Thursday, August 15, 2013
Stress and force factors in Implants
¨ Stress factors
¨ Early crestal bone loss
¨ Various hypotheses related to early crestal bone loss
¨ Force factors
¨ Para function
¨ Masticatory dynamics
¨ Position of the abutment in the arch
¨ Direction of load forces
¨ Nature of the opposing arch
¨ Effect on treatment planning
¨ An understanding of the etiology of early crestal bone loss, unretained restorations, and fracture of components enables the practitioner to develop a treatment plan capable to reduce force factors.
¨ These factors are evaluated in magnitude, duration, direction, type, and magnification effects.
¨ Various methods to reduce these factors are employed.
Implant complications from stress:
1. Implant failure
2. Early crestal bone loss
3. Occlusal overload bone loss
4. Screw loosening (prosthesis or abutment)
5. Implant fracture (body or component)
6. Prosthesis fracture (occlusal material or framework)
Early crestal bone loss:
¨ It varies in amount and dramatically decreases after the first year. This phenomenon is described as saucerization.
¨ The initial transosteal bone loss around an implant forms a v- or a u-shaped pattern,which has been described as ditching or saucerization around the implant.
The current hypotheses for the early crestal bone loss:
1. Periosteal Reflection Hypothesis.
2. Implant Osteotomy Hypothesis.
3. Autoimmune Response of Host Hypothesis.
4. Biological Width Hypothesis.
5. Stress Factors Hypothesis.
Periosteal Reflection Hypothesis:
¨ It causes a transitional change in the blood supply to the crestal cortical bone. Cutting cones develop from monocytes in the blood and precede new blood vessels into the crestal regions of bone.
¨ The greater the amount of trabecular bone under the crestal cortical bone, the less crestal bone loss is observed.
¨ To place the implant in sufficient available bone, an implant ridge is usually 5mm or wider at the crest.
¨ This theory would lead to a generalized horizontal bone loss of the entire residual ridge reflected not the localized ditching pattern around the implant.
Implant Osteotomy Hypothesis:
¨ The implant osteotomy causes trauma to the bone in immediate contact with the implant, and a devitalized bone zone of about 1mm is created around the implant.
¨ The crestal region is more susceptable to bone loss during initial repair because of its limited blood supply and the greater heat generated in this denser bone.
¨ If heat and trauma during implant osteotomy preparation were responsible for early crestal bone loss, the average bone loss of 1.5mm from the first thread is not observed at second-stage uncovery surgery 4 to 8 months after implant placement.
Autoimmune Response of Host Hypothesis:
¨ The primary cause of bone loss a round natural teeth is bacteria induced. Bacteria are the causative element for vertical defects around implants.
¨ Occlusal trauma may accelerate the process, but trauma alone is not a determining factor.
¨ If bacteria were causal agent for initial bone loss, why does most bone loss occur the first year (1.5mm) and less (0.1mm) each successive year?
The bacteria theory does not explain adequately the early crestal bone loss phenomenon.
Biological Width Hypothesis:
¨ Average biological width-2.04mm
¨ The periimplant tissues exhibit histologic sulcular and junctional epithelial zones similar to a natural tooth.
¨ The primary difference is the lack of connective tissue attachment and the presence of primarily 2 fiber groups, rather than 11 with the natural tooth.
¨ James and keller explained biological seal phenomenon.
¨ Hemidesmosomes help from a basal lamina-like structure on the implant, which can act as a biological seal.
¨ Hemidesmosomal seal only has a circumferential band of gingival tissue to provide mechanical protection against tearing.
¨ Biological seal around dental implants can prevent the migration of bacteria and endotoxins into the underlying bone, but it is unable to constitute junctional epithelial component of the biologic width similar to the natural tooth.
¨ Components of the linear body cannot physiologically adhere to or become embedded into the implant body.
Stress Factors Hypothesis:
¨ Bone modeling and remodeling are controlled by the mechanical environment of strain.
¨ Remodeling also is called bone turnover and allows the implant surface to adapt to its biomechanical situation.
¨ Dental implants are fabricated from titanium or its alloy.
¨ Modulus of elasticity of titanium is 5 to 10 times greater than bone.
¨ When two materials of different moduli are placed together with no intervening material and one is loaded, a stress contour increase will be observed where the two materials first come into contact.
¨ The stress contours form a v- or u-shaped pattern, with greater magnitude near the point of the first contact.
¨ The stresses found at the crest when beyond physiologic limits may cause microfracture of bone or strain in the pathologic overload zone and resorption.
¨ Occlusal loads on an implant may act as a bending moment that increases stresses at the crest.
¨ Screw loosening and crestal bone loss are repeated with increased frequency before the fracture of the implant body.
¨ The bone is less dense and therefore weaker at implant uncovery than it is after 1 year of prosthetic loading.
¨ Bone is 60% mineralized at 4 months and takes 52 weeks to completely mineralize.
¨ Partially mineralized bone is weaker than completely mineralized bone.
¨ Woven bone first forms around an implant.
¨ Woven bone is unorganized and weaker than lamellar bone, which is organized and load bearing structure.
¨ Lamellar bone forms several months after the woven bone has replaced the devitalized zone around the implant at insertion.
¨ The bone changes from a fine trabecular pattern after initial healing to a coarse trabecular pattern after loading, especially in the crestal half of the implant interface.
¨ Density of the bone is related directly to the strength and elastic modulus, the crestal bone strength may increase in relation to the functional loading.
¨ Absence of radiographic bone loss is most often observed when stress factors are reduced.
¨ The stress is greatest at the crest, compared with other regions of the implant body.
¨ The denser the bone, the less crestal bone loss observed.
¨ The maxillary arch often exhibits greater bone loss than the mandibular arch.
¨ A very dense bone captures the stress closer to the crestal region. Avery soft bone allows the stress to be transmitted farther along the implant interface.
¨ The softer the bone, the farther the stress pattern apical progression.
¨ Implants that maintain crestal bone negate the hypotheses of periosteal reflection, osteotomy preparation, and biological width.
¨ The etiology of early crestal bone loss and early implant failure after loading is primarily from excess stress transmitted to the immature implant-bone interface.
¨ One biomechanical approach to decrease stress is to increase surface area.
¨ Another method to decrease stress is to decrease forces.
Force may be decreased in
¨ 1. Magnitude
¨ 2. Duration
¨ 3. Type
¨ 4. Direction
¨ 5. Multiplication factors.
¨ Stress is directly related to force.
¨ As a result any force factor magnifies the stress.
¨ Once the prosthesis type is determined, the potential force levels that will be exerted on the prosthesis should be evaluated and accounted for in the over all treatment plan.
¨ The initial implant survival, early loading survival, early crestal bone loss, incidence of abutment or prosthetic screw loosening, and unretained restorations, porcelain fracture, and component fracture are influenced by the factors of force.
Dental factors that affect stress primarily include:
· Tongue thrust
¨ 2. Masticatory dynamics
¨ 3. The position of the abutment in the arch
¨ 4. The nature of the opposing arch
¨ 5. Direction of load forces
¨ 6. The crown-implant ratio.
NORMAL BITE FORCE:
¨ Perpendicular to occlusal plane
¨ short duration
¨ Brief total period(9min/day)
¨ Force on each tooth:20 to 30 psi
¨ Maximum bite force:50 to 500 psi
¨ More constant
¨ Maximum when swallowing(3 to 5 psi)
¨ Brief total swallow time(20 min/day)
¨ Parafunctional forces on teeth or implants are characterized by repeated or sustained occlusion and have long been recognized as harmful to the stomatognathic system.
¨ The most common cause of implant failure after successful surgical fixation or early loss of rigid fixation during the first year of implant loading is the result of parafunction.
¨ Complications occur with greater frequency in the maxilla, because of a decrease in bone density and an increase in the moment of force.
Ø Nadler has classified the causes of parafunction or non functional tooth contact into the following six categories:
¨ Local factors include:
Tooth form or occlusion and soft tissue changes such as ulcerations or pericoronitis.
¨ Systemic factors include:
Cerebral palsy, epilepsy, and drug related dyskinesia.
¨ Psychological causes include:
The release of emotional tension or anxiety.
They occur with greatest frequency.
¨ Occupational factors:
Concern professionals such as dentists, athlets, and precision workers, musician who develops altered oral habits.
¨ Involuntary movement:
That provokes bracing of the jaws.
¨ Voluntary causes:
Chewing gum or pencils,pipe smoking.
¨ The parafunction may be categorized as:
It is vertical or horizontal, non functional grinding of teeth.
Biting force was greater (4 to 7times normal).
Ø Symptoms include repeated headaches, a history of fractured teeth or restorations, repeated uncemented restorations, and jaw discomfort on awakening.
Ø Signs include an increase in size of the temporal and masseter muscles, deviation of the lower jaw on opening, limited occlusal opening, increased mobility of teeth, cervical abfraction of teeth, fracture of teeth or restorations, and uncemented crowns or fixed prosthesis.
Ø The best and easiest way to diagnose bruxism is to evaluate the wearing of teeth.
Ø Severe bruxism changes normal masticatory forces by magnitude (higher bite forces), duration (hours rather than minutes), direction (lateral rather than vertical), type (shear rather than compression), and magnification (4 to 7 times normal).
¨ It generates constant force exerted from one occlusal surface to the other without any lateral movement.
¨ The direction of load may be vertical or horizontal.
¨ Signs include tooth mobility, muscle tenderness and hypertrophy, deviation during occlusal opening, limited opening, stress lines in enamel, cervical abfraction, and material fatigue.
¨ The clenching patient has the “sneaky disease of force”.
¨ Fremitus, a vibration type of mobility of a tooth, is often present in the clenching patient.
¨ Other signs stress lines in enamel, stress lines in alloy restorations.
¨ A common clinical finding of clenching is a scalloped border of the tongue.
¨ The tongue is braced against the lingual surfaces of the teeth during clenching, exerting lateral pressures and resulting in the scalloped border.
¨ Increase in force magnitude and duration.
¨ Clenching patient suffer from a phenomenon called creep, which also results in fracture of components.
Tongue Thrust and Size:
¨ Parafunctional tongue thrust is the unnatural force of the tongue against the teeth during swallowing.
¨ A force of 41 to 709g/cm on the anterior and lateral areas of the palate has been recorded.
¨ The force of tongue thrust is of lesser intensity than in other parafunctional forces, it is horizontal and can increase stress at the permucosal site of the implant.
¨ The placement of implants and prosthetic teeth in patients with large tongue results in an increase in lateral force, which may be continuous.
¨ A prosthetic mistake is to reduce the width of the lingual contour of the mandibular teeth.
¨ The lingual cusp of the restored mandibular posterior teeth should follow the curve of Wilson and include proper horizontal overjet to protect the tongue during occlusion.
¨ They are responsible for the amount of force exerted on the implant system.
¨ The force is related to the amount and duration of function.
¨ The size of the patient can influence the amount of bite force.
¨ Forces recorded in women are 20lb less those in men.
¨ The sex, muscle mass, exercise, diet, state of the dentition, physical status, and age may influence muscle strength, masticatory dynamics, and therefore maximum biteforce.
POSITION WITH IN THE ARCH:
¨ The maximum biting force is greater in molar region and decreases as measurements progress anteriorly.
¨ Mansour et al. evaluated occlusal forces and moments mathematically using a ClassIII lever arm, the condyles being the fulcrum and the masseter and temporalis muscles supplying the force.
¨ The anterior biting force is decreased in the absence of posterior tooth contact and greater in the presence of posterior occlusion or eccentric contacts.
DIRECTION OF LOAD:
¨ The direction of occlusal load results in significant differences in the amount of force exerted on an implant.
¨ Forces are tensile, compressive, or shear to the implant system.
¨ Bone is strongest to compressive forces, 30% weaker to tensile loads, and 65% weaker to shear loads.
¨ All the stresses occur in the coronal half of implant bone interface.
¨ Much less stress occur with vertical loads compared with an angled load on an implant.
¨ A lateral load on an implant crown makes the crown height act as a lever and force magnifier.
¨ Lateral forces represent a 50% to 200% increase in stress compression compared with vertical loading, and tensile streses may increase more than tenfold.
¨ The direction of forces may be one of the more critical factors to be evaluated during implant treatment planning.
¨ The average occlusal load of natural dentition is at 12 degrees to the tooth root.
¨ Mandibular premolar implants are best positioned for axial loading.
¨ Mandibular posterior implants are placed with a facial inclination of the implant apex, to avoid perforation of the submandibular fossa.
¨ If the forces of occlusion are not axial to the implant body, additional implants, wider implants, stress relievers in the prosthesis or overdentures should be considered.
¨ Natural teeth transmit greater impact forces through occlusal contacts than do softer-tissue borne complete dentures.
¨ Implant overdentures improve the masticatory performance and permit a more consistent return to centric relation occlusion during function.
¨ The maximum force is related to the amount of tooth or implant support.
¨ It affects the amount of forces distributed to the implant-prosthetic system in the presence of lateral or cantilevered forces.
¨ The greater the crown height, the greater the moment of the force under the lateral loads.
¨ The crown height acts as a lever with any lateral force.
¨ Since stresses are concentrated at the crest of the rigidly fixated implant, the crown height multiplier increases stress rapidly.
¨ For every 1 mm crown height increase, a force increase may be 20%.
¨ An indirect relationship is found between the crown and implant height.
¨ The lesser the bone volume, the greater the crown height and the greater the number of implants indicated.
¨ The overall stress to the implant system may be reduced by increasing the surface area over which the force is applied.
¨ Most effective method to increase the number of implants used to support the prosthesis.
¨ The retention of prosthesis is improved with greater no. of splinted abutments.
¨ With this the amount of stress to the system is reduced, and the marginal ridges on the implant crowns are supported by the connectors of the splinted crowns, which applies compressive forces rather than shear loads on the porcelain.
¨ One implant for each tooth missing may be indicated in the posterior regions of the mouth, for a large, young, male patient with severe parafunction.
¨ Implant positioning is related to implant number because more than two implants are needed to form a biomechanical tripod.
¨ Cantilevers are a force magnifier and represent a considerable risk factor in implant support, screw loosening, crestal bone loss, fracture.
¨ Therefore implant no. & position should aim at eliminating cantilevers, especially when other force factors are increased.
¨ The best way to reduce risk factors is to increase implant no.
¨ The surface area of implant support may also be increased by the size of the implant.
¨ Each 3mm increase in height can improve surface area support by more than 20%.
¨ The significance in increased length is not found at the crestal bone interface but rather in initial stability and the overall amount of bone implant interface.
¨ The increased length also provides resistance to torque or shear forces when abutments are screwed into place.
¨ The surface area of implant support system is directly related to the width of the implant.
¨ Each 0.25mm increase in implant diameter may increase the overall surface area app. 5 to 10%.
¨ Bone augmentation in width may be indicated to increase implant diameter by 1mm or app. 25% increased surface area.
¨ Implant macrodesign may affect surface area even more than an increase in width.
¨ A cylinder implant provides 30% less surface area than a conventional threaded implant of same size.
¨ A threaded implant with 10 threads for 10mm has more surface area than one with 5 threads.
¨ A thread depth of 0.2mm has less surface area than an implant with 0.4mm.
¨ The platform of the implant body is larger in the larger diameter implant. So less force is transmitted through screws during occlusal loads.
¨ Screw loosening may be decreased by a preload with a torque wrench on the screw.
¨ The threads of the screw form a 30 degree angle.
¨ A rotational force on the screw places a shear force on the incline of the thread.
¨ Most systems use a 30 to 35 Ncm rotational force on the abutment screw to preload or stretch the screw without risk of fracture.
¨ A more effective method to preload the screw is to tighten the screw to the recommended amount and then untighten the screw after a few minutes and retighten it to the required torque force again.
¨ Screw loosening is affected by the no. of threads.
¨ The height of the antirotational component of the implant body also can affect the amount of the force applied to the abutment screw.
¨ The higher the hexagonal height, the less stress applied to the screw.
¨ Materials follow a fatigue curve, which is related to the number of cycles and the intensity of force.
¨ The magnitude of the force increases over time because the muscles become stronger and the number of cycles on the prosthetic components is greater as a result of the parafunction.
¨ The density of bone is in direct relationship with the amount of implant-bone contact.
¨ The less area of bone contacting the implant body, the greater the overall stress.
¨ Progressive bone loading changes the amount and density of the implant-bone contact.
¨ The body is given time to respond to a gradual increase in occlusal load.
¨ This increases the quantity of bone at the implant interface, improves the bone density, and improves the overall support system mechanism.
¨ The very dense bone of resorbed anterior mandible (D1) has the highest percentage of lamellar bone in contact with an endosteal implant.
¨ Amount of stress to the implant increases in D4 bone because fewer regions of bone contact are present.
EFFECTS ON TREATMENT PLANNING:
¨ Solution is an increase in implant-bone surface area.
¨ Additional implants are the solution of choice to decrease stress, rather than only an increase in implant width or height.
¨ The amount of bone in contact with the implant is also increased as a multiple of the no. of implants.
¨ The greater the diameter of the implant, the lesser the stress transmitted to the surrounding crestal bone.
¨ An increase of 0.5mm of the abutment post diameter may increase the fatigue strength by 30%.
The implant treatment plan is modified primarily in two ways when implants are inserted in the posterior region.
1. additional implants.
2. occlusal considerations.
¨ The elimination of posterior lateral occlusal contacts during excursive movements is recommended when opposing natural teeth or an implant or tooth supported fixed prosthesis.
¨ This benefits in two aspects:
¨ Use of a night guard is helpful for the bruxism patient with a fixed prosthesis to transfer the weakest link of the system to the removable acrylic appliance.
¨ Anterior guided disocclusion of the posterior teeth in excursions is strongly suggested in the night guard, which may be designed to fit the maxilla or mandible.
¨ A soft night guard, which is slightly relieved over the implants, is used in clenching patient.
¨ A night guard with a hard acrylic outer shell and soft resilient liner has biomechanical advantage to reduce the impact of the force during parafunction.
¨ Unlike teeth, implants do not extrude when no occlusal force is present. As a result, the night guard can be relieved around an immediate implant, so the teeth bear the entire load.
¨ Implant failure during healing is parafunction found with a patient wearing a soft tissue supported prosthesis over a submerged implants.
¨ The time intervals between prosthodontic restoration appointments may be increased through progressive bone loading techniques.
¨ Anterior implants submitted to lateral parafunction forces require further treatment considerations.
¨ Additional implants are indicated with greater diameter.
¨ The excursions are canine guided if natural, healthy canines are present.
¨ Mutually protected occlusion, is developed if the implants are in the canine position or if this tooth is restored as a pontic.
¨ The forces must be disturbed along the long axis of the implant, narrow occlusal tables to prevent inadvertent lateral forces, decrease the forces necessary for mastication, and leave greater space for the tongue.
¨ Enameloplasty of the cusp tips of the opposing natural teeth is indicated to improve the direction of vertical forces, within the guidelines of the intended occlusion.
¨ Submerged, two-phase protocols are recommended in patients with horizontal force factors such as lateral tongue thrust.
¨ Myofunctional therapy and autogenous bone grafts to modify the bone division for endosteal-two stage implant placement, cantilevered bridges from the anterior teeth, or conventional removable partial dentures are valid treatment objectives.
¨ If the anatomical conditions do not permit the placement of implants, a removable overdenture (RP-4 or RP-5) is indicated.
¨ RP-4 or RP-5 may be removed during periods of parafunction.
¨ Stress distributors may be used in the attachment system.
¨ Additional implants are the solution of choice to decrease, along with an increase in implant width or height to decrease the no. of pontics and dissipate stresses more effectively to the bone structure, especially at the crest.
1. Dental implant prosthetics – Carl E. Misch
2. Principles and practice of implant dentistry – Charles Weiss, Adam Weiss.
3. Tissue – integrated prosthesis. Osseointegration in clinical dentistry – Branemark, zarb, Albrektsson
4. Oral rehabilitation with implant supported prosthesis -Vincente
5. ITI dental implants- Thomas G.Wilson