chapter 11 carl e misch

cylinder type

screw type

press fit

Combination of features

friction-fit insertion may have less risk of pressure necrosis easiest to insert high initial success rates after 5 years of loading, reports of the loss of

crestal bone and implant failure more often observed

A report by Zechner et aJ. Evaluated the peri-implant bone over a 3- to 7-year period around functionally loaded, screw-type implants with a machined surfaced V-thread and sandblasted, acid-etched, squarethreaddesign.

The range of bone loss in the study was 0.1 to 8.5 mm for the machined V-thread and 0.2 to 4.8 mm for the rough surface, square-threaded implant.

The range of bone loss and the incidence of bone loss both indicate implant design or surface condition made a difference in this report.

A prospective report by Karoussis et al. also indicated that implant survival and marginal bone loss is related to implant design.

In other words, several clinical reports found different implant designs influence not only the implant survival, but also the amount of early crestal bone loss after loading.

Dental implants function to transfer loads to surrounding biological tissues.

Thus the primary functional design objective is to manage (dissipate and distribute) biomechanical loads to optimize the implant-supported prosthesis function.

Three types of forces may be imposed on dental implant : Compression(strongest) tension(30% weaker)Shear( 65% weaker)

limit shear forces on bone, because it is least resistant to fracture under these loading conditions.

An implant has a macroscopic body design and a microscopic component to implant design:

1 .The microscopic features(important during initial implant healing and the initialloading period).

2.The macroscopic implant body design is most important during early loading and mature loading periods.

Smooth-sided, cylindrical implants :ease in surgical placement larger shear forces

smooth-sided, cylindrical, tapered implant:compressive load to the bone-implant interface, depending on the degree of taper

The greater the taper, the greater the component of compressive load. (not more 30 degree)

the greater the taper:1. the less the overall surface area of the implant body under load2. the less initial stability

1.no functional surface area advantage, because the threads of a screw bear the compressive loads to the bone.

2. some surgical advantage during initial insertion, because it inserts down within the osteotomy halfway before engaging bone.

3.the lesser surface area of a tapered implant increases the amount of stress at the crestalportion

4.at the apical half are often less deep,becausethe outer diameter continues to decrease. (limits the initial fixation).

A smooth-cylinder implant body results in essentially a shear load at the implant-bone interface.

Bone grows to a cylinder-shape implant during initial healing.

.

However, this type of body geometry must rely on a microscopic retention system such as roughening or coating for the initialloading period:

1.Etch (acidetch, mechanical etch)

2.coatings (titanium plasma spray or HA)

If the The quality of the coating is altered:

1.from friction during surgery2.infection3.mechanically removed during treatment of periimplantitis4.from bone remodeling over years

the remaining smooth-sided cylinder is severely compromised for healthy load transfer to the surrounding tissues.

The surface conditions of an implant may enhance :

1.bone-implant contact (BIC)2.adhesion qualities to the bone

However, the surface coatings on cylinders do not permit compressive forces to be effectively transmitted to the bone cells, because the microfeatures of the coating are too small for the cells to be loaded in compression."

Therefore the surface area-bone contact percentage is greater during initial healing.

but the functional surface area during long-term loading is most dependent on the macroscopic design of the implant body.

Numerous reports demonstrate roughened surfaces have higher BIC compared with machined surfaces.

implant body design was more important than the surface condition of the implant for crestal bone loss and overall BIC after loading.

Any smooth shear surface on an implant body increases the risk of bone loss because of inadequate load transfer.

The crestal bone loss contribute to an increase in crown height which further magnifies stress from bending moments.

The greater the angle of load, the greater the stresses to the implant-bone interface.

A 30-degree angled load will increase the overall stress by 50% compared with a long axis load.

the long axis should be perpendicularto the curve of Wilson and curve of Spee to apply a long axis load to the implant during occlusal load in co.

thread shape is particularly important when considering longterm load transfer to the surrounding bone interface. Figure 11-10

functional surface area is defined as the area that actively serves to dissipate compressive loads to the implant-bone interface.

Functional thread surface area, therefore, is that portion of the thread that participates in compressive load transmission under theaction of an axial (or near-axial) occlusal load.

total theoretical surface area, which may include a "passive" area on the implant that does not participate in load transfer , or has a feature so small bone cannot adapt to loadtransfer.

Duyck et al. also found that the bone density was equally distributed above and below a threaded implant after initial bone healing.

However, after dynamic loading,the bone implant density was greater on the bottomof the thread face angle and less on top of the thread.

Bolind et al. confirmed that The bone contact was:

1.least at the tip of each thread (where the highest strain occurs)

2. the greatest under the thread face angle (where the bone is loaded more in compression).

Therefore the design of the implant notonly governs the initial stability of the implant, but as important determines the BIC percent and location of contact available for effective load transfer to the bone after occlusal loading.

There are several parameters of an implant that may alter the functional surface area. Three of these include:

thread pitch thread shape thread depth

distance measured parallel betweenadjacent thread form features of an implant.

Figure 11-19).

The smaller (or finer) the pitch:

1.the more threads on the implant body

2. the greater surface area per unit length of the implant body.

implant pitch may be made smaller when:

1. great forces

2. bone quality is poor

3. inadequate length

The thread number is most significant for the shorter length implants. For example, the Straumann ITI 6- and 8-mm-long implants may only have three threads to carry the compressive load

The thread number may be affected by the implant crest module design. When the implant body has an extended smooth crest module, the number of the thread to support the occlusal load is reduced

The surgical ease of implant placement is related to thread number.

The fewer the threads, the easier to insertthe implant.

V-thread design is called a fixture and is primarily used for fixating metal parts together.

The reverse buttress thread shape was initially designed for pullout loads.

The square or power thread provides an optimized surface area for intrusive, compressive load transmission.

A buttress thread shape may also load the bone with primarily a compressive load transfer.

The V-shaped and reverse buttress thread shapes had similar BIC percent and similar reverse torque values to remove the implant after initial healing .

The square thread design had a higher BIC percent and a greater reverse torque test.

Figure 11-25 A, A long axis load to an implant body with V-thread with a 30-degree thread face converts the load direction to a 30-degree angle at the implant interface

B, A plateau or square-thread design can deliver a compressive force to the bone.

The shear force on a V-thread face that is 30 degrees Is approximately 10 times greater than the shear force on a square thread.

The shear component of a 15-degree face angle is five times greater than the shear force on a square thread.

The thread depth of an implant refers to the distance between the outer (or major) diameter and the inner (or minor) diameter of the thread.

The deeper the thread depth, the greaterthe functional surface area.

The more shallow the thread depths, the easier it is to thread the implant in dense bone, and the less likely bone tapping is required prior to implant insertion.

the implant increases in surface area by 15% to 25% for every 1-mm increase indiameter.

is the transosteal region, which extends from the implant body and often incorporates the antirotation components of the abutment implant connection.

The crest module of the implant has a surgical influence, a biological width influence, a loading profile consideration and a prosthetic influence.

The crest module of an implant should be slightly larger than the outer thread diameter of the implant body:

1. Provides a barrier for the ingress of bacteriaor fibrous tissue during initial healing

2.And provides greater initial stability of the implant following placement.

The larger crest module diameter also increases surface area, which can further decrease stress at the crestal region.

The increase in crest module diameter increases the platform of the abutmentconnection with a stress reduction to the abutment screw during lateral loading.

figure 11-37 The crest module with a cylinder metal collartransfers primarily shear forces to the bone (for left).

Figure 11-41 the The external hex designs allow the fabrication of threads closer to the crestal region of the irnplant.

The apical portion of a root form implant is most often tapered to permit the implant to seat within the osteotomy before the implant body engages the crestal bone region.

As a result, the patient does not need to open the mouth as wide, which is especially of benefit in the posterior regions of dentatepatients.

Most root form implants are circular in cross section.This permits a round drill to prepare a round hole,precisely corresponding to the implant body.

however, do not resist torsion/shear forces when abutment screws are tightened,orwhen single-tooth implants receive a rotational(torsional) force.

As a result, an antirotational feature is incorporated into the implant body, usually in the apical region.

The most common design is a hole or vent.

In theory, bone can grow through the apical hole and resist torsional loads applied to the implant.

The apical hole region may also increase the surface area available to transmit compressive loads to the bone.

A disadvantage of the apical hole occurs when the implant is placed through the sinus floor or becomes exposed through a cortical plate.

The apical hole may fill with mucus and become a source of retrograde contamination.

Another antirotational feature of an implant body may be flat sides or grooves along the body or apical region of the implant body.

The apical end of each implant should be flatrather than pointed.

Pointed geometry has less surface area , thereby raising the stress level in that region of bone.

Titanium alloy (Ti-6Al-4V) has been shown to exhibit the most attractive combination of mechanical and physical properties, corrosion resistance, and biocompatibilityof all metallic biormaterials.

The primary advantage of titanium alloy as compared with other grades of titanium is its strength.

Titanium and its alloy represent the closest approximation to the stiffness of bone of any surgical grade metal used as an artificial replacement for skeletal tissue.

even though it is almost 6 times stiffer than dense cortical bone.