Available Bone

Available bone has to be assessed in terms of the ridge morphology and bone density.

These are important factors to be considered when implants are to be used as treatment options.

Ridge morphology
The implant-supported prosthesis is affected by the ridge morphology especially implant-supported overdentures which gain dual support from implants and edentulous ridges. Alveolar ridge shape may be classified related to retention.

•  The ideal ridge form (square) has high, broad parallel walls, which also improve lateral stability.
•  A flat, atrophic ridge provides poor retention and stability but adequate support for the maxillary denture.
•  The least stable is the V-shaped ridge at an angle to the opposing force. It is the poorest for retention or support

Bone Density
The compact bone surrounding dense evenly spaced trabeculae with small cancellous spaces is ideal/suitable for implant placement. Dense or porous cortical bone is found on the outer surfaces of bone and include the crest of an edentulous ridge. Coarse and fine trabecular bone are found within the outer shell of cortical bone and occasionally on the crest.

Misch classified bone density into following types (1988):

D1. Dense cortical bone- which is almost never observed in maxilla and
approximately 8% of time in mandible , where twice as often in anterior mandible than the posterior mandible.

D2. Dense to thick porous cortical bone on the crest and coarse trabecular bone within. It is the most common density observed in mandible and is less often in maxilla, where it is more likely to occur in partially edentulous patient’s anterior
and premolar region.

D3. Very common in maxilla followed by posterior mandible and then anterior mandible

D4. The softest bone, is most often found in the posterior maxillae, especially in molar region or after a sinus graft augmentation. Anterior maxilla has D4 bone less than 10% of time and mandible less than 3%.

The type of implant macrogeometry, design and whether it is to be coated or not will be influenced by the corresponding bone density at the site of implant placement.Another factor to be kept in mind is the anatomic concavities present in the lingual surface of the mandible, the sublingual fossa anteriorly and the submandibular fossa posteriorly. Normally assessment of the bone quantity is done by panaromic x-rays and since, it is a two dimensional representation, the concavities cannot be assessed.It is possible to see them only in the oblique sections in a dental CT- Scan. Especially in the mandibular second molar region extreme caution should be taken while planning an implant as the prominent internal oblique ridge gives a false sense of abundant bone, however, just inferior to the ridge lies the submandibular fossa. If a wide implant is placed directed in a straight axis it can perforate the lingual cortex.Therefore, it is prudent to evaluate the fossa by deep palpation, assess on oblique views on CT-scan and then plan the axis of the implant following that of the tooth to be replaced and not straight as will be discussed in the following units on treatment planning.

Following the discussion of some of the important anatomic landmarks in implant surgery , lets now understand the basic bone physiology and why it is important for an implant dentist to possess its knowledge.