Chapter 3 Implant and prosthodontics
Professor emeritus at Okayama University
Atsushi Yamashita
I. Basic implant concepts
Medical care in the twentieth century has progressed in the form of a system that maintains a focus on
the role of doctors and disease intervention, the so-called doctor/disease-oriented system (DOS). The DOS
concept placed the disease at the center, and the doctor treated patients in a way that was believed by thedoctor to be the best for the patient. However, this concept that placed emphasis on the doctor’s discretion
carried the danger of the doctor ignoring the views of the patient. Entering the twenty-first century, theparadigm shifted to the patient/problem-orientated system (POS) where the emphasis was on the patient,and treatment of the problems presented by the patient. Here, the patient’s autonomy, right ofself-determination, inviolability, and confidentiality were respected. Concepts such as quality of life
(QOL) and informed consent were emphasized, and medical care evolved to emphasize avoidance of harmto the patient.
Where DOS treatment prevailed in prosthodontics, especially with crown prosthodontics emphasis wasplaced on the dynamic stability of prosthetic appliances for the replacement of missing teeth. Treatments
largely involved inactivating the dental pulp of the abutment by means of mass removal of teeth, which
may result in destruction or loss of biological tissues, without any regard for conservation of teeth. This
often resulted in a poor long-term prognosis and led to the development of conditions such as periapical
pathosis and root fracture.
With POS treatment, the merits and demerits of each treatment option, including no intervention, arediscussed with the patient to arrive at a conclusion that the patient agrees to. The treatment options areunderpinned by the idea of minimal intervention (MI), where mass removal of teeth is replaced byemphasis on reducing the burden on the patient, with minimal damage to the body during treatment. The
idea of MI goes along with the emphasis on progress in dental adhesive materials, and advances inprosthodontics and conservation treatments. One clinical application, the resin-bonded, fixed, partial
denture (adhesive bridge) was developed at the forefront of partial dentures that adhere to the POS
concept.
Meanwhile, the implant prosthesis has the advantage of not encroaching upon neighboring units in therecovery of missing teeth. Therefore, the prognosis for the neighboring teeth is favorable, and treatment
results in better outcomes than would be the case without intervention, such as extending the life
expectancy of neighboring teeth. This favorable outcome of implant treatment has had a great impact andhas been confirmed to support

progress of the “8020 campaign” (have 20 teeth left at the age of 80),

which was set up by the Ministry of Health and Welfare with the cooperation of with the Japan Dental
Association, to improve the dental health of the nation. Implant treatment in keeping with MI can stop
further degradation of teeth and has a positive effect on the QOL of patients with missing teeth. It is thus
a treatment option that complies with the practice of POS.
There are additional benefits of implant treatment such as a high level of aesthetic restoration, andrestoration of mastication and articulation.
However, a disadvantage of this high level of restoration of mastication is that it can lead to mechanical1

problems with the implant itself. The greater mastication force can exert extensive stress on the implant,
resulting in loosening of screws, implant body rupture and acceleration of bone resorption. Accordingly, inorder to improve the prognosis, an extensive understanding of maxillofacial biomechanics and its basis occlusion - are an essential requirement.

II. Establishing accurate occlusion
A. The survival rate of implant prostheses and reasons for failures
The implant prosthesis, if established correctly, complies well with the concept of POS-based medical care,and recent survival rates of these treatments have been 98% after five years, and 93% after ten years. In
order to increase the twenty-year survival rate to over 50%, the figure for crown bridges, it is essential totake into consideration minute details and to implement these in practice to a degree far beyond that ofprevious prosthetic methods.
The factors that influence the prognosis of the implant prosthesis are as follows: missing teeth, quantityand quality of bone, contamination, and mechanical overload. The most prevalent concomitant disease isperi-implantitis, followed by mechanical overload, which is further divided into that affecting thesuperstructure and that affecting the body.
In this book, we will discuss ways to alleviate exertion of pressure on the prosthesis, in order to reduce
the risks that may arise later.
As you likely already know, implants were not developed by research organizations such as universities,
but by general clinicians. Consequently, there are only a limited number of well-designed prospective
clinical studies evaluating factors such as occlusion of implant and parafunction. We hope that theuniversities and research organizations will take the incentive and follow up the outcomes of mechanicaloverload on the prosthesis and those affected by bruxism, and summarize these findings in a systematic
review. It is important to note that lack of evidence does not indicate that the subject matter of“investigation of the nature of occlusion” or “what occlusion should be” should be taken lightly. I report
here my findings from clinical practice and past prostheses, but must apologize for some of these, which
may be based on poor scientific evidence.

B. An understanding of functional occlusion is essential in order to establish successful occlusion
with an implant
The best documentation of physiological mechanisms involved in the stomatognathic system wascompiled by Kawamura in 1972. He concluded that precise occlusion is established as a result of “anassociation between three components involved in functional occlusion: the masticatory muscles,
temporomandibular joint and teeth/dentition, with the combined effect of neurological control by thebrain and the central nervous system” (Fig. 3-3-1).
At the same time, Jankelson B. developed a method of measuring mandibular movement electrically. The
Mandibular Kinesiograph, MKG, was used to objectively evaluate and differentiate correct occlusion fromabnormal occlusion, which in turn showed the true nature of a physiological occlusion. In the past,assessment of occlusion had placed emphasis on the tooth occlusal pattern, the plane of the occlusal
surface, and occlusal form, and it was believed that construction of a prosthesis on an articulator, that
imitated jaw movement, would harmonize with the functions of the oral cavity. Present practice has
2incorporated the functions of the neuromuscular system to arrive at physiological occlusion, and has led
to progress. I trust that this paragraph will be of some use in the long-term practice of “implant
treatments that are beneficial for patients”.

Fig.

Functional Occlusion System
Brain and the central
nervous system

3-3-1

Understanding

thefunctional

occlusion that is essential for establishingsuitable occlusion with implant prosthesis

The Masticatory muscles
Occlusion

Teeth・Dentition

Temporomandibular joint(1972, Yojiro Kawamura)

C. Why some successful occlusions collapse
A natural occlusion has to arise from the interplay of anatomically and physiologically functioning
masticatory muscles and the temporomandibular joint, and the strong occlusion of the upper and lower
mandibles is established by the coordinated activity of the central nervous system including the brain.
The mandibular position in a functional occlusion system is sustained by being suspended by themasticatory muscles and the temporomandibular joint. Fig. 3-3-2 displays the natural mandibular
position termed the “muscular contact position (MCP)” determined by Brill8). He defined the MCP as “themandibular position where the mouth is closed as the result of the lower mandible teeth slowly meetingthe upper maxillary teeth, after establishing the muscular path of closure with slight muscle movement
where the head has been held erect. This positioning of the jaw is the same as the maximum intercuspal
position (centric occlusion) in terms of anatomical expression, and has been named from a physiologicalpoint of view.” This position has been verified physiologically to be present in 90% of people with complete
permanent dentition. Brill stated that a physiologically adequate mandibular position is achieved in an
occlusion where all the teeth contact with the opposing teeth at a time without dislocation of the mandible
when gently closing the mouth after repeating a small open-and-close movement several times without
contact of teeth, and the ideal occlusion is achieved when the MCP and the intercuspal position arematched. During this series of movements, each masticatory muscle shows coordinated movements, andthe mandibular condyle is centered in the glenoid fossa. The MCP relative to the cranium is the ideal
mandibular position. The chewing pattern encountered with the mandible in the MCP and intercuspal
positions match is unlikely to vary (less than 1.0 mm), as seen in Fig. 3-3-2) and tends to be centered on
the MCP 9). If this chewing pattern is presented at the time of investigation, it can be predicted that the
implant prosthesis is unlikely to experience a lateral force or rotatory force, and therefore overload
related to occlusion is not likely to become a problem.

3

Fig. 3-3-2 Chewing pattern at theIntercuspal Position
Muscularcontact
position

ideal mandibular position “muscular
contact
muscularposition”
contact

at

which

the

positionandintercuspal position are matched

Small open‐and‐close movement
without contact ofteeth

D. Why the mandibular position shifts relative to the skull
The mandible is held in a sling below the skull. Because of this, people with complete permanentdentition are prone to displacement due to the factors mentioned in Fig.3-3-3, which may alter the way inwhich the upper and lower teeth interact. In addition, emotional stress has been reported to influence themasticatory muscles and result in a shift in the lower mandible. An excessive inlay height of just a few
microns can lead to activation of a recovery reflex that consequently results in unilateral mastication or acharacteristic chewing habit in some individuals, and thus a shift in the position of the mandible relative
to the cranium. Teeth, therefore constitute the factor that most influences occlusion. A shift in themandible influences the masticatory muscles and the jaw joint that form the functional occlusion system.
These two components can either accommodate this change or manifest with symptoms of pain anddiscomfort (Fig. 3-3-4). It is generally known that individuals with strong resistance are unlikely toexperience masticatory muscle pain or develop temporomandibular dysfunction syndrome. This concept issimilar to that encountered with cases where poor oral hygiene does not necessarily lead to periodontal
disease. In some patients, it cannot be ruled out that general malaise might arise as a result of a slightdifference in the height of the bridge. Another example is where two molars that were removed to treat
periodontal disease were not replaced. The neighboring teeth started to incline towards the empty spaceresulting in premature contact. As the patient started to avoid contact with these affected teeth, themandible was gradually displaced from its original position.
In general practice, even with this poor understanding of occlusion and its effects, it is suggested that
there are still few cases that present with malfunction of the masticatory muscles or temporomandibular
joint because of the presence of protective factors in some individuals. Even with the prevalence ofstrongly adverse factors, the resistance of the functional occlusion system and factors that preserve it
prevail. When the adverse factor and protective factor are balanced, this is termed individual, normal
occlusion; when the adverse factor is stronger than the protective factor, this is apparent malocclusion;
and when the protective factor predominates over the adverse factor, this is called inapparent
malocclusion. (Fig. 3-3-5).
4Changes to the Teeth(Contact) Tooth attrition

Teeth constitute the factor that mostinfluences occlusion(Attack factor)

 Abrasion Bruxism
 Slope of adjacent tooth loss Extrusion of opposing tooth loss High restoration
 Lack of occulusal adjustment for dentures
 Mistake of bite taking
 External wound Orthodontic therapy

Masticatory muscle

Jaw joint(Accmodator)(Accomodator)

 Emotional stress etc

Fig. 3-3-3
VariousFig. 3-3-4
changesto teeth can shiftthemandibular position “muscular position”

Changes to teeth affect the manticatory
muscle and jaw joint, resulting in the shifts
in the mandibular position.
Fig. 3-3-5 By the shifts in the balance of theprotective and adverse factors give rise to avariety of adjustments in the mandibular

Balance

Attackfactor(Change of teeth
and muscles)Normal occlusion(Individual normal occlusion)>=<

Protection
factor(Resistance offunctional occlusion
system)positions:individual

occlusion,

apparent

malocclusion, and inapparent malocclusion.

Apparent malocclusion

Inapparent malocclusion[A shift in the mandibular position relative to the cranium overloads the implanted prosthesis]
Fig. 3-3-6 illustrates an example of tapping where contact sliding is performed in centric occlusion (CO)caused by dislocation of the mandible after occlusal contact in the MCP. The chewing pattern which
results in cases such as this, a gliding motion known as “hit and slide”10), can be observed as the mandible
slides from the MCP to centric occlusion, as shown in Fig. 3-3-6. If this pattern is observed at the time of
consultation, this is an indication that there is a possibility that the implant prosthesis will repeatedly
experience lateral stress and rotational force. By modifying the occlusal interactions or the width of theocclusal plane, it should be possible to control the lateral force or the rotational forces to some extent,
however, there is a general consensus that they should be treated to restore the MCP so that functional
occlusion is not affected.

5Intercuspal Position

Muscularcontact
position

MandibleslidesTapping at
intercuspal
position

Small open‐and‐close
movement without
contact of teeth

Fig. 3-3-6
Upon dislocation of the mandible relative to the position of the skull, the chewing patter that often follows
this is where the mandible slides in a gliding motion from the MCP to centric occlusion after occluding to
the muscular position. This may not appear to be a major problem, but this motion acts as a lateral, androtational occlusal force on the implant body

E. A case study in which physiological occlusion therapy improved the occlusion, with placement of
an implant prosthesis
This chapter discusses the examination, treatment methods, and results, supported by a case study of apatient (referred to as Y.T), in order to further the understandings of functional occlusion that will be
useful in placement of implant prosthetics.
This 34 year-old female patient (a housewife) arrived with concerns as which part of the mouth she canchew with, and requested overall treatment of her jaw including implant prosthetic surgery (Fig. 3-3-7).
From the dental and panoramic radiograph, the No. 27 is shown to have P3 and a apical lesion in the
maxilla frontal teeth (Fig. 3-3-8). Currently, she also has tension in the back of her neck, stiff shoulders,
body strain and dry eyes.

Preoperative

Fig. 3-3-7

Fig. 3-3-8

The images of the intra-oral cavity of Y. T. at

Dental radiograph and panoramic radiograph ofthe time of first medical examination

Y.T. at the first medical examination

6

[Examination of occlusion (Rapid movement of opening and closing of the mouth)]
The examination of the occlusion, in conducting implant prosthetic treatment, should be done with theaim to avoid any excessive occlusal force to act on the implant prosthesis, with objective evaluation of thefunctional occlusion. Each elements of the functional occlusion system were examined using computerized
mandibular scanning (CMS) K7 Evaluation system11) that had been incorporated with the dental
electromyography and joint sonography that was a further developed from the MKG that were usedpreviously.
The recording of the case Y.T. undergoing rapid opening and closing of the mouth movement is shown in
Fig. 3-3-9, with the velocity curve shown (left) that is the differential of the vertical component taken fromthe opening and closing of the mouth movement. The state in which the masticatory muscle group acts insync with each other during opening and closing the mouth, a smooth lissajous curve is drawn. The
recordings of the present case thus show no problems with the masticatory muscles. However, thetracking of the pathway in the coronal plane show no consistency and do not conform to one mandibuolar
position, which is reflected in patient’s complaint of not knowing in which position to chew.
The kinesiograph recorded while tapping twice at the centric occlusion from the mandible rest position isshown in Fig. 3-3-10. The three graphs show the up-down movement (top), the front-back movement(middle) and left-right movement (bottom).
In a standard functional occlusion system, the free-way space from the muscular contact position isroughly 1.0 mm (±0.5 mm) and the ratio of vertical and longitudinal motion do not deviate from 1:1 to 3:1.
In this patient however, when shifting from the mandible rest position to the occlusal position, thelongitudinal movement was found to be more significant than the vertical movement, with a relatively
small free-way space that was particularly unstable at the centric position. With regard to the horizontal
movement, there is cuspal interference to the left of the muscular contact position, and the tapping was
performed by avoiding this, in an unstable state.

Fig. 3-3-9
The preoperative occlusion examination of
Y.T. (rapid switching aperture movement)
mandibular position changes every time
she chews, and is therefore unstable

7

Fig. 3-3-10
The preoperative occlusion examination ofY.T.

(tapping

and

rest

position)

mandibular position is extremely unstable
as shown in three dimensions

Left

Right

[Examination of the jaw joint]
The examination of the temporomandibular joint is essential component of functional occlusion system.
At the medical examination conducted ten years ago, an arthrosound was presented. However as there
were no pain symptoms, it was left untreated. From this, the disc displacement was suspected. Frompalpation, a clicking was heard in the right temporomandibular joint during the mouth opening andclosing therefore a MRI scan was conducted for further examination.
The MRI recording of the present case is shown in Fig. 3-3-11. A non-restorative articular disc anterior
dislocation is featured on the left, and restorative articular disc anterior dislocation on the right. Taking
into consideration, the absence of subjective symptoms, the age, and the degree of deformation, it was
decided that the correction of the articular joint was not practical, and with consent from the patient, themandibular position (muscular contact position) that was best for the functional occlusion in this state
was attempted to be achieved.

Left intercuspal position

Left open mouth position

Fig.3-3-11MRI record of Y.T. Left temporomandibular joint
disk is a non-articular disc with reduction andright disc is an articular disc with reduction

A non‐articular disc with reduction
Right intercuspal position

Right open mouth position

An articular disc with reduction[Examination and treatment plan]
The findings indicated that the masticatory dysfunction in this case to have resulted from the shift in themandibular position relative to the skull. The treatment for this was decided to be conducted in the8following order, after undergoing the diagnostics:a) Examination of occlusion with K7 Diagnostic systemb) MRI examination of the jaw jointc) Extraction of No. 2 toothd) Intrtadontal treatment of the maxillary frontal, No. 5 and No. 21 teethe) Construction of removable orthosis at the muscular contact positionf)Re-fabrication of crowns to the teeth No. 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 21, 30 and 31.

g) Placement of unilateral distal extension bar removable partial denture (RPD) with attachmenth) Perioperative examination of the occlusion with K7 Diagnostic system (for confirmation of thecorrection of occlusal position and re-evaluation of functional occlusion).
i)

Implant treatment for No. 19 and 20.

j)

The postoperative examination of the occlusion with K7 Diagnostic system.

The removable orthosis (Fig. 3-3-12) was fabricated by impression taking at the muscular contact position
after the intradontal treatment. Orthosis is a device that is employed to correct the displacement of themandibular position in relation to the skull. The different types consist of: provisional restoration12),
removable orthosis to replace the missing teeth, Auto-repositioning appliances (Shore) 13).

Fig. 3-3-12
Occlusal impression taking at the muscular
position and removable mouthpiece[Perioperative examination]
The position of the mandible was analyzed to determine whether its position had been corrected inrelation to the skull with the application of the removable orthosis.
The recording of a number of opening and closing movements of the mouth with the placement ofremovable orthosis that was fabricated at the muscular contact position, to adjust it to occlude at themuscular contact position is shown in Fig. 3-3-13. Here, the curve shows that the mandibular position tohave been corrected, and the jaw movement to have been restored close to the norm.
The coordination of the masticatory muscles during the rapid mouth opening and closing showed nosignificant differences between before and after the surgery. By adjusting the displaced mandibular
position to the muscular contact position, the occlusal position became stabilized at the muscular contact
position and narrowing the opening and closing of the mouth.
The recordings after the placement of the crown bridge, key & way attachment unilateral extension bar
RPD is shown in Fig. 3-3-15. The prosthetics were placed at the muscular contact position since theapplication of orthosis had restored the displacement of the mandibular position in relation to the skull.
The ratio of vertical movement as well as the anterioposterior movement were improved close to the norm,
but due to the fact that there has not been much time since the attachment of the crown or RPD,
9

instability can be seen in the anterioposterior direction with a slight displacement to the left side.

Fig. 3-3-13

Fig. 3-3-14

Intraoperative occlusal examination – theIntraoperative occlusal examination – therapid switching aperture movement at

rapid switching aperture movement always

the muscular position with the placement

returned to the muscular position andof removable mouth piece has returned toreturned to the norm

the norm
Fig. 3-3-15
Intraoperative

occlusal

examination–

instability in the longitudinal direction, witha shift of 0.3mm from muscular position but
the condition has been improved overall[Perioperative examination of the jaw joint]
TMJ radiogram taken by a standardized technique, the individualized anatomic X-ray aligner “condyray”,
after the occlusal correction with orthosis is shown in Fig. 3-3-16.
Although the left articular disc is shown to have been dislocated, the mandibular condyles on both sides
are positioned at the centre of the temporomandibular fossa in the occlusal position as well as in the rest
position. The articular joint spaces are also evenly distributed longitudinally.
The left two images of Fig. 3-3-17 features the electrical discharge of the anterior bellies of the temporalmuscles and the masseter muscles on the left and right, recorded at the time of strong masticatory force
application with the natural teeth without any mediators. The images on the right shows the recordingwith the application of strong masticatory force in the presence of roll of cotton placed in between theteeth. The amount of electrical discharge and the ratio both indicate its recovery to the norm. The results
confirmed the mandibular position to have been corrected to the position of the muscles, and that themasseter muscle group to be functioning in a coordinated manner.
From these findings it was concluded that the mandibular position in relation to the skull had been10

corrected, and that unwarranted occlusal pressure would not be exerted onto the implant body installed,therefore implant prosthethetic treatment was performed (Fig. 3-3-18).

Right intercuspal position

Right rest position

Left intercuspal position

Left rest position

Fig. 3-3-16
Intraoperative occlusal examination – the left
disc is non-articular but the extent of the gap isthe same in both sides, front and back, and the
mandibular condyle is placed in the middle ofglenoid cavity.

Fig. 3-3-17
Occlusal

examination

after

placementoftheremovable mouthpiece (the magnitude of muscle
discharge

when

clenched)–The

dislocatedmandibular position was corrected. Both the natural
teeth (two on the left), mediated with cotton roll(one on the right) were corrected in the magnitudeof muscle discharge, and the ratio of temporalmuscle anterior belly discharge and massetermuscle discharge.

Fig. 3-3-18
Implant prosthetic surgery was performed once the shift in the mandibular position with respect to thecranium was restored to the muscular position.
F. The effect that an implant prosthesis has on occlusion
This part will describe the care required after treatment of malocclusion to restore function in a mannerthat will last safely and stably, for a long duration.
[The fundamental differences between implants and natural teeth]
The differences between natural teeth and implants is that implants have no sensory receptors andinterference mechanism. Therefore, the feedback information from the periodontal sensory receptors
present in the natural teeth is lost. The reason why people who have a full denture on both the upper andlower jaws are still able to chew, articulate and swallow, is mainly due to the sensory receptors present in
the masticatory muscles and the temporomandibular joint (Fig. 3-3-19). The graphs shown in Fig. 3-3-20
illustrate a significant difference in the mobility of the implant with applied occlusal force, compared to11that in the natural teeth. The natural tooth is greatly affected initially by a small load, followed by more
gradual change. The periodontal membrane undergoes viscoelastic deformation, so slight loading canresult in an abrupt change, and then, with an additional load, a more gradual change. Therefore, up toloads of 1000g, the main modification will be to the periodontal membrane, with no effect on the alveolar
bone.

Burden ofocclusion
Size
Direction
Frequency

Mandibular movement

Fig. 3-3-19

Fig. 3-3-20

The implant that lacks ligaments in

The difference in transmission of the occlusal

the surrounding structures and theforces between natural teeth and implant

functional occlusion

In contrast, with an implant the load will directly affect the alveolar bone from the beginning. Thisimplies that possibility that the alveolar bone surrounding the implant will be affected by external forces14), 15). In locations where both natural teeth and implants exist, it is usually the implant that experiences

masticatory forces. Therefore, theoretically, the implant prosthesis should be fixed so that it receives thesame amount of occlusal force as the natural teeth with slight chewing, in a manner referred to as,“implant protected occlusion”, where the occlusal contact of the implant prosthetics is slightly less thanthat experienced by the adjacent natural tooth (Fig. 3-3-21).

Fig. 3-3-21
The difference in degree of sinkage between
natural teeth and implant

G. Dental overload and implant prostheses
There must be numerous people with two-piece type implants who have experienced loosening of thescrew or damage to the frame after a few years. Vertical force on the screw axis of the superstructure, or12lateral forces, which act directly on the structure, have been considered the responsible factors. The
solutions to these failures are as follows: avoid placing the occlusal contact at a point where it is distantfrom the central axis of the implant, and where the area of contact is large; in particular, take great carewhen the point of contact is located in a site that works as a cantilever; improve the accuracy with which
the framework is fitted; treat bruxism; change to a one-piece type.
In my clinical experience, when using the two-piece type, there were a number of cases where loosening of
the screws in the implant connection occurred after several years. Once the HA-coated one-piece type wasadopted, there have not been any problems, resulting in a safe and secure treatment option that is
pleasant and advantageous for the patient.
[The effects of excessive load on osseointegration of the implant]
Is excessive load, one of the causative factors for resorption of the bone, present around the implant? The
answer to this question cannot be obtained easily, as those clinical studies that have looked into thecausative mechanisms contain contrasting observations 16). Some studies have addressed the biologicalcomplications, such as infection as a factor in addition to excess load. For example, in one report 17), bone
resorption arose as a result of excess loading, and then progressed due to contamination with bacteria
from periodontal disease present in the depth of the pocket. In animal studies that used monkeys or dogs,
on placing a taller superstructure, it was found that loss of osseointegration occurred, even though initial
osseointegration was established 19). In contrast, in another study where a superstructure with an excess
height of 3 mm was placed in human subjects, the amount of bone resorption did not differ significantly
from that seen in controls 20).
It can at least be said that implant prostheses installed in patients with bruxism are likely to undergobone resorption, and the bones surrounding the implant may be damaged in the initial stages. The use ofnight guards is essential in these patients. This fact has become a consensus opinion for management of
those with extremely severe bruxism, but even in those who have very mild or no bruxism, the possibilityof occurrence of marginal bone resorption cannot be ignored in the long run.
From a general point of view, it cannot be said that normal loads exerted on the implant from normal
occlusion are without effect on the bones in the margin of the implant. To secure long-term stability, it isimportant to decrease the risks of excessive loading as much as possible.
[Anti-axial loading and bone resorption]
During preparation of the implant prosthesis, even if the appliance is designed and moulded on the model,
and the location and direction of implantation are determined with the use of stents, this does not
automatically result in implantation in an ideal position and in the direction that was initially planned.
Anti-axial loads can exert a large bending moment on the implant, and this results in bone absorption as
has been proven in an experiment using dogs 21). What is an acceptable amount of tilt? There have been
no studies of single implants installed at angles between 15° and 45°, but there has been a report thatbone resorption does not result if the superstructure is connected. It is best to implant single implants
without any tilt, and for the occlusal force to act on the long axis of the implant, as much as possible.
Where implantation was done at an angle, it is thought that the only solution to control the overload is tomodify the occlusal contact.
13

[The width of occlusal contact] (Fig. 3-3-22)
Off-axis excess loading can occur as a result of designing a occlusal surface that is larger than the
diameter of the implant axis, so that all pressure is exerted on this occlusal contact. Twil et al.
investigated resorption of the marginal bones by dividing occlusal contact areas into four different widths
that varied from less than 6 mm to more than 8 mm. They reported that there was no significant
correlation between the width and the extent of bone resorption observed. It has to be noted that thisstudy used two implants that were connected to each other, instead of a single implant. I believe that evenwhen a molar is being replaced, the width of the occlusal plane should be the same as that of the premolar,
as in the diagram. There are instances where the width of the occlusal plane of the premolar prosthesis is
not much different from that of the natural teeth. However, even in cases where the implants areconnected to each other, the width of occlusal contact should be limited, and the focus of the force should
be applied as close to the axis as possible. If an esthetic problem arises as a result of this, then it will
become important to control excessive load by altering the location of the occlusal contact.
Fig. 3-3-22
The width of occlusal plane of implant prosthesis[Connection with natural teeth] (Fig. 3-3-23)
As shown in Fig. 3-3-23, there is a difference in the extent to which the tooth sinks when a load is appliedin an axial direction to the natural tooth which has a dental annular ligament, and implants. With
replacement of the last maxillary molar, the question arises as to whether it is appropriate to bridge thecrown with the natural adjacent tooth using either rigid fixation or semi-fixation. The intention here is ofcourse to strengthen the crown and implant prosthesis, but various authors are divided in their views. On
one hand, both rigid fixation and semi-fixation are considered acceptable 24), but others suggest that increating a connection, both the adjacent teeth and the implant will result in subsidence 25). The results ofmeta-analysis indicated that in the long run, the link will lead to a decreased implant survival rate 26). In
my experience too, loosening of the screw in a semi-fixed implant has resulted after ten years. It can besaid that short-term use of stabilization with the natural adjacent teeth does not cause much of a problem,
however, long-term use decreases the implant survival rate and therefore, to be on the safe side, its use
should be avoided.

Fig. 3-3-23
The difference in degree of sinkage between
natural teeth and implant

14

[Occlusion of the single-tooth implant prosthesis]
Install the implant prosthesis so that only light contact occurs with centric occlusion with forcefulmastication, and direct the occlusal force along the long axis of the implant as much as possible. Also
avoid eccentric occlusion in so far as possible. If the implant is installed at an angle, set the prosthesis sothat the only contact is at position A, as illustrated in Fig. 3-3-24.

Fig. 3-3-24
The point of contact that controls the excessive
loading. The distances of less than 13 mm, excess
loading can become a problem if the contacts areformed on points A, B and C. Contacts formed on
points A and B, or only on A is favorable[Difficulties with implant prostheses placed posteriorly, and their occlusion] (Fig. 3-3-24)
As is well known, the loss of natural dentition occurs in the following order: maxillary second molar,
maxillary first molar, mandibular molars. Many people have likely experienced breakage of porcelainsuperstructures with exposure of the frame, within a few years. The reason for this is that masticatory
movement occurs geometrically, with the jaw joint as the 'axle'. Visualization of the operation of a pair ofscissors might be the easiest example to illustrate this concept. The quadrate portion of the massetermuscle, which is the generator of occlusion, is located in a position that resembles the pivot of the scissors,
and therefore the teeth close to this pivot experience a greater, continuous occlusal force than the frontal
teeth. The posterior teeth are also difficult to brush due to their location, therefore, the periodontal
tissues are generally much weaker than those surrounding the other teeth. In addition, if the patient has
a habit of chewing with greater masticatory force than normal, and added effects are present, such as a
distal shift of the mandibular position, tooth loss can result from strain due to overload.
Implementing consistent tooth brushing or fixing the mandibular position in relation to the skull can bedone in order to improve the situation, however, the strength of the masticatory force and the location of
the masseter muscle in centric occlusion is fixed and cannot be modified. Possible solutions are thus to
examine the extent of tooth abrasion and attrition, bulging of bone, and occlusion before the operation; toreserve clearance of over 2 mm between the prosthesis and the opposing teeth; to fit crowns made of alloy
metals, rather than porcelain, with appropriate explanation, and unless the patient is in show business;
and not to deceive the patient by telling him/her that the popular hybrid resin is a ceramic crown. Even afrontal replacement can be subject to early attrition, exposing the metallic part, and this can result in
shift of the mandible.
The occlusion should be set so that the molars are in disclusion during any mandibular movement. If
disclusion cannot be established, resolve this with the use of cuspid rise 27). The plane of occlusion should
be minimized as much as possible, and the occlusal plane at the maxillary second molar should be
reduced from a quarter to a half.
15

[Occlusion of the prosthesis of full denture implants]
Forms of “mutually protected occlusion” or “lingualized occlusion” should be used where full denture
prosthesis is created, in order to protect the frontal teeth from experiencing excessive contact.
[Occlusion of an overdenture]
When supporting a maxillary overdenture with four implants, this requires the support of the palate,similarly to normal artificial dentures. In the case of poor bone quality, or where bone has been
augmented, six implants are required for support.
For a mandibular overdenture, either support it by connecting two ball anchors (zest attachment, locator
attachment) or with a bar. Fully balanced occlusion should be provided where the extent of bone
resorption of the alveolar ridge is slight, and where there are no problems in the relationship with theocclusive teeth. However, a progressive increase in the bone resorption of the alveolar ridge is morecommon where the relationship with the occlusive teeth is problematic. Lingualized occlusion, as shownin the diagram, should be provided (Fig. 3-3-25).
Fig. 3-3-25
The occlusion applied on the overdenture that supports
the implant[Bruxism and implant prostheses]
There is consensus that the phenomenon of bruxism occurs as a result of people releasing theirfrustration. No effective treatment has been developed, and bruxism is a contraindication for implant
treatment. Recently, a simplified measuring device, called the Bite Strip (S.I.P. Ltd) was developed andhas been marketed as a way of determining whether a person is a bruxer and the extent to which thisoccurs. If it is determined with a Bite Strip that the person has bruxism, avoid an implant or if thepatient persists in demanding one, he/she should agree to wear a protector. The patient may not comply,
and in these circumstances it may be impossible to determine the reason for a problem, and where theresponsibility lies. For this reason, if it has been determined on testing that the patient is a strong bruxer,
the condition should be regarded as a contra-indication for implant treatment.
[The pros and cons of immediate loading]
As previously mentioned, at the start of this century the main aim of implant treatment clearly shifted to
improvements in patient QOL, which in turn has made shortening of the treatment period the subject offurther development. One way to reduce the period of treatment is to adopt immediate loading, withapplication of occlusion immediately following installation. The major advantage of this method is that
healing can be stimulated by applying pressure. A tooth that is attractive to the patient can also be
formed in the space of one day.
The one disadvantage of this method is that osseointegration has not yet been established, therefore, an
unstable period can persist for several weeks after the onset of loading. In the research conducted by Oh
16and Ericsson et al. 28), 29), roughly 20% of such implants failed. If an aesthetic implant is a requirement for
replacement therapy, the implanted body should be completely devoid of any occlusal pressure, and not befunctional. Regarding implants of anterior teeth, the bones on the buccal side tend to be thin, therefore,
the structural change from before to after surgery is much more distinct than with the molars. It is
generally believed that two months is adequate for molar replacements to stabilize, but with the frontal
teeth, a period of at least three months is required. A longer period is required if bone augmentation withguided bone regeneration (GBR) has been applied. Consequently, it can be said that final restorations
should not be put in place initially, before changes to the interdental papilla can be predicted. In addition,in the absence of any support for the soft tissues, it has been said that loss of the interdental papilla canbe accelerated. It can therefore be concluded that there is a significant number of problems related to theimmediate loading method of implant insertion, with regards to the effect it has on occlusal contact.

III. Postscript
The best occlusion that can be accomplished with an implant prosthesis can be said to be one that has
been controlled biomechanically, and within the tolerance limits of the prosthetics, so as to establish
long-term stability of the implant body and the superstructure. In other words, long-term stability of theimplant prosthesis is dependent on the ability to exclude those factors that augment occlusal overloading.

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