Implant Compression Necrosis: A Scrupulous Review

Introduction

Bone is a distinct structural material which is dynamic in nature, and undergoes constant renewal in response to various factors that might be mechanical, nutritional, and hormonal influences, among which applied mechanical stresses play a key role in its response. Although bone can and will form, remodel even if there is a complete lack of stress like bone formation in osseointegration. It has been noticed by various studies that employing light mechanical load may develop conducive stresses within the bone and enhance bone formation, but on the other hand, if mechanical stresses go beyond a certain threshold level may have a negative impact and the implant would fail to osseointegrate^.1

Looking at the crestal bone level which surrounds an osseointegrated implant, its physiologic reaction may be in three phases, the first one being associated with primary site preparation and the bone loss in this phase may be linked to substandard surgical procedures, insufficient quantity and quality of bone or an osseous defect. The second phase is the uncovering phase which due to its non-invasive nature of the procedure isn’t linked with any crestal bone loss and is rarely reported in literature. And the third phase is prosthetic loading and function, which well reports a crestal bone loss in studies. A new principle of ‘Ailing and Failing’ implants has been put forth which might reverse or repair the clinical problems of crestal bone loss. Previous reports suggest excessive crestal bone loss due to plaque-induced peri-implantitis or occlusal overload, the latter producing excessive stresses around the implant.¹

Mainly 3 main biomechanical parameters regulate the distribution of stress and optimal stability of an implant, firstly the placement procedures, i.e., drilling of the osteotomy site and compression technique used to attain local stability, secondly the design feature of implant and the loading condition to which the implant is subjected.²

For successful osseointegration, the primary stability of an implant is of utmost importance and the level of stability should be high enough. It is mainly measured by insertion torque which is the torque required for the implant to advance in the prepared osteotomy, but excess torque at the insertion time may lead to bone compression which if crosses its physiologic limit may lead to a bloodless field i.e. ischemia following necrosis or sequestrum formation which in turn leads to implant failure. Too much torque levels may result in high levels of strain transmitted to adjacent bone. Since the crestal regions around the implant with dense cortical bone may have a minimal blood supply, comes across maximum strain on insertion hence making it more vulnerable to bone necrosis on application of excessive pressure.³

Compression necrosis

The literature reported the first case of suspected compression necrosis around dental implants in 1991 by Haider et al. ⁴ Moreover a case report suggested an implant failure with suspected compression necrosis as a consequence of bone overheating and too much tightening of the implant with bone chip compression at the apex of implant. And since the crestal bone is faintly vascular and barely resists shear forces, it makes it vulnerable to early bone loss averaging 1.2 mm.^5,6 Studies reveal microfractures in bone development on implant placement which occur due to sheer stress acting along the long axis of the implant on its installation. This stress intensifies as the implant goes farther into the bone and to some extent increases the friction and torque. And as a consequence, this type of stress and friction leads to necrosis at bone-to--to-implant interface.⁷

On evaluating the fracture dynamics of trabecular bone, elevated local strain levels were seen during compression resulting in microdamage of alveolar bone.⁸ A previous study analysed how does an increase in the compressive strain level accumulates microdamage in bovine trabecular bone. The results showed that on raising the maximum compressive strain, all microdamage parameters increased.⁹ After implant insertion in the bone, the tissue reaction that comes after can be compared to fracture healing. It starts with a blood clot formation in between the remaining bone and the implant surface. The pluripotent mesenchymal cells may either differentiate into fibroblasts or osteoblasts depending on the environment and the relative immobility of bone-to-implant interface forming a scar tissue or bone respectively.¹⁰ The mesenchymal cells may show chondrogenic differentiation owing to poor vascularity and low oxygen tension conditions. Moreover, the cellular differentiation may be affected by the mechanical stress to which the tissues are subjected. Healing phase of an implant can be affected by micromovement as the distortional stresses may impair cells, altering their genetic expression and synthetic activity, which may lead to fibrous scar tissue formation.¹¹

Necrosis, or accidental cell death usually occurs seconds to minutes following an injury and is distinguished by cell swelling and cell membrane integrity loss. Necrosis is cell death caused by unrecoverable external injury and changes in the nucleus (swelling, pyknosis, karyorrhexis, karyolysis) and can be seen under the microscope as well in the cytoplasm which shows eosinophilic staining. Osteonecrosis is basically bone death that may disintegrate the architectural structure of bone, which may give rise to pain in joints, bone damage, and loss of function.

Osteonecrosis may also be termed as avascular necrosis, ischemic necrosis, subchondral avascular necrosis, and aseptic necrosis of bone. It is not a specific disease entity, but rather it is the final common pathway of several conditions leading to bone death.¹²

Histopathology of osteonecrosis

From previous literature, it was accepted that empty osteocytic lacuna is histologically suggestive of osteonecrosis, but it’s now comprehended that suboptimal tissue fixation may be the cause of artefactual loss of stain in osteocytic nuclei. On the contrary, studies performed on both human and experimental bone infarction in animals have shown that loss of osteocytes in incomplete until 2-4 weeks after the onset of ischemia, which implies that even in properly processed tissue, histologic bone death may be hampered if based on this feature alone. Moreover, sometimes there has been seen empty lacunae in the interstitial lamellae between osteons of cortical bone whose numbers rises with ageing and probably as a consequence of diminished blood supply. Although in severe ischemia cases a complete absence of osteocytes has been seen compared to the patchily distribution of osteocytes seen within cortical bone.¹²

Bone ischemia is indicated by the few early signs seen in marrow spaces microscopically, on starting from the 2nd day there is nuclear stain loss of marrow cells and the appearance of large round and ovoid spaces filled with fat, following which this fatty and haemopoietic marrow is ghosted with small vessels showing signs of necrosis. After 15 days, empty osteocytic lacunae and trabecular surface lacking cells were seen. Necrotic zone boundary shows proliferation of capillaries along with fibroblasts and foamy histiocytes, which accounts for necrotic fatty marrow breakdown, while dead bone is eliminated somewhat by osteoclasts and replaced by newly formed trabeculae; alternates with woven bone which lay down on dead trabeculae surface.¹²

Factors leading to compression necrosis

Excessive torque

The amount of torque needed to tap the implant into the prepared osteotomy is the insertion torque and the measure of torque noted when the implant gets as far as its final apico-occlusal position is its final seating torque. Insertion torque is a qualitative parameter which is mostly taken in account during implant placement Surgical technique, implant pattern and form and bone quality of implant placing site are factors which mainly affect the implant insertion torque.

The maximum Insertion Torque (IT) is essential to embed the implant into the prepared bone site; the reported values differ from 5 to 50 ncm. Currently it’s unknown how much torque is required to gain adequate primary stability. IT is directly proportional to bone density values which reported higher implant stability.¹⁴

A previous study which investigated how the bone temperature change was affected by the surgical technique, implant macrodesign, and insertion torque during implant placement showed a significant increase in cortical bone temperature at a depth of 1 mm with increasing IT values, while in deeper parts of the osteotomy no significant changes in IT value were found. Moreover, a temperature increase of 10°C above body temperature showed a clinically significant outcome which precipitated bone necrosis, and an increase of 4.3°C depleted the quality of “de novo” formed bone. However, there was a temperature rise from 0.3°C to 0.43°C on increasing IT from 30 to 40 Ncm. Therefore, in spite of the fact that a 30Ncm torque is advantageous but insufficient for smooth implant placement, 40 ncm torque can be utilized without any detrimental effects on the bone.¹⁵ Literature exist which shows that high insertion torques increases bone remodelling rather than inducing peri-implant fibrosis in hard bone, which then modifies the healing of peri-implant bone. Therefore, high insertion torque may be beneficial for immediate implant loading, but may be contraindicated for delayed loading.¹⁶

It has been suggested that the implant insertion should be done at a minimum of 30 ncm torque and a maximum of 40–45 ncm to gain adequate primary stability without generating excessive compression in the peri-implant bone and the torque above this might disturb the microcirculation of the surrounding bone and cause necrosis of the osteocytes. Implants if inserted with higher torque might cause decreased mechanical strength and hence early implant failure. The torque was affected most by the diameter, followed by the length, and then the shape of implant. Another way to decrease the insertion torque would be to reduce the friction at the bone and implant interface with lubricants.⁷

Elastic and plastic strain

The development of strains has a pivotal role in both modelling and remodelling of alveolar bone. Beyond a certain threshold, strain levels might cause destruction within the tissue in the form of microcracks which is linked to elevated elastic strain levels and accompanied by “plastic” strain development. The latter strain is irrecoverable or residual strain increases the probability to subside or implant failure.¹⁷

Frost`s mechanostat theory is reported as a model of four zones of compact bone as it relates to mechanical adaptation to strain. First zone is the “Acute disuse window” with the microstrain of trivial bone being 0-50 microstrains. In this window there is a gradual net bone loss as the bone loses mineral density and non-use atrophy occurs due to inhibition of modelling of new bone. Second comes the “Adapted Window” (50-1500 microstrain) called the homeostatic window of health since there is a balance maintained between bone modelling and remodelling and it’s the ideal strain required around an endosteal implant. Third zone is the “Mild Overload Zone” (1500-3000 microstrain) with decreased bone strength and density due to remodelling inhibition, and hence histologic bone repair/woven bone. Therefore, care should be taken while loading as bone strength may not be adequate to repair. Lastly “Pathologic Overload” (>3000 microstrains) in which only woven bone is formed, cortical fractures occur, and there is evidence of crestal bone loss.¹⁸

^{Implant stability}

For successful osseointegration, i.e, a direct structural and functional link which is there between the bone and implant surface, implant stability has a crucial role. Implant stability is divided as primary and secondary stability. In essence ‘primary stability’ is a clinical parameter of micromotion and defined as a mechanical locking of bone and implant upon placement which depends on the quality and quantity of local bone. Secondary stability is accomplished when the implant interface is surrounded by new bone and is linked to remodelling of tissues and bone around the bone-implant interface. Secondary stability may not be attained without good primary stability. Poor primary stability may cause small movements of the implant, which might hinder necessary bone remodelling affecting secondary stability. This leads to back and forth motion of the implant causing microfractures followed by necrosis.7 Currently, two methods, namely, cutting-torque measurement and resonance frequency analysis (RFA), are frequently used to assess primary implant stability.¹⁹

Use of self-tapping screws

Recently a research was done to compare cortical tapping and cortical widening as methods to impede overcompression of the bone. Cortical tapping is to prepare a thread profile of the implant before its insertion to seat the implant without putting pressure. For pretapping the drill used is shorter than the length of the implant with a matching coronal part of implant. And for cortical widening, a countersink drill is used to expand the crest area which has a bit larger diameter than implant, which in turn prevents overcompression of dense cortical bone. The study inferred an upper hand of cortical tapping biomechanically for implants during immediate loading and when the bone density is high.²⁰ The energy which is employed in tapping the site either before or during the placement of implants is the union of the thread placement force from the tip of the instrument and the friction created as the remaining part of the tip or implant enters the site.²¹

Self-tapping screws possess flutes cut out of the screw tip to permit cutting to escape, also the insertion torque in pretapped sites differs from thread-cutting forces measured with a screw tap or a self-tapping implant. The relative thread profiles of the tap and implant fixture are the variables which majorly influence the insertion torque when a threaded implant is placed into a pretapped hole in bone, and hence if thread-cutting forces are removed, then the insertion torque is a function of the compressive stresses applied locally to the surrounding bone and friction at the implant-bone interface.²²

Previous literature suggests a significant higher primary stability in medium density of bone and implant design without self-tapping blades in contrast to that with selftapping blades, and this variance can be elucidated by the difference in implant geometry, especially the presence of a self-tapping blade, even if the implant design has the same geometry.²³

Poor quality of bone

The entire body comprises of about 85% of the bone which is cortical and the rest is spongy in nature, out of which cortical bone remodels approximately remodelling 3% and spongy bone about 25% of its mass each year. Cortical bones seem to have a mechanical/protective function while spongy bone shows a metabolic role. Cancellous bone has double strength in compression as in tension and has the same nature to man-made rigid cellular foams. Therefore, its energy absorption capacity is much lower in tension than in compression.^24,25,26

There are evidence to suggest that implants can overheat the bone with a thicker cortical plates of the mandible leading to high failure rates.⁷ Moreover precise implant placement with increased primary stability without overheating risk can be avoided with the use of bone condensing technique to prepare the implant site in soft maxillary bone. A comparative study was done between conventional implant placement technique and Summer's osteotome technique to evaluate the crestal bone loss around the early non-functional implant and better primary stability which inferred that the osteotome procedure benefitted the knife edge ridges but shouldn't be considered a substitution for the conventional procedures for implant placement.²⁷

Keeping in mind that the interplay between implant design and the adjacent bone affect the implant stability, various authors suggest a high insertion torque is desired for better integration of implant,^16,28 as many studies suggested prevention of adverse micromovements under loading above 100 mm when insertion torque value are in the range of 25– 45 ncm but on the contrary it may not be true for all implant designs and drilling techniques.^29,30

Although the change in design parameters may be wise from an engineering view point, it must be pondered that bone is a dynamic tissue which will react to surgical procedure stimulation and/or the association between the implant macrogeometry and its drilling dimensions.³¹ Thus, while reduced micromotion under loading is desirable, low degrees of bone stress are also desirable since a lower amount of remodelling would be necessary during osseointegration, potentially resulting in slight decreases in implant stability over time.³²

Undersizing the osteotomy site

Majority of implant systems fabricate a preparation site which is a bit smaller than the implant being placed, which will clearly produce low levels of insertion stress. Evidence suggests a high rate of implant success if the bone quality is average with good and sufficient compression and stability with no overcompression. On the contrary, high compression will be induced if tapered implants are used or if the drill size is much smaller than the implant diameter, which might cause early implant failure. Clearly, an association exists between compression of the osteotomy site and insertion torque.^2,33,34

A slightly narrower final drill with a tapered implant design often elevates the insertion torque as well as localized bone compression, and both factors might increase the primary implant stability. There is histologic evidence suggesting an improved peri-implant formation with bone condensation. But if the local stress is way too high may lead to ischemia and localized bone necrosis at the implant-bone interface. Literature demonstrated that undersizing an osteotomy site may improve primary stability and implant survival, implying that top-to-bottom initial stability is a desirable outcome. Conclusion was made that using smaller diameter drills for implant placement in the posterior maxillae, where bone quality is generally poor, may improve primary implant stability.^35,36,37

Macrogeometry of the implant and aggressively cutting edges (Implant design)

The fundamental criteria to attain osseointegration in early and delayed loading of the implant is the initial stability which depends on bone quality and the surgical technique. Such anchorage may also be affected by implant design factors such as overall surface area, length, and thread configuration, which may be significant when anticipating immediate or early loading in order to reduce micromotion of greater than 150µm. The principle of implant design basically reduces the threshold for ‘tolerated micromotion’ and gains initial stability, incorporating design factors that minimise the sheer force effect on the interface so that the marginal bone is safeguarded and lastly include design features that may facilitate bone formation and bone healing (secondary osseointegration).³⁸

Literature reveals that the tapered implants can be chosen to improve primary stability and, when placed induce a degree of lateral compression of the cortical bone layer in critical bone quality. Therefore, it is desired to have an implant with a positive taper which will cause optimal compression in poor bone qualities.^23,39 Moreover the surface geometry needs to be considered in order to address the seating stability and compression levels within good bone. Most of the present implant systems use a thread-cutting geometry to tap the thread in the bone while placing the implant, which establishes a very effective bone-to-implant contact. Current implants can change the bone volume ratio within threads by decreasing the implant thread thickness. The main feature of the geometry of the thread-cutting face is that there is much room or volume in the relief chambers for clearance of bone.³⁸

Although the bone clearance chambers are fabricated to boost the volume for bone chip entrapment, they should also have maximum implant contact with the threaded area. In lieu of optimizing the insertion of the threaded implants, a secondary cutting feature can be introduced along the side of the implant. The secondary cutting face is shallower than the fore-mentioned face, doesn’t engage with soft quality bone, will engage the dense bone but a minimal amount of bone is removed, won’t impair implant stability, and fits in all bone qualities differently without over-compressing good bone.²

Previous research work proposed that threads with their uneven contour will produce a stress field that in turn matches with the ‘physiologic overload zone’ and initiate new bone formation that may support ‘cuplike bone formation’ at the crest of the implant thread and affect the amplitude of stresses in the bone.

Studies have shown a decrease in shear force and increased compressive load when a square crest thread with a flank angle of 3 degrees is used and the thread pitch and depth of square thread varied in each of the 4 known densities of bone. This discrete level of stress produced by varied thread shapes prompts new bone formation in order to attain a similar microstrain in all bone types.⁴⁰

Discussing the thread patterns in dental implants presently they range from micro threads near the neck of the implant (AstraTech, Lexington, MA) to broad macrothreads on the mid-body Bio Horizons, Birmingham, AL; Steri-Oss, Nobel Biocare) and a number of altered pitch threads that induce self-tapping and bone compression. Many other modifications have been used to emphasize the effect of threads, but very few are documented scientifically. Looking at the evidence, about the effect of implant thread shape on implant stability is unclear and may be inferred that thread design might influence poor quality bone rather than good quality bone.³⁹

Experimental studies have reported the maximum bone stresses localised in the cortical bone in the region of the implant neck, and the bone loss starts around the implant neck. Therefore, it is said that the crest region shouldn’t be designed for load bearing and suggested that the implant neck should be smooth/ polished. However, significant loss of crestal bone has been reported for implants with 3 mm long smooth polished necks.⁴¹

Compared to cylindrical implants, no initial insertion curve is seen in tapered implants. Owing to the cylindrical design, the effective surface to be cut increases, which in turn increases the cutting torque with a constantly increasing frictional torque. As the implant goes deeper due to high torque, there is a risk of decreased circulation in the bone which may result in necrosis of the connective tissue carrying the implant incision. In cylindrical implants, the complete thread length must be passed during insertion, which extends the insertion time markedly, while tapered implants, on the contrary, have the added advantage of being placed to a certain depth in the predrilled bone cavity before the turning of the implant for insertion starts which lessens the vertical space needed in the beginning and reduces the risk of the implant being placed into the opening at a crooked angle, allowing the threads to be cut more easily.¹⁴

In a finite element analysis study, the outcomes indicated that the implant design, i.e., both in the macro and micro geometry aspect, significantly affected implant stability. Theoretically, implant threads with shortened pitch may have two advantages for enhanced bone response; macro-threads may tend to impede micromovements during healing and thus firm implant stability is attained, and the inclusion of altered threads with shortened pitch in between the macro threads may be effective under compression forces and allow for enhanced bone healing. Secondly; the application of threads with a shortened pitch to the implant will have a surface enlargement on the macro level (mm level) compared with the control implants, thus enhancing bone formation.^42,43

Conclusion

Factors such as application of excessive torque, use of self-tapping implant, under-sizing osteotomy preparation, short implants with aggressive cutting edges of implants can lead to compression necrosis which leads to implant failure. Implant failure may occur due to various reasons and sometimes the aetiology may not be known. Putting excessive torque for placing an implant may increase the risk of failure due to compression. Precision in surgical techniques with necessary irrigation may prevent implant failure because of overcompression. Proper attention should be given to implant insertion torque values which shouldn’t go beyond the recommended torque for a particular implant system. Moreover, reversing the implant by one quarter turn after insertion may cut back the stress in adjacent bone, especially while using tapered implants. Pre tapping is essential in placing implants into dense bone and it may prevent the need to use high torque values to place the implant.

Source(s) of Support

Nil

Conflicts of Interest

Nil