NEET MDS Synopsis
Structure of Orbital Walls
Oral and Maxillofacial SurgeryStructure of Orbital Walls
The orbit is a complex bony structure that houses the eye and its associated
structures. It is composed of several walls, each with distinct anatomical
features and clinical significance. Here’s a detailed overview of the structure
of the orbital walls:
1. Lateral Wall
Composition: The lateral wall of the orbit is primarily
formed by two bones:
Zygomatic Bone: This bone contributes significantly
to the lateral aspect of the orbit.
Greater Wing of the Sphenoid: This bone provides
strength and stability to the lateral wall.
Orientation: The lateral wall is inclined at
approximately 45 degrees to the long axis of the skull,
which is important for the positioning of the eye and the alignment of the
visual axis.
2. Medial Wall
Composition: The medial wall is markedly different from
the lateral wall and is primarily formed by:
Orbital Plate of the Ethmoid Bone: This plate is
very thin and fragile, making the medial wall susceptible to injury.
Height and Orientation: The medial wall is about half
the height of the lateral wall. It is aligned parallel to the
antero-posterior axis (median plane) of the skull and meets the floor of the
orbit at an angle of about 45 degrees.
Fragility: The medial wall is extremely fragile due to
its proximity to:
Ethmoid Air Cells: These air-filled spaces can
compromise the integrity of the medial wall.
Nasal Cavity: The close relationship with the nasal
cavity further increases the risk of injury.
3. Roof of the Orbit
Composition: The roof is formed by the frontal bone and
is reinforced laterally by the greater wing of the sphenoid.
Thickness: While the roof is thin, it is structurally
reinforced, which helps protect the contents of the orbit.
Fracture Patterns: Fractures of the roof often involve
the frontal bone and tend to extend medially. Such fractures can lead to
complications, including orbital hemorrhage or involvement of the frontal
sinus.
4. Floor of the Orbit
Composition: The floor is primarily formed by the
maxilla, with contributions from the zygomatic and palatine bones.
Thickness: The floor is very thin, typically measuring about
0.5 mm in thickness, making it particularly vulnerable to
fractures.
Clinical Significance:
Blow-Out Fractures: The floor is commonly involved
in "blow-out" fractures, which occur when a blunt force impacts the eye,
causing the floor to fracture and displace. These fractures can be
classified as:
Pure Blow-Out Fractures: Isolated fractures of
the orbital floor.
Impure Blow-Out Fractures: Associated with
fractures in the zygomatic area.
Infraorbital Groove and Canal: The presence of the
infraorbital groove and canal further weakens the floor. The
infraorbital nerve and vessels run through this canal, making them
susceptible to injury during fractures. Compression, contusion, or
direct penetration from bone spicules can lead to sensory deficits in
the distribution of the infraorbital nerve.
Clotrimazole
Pharmacology
Clotrimazole: Clotrimazole is a potent, specific inhibitor of p450 enzymes.
It is used in some antifungal medications, and in the treatment of yeast infections.
Water Acid Bases & Buffers
Biochemistry
Keq, Kw and pH
As H2O is the medium of biological systems one must consider the role of this molecule in the dissociation of ions from biological molecules. Water is essentially a neutral molecule but will ionize to a small degree. This can be described by a simple equilibrium equation:
H2O <-------> H+ + OH-
This equilibrium can be calculated as for any reaction:
Keq = [H+][OH-]/[H2O]
Since the concentration of H2O is very high (55.5M) relative to that of the [H+] and [OH-], consideration of it is generally removed from the equation by multiplying both sides by 55.5 yielding a new term, Kw:
Kw = [H+][OH-]
This term is referred to as the ion product. In pure water, to which no acids or bases have been added:
Kw = 1 x 10-14 M2
As Kw is constant, if one considers the case of pure water to which no acids or bases have been added:
[H+] = [OH-] = 1 x 10-7 M
This term can be reduced to reflect the hydrogen ion concentration of any solution. This is termed the pH, where:
pH = -log[H+]
Prognosis After Traumatic Brain Injury
Oral and Maxillofacial SurgeryPrognosis After Traumatic Brain Injury (TBI)
Determining the prognosis for patients after a traumatic brain injury
(TBI) is a complex and multifaceted process. Several factors can
influence the outcome, and understanding these variables is crucial for
clinicians in managing TBI patients effectively. Below is an overview of the key
prognostic indicators, with a focus on the Glasgow Coma Scale (GCS) and other
factors that correlate with severity and outcomes.
Key Prognostic Indicators
Glasgow Coma Scale (GCS):
The GCS is a widely used tool for assessing the level of
consciousness in TBI patients. It evaluates three components: eye
opening (E), best motor response (M), and verbal response (V).
Coma Score Calculation:
The total GCS score is calculated as follows: [ \text{Coma
Score} = E + M + V ]
Prognostic Implications:
Scores of 3-4: Patients scoring in this range
have an 85% chance of dying or remaining in a vegetative
state.
Scores of 11 or above: Patients with scores in
this range have only a 5-10% chance of dying or remaining
vegetative.
Intermediate Scores: Scores between these
ranges correlate with proportional chances of recovery, indicating
that higher scores generally predict better outcomes.
Other Poor Prognosis Indicators:
Older Age: Age is a significant factor, with older
patients generally having worse outcomes following TBI.
Increased Intracranial Pressure (ICP): Elevated ICP
is associated with poorer outcomes, as it can lead to brain herniation
and further injury.
Hypoxia and Hypotension: Both conditions can
exacerbate brain injury and are associated with worse prognoses.
CT Evidence of Compression: Imaging findings such
as compression of the cisterns or midline shift indicate significant
mass effect and are associated with poor outcomes.
Delayed Evacuation of Large Intracerebral Hemorrhage:
Timely surgical intervention is critical; delays can worsen the
prognosis.
Carrier Status for Apolipoprotein E-4 Allele: The
presence of this allele has been linked to poorer outcomes in TBI
patients, suggesting a genetic predisposition to worse recovery.
Beveled Conventional Preparation
Conservative DentistryBeveled Conventional Preparation
Characteristics
External Walls: In a beveled conventional preparation,
the external walls are perpendicular to the enamel surface.
Beveled Margin: The enamel margin is beveled, which
helps to create a smooth transition between the restoration and the tooth
structure.
Benefits
Improved Aesthetics: The beveling technique enhances
the aesthetics of the restoration by minimizing the visibility of the
margin.
Strength and Bonding: Beveling can improve the bonding
surface area and reduce the risk of marginal leakage, which is critical for
the longevity of the restoration.
RESPIRATORY DISORDERS - SURFACTANT DEFICIENCY
Physiology
(RDS) Respiratory distress of Newborn
1. hyaline membrane disease of the new born
2. decrease in surfactant, Weak, Abnormal complience of chest wall
3. Small alveoli, difficult to inflate, Alveoli tent to collapse, many of varied sizes
4. decrease in O2 diffusion area, lung difficult to expand, in compliance
Cardiac Conduction
Physiology
A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.
Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.
Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.
Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.
Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects
Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.
1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.
2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.
3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.
4) Ventricular repolarization occurs.
Adrenergic Agents
Pharmacology
ISOPRENALINE
It is beta-receptor stimulant, which stimulates the heart and causes tachycardia.
It relaxes the smooth muscles particularly the bronchial and GIT. It is mainly used in bronchial asthma, in the treatment of shock and as a cardiac stimulant in heart block.
ORCIPRENALINE
Is a potent β-adrenergic agonist.
Receptor sites in the bronchi and bronchioles are more sensitive to the drug than those in the heart and blood vessels.
AMPHETAMINE
increases the systolic and diastolic blood pressure. Amphetamine is a potent CNS stimulant and causes alertness, insomnia, increased concentration, euphoria or dysphoria and increased work capacity.
Amphetamines are drugs of abuse and can produce behavioural abnormalities and can precipitate psychosis.
PHENYLEPHRINE
It is used as a nasal decongestant and mydriatic agent and also in the treatment of paroxysmal supraventricular tachycardia.
UTERINE RELAXANTS (TOCOLYTICS)
ISOXSUPRINE
Isoxsuprine has a potent inhibitory effect on vascular and uterine smooth muscle and has been used in the treatment of dysmenorrhoea, threatened abortion, premature labour and peripheral vascular diseases.