1. PULPAL REACTION TOPULPAL REACTION TO
CAVITY AND CROWNCAVITY AND CROWN
PREPARATIONPREPARATION
DR. BAHJAT ABU HAMDANDR. BAHJAT ABU HAMDAN
CONSULTANT PROSTHODONTISTCONSULTANT PROSTHODONTIST
DDS, CES, DSODDS, CES, DSO

2. Pulpal Reaction to C.. and C.. PrepPulpal Reaction to C.. and C.. Prep
 1. Introduction .
 2. Thermal injury .
 3. Transection of the Odontoblastic Processes and its
Implication in the Dentin and Pulp Complex .
 4. Crown Preparation .
 5. Vibratory Phenomenon .
 6. Desiccation of Dentin .
 7. Pulp Exposure .
 8. Smear Layer .

3. Pulpal Reaction to C.. And C.. PrepPulpal Reaction to C.. And C.. Prep
 9. Remaining Dentin Thickness .
 10. Acid Etching .
 11. Immunodefense of the pulp to Tooth Preparation .
 12. Comparison of Cavity Preparation by High-Speed
Handpiece and Bur and Er:YAG Laser .
 13. Pin Insertion .
 14. Pulp Horn Extension .
 15. Prevention of pulp injury.

4. 1. Introduction1. Introduction
 The vital pulp tissues are the best filling for the
root canal. (Marmasse) .
 These tissues have the following advantages:
 a: Play a role of a warning system.
 b: They form a protection and defense system .
 c: They give the indication of sound periapical
tissues .
 d: They play an important role in the formation of
normal root (apexigenesis).
 e . A vital tooth has normal esthetic appearance.

5. 1. Introduction .1. Introduction .
 Of the various forms of treatment, operative
procedures are the most frequent cause of pulpal
injury .
 Trauma to the pulp can’t always be avoided,
particularly with extensive restorations .
 A competent clinician, recognizing the hazards
associated with each step of the restorative
process, can often minimize if not prevent, trauma
to preserve the vitality of the tooth .

6. 2. Thermal injury2. Thermal injury
 Cutting of dentin with bur or stone produce a
considerable frictional heat .
 This operation can be presented in the following
equation ;
 Mech.E >>>> Cut + Heat .
 Based on that equation several factors influence
the quantity of the heat produced;
 a .Speed of rotation . A higher speed has more
mechanical energy, so it will produce more cut
and more heat .

7. 2. Thermal injury .2. Thermal injury .
 b. Size and shape of cutting instrument . More
important the contact with tooth tissues higher the
mechanical energy transferred is , so the heat
produced is more important .
 c. Quality of the instrument. A new instrument
produce less heat than old one.
 Mech.E-----> Cut + Heat
 New bur ---> More Cut + Less Heat
 Old bur (more friction)---> Less Cut + More Heat

8. A. TUNGSTEN CARBIDE ROUND BUR BEFORE USE B. SAMEA. TUNGSTEN CARBIDE ROUND BUR BEFORE USE B. SAME
BUR AFTER PREPARING 5 CAVITIES. C. EXTRA-COARSEBUR AFTER PREPARING 5 CAVITIES. C. EXTRA-COARSE
DIAMOND BUR BEFORE USE. D. SAME AFTER PREPARING 2DIAMOND BUR BEFORE USE. D. SAME AFTER PREPARING 2
CAVITIES. LOSS OF ABRASIVEON DOME END. TUNGSTENCAVITIES. LOSS OF ABRASIVEON DOME END. TUNGSTEN
CARBIDE ALWAYS CHATTERED. THE COARSE DIAMONDCARBIDE ALWAYS CHATTERED. THE COARSE DIAMOND
WAS THE ONLY ONE THAT CUT SMOOTHLY AND REMAINEDWAS THE ONLY ONE THAT CUT SMOOTHLY AND REMAINED
EFFECTIVE AFTER 5 CAVITIES PREP(Stokes AN and TidmarchEFFECTIVE AFTER 5 CAVITIES PREP(Stokes AN and Tidmarch
BG.)BG.)

9. 2. Thermal injury2. Thermal injury
 d. Length of time the instrument in contact with
the dentin. Interrupted touches produce less heat
than long contact.
 This situation could be compared to one who try
to touch a hot instrument or try to drink a hot
coffee or tea (energy source), a touch for short
time doesn’t harm and he doesn’t feel the heat, but
a touch for long time will cause burn.

10. DIAGRAM ILLUSTRATING THE DIFFERENCE IN SIZE ANDDIAGRAM ILLUSTRATING THE DIFFERENCE IN SIZE AND
DENSITY IN TUBULES IN THE DENTINAL FLOOR BETWEEN ADENSITY IN TUBULES IN THE DENTINAL FLOOR BETWEEN A
SHALLOW (A) AND A DEEP (B) CAVITY PREPARATION.SHALLOW (A) AND A DEEP (B) CAVITY PREPARATION.
(Trowbridge HO: Dentistry 22(4):22-29 1982)(Trowbridge HO: Dentistry 22(4):22-29 1982)

11. 2. Thermal injury2. Thermal injury
e. Amount of pressure exerted on the
handpiece. This situation could be a reflex
when the bur doesn’t cut anymore (dull
bur).
f. The thermal conductivity of the dentin is
relatively low, so the heat produced in the
shallow preparation is less likely to injure
the pulp than a deep cavity prep.

12. 2. Thermal injury.2. Thermal injury.
 The greatest potential for damage (during dry
prep.) was within 1-2 mm radius of the dentin
being cut; so deeper the cavity is more dentinal
tubules and more odontoblast will be involved in
the heat effect.
 So how to control this produced heat. In a study
using a low speed (11000 RPM) with and without
coolant;
 With water and air spray---> only minor change in
blood pulpal flow.

13. RISE IN TEMPERATURERISE IN TEMPERATURE AT THE DENTIN-ENAMEL JUNCTIONAT THE DENTIN-ENAMEL JUNCTION
AS RELATED TO TYPE OF COOLANT USED .(Thompson, R,E;AS RELATED TO TYPE OF COOLANT USED .(Thompson, R,E;
Thermal effects in teeth. Thesis ,University of Utah, June 1971)Thermal effects in teeth. Thesis ,University of Utah, June 1971)

14. PULPAL TEMPERATURE RISE DURING TOOTHPULPAL TEMPERATURE RISE DURING TOOTH
PREPARATION. GROUP I, AIR TURBINE, WATER COOLED;PREPARATION. GROUP I, AIR TURBINE, WATER COOLED;
GROUP II, AIR TURBINE, DRY;GROUP III,LOW SPEED WATERGROUP II, AIR TURBINE, DRY;GROUP III,LOW SPEED WATER
COOLED; GROUP IV, LOW SPEED, DRY.( Zach L, Cohen G: oralCOOLED; GROUP IV, LOW SPEED, DRY.( Zach L, Cohen G: oral
surgery 19:515 1965.)surgery 19:515 1965.)

15. 2. Thermal injury.2. Thermal injury.
 Without coolant ---> heat generation---> the flow
was further reduced 1 hour after cutting
cessation---> suggesting severe damage
underneath the cutting site--->irreversible damage.
 g. Effect of local anesthesia. Vasocontrictor
potentiate and prolong the anesthesia effect by
reducing blood flow, which place the pulp in
jeopardy (danger).
 B A or infiltration cause significant decrease in
pulpal blood flow last for short time.

16. EFFECTS OF CROWN PREP. IN DOGS, WITH AND WITHOUTEFFECTS OF CROWN PREP. IN DOGS, WITH AND WITHOUT
WATER AND AIR SPRAY (AT 350,000 RPM) ON PULPALWATER AND AIR SPRAY (AT 350,000 RPM) ON PULPAL
BLOOD FLOW. (Pathways of the pulp, Eight Edition)BLOOD FLOW. (Pathways of the pulp, Eight Edition)

17. 2. Thermal injury.2. Thermal injury.
 Ligamental injection of (lidocain 1/100000) will
decrease the pulpal blood flow for 30 minutes.
Irreversible pulpal injury apt to occur when dental
procedure such as full crown prep performed
immediately after ligamental injection.
 However healthy pulp can withstand a period of
reduced blood flow which return to normal with
the sensory nerve activity after 3 h of total
cessation of blood flow.

18. EFFECTS OF INFILTRATION ANESTHESIA (2% LIDOCAINEEFFECTS OF INFILTRATION ANESTHESIA (2% LIDOCAINE
WITH 1:100,000 EPINEPHRINE) ON PULPAL BLOOD FLOW INWITH 1:100,000 EPINEPHRINE) ON PULPAL BLOOD FLOW IN
THE MAXILLARY CANINE TEETH OF DOGS (Kim S: Effects ofTHE MAXILLARY CANINE TEETH OF DOGS (Kim S: Effects of
local anesthetic on pulpal blood flow in dogs, J Dent Res.63 (5)local anesthetic on pulpal blood flow in dogs, J Dent Res.63 (5)
650 1984.)650 1984.)

19. LIGAMENTAL INJECTION (2% LIDOCAINE 1:100,000LIGAMENTAL INJECTION (2% LIDOCAINE 1:100,000
EPINEPHRINE) CAUSED TOTAL CESSATION OF PULPALEPINEPHRINE) CAUSED TOTAL CESSATION OF PULPAL
BLOOD FLOW FOR 30 MINUTES.BLOOD FLOW FOR 30 MINUTES.

20. 2. Thermal injury2. Thermal injury
 So cessation of blood flow for 30min +tooth prep
for full C which release--->vasoactive agent,
substance p, will result in the accumulation this
substance and other metabolic waste products
which may result in permanent damage to the
pulp.
 Based on that it is advisable to avoid ligamental
injection for cavity and crown preparation and
keep this type of injection for tooth extraction and
pulp extirpation.

21. 2. Thermal injury2. Thermal injury
 h. Blushing of teeth is attributed to frictional heat.
It is the result of vascular stasis in the
subodontoblastic capillary plexus flow, if the
dentin with pinkish hue the case could be
reversible under favorable conditions. If it is
purplish color, it indicates a thrombosis, so a poor
prognosis should be expected.
 Goodacre summarized (to minimize the thermal
effects, tooth preparation should be performed
using an ultra highspeed handpiece (250,000-
400,000 rpm) with an air-water spray from
multidirectional water ports

22. 2. Thermal injury.2. Thermal injury.
Waetr flow rate should be at 50 ml/minute
and the water should be regulated to be
below body temperature (ideally 30-34 C).
Excavation of soft tissues in the deep part of
the tooth should be done by slower speed
(160,000 rpm or less) using a new carbide
bur.

23. DIAGRAMMATIC REPRESENTATION OF THE ODONTOBLASTDIAGRAMMATIC REPRESENTATION OF THE ODONTOBLAST
LAYER AND SUBODONTOBLAST REGION OF THE PULP.LAYER AND SUBODONTOBLAST REGION OF THE PULP.
(Pathways of the Pulp, Eight Edition)(Pathways of the Pulp, Eight Edition)

24. 3. Transection of the3. Transection of the
odontoblastic processodontoblastic process
 Determining the exact cause of the death when the
odontoblasts disappear after restorative procedure
is not possible.
 Seltzer et al. showed that damaging pulp changes
may develop when impressions are taken under
pressure.
 Bacteria placed into a freshly prepared cavity were
forced into the pulp. Moreover, the negative
pressure created in removing an impression may
also cause odontoblastic aspiration.

25. SCHEMATIC ILLUSTRATION OF FACTORS THAT MIGHTSCHEMATIC ILLUSTRATION OF FACTORS THAT MIGHT
CAUSE PULPAL REACTION. (Pathways of the Pulp, EightCAUSE PULPAL REACTION. (Pathways of the Pulp, Eight
Edition).Edition).

26. 4. Crown preparation4. Crown preparation
Studies of long term effects of crown prep
on the pulp vitality found a higher incidence
of pulp necrosis.
With full crown prep 13.3%.
With partial veneer prep 5.1%.
Unrestored control tooth 0.5%.

27. 5.Vibratory phenomenon.5.Vibratory phenomenon.
 Shock waves produced by vibration are
particularly pronounced when:
 The cutting speed is reduced.
 Distorted bur.
 Loose bur clutch.
 Eccentric rotation looseness of handpiece tip.
 Crazing of enamel can be caused by eccentric
rotation of the bur.
 The vibration across enamel or early in the
dentinoenamel junction may induce slight
inflammation in the underlying pulp.

28. 6. Desiccation of dentin.6. Desiccation of dentin.
 In cavity or crown prep, about 1mm in the dentin,
(2.1--->2.5mm away from the pulp) the number of
dentinal tubules is 11000---> 36000/mxm.
 When the surface of freshly cut dentin is dried
with a jet of air, or a cavity drying agent, a strong
hydraulic forces are created on the dentinal
tubules, causing a phenomenon of odontoblast
displacement.
 In this reaction the cell bodies of odontoblast are
displaced upward in the dentinal tubules.

29. DIAGRAM ILLUSTRATING MOVEMENT OF FLUID IN THEDIAGRAM ILLUSTRATING MOVEMENT OF FLUID IN THE
DENTINAL TUBULES RESULTING FROM THE DEHYDRATIONDENTINAL TUBULES RESULTING FROM THE DEHYDRATION
EFFECTSOF A BLAST OF AIR FROM AN AIR SYRINGE.EFFECTSOF A BLAST OF AIR FROM AN AIR SYRINGE.
(Pathways of the Pulp,Eight Edition).(Pathways of the Pulp,Eight Edition).

30. ODONTOBLASTS (ARROW) DISPLACED UPWARD IN THEODONTOBLASTS (ARROW) DISPLACED UPWARD IN THE
DENTINAL TUBULES.(Pathways of the Pulp,Eight Edition).DENTINAL TUBULES.(Pathways of the Pulp,Eight Edition).

31. 6. Desiccation of dentin.6. Desiccation of dentin.
 Such displacement result in the loss of
odontoblasts which undergo autolysis and
disappear from the tubules.
 Desiccation of dentin by cutting procedures or air
does not injure the pulp.
 The destroyed odontoblasts are replaced by new
ones arise from the cell-rich zone of the pulp, and
in 1-3 month reparative dentin is formed.

32. 7. Pulp exposure.7. Pulp exposure.
 Exposure of the pulp may occur during :
 a. Cavity preparation.
 b. Excavation of carious dentin.
 c. Accidental mechanical exposure may result
during placement of pins or retention point in
dentin.
 In general, if it is sterilized exposure vitality of the
tooth could saved.
 Occasionally a pulp exposure is made unknown to
the dentist because there is no bleeding.

33. Cement forced into the pulp duringCement forced into the pulp during
cementation. pulpitis and severe pulpalgiacementation. pulpitis and severe pulpalgia
resulted.(Ingle. Blackland Endodontics 5resulted.(Ingle. Blackland Endodontics 5thth
Ed.)Ed.)

34. 8. Smear layer.8. Smear layer.
 It is amorphous smooth layer of microcristalline
debris.
 This layer may interfere with the adaptation of
restorative material to dentin, it may not be
desirable to remove the entire layer but leaving
plugs in aperture of the dentinal tubules.
 Brannstrom believes that most of restorative
materials do not adhere (has poor adaptation) to
the dentinal wall which leaves gaps invaded by
bacteria from oral cavity or the contaminated
smear layer causing pulp irritation.

35. 8. Smear layer.8. Smear layer.
 Removal of the entire smear layer by acid etching
increase the permeability of the dentinal tubules,
so the incidence of hypersensitivity and pulpal
inflammation may be increased unless a cavity
liner, base, or dentin bonding is used.
 However, current treatment of hypersensitive teeth
is directed toward reducing the functional of the
dentinal tubules to limit fluid movement.
 Four possible treatment modalities are considered
to achieve this goal:

36. 8. Smear layer.8. Smear layer.
 A. formation of smear layer on the sensitive
dentin by BURNISHING the exposed dentin.
 B. application of agents, such as OXALATE
COMPOUNS that form insoluble precipitates
within the tubules.
 C. impregnation of the tubules with PLASTIC
RESINS.
 D. application of DENTIN BONDING AGENTS
to seal off the tubules.
 LASER irradiation can modify dentin sensitivity,
but care should be considered of its pulp effects.

37. 9. Remaining dentin9. Remaining dentin
thickness.thickness.
 Dentin permeability increases almost
logarithmically with increasing cavity depth
(difference in number and size of dentinal tubules.
 Increasing dentin permeability means increasing
of dental pulp injury from restorative procedures.
 2mm of dentin thickness would protect the pulp
from the effects of restorative procedures.
(Stanley)
 In carious teeth a distance of 1.1mm or more
between the invading bacteria and the pulp the
inflammatory responses are negligible.

38. REPARATIVE DENTIN (RD) DEPOSITED IN RESPONSE TO AREPARATIVE DENTIN (RD) DEPOSITED IN RESPONSE TO A
CARIOUS LESION IN THE DENTIN. (Trowbridge HO;CARIOUS LESION IN THE DENTIN. (Trowbridge HO;
Pathogenesis of pulpitis resulting from dental caries, J Endod. 7;52Pathogenesis of pulpitis resulting from dental caries, J Endod. 7;52
1981)1981)

39. 9. Remaining dentin9. Remaining dentin
thickness.thickness.
 Reversible pulpitis can be noticed when the
remaining sound dentin between the lesion and the
pulp is about 0.5mm. The irreversible pulpitis
takes place when this reparative dentin is invaded.
 Reparative dentin has low permeability so it
reduces the incoming bacterial antigens,but it must
be considered that this not always the case.
Reparative dentin can be deposited in a pulp
which is irreversibly injured, SO PRECAUTIONS
SHOULD BE DONE IN THE DIAGNOSIS AND
TREATMENT.

40. 9. REMAINING DENTIN TH.9. REMAINING DENTIN TH.
 In young teeth,try to limit the preparation,so using
the resin bonded bridge (Maryland) is advised.
 Consider the preparatory treatment to correct the
position ( inclination, buccal or lingual position)
of the teeth to be prepared.
 Consider the tooth anatomy when you prepare.
 Use a carbide bur to excavate the soft tissues
when there is no symptoms indicating irreversible
pulpitis, but don’t excavate completely the soft
tissues over the pulp.
 Protect the dentin freshly cut by using temporary
filling or temporary crown.

41. RELATIONSHIP BETWEEN TOOTH PREP. AND PULPRELATIONSHIP BETWEEN TOOTH PREP. AND PULP
CHAMBER SIZE. THE DOTTED LINES REPRESENT PULPCHAMBER SIZE. THE DOTTED LINES REPRESENT PULP
CHAMBER MORPHOLOGY AT VARIOUS AGES. (OhashiCHAMBER MORPHOLOGY AT VARIOUS AGES. (Ohashi
Y:Shikagakuho 68:726, 1968.)Y:Shikagakuho 68:726, 1968.)

42. 10. Acid etching.10. Acid etching.
 Its designed to enhance the adhesion of the
restorative materials.
 It increases the permeability and bacterial
penetration of the dentin.
 Results of one physiologic investigation have
shown that acid etching with a remaining of 1.5m
has little effect on pulpal blood flow.
 In practice etching dentin for 15sec has no pulp
effects, but protecting the pulp when the cavity is
deep should be considered.

43. 10. Acid etching.10. Acid etching.
 Acid etching is necessary to improve bonding to
the enamel as a part of the composite technique.
 On the dentin, it is believed that it may improve
bonding by the removal of the smear layer,
grinding debris, bacteria and denaturated collagen.
 Citric and phosphoric acids were used, these
experiments were followed by pulpal
inflammatory responses.
 Apparently these reactions were because of the
strength of the acid 50%, length of application
5min, remaining dentin, micro leakage under the
resin and the bacterial invasion.

44. 10. Acid etching.10. Acid etching.
 Brannstrom, Pashley, White and Cox, concluded
that acid etching on vital dentin doesn’t cause pulp
inflammation when the acid is diluted and applied
for short time.
 Fusayama in Japan, and Kanca and Bertalotti in
U.S.A. popularized dentin acid treatment claiming
no deleterious pulpal effects, taking in
consideration the application of a dentin bonding
agent, thus eliminating the micro leakage.
 Kanca used 37% ph.acid gel for 15 sec.only,
others 10% polyacrylic, or citric acid for 10 sec .
 Acid etching open dentinal tubules so a liner or
base, or better dentin bonding agent to be used.

45. 11. Immunodefense of the11. Immunodefense of the
pulp to tooth preparationpulp to tooth preparation
 It is related to the depth and the extension of the
preparation.
 Negligible changes follow shallow preparation
with copious water coolant.
 A deep prep impact the pulp more severely with
stronger pulp cell reaction, release of substance p
and by the sensory nerve cells.
 Increasing of the pulp flow initially then decreased
severely because of the low –compliance
environment of the pulp.

46. 11. Immunodefense11. Immunodefense……
 Finally, immunodefense cells of the type that
appeared in response to caries accumulate
underneath the area of tooth preparation.
 Reaction to cavity or crown prep relatively deep
result in loss of of primary odontoblasts.
 Formation of reparative dentin by new
odontoblasts which due to a mitotic activity an
differentiation of the fibroblast in the cell-rich
zone.

47. 12. Comparison of cavity prep12. Comparison of cavity prep
by high speed handpiece andby high speed handpiece and
bur and Er.YAG laserbur and Er.YAG laser
In general there were no noticeable
histopathologic difference between the laser
and the high speed.
The question to be asked is whether the
laser is more efficient than the high speed
handpiece for this purpose.
(Erbium,chromium:yetterium-scandium-
gallium-garnet laser hydrokinetic system)

48. 13. Pin insertion.13. Pin insertion.
 Increase in pulp inflammation and death has been
noted since the use of dentinal pins.
 In a study on on extracted molar teeth, researchers
found that placing 1 pin in its correct position
caused cracks extended into the pulp in73%,2 pins
would cause pulp exposure in 93%,3 pins 98%.
 Placing the pins, close to the pulp,in presence of
calcium hydroxide protect the underlying pulp
which remained normal, otherwise it may cause
pulp inflammation. (Suzuki and colleagues)
 Pins are gradually replaced by adhesive.

49. CRACKS CAUSED BY INSERTION OF DENTINAL PIN.CRACKS CAUSED BY INSERTION OF DENTINAL PIN.
(Courtesy of Schlissell et al, J. Dent. Res.).(Courtesy of Schlissell et al, J. Dent. Res.).

50. Pin placement with calcium hydroxide.notePin placement with calcium hydroxide.note
dentinal cracks from the force of insertion.dentinal cracks from the force of insertion.
Cracks filled with CH.with moderate pulpCracks filled with CH.with moderate pulp
inflammation under affected tubules. (Suzuki M,inflammation under affected tubules. (Suzuki M,
Goto G, Jordan RE. J Am Dent.Goto G, Jordan RE. J Am Dent.
Assoc.1973;87;636).Assoc.1973;87;636).

51. Pin placement with calcium hydroxide and noPin placement with calcium hydroxide and no
dentinal fracture. Irritation dentin response isdentinal fracture. Irritation dentin response is
apparent in 28 days. The remaining dentinapparent in 28 days. The remaining dentin
thickness is 0.5mm. (Suzuki M, Goto G, Jordanthickness is 0.5mm. (Suzuki M, Goto G, Jordan
RE. J Am Dent Assoc 1973;87:636.)RE. J Am Dent Assoc 1973;87:636.)

52. 14. Pulp horn extension.14. Pulp horn extension.
 In a remarkable investigation of the coronal pulp
chamber of U/L molars, Sproles discovered
cervical pulp horns in about 66.8---->96.3%.
 This extra pulp horn, in the furcal plane area
where the pulp is only 1.5-2mm away before prep,
is considered a real danger in crown and cavity
prep.
 It is found in the mesio-buccal of the U/molars
65%,and 61%in the /L molars.

53. SPROLESPROLE’’S CERVICAL PULP HORNS, FOUN IN MULTIPLES CERVICAL PULP HORNS, FOUN IN MULTIPLE
LOCATION IN UP TO 96.3% OF MOLAR TEETH, EXTENDLOCATION IN UP TO 96.3% OF MOLAR TEETH, EXTEND
PERILOUSLY CLOSE TO THE TOOTH SURFACE NEAR THEPERILOUSLY CLOSE TO THE TOOTH SURFACE NEAR THE
CEMENTOENAMEL JUNCTION.(Sprole RA.)CEMENTOENAMEL JUNCTION.(Sprole RA.)

54. 15. Prevention of pulp injury.15. Prevention of pulp injury.
 The University of Connecticut reported that
“previous restorative treatment was the major
etiologic factor leading to root canal therapy”
 There are many day-to-day insults levied against
the pulp that can be PREVENTED: (1) depth of
cavity and crown prep, (2) width and extension of
cavity and crown prep, (3) heat damage and
desiccation during cavity prep, (4) chemical injury
through medicaments, (5) toxic cavity liner and
base, (6) toxic filling materials, and (7) prevention
of micro leakage.

55. THANK YOUTHANK YOU