BONE DENSITY A key determinant in clinical implant success ROHIT YADAV J.R III

INTRODUCTION External as well as internal architecture of bone is important for clinical implant success . External architecture is described as AVAILABLE BONE VOLUME, and internal bone architecture is described in terms of BONE QUALITY or BONE DENSITY.

Bone density(bone mineral density) is a medical term normally referring to the amount of mineral matter per squre centimeter of bones. Determining factor in treatment planning, implant design, surgical approach, healing time, and initial progressive bone loading.

The range of implant success documented should be considered in terms of the location concerned, as different regions of the jaws have different bone conditions i.e. bone volume and density. Higher implant success are well documented in denser bony regions as compared to less denser locations.

But this doesn't mean that clinical implant success cannot be achieved in regions with less dense bones. Factors from initial treatment planning to loading protocols have to be addressed carefully in these conditions.

ETIOLOGY OF VARIABLE BONE DENSITY As per the WOLFF’S LAW ( 1892) “ Every change in the form or function of bones or of their function alone is followed by certain definite changes in their internal architecture and equally definite alterations in the external conformation, in accordance with mathematical laws. Bone is an organ that changes in relation to no. of variable factors including hormones, vitamins, and mechanical factors.

Macmilllan & Parfitt reported the variation of trabeculae in the alveolar region- Mandible - force absorption unit Maxilla - force distribution unit

Cortical and trabecular bone are modified constantly by modelling and remodelling. MODELING - has independent sites for formation and resorption and results in change in shape or size of bone. REMODELING – is resorption and formation at the same site that replaces existing bone and primarlily affects the internal turnover of the bone, including next to the endosteal implants.

Bone modeling and remodeling are primarily controlled by mechanical strain.

Bone density evolves as a result of mechanical deformation from strain Bone density evolves as a result of mechanical deformation from strain .Bone was found to be most dense around teeth and more dense around the teeth at the crest compared to the regions around apex. Decrease in bone density – - occurs after tooth loss - occurs after loss of opposing occlusal contacts - related to the time of edentulism, original density, muscular attachments, flexure and torsion of mandible, parafunction ,hormonal influences and systemic conditions.

Frost has modeled 4 zones for cortical bone related to mechanical adaptation to strain before spontaneous fracture.

ACUTE DISUSE WINDOW The bone density decreases and disuse atrophy occurs, as modeling for new bone is inhibited and remodeling is stimulated, resulting in net loss of the bone. The microstrain levels are 0 to 50 microstrains.

ADAPTED WINDOW ( Physiologic loading zone) Represents the equilibrium of modeling and remodeling and bone conditions are maintained in this level. Steady state levels are maintained and regarded as homeostatic window of health. Histologically it is primarily lamellar or load bearing bone type. Microstrain levels are 50 to 1500 microstrains. The ideal desired zone around the implants.

MILD OVERLOAD WINDOW The microstrain levels are 1500 to 3000 microstrains. This zone corresponds to bone modeling stimulation and remodeling inhibition leading to decrease in bone density and strength. Histologically this zone corresponds to woven or repair bone type.

Overloaded implants may have this type of bone around them, when the bone get damaged due to slight overload conditions and then tries to repair itself. The woven bone is definitely weaker than the more mature lamellar bone

The microstrain levels are >3000 microstrains. PATHOLOGIC OVERLOAD WINDOW The microstrain levels are >3000 microstrains. Spontaneous fracture occurs at 20,000 to 30,000 microstrains.that means the pathologic overload zone begins at 20-40% of ultimate fracture strength. The only type of bone present is woven in nature

Initial crestal bone loss evident after implant placement shows this type of overload zone. Different zones just described are basically of cortical bone and varies according to bone density i.e all levels have lower values for corresponding lower densities of bones.

MISCH - BONE DENSITY CLASSIFICATION DESCRIPTION TACTILE SENSES COMMON LOCATION D1 Dense cortical Oak or Maple Anterior mandible D2 Porous cortical and coarse trabecular White pine or spruce wood Ant – post mandible. Ant. Maxilla. D3 Porous cortical (thin) and fine trabecular. Balsa wood Ant.- post. Maxilla. Post. Mandible. D4 Fine trabecular Styrofrom Posterior maxilla

ANATOMICAL LOACTION OF BONE DENSITY TYPES IN PERCENTAGE ANT. MAXILLA POST MAXILLA ANT. MANDIBLE POST MAND. D1 6 3 D2 25 10 66 50 D3 65 46 D4 40 1

Bone density D2 is most common density observed in mandible as a whole, with anterior mandible showing D2 bone 2/3rd of times. Single-tooth or two-tooth partially edentulous span almost always have D2 bone.

As far as D1 bone is concerned it is found mainly in mandible ( anterior > posterior regions). D3 bone is common in maxilla, more than half of the patient have D3 bone in upper arch, mainly in anterior maxilla. D4 bone is mostly found in posterior maxilla especially in molar region.

RADIOGRAPHIC BONE DENSITY COMPUTERIZED TOMOGRAPHY Bone density can be determined precisely by using computerized tomograms (CT)

CT produces axial images of the patient's anatomy, perpendicular to the long axis of the body. Each CT axial image has 260,000 pixels, and each pixel has a CT no. called HOUNSFIELD UNIT, related to the density of the tissue . Higher the no. of Hounsfield unit / CT no. , higher the density. Bone density is different in crestal region as compared to apical region, and it is the crestal density ( i.e. creatal 7 – 10 mm ) which is important to treatment protocol.

COMPUTED TOMOGRAPHY DETERMINATION OF BONE DENSITY > 1250 HOUNSFIED UNIT D2 850 – 1250 HOUNSFIED UNIT D3 350 – 850 HOUNSFIED UNIT D4 150 – 350 HOUNSFIED UNIT D5 < 150 HOUNSFIED UNIT

As far as panoramic radiographs or periapical radiographs are concerned , they are not reliable sources as lateral cortical plate obsure the trabecular bone density. In addition to this little diffrences between different bone types are difficult to assess , esp. between D2 and D3.

D1 bone is found to be 10 times stronger to D4 bone. BONE STRENGTH AND DENSITY Bone density is directly related to the strength of the bone before microfracture, i.e ultimate fracture strength. D1 bone is found to be 10 times stronger to D4 bone. D2 bone is 47 to 68% greater than D3 bone.

Elastic modulus and density Relate to the stiffness of material. Elastic modulus of bone is more flaxible than titanium. The difference between the two materials may create microstrain condition of pathologic overload and cause implant failure.

INFLUENCE OF BONE DENSITY ON LOAD TRANSFER The mechanical distribution of stress occurs primarily where bone is in direct contact to the implant. Smaller the area of bone contact with the implant , greater the overall stress. Initial bone density is thus important, not only for initial immobilization of the implant during healing but also for the wide distribution of stress to the bone.

Bone density influences the amount of contacting bone with the implant surface, at both the 1st and 2nd stages. For areas with lesser bone density like posterior mandible greater implant surface area is required to obtain a similar amount of bone implant contact ( as compared to greater bone densities). For same amount of load applied ,as the bone density decrease the amount of crestal stress increases , with increase in of penetrence of stress towards the apical end.

TREATMENT PLANNING FACTORS REALTED TO VARIED BONE DENSITIES. Bone density is implant treatment modifier in many ways – -Prosthetic factors -Implant size -Implant design -Implant surface condition -Implant number

Prosthetic factors As bone density decreases , the strength of the bone decreases , and to reduce microfracture of the bone the stress to the bone should be reduced. Prosthesis design factors for less dense bones (D3/ D4) Shortened cantilever length. Narrower occlusal table. Preventing offset loading.

To minimize wearing time RP-4 restorations can be provided instead of fixed restorations. RP-5 restorations help to transfer some load to soft tissue sparing the implants. Night or occlusal / guards to prevent parafunctional forces on implant system.

Implant size Width of the implant also decrease the stress by increasing the surface area, which may also reduce the required implant length. For every .5mm increase in width , increase in surface area is between 10- 15 %. Also greatest stresses are concentrated at the crestal region of implant, width is more important than implant length. E.g. D4 bone requires wider implants than D1 or D2 bone.

Implant length is also important for initial fixation of the implants . However after complete healing, implant length is not of much importance as much of the stresses are concentrated at the crestal region. The minimum implant length are – D1 --- 10mm D2 --- 12mm D3 --- 14mm

For D4 bones, implants with greatest surface area should be used. E.g. classic V thread screw design has 30 % more surface area than cylindrical implants. The no. as well as the depth of the threads also influences the total surface area of the implants.

Greater the no. of threads, greater the functional surface area at the time of immediate load

Surface Coatings on the implants help to increase surface area and thus total bone contact. Hydroxypatite coating are strongly recommended in D4 implants as they have shown to increase short term survival rates. In long term total mechanical loads transfer to the implants are more critical than surface coatings.

Finally as far as the loading protocols are concerned, progressive loading is recommended for softer bones which allows bone to accommodate according to functional loads.. Over time progressive loading changes the amount and density of the bone- implant contact.

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