDLC developed pupillary distortion, and gonioscopy revealed damage to the peripheral iris. Histological examination of case 1 revealed the staphyloma covered by a thin layer of conjunctival epithelium, collagen and vitreous condensation. We also observed cicatricial cilary body contraction causing distortion of the pupil and lens.
Therapeutic TCDLC can produce scarring of the iris root, anterior chamber angle, draining structures and ciliary body, and may result in pupillary distortion. Pre-existing scleral scars may predispose to scleral damage following TCDLC. We discuss a simple strategy to avoid this complication of TCDLC.