# Failure Length and Primary Stability: An In Vitro Study on Polyurethane Foam
## 4 Discussion  
In order to evaluate the impact of the length of dental implants on the primary stability, the measurements of short vs. long implants on RFA, ITV, and RTV were measured on polyurethane blocks of 20 and 30 PCF that mimic the bone density of the Misch's classes D3 and D2, respectively, of 0.32 and 0.48 g/cm\({}^{3}\)[9; 12; 14; 25; 31; 32]. Moreover, the compressive strength range (MPa) of the 20 and 30 PCF polyurethane block are very similar to the cortical bone strength that ranged between 130 and 200 MPa, while lower values are reported for the trabecular bone that ranges between 0.1 and 16 MPa. The use of polyurethane foam as a bone substitute to perform orthopedic tests has been validated since 2001 by the ASTM F1839-01 Standard Specification. Since then, many studies have been published on this material, especially in orthopedics. As shown by [PERSON] et al., the use of polyurethane as a bone substitute for in vitro studies permits the use of a material characterized by similar mechanical properties to trabecular bone, such as the stress-strain curve, but, at the same time, the availability of a material that is always the same from lot to lot. In this way, it is possible to repeat the tests without the risk of bias connected with the use of a substrate that changes properties from one sample to another, such as natural bone [33; 34; 35; 36]. This type of fixture has a novel multiscale roughness and double-etched surface that is characterized by a remarkable decrease in _Streptococcus oralis_ biofilm formation in respect to machined and single-etched titanium [20]. The measurements of the resonance frequency analysis by using the Ostell device permitted us to obtain an objective evaluation of the primary stability by monitoring the changes in the stiffness and stability at the implant-tissue interface [1]. Considering that we found no significant differences between the RFA measurements in the meso-distal (MD) and vestibulo-lingual (VL) direction, we here discuss these results as a single value. The RFA is influenced by many parameters, firstly by the specific implant system adopted, and, currently, no specific thresholds of ISQ ranges permit to distinguish between failures or successful implants [37]. The literature shows that an ISQ < 45 indicates a poor primary stability, and values >65 are considered to be the most favorable for implant stability [1; 38]. In particular, in accordance with the manufacturer and the previous literature, the Ostell measurements can be used to classify the implant stability as good with ISQ > 70, medium with ISQ 60-69, and low with ISQ < 60 [24; 39; 40]. The results of this work showed significantly higher RFA values for long implants, with respect to the short ones, in all conditions. In particular, on D2 bone density, the RFA values were 74 ISQ for long implants, which could be also immediately loaded as single implants; on the contrary, short implants showed average values of 64 ISQ and would be considered for immediate loading only for total splinted restorations. On the contrary, for low bone densities (20 PCF), although there were significant differences between the RFA values of the long and short implants, these results should be considered with caution. Indeed, the average RFA value of short implants was 56.8 ISQ, which is very near the threshold of 60 ISQ that distinguishes between low- and medium-stability implants. So, independently, by the length on D3 bone, the implant stability of all fixtures could be considered as medium, and, consequently, they could be immediately loaded only in the case of total splinted restorations; on the contrary, a second-stage protocol should be considered. A recent review of the literature showed no significant differences in the survival rate at 1 and 3 years between low- and high-ITV implants [41].