Scientic Rationale for Dental Implant Design

A PRESENTATION OF:VIRENDRA VIKRAM SINGH

MDS FINAL YEAR

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.

Biomechanical load management is

dependent on two factors : The character of applied force and the functional surface area over which the load is dissipated, fundamental scientific principles related to force and surface area may be combined with engineering principles to pursue the desired clinical goals.

Bone is strongest when loaded in

compression,30% weaker when subjected to tensile force and 65% weaker when loaded in shear (attempt be made to limit shear forces on bone since it is least resistance to fracture under these loading conditions).

Force Type and Influence On Implant Body Design

An Implant has a Macroscopic body design

and a Microscopic component to Implant design.Both design features although independent are relevant for the clinical behavior .The microscopic features are most important during initial Implant healing and the initial loading period.The macroscopic implant body design is most important during early loading and mature loading periods.

Forces applied to the Implant may be

evaluated in magnitude,duration type and direction. The surface area over which the forces are applied is also relevant and is inversely proportional to the stress observed with in the Implant system. Mathematically,

Stress= Force/Surface area.

Implant Body : Functional vs Theoritical Surface area.

It can be clearly seen from this basic engineering

that, to reduce Stress force must decrease. As mentioned earlier Bone is strongest when loaded in compression,30% weaker when subjected to tensile force and 65% weaker when loaded in shear (attempt be made to limit shear forces on bone since it is least resistance to fracture under these loading conditions.

Functional thread surface area therefore is that portion of thread that participates in compressive load transmission under the action of axial occlusal load.

Theoritical surface area may include a Passive

area on the Implant not participating in load transfer or has a so small a feature such that bone cannot adapt to load transfer .Sample this: A plasma spray coatings are often reported to provide up to 600% more total surface area for the potential bone Implant contact.However size of each plasma spray particle is less than 8 microns and the 120 microns meter bone cell does not receive a transfer of mechanical stress from this feature.

The amount of actual BIC that can be used for

compressive loading may be less than 30% of the total theoretical surface area.

An improved functional surface area per unit length of the Implant ( in contrast to total surface area) is beneficial to reduce the mechanical stress to bone.

Most Stress to the Implant bone interface in D1 to D3 bone is in the crestal 5-9mm of the Implant.

The design of the Implant body in the coronal 9mm is most important to appropriately distribute occlusal stresses to bone .

Cortical & trabecular bone are modified by

modeling or remodeling .Modeling is the result of independent sites of formation and resorption that change the shape or size of bone.Remodelling is a process or resorption and formation at the same site that replaces previously existing bone and is responsible for a change in bone quality.

Scientific Rationale For Implant Design

Histologic description of bone includes – Lamellar bone,

Woven bone, Composite bone ,Bundle Bone. Lamellar bone( 1-5 micron meters per day) is most organized ,highly mineralized & strongest of the bone type.It has been called Load Bearing Bone. Woven bone / Immature (600 micron meters per day) bone since it is unorganized ,less mineralized has less strength than the other types.Therefore a higher Bone remodeling rate is directly proportional to the amount of woven bone formation.Therefore bone remodelling rate is directly proportional to the strength of the Implant interface & the degree of risk for the Implant bone interface.

Threads are designed to maximize initial

contact ,enhance surface area and facilitate dissipation of bone at the bone Implant interface. Functional surface area perunit length of Implant may be modified by varying three geometric thread parameters :

Thread Pitch,Thread Shape& Thread Depth

Thread Geometry-

Thread Pitch is the distance measured parallel

between adjacent thread form features of an Implant. The height of the threaded portion of the Implant

body divided by the pitch equals the thread per unit length.

The smaller( finer) the pitch the more threads on Implant body for a given unit length and thus the greater surface area per unit length of the Implant body if all other factors are equal.(When the magnitude of force is greater than usual finer pitch can be incorporated ).

Roberts observed in an animal model that the thread

number may effect the bone Implant contact %.When two different Implant designs were placed in the same animal ,a higher BIC was observed with the Implant with the greater thread number.Amongst all design variables, Pitch has the most significant effect on changing the surface area on a threaded Implant.

An ideal Implant length cannot be planned without advanced bone augmentation, an Implant with greater thread numbers may improve the functional surface area for the height dimension compromise.

The thread number is most significant for the

shorter length Implants.Straumann ITI 6 & 8mm long Implants may have three threads to carry the compressive loads , the thread Pitch of other Implant designs feature 7-10 threads for a similar length .The greater the thread number the greater the Initial fixation and greater the overall surface area after loading .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 insert the Implant.If fewer threads are used in dense bone , the ease of placement is improved.since hard bone is more difficult to tap and insert a threaded Implant.

Thread shape in Dental design include

Square ,Vshaped Buttress, Reverse buttress. Conventionally V shaped thread design is

called a fixture used for fixing metal parts together.Krupp’s ‘Reverse Buttress were initially designed for pull out loads.Therefore transfer for occlusal loads to the bone is similar to that of V thread design.

THREAD SHAPE-

THREAD PROFILING

Dental Implant application dictate the need for

a thread shape optimized for long term function under occlusal, intrusive load directions.

Square or power thread provide an optimized surface area for intrusive compressive load transmission .Most automobile jack or engineering design built to bear a load use some form of square design yet very few Implant design have incorporated a square thread design.

A buttress thread shape may also load the

bone with primarily a compressive load transfer.V shaped and reverse buttress thread shape had similar BIC% and similar reverse torque value to remove the Implant after initial healing.Square thread design had higher BIC % and a greater reverse torque test value.Hence thread shape is a parameter in Implant design for initial healing phase of Implant.

The face angle of the thread or plateau in an Implant

body can modify the direction of the occlusal load imposed on prosthesis and abutment connection to a different direction at bone interface.Face angle of a V shaped thread is 30º off the long axis and for a square thread is at right angle to the long axis.Occlusal loads in axial direction of Implant body may be compressive at the bone interface when Implant body incorporates square or /plateau design. Occlusal loads in axial direction of Implant body may be converted to high shear loads when Implant body incorporates V shaped thread.

Is the distance between the major and minor

diameter of the thread.Conventional Implant provide a uniform thread depth throughout the length of the Implant.A straight minor diameter used in almost every screw type result in uniform cross sectional area throughout a parallel walled Implant length.A tapered Implant has a similar minor diameter but outer diameter decrease in relationship to the taper so thread depth decrease.

Thread Depth

Yet most Implant design increase surface area by

10-30% with larger diameter Implant body ,decrease towards apical region hence tapered Implant has overall less surface area .Thread depth may be modified relative to the Diameter of the Implant and thereby overall surface area may be increased by 150% for every 1mm diameter increase.Therefore, the overall functional surface area of an Implant body is therefore related to the thread Pitch, thread shape & Thread Depth.Although molars of natural teeth have 200% more surface area than Premolars.