Chapter 29 Density of Bone: Effect on Surgical Approach and Healing

The density of available bone in an edentulous site has a primary influence on treatment planning, implant design, surgical approach, healing time, and initial progressive bone loading during prosthetic reconstruction.^1,² A direct bone-implant interface has been demonstrated with a variety of endosteal implant designs and materials. Three requirements necessary for initial rigid fixation—atraumatic bone preparation,³ close approximation of living bone to the biocompatible implant surface,⁴ and absence of movement at the interface during healing⁵—are all closely dependent on the bone density at the implant site. The quality of the recipient bone directly influences the amount of trauma generated during osteotomy preparation. This in turn provokes a cascade of reactions at the bone-implant interface that affect the quality of the load-bearing surface.

Once the implant is initially integrated with the bone, the bone-loading process from occlusal forces becomes a critical factor in long-term implant survival. The bone density under load is directly related to the bone strength and is therefore a critical parameter for long-term survival.^6,⁷ The occlusal stresses applied through the implant to the bone must remain within the physiologic to mild overload zone; otherwise, pathologic overload with associated bone loss and microfracture, and implant mobility may occur. The treatment planning and scientific rationale of strength, modulus of elasticity, bone-implant contact percent, and stress transfer difference related to bone density has been addressed in a previous chapter. This chapter addresses the modifications of the surgical and healing aspects related to each bone density in the oral environment.

LITERATURE REVIEW

Lekholm and Zarb ⁸ listed four bone qualities found in the anterior regions of the jawbone: quality 1 comprises homogeneous compact bone; quality 2, a thick layer of compact bone surrounding a core of dense trabecular bone; quality 3, a thin layer of cortical bone surrounding dense trabecular bone of favorable strength; and quality 4, a thin layer of cortical bone surrounding a core of low-density trabecular bone. Irrespective of the different bone qualities, all bone was treated with the same implant design and standard surgical and prosthetic protocols.

Following the proposed protocols of Brånemark et al., it was found that implant survival in initial surgical success was related to the quality of bone.⁴ A higher surgical failure was observed in softer bone types, especially in the maxilla. For example, Engquist⁹ reported the surgical loss of 38 of 191 implants in the maxilla in D4 bone (20% loss), compared with 8 of 148 mandibular implants (5% loss) before stage II surgery. Jaffin and Berman reported an overall 8.3% surgical and initial healing loss in 444 maxillary implants with softer bone.¹⁰ Friberg et al. reported a 4.8% implant failure at stage II uncovery for 732 maxillary posterior implants, which was greater than mandibular failure.¹¹ Quirynen et al. also reported a 4.1% implant loss at stage II uncovery out of 269 implants in the maxilla.¹² Fugazzotto et al. reported 22 failures out of 34 implants placed in quality 4 bone.¹³ Hutton et al. identified poor bone quantity and quality 4 as the highest risk of implant failure in a study of 510 implants with an overall failure rate in the maxilla nine times greater than in the mandible.¹⁴ Sullivan et al. indicated a 6.4% stage II failure rate in the maxilla (12/188) and a 3.2% failure in the mandible (7/216).¹⁵ Snauwaert et al. reported more frequent early failures in poor density maxillae.¹⁶ Herrmann et al. correlated failure factors such as poor bone quality and volume.¹⁷ A number of reports in the literature demonstrated the greatest risk of surgical failure was observed in the softest bone type (D4), especially when found in the maxilla.

On the other extreme, a large clinical study from 33 U.S. Department of Veterans Affairs (VA) hospitals by the Dental Implant Clinical Research Group (DICRG) states quality 1 bone had the highest surgical failure rate (4.3%), followed by quality 4 (3.9%), quality 2 (2.9%), and quality 3 with the fewest failures at 2.6% (Figure 29-1). The overall implant surgical failure was 3%; the maxilla had better success at stage II surgery (98.1%) than the mandible (96.4%).¹⁸ It must be emphasized that these reports only present implant failures up to stage II uncovery. The DICRG also noted that the failure rate was twice as great for surgeons who had placed fewer than 50 implants, compared with more experienced surgeons. The literature contains many published reports that indicate an implant surgical failure range from 3.2% to 5% in the mandible and 1.9% to 20% in the maxilla, with most reports indicating the greatest failure rates in maxillary implants with soft bone. It is clear from these reports that a wide range of results may be achieved and therefore consideration should be given to methods that improve surgical survival.

Figure 29-1 The Dental Implant Research Group represents 33 different hospitals that place dental implants. The highest surgical failure rate was quality 1 bone, followed by quality 4 bone. The fewest surgical failures were observed in quality 3 bone.

Misch developed a different surgical protocol for different bone qualities in 1988. Following these specific methods, prospective and retrospective multicenter clinical studies in a wide range of office settings found surgical survival above 99%, regardless of the density type of bone, the arch (mandible versus maxilla), sex and age of the patient.¹⁹^–²⁴ Therefore a different surgical protocol for different bone densities appears warranted.

The implant design, surgical protocol, healing, treatment plans, and progressive loading time spans are unique for each bone density type.^1,^2,²⁵ More recently the use of improved rotary instruments, implant designs, and surgical approaches for different bone qualities has been recognized as a valid recommendation. A multicenter prospective clinical study by Misch et al. of 364 consecutive implants in 104 consecutive patients found a surgical survival rate (up to abutment connection) at stage II of 100% for D1, 98.4% for D2, 99.8% for D3, and 100% for D4 implants¹⁹ (Figure 29-2). Altering the surgical approach and implant design for each bone density can yield an overall implant surgical survival of 99.8% and a 2-year survival of 99.4%. In this chapter the surgical considerations and optimal healing time are discussed relative to each bone category, based on the literature, prospective clinical studies, and long-term experience.

Figure 29-2 A multicenter study reported a surgical success of 99.6%, regardless of bone quality.

(Data from Misch CE, Hoar JB, Beck G et al: A bone quality based implant system: a preliminary report of stage I and stage II, Implant Dent 7:35-42, 1998.)

BONE DENSITY CLASSIFICATIONS

Misch defined four bone density groups in all regions of the jaws that vary in both macroscopic cortical and trabecular bone types.² The regions of the jaws are divided into: (1) the anterior maxilla (second premolar to second premolar); (2) posterior maxilla (molar region); (3) anterior mandible (first premolar to first premolar); and (4) posterior mandible (second premolar and molars). The regions of the jaws often have similar bone densities.

In general, the anterior mandible is usually D2 bone; the posterior mandible, D3 bone; the anterior maxilla, D3 bone; and the posterior maxilla, often D4 bone. This generalization is used for the initial treatment plan. However, resorbed anterior mandibles may be D1 bone in approximately 25% of male patients and the posterior maxilla may have D3 bone after 6 months in the majority of sinus graft patients. The regional locations of the different densities of cortical bone are more consistent than the highly variable trabecular bone. Bone density may be most precisely determined before surgery by a computed tomography (CT) scan of the edentulous site (accompanied by Hounsfield values of the bone). Reformatted software allows “electronic surgery” of the CT images and relates the Hounsfield values at the implant-bone interface. It may also be grossly estimated by radiographic evaluation of tomograms. Conventional dental radiographs, such as periapical, panoramic, or lateral cephalometric images, are usually not diagnostic.

A common point at which to evaluate bone quality is during surgery. The presence and thickness of a crestal cortical plate and the density of trabecular bone are easily determined during implant osteotomy preparation. The density of bone is determined by the initial bone drill, and evaluation continues until the final osteotomy preparation.

It should be emphasized that the bone density (D1 to D4) classification of Misch is slightly different from Lekholm and Zarb’s bone quality types (Q1 to Q4). According to Misch et al., D3 bone has fine trabeculae and is 47% to 68% weaker than D2 trabeculae, and 20% stronger than D4 trabeculae, whereas Lekholm and Zarb stated that Q3 bone has favorable-strength trabeculae similar to Q2.^7,⁸ In other words, the actual strength of the trabecular bone is different for each bone density, regardless of the presence or absence of cortical bone adjacent to the implant. In addition, the Lekholm and Zarb bone quality only evaluated bone in the anterior maxilla and mandible. The Misch bone density scale also evaluated the posterior molar regions of the jaws. As a result, a primary difference between D3 and D4 bone is also the presence of cortical bone in D3, which increases its overall strength and modulus of elasticity.^6,⁷ The quality 4 bone of Lekholm and Zarb is similar to Misch’s D3, whereas D4 bone is even weaker because little to no cortical bone is present to improve the strength or the elastic modulus of the fine trabecular bone.

Dense Cortical (D1) Bone

The very dense (oak- or maplelike) D1 bone is composed of almost all dense cortical bone. The maxilla almost never presents D1 bone. In Division A bone, approximately 4% of the anterior mandibles and 2% of the posterior mandibles have this dense bone category. In Division C–h bone of an anterior mandible, these numbers increase and may reach 25% in males, whereas weaker bone density D3 and D4 bone are less commonly encountered.

Advantages of D1 Bone

The homogeneous, dense D1 bone type presents several advantages for implant dentistry. Composed histologically of dense lamellar bone with complete haversian systems, it is highly mineralized and able to withstand great occlusal loads. The cortical lamellar bone may heal with little interim woven bone formation, ensuring excellent bone strength while healing next to the implant.^26,²⁷ Four splinted, threaded, titanium plasma spray implants immediately loaded in the anterior mandible have yielded very predictable long-term success (above 94%), to support an overdenture.²⁸ Other screw-type implants inserted with a similar surgical and prosthetic approach also resulted in similar long-term survival in this bone region.²⁹

D1 bone is more often found in anterior mandibles with moderate to severe resorption and greater crown/implant ratios. A threaded implant design provides greater surface area than a cylinder, especially in shorter lengths, and improves the dissipation of stresses in the crestal cortical region despite higher moments of force from the greater crown height, to sustain long-term functional stress.

The percentage of light microscopic contact of bone at the implant interface is greatest in D1 bone type and greater than 80% (Figure 29-3). In addition, this bone density exhibits greater strength than any other type.^6,⁷ The strongest bone also benefits from the greatest bone-implant contact (BIC). Less stresses are transmitted to the apical third of the implants than in other bone types. As a result, shorter implants can better withstand greater loads than in any other bone densities. In fact, the placement of longer implants may decrease surgical survival rates, as overheating during osteotomy preparation is a primary concern in this bone type. Greater heat is often generated at the apical portion of the osteotomy, especially when preparing dense cortical bone.³⁰

Figure 29-3 D1 bone has the highest bone-implant contact (BIC), which is usually greater than 80% after initial bone healing. Thus the strongest bone is also the bone with the greatest BIC. Both these conditions make D1 bone most suitable for occlusal loading.

Disadvantages of D1 Bone

Dense cortical bone also presents several disadvantages. The implant height is often limited to less than 12 mm in the atrophic mandible, and the crown height space is often greater than 15 mm. As a result, additional force-multiplying factors (such as cantilevers or lateral forces) are further magnified on the implant-prosthetic system. Stress-reducing factors may be incorporated in the prosthesis design to reduce these effects, not only on the bone, but also on the prosthetic components.

D1 bone has fewer blood vessels than the other three types, and therefore it is more dependent on the periosteum for its nutrition. The cortical bone receives the outer one third of all its arterial and venous supply from the periosteum.^31,³² This bone density is almost all cortical, and the capacity of regeneration is impaired because of the poor blood circulation.³³ Therefore delicate and minimal periosteal reflection is indicated. When D1 density is present, the bone width is usually abundant. Fortunately, there are few occurrences when facial or lingual undercuts are observed with D1 bone densities, and flap reflection can be safely kept to a minimum. The precise closure of the periosteum and the overlaying tissue has been shown to help recover the blood supply and is encouraged.³⁴

Implant Osteotomy

The primary surgical problem of D1 bone is that the dense cortical bone is more difficult to prepare for endosteal implants than any other bone density (Box 29-1). The most common cause of implant failure in this bone quality is surgical trauma resulting from overheating the bone during the implant osteotomy procedures, because rotary drills progress with more difficulty. The zone of devitalized bone that forms around the implant is larger in this bone density and must be remodeled and replaced by vital bone for the interface to be load bearing (Figure 29-4). As a result, implant surgical failure may be greater in D1 bone than any other bone density.^2,¹⁸ Therefore the dental surgeon must strive to minimize the thermal trauma. The heat generated during an implant osteotomy is related to the presence and temperature of irrigation,³⁵^–³⁷ amount of bone being prepared,^38,³⁹ drill sharpness and design,⁴⁰^–⁴³ time of preparation,⁴⁴ depth of the osteotomy,^37,^30,^41,⁴⁵ pressure on the drill,⁴⁰ drill speed,^39,^46,⁴⁷ and variation in cortical thickness.^35,⁴⁸

Box 29-1 Implant Osteotomy Preparation in Dense Compact (D1) Bone

Overheating during Osteotomy

Abundant external or internal irrigation

Cooled saline irrigation

Intermittent pressure on drill

Pause every 3 to 5 seconds; keep irrigating

New drill designs, flutes, geometry

Incremental drill sequence (more drills; pass same drill more than once)

Blood Supply

Primarily from periosteum

Minimal reflection

Final Osteotomy Drill

Greater width

Greater height

Slower speed used

Bone Tap

Short of full osteotomy depth

Allows passive implant fit

Prevents internal implant-body/implant-bone interface microfracture

Removes drill remnants

Final Implant Placement at or above Bone Level

Unthread ½ turn to relieve internal stresses

Slower Healing Rate

Lamellar bone

Fewer blood vessels

Five months to achieve mature interface

Stage II Uncovery

Three to 4 months

May often use immediate loading (when prosthesis is biomechanically stable)

Figure 29-4 Devital zone (D) of bone next to the implant (I) is primarily created by heat generated during surgery, which radiates from the site, especially in cortical bone. Other contributing factors include lack of blood supply, pressure necrosis from implant placement, microfracture from bone tapping, and implant insertion. V, Vital bone.

Bone cell survival is very susceptible to heat. Eriksson has demonstrated that in rabbit, bone temperature as low as 3° C above normal (40° C) can cause bone cell necrosis.⁴⁹ Therefore a conscious effort is made to control temperature elevation every time a rotary instrument is placed in contact with bone. At least 50 mL/min of cooled irrigation, such as sterile physiologic saline, is used as a profuse irrigant and is a critical element to reduce heat.³⁵–^37,⁵⁰ Distilled water should not be used, as rapid cell death may occur in this medium.⁵¹ Intravenous dextrose solution (D₅W) also may be used, with the clinical advantage of decreasing hand piece breakdown occurring from the effects of the salt in a saline solution, although the surgical gloves often feel sticky near the conclusion of the surgery. The irrigant also acts as a lubricant and removes bone particles from the implant osteotomy site. Without irrigation, drill temperatures above 100° C are reached within seconds during the osteotomy,⁵² and consistent temperatures above 47° C are measured several millimeters away from the implant osteotomy. The temperatures of the irrigant can also affect the bone temperature.^36,³⁹ Copious irrigation is suggested, especially in D1 bone.

The amount of heat produced in the bone is directly related to the amount of bone removed by each drill.⁵³ A 3-mm pilot drill generates greater heat than a 2-mm pilot drill.^39,⁵⁴ As a result, most manufacturers suggest the first drill be 2 mm in diameter. In a similar fashion, the amount of heat generated by successive drills is also directly related to the increase in drill diameter.^52,⁴³ A 3-mm drill after a 2-mm drill cuts 0.5 mm on each side of the drill. A 2.5-mm drill after a 2-mm drill only cuts 0.25 mm of bone on each side of the osteotomy. The smaller incremental drill size allows the surgeon to prepare the site faster, with less pressure and less heat.⁴² In addition, when large increases in drill diameter are used to prepare bone, the surgeon may inadvertently change the angulation of the drill, because the larger drill is removing a greater bone volume and the tactile sense is decreased. As a result, an elliptical osteotomy may be prepared that does not correspond accurately to the round implant diameter. The gradual increase in osteotomy size also reduces the drill shatter at the crestal opening, which can inadvertently chip away pieces of bone on the crest where complete bony contact is especially desired. The gradual increase in drill diameter also keeps each drill sharper for a longer period, which also reduces the heat generated (Figure 29-5).

Figure 29-5 The number of steps in the preparation of the osteotomy is related to the bone density. D1 bone requires four drills, a crestal bone drill and a bone tap. D3 requires only three drills and no crestal drill or bone tap.

In the past, the rotational speed of the drill has been suggested to be less than 2000 rpm, and several manufacturers have recommended speeds al low as 750 rpm. It has been an accepted theory that the higher the speed, the higher the bone temperature during preparation.⁴ This is false; otherwise, slow-speed hand pieces would be used to prepare natural teeth. A high-speed hand piece (300,000 rpm) can remove bone over an impacted tooth or during an apicoectomy and still allow bone regeneration. Rafel prepared bone at 350,000 rpm in a human mandible and found a temperature of only 23.5° C at a distance of 3 mm from the drill periphery.⁴⁷ High-speed drills at 300,000 rpm have been used to prepare blade implant osteotomies, yet bone grew over the blade shoulder and was in direct contact with the implant.⁵⁵

A study by Sharawy et al. compared four drill designs (two internal irrigated and two external irrigated) at speeds of 1225, 1667, and 2500 rpm.³⁹ Thermocouples connected to a computer to record temperature and time were placed within 1 mm of the osteotomy site in D2-type bone (Figure 29-6). All drill designs in the study recorded lower bone temperatures with the greatest rpm and, conversely, found the highest bone temperatures with the lowest rpm (Figures 29-7 and 29-8). As important, the slowest rpm resulted in bone temperatures at or above 40° C, which may be a threshold of bone-cell death. The highest rpm (2500) increased the bone temperature by 2° to 3.5° C, whereas the 1225 rpm recorded a bone temperature above 41° C. Therefore the higher speed of 2500 rpm may prepare bone at a lower temperature than 1500 rpm, especially when in dense bone.³⁹ The rotational speed of the drill is one of the more critical criteria to reduce bone temperatures.

Figure 29-6 Thermocouples were positioned within 1 mm of the drill site and inserted into bone for a depth of 8 mm. The wires were connected to a computer to measure the temperature, time of preparation, and time the bone temperature was elevated.

Figure 29-7 The internally cooled drill of the Paragon implant system recorded 41° C with the first drill at 1225 rpm. A rpm of 2500 reduced the bone temperature preparation for all drill diameters.

Figure 29-8 The externally cooled drills of the Brånemark implant system recorded reduced temperatures at 2500 rpm compared with slower speeds.

Eriksson reported bone cell death when a temperature of 40° C was applied for 7 minutes, or when a temperature of 47° C was applied for 1 minute.⁴⁹ In other words, time and temperature are interrelated critical factors in implant site preparation. As the temperature increases, the time the bone temperature is elevated must be reduced. In the study by Sharawy et al., the time the bone temperature remained elevated was recorded for each rpm evaluated.³⁹ When the drill prepared an 8 mm depth osteotomy, the temperature remained elevated for 45 to 58 seconds (Figure 29-9). The slower the rpm (1225), the longer the bone temperature remained above the baseline. Because two to three drills are used to prepare an implant site, at 1225 rpm the first drill may increase the temperature to 41° C, the second drill to 45° C, and the third drill to 49° C, when the time between each sequence is not extended more than 1 minute. Therefore, after each drill is used within an osteotomy, a 1-minute pause should be observed before the next drill. In the Sharawy et al. study, the first drill diameter recorded the longest preparation time and the highest temperature, and the longest recovery time.³⁹ Therefore to reduce the preparation time within the bone to a minimum in D1 bone, the surgeon should not apply constant pressure to the drill, but “bone dance” with intermittent pressure for 1 second in the D1 bone and 1 to 2 seconds out of the bone while the cooled irrigation is allowed to perfuse the site. The surgeon should also pause approximately every 5 to 10 seconds of bone preparation for 1 minute to allow the limited blood supply access to the surgical site, permit dissipation of heat, irrigate the area to reduce bone temperatures, and remove debris from the osteotomy site.

Figure 29-9 The internally cooled drills of the Nobel Biocare, Steri-Os implant system demonstrate the temperature in the bone remains elevated for an extended period (up to 58 seconds) after site preparations in D2 bone. The lower the rpm, the longer the temperature remains elevated.

The pressure used against the bone should neither be so hard as to stall the drill, nor so light that the drill only creates heat and does not prepare the bone. Hobkirk and Rusiniak found that the average force placed on a hand piece during preparation of an osteotomy is 1.2 kg.⁴⁸ Matthews and Hirsch concluded that the force applied to the hand piece was more influential than the drill speed in temperature elevation.⁴⁰ When the pressure on the hand piece was increased appropriately, drill speeds from 345 to 2900 rpm did not affect the temperature. They found that increasing both speed and pressure allowed the drill to cut more efficiently and generated less heat. The effect of drill speed and pressure related to bone temperature was also reported by Brisman.⁴⁶ In cortical bone, speeds of 1800 rpm with a load of 1.2 kg produced the same heat as when speed increased to 2400 rpm with pressure of 2.4 kg. The greater speed and greater pressure was more efficient than low speeds. Increasing pressure alone increased heat; increasing speed alone also increased heat. Different amounts of pressure are therefore used in response to the density of the bone. Sufficient pressure should be used on the drill to proceed at least 2 mm every 5 seconds. If this is not achieved, new (sharper) or smaller-diameter drills are indicated for each site preparation. The pressure on the drills should not reduce the rpm, which makes the drill less efficient and increases heat. Hand pieces of sufficient torque should be used to prevent this complication.

Lavelle has demonstrated lower drilling temperatures in cortical bone with internally irrigated drills.⁵⁶ Thus many manufacturers suggest that an internally irrigated drill be used during implant osteotomy preparation. However, the bone temperature in the study by Sharawy et al. compared two different internal irrigated drill systems (Zimmer and Nobel Biocare) with two different external irrigated systems (Nobel Biocare and BioHorizons).³⁹ The two external irrigated drill systems prepared the bone osteotomy at lower bone temperatures than the two internal drill systems. The difference in the bone temperatures between the four systems ranged from 1° to 3° C (depending upon the drill diameter). This study did not use the same drill design when comparing internal versus external irrigation. Therefore, the difference in bone temperature elevation may be related more to drill design than to the method of irrigation. Benington et al. tested internal and external irrigation systems on bovine bone. At a constant load of 1.7 kg, no difference was found between the two designs.⁴³

Internal irrigation of drills may present several disadvantages. The portal of the internal-cooled drill that exits at the drill apex often becomes clogged once the drill is in the bone deeper than 5 mm.⁵⁷ As a result, no irrigant escapes the portal and the drilling must be repeatedly stopped to remove debris. The pressure required during drilling dense bone also may completely prevent saline from flowing out of the drill in deeper preparations, with bone debris obturating the portal of the irrigation chamber. In a histologic study by Watzek et al., the BIC percentages between internal and external irrigated drill systems in cortical and trabecular bone were compared 4 weeks after implantation. In cortical bone, the BIC was highest in internal irrigated drills when the osteotomy was deep. In trabecular bone or less deep cortical sites, the external by irrigated dills recorded greater BIC values⁴⁵ (Figure 29-10).

Figure 29-10 The externally irrigated drill had more bone-implant contact (BIC) after 4 weeks of healing in trabecular bone (metaphysics) and for shallow osteotomies in cortical bone (diaphysis). The internally irrigated drill had a higher BIC in deeper osteotomies of cortical bone.

Complete cleaning of the internal chamber of the internal irrigated drill is virtually impossible, and the drill usually retains organic contaminants in the chamber from a previous surgery ⁵⁷ (Figure 29-11). Although these organic remnants may be sterilized as the drills are autoclaved after each surgery, it seems unwise to allow organic debris to be transmitted from one patient to another. The external irrigated drill may be sterilized for a following surgery. In addition, the internally cooled drill fabrication in smaller diameters poses technical problems and may cause fracture during use in more dense bone. The externally irrigated drill is also less expensive to fabricate and can be made in smaller diameters.