**7. Bone healing**

Osseointegration follows a common, biologically determined program that is subdivided into 3 stages:


Osseointegration is also a measure of implant stability, which can occur at 2 different stages: primary and secondary. Primary stability of an implant mainly comes from mechanical engagement with compact bone. Secondary stability, on other hand, offer biological stability through bone regeneration and remodeling. The former is a requirement for secondary stability. The latter, however, dictates the time of functional loading [6].

In order to understand RFA and what it is measuring, one must understand implant stability and the factors that influence it. At the time of implant placement, 100% of the stability comes from the relationship between the geometry of the implant and the peri-implant bone. The implants macro geometry; such as thread patter and size, shape of the body of the implant (tapered or parallel walled) and the roughness of the surface coating, or micro-geometry all effect the mechanical locking ability in bone. On the other side of this equation there is the bone itself. The initial stability, or mechanical retention is directly affected by the quality of the bone that the implant is inserted into, with bone types ranging from D1 to D4.

D1 bone has very little medullary bone and therefore mainly consists of cortical bone which is very dense and highly mineralized. As we move down the progression scale towards poorer bone quality, we see an increase in medullary or cancellous bone, with its large non-mineralized spaces between the trabeculae. On the opposite side of the spectrum there is D4 bone which has a very thin mineralized cortical shell that surrounds spongy bone with very little mineralized content and a higher percentage of collagen.

As expected, mechanical stability is usually very high in D1 bone and for the most part very poor in D4 bone with ISQ values supporting this [7].

Biologic stability is attributed to the surface interaction and maturation of bone surrounding the dental implant. The bone, as it is moving through its three stages of change has a direct effect on the stability of the implant. Even after high initial mechanical stability, there is usually a drop off in overall stability during the 3–6 week time period post-placement. This is due to the natural remodeling that occurs as the healing process progresses. There is resorption of bone by osteoclastic activity followed by the deposition of osteoblasts on the surface of the implant. At this point the un-organized

**Figure 4.** *Comparison of mechanical vs. biologic stability.*

woven bone starts to become organized and the formation of lamellar bone occurs. It is at this point that we see an increase in implant stability afforded by the progression of osseointegration. The next step in the process is maturation and then it is followed by the adaptation of the peri-implant bone to the forces of mastication and load. This cross-over can be seen in the diagram above (**Figure 4**).
