Since the introduction of the concept osseointegration, remarkable changes have taken place in implant dentistry. Bone quality and quantity plays an important role in long term success of implant therapy. Bone is a dynamic biological tissue composed of metabolically active cells that are integrated into a rigid framework. A lot of research work has taken place to determine the success of implants placed in different quality and quantity of bone. Primary stability of implant has been proposed to be one of the most important factors responsible for the success of implant therapy which is primarily determined by the density of the bone. The detailed knowledge of different aspects of bone is essential for long term implant success. In the following sections we shall study in detail the qualitative and quantitative aspects of bone and its importance in implantology.
2) Biological aspect of bone:
Bone is a dynamic tissue. The cellular components of bone consist of osteogenic precursor cells, osteoblasts, osteoclasts, osteocytes, and the hematopoietic elements of bone marrow 1. The bone is covered by periosteum a layer known as the mother of bone. The periosteum is a tough, vascular layer of connective tissue that covers the bone but not it’s articulating surfaces. The thick outer layer, termed the “fibrous layer,” consists of irregular, dense connective tissue. A thinner, poorly defined inner layer called the “osteogenic layer” is made up of osteogenic cells. The endosteum is a single layer of osteogenic cells lacking a fibrous component 2. Osteoblasts are involved in bone formation, osteoclasts in bone resorption and osteocytes are mature osteoblasts trapped within the bone matrix. There are three primary types of bone: woven bone, cortical bone, and cancellous bone 1.
It is composed of randomly arranged collagen bundles and irregularly shaped vascular spaces lined with osteoblasts. It is formed during embryonic development, during fracture healing (callus formation), and in some pathological states such as hyperparathyroidism and Paget disease.
Cortical bone/compact/lamellar bone:
It is remodelled from woven bone by means of vascular channels that invade the embryonic bone from its periosteal and endosteal surfaces. It is well organized lamellar deposition of mineralised layers. It primarily consists of a Haversian system which is a cylindrical shaped system that surrounds longitudinally oriented vascular channels called haversian canals as well as horizontally oriented canals system the Volkmann canals.
Cancellous bone /trabecular bone:
It lies between cortical bone surfaces and consists of a network of honeycombed interstices containing hematopoietic elements and bony trabeculae. The trabeculae are oriented in such a way that they provide the best support to the forces exerted on the bone.
3) Bone resorption after tooth extraction:
After tooth extraction there is bone remodelling which involves the osteoclastic resorption of the residual alveolar ridge. Bone resorption is particularly more on the labial wall as well as there is bone deposition within the extraction socket. During the first 3 months, the rate of bone resorption is highest which gradually reduces after 6 months. Re-modeling is generally complete and stabilized after 1-2 years 3. The rate of bone resorption in mandible is 3-4 times higher than in the maxilla.
The bone resorption pattern is different in maxilla and mandible. In the maxilla resorption of the alveolar ridge occurs mainly on the buccal aspect. This is because of the reason that buccal cortical bone of maxilla is thinner as compared to the palatal bone. So, the remodelling of maxilla is in a centripetal fashion which makes the maxilla smaller with due course of time. In case of mandible, the buccal cortical plate is thinner than the lingual cortical plate in all areas except in the molar region. So, bone resorption in the mandible occurs primarily lingually in the premolar areas and buccally in the molar segments. Thus, this kind of resorption in maxilla and mandible make maxilla smaller that the mandible with due course of time.
The availability of sufficient bone is must for implant placement. If the bone is in-sufficient, bone augmentation procedures are carried out. Various classifications have been proposed to calssify the available bone both quantitatively and qualitatively.
4) Based on volume of the bone available following classifications have been proposed,
A) Atwood’s classification:
Atwood (1979) 4 proposed a classification for edentulous mandible. In this classification system ridge resorption in a horizontal direction was considered. Measurements were made in a cross section of the mandible at the site of the central incisor. His classification of resorption stages is as follows:
Class I : Tooth bearing alveolus.
Class II : Alveolus after extraction.
Class III : High alveolar process.
Class IV : High, narrow alveolar process.
Class V : Rounded flat alveolar process.
Class VI : Concave flat alveolar process.
Out of these six classes Class III and above actually represent the stages of bone resorption.
B) Seibert’s classification:
Seibert 5 classified ridge defects are based on the location of the deformity.
Class I ridge defects
It involves bone loss in the bucco-lingual width only.
Class II ridge defects
It involve a loss in the apicocoronal height only.
Class III ridge defects
It is a combination of both bucco-lingual and apicocoronal loss (both width and height).
C) Cawood and Howell’s classification:
Cawood and Howell 6 also gave classification of the resorbed maxilla and mandible. In their study published in 1988, it was found that the shape of the basilar process of the mandible and maxilla remains relatively stable, whereas changes in shape of the alveolar process are significant in both the vertical and horizontal axes and follow a predictable pattern. They proposed the following resorption stages:
Class I : Dentate ridge.
Class II : Ridge directly after extraction.
Class III : Broad and rounded ridge with adequate height and width.
Class IV : Knife-edge ridge with sufficient height but insufficient width.
Class V : Flat ridge with insufficient height and width.
Class VI : Depressed ridge with a cup-shaped surface.
D) Fallschssel’s classification:
Fallschssel’s classification 7 is based on the examination of 19 skulls. The classification refers only to the shape of the alveolus in the canine and in the incisor region of the edentulous maxilla. Therefore practical application is limited.
Class I : Dentulous ridge.
Class II : Wide high alveolar process.
Class II : Narrow high alveolar process.
Class III : Sharp high alveolar process.
Class IV: Wide alveolar process with reduced height.
Class V : Completely resorbed alveolar process.
E) Lekholm and Zarb classification:
Lekholm and Zarb 8 in 1985 gave a new classification for various degrees of atrophy for both mandible and maxilla.
A : Virtually intact alveolar ridge
B : Minor resorption of the alveolar ridge
C : Advanced resorption of the alveolar ridge to the base of the dental arch
D : Initial resorption of the base of the dental arch
E : Extreme resorption of the base of the dental arch
Lekholm and Zarb classification
(The line in the diagram represents the boundary between the alveolar ridge and the base of the dental arch.)
F) Misch and judy classification:
In 1985, Misch and Judy 12 proposed a classification system for the available bone with treatment options for each category. This classification system is one of the most commonly used system for classify bone volume. For ease of understanding they have based their classification system on Kennedy’s classification for partially edentulous arches. Kennedy’s classification is one of the most commonly followed classification system used for partially edentulous arches. Before going to Misch and Judy classification lets first discuss briefly Kennedy’s classification.
The Kennedy-Applegate-Fiset Classification System for partially edentulous arches:
Class I: Bilateral posterior edentulous areas.
Class II: An unilateral posterior edentulous area.
Class III: Unilateral bounded partially edentulous.
Class IV: Bilateral bounded anterior partially edentulous.
Kennedy’s classification for partially edentulous arches
Based on these four classes of partial edentulism Misch and Judy classified the available bone volume into 4 divisions A, B, C and D. They have evaluated the available bone volume in terms of the following parameters:
- Crown height / Bone Height (Implant body) ratio
Let us discuss these parameters before we discuss the classification in detail.
a) Available Bone Height:
The selection of the length of the implant fixture depends upon the available bone height in an edentulous site. The available bone height is measured from the crest of the edentulous ridge to the opposing limiting anatomical landmarks such as inferior alveolar canal in the mandible and the floor of the nasal cavity and maxillary sinus in the maxilla which can be identified during radiographic assessment. Maximum bone height is available in anterior regions of the jaws. The canine eminence region in the maxilla and the first premolar region in the mandible offer the greatest bone height column of bone 9. More the height of the implant more is its surface area. An implant 3 mm longer provides more than 10% increase in surface area. Increase in length of the implant provides better initial stability which is one of the most important factors responsible for implant success. The requirement of length is reduced in cases of dense bone where good initial stability can be achieved even with shorter implants.
In the maxillary and mandibular posterior regions due to presence of limiting structures i.e. maxillary sinus and mandibular canal respectively the available bone height is less. The use of narrower and shorter implants is indicated in these regions. It is recommended that 2-mm of bone between a critical structure (inferior alveolar canal or maxillary sinus) and the implant should be maintained to avoid any surgical error.
b) Available Bone Width:
The available bone width is measured between the facial and lingual plates at the crest of potential implant site. Usually the crest of the alveolar ridge is triangular in shape with a broad base. Osteoplasty in this situation provides a wide platform for the placement of implant. The problem of availability of bone width in more common with long standing edentulous area, especially the mandibular incisor areas. As a rule minimum of 1 mm of bone is must all around implant t provide adequate blood supply during healing. If this minimum dimension is maintained at the crest, the dimensions increase as we move apically because of increase in bone volume. It must be remembered that ideal implant diameter corresponds to the width of the natural tooth 2 mm below the CEJ, so that implant crown emergence through the soft tissue is similar to a natural tooth 10.
c) Available Bone Length:
It is the mesio-distal length of available bone in the edentulous area limited by adjacent teeth or implants. As a guideline it is recommended that the minimum distance between an implant and tooth should be 1.5-2 mm and minimum distance between adjacent implants should be 3-4 mm. The diagnostic cast articulation is very helpful in accurate measurement of available bone length.
d) Available Bone Angulation:
The angulation of the bone plays an important role during implant placement. The bone angulation refers to the root trajectory in relation to the occlusal plane. It is a significant feature because the occlusal forces on implant are directed from implant body to the surrounding bone. In mandible the angulation of implant placement progressively increases from premolar to second molar region because of presence of submandibular fossa creating a deep lingual undercut. In premolar region it is approximately 10⁰, molar region 15⁰ and in second molar region around 20-25⁰. The angulation at which the maxillary central incisors can be loaded in 12⁰ 11.
Because of bone angulation some areas in the bone do not allow us place implant in desired position. The use of angled abutments in these areas helps us to achieve parallelism as well as desired positioning of the abutment.
e) Crown / Implant body ratio:
This ratio plays an important role in long term success of the implant. Ideally the crown/implant body ratio should be < 1 i.e. the fixture should be longer than the crown portion. In cases where it is difficult to achieve, the treatment plan should be modified to incorporate more number of implants or wider implants to counter the increased stress.
Let us now discuss the Misch and Judy classification in detail.
Classification for partially edentulous arches:
Misch and Judy 12 classiﬁed available bone into 4 divisions: abundant, barely sufficient, compromised, and deﬁcient (A–D). To extend this specific organized approach to classify the bone volume, the basic four divisions have been expanded to following categories,
Division B + w (width)
Division B – w (width)
Division C – w (width)
Division C – h (height)
Division C – a (angulation)
Division A (Abundant Bone):
This type of bone is available soon after tooth extraction where the bone is abundant in all the dimensions. This type of bone is usually available in anterior regions. Less bone height is available in the posterior areas of mandible and maxilla due to limiting structures. In such situations, wider implants (5 to 6 mm) may be considered in the molar regions as suitable alternatives. Due to availability of abundant bone in all dimensions, use of root-form implants with height > 12 mm and width > 4 mm is indicated in this category. Their advantages include:
- Greatest surface area
- Improved stress distribution
- Greatest range of prosthetic options
- Less fracture of implant and components
- Less abutment screw loosening
Division A Dimensions:
- > 10-13 mm height
- > 5 mm width
- > 7 mm length
- < 30 degrees of angulation
- < 15 mm crown height
- C/I ratio < 1
This division of bone allows the ideal implant placement. FP-1 prosthesis (Read in detail about prosthesis in “Treatment planning for implant patient: A general overview”) can be placed in this type of bone most of the time which gives the most esthetic results. FP-2 or FP-3 prosthesis may be considered depending on amount of bone loss and lip positions. If RP-4 or RP-5 prosthesis are planned, osteoplasty is required to make sufficient interarch space to accommodate the metal framework and the acrylic superstructure and overdenture attachments.
Division B (Adequate Bone):
In this type of bone there is slight to moderate atrophy of alveolar bone. It is commonly found in posterior regions of maxilla and mandible. In this division, two sub types (B + and B – w) exist depending on the extent of resorption. The available mesio-distal bone length and angulation criteria also differ as a consequence of the reduced width of bone. Criteria of available bone height and crown height remain the same.
Division B Dimensions:
- > 10-13 mm height
- B + w 4 to 5 mm width
- B – w 2.5 to 4 mm width
- > 12 mm length
- < 20 degrees of angulation
- < 15 mm crown height
- C/I ratio < 1
The major difference between division A and B is the width of available bone. In case of division B the ridge is narrower as compared to division A. In this situation osteoplasty is recommended to reduce the crest of the bone to achieve a wide platform. If by doing so, the bone height attained is greater than 12 mm, the division has been altered to a Division A with width > 5 mm. The osteotomy done increases interarch distance which indicates the use of FP-2 or FP-3 prosthesis to compensate for the increased interarch distance. However, the crown height/implant body ratio remains < 1 after the osteoplasty due to sufficient available bone height. If after osteoplasty the available height of bone becomes < 10 mm and C/I ratio ≥ 1, it is then considered Division C- h type of bone and treatment options will then follow those available in the Division C – h bone.
Another option is to go for narrow Division B root form implants (diameter of 2.7 to 3.5 mm). But, there are some inherent problems with Division B root form implants 13- 15 which include: nearly 25% reduction in surface area, stress concentration almost twice at the crestal region of the implant, lateral loads cause almost thrice the stress to the implant as compared to Division A implants, fatigue fractures of the abutment post, poor esthetics and often requirement of two implants for proper prosthetic support. It is recommended that angle of load must be reduced to less than 20 degrees to compensate for the smaller diameter of implant.
Bone augmentation can be done to convert the Division B bone to Division A bone and treatment options will then follow those available in the Division A bone. The block graft either autogenous and/or demineralized freeze dried bone (DFDB) and/or synthetic bone substitutes can be used with or without barrier membrane to increase the overall bone volume to achieve Division A bone. The augmentation requires a 4 to 6 months healing period before placement of endosteal implants
Division C (Compromised Bone):
Moderate to advanced atrophy is used to describe this clinical condition. The bone may be deficient in one or more dimensions. After extraction of teeth the resorption of the bone first takes place along the width and then along the height of available bone. Keeping this thing in mind, they have given C-w category to describe the reduced bone width. Continued resorption process causes reduced bone height which is described by category C-h.
They have described one more category C-a in which there is adequate bone both in height and width but the angulation is greater than 30⁰ regardless of implant placement. This situation is most commonly found in the anterior mandible and maxilla.
Division C Dimensions:
- < 10 mm height
- < 2.5 mm width
- ≥ 30 degrees of angulation(Ca)
- C/I ratio ≥ 1
According to Misch 16 there are six treatment options for Division C bone which include
- Root form implants
- Augmentation procedures
- Ramus frame implants
- Subperiosteal implants
- Transosteal implants
Out of these treatment options, in present scenario bone augmentation procedures and root form implants are most commonly used in the division C-h of bone. In C-h type of bone the sub-periosteal implants can also be considered. But this treatment is not is a treatment of choice for many clinicians because of multiple problems involved.
In case of C-w bone osetoplasty or bone augmentation procedures are done. The augmentation procedures with block graft provide good results. A healing period of 4-6 months is required before going for implant placement. It is better to place more number of implants to achieve better support for the prosthesis. The situation varies from case to case, so the prosthesis with least invasive and most durable results should be selected for a patient.
Division D (Deficient Bone):
This clinical condition is used to describe the severe atrophy of bone. There is complete resorption of the alveolar bone and continued resorption of the basal bone results in a completely flat maxilla. The resorption of the nasal spine may also be evident. In case of mandible the continued resorption upto superior genial tubercles may be evident. The completely edentulous Division D patient is the most difficult to treat because of challenging clinical procedures involved. Bone augmentation procedures have to be carried out to have sufficient support for implant placement. The surgical skill required is greater and the prosthetic outcome has a guarded prognosis. Fixed restorations are almost always contraindicated due to significant crown height. Because of the complexities involved the complications during surgery as well as implant failures are more likely to happen in this bone division.
Division D (deficient bone) is most difficult to treat. In this bone type, the bone loss is severe which increases the probability of implant failure in absence of bone augmentation to provide adequate bone support. The bone augmentation to upgrade the division of bone is highly recommended in these cases. Autogenous block graft can be done with a healing period of around 6 months before implants are placed. In maxillary posterior region sinus uplift is necessary to provide adequate bone support to implants. Care must be taken during the surgery not to perforate the Schneiderian membrane.
Subperiosteal implants are indicated in advanced ridge resorption cases. They are usually done in the mandible because the division D maxilla does not provide enough support to the metal framework. Another option is RP-5 type of prosthesis. It is a implant as well as tissue supported prosthesis, so it allows the continued bone resorption in the posterior region. Patient should be informed about the limitations as well as the complication before we go for any surgical procedure.
It must be remembered that operative as well as post-operative complications are most commonly involved with treatment in Division D bone. A carefully designed treatment plan and operator’s skill are the cornerstones of success of implant therapy in these patients.
Misch and Judy classification for Kennedy’s class I situation
Misch and Judy classification for Kennedy’s class II situation
Misch and Judy classification for Kennedy’s class III situation
Misch and Judy classification for Kennedy’s class IV situation
Classification for completely edentulous arches:
Misch has classified completely edentulous arches into 3 Types. It should be noted that the classification for partially edentulous arches uses the term “Class” same as the Kennedy’s classification but classification for completely edentulous arches uses term “Type”. The divisions of the bone volume i.e. Division A, B, C, D are same in both the cases. The maxilla and the mandible have been divided into three regions: the anterior region, the right region and the left region. The anterior region is the area between first premolar in an arch. In the mandible this area is between the mental foramen and in maxilla this area is present between the anterior limit of maxillary sinuses of both sides. The edentulous areas posterior to anterior region are identified as right and left posterior regions accordingly.
Misch’s classification for edentulous arches: division of the edentulous arches
In this type of edentulous arch the bone is symmetrical and is similar in all three segments. According to the volume of bone present the division of the bone is written with type 1. For example, Type 1 Division A means symmetrical bone in all three segments with abundant bone present. Type 1 Division C-h means symmetrical bone present in all three segments with inadequate bone having deficiency in height. Type 1 C-w indicates symmetrical bone present in all three segments with inadequate bone with deficient bone width.
Misch’s Type 1 edentulous arch
Here, the bone is similar in the posterior segments but differ from the anterior segment. Usually, the bone in the posterior segments is less as compared to the anterior segment. The classification of the bone is written as Type 2 + division of the bone in anterior segment + division of the bone in the posterior segments. For example Type 2 Division A, B arch indicates bone is similar in the posterior segments but different in anterior segment having division A bone in the anterior segment and division B bone in the posterior segments. Type 2 Division A, C arch indicates bone is similar in the posterior segments but different in anterior segment having division A bone in the anterior segment and division C (inadequate) bone in the posterior segments.
Misch’s Type 2 edentulous arch
In this type, the posterior segments have different bone levels. This condition is not so common but if present should be classified as Type 3. After writing the type, the anterior bone volume division is listed first followed by the division of the bone in the right posterior segment and then the left posterior segment. For example, Type 3 division A, B, C indicates different bone volume in the posterior quadrants with anterior segment having Division A bone volume, right posterior segment with B bone volume and division C bone volume in left posterior segments.
Misch’s Type 3 edentulous arch
5) Based on quality of the bone (bone density) following classifications have been proposed,
A) Linkow classification:
Linkow 17 in 1970 classified bone density into following classes,
Class I bone structure : Ideal bone type consisting of evenly spaced trabeculae and small cancellous space.
Class II bone structure: Bone with slightly large cancellous space with less uniformity of the osseous pattern.
Class III bone structure: Large marrow filled space present between bone trabeculae.
B) Leckholm and Zarb classification:
Leckholm and Zarb 18 have classified the bony architecture based on the relative proportion of cortical to trabecular bone as,
Type I bone: Homogeneous cortical bone.
This type of bone has less blood supply than all of the rest of the types of bone. The blood supply is required for the bone to harden or calcify the bone next to the implant. Therefore, it takes approximately 5 months for this type to integrate with an implant as opposed to 4 months for type II bone.
Type II bone: Thick cortical bone with marrow cavity.
It is considered as an ideal bone quality for implant healing. It has got adequate blood supply and usually takes 4 months to integrate with an implant.
Type III bone: Thin cortical bone with dense trabecular bone of good strength.
Here, the bone quality is not as good as type II bone and 6 months time is suggested before loading an implant placed in this type of bone. Extended gradual loading of the implant can, however, improve the bone density.
Type IV bone: Very thin cortical bone with low density trabecular bone of poor strength.
This type of bone is least favourable for implant placement. It takes the longest length of time to integrate with the implant after placement, which is usually 8 months. Additional implants should be placed to improve implant/bone loading distribution. Incremental loading of the implants over time will improve bone density. Bone grafting or augmentation of bone is often required. Bone expansion and or bone manipulation can improve initial implant fixation.
Leckholm and Zarb classification for quality of bone
C) Micsh’s classification:
Misch 16 in 1988, classified bone into 4 types based on macroscopic cortical and trabecular characteristics. These are as follows,
D1 type bone:
D1 type of bone is primarily composed of almost all cortical bone. A Hounsfield unit reading of 1250 and above indicates D1 bone. This type of bone is primarily present in anterior mandible. In this type of bone, the bone to implant contact (BIC) is greatest and exceptional initial implant stability can be achieved. But, due to its high density there are fewer blood vessels providing significant portion of nutrient and blood supply on the periosteum.
A conservative flap refection id advised in this case to minimize the impact of disruption of blood vessels during and after the surgery. Another problem is the heat generation during osteotomy. To minimize the heat production during drilling a pre-cooled sterile saline solution should be used with smaller increments in drill size with force around 2 Kg and drill speed around 2000 rpm. Drill should be used in slow pumping motion which further reduces heat generation.
D2 type bone:
In this type of bone there is thick cortical bone which surrounds the coarse trabecular bone. It is most suitable for implant placement and post-operative healing. This type of bone is primarily found in anterior and posterior mandible. A Hounsfied reading between 850 to 1250 units is indicative of D2 bone.
Bone to implant contact (BIC) is excellent in this case and good vascular supply due to coarse structure of the trabecular bone allows excellent post operative healing. Fewer drills may be necessary to achieve final osteotomy dimensions. Drill speed, exerted force, pumping motion and pre cooled, sterile saline solution irrigation are the same as D1 bone.
D3 type bone:
In this type of bone central fine trabecular bone is covered by porous crestal layer of cortical bone. Commonly this type of bone is found in anterior and posterior maxilla but also in the posterior mandible. A Hounsfield reading between 350 and 850 units is indicative of type D3 bone.
The bone to implant contact is significantly reduced in D3 type of bone. A modification in the drill pattern is advised to overcome this problem. Reduced number of drills as well as reduced drill speed (1500 rpm) is advocated in this type of bone. Care should be taken while using drills. Because of less density there are chances of perforation of the buccal plate especially in the maxillary anterior region. One advantage of this type of bone is high vascularity due to fine trabecular bone. It aids in healing around implant. Because the bone implant contact is less, titanium plasma spray (TPS) or hydroxylapatite (HA) coated implants are advised.
D4 type bone:
This type of bone is a nightmare for implantologist. Bone is formed of fine trabecular pattern with often absence of cortical bone. This type of bone is commonly found in posterior maxilla. A Hounsfield reading between 150 and 350 units is indicative of D4 bone.
In this case also a modified drill protocol is advised because the bone to implant contact is least in this case. An undersized osteotomy with use of osteotomes to condense rather than remove bone is indicated. Titanium plasma spray (TPS) or hydroxylapatite (HA) coated implants may increase the predictability and success of implant case.
D5 type bone:
This is immature and non-mineralized bone not suitable for implant placement. A Hounsfield reading between < 150 units is indicative of D5 bone.
The availability of bone is the prime requirement for implant placement. The bone quantity as well as quality is important determinants of the success of implant therapy. Many classifications have been proposed to classify the bone on the basis of its quality and quantity to help the clinicians to formulate a treatment plan. From the above discussion it is clear that if the bone is deficient, the treatment plan is modified to change the design of prosthesis or to achieve adequate bone support for the implants by performing the bone augmentation procedures. We should consider treatment options which are most suitable for a particular patient with high prediction of success.
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