# Subperiosteal Anchorage in Orthodontics: A Narrative Review
## 4 Discussion
### Biomechanical Properties  
Onplant devices are placed on relatively inactive bone surfaces, whereas dental implants are placed in freshly prepared cavities in the alveolar bone: this difference leads to a different possibility for osseointegration. The first in vivo study was conducted by [PERSON] and [PERSON] in dogs and monkeys [9]. It was shown that loaded and unloaded onplants revealed no significant histological differences; in all instances, bone was found directly fixed to the hydroxyapatite coating. In some regions, dense bone tissue was also found connecting the palatal bone to the device, and trabecular-type bone was found to underlay the titanium surface. Histological examination also did not reveal any presence of inflammatory cells such as macrophages or osteoclasts after osseointegration. In the same study, ex vivo mechanical tests were conducted. The shear force needed to dislodge onplants from the underlying mandible was around 72.5 kg. In addition, after the application of a continuous force of 0.3 kg, none of the onplants moved or failed [9]. The rate of success of the onplant as an orthodontic anchor unit depends on different factors, but the amount of force it can withstand before failure is one of the most important. Research on healing after osseointegration can provide clinicians with an optimum time for loading onplants. The research group of [PERSON] et al. reported on the experimental use of an alphatricalcium phosphate coating on perforated subperiosteal implants to reduce the time for starting the loading when using the device as an absolute anchorage; the results of the study conducted on rabbits showed that the new coating provides greater tensile strength compared to a hydroxyapatite coating after a period of 3 weeks. This method could ensure reduced osseointegration time and faster orthodontic therapies [19]. Many improvements of surface coatings have been proposed since onplants were first commercialized. In order to reduce the healing time, a hydroxyapatite collagen bone-like nanocomposite coating was developed by [PERSON] et al. [18] and applied on the bone surface by [PERSON] et al. [19]; this surface reached rapid osseointegration onto rat calvaria in 4 weeks, with a bone-bonding strength of 16.4 N for a 6 mm circular Onplant [11; 21]. Recently, [PERSON] et al. have tried to develop a novel bone-like nanocomposite coating of hydroxyapatite/collagen by modified electrophoretic deposition to surround bone formation [25]; the authors concluded that this kind of titanium coating showed excellent biological properties that may accelerate the osseointegration of subperiosteal \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Authors (Year)** [Reference] & **Type of Study** & **No. of Patients** & **Mean Patient Age (Years)** & **No. of Onplants** & **Percentage Failure** & **Cause of Failure (No. of Failed Onplants)** \\ \hline [PERSON] et al. (2006) [6] & Pilot study & 8 & 30.1 \(\pm\) 15.6 & 8 & 12.5\% & No osseointegration (1) \\ \hline [PERSON] and [PERSON] (2008) [18] & Randomized clinical trial & 29 & 14.0 \(\pm\) 1.53 & 29 & 17.2\% & No osseointegration (1) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Failure rates of Onplant studies in humans. implants threefold over previous techniques. Semicircular onplants were also tested and they have proved to have greater strength than circular onplants [22]. [PERSON] et al. performed a finite element analysis to determine the shape optimization of the subperiosteal devices; the study group concluded that a rectangular cross-section is the most mechanically favorable to guarantee higher resistance to bonding strength in simulated rat calvarium bone [24]. The use of bone morphogenetic protein-2 and dentin matrix protein-1 were tested like coating surface by [PERSON] at al. Bone morphogenetic protein-2 has an osteoinductive effect and it promotes the differentiation of mesenchymal stem cell to osteoblast and that proteins are involved in bone remodeling.