John D. Landoll

A comparison of fatigue failure responses of old versus middle-aged lumbar motion segments in simulated flexed lifting

Study Design. Survival analysis techniques were used to compare the fatigue failure responses of elderly motion segments to a middle-aged sample. Objectives. To compare fatigue life of a middle-aged sample of lumbosacral motion segments to a previously tested elderly cohort. An additional objective was to evaluate the influence of bone mineral content on cycles to failure. Summary of Background Data. A previous investigation evaluated fatigue failure responses of 36 elderly lumbosacral motion segments (average age, 81 ± 8 years) subjected to spinal loads estimated when lifting a 9-kg load in 3 torso flexion angles (0°, 22.5°, and 45°). Results demonstrated rapid fatigue failure with increased torso flexion; however, a key limitation of this study was the old age of the specimens. Methods. Each lumbosacral spine was dissected into 3 motion segments (L1–L2, L3–L4, and L5–S1). Motion segments within each spine were randomly assigned to a spinal loading condition corresponding to lifting 9 kg in 3 torso flexion angles (0°, 22.5°, or 45°). Motion segments were statically loaded and allowed to creep for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading). Results. Compared with the older sample of spines, the middle-aged sample exhibited increased fatigue life (cycles to failure) in all the torso flexion conditions. Increased fatigue life of the middle-aged specimens was associated with the increased bone mineral content (BMC) in younger motion segments (mean ± SD, 30.7 ± 11.1 g per motion segment vs. 27.8 ± 9.4 g). Increasing bone mineral content had a protective influence with each additional gram increasing survival times by approximately 12%. Conclusion. Younger motion segments survive considerably longer when exposed to similar spine loading conditions that simulate repetitive lifting in neutral and flexed torso postures, primarily associated with the increased bone mineral content possessed by younger motion segments. Cycles to failure of young specimens at 22.5° flexion were similar to that of older specimens at 0° flexion, and survivorship of young specimens at 45° flexion was similar to the older cohort at 22.5°.

Comparison of Absorptiometric Evaluations from Total-Body and Local-Region Skeletal Scans

The most common measurement sites for dual-energy absorptiometry (DXA) in clinical practice are posteroanterior (PA) spine and femur. However, other skeletal regions may provide different bone density information. The purpose of this study was to establish the least number of DXA measurements needed to obtain complete information about bone. A total of 262 normal female subjects, 8-50, were measured on a Lunar DPX-L scanner under total body, PA spine, lateral spine, and femur protocol. Forearm measurements were performed with a Lunar SP2 single-photon absorptiometry scanner. The various measurements were compared based on a linear regression model. The correlation coefficients for bone mineral density (BMD) between adjacent vertebrae were 0.92-0.95, and the associated standard errors of the estimate (SEE) were 4.5-5.5%. Total-body BMD can best predict BMD of the trunk, arms, and legs (SEE<4.3%), but least that of the lateral view of the spine (SEE>13.9%). BMD values of the leg from total-body scans predict those from the femoral neck with an error of 9.0%, and those of the trochanteric region with 11.1%. The error between adjacent vertebrae (6%) is considered acceptable, then a total-body measurement combined with a lateral view of the spine and a femur scan are adequate.

Selenium Intakes, Absorption, Retention, and Status in Adolescent Girls

OBJECTIVES To assess selenium intakes, absorption, retention, and status in healthy adolescent girls and the effect of calcium supplementation on selenium parameters. DESIGN Annual 2-week study conducted each year for 3 consecutive years in which yearly selenium intakes, absorption, and retention and blood selenium status were measured. SETTING A metabolic unit in a large metropolitan hospital located in Columbus, Ohio--a low selenium region of the United States. SUBJECTS Healthy white girls aged 11 to 14 years (n=16) enrolled in a calcium balance study and randomly assigned to receive a placebo of methylcellulose (n=9) or a calcium supplement containing 1,000 mg supplemental calcium as calcium citrate malate (n=7). INTERVENTIONS Each subject consumed a diet with approximately 100 microg selenium/day during the yearly 2-week balance studies. RESULTS Selenium status measurements (serum and erythrocyte selenium and glutathione peroxidase activity) were all within normal ranges for adults during the study. Apparent selenium absorption averaged 71%, 76%, and 74% for years 1, 2, and 3 of the study, respectively, and did not vary significantly (P>.05). Average daily selenium retention did not differ among the years of the study (P>.05) and indicated that the usual selenium intake was approximately 100 microg daily. Measurements of selenium status and retention did not differ between calcium-supplemented and placebo groups. CONCLUSIONS An intake of approximately 100 microg selenium/day is the typical intake of the mineral among the subjects and appeared adequate to maintain selenium status in these healthy adolescent girls; in addition, calcium supplementation of 1,000 mg daily does not have a negative impact on selenium parameters.

Role of calcium in maximizing peak bone mass development

Calcium intake below requirement level during bone modeling and skeletal consolidation could lower peak bone mass in the population

Skeletal Development in Young Females: Endogenous Versus Exogenous Factors

The most critical period in skeletal development is during the time of the most rapid bone modeling and turnover of the adolescence. The process of bone modeling that takes place from birth until the cessation of longitudinal bone growth is characterized by changes in the volume and the shape of the bones. Thereafter, bone tissue within the existing skeletal structure is continuously being formed and resorbed with minimal change in bone size through the remodeling process (1). From infancy through late adolescence the activity of bone formation predominates, resulting in a steady accumulation of bone mass. On average, most of the skeletal mass is accumulated by the age of 18 (Fig. 3.1) (2, 3). Thereafter, there is a minimal change in bone mass and density with age up to the time of menopause. Some skeletal sites begin to lose bone immediately after the age of 18 (proximal femur and trabecular bone in the vertebrae), and the other sites show continuous apposition of bone up to the time of menopause (forearm and total spine) (3).