Recently, one of my friends showed me this picture from his implant planning software and asked me if maybe he should abort this implant surgery:
In several different implant planning software, there is this little module that allows you see the “Hounsfield Units”, or H.U, of the bone around your planned implant site.
What are H.U?
In its true form, H.U. is a measure of radiodensity of an object. However, in cone beam CT the H.U. units that you see are not the true H.U.
Instead, an H.U. value is estimated from greyscale values.
Although there are numerous factors that can introduce inaccuracies, the predicted H.U. values can actually be very close to the true H.U. values. (Mah et al. 2010)
Digging up some Info, on LinkedIn
While the RED colors all over the implant can look quite alarming, I have never really paid much attention to this factor.
Instead, I have mainly used the CBCT to view the sagittal, coronal, and axial, slices to study the bone volume.
But I wanted to learn how H.U. could help improve my surgeries, so I did what any responsible, evidence-based, academic would do… I asked on social media 😉
On Linkedin, I found that some implant surgeons find HU on CBCT useful and some ignore it. These are some of the comments:
- “I pay very little attention to this – if at all.”
- “I will admit that this parameter is something I have not closely monitored.”
- “The more I pay attention to this, the more prepared I am surgically.”
- “I use thin 3D density depiction as a warning, but rely more on visualization of nearby anatomical structures.”
- “I personally pay good attention to this parameter but I try to keep in mind the standard deviation of HU due to CBCT inaccuracies…”
What’s the Literature Say?
I found a great article by Wada et al (2016) [download the article]. They look at whether it is possible to use the H.U. value to predict insertion torque during implant surgery,
Here’s what they found:
“A significant positive correlation between insertion torque values and mean CT values around the implants…”
“The results revealed that bone density around the implant is a useful index.”
This finding is common sense (more dense bone –> higher insertion torque).
But, it is very important because now we have some concrete evidence that we can use H.U. in our data collection when planning our implant surgeries.
They even go on to provide a numerical formula to use H.U. to help you predict your insertion torque!
I don’t know that I would personally depend on this formula, but I am convinced now that H.U. can be an additional useful piece of information.
- HU from CBCT is not the true HU value, but it’s useful anyway
- A high HU correlates with a high insertion torque, and vice versa
- I am not likely to abort a surgery because of low H.U. values, but a low value may indicate to me that I should undersize the implant site preparation to help gain a more acceptable insertion torque value and avoid the dreaded “spinning implant.”
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Mah, P et al. Hounsfield units from grey levels in CBCT. Dentomaxillofacial radiology (2010) 39, 323-335.
M. Wada, T. Suganami, M. Sogo Y. Maeda: Can we predict the insertion torque using the bone density around the implant?. Int. J. Oral Maxillofac. Surg. 2016; 45:221–225.