BV/TV, Tb.Th., Tb.Sp. and the other usual suspects: what do they really mean?
If you are asking yourself this question, you should definitely read this! Coming soon in the same series: Trabecular Thickness (Tb.Th) and Trabecular Spacing (Tb.Sp), Bone surface density (BS/TV), Structure Model Index (SMI), Connectivity Density (Conn. D), and others. The “Light version” series is intended to those who are just approaching the field, or to those who are looking for a simple way to explain these concepts to others. If you are a professional, please insert your comments to help us making this post more complete and interesting.
So let’s start from BV/TV!
Bone volume density
BV/TV stands for Bone Volume over Total Volume.
It indicates the fraction of a given volume of interest (VOI, i.e. the Total Volume TV) that is occupied by mineralized bone (Bone Volume). BV/TV is usually reported as a % value. This index is probably the most known and the most used output of a microCT analysis performed on bone.
To make it simple, if the glass is your TV, the glass with no water has 0% water volume density, the middle glass has 50% water volume density, and the full glass has 100% water volume density (ok, maybe 98%….). I guess you never wish a 100% bone volume density though… It is clear that in the calculation of BV/TV, the appropriate selection of the TV plays a key role (read the post Choose the right Volume of Interest!).
Why is BV/TV useful?
It can be used to evaluate relative changes in bone volume density following a given treatment.
Let’s take some examples:
EXAMPLE 1: Evaluation of the effects of an anti-osteoporosis drug
Your preclinical study will most likely comprise a negative control group (e.g. OVX) and a treatment group to whom the drug is administered. One way to determine the efficacy of the drug is to compare BV/TV in defined VOIs in the control vs treatment group. You would expect that if your drug works, the treatment group shows higher BV/TV values than the control group. In the figure below a similar case is presented where one group was ovariectomized and the other one underwent sham control (the image is property of b-cube)
EXAMPLE 2: Integration of implant in bone
You have a great idea for a new coating that should promote the implant integration in bone. To prove the concept, you run a preclinical study where a group is treated with the non-coated and another group with the coated implant. A possible way of evaluating the efficacy of the coating is to analyze BV/TV in a VOI surrounding the implant. Hopefully, you will see that the non-coated implant group has lower BV/TV than the coated implant one. There is much more you could look at, like for example implant/bone contact surface, but we will talk about that in another post. In the figure here below we are showing the case of a dental implant (blue) and the bone/implant contact surface (brown) (the image is property of b-cube).
What are typical values for BV/TV?
BV/TV depends on the species (human, mouse, sheep, etc.), on the type of bone, on the location within a bone, and on the health status of the subject. You can get a good idea of typical values for human bone from this publication.
Qualitative vs quantitative analysis:
We all like a nice graph showing statistical difference between two bars. Mission accomplished, next task.
But…it is almost never that easy. BV/TV and microCT indices alltogether are only a piece of the whole picture. And microCT “numbers” are only a piece of a piece of a picture.
Sometimes the value of images is underestimated. Looking at representative 2D slices of the microCT scan can actually reveal lots of interesting information that in some cases can be even more useful than the straight BV/TV number. The images may give you qualitative input on the bone healing mechanism of action, on how a spinal fusion looks like, on where bone is localized within the VOI, on how microCT images compare with histology sections. Sure, it is more work, but it is worth if you really want to understand “what’s going on”.
Not always straight-forward:
For microCT professionals this may sound obvious, however it is important to point out that the “magic” BV/TV number spit out by the scanner can vary significantly with the methods used to perform the analysis. So, for different measurements on the same sample, BV/TV will vary if you select one or another Volume of Interest, if another material present in the VOI creates x-ray absorption artifacts, if different scanning settings have been used, etc. The best way to reduce the risk of result misinterpretation, is to use consistent methods, and possibly an automated analysis software that eliminates the bias of the operator. When you compare results from different studies, make sure you check how the results were obtained and if a comparison makes sense.
We hope you have found this post useful. By posting a question, suggestion, or criticism you’ll help us and the other readers. Thanks!