Aesthetic intramobile element for dental implants

An intramobile element for dental implants that includes an elastic, flat, circular washer having rounded edges, being 0.75 mm in height and designed to fit neatly on existing abutments and a prosthesis that comprises a gold member to which a porcelain cover is cast. The gold member has a cylindrical head with an internal cradle to precisely support a gold retaining screw with a rounded head and a screw channel spaced from the screw threads to allow for rotation of the prosthesis and making use of existing specific length abutments to control placement of the washer either supragingivally for hygienic reasons or supragingivally for esthetic reasons.

FIELD OF THE INVENTION 
The invention relates to a dental implant single intramobile element (IME) 
and to a special prosthesis designed to dampen the transmission of 
occlusal forces into the jawbone. More specifically, the invention is an 
implant system which: 
1) Allows aesthetic fabrication of implant restorations. 
2) Is hygienic. 
3) Is elastic and yet allows precise, predictable seating of a restoration 
in three-dimensional space, in other words, it establishes a precise 
vertical dimension of occlusion. 
4) Utilizes existing fixture and abutment components, eliminating the need 
for retooling by manufacturers. 
BACKGROUND OF THE INVENTION 
Root-form dental implants have been used with success rates of over 90% to 
replace teeth lost to various disease processes. The initial clinical use 
was to replace 12 to 14 teeth per arch on 6 to 8 implants. This is in the 
fully edentulous situation, and is rigidly fixed in place by retaining 
screws. In recent years many more applications have evolved in the 
partially edentulous patient. These include fixed bridges on as few as two 
implants, but lacking the tripod effect provided by three or more 
implants. These also include single tooth restorations, that is one tooth 
on one implant. The long-term success rates of these more recent types of 
restorations are not known. 
Current implant restorations typically conduct occlusal forces through 
porcelain, gold alloy, titanium, then to bone. All of these structures are 
rigidly fixed together with screws. 
Implants generally consist of three components. The fixture, fabricated in 
titanium, is surgically placed into the jawbone. A process called 
osseointegration occurs, in which bone forms a direct contact with the 
titanium fixture. Any fibrous tissue or other tissue type is considered to 
be a lack of osseointegration and constitutes a clinical failure of the 
implant fixture. In fully edentulous cases, failure of one or two of, say, 
eight implant fixtures does not cause the entire prosthesis to fail. 
However, in partially edentulous and single tooth applications, failure of 
even one implant fixture normally results in failure of the entire case. A 
new fixture can subsequently be placed, but this considerably delays the 
case and there is no guarantee that the new fixture will work either. It 
should be noted that approximately 50% of failures occur before loading 
with occlusal forces via a prosthesis, and most of the remaining failures 
occur within one year of each loading. 
The second implant component is the abutment, which is a cylindrical 
titanium construct with a large titanium abutment retaining screw. The 
abutment is precisely machined to fit the fixture; any abutment should fit 
any fixture with the same degree of exactness. The abutment passes from 
the distal surface of the fixture through the gingival tissue. Various 
abutment types are available. Some pass entirely through the gingiva so 
that their distal surface is visible in the mouth. Others end 
subgingivally for better aesthetics. Aesthetics depends on the contour of 
the restoration and the exact position of the distal surface of the 
abutment. 
The third implant component is the prosthesis. This is custom-cast gold 
alloy with a veneer of porcelain, and forms the actual "teeth". Casting is 
accomplished by either: 
1) A plastic cylinder, fitting to the distal surface of the abutment, which 
is cast in the lost wax technique, or 
2) A machined gold alloy to which additional gold can be cast to form the 
tooth. 
Porcelain is added after the metal frame is cast. The prosthesis is rigidly 
held in place by one or more prosthesis retaining screws. These are 
smaller than the abutment retaining screws and are made from gold, not 
titanium. Both screws are tightened with 5 to 10 Newtons of torque; this 
process causes the fixture-abutment-prosthesis assembly to be held rigidly 
together. 
Restorations on implants can be esthetic enough that they resemble natural 
teeth in color, contour and relationship to the gingiva. Titanium and gold 
can be hidden in the subgingival area, the sulcus. In these restorations, 
contoured and shaded porcelain can be carried below the gingival crest, 
into the subgingival area. For technical reasons, this porcelain must end 
1 mm above the proximal surface of the abutment. Consider the example of a 
4 mm sulcus (the distance from the crest of the gingiva to the distal 
surface of the fixture). If the desired result is a porcelain, natural 
tooth growing out of the gingiva, the abutment can be no more than 2 mm in 
height. This leaves 1 mm for the minimum metal showing on the prosthesis 
and 1 mm of subgingival porcelain as a reserve in the case of recession or 
shrinkage of the gingiva. 
Thus, many of the drawbacks of earlier implant restorations, particularly 
aesthetics deficits, have been overcome. However, concern has arisen over 
the rigidity of the complete implant system. The system possesses a very 
high modulus of elasticity, which means that it has very little 
dimensional change per unit of force applied. Occlusal forces are 
transmitted in a direct manner through the entire system into the bone. 
Some concerns are: 
1) The natural tooth possesses a periodontal ligament, which dampens 
occlusal forces. Lacking this dampening may cause failure (fracture or 
bending) of implant components. Indeed, the prosthesis retaining screw is 
gold and smaller than the titanium abutment retaining screw. This provides 
a "weak link"; the easiest component to break is also the easiest to 
remove and replace. Thus, implant design anticipates some percentage of 
component failure. 
2) Fixed bridges from natural teeth to implants have historically had poor 
success rates. The implants are so rigid that they support the restoration 
while the teeth drift out of function. The resulting torque may cause 
restoration failure or implant component fracture. 
3) The different pattern of stress induced by the more direct force 
transmission may cause failure of the bone-titanium interface 
(osseointegration). Long-term (&gt;10 year) studies on this aspect of the 
partially edentulous and single tooth situations do not exist. 
4) Extreme accuracy of fit is required between the prothesis and the 
superior surface of the abutment. Any minor imperfections in impression 
making and prosthesis fabrication become exceedingly damaging due to the 
lack of elasticity. The strain induced by these inaccuracies may cause 
failure of the bone-titanium interface. 
Various intramobile elements have been designed to simulate the function of 
the healthy periodontal ligament as occlusal function is transmitted 
through the restoration. Such function in humans varies on a range of 
100-2,440 Newtons in an axial direction, and is on the order of 20 Newtons 
in a lateral direction. A healthy periodontal ligament may depress and 
rebound 0.1 mm. An implant prosthesis, being rigid, will depress several 
orders of magnitude less. The goal of an intramobile element is to provide 
an implant system with a degree of movement under function which is 
similar to that of a natural tooth. 
DESCRIPTION OF THE PRIOR ART 
A number of patents and publications which illustrate the state of the art, 
including fixtures, abutments, prostheses and accessories such as 
tightening screws, alloys, metals and dampening gaskets will be discussed 
below. Patents which are deemed to be most pertinent will be related in 
terms of the following features, deemed illustrative of the invention. 
1) Aesthetics, 2) Location, 3) Rotational movement, 4) Changes in vertical 
dimension of occlusion, 5) Use of existing vs new fixtures/abutments, 6) 
Relative ease of replacement. 
1) Aesthetics 
U.S. Pat. Nos. 4,950,161, 4,993,950, 5,026,280, 5,040,982, 5,098,294 and 
5,174,755 all show large areas of their elastic components 
supragingivally. 
U.S. Pat. Nos. 4,488,874, 4,547,156, 4,631,031, 4,756,689, 4,993,950 
5,026,280, 5,040,982, 5,098,294 and 5,174,755 cannot maintain a natural, 
physiologic, aesthetic emergence profile, defined as the contours of the 
implant prosthesis resembling a natural tooth in its relation to the 
gingival contours as it passes through the gingiva as previously 
described. The involves bringing porcelain subgingivally. 
2) Location-subgingival vs supragingival 
Placement of an elastic member on the distal surface of the various 
abutments provides control of its final position, supragingival or 
subgingival to an exact depth. Supragingival is more hygenic; subgingival 
is more aesthetic. Logical clinical decisions could be made based on 
esthetic and oral hygiene access perameters in various areas of the mouth. 
U.S. Pat. Nos. 4,631,031, 4,993,950, 5,026,280, 5,049,982, 5,098,294 and 
5,174,755 are all supragingival by nature. 
U.S. Pat. No. 4,950,161 is possibly subgingival, but by a standard, 
non-adjustable amount. 
3) Rotational movement during lateral function 
In U.S. Pat. No. 4,631,031 threaded stud 40 and crown connector 18 do not 
allow rotation. In U.S. Pat. No. 4,756,689 flat surfaces between locking 
screw 4 and matrix 6 do not allow rotation. In U.S. Pat. No. 4,993,950 
slight rotation is allowed by the screw, but it is minimal. Flat surfaces 
between screw 30 and keeper 32 are too long and parallel. U.S. Pat. Nos. 
5,026,280 and 5,174,755 allow too much rotational movement. It is 
excessive compared to natural teeth. In U.S. Pat. No. 5,098,294 the 
prosthesis is locked in place by a screw. It relies on loose fit to rotate 
somewhat. 
4) Changes in vertical dimension of occlusion with different degrees of 
retention screw tightening. 
U.S. Pat. Nos. 4,631,031, 4,950,161, 4,993,950 and 5,098,294 would appear 
to compress to various degrees depending on the degree of retaining screw 
tightening, due to the relatively thick cross sections of their elastic 
members. U.S. Pat. No. 5,040,982 would compress to various degrees due to 
the composite nature of its elastic bushing. U.S. Pat. Nos. 5,026,280 and 
5,174,755 would also compress to varying degrees due to the large, 
irregular shape and imprecise fit of their elastic components. For 
example, a 5% dimensional change in a 5 mm elastic component is much more 
severe than a 5% change in a 0.75 mm elastic component, when considered as 
a linear measurement. Such dimensional changes would affect the height of 
the restoration in an unpredictable manner, which is unacceptable from a 
functional standpoint. 
5) Use of existing vs. new fixtures/abutments. 
U.S. Pat. Nos. 4,547,156, 4,631,031, 4,950,161 and 5,098,295 would require 
new fixture, abutment and prosthesis components, an entirely new system. 
U.S. Pat. Nos. 4,488,974, 4,756,689, 4,993,950, 5,026,280, 5,040,982, 
5,098,294, and 5,174,755 would require new abutment and prosthesis 
components, while retaining existing features. Thus no previous 
intramobile elements have utilized the newer, more esthetic abutment 
designs. 
6) Relative ease of replacement when worn. 
U.S. Pat. Nos. 4,547,156, 4,631,031, 5,040,982, 5,098,284, and 5,098,295 
appear cumbersome and time-consuming to change in clinical dentistry. 
Placing very small parts accurately into position while stabilizing and 
assembling the rest of the prosthesis would be difficult. 
Considering all the limitations of the prior art as shown by the above 
cited patents, there is clearly a need for an esthetic, precisely 
positioned intramobile element with potential for rotational as well as 
axial movement of the proper amount. 
SUMMARY OF THE INVENTION 
The invention utilizes the existing conventional features and abutments of 
a number of dental manufactures these products could be used. It is the 
object of this invention to provide for these existing systems a resilient 
or elastic member and a prosthesis design which allows axial and 
rotational movement similar to a healthy periodontal ligament on a natural 
tooth. 
It is a further object of this invention to maintain a high degree of 
control of aesthetic, meaning the concealment of metals and elastic 
materials where their visibility would be objectionable to the patient. 
It is a further object of the invention to allow precise positioning 
(especially in height or vertical dimension of occlusion) when the 
prosthesis retention screw is tightened intra-orally. 
The invention consists of an elastic member, hereinafter referred to as the 
gasket, and a specific prosthesis/prosthesis retaining screw assembly. The 
majority of abutments have a flat surface at the perimeter of their distal 
surface. The central portion of this distal surface is contoured in 
various complex fashions to allow for placement of the abutment retaining 
screw and for the particular fit of the prosthetic component. 
The gasket portion is a flat, washer-shaped circular construct whose 
proximal surface rests on the flat portion at the perimeter of the distal 
surface of the abutment. The gasket is fixed in space by both its inner 
circular aspect engaging the abutment center and by being compressed 
between the prosthesis and the abutment distal surface. 
The prosthesis portion has a centrally located non-threaded channel for a 
retention screw. Because there is a space between the threads of the screw 
and the walls of the channel, rotation of the entire prosthesis can occur 
under lateral, occlusal forces. 
When a direct axial load is applied to the prosthesis, the gasket will 
compress. Its modulus of elasticity is such that the compression is 
approximately 0.1 mm, the same as that of a healthy periodontal ligament 
during an average occlusal load. When lateral components of force occur, 
the round shape of the prosthesis/prosthesis retention screw interface 
will allow rotation and a differential compression of the gasket, more on 
the side of greater occlusal loading. This combination of the round shape 
and the shaped gasket will dampen the force transmitted into the titanium 
fixture and the bone. 
An inherent problem with elastic members of implant systems is that they 
have the potential to alter the vertical dimension of occlusion in an 
unpredictable manner. In other words, if a clinician overtightens a 
retaining screw, the prosthesis will depress and be in hypofunction. In 
the present invention the gasket is intended to have a high enough modulus 
of elasticity so that the 10 Newtons of average torque that a clinician 
applies to the prosthesis retention screw will not cause significant 
gasket compression. However, the direct axial loads applied by patient's 
occlusal forces, being 10 to 200 times greater, will cause the desired 
gasket compression. The desired precise positioning of implant components 
is still achieved, since the gasket adds a 0.75 mm component to the 
system. This component cannot be altered by variations in tightness of 
prosthesis retention screws, as occurs with different clinicians. 
Aesthetic is controlled by placment of the gasket at various levels, using 
the distal surface of the abutment as a reference point. If this surface 
is 2 mm below the top of the subgingival surface, the gasket will be 
invisible. On the other hand if aesthetic is not a concern, the gasket can 
be in the supragingival, most hygienic position. In any event, the 
clinician has complete control on a tooth-by-tooth basis, something not 
previously achievable with previous intramobile elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 there is seen an implant fixture 1 which is screwed into the 
jawbone J to the level indicated by line 10. The abutment 2 is screwed 
into fixture 1 by an abutment retaining screw having outwardly tapered 
head 4 and threaded shaft 4a. In the case of a 2 mm abutment the height of 
the crest of the gingiva would typically be at line 11. Note that the 
components described thus far constitute the prior art. The abutment is 
the "tapered" type. The invention is designed to be adapted to other 
abutment types for example, the standard and the conical. 
Gasket 7 rests on the outer perimeter of the distal surface of the 
abutment. The gasket is made of an elastic material such as 
polyoxymethylene. The inner aspect of the gasket is round, precisely 
matched to fit around the abutment retaining screw head as seen in FIG. 3. 
The gasket is a flat washer-shaped circular construct with slightly 
rounded edges and is 0.75 mm in height. The gasket will have a modulus of 
elasticity such that the 0.75 mm height compresses approximately 0.1 mm in 
response to normal occlusal forces. This degree of stiffness would not 
allow any significant compression during normal hand tightening of the 
prosthesis retention screw. The gasket will have minimal water solubility 
and minimal water absorption so as to avoid dimensional change. It will 
have minimal porosity, to eliminate bacterial growth. It will have a 
lifetime retaining its full elastic properties measured in 10 to 10.sup.10 
cycles. This gives a clinical lifetime of one to three years before any 
loss of elastic properties occurs. The gasket material will be white or 
grey in color if compatible with the other properties. 
The prosthesis includes an assembly having a portion 5 made of precision 
machined gold alloy construct and a portion 3 made of porcelain and cast 
to portion 5. The head 5a of the prosthesis is cylindrical and the 
remainder 5b devolves into a truncated cone. The truncated cone has a 
proximal surface 12 which is flat and rests on the distal surface of the 
gasket. This bearing surface is responsible for transmitting occlusal 
forces to the gasket. The prosthesis has a non-threaded channel 15 running 
through its center for the prosthesis retaining screw 6. At the distal 
surface of this channel is a cradle 13 which is countersunk and fits 
precisely to the proximal rounded surface of the screw head 6a when screw 
shaft 6b has engaged the abutment head. The channel 15 is also free from 
any contact with the prosthesis retaining screw. The rounded screw head 6a 
takes the form of an arc of a circle, the center 8 of the circle lying 2 
mm from the point where the screw head meets shaft 6b, thus forming the 
screw head on the arc of a 4 mm circle. A screw access channel 14 directly 
above the cradle provides an opening to allow placement and retrieval of 
the retention screw, which in turn allows placement and removal of 
prosthesis 3. The internal contours of the prosthesis provide a 0.5 mm 
relief from the centrally located tapered abutment screw and the shaft of 
the retention screw. 
Under function resulting from occlusal forces, the fixture, the abutment, 
the abutment screw, and the prosthesis retention screw all remain rigid as 
in any current implant system. The prosthesis 5 is, however free to move 
axially, rotationally or any combination thereof. The cradle of the 
prosthesis 5 either moves bodily away from the prosthesis retention screw 
head or it rotates along the curve of such head, in both cases compressing 
the gasket. This action serves to mimic the function of the periodotal 
ligament in natural teeth. 
It will be understood by those skilled in the art that various 
modifications may be made in the components described in this invention 
without departing from the scope of this invention as claimed.