Indirect way to estimate peak joint loads in life and in skeletal remains (insights from a new paradigm)

DENSITY-BASED LOAD ESTIMATION PREDICTS ALTERED FEMORAL LOAD DIRECTIONS FOR COXA VARA AND COXA VALGA

Quantifying differences in joint loading for coxa vara and coxa valga is important for understanding what constitutes a pathological deformity. Prior free-body analyses for varus and valgus femora suggest that the loading direction in single-leg stance becomes more vertical for coxa valga and more horizontal for coxa vara. The objectives of this study were: 1) to apply a density-based load estimation technique to varus and valgus femora; 2) to infer potential differences in femoral loading for varus and valgus femora from the density; and 3) to compare the results with previous studies of femoral loading for single-leg stance. Representative valgus, normal, and varus femora from male cadavers were scanned in the plane of the femoral neck using computed tomography. A two-dimensional finite element model, including the density data from the CT scans, was constructed for each femur. A density-based bone load estimation method was used to determine the dominant loading pattern, and an average load direction was calculated. The average load direction varied consistently from more vertical for coxa valga to more horizontal for coxa vara. The results indicate that the differences in loading directions reduce the risk of epiphyseal slip or neck fracture in coxa vara and increase the tendency for subluxation or dislocation in coxa valga. Agreement between relative load angles from the density-based load estimation and free-body analyses confirms that internal femoral density is adapted to applied loads regardless of external femoral geometry.

Olympic fencers: adaptations in cortical and trabecular bone determined by quantitative computed tomography

We investigated how cortical bone, trabecular bone, and muscle adapt in US Olympic Fencing Team members. These athletes demonstrate femoral cortical bone expansion, greater distal femoral trabecular bone density, and greater muscle mass compared to controls. This is the first study to investigate musculoskeletal adaptations in Olympic fencers.

PURPOSE

Wolff's law states that bone remodels according to mechanical forces placed upon it. Our goal was to determine how cortical and trabecular bone adapt in Olympic athletes who perform intermittent high-impact activity.

MATERIALS AND METHODS

Nine males from the 2004 US Olympic Fencing Team and nine matched controls were evaluated by quantitative computed tomography. Femurs were scanned at 50% and 75% along the shaft. We evaluated cortical thickness (C.Th), cortical (C.Ar), trabecular (Tb.Ar), and total bone areas (Tot.Ar), proportions of C.Ar and Tb.Ar to Tot.Ar, cortical (C.BMD.), trabecular (Tb.MBD), and total bone densities (Tot.BMD), muscle (M.Ar), and thigh areas (Th.Ar).

RESULTS

Fencers had greater C.Th (+24.5 to 38.8%), C.Ar (+16.9 to 19.6%), C.Ar/Tot.Ar (+6.3 to 16.3%), and lower Tb.Ar/Tot.Ar (-23.5% to -23.8%; p<0.05). Fencers demonstrated a positive difference in C.Th in the dominant vs. nondominant thigh at 50% (+5.4%, p = 0.040) and at 75% (+13.8%, p = 0.048 by analysis of covariance). Fencers had 54% greater Tb.BMD at 75% (p = 0.025), but not at 50% (p = 0.63). There was no difference between groups for C.BMD (p = .66 at 50%, p = 0.88 at 75%). Fencers had greater M.Ar (+30%) and asymmetrically greater M.Ar (+12.2%) in the dominant thigh (p < 0.004).

CONCLUSION

In world-class athletes who perform intermittent, high-impact activity, cortical bone expands, trabecular bone density is greater, and muscle mass is greater. This is the first study to examine musculoskeletal adaptations in Olympic fencers.

Spinal degenerative disk disease (DDD) in female macaque monkeys: epidemiology and comparison with women

Spinal degenerative disk disease (DDD) in a radiographic, cross-sectional sample of 192 female macaque monkeys, approximately 5-30 years old, is described. The presence and extent of disk space narrowing (DSN) and anterior osteophytosis were assessed with reference to age, average lifetime body mass. and distribution within the thoracolumbar spine. Age was a strong correlate of disk narrowing and osteophytosis, with early signs appearing at equivalent ages in both species and increasing in prevalence thereafter. Macaques showed a far greater prevalence of DDD, especially in the oldest age group, than has been reported in the human data. Body mass was associated with disk narrowing in the macaque, but not with osteophytosis. The two species differed little in the pattern of distribution of DDD along the spine. Our results suggest that bipedality is not the singular, or even the most important, biomechanical factor in the development of human DDD. Rather, others shared postural regimes, e.g., sitting, may be responsible for the onset and progression of DDD in both species. The macaque model could substantially add to the understanding and, potentially, treatment of this oftentimes debilitating condition.

An approach to estimating bone and joint loads and muscle strength in living subjects and skeletal remains

Skeletal physiology that clarified after 1990 shows that bone modeling normally makes a bone strong enough to keep its loads from causing strains above a "modeling threshold". That arrangement adapts bone strength to the largest loads on a bone, which are usually brief and infrequent. Accordingly, in bone adapted chiefly to uniaxial compression loads, the modeling threshold's value and the cross-sectional amount of that bone could suggest the size of those loads. Bone loaded in that way does support the articular surfaces of synovial joints as their "supporting bone", so its amount could suggest the size of the loads it had adapted to, and therefore the loads on the joint that it supports. During growth a joint's size is proportional, directly but not linearly, to the size of its total loads, so that its size at skeletal maturity could be an index of those loads at that time. Joints cannot decrease in size. Yet throughout life their supporting bone can decrease or increase in strength and "mass" to adapt to changes in a joint's loads. Thus, an adult joint's size could suggest the size of the loads it adapted to by skeletal maturity, while the cross-sectional amount of its supporting bone at any later age could reveal the size of those loads at that later age, and thus suggest any change in those loads that might have occurred after skeletal maturity. Since the bone modeling threshold, and the relationships between bone strain, stress, and unit loads are now known, it is possible with this procedure to estimate the total loads on joints, and how body weight and muscle strength contribute to those loads in both living subjects and skeletal remains. To make a reliable technology of the idea involves some problems which this paper identifies and suggests how to resolve. Am. J. Hum. Biol. 11:437-455, 1999. Copyright 1999 Wiley-Liss, Inc.

Mineral mass, size, and estimated mechanical strength of triple jumpers' lower limb

This study was designed to examine the anticipated strong influence of extreme impact loading on the mineral mass, size, and gross structural properties of triple jumpers' lower limb bones. We compared the bone data obtained with peripheral quantitative computed tomography (pQCT) and dual-energy X-ray absorptiometry (DXA) from 8 Finnish triple jumpers with corresponding data from gender-, age-, height-, and weight-matched nonathletic peers. The volumetric (trabecular) density was significantly higher in the jumpers than in controls (from an average 18% difference at the distal tibia to a 41% difference at the proximal tibia), whereas the cortical density did not differ between groups. The DXA-derived areal bone mineral density of the femoral neck and lumbar spine was 31% higher in the jumpers than in controls, compared with a 16% difference between groups at the less-loaded distal radius. The lower limb bones were comparable in size between groups except at the distal femur where a significant 4%-6% difference was observed in favor of the triple jumpers. Mean tibial cortical wall thickness and area were substantially greater in the triple jumpers; the mean group difference ranged from about 20% at the shaft sites, to over 50% at the distal tibia. Given the apparently stronger cortices in the triple jumpers, the section moduli (bone strength index) of their femoral necks and tibiae were 19%-31% higher compared with the control group. Our findings indicate the ability of extreme impact loading to considerably improve bone's mechanical competence. Adaptation to loading seems to occur in a site-specific fashion by gross geometric changes, structural or architectural changes, or by their combination. The loading effect was best seen as enlarged bone cortices, probably after the trabecular density had reached its ceiling.

A physical model for dual-energy X-ray absorptiometry--derived bone mineral density

RATIONALE AND OBJECTIVES

Dual-energy X-ray absorptiometry (DXA)-derived areal bone mineral density (BMD) is an established predictor of osteoporotic fractures and reflects bone strength as well. The goal of this study was to develop and validate a physical model for appropriate interpretation of BMD.

METHODS

DXA and peripheral quantitative computed tomography investigations of the distal tibia (n = 45), proximal tibia (n = 12), distal femur (n = 26), and distal radius (n = 34) were carried out. The DXA-derived BMD was analytically modeled as a nonlinear function of volumetric bone mineral apparent density and the cross-sectional area (eCSA) of given bone; ie, BMD(mod) = apparent BMD x square root of eCSA.

RESULTS

At every measured skeletal site, the relationship between BMD and BMD(mod) was systematically stronger than that observed separately between BMD and apparent BMD or cross-sectional area. The models (r2) explained 85%, 94%, 87%, and 74% of the variability in BMD at the distal tibia, proximal tibia, distal femur, and distal radius, respectively.

CONCLUSIONS

The mutual contributions of bone density and size to BMD can vary to some extent in a site-dependent fashion. This dual nature of BMD on one hand provides a reasonable mechanical explanation for why BMD is a good surrogate of bone strength and a predictor of osteoporotic fractures but on the other hand, complicates its detailed interpretation.

Calcified cartilage morphometry and its relation to subchondral bone remodeling in equine arthrosis

The calcified layer of articular cartilage is known to be affected by age and mechanical factors that may play a role in the development of arthrosis. Because these factors are also related to subchondral remodeling and sclerosis, a morphometric study was carried out in fluorochrome-labeled animals to determine whether the level of subchondral remodeling affected the thickness of the calcified cartilage layer and its irregularity and vascularity at the interface with subchondral bone. These parameters were also studied at a site of increased mechanical stress. The area and thickness of the calcified cartilage layer was determined in basic fuchsin-stained ground sections (120 microm). The irregularity of the chondro-osseous interface was expressed as the ratio of its length to that of the relatively straight tidemark (Int/Tid) and the number of abutting vessels with and without fluochrome labels were counted (N.Ves/Tid,%L.Ves/Tid). These were compared with single-labeled surface (sLS/BS, %) in subchondral bone, which was used as an index of remodeling. In a group of 12 horses, in which one carpus had an osteochondral fragment surgically created 10 weeks earlier, there was activation of subchondral remodeling in the third carpal bone opposite the fragment. An increase in %L.Ves/Tid (p < 0.01) at the interface was correlated with the increase in %sLS/BS in subchondral bone (r=0.431, p=0.035). The number of abutting vessels and the interface irregularity were not significantly changed on the fragmented side. In the metacarpal condyles from the fetlock joints of the same horses there were no differences associated with the surgically created fragment in the carpus and no correlation of %L.Ves/Tid with subchondral %sLS/BS. At a site where mechanical overload and traumatic osteochondrosis is known to occur on the palmar surface, the calcified cartilage was thinner, and the interface irregularity tended to be greater. These findings indicate that activated subchondral remodeling extends to involve the calcified layer, but the thickness and irregularity of the calcified cartilage are not consistently related to current subchondral remodeling. At sites of mechanical overload the calcified cartilage was thinner and the interface tended to be more irregular, suggesting previous increased remodeling.

Peripheral quantitative computed tomography in human long bones: evaluation of in vitro and in vivo precision

Despite the excellent performance in clinical practice and research, the dual-energy X-ray absorptiometry is restricted by the inherent planar nature of the measurement and the inability to discriminate between trabecular and cortical components of bone. Recently, a new peripheral tomographic scanner (Norland/Stratec XCT 3000) was introduced for versatile measurements of human long bone characteristics in vivo, including trabecular and cortical density (TrD and CoD, respectively), respective cross-sectional areas (TrA and CoA), bone strength index (BSI), and bone mineral content (BMC). We evaluated the technical performance of the scanner using different phantoms and determined the in vivo precision of the above-noted applications by measuring twice several sites of upper and lower limbs of 19 and 36 volunteers aged 23-60 years. The bone scans were performed, with intermediate positioning of the subject, at two different anatomic sites of the forearm, three sites of the upper arm, three sites of the shank, and two sites of the thigh, with the respective skeletal sites representing different bone compositions and sizes. According to phantom measurements, the XCT 3000 appeared to be a highly linear, stable, and precise (coefficient of variation [CV] about 0.2%) system in vitro. The soft tissue thickness, however, had a linear effect on density values and a nonlinear effect on BMC, whereas the effect on cross-sectional area was marginal. The in vivo root mean square CV (CVrms) values for the long bone ends ranged from 0.9% (distal tibia) to 2.7% (distal femur) for TrD, from 1.8% (distal femur) to 7.6% (distal radius) for TrA, from 2.0% (distal tibia) to 6.8% (proximal tibia) for CoD, from 1.8% (distal femur) to 4.9% (proximal tibia) for CoA, and from 4.2% (distal tibia) to 7.7% (distal radius) for BSI. The corresponding CVrms values for the long bone shafts ranged from 0.5% (midshaft of humerus) to 1.4% (midshaft of fibula) for CoD, from 1.7% (midshaft of tibia) to 4.6% (proximal shaft of humerus) for CoA, and from 2.5% (midshaft of tibia) to 7.5% (proximal shaft of humerus) for BSI. There was no interoperator effect on precision. This study provided, for the first time, independent precision data for the new XCT 3000 peripheral quantitative computed tomography (pQCT) scanner in various applications of human long bones (radius, ulna, humerus, tibia, fibula, and femur) and gave practical guidelines and procedures on how to employ this versatile method in clinical and research applications. The technical performance of the tested system was excellent and it allowed, with a low radiation dose, precise in vivo evaluation of trabecular and cortical density, cross-sectional area, and BMC of selected skeletal sites. The potential effect of the soft tissue thickness on density and mineral content values need to be recognized. The pQCT measurement seems to be useful in supplementing the integral, planar DXA data and obviously opens new possibilities for clinical practice and research.