Measuring Marrow Density and Area Using Peripheral Quantitative Computed Tomography at the Tibia: Precision in Young and Older Adults and Individuals With Spinal Cord Injury

Relationships between T-scores at the hip and bone mineral density at the distal femur and proximal tibia in persons with spinal cord injury

Objective: To identify T-score values at the total hip (TH) and femoral neck (FN) that correspond to the cutoff value of <0.60 g/cm² for heightened risk of fracture at the distal femur (DF) and proximal tibia (PT).Design: Retrospective analysis of data in a research center's database. Setting: Community-based individuals with spinal cord injury (SCI). Participants: 105 unique individuals with SCI. Outcome Measurements: DXA derived areal BMD (aBMD) and T-score of the DF, PT, TH, and FN. Results: The aBMD at the DF and PT regions were predictors of T-scores at the TH (R² = 0.63, P < 0.001 and R² = 0.65, P < 0.001) and FN (R² = 0.55, P < 0.001 and R² = 0.58, P < 0.001). Using the DF and PT aBMD of 0.60 g/cm² as a value below which fractures were more likely to occur, the predicted T-score was -3.1 and -3.5 at the TH and -2.6 and -2.9 at the FN, respectively. However, when the predicted and observed T-score values disagree outside the 95% limit of agreement, the predicted T-score values are lower than the measured T-score values, overestimating the measured values between -2.0 and -4.0 SD. Conclusion: The DF and PT cutoff value for aBMD of 0.60 g/cm² was a moderate predictor of T-score values at the TH and FN, with considerable inaccuracies outside the clinically acceptable limits of agreement. As such, the direct measurement of knee aBMD in persons with SCI should be performed, whenever possible, prior to prescribing weight bearing upright activities, such as robotic exoskeletal-assisted walking.

Fat and bone: the multiperspective analysis of a close relationship

The study of bone has for many years been focused on the study of its mineralized component, and one of the main objects of study as radiology developed as a medical specialty. The assessment has until recently been almost limited to its role as principal component of the scaffolding of the human body. Bone is a very active tissue, in continuous cross-talk with other organs and systems, with functions that are endocrine and paracrine and that have an important involvement in metabolism, ageing and health in general. Bone is also the continent for the bone marrow, in the form of "yellow marrow" (mainly adipocytes) or "red marrow" (hematopoietic cells and adipocytes). Recently, numerous studies have focused on these adipocytes contained in the bone marrow, often referred to as marrow adipose tissue (MAT). Bone marrow adipocytes do not only work as storage tissue, but are also endocrine and paracrine cells, with the potential to contribute to local bone homeostasis and systemic metabolism. Many metabolic disorders (osteoporosis, obesity, diabetes) have a complex and still not well-established relationship with MAT. The development of imaging methods, in particular the development of cross-sectional imaging has helped us to understand how much more laid beyond our classical way to look at bone. The impact on the mineralized component of bone in some cases (e.g., osteoporosis) is well-established, and has been extensively analyzed and quantified through different radiological methods. The application of advanced magnetic resonance techniques has unlocked the possibility to access the detailed study, characterization and quantification of the bone marrow components in a non-invasive way. In this review, we will address what is the evidence on the physiological role of MAT in normal skeletal health (interaction with the other bone components), during the process of normal aging and in the context of some metabolic disorders, highlighting the role that imaging methods play in helping with quantification and diagnosis.

Bone Marrow and Muscle Fat Infiltration Are Correlated among Postmenopausal Women With Osteoporosis: The AMBERS Cohort Study

Bone and muscle have shown to interact, but little is known about fat within bone and muscle. Clinical studies have isolated fat within bone and muscle using MRI. In this cross-sectional study, we hypothesized that bone marrow adiposity and muscle adiposity are related and that this relationship is associated with osteoporosis. Postmenopausal women aged 60 to 85 years were recruited as part of the Appendicular Muscle and Bone Extension Research Study (AMBERS). Participants completed dual-energy X-ray absorptiometry (DXA) of the hip and spine to diagnose osteoporosis. Muscle adiposity was measured with MRI at the 66% site of the leg. Fat segmentation was achieved using a semi-automated iterative threshold-optimizing algorithm (error < 5%). Peripheral quantitative computed tomography measured marrow density of the 4% distal tibia (surrogate for marrow fat) by threshold-based, edge-detection segmentations and by examining residuals from trabecular bone density regressed on trabecular tissue mineral density. Muscle adiposity from MRI was regressed on marrow density using linear regression. Models were further examined with an interaction with osteoporosis status. Among 312 women (aged 75.4 ± 5.9 years, body mass index [BMI] 29.5 ± 5.7 kg/m² ), a larger amount of muscle fat was associated with lower marrow density at the 66% mid-tibia (B = 84.08 [27.56], p = 0.002) and at the 4% distal tibia (B = 129.17 [55.96], p = 0.022) after accounting for age, height, weight, average daily energy expenditure, hypertension, and diabetes. Interactions of this relationship with osteoporosis status were also significant. Upon probing these interactions, the relationships were significant only in women with osteoporosis but not in those without osteoporosis. Fat from bone marrow and muscle may be related to one another through the same phenomenon, which is likely also responsible for osteoporosis, but independent of hypertension and diabetes. More research should focus on the potential abnormalities in muscle and bone fat metabolism and mesenchymal cell commitment to fat within patients with osteoporosis. © 2019 American Society for Bone and Mineral Research.

Progressive Sublesional Bone Loss Extends into the Second Decade After Spinal Cord Injury

OBJECTIVE

The rate of areal bone mineral density (aBMD) loss at the knee (distal femur (DF) and proximal tibia ) and hip (femoral neck (FN) and total hip (TH)) was determined in persons with traumatic spinal cord injury (SCI) who were stratified into subgroups based on time since injury (TSI).

DESIGN

Cross-sectional retrospective review.

SETTING

Department of Veterans Affairs Medical Center and Private Rehabilitation Hospital.

PARTICIPANTS

Data on 105 individuals with SCI (TSI ≤12 months, n = 19; TSI 1-5 years, n = 35; 6-10 years, n = 19; TSI 11-20 years, n = 16; TSI >20 years, n = 15) and 17 able-bodied reference (AB_ref) controls.

INTERVENTIONS

NA Main Outcome Measures: The knee and hip aBMD values were obtained by dual energy X-ray absorptiometry (GE Lunar iDXA) using standard clinical software for the proximal femur employed in conjunction with proprietary research orthopedic knee software applications. Young-normal (T-score) and age-matched (Z-scores) standardized scores for the FN and TH were obtained using the combined GE Lunar/National Health and Nutrition Examination Survey (NHANES III) combined reference database.

RESULTS

When groups were stratified and compared as epochs of TSI, significantly lower mean aBMD and reference scores were observed as TSI increased, despite similar mean ages of participants among the majority of TSI epoch subgroups. Loss in aBMD occurred at the distal femur (DF), proximal tibia (PT), FN, and TH with 46%, 49%, 32%, and 43% of the variance in loss, respectively, described by the exponential decay curves with a time to steady state (t_ss) occurring at 14.6, 11.3, 14, and 6.2 years, respectively, after SCI.

CONCLUSIONS

Sublesional bone loss after SCI was marked and occurred as an inverse function of TSI. For aBMD at the hip and knee, t_ss extended into the second decade after SCI.