Biomechanics, Treatment planing and prosthetic

considerationsDr. Nitika Jain

Biomechanical considerations◦ Bone density◦ Load bearing capacity◦ Linear configurations and implant overload

Treatment planing with dental implants◦ Edentulous maxilla◦ Edentulous mandible◦ Partially edentulous patients

Single unit Multi unit

Contents

Clinical strategies to avoid implant overload and other prosthetic considerations◦ Connecting implants with natural dentition◦ Immediate or early loading in posterior dentition

Conclusion

Bone density – a key determinant for treatment planing

Available bone is particularly important in implant dentistry and describes the external architecture or volume of the edentulous area considered for implants.

The internal structure of bone is described in terms of quality or density - biomechanical properties like ◦ Strength ◦ Modulus of elasticity

Most dense bone is ◦ Anterior mandible◦ Anterior maxilla◦ Posterior mandible◦ Posterior maxilla ( least dense bone)

Influence of bone density on implant success rates

Adell et al ◦ 10% greater success rate in anterior mandible as

compared to anterior maxilla Schnitman et al

◦ Reported lower success rate in posterior mandible as compared with anterior mandible

Highest clinical failure – posterior maxilla ◦ Where the force magnitude is greater and bone

density is poorer

Various studies

Linkow 1970◦ Class I bone structure: The ideal bone type

consists of evenly spaced trabeculae with small cancellated spaces.

◦ Class II bone structure: The bone has slightly larger cancellated spaces with less uniformity of the osseous pattern.

◦ Class II bone structure: Large marrow-filled spaces exists between bone trabeculae.

Bone Classification

Lekholm and Zarb 1985

Misch bone density classification

Radiographic bone density

The bone density may be different near the crest, compared with the apical region where the implant is planned.

The most critical region of bone density is the crestal 7 to 10mm of bone.

Therefore, when the bone density varies from the most crestal to apical region around the bone, the crestal 7 to 10mm determines the treatment plan protocol.

Bone density is directly proportional to the strength of the bone before micro fracture.

A ten fold difference in bone strength from D1 to D4.

D2 bone exhibited a 47% to 68% greater ultimate compressive strength compared with D3 bone.

Bone strength and density

Elastic modulus describes the amount of strain ( changes in length divided by the original length) as a result of a particular amount of stress.

Relates to stiffness of the material. The elastic modulus of bone is more flexible

than titanium.

Elastic modulus and density

When higher stresses are applied to an implant prosthesis, the titanium has lower strain (change in shape) compared with the bone. ◦ The difference between the two materials may create

micro strain conditions of pathologic overload and cause implant failure.

But when stresses applied are low, the micro strain difference between titanium and bone is minimized and remains in the adapted window zone, maintaining load bearing lamellar bone at the interface.

The initial bone density not only provides mechanical immobilization of the implant during healing, but after healing also permits distribution and transmission of stresses from the prosthesis to the implant –bone interface.

Open marrow spaces or zones of unorganized fibrous tissue do not permit controlled force dissipation or micro strain conditions to the local bone cells.

Bone density and bone-implant contact percentage

More the area is in contact with the implant interface so more force dissipation. (BIC)

BIC % is more in cortical bone as compared to trabecular bone.

D1 – 85% BIC D2 – 65 to 75% BIC D3 – 40 to 50% BIC D4 – fewer areas of BIC

Crestal bone loss and early implant failure after loading results may occur from excess stress at the implant-bone interface.

As a result of the correlation of bone density, elastic modulus bone strength, and bone implant contact percent, when a load is placed on the implant, the stress contours in the bone are different for each bone density.

Bone density and stress transfer

D1 – strains are near the crest, stress in this region are of less magnitude

D2 – sustains a highly greater crestal strain, intensity of stress extends apically

D4 – greatest crestal strains and stress are extended farthest apically along the implant body.

Treatment planing

Four facts form the basis for treatment plan modification in functioning of the bone quality:◦ Each bone has a different strength◦ Bone density affects the elastic modulus◦ Bone density result in different amount of bone-

implant contact percentage◦ Bone density differences result with a different

stress-strain distribution at the bone implant interface.

A thorough understanding of implant biomechanics is essential if implant –retained restorations are to be employed predictably.◦ The load bearing capacity of implants supporting

the restoration must be greater than the anticipated loads during function.

◦ If the loads applied exceed load bearing capacity of the implants, the prosthesis, or the supporting bone, implant overload may result in mechanical or biologic failure.

Biomechanical considerations

Types of failure

Mechanical failure

Biological failure

Biological failure◦ A resorption –remodeling response of the bone around

the implant is provoked , leading to progressive bone loss.

◦ In some cases, bone loss around the implant progresses until the implant is no longer supported and osseointegration is lost. Brunski J et al 2000

Mechanical failure◦ Screws that secure the restoration may bend, loosen,

or fracture. ◦ The most devastating type – fracture of the implant

Osseo integrated implants and prosthesis are rigidly connected with the jawbone, and no movement is possible.

Any movement of a dental implant is indicative of failure or loss of osseointegration ( fibrous encapsulation).

As a result of this rigid relationship, the dental implant, the attached implant –retained restoration, and the surrounding bone are not adaptive to adverse or excessive forces.

If occlusal loads exceeds the tolerance of the implant, the connecting components, the attached prosthesis, or the supporting bone to withstand the stress, then fatigue, fracture, or failure will occur.

Implant biomechanics

Implant biomechanics

Cantilevers on the prosthesis should be reduced or preferably eliminated; therefore the terminal abutments in the prosthesis are the key positions.◦ Force magnifiers

Rule 1

Three adjacent pontics should not be designed in the prosthesis

Rule 2

The canine and the first molar sites are the key positions, especially when adjacent teeth are missing.

Rule 3

An arch is divided into 5 segments. When more than one segment of the arch is being replaced, a key one implant position is at least one implant in each segment.

Rule 4

The quantity and quality of bone support around the dental implants ◦ Influence load bearing capacity◦ Resistance to occlusal loading

Bone appositional index◦ Percentage of bone-to-implant contact

The lower the bone-to-implant contact and the lower the bone density surrounding the implants and the resistance to occlusal loading the lower will be the support of the implants and the resistance to occlusal loading.

Quality of bone

The bone appositional bone index in the post. Maxilla ranges from 30-60% whereas in ant. Mandible its 65-90%

Anatomic structures and lack of bone height in the posterior mandible and maxilla limit the amount of available bone for placement of long implants and thus reduce the potential for bone-to-implant contact.◦ Techniques like lateral nerve repositioning is

possible but has a moderately high morbidity.◦ Sinus floor elevation and bone augmentation

procedures - enabled to increase the height of bone available in the post. Maxilla thus allowing for the placement of longer implants with improved results.

Implant length

The implants with an altered microtopography (acid etched) can achieve a greater bone-to-implant contact in poor quality bone (eg. Trabecular bone of posterior maxilla) than implants with a machined surface.◦ Lazzara Rj et al IJPRD 1999◦ Trisi P et al JP 2003

Implant surface

Implant diameter

Implant diameter

Earlier in 1980 and 1990, posterior maxilla were restored with one or two implants or in some pts. 2 implants were used to support with three of four dental units.

◦ Currently it is imperative, that treatment of posterior segments with one implant for every missing tooth that will be restored.

◦ Also if space permits, it is desirable to use a minimum of three implants to replace the missing posterior teeth in the maxilla.

Implant number

Implant number

When implants are arranged in a linear fashion, the biomechanics with respect to anticipated bone response are quite unfavorable compared with a configuration where the implants are arranged in a non-linear (curvilinear or staggered) fashion.

Linear configuration and implant overload

Arranging implants in a nonlinear manner creates a more stable base that is more resistant to the torquing forces created by off centre contacts and lateral loads.◦ This is particularly true when loads are not

applied along the long axis of the implant.

Implant supported FPD restoring partial posterior quadrants – nonaxial loads can cause sufficient load magnification at the bone-to-implant interface, resulting in bone resorption and higher rates of implant failure.

This has been supported by numerous FEA studies, which clearly demonstrate that non-axial forces significantly increase the stress concentration to the cortical bone around the neck of the implant.

Finite element analysis (FEA) is a computerized investigative method that uses a mathematic model to assess stress in various objects and their surroundings when subjected to forces. It is useful in generating a hypothesis and testing basic biomechanical mechanisms but cannot be relied on for definitive answers.

Only hard clinical evidence is undisputed and any assumption or predictions that are made by FEA needs to be validated clinically.

Finite element analysis

Using the finite element analysis (FEA), Pierrisnard and colleagues showed that greater implant length did not positively affect the way stresses were transferred to the implant but found that increasing implant diameter reduced the intensity of stress along the length of the implant.

Iplikcioglu and Akca using the same method observed that wider implants rather than longer implants registered lower stress value to the whole system, suggesting that the use of short, wide implants could increase the load-bearing capacity of implants and implant prosthesis.

Baggi and colleagues also used FEA to show that increases in implant width reduced stress more than increases in length.

FEA studies

Non axial loads can lead to implant overload (load magnification)

Precipitates a resorptive remodeling response of the bone around the neck of the implant

When load persists, the bone loss progresses and can lead to implant failure.

Brunski et al proposed that excessive occlusal loads lead to micro damage (fractures, cracks) of the bone adjacent to the implant, which provokes a resorptive remodeling response.

Linear implant configuration in the posterior mandible and posterior maxilla are particularly prone to bone loss when loads are not applied axially.

Bone loss in posterior Implants is more damaging because implants in these areas are primarily supported by the cotical bone around the coronal aspect.

Therefore, posterior implant should be positioned such that occlusal forces can be directed down the long axis of the implant (axial loads).

Also, the final restoration will be more simple and more cost effective to fabricate when angled or custom abutments are not required.

Extreme damage is seen in cases of posterior Implant supported restoration with a cantileverd pontics when nonaxial occlusal forces are present.

Because occlusal forces were directed to the pontic created torquing forces around the neck of the implant closest to the cantilever.

Therefore, cantilevered pontics are contraindicated for unilateral posterior, implant supported restorations.

Angulation of the implants in relation to the plane of occlusion and the direction of the occlusal load - important factor in optimizing the transfer of occlusal forces to implants.

Earlier in 1980s, many implants placed in posterior Maxilla exhibited buccal angulation or resulted in restorations with buccal cantilever or may be excessive distal angulation.

Angulation

Minor discrepancy in angulations are not significant, but if loads are at an angle of 20 degrees or more to the axis of the implant, load magnification resulting in resorptive remodeling response of the adjacent bone.

Edentulous maxilla Edentulous mandible Partially edentulous patients

◦ Multiunit restoration in post quadrants◦ Single – tooth implants in post. Quadrants

Treatment planning with dental implants

◦ Poor ridge form, conventional maxillary denture is marginally stable.

◦ 2 or 4 implants will provide greater stability and security of maxillary denture in function when the maxillary ridge is severely resorbed and lacks resistance to lateral forces.

◦ Intact mandibular anterior dentition but lacks posterior Support. Implants in the maxilla can offset the potentially destructive effects on the premaxillary region when a mandible with natural anterior teeth and missing posterior teeth opposes and edentulous maxilla.

Edentulous maxilla

If pt. Cannot tolerate palatal coverage. ◦ The palateless denture, which may enhance their

sensation of taste and texture or may simply provide a psychological advantage.

◦ To inhibit gag reflex ◦ Large palatal tori

Then minimum of 4 implants with adequate A-P spread allows the fabrication of an implant assisted over denture without palatal coverage.

The maxillary sinus limits the height of bone available for implant placement in the posterior region. As a result, the A-P spread is limited. If the A-P spread is inadequate to provide support, a full-palatal-coverage overlay denture is recommended.

Due to alveolar ridge resorption after tooth loss in premaxillary region, the adequate support for the upper lip is lacking.

Thus, in most pts. Its advisable to construct an implant – assisted maxillary overdenture (not an implant supported fixed prosthesis.)

Lower cost, improved hygiene access, and predictable speech articulation benefits that favor the use of an overlay denture in the edentulous maxilla over an implant – supported fixed prosthesis.

Implant-assisted overlay denture. A, Clinical photograph of four-implant bar in the maxilla designed to retain a palateless overlay denture. B, Photograph of clip and attachment design of palateless overlay denture. C, Cross-section of Hader bar clip attached to anterior bar (inset). D, Axis of rotation and function of resilient attachment.

Mandibular complete denture is more problematic as compared to maxilla◦ Specially for pts. With severely resorbed atrophic

ridges Lack of stability and retention

The 2 implant assisted over denture is the best treatment for such patients

Edentulous mandible

Place two implants in the anterior mandible with a connecting bar.

One or two clips retain the denture over the bar.

Fixed implant supported prosthesis require 4,5 or 6 implants arranged in an appropriate arc of curvature with at least 1cm of A-P spread.

Implant-assisted overlay denture. A, Clinical view of overdenture in occlusion.B, Photograph of mandibular overlay denture (tissue-bearing surface) designed for an implant bar attached to two implants in the anterior mandible. C, Clinical view of bar attached to two implants in the anterior mandible. D, Illustration demonstrating how axis of rotation allows denture to rotate around the bar.

Multiunit restorations in posterior quadrants

◦ Lowest success for short span restorations in posterior maxilla Maxillary sinus. Inferior nerve position and also quality

of bone in posterior ◦ Therefore, rough implants will improve the bone

anchorage but in some pts. It may not provide anchorage to support unilateral, implant supported, FPD if implants are too short.

◦ Also, the acid etched surfaces – much better anchorage ( bone deposited is harder and denser and more resistant to resorptive remodeling)

Partially edentulous patients

Single tooth implants in posterior quadrants◦ Maxilla Vs mandible◦ In mandibular first molar - conventional diameter

3.75 or 4.0 mm - unfavorable results

◦ When external hex-headed implants were used – loosening of the screw

because the diameter of the implant head is much smaller than the size of the occlusal surface.

Tipping of restoration – leads to stretching and loosening of the screw

Single-tooth restoration in the posterior mandible supported by a wide-diameter implant. A, Clinical photograph of healing abutment on wide-diameter implant. B, Photograph of laboratory model with single molar. C, Clinical photograph of molar crown supported by wide-diameter implant. The use of wide-diameter (external hex) implants eliminates the problem of screw loosening for single-tooth, posterior, implant-supported crowns.

1. Place implants perpendicular to the occlusal plane

2. Place implants in tooth positions3. Use an implant for each unit being replaced4. Avoid the use of cantilevers in linear

configurations5. Avoid connecting implants to teeth6. If connecting implants to teeth, use a rigid

attachment7. Control occlusal factors such as cusp angles and

width of occlusal table8. Restore anterior guidance if possible

Strategies to avoid implant overload

Multiunit implant restorations should be splinted to maximize implant support (sharing the loads), and emergence profiles should be developed with open embrasure spaces to facilitate oral hygiene.

Occlusal design for implant-supported prostheses is an essential and integral determinant of overall treatment planning.

The risk of implant overload can be minimized by ◦ limiting the width of the occlusal table of the

implant-supported fixed partial denture, ◦ flattening the cusp angles, ◦ avoiding the use ofcantilevered restorations, ◦ and restoring the anterior guidance provided by

the anterior dentition.

Occlusal design

it is advisable to keep implant-supported restorations separate from natural teeth◦ Implants and teeth function differently and

connecting them can lead to complications such as screw loosening and intrusion of natural dentition.

◦ Teeth have the capacity to move under functional occlusal loads while implants do not.

Connecting implant to teeth

Specifically, if implants are to be connected to the natural dentition, it should be done in a rigid manner, ◦ either with screw-retained attachments or ◦ with copings secured by permanent cement. ◦ Tooth preparation should allow good retention,

teeth should be periodontally healthy and stable, and the occlusal scheme should be good.

Feasible when ◦ implants are placed in good quality bone◦ Are used to retain implant assisted overlay

denture

But in cases of posterior quadrants, the immediate or early loading is inadvisable

Immediate or early loading in posterior quadrants

The importance of biomechanics and the limitations of implant systems were initially underestimated. Over the years, clinical experience and research underscored the importance of biomechanics in the success and predictability of implant-retained prostheses.

The rigid nature of implant-retained restorations and the lack of forgiveness in these systems demands a revised approach to treatment planning that is now applied.

The biomechanics must be factored into the planning at the beginning of any implant treatment to achieve long-term, predictable success.

Conclusion

Newman, Takei, Klokkevold, Carranza. Carranza’s Clinical Periodontology, 10th Edition and 11th Edition

Lindhe, Lang, Karring. Clinical Periodontology & Implant Dentistry, 5th Edition.

Carle E. Misch. Contemporary Implant Dentistry. 3rd edition.

References

Robert