Brothers Mario and Luigi are at the restaurant and both order a glass of water. The waiter serves Mario a full glass, whereas poor Luigi only gets it half full. “hey, waiter!” Luigi starts complaining…until the garçon makes him innocently notice that his glass is double as big as Mario’s one, and that he poured the exact same quantity of water into the two glasses. Happy and reassured, Mario and Luigi continue talking about their fantastic adventures…
What really matters to Luigi is how much water he got (the Water Volume, WV), as the volume of his glass (the Total Volume, TV) won’t play a key role in satisfying his thirst. In other words, he is not interested in the fraction of his glass that is filled with water (the Water Volume Density, WV/TV). But is it the same when we talk about bone?
After lunch, Mario and Luigi have an appointment at the hospital for a spine check. Surprisingly, the radiologist finds out that Luigi has an anomalous L5 vertebral body that is double as big as Mario’s one. Both vertebral bodies contain the same absolute trabecular Bone Volume, but Luigi’s is half as dense. Same conclusion than at the restaurant? Unfortunately not, as in this case Luigi has a reason for being worried: his vertebral body is weaker than it should be, i.e. it is at risk of fracture.
The absolute BV value does not usually tell much, as the obvious question would be “yes, but what is your reference volume?”. It is like somebody asking you: “Is ten people a lot?”. Well, hard to say! It is a lot if you are in an elevator, whereas you would feel pretty lonely if you are at the stadium. That’s why it is important to choose an appropriate TV, i.e. Volume Of Interest. Answering the following questions may help you in this process:
1. Is the region I am analysing the one where I expect to see the effects of my treatment?
2. How large is the volume where I expect an effect?
3. What shape should the Volume Of Interest have?
4. Should the analysis include only trabecular bone, or cortical bone, or both?
5. If the treatment includes a biomaterial or an implant, should this be included or excluded in the TV? What is the rationale for your choice?
Before you start complaining to a waiter or more likely to your R&D manager that you see too little bone, or before being excited because “there is a lot of bone”, you should better make sure you have a good rationale for the choice of the VOI!
You may also be interested in Bone Volume Density (BV/TV) – Light version and other Tutorial posts.