Bone Density Comprehensive Notes

Bone Density: A Key Determinant for Treatment Planning

  • Available bone is the external architecture/volume of the edentulous area for implants.
  • Early implantology:
    • Available bone wasn't modified.
    • Existing bone volume was the primary treatment planning factor.
    • Less available bone: short, fewer implants.
    • Larger bone volumes: long implants in greater numbers.
  • Modern approach:
    • Treatment plan starts with the final prosthesis.
    • Evaluate patient force factors.
    • Assess bone density.
  • Bone density (quality) reflects biomechanical properties (strength, elasticity).
  • Bone architecture (internal & external) controls implant dentistry aspects.
  • Bone density impacts:
    • Treatment planning.
    • Implant design.
    • Surgical approach.
    • Healing time.
    • Initial bone loading during reconstruction.

Influence of Bone Density on Implant Success Rates

  • Bone quality varies by arch position:
    • Densest: anterior mandible.
    • Next: anterior maxilla and posterior mandible.
    • Least dense: posterior maxilla.
  • Adell et al. reported ~10% greater success in the anterior mandible vs. anterior maxilla with standard protocols.
  • Schnitman et al. noted lower success in the posterior mandible vs. anterior mandible with the same protocol.
  • Highest failure rates: posterior maxilla (greater force, poorer density).
  • Engquist et al. observed 78% of implant failures in soft bone types.
  • Friberg et al. observed 66% of failures in the resorbed maxilla with soft bone.
  • Jaffin and Berman:
    • 5-year study: 44% failure rate with poor-density bone in the maxilla.
    • 35% implant loss in any region with poor bone density.
    • 55% of failures in soft bone.
  • Johns et al.:
    • 3% failure in moderate bone densities.
    • 28% failure in the poorest bone type.
  • Smedberg et al. reported a 36% failure rate in the poorest bone density.
  • Reduced implant survival relates more to bone density than arch location.
  • Snauwaert et al. (15-year study) reported early and late failures more frequently in the maxilla.
  • Herrmann et al. found implant failures strongly correlated to patient factors, including bone quality and poor bone volume (65% failure rate).
  • Failures occur after prosthetic loading, not primarily from surgery/healing.
  • Misch's protocol (treatment plan adaptation, implant selection, surgical approach, healing, loading) resulted in similar success in all bone densities and arch positions.

Etiology of Variable Bone Density

  • Bone adapts to factors like hormones, vitamins, and mechanical influences, with biomechanical parameters (duration of edentulous state) being predominant.
  • Meier (1887) described trabecular bone architecture in the femur qualitatively.
  • Kulmann (1888) noted similarity between trabecular bone patterns in the femur and tension trajectories in construction beams.
  • Wolff's Law (1892): "Every change in the form and function of bone or of its function alone is followed by certain definite changes in the internal architecture, and equally definite alteration in its external conformation, in accordance with mathematical laws."
  • Murry reported that the external architecture of bone changes in relation to function and the internal bony structure is also modified.
  • MacMillan and Parfitt reported on trabeculae variations in the alveolar regions of the jaws.
  • Mandible vs. Maxilla:
    • Mandible: force absorption unit, denser/thicker cortical bone, coarser/denser trabecular bone.
    • Maxilla: force distribution unit, thin cortical plate, fine trabecular bone.
    • Bone is densest around teeth (cribriform plate), denser at the crest than around the apices.
  • Alveolar bone resorption with orthodontic therapy illustrates biomechanical sensitivity.
  • Trabecular bone loss occurs from decreased mechanical strain.
  • Orban demonstrated decreased trabecular bone around a maxillary molar without opposing occlusion.
  • Bone density decreases after tooth loss, related to:
    • Length of edentulous time without appropriate loading.
    • Initial bone density.
    • Flexure/torsion in the mandible.
    • Parafunction before/after tooth loss.
    • Change is greatest in the posterior maxilla, least in the anterior mandible.

Bone Modeling and Remodeling

  • Cortical and trabecular bone are constantly modified by modeling or remodeling.
  • Modeling: independent sites of formation and resorption, changing bone shape/size.
  • Remodeling: resorption and formation at the same site, replacing existing bone (internal turnover/next to implants).
  • Adaptive phenomena are associated with mechanical stress and strain.
  • Stress: ForceFunctionalArea\frac{Force}{Functional Area}
  • Strain: ChangeinLengthOriginalLength\frac{Change in Length}{Original Length}
  • Greater stress leads to greater strain.
  • Bone modeling and remodeling are controlled by the mechanical environment of strain.
  • Alveolar bone density evolves due to mechanical deformation from microstrain.
  • Frost proposed a model of four histologic patterns for compact bone related to mechanical adaptation to strain:
    • Pathologic overload zone.
    • Mild overload zone.
    • Adapted window.
    • Acute disuse window.
    • These categories describe trabecular bone response in the jaws.
Acute Disuse Window (0-50 microstrain)
  • Bone loses mineral density, disuse atrophy occurs.
  • Modeling is inhibited, remodeling is stimulated, net loss of bone.
  • E.g., 15% decrease in cortical plate and extensive trabecular bone loss in immobilized limbs for 3 months.
  • Cortical bone density decrease of 40% and trabecular bone density decrease of 12% have been reported with disuse of bone.
  • Similar bone loss occurs in microgravity environments due to lack of Earth's gravity.
  • Astronauts lost nearly 12% of bone minerals after 111 days on the Russian Mir space station.
Adapted Window (50-1500 microstrain)
  • Equilibrium of modeling and remodeling, bone conditions maintained.
  • Bone remains in a steady state, homeostatic window of health.
  • Histology: primarily lamellar/load-bearing bone.
  • ~18% of trabecular bone and 2-5% of cortical bone are remodeled each year in the physiologic loading zone.
  • This is the desired strain range around an endosteal implant.
  • Bone turnover is required, as Mori and Burr provided evidence of remodeling in regions of bone microfracture from fatigue damage within the physiologic range.
Mild Overload Zone (1500-3000 microstrain)
  • Greater rate of fatigue microfracture and increase in cellular turnover rate of bone.
  • Bone strength and density may eventually decrease.
  • Histology: usually woven/repaired bone.
  • May occur when an endosteal implant is overloaded, and the bone interface attempts to change the strain environment.
  • Woven bone is weaker than lamellar bone.
  • Care must be taken because the “safety range” for bone strength is reduced during the repair.
Pathologic Overload Zones (greater than 3000 microstrain)
  • Cortical bone fractures occur at 10,000 to 18,000 microstrain (1-2% deformation).
  • Pathologic overload may begin at microstrain levels of only 18% to 40% of the ultimate strength or physical fracture of cortical bone.
  • Bone may resorb, form fibrous tissue, or repair woven bone.
  • Marginal bone loss during implant overloading may be due to bone in this zone.
  • Implant failure from overload may also be a result of bone in the pathologic overload zone.

Bone Classification Schemes Related to Implant Dentistry

  • Linkow and Chercheve (1970) classified bone density into three categories:
    • Class I: Evenly spaced trabeculae with small cancellated spaces (ideal).
    • Class II: Slightly larger cancellated spaces with less uniformity (satisfactory).
    • Class III: Large, marrow-filled spaces between trabeculae (loose-fitting implant).
  • Lekholm and Zarb (1985) listed four bone qualities in the anterior regions of the jawbone:
    • Quality 1: Homogeneous compact bone.
    • Quality 2: Thick layer of compact bone surrounding dense trabecular bone.
    • Quality 3: Thin layer of cortical bone surrounding dense trabecular bone of favorable strength.
    • Quality 4: Thin layer of cortical bone surrounding a core of low-density trabecular bone.
  • Schnitman et al. observed a 10% difference in implant survival between Quality 2 and Quality 3 bone, and 22% lower survival in the poorest bone density following Lekholm & Zarb protocol.
  • Johns et al. Reported 3% failure in type III bone, but 28% in type IV bone.
  • Smedberg et al. reported a 36% failure rate in type IV bone.
  • A standardized surgical, prosthetic, and implant design protocol does not yield similar results in all bone densities.
  • These reports are for implant survival, not the quality of health of surviving implants. Crestal bone loss is also related to bone density.

Misch Bone Density Classification

  • Proposed four bone density groups independent of jaw regions based on macroscopic cortical and trabecular bone characteristics in 1988.
  • Treatment plans, implant design, surgical protocol, healing, and progressive loading time spans are described for each bone density type.
  • Following this regimen, similar implant survival rates have been observed for all bone densities.
Bone Density Types
  • Dense or porous cortical bone is found on the outer surfaces of bone and includes the crest of an edentulous ridge.
  • Coarse and fine trabecular bone types are found within the outer shell of cortical bone and occasionally on the crestal surface of an edentulous residual ridge.
  • The four increasing macroscopic densities constitute four bone categories described by Misch (D1, D2, D3, and D4) located in the edentulous areas of the maxilla and mandible.
    • D1: Primarily dense cortical bone.
    • D2: Dense-to-porous cortical bone on the crest and coarse trabecular bone within.
    • D3: Thinner porous cortical crest and fine trabecular bone in the region next to the implant.
    • D4: Very little to no crestal cortical bone, fine trabecular bone composes almost all of the total volume of bone next to the implant.
    • D5: (very soft) Immature bone in a developing sinus graft.

Determining Bone Density

Bone density can be determined by various techniques including tactile sensation, during surgery, the general location, or radiographic evaluation (CBCT).

Location

The location of different bone densities often may be superimposed on the different regions of the mouth.

  • D1 bone is almost never observed in the maxilla and is rarely observed in most mandibles (6% of the time in the Division A anterior mandible and 3% of the time in the posterior mandible).
  • The bone density D2 is the most common bone density observed in the mandible (the anterior mandible consists of D2 bone approximately two-thirds of the time. Almost half of patients have D2 bone in the posterior mandible).
  • Bone density D3 is common in the maxilla (More than half of patients have D3 bone in the upper arch. The anterior edentulous maxilla has D3 bone approximately 75% of the time).
  • The softest bone, D4, is most often found in the posterior maxilla (approximately 40%), especially in the molar regions or after a sinus graft augmentation (where almost two-thirds of the patients have D4 bone).

Generalizations for treatment planning can be made prudently based on location.

The bone density by location method is the first way the clinician can estimate the bone density in the implant sites to develop an initial treatment plan. It is safer to err on the side of less-dense bone during treatment planning, so the prosthesis will be designed with slightly more, rather than less, support.

Radiographic Evaluation
  • Periapical or panoramic radiographs are minimally beneficial in determining bone density because of their two-dimensional nature and the lateral cortical plates often obscure the trabecular bone density.
  • Bone density may be more precisely determined using cone beam computerized tomography (CBCT).
  • With conventional computerized tomography (CT), each image is comprised of pixels. Each pixel in the CT image is assigned a number, also referred to as a Hounsfield or CT number.
  • The CT Hounsfield scale is calibrated such that the Hounsfield unit values are based on water (0 HU) and air (−1000 HU). In general the higher the CT number, the denser the tissue.
  • The HU is a quantitative measurement used in CT scanning to express CT numbers in a standardized form.
  • When evaluating dental cone beam computed tomography (CBCT) images in regard to bone density, there does not exist a direct correlation (accuracy of measurement) compared with medical CT.
  • Most dental CBCT systems inherently have an increased variation and inconsistency with density estimates.
  • Dental imaging software frequently provides attenuation values (HU); however, such values should be recognized as approximations lacking the precision of HU values derived from medical CT units.
  • HUs have been correlated with bone density and treatment planning for dental implants.
  • The Misch bone density classification may be evaluated on the CT images by correlation to a range of HUs:
    • D1: >1250 HU
    • D2: 850 to 1250 HU
    • D3: 350 to 850 HU
    • D4: 0 to 350 HU
    • D5: <0 HU
  • In the mandible, failed sites exhibited higher HUs than usual. This was correlated with failure in dense bone, possibly because of the lack of vascularization or overheating during surgery. By contrast, in the maxilla the bone density was low for the failed sites.
  • The most critical region of bone density is the crestal 7 to 10 mm of bone, because this is where most stresses are applied to an osteointegrated bone–implant interface.
Tactile Sense
  • There is a great difference in the tactile sensation during osteotomy preparation in different bone densities, because the density is directly related to its strength.
    • D1 bone: similar to the resistance on a drill preparing an osteotomy in oak or maple wood (e.g., hard wood).
    • D2 bone: similar to the tactile sensation of drilling into white pine or spruce (e.g. soft wood).
    • D3 bone: similar to drilling into a compressed balsa wood.
    • D4 bone: similar to drilling into a compressed Styrofoam.
  • When an implant drill can operate at 1500 to 2500 rpm, it may be difficult to feel the difference between D3 and D4 bone.

Scientific Rationale of a Bone Density–Based Treatment Plan

Bone Strength and Density
  • Bone density is directly related to the strength of bone before microfracture.
  • A tenfold difference in bone strength may be observed from D1 to D4 bone.
  • D2 bone exhibited a 47% to 68% greater ultimate compressive strength, compared with D3 bone.
Elastic Modulus and Density
  • The elastic modulus describes the amount of strain (changes in length divided by the original length) as a result of a particular amount of stress. It is directly related to the apparent density of bone.
  • Misch et al. found the elastic modulus in the human jaw to be different for each bone density.
  • As a result, when a stress is applied to an implant prosthesis in D1 bone, the titanium–D1 bone interface exhibits a very small microstrain difference. In comparison, when the same amount of stress is applied to an implant in D4 bone, the microstrain difference between titanium and D4 bone is greater and may be in the pathologic overload zone.
  • As a result, D4 bone is more likely to cause implant mobility and failure.
Bone Density and Bone–Implant Contact Percentage
  • 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.
  • The mechanical distribution of stress occurs primarily where bone is in contact with the implant. Open marrow spaces or zones of unorganized fibrous tissue do not permit controlled force dissipation or microstrain conditions to the local bone cells.
  • Because stress equals force divided by the area over which it is applied, the less the area of bone contacting the implant body, the greater the overall stress, all other factors being equal.
  • Misch noted in 1990 that the bone density influences the amount of bone in contact with the implant surface, not only at first stage surgery but also at the second stage uncovery and early prosthetic loading.
Bone Density and Stress Transfer
  • Crestal bone loss and early implant failure after loading results may occur from excess stress at the implant–bone interface.
  • In D1 bone, highest strains are concentrated around the implant near the crest, and the stress in the region is of lesser magnitude.
  • D2 bone sustains a slightly greater crestal strain, and the intensity of the stress extends farther apically along the implant body (Fig. 18.31).
  • D4 bone exhibits the greatest crestal strains, and the magnitude of the stress on the implant proceeds farthest apically along the implant body (Fig. 18.32)

Treatment Planning

  • There exist four key principles that help form the basis for treatment-plan modification in function of the bone quality:
    • each bone density has a different strength
    • bone density affects the elastic modulus
    • bone density differences result in different amounts of BIC percent
    • bone density differences result in a different stress/strain distribution at the implant–bone interface when implants are loaded
  • As the bone density decreases, the strength of the bone also decreases. To decrease the incidence of microfracture of bone, the strain to the bone must be reduced. Strain is directly related to stress. Consequently, the stress to the implant system should also be reduced as the bone density decreases.

Strategies to reduce biomechanical loads on implants includes prosthesis design to decrease force the most ideal technique is the splinting of multiple implants.