CONTENTS:
¨ Introduction
¨ Stress
factors
¨ Early
crestal bone loss
¨ Various
hypotheses related to early crestal bone loss
¨ Force factors
¨ Para
function
¨ Masticatory
dynamics
¨ Position
of the abutment in the arch
¨ Direction
of load forces
¨ Nature
of the opposing arch
¨ Effect
on treatment planning
¨ Summary
¨ References
Introduction:
¨ An
understanding of the etiology of early crestal bone loss, unretained
restorations, and fracture of components enables the practitioner to develop a
treatment plan capable to reduce force factors.
¨ These
factors are evaluated in magnitude, duration, direction, type, and
magnification effects.
¨ Various
methods to reduce these factors are employed.
Implant complications from stress:
1. Implant failure
2. Early crestal bone loss
3. Occlusal overload bone loss
4.
Screw loosening (prosthesis or abutment)
5.
Implant fracture (body or component)
6. Prosthesis fracture (occlusal material or
framework)
Early
crestal bone loss:
¨ It
varies in amount and dramatically decreases after the first year. This
phenomenon is described as saucerization.
¨ The
initial transosteal bone loss around an implant forms a v- or a u-shaped
pattern,which has been described as ditching or saucerization around the
implant.
The current hypotheses for the early crestal
bone loss:
1. Periosteal Reflection Hypothesis.
2. Implant Osteotomy Hypothesis.
3. Autoimmune Response of Host Hypothesis.
4. Biological Width Hypothesis.
5. Stress Factors Hypothesis.
Periosteal Reflection Hypothesis:
¨ It causes a transitional change in the blood
supply to the crestal cortical bone. Cutting cones develop from monocytes in
the blood and precede new blood vessels into the crestal regions of bone.
¨ The
greater the amount of trabecular bone under the crestal cortical bone, the less
crestal bone loss is observed.
¨ To
place the implant in sufficient available bone, an implant ridge is usually 5mm
or wider at the crest.
¨ This
theory would lead to a generalized horizontal bone loss of the entire residual
ridge reflected not the localized ditching pattern around the implant.
Implant
Osteotomy Hypothesis:
¨ The
implant osteotomy causes trauma to the bone in immediate contact with the
implant, and a devitalized bone zone of about 1mm is created around the
implant.
¨ The
crestal region is more susceptable to bone loss during initial repair because
of its limited blood supply and the greater heat generated in this denser bone.
¨ If
heat and trauma during implant osteotomy preparation were responsible for early
crestal bone loss, the average bone loss of 1.5mm from the first thread is not
observed at second-stage uncovery surgery 4 to 8 months after implant
placement.
Autoimmune
Response of Host Hypothesis:
¨ The
primary cause of bone loss a round natural teeth is bacteria induced. Bacteria
are the causative element for vertical defects around implants.
¨ Occlusal
trauma may accelerate the process, but trauma alone is not a determining
factor.
¨ If
bacteria were causal agent for initial bone loss, why does most bone loss occur
the first year (1.5mm) and less (0.1mm) each successive year?
The
bacteria theory does not explain adequately the early crestal bone loss
phenomenon.
Biological
Width Hypothesis:
¨ Average
biological width-2.04mm
¨ The
periimplant tissues exhibit histologic sulcular and junctional epithelial zones
similar to a natural tooth.
¨ The
primary difference is the lack of connective tissue attachment and the presence
of primarily 2 fiber groups, rather than 11 with the natural tooth.
¨ James and keller
explained biological seal phenomenon.
¨ Hemidesmosomes
help from a basal lamina-like structure on the implant, which can act as a
biological seal.
¨ Hemidesmosomal
seal only has a circumferential band of gingival tissue to provide mechanical
protection against tearing.
¨ Biological
seal around dental implants can prevent the migration of bacteria and
endotoxins into the underlying bone, but it is unable to constitute junctional
epithelial component of the biologic width similar to the natural tooth.
¨ Components of the linear body cannot
physiologically adhere to or become embedded into the implant body.
Stress Factors Hypothesis:
¨ Bone
modeling and remodeling are controlled by the mechanical environment of strain.
¨ Remodeling
also is called bone turnover and allows the implant surface to adapt to its
biomechanical situation.
¨ Dental
implants are fabricated from titanium or its alloy.
¨ Modulus
of elasticity of titanium is 5 to 10 times greater than bone.
¨ When
two materials of different moduli are placed together with no intervening
material and one is loaded, a stress contour increase will be observed where
the two materials first come into contact.
¨ The
stress contours form a v- or u-shaped pattern, with greater magnitude near the
point of the first contact.
¨ The
stresses found at the crest when beyond physiologic limits may cause
microfracture of bone or strain in the pathologic overload zone and resorption.
¨ Occlusal
loads on an implant may act as a bending moment that increases stresses at the
crest.
¨ Screw
loosening and crestal bone loss are repeated with increased frequency before
the fracture of the implant body.
¨ The
bone is less dense and therefore weaker at implant uncovery than it is after 1
year of prosthetic loading.
¨ Bone
is 60% mineralized at 4 months and takes 52 weeks to completely mineralize.
¨ Partially
mineralized bone is weaker than completely mineralized bone.
¨ Woven
bone first forms around an implant.
¨ Woven
bone is unorganized and weaker than lamellar bone, which is organized and load
bearing structure.
¨ Lamellar
bone forms several months after the woven bone has replaced the devitalized
zone around the implant at insertion.
¨ The
bone changes from a fine trabecular pattern after initial healing to a coarse
trabecular pattern after loading, especially in the crestal half of the implant
interface.
¨ Density
of the bone is related directly to the strength and elastic modulus, the
crestal bone strength may increase in relation to the functional loading.
¨ Absence
of radiographic bone loss is most often observed when stress factors are
reduced.
¨ The
stress is greatest at the crest, compared with other regions of the implant
body.
¨ The
denser the bone, the less crestal bone loss observed.
¨ The
maxillary arch often exhibits greater bone loss than the mandibular arch.
¨ A
very dense bone captures the stress closer to the crestal region. Avery soft
bone allows the stress to be transmitted farther along the implant interface.
¨ The
softer the bone, the farther the stress pattern apical progression.
¨ Implants
that maintain crestal bone negate the hypotheses of periosteal reflection,
osteotomy preparation, and biological width.
STRESS FACTORS:
¨ The
etiology of early crestal bone loss and early implant failure after loading is
primarily from excess stress transmitted to the immature implant-bone
interface.
¨ One
biomechanical approach to decrease stress is to increase surface area.
¨ Another
method to decrease stress is to decrease forces.
Force may be decreased in
¨ 1. Magnitude
¨ 2. Duration
¨ 3. Type
¨ 4. Direction
¨ 5. Multiplication factors.
Force factors:
¨ Stress
is directly related to force.
¨ As
a result any force factor magnifies the stress.
¨ Once
the prosthesis type is determined, the potential force levels that will be
exerted on the prosthesis should be evaluated and accounted for in the over all
treatment plan.
¨ The
initial implant survival, early loading survival, early crestal bone loss,
incidence of abutment or prosthetic screw loosening, and unretained
restorations, porcelain fracture, and component fracture are influenced by the
factors of force.
Dental factors that affect stress primarily include:
¨ 1.Parafunction
·
Bruxism
·
Clenching
·
Tongue thrust
¨ 2. Masticatory dynamics
¨ 3. The position of the abutment in the
arch
¨ 4.
The nature of the opposing arch
¨ 5. Direction of load forces
¨ 6. The crown-implant ratio.
NORMAL BITE FORCE:
Bite Forces
¨ Perpendicular to occlusal plane
¨ short duration
¨ Brief total period(9min/day)
¨ Force on each tooth:20 to 30 psi
¨ Maximum bite force:50 to 500 psi
Perioral Forces
¨ More constant
¨ Lighter
¨ Horizontal
¨ Maximum when swallowing(3 to 5 psi)
¨ Brief total swallow time(20 min/day)
PARAFUNCTION:
¨ Parafunctional forces on teeth or implants are
characterized by repeated or sustained occlusion and have long been recognized
as harmful to the stomatognathic system.
¨ The most common cause of implant failure after
successful surgical fixation or early loss of rigid fixation during the first
year of implant loading is the result of parafunction.
¨ Complications occur with greater frequency in the
maxilla, because of a decrease in bone density and an increase in the moment of
force.
Ø
Nadler
has classified the causes of parafunction or non functional tooth contact into
the following six categories:
1. local.
2.
systemic.
3. psychological.
4.
occupational.
5.
involuntary.
6. voluntary.
¨
Local factors
include:
Tooth form or occlusion and soft
tissue changes such as ulcerations or pericoronitis.
¨
Systemic factors
include:
Cerebral palsy, epilepsy, and drug related
dyskinesia.
¨
Psychological
causes include:
The release of
emotional tension or anxiety.
They occur with
greatest frequency.
¨
Occupational
factors:
Concern
professionals such as dentists, athlets, and precision workers, musician who
develops altered oral habits.
¨
Involuntary
movement:
That provokes
bracing of the jaws.
¨
Voluntary causes:
Chewing gum or
pencils,pipe smoking.
¨
The parafunction
may be categorized as:
Ø
Absent.
Ø
Mild.
Ø
Moderate.
Ø
Severe.
Bruxism:
It is vertical or horizontal, non functional
grinding of teeth.
Biting force was greater (4 to 7times normal).
Diagnosis:
Ø
Symptoms include repeated headaches, a history of fractured
teeth or restorations, repeated uncemented restorations, and jaw discomfort on
awakening.
Ø
Signs include an increase in size of the temporal and
masseter muscles, deviation of the lower jaw on opening, limited occlusal
opening, increased mobility of teeth, cervical abfraction of teeth, fracture of
teeth or restorations, and uncemented crowns or fixed prosthesis.
Ø
The
best and easiest way to diagnose bruxism is to evaluate the wearing of teeth.
Ø
Severe
bruxism changes normal masticatory forces by magnitude (higher bite forces),
duration (hours rather than minutes), direction (lateral rather than vertical),
type (shear rather than compression), and magnification (4 to 7 times normal).
Clenching:
¨ It generates
constant force exerted from one occlusal surface to the other without any
lateral movement.
¨ The direction of load may be vertical or horizontal.
Diagnosis:
¨ Signs include tooth mobility, muscle tenderness and
hypertrophy, deviation during occlusal opening, limited opening, stress lines
in enamel, cervical abfraction, and material fatigue.
¨ The clenching patient has the “sneaky disease of
force”.
¨ Fremitus, a vibration type of mobility of a tooth, is
often present in the clenching patient.
¨ Other signs stress lines in enamel, stress lines in
alloy restorations.
¨ A common clinical finding of clenching is a scalloped
border of the tongue.
¨ The tongue is braced against the lingual surfaces of
the teeth during clenching, exerting lateral pressures and resulting in the
scalloped border.
FATIGUE FRACTURES:
¨ Increase in force magnitude and duration.
¨ Clenching patient suffer from a phenomenon called
creep, which also results in fracture of components.
Tongue Thrust and Size:
¨ Parafunctional tongue thrust is the unnatural force of
the tongue against the teeth during swallowing.
¨ A force of 41 to 709g/cm on the anterior and lateral
areas of the palate has been recorded.
¨ The force of tongue thrust is of lesser intensity than
in other parafunctional forces, it is horizontal and can increase stress at the
permucosal site of the implant.
¨ The placement of implants and prosthetic teeth in
patients with large tongue results in an increase in lateral force, which may
be continuous.
¨ A prosthetic mistake is to reduce the width of the
lingual contour of the mandibular teeth.
¨ The lingual cusp of the restored mandibular posterior
teeth should follow the curve of Wilson and include proper horizontal overjet
to protect the tongue during occlusion.
MASTICATORY DYNAMICS:
¨ They are responsible for the amount of force exerted
on the implant system.
¨ The force is related to the amount and duration of
function.
¨ The size of the patient can influence the amount of
bite force.
¨ Forces recorded in women are 20lb less those in men.
¨ The sex, muscle mass, exercise, diet, state of the
dentition, physical status, and age may influence muscle strength, masticatory
dynamics, and therefore maximum biteforce.
POSITION WITH IN THE ARCH:
¨ The maximum biting force is greater in molar region
and decreases as measurements progress anteriorly.
¨ Mansour et al. evaluated occlusal forces and moments
mathematically using a ClassIII lever arm, the condyles being the fulcrum and
the masseter and temporalis muscles supplying the force.
¨ The anterior biting force is decreased in the absence
of posterior tooth contact and greater in the presence of posterior occlusion
or eccentric contacts.
DIRECTION OF LOAD:
¨ The direction of occlusal load results in significant
differences in the amount of force exerted on an implant.
¨ Forces are tensile, compressive, or shear to the implant
system.
¨ Bone is strongest to compressive forces, 30% weaker to
tensile loads, and 65% weaker to shear loads.
¨ All the stresses occur in the coronal half of implant
bone interface.
¨ Much less stress occur with vertical loads compared
with an angled load on an implant.
¨ A lateral load on an implant crown makes the crown
height act as a lever and force magnifier.
¨ Lateral forces represent a 50% to 200% increase in
stress compression compared with vertical loading, and tensile streses may
increase more than tenfold.
¨ The direction of forces may be one of the more
critical factors to be evaluated during implant treatment planning.
¨ The average occlusal load of natural dentition is at
12 degrees to the tooth root.
¨ Mandibular premolar implants are best positioned for
axial loading.
¨ Mandibular posterior implants are placed with a facial
inclination of the implant apex, to avoid perforation of the submandibular
fossa.
¨ If the forces of occlusion are not axial to the
implant body, additional implants, wider implants, stress relievers in the
prosthesis or overdentures should be considered.
OPPOSING ARCH:
¨ Natural teeth transmit greater impact forces through
occlusal contacts than do softer-tissue borne complete dentures.
¨ Implant overdentures improve the masticatory performance
and permit a more consistent return to centric relation occlusion during
function.
¨ The maximum force is related to the amount of tooth or
implant support.
CROWN HEIGTH:
¨ It affects the amount of forces distributed to the
implant-prosthetic system in the presence of lateral or cantilevered forces.
¨ The greater the crown height, the greater the moment
of the force under the lateral loads.
¨ The crown height acts as a lever with any lateral
force.
¨ Since stresses are concentrated at the crest of the
rigidly fixated implant, the crown height multiplier increases stress rapidly.
¨ For every 1 mm crown height increase, a force increase
may be 20%.
¨ An indirect relationship is found between the crown
and implant height.
¨ The lesser the bone volume, the greater the crown
height and the greater the number of implants indicated.
AREA FACTORS:
ABUTMENT NUMBER:
¨ The overall stress to the implant system may be
reduced by increasing the surface area over which the force is applied.
¨ Most effective method to increase the number of
implants used to support the prosthesis.
¨ The retention of prosthesis is improved with greater
no. of splinted abutments.
¨ With this the amount of stress to the system is
reduced, and the marginal ridges on the implant crowns are supported by the
connectors of the splinted crowns, which applies compressive forces rather than
shear loads on the porcelain.
¨ One implant for each tooth missing may be indicated in
the posterior regions of the mouth, for a large, young, male patient with
severe parafunction.
ABUTMENT POSITION:
¨ Implant positioning is related to implant number
because more than two implants are needed to form a biomechanical tripod.
¨ Cantilevers are a force magnifier and represent a
considerable risk factor in implant support, screw loosening, crestal bone
loss, fracture.
¨ Therefore implant no. & position should aim at
eliminating cantilevers, especially when other force factors are increased.
¨ The best way to reduce risk factors is to increase
implant no.
IMPLANT SIZE:
¨ The surface area of implant support may also be
increased by the size of the implant.
¨ Each 3mm increase in height can improve surface area
support by more than 20%.
¨ The significance in increased length is not found at
the crestal bone interface but rather in initial stability and the overall
amount of bone implant interface.
¨ The increased length also provides resistance to
torque or shear forces when abutments are screwed into place.
¨ The surface area of implant support system is directly
related to the width of the implant.
¨ Each 0.25mm increase in implant diameter may increase
the overall surface area app. 5 to 10%.
¨ Bone augmentation in width may be indicated to
increase implant diameter by 1mm or app. 25% increased surface area.
IMPLANT DESIGN:
¨ Implant macrodesign may affect surface area even more
than an increase in width.
¨ A cylinder implant provides 30% less surface area than
a conventional threaded implant of same size.
¨ A threaded implant with 10 threads for 10mm has more
surface area than one with 5 threads.
¨ A thread depth of 0.2mm has less surface area than an
implant with 0.4mm.
SCREW LOOSENING:
¨ The platform of the implant body is larger in the
larger diameter implant. So less force is transmitted through screws during
occlusal loads.
¨ Screw loosening may be decreased by a preload with a
torque wrench on the screw.
¨ The threads of the screw form a 30 degree angle.
¨ A rotational force on the screw places a shear force
on the incline of the thread.
¨ Most systems use a 30 to 35 Ncm rotational force on
the abutment screw to preload or stretch the screw without risk of fracture.
¨ A more effective method to preload the screw is to
tighten the screw to the recommended amount and then untighten the screw after
a few minutes and retighten it to the required torque force again.
¨ Screw loosening is affected by the no. of threads.
¨ The height of the antirotational component of the
implant body also can affect the amount of the force applied to the abutment
screw.
¨ The higher the hexagonal height, the less stress
applied to the screw.
FATIGUE FRACTURES:
¨ Materials follow a fatigue curve, which is related to
the number of cycles and the intensity of force.
¨ The magnitude of the force increases over time because
the muscles become stronger and the number of cycles on the prosthetic components
is greater as a result of the parafunction.
BONE DENSITY:
¨ The density of bone is in direct relationship with the
amount of implant-bone contact.
¨ The less area of bone contacting the implant body, the
greater the overall stress.
¨ Progressive bone loading changes the amount and
density of the implant-bone contact.
¨ The body is given time to respond to a gradual
increase in occlusal load.
¨ This increases the quantity of bone at the implant
interface, improves the bone density, and improves the overall support system
mechanism.
¨ The very dense bone of resorbed anterior mandible (D1)
has the highest percentage of lamellar bone in contact with an endosteal
implant.
¨ Amount of stress to the implant increases in D4 bone
because fewer regions of bone contact are present.
EFFECTS ON TREATMENT PLANNING:
¨ Solution is an increase in implant-bone surface area.
¨ Additional implants are the solution of choice to
decrease stress, rather than only an increase in implant width or height.
¨ The amount of bone in contact with the implant is also
increased as a multiple of the no. of implants.
¨ The greater the diameter of the implant, the lesser
the stress transmitted to the surrounding crestal bone.
¨ An increase of 0.5mm of the abutment post diameter may
increase the fatigue strength by 30%.
The implant treatment plan
is modified primarily in two ways when implants are inserted in the posterior
region.
1.
additional implants.
2. occlusal
considerations.
¨ The elimination of posterior lateral occlusal contacts
during excursive movements is recommended when opposing natural teeth or an
implant or tooth supported fixed prosthesis.
¨ This benefits in two aspects:
¨ Use of a night guard is helpful for the bruxism
patient with a fixed prosthesis to transfer the weakest link of the system to
the removable acrylic appliance.
¨ Anterior guided disocclusion of the posterior teeth in
excursions is strongly suggested in the night guard, which may be designed to
fit the maxilla or mandible.
¨ A soft night guard, which is slightly relieved over
the implants, is used in clenching patient.
¨ A night guard with a hard acrylic outer shell and soft
resilient liner has biomechanical advantage to reduce the impact of the force
during parafunction.
¨ Unlike teeth, implants do not extrude when no occlusal
force is present. As a result, the night guard can be relieved around an
immediate implant, so the teeth bear the entire load.
¨ Implant failure during healing is parafunction found
with a patient wearing a soft tissue supported prosthesis over a submerged
implants.
¨ The time
intervals between prosthodontic restoration appointments may be increased
through progressive bone loading techniques.
¨ Anterior implants submitted to lateral parafunction
forces require further treatment considerations.
¨ Additional implants are indicated with greater
diameter.
¨ The excursions are canine guided if natural, healthy
canines are present.
¨ Mutually protected occlusion, is developed if the
implants are in the canine position or if this tooth is restored as a pontic.
¨ The forces must be disturbed along the long axis of
the implant, narrow occlusal tables to prevent inadvertent lateral forces,
decrease the forces necessary for mastication, and leave greater space for the
tongue.
¨ Enameloplasty of the cusp tips of the opposing natural
teeth is indicated to improve the direction of vertical forces, within the
guidelines of the intended occlusion.
¨ Submerged, two-phase protocols are recommended in
patients with horizontal force factors such as lateral tongue thrust.
¨ Myofunctional therapy and autogenous bone grafts to
modify the bone division for endosteal-two stage implant placement,
cantilevered bridges from the anterior teeth, or conventional removable partial
dentures are valid treatment objectives.
¨ If the anatomical conditions do not permit the
placement of implants, a removable overdenture (RP-4 or RP-5) is indicated.
¨ RP-4 or RP-5 may be removed during periods of
parafunction.
¨ Stress distributors may be used in the attachment
system.
CONCLUSION:
¨ Additional implants are the solution of choice to
decrease, along with an increase in implant width or height to decrease the no.
of pontics and dissipate stresses more effectively to the bone structure,
especially at the crest.
REFERENCES:
1.
Dental implant prosthetics – Carl E. Misch
2.
Principles and practice of implant dentistry – Charles Weiss, Adam Weiss.
3.
Tissue – integrated prosthesis. Osseointegration in clinical dentistry –
Branemark, zarb, Albrektsson
4.
Oral rehabilitation with implant supported prosthesis -Vincente
5.
ITI dental implants- Thomas G.Wilson
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