NEET MDS Synopsis
Berkson's Bias
Public Health DentistryBerkson's Bias is a type of selection bias that occurs in
case-control studies, particularly when the cases and controls are selected from
a hospital or clinical setting. It arises when the selection of cases
(individuals with the disease) and controls (individuals without the disease) is
influenced by the presence of other conditions or factors, leading to a
distortion in the association between exposure and outcome.
Key Features of Berkson's Bias
Hospital-Based Selection: Berkson's Bias typically
occurs in studies where both cases and controls are drawn from the same
hospital or clinical setting. This can lead to a situation where the
controls are not representative of the general population.
Association with Other Conditions: Individuals who are
hospitalized may have multiple health issues or risk factors that are not
present in the general population. This can create a misleading association
between the exposure being studied and the disease outcome.
Underestimation or Overestimation of Risk: Because the
controls may have different health profiles compared to the general
population, the odds ratio calculated in the study may be biased. This can
lead to either an overestimation or underestimation of the true association
between the exposure and the disease.
Example of Berkson's Bias
Consider a study investigating the relationship between smoking and lung
cancer, where both cases (lung cancer patients) and controls (patients without
lung cancer) are selected from a hospital. If the controls are patients with
other diseases that are also related to smoking (e.g., chronic obstructive
pulmonary disease), this could lead to Berkson's Bias. The controls may have a
higher prevalence of smoking than the general population, which could distort
the perceived association between smoking and lung cancer.
Implications of Berkson's Bias
Misleading Conclusions: Berkson's Bias can lead
researchers to draw incorrect conclusions about the relationship between
exposures and outcomes, which can affect public health recommendations and
clinical practices.
Generalizability Issues: Findings from studies affected
by Berkson's Bias may not be generalizable to the broader population,
limiting the applicability of the results.
Mitigating Berkson's Bias
To reduce the risk of Berkson's Bias in research, researchers can:
Select Controls from the General Population: Instead of
selecting controls from a hospital, researchers can use population-based
controls to ensure a more representative sample.
Use Multiple Control Groups: Employing different control
groups can help identify and account for potential biases.
Stratify Analyses: Stratifying analyses based on
relevant characteristics (e.g., age, sex, comorbidities) can help to control
for confounding factors.
Conduct Sensitivity Analyses: Performing sensitivity
analyses can help assess how robust the findings are to different
assumptions about the data.
The Optic Nerve
Anatomy
This is the second cranial nerve (CN II) and is the nerve of sight.
Biology of tooth movement
OrthodonticsBiology of tooth movement
1. Periodontal Ligament (PDL)
Structure: The PDL is a fibrous connective tissue that
surrounds the roots of teeth and connects them to the alveolar bone. It
contains various cells, including fibroblasts, osteoblasts, osteoclasts, and
immune cells.
Function: The PDL plays a crucial role in transmitting
forces applied to the teeth and facilitating tooth movement. It also
provides sensory feedback and helps maintain the health of the surrounding
tissues.
2. Mechanotransduction
Mechanotransduction is the process by which cells convert mechanical
stimuli into biochemical signals. When a force is applied to a tooth, the
PDL experiences compression and tension, leading to changes in cellular
activity.
Cellular Response: The application of force causes
deformation of the PDL, which activates mechanoreceptors on the surface of
PDL cells. This activation triggers a cascade of biochemical events,
including the release of signaling molecules such as cytokines and growth
factors.
3. Bone Remodeling
Osteoclasts and Osteoblasts: The biological response to
mechanical forces involves the coordinated activity of osteoclasts (cells
that resorb bone) and osteoblasts (cells that form new bone).
Compression Side: On the side of the tooth where
pressure is applied, osteoclasts are activated, leading to bone
resorption. This allows the tooth to move in the direction of the
applied force.
Tension Side: On the opposite side, where tension
is created, osteoblasts are stimulated to deposit new bone, anchoring
the tooth in its new position.
Bone Remodeling Cycle: The process of bone remodeling
is dynamic and involves the continuous resorption and formation of bone.
This cycle is influenced by the magnitude, duration, and direction of the
applied forces.
4. Inflammatory Response
Role of Cytokines: The application of orthodontic
forces induces a localized inflammatory response in the PDL. This response
is characterized by the release of pro-inflammatory cytokines (e.g.,
interleukins, tumor necrosis factor-alpha) that promote the activity of
osteoclasts and osteoblasts.
Healing Process: The inflammatory response is essential
for initiating the remodeling process, but excessive inflammation can lead
to complications such as root resorption or delayed tooth movement.
5. Vascular and Neural Changes
Blood Supply: The PDL has a rich blood supply that is
crucial for delivering nutrients and oxygen to the cells involved in tooth
movement. The application of forces can alter blood flow, affecting the
metabolic activity of PDL cells.
Nerve Endings: The PDL contains sensory nerve endings
that provide feedback about the position and movement of teeth. This sensory
input is important for the regulation of forces applied during orthodontic
treatment.
6. Factors Influencing Tooth Movement
Magnitude and Duration of Forces: The amount and
duration of force applied to a tooth significantly influence the biological
response and the rate of tooth movement. Light, continuous forces are
generally more effective and less damaging than heavy, intermittent forces.
Age and Biological Variability: The biological response
to orthodontic forces can vary with age, as younger individuals tend to have
more active remodeling processes. Other factors, such as genetics, hormonal
status, and overall health, can also affect tooth movement.
Muscles acting on the Temporomandibular Joint
AnatomyMuscles acting on the Temporomandibular Joint
Movements of the temporomandibular joint are chiefly from the action of the muscles of mastication.
The temporalis, masseter, and medial pterygoid muscles produce biting movements.
The lateral pterygoid muscles protrude the mandible with the help from the medial pterygoid muscles and retruded largely by the posterior fibres of the temporalis muscle.
Gravity is sufficient to depress the mandible, but if there is resistance, the lateral pterygoid, suprahyoid and infrahyoid, mylohyoid and anterior digastric muscles are activated.
Actions
Muscles
Depression (Open mouth)
Lateral pterygoid
Suprahyoid
Infrahyoid
Elevation (Close mouth)
Temporalis
Masseter
Medial pterygoid
Protrusion (Protrude chin)
Masseter (superficial fibres)
Lateral pterygoid
Medial pterygoid
Retrusion (Retrude chin)
Temporalis
Masseter (deep fibres)
Side-to-side movements (grinding and chewing)
Temporalis on same side
Pterygoid muscles of opposite side
Masseter
The Eye and Orbit
AnatomyThe Eye and Orbit
The orbit (eye socket) appears as a bony recess in the skull when it is viewed from anteriorly.
It almost surrounds the eye and their associated muscles, nerves and vessels, together with the lacrimal apparatus.
The orbit is shaped somewhat like a four-side pyramid lying on its side, with its apex pointing posteriorly and its base anteriorly.
Keratoses (Horny Growth)
General Pathology
Keratoses (Horny Growth)
1. Seborrheic keratosis is a common benign epidermal tumor composed of basaloid (basal cell-like) cells with increased pigmentation that produce a raised, pigmented, "stuck-on" appearance on the skin of middle-aged individuals.
- they can easily be scraped from the skin's surface.
- frequently enlarge of multiply following hormonal therapy.
- sudden appearance of large numbers of Seborrheic keratosis is a possible indication of a malignancy of the gastrointestinal tract (Leser-Trelat sign).
2. An actinic keratosis is a pre-malignant skin lesion induced by ultraviolet light damage.
- sun exposed areas.
- parakeratosis and atypia (dysplasia) of the keratinocytes.
- solar damage to underlying elastic and collagen tissue (solar elastosis).
- may progress to squamous carcinoma in situ (Bowen's disease) or invasive cancer.
3. A keratoacanthoma is characterized by the rapid growth of a crateriform lesion in 3 to 6
weeks usually on the face or upper extremity.
- it eventually regresses and involutes with scarring.
- commonly confused with a well-differentiated squamous cell carcinoma.
THE THYROID GLAND
General Pathology
THE THYROID GLAND
The thyroid gland develops embryologically from the developing pharyngeal epithelium that descends from the foramen cecum at the base of the tongue to its normal position in the anterior neck. This pattern of descent explains the occasional presence of ectopic thyroid tissue, most commonly located at the base of the tongue (lingual thyroid) or at other sites abnormally high in the neck.
Pneumoconioses
General Pathology
Pneumoconioses—are environmentally related lung diseases that result from chronic inhalation of various substances.
1. Silicosis (stone mason’s disease)
a. Inhalant: silica dust.
b. Associated with extensive fibrosis of the lungs.
c. Patients have a higher susceptibility to tuberculosis infections.
2. Asbestosis
a. Inhalant: asbestos fibers.
b. Associated with the presence of pleural plaques.
c. Consequences include:
(1) Mesothelioma (malignant mesothelial tumor).
(2) Bronchogenic carcinoma.
3. Anthracosis
a. Inhalant: carbon dust.
b. Usually not as harmful as silicosis or asbestosis.
c. Associated with the presence of macrophages containing carbon.