Trabecular Bone Score (TBS) + DEXA
Trabecular Bone Score Sheds Light on DEXA Scan
Trabecular bone consists of a network of interconnected plates and rods.
A Trabecular Bone Score (TBS) enhances the interpretation of DEXA by shedding light on bone quality and resilience to fracture. A low Bone Mineral Density (BMD) as determined by DEXA increases fracture risk, but poor bone quality can also make bones fragile even when bone density appears normal.
Bone strength is more than just bone density. Bone quality describes aspects of bone composition and structure that contribute to bone strength independently of bone density.
But before we get into bone density vs. bone quality, let’s look at what trabecular bone is.
What is Trabecular Bone?
Trabecular bone, also known as cancellous or spongy bone, is a type of bone tissue characterized by its honeycomb-like structure. It is porous and lightweight, less dense than cortical (compact) bone.
Trabecular bone consists of a network of interconnected plates and rods, called trabeculae. These trabeculae form a three-dimensional framework that supports the bone. It is typically found in the following areas:
Ends of the long bones (e.g. femur, humerus)
Vertebral bodies
Pelvis
Ribs
Functions of Trabecular Bone
Understanding the structure and function of trabecular bone is important for diagnosing osteoporosis and other bone diseases. It plays a crucial role in:
Weight-bearing: It distributes forces evenly thought the bone, reducing stress and minimizing weight.
Shock absorption: Its porous structure helps absorb impact and vibrations.
Bone remodeling: The trabeculae are constantly being remodeled in response to changes in load and activity.
How is Trabecular Bone Measured?
Trabecular Bone Score, or TBS, is a software-based tool that analyzes the pixel variations in a standard DEXA lumbar spine scan. It does not require additional imaging; instead, it extracts structural information from the DEXA image.
The score reflects trabecular connectivity and microarchitecture, meaning how well the internal bone structure is organized. The numerical scores are typically categorized into three ranges of bone quality and fracture risk:
Normal (Good microarchitecture) Low risk of fracture
Partially degraded microarchitecture Intermediate risk
Degraded microarchitecture High risk
Higher TBS values suggest well-connected trabecular bone, which provides strength and flexibility. Lower TBS values indicate degraded bone microarchitecture, increasing the risk of fractures, even if BMD appears normal. Unlike BMD, which may take years to change significantly, TBS can show earlier changes in bone quality.
TBS does not replace DEXA but enhances its interpretation. It is particularly helpful in borderline cases, such as:
A person with osteopenia (low BMD but not osteoporosis) who has a low TBS may be treated more aggressively.
A person with osteoporosis but a high TBS may have a lower fracture risk than expected.
Limitations of TBS
Cannot be used on its own, and must be used alongside DEXA
Limited to lumbar spine (does not assess hip or other skeletal sites)
May be artificially elevated by degenerative changes, such as osteoarthritis
To sum it up, combining TBS with DEXA provides a more complete picture of bone health, leading to better treatment options.
How Does Bone Density Compare with Bone Quality?
What is Bone Density (BMD)?
BMD is the amount of mineral (primarily calcium and phosphorus) per unit of bone volume, i.e. the quantity of bone mass. It is the key measurement used to diagnose osteoporosis and osteopenia, typically assessed by a DEXA (Dual-Energy X-ray Absorptiometry) scan.
While it is considered the gold standard of measurement, it alone does not provide information about bone strength or resistance to fracture since it does not account for bone’s internal structure or composition.
What is Bone Quality? (structural and material properties of bone, separate from bone density)
Aspects of bone quality include microarchitecture, collagen properties, bone turnover, and mineral crystal size. Unlike BMD, bone quality is harder to measure directly. New techniques to assess these components of bone quality are being developed.
Microarchitecture: The arrangement and connectivity of the internal struts of trabecular (spongy) bone. This type of bone is vulnerable to losing its support structure, becoming porous and weak.
Collagen Properties: The structural proteins that provide bone flexibility.
Bone Turnover: The balance between bone formation and resorption.
Mineral Crystal Size and distribution within the bone.
Accumulated Microscopic Damage
How is Bone Quality Measured?
Trabecular Bone Score (TBS) analyzes the microarchitecture of the spine using DEXA images.
REMS (Radiofrequency Echographic Multi-Spectrometry) uses ultrasound to assess bone elasticity and structure.
For more about REMS, please see Bone Density Tests: DEXA vs. REMS Explained
Shedding Light on Resilience to Fracture
While DEXA gives us the quantity of bone mass, it does not shed light on other characteristics of the bone, such as bone strength, flexibility, and integrity. This explains why some people with normal BMD or osteopenia suffer fractures due to poor bone quality.
Aging, certain diseases (e.g. diabetes, chronic kidney disease, hyperparathyroidism) and medications (glucocorticoids) can reduce bone quality even if BMD remains stable.
A comprehensive bone health assessment should ideally consider both BMD and bone quality, using tools like TBS and REMS in addition to standard DEXA scores.
Disclaimer: The information presented in this article should not be construed as medical advice. It is not intended to replace consultation with your physician or healthcare provider.
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