Scientists can now look at the microarchitecture of bone through the use of High Resolution peripheral Quantitative Computed Tomography (HR-pQCT). This allows researchers to study the quality of bone structure at the micro level and, hopefully, understand what activities promote improved bone quality and are effective strategies for osteoporosis prevention and treatment.
Two recent studies provide more insight into trabecular architecture as well as the mechanical behavior and fragility of bone. In the following sections, I provide a summary of each of these two studies.
1. Trabecular Microarchitecture and Mechanical Behavior of L3
At the American Society of Bone Mineral Research (ASBMR) meeting in Toronto in October of 2010, research outlining the contribution of trabecular microarchitecture to the mechanical behavior of L3 was presented. (1)
Before I get into the details of the study and its results, I need to provide some definitions for the reader.
Trabecula Identification Based on Shape
Trabecular Bone
Trabecula fall into two broad categories based on the shape of the trabecula. These are defined as:
- Plates: Plates are the trabecula that are thicker and more “plate-like” in shape. In the image to the right, the plates are the wide, flat light colored parts of the bone. Since they are wider and thicker than the rod-like connections, they provide more support and contribute to the overall quality of the bone structure.
- Rods: Rods are the trabecula that are “rod-like” in shape. With loss of bone mineral density, the trabecula that are plate-like can become rods as they become thinner. This change can reduce the quality of the trabelula microarchitecture. In the image to the right, the rods are the light colored, thinner rod-like connections.
Orientation
Both rods and plates occur in different orientations. The following are the three orientations:
- Transverse – Horizontal alignment
- Oblique – Oblique alignment
- Longitudinal – Vertical alignment
Segmentation
Morphological analysis of individual trabeculae segmentation can be broken into the following categories:
- Plate/rod volume fraction
- Axial bone volume fraction
- Plate/rod number density
- Plate/rod thickness
- Plate surface area/rod length
- Plate-plate/plate-rod junction density
Analysis
An analysis of L3 was undertaken from 21 fresh donors. The average age was 76 years and the group was composed of 10 men and 11 women. Bone mineral content (BMC, g) and bone mineral density, (BMD, g/cm2) were assessed by lateral DXA.
Two vertically oriented virtual biopsies were obtained – one from the anterior vertebral body and one from the posterior vertebral body. Mechanical behavior was assessed using static compression testing involving the following three metrics:
- Failure load (N)
- Stiffness (N/mm)
- Work to Failure (N.mm)
Results & Conclusion
Trabecular architecture was found to have greater deterioration in the anterior versus posterior region. Researchers found that BMD explained up to 44% of the variability in vertebral mechanical behavior. The microarchitecture explained up to 66% of the variability in vertebral mechanical behavior. The combination of bone mass (BMD) with microarchitecture explained up to 86% of the variability in vertebral failure load.
These findings explain why flexion exercises, ADLs and other activities that increase the load on the anterior vertebral body can increase the risk and rate of vertebral fractures.
2. Microstructural Changes of Trabecular Bone
In another study (2) presented at the same American Society of Bone Mineral Research (ASBMR) meeting in Toronto, researchers explored the microstructural and mechanical changes of trabecular bone in postmenopausal women who had fragility fractures.
Researchers compared 101 control subjects who had not fractured to 68 women who had sustained fractures from a fall or slip from a standing height or lower position. The research team discovered that the DXA scores of both groups were similar. However, the quality of the bone in the fracture group was much lower than that of the group that did not fracture.
The trabecular microstructure and the whole bone stiffness of the women who fractured was lower that that of the control group. The following characteristics of trabecular bone were found among the group that fractured:
- Reduced plate-like bone volume.
- Less plate-like structure.
- Less axially aligned trabecula.
- Longer trabecular rods indicative of more separated trabecula.
- Decreased connectivity between plates and plates and between plates and rods.
Although the BMD of the group who fractured was similar to that of the non-fracture group, the bone quality was the determining factor as to the strength of the bones.
References
- Wegrzyn J et al. Contribution of Trabecular Microarchitecture and its Heterogeneity to the Mechanical Behavior of Human L3 Vertebra. Univ. Lyon, France and Harvard, Boston. Poster presentation at the 2010 ASBMR Toronto.
- Liu XS et al. Fewer Trabecular Plates and Decreased Connectivity Between Plates and Rods is associated with Reduced Bone Stiffness in Postmenopausal Women with Fragility Fractures. Columbia Univ. NY. Poster presentation at the 2010 ASBMR Toronto.
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{ 2 comments… read them below or add one }
Dear Margret, thank you so much for that extra work of summarizing the studies from Toronto. I am so glad that we finally get to hear more about “bone quality” which is different compared to “bone density”. Yet in the mainstream medical world it is all about “bone density”. Even though both groups had similar DXA’s, their fracture rate differed. This is so interesting! This shows that DXA tests only measure the bone mass but leaves out the very important feature of bone quality: bone size, shape and the trabecular structure. DXA testing does not show any information about bone micro-architecture. Therefore it cannot tell us about bone quality or strength. For us as osteoporosis specialists it is good to know that our clients may have low bone density but may have good bone quality and normal bone strength. We can hope that new tests will be created that can actually test “bone strength” and the overall bone quality. In the meantime, it is so important to teach our clients the “How’s” of bone strengthening and density loss; no matter what their DXA test shows. Thank you for a great program, Margret!
Hi Marion – Great summary. As a Physical Therapist who deals with clients with low bone density and osteoporosis, you are a Health Professional who is familiar with the importance of bone quality and bone strength. And you know how to get your clients on the right track. Keep up the good work in California!
- Margaret