Interpretation of Calcaneus Dual-Energy X-Ray Absorptiometry Measurements in the Assessment of Osteopenia and Fracture Risk

ABSTRACT Dual-energy X-ray absorptiometry (DXA) of the calcaneus is useful in assessing bone mass and fracture risk at other skeletal sites. However, DXA yields an areal bone mineral density (BMD) that depends on both bone apparent density and bone size, potentially complicating interpretation of the DXA results. Information that is more complete may be obtained from DXA exams by using a volumetric density in addition to BMD in clinical applications. In this paper, we develop a simple methodology for determining a volumetric bone mineral apparent density (BMAD) of the calcaneus. For the whole calcaneus, BMAD ‫؍‬ (BMC)/A DXA 3/2 , where BMC and A DXA are, respectively, the bone mineral content and projected area measured by DXA. We found that A DXA 3/2 was proportional to the calcaneus volume with a proportionality constant of 1.82 ؎ 0.02 (mean ؎ SE). Consequently, consistent with theoretical predictions, BMAD was proportional to the true volumetric apparent density () of the bone according to the relationship ‫؍‬ 1.82 BMAD. Also consistent with theoretical predictions, we found that BMD varied in proportion to V 1/3 , where V is the bone volume. We propose that the volumetric apparent density, estimated at the calcaneus, provides additional information that may aid in the diagnosis of osteopenia. Areal BMD or BMD 2 may allow estimation of the load required to fracture a bone. Fracture risk depends on the loading applied to a bone in relation to the bone's failure load. When DXA is used to assess osteopenia and fracture risk in patients, it may be useful to recognize the separate and combined effects of applied loading, bone apparent density, and bone size. (J Bone Miner Res 2000;15:1573-1578

Tyrannosaurus Osborn 1905

Questions have been raised about the methods used and conclusions reached in this Letter 1. In revisiting the work, we realized that we did not provide sufficient methodological details regarding the many steps that went into our growth curve analysis, although the main conclusions of the paper were not affected. we regret any misunder- standing that might have resulted. A detailed rationale is available in the Supplementary Methods and Discussion of this Corrigendum and the source data are provided as Supplementary Data. we thank N. Myhrvold for bringing these issues to our attention. In our reanalysis we found a minor translational mistake affect- ing the reported growth for Tyrannosaurus, which does not appear to have contributed to Myhrvold’s concerns (details can be found in the Supplementary Methods and Discussion to this Corrigendum.) The correct equation is Mass = (5,649/[1 +e −0.55(Age−16.2)]) + 5. This produces a maximal growth rate of 758 kg yr −1 using points closely bounding the inflection point and 774 kg yr −1 using the instantaneous equation. The reported value was 767 kg yr −1. This slight discrepancy (see the corrected Fig. 2 in the Supplementary Methods and Discussion to this Corrigendum) does not compromise our conclusion that Tyrannosaurus primarily achieved gigantism through evolutionary acceleration.Published as part of Gregory M. Erickson, Peter J. Makovicky, Philip J. Currie, Mark A. Norell, Scott A. Yerby & Christopher A. Brochu, 2016, Corrigendum: Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs, pp. 538 in Nature 531 on page 1, DOI: 10.1038/nature16487, http://zenodo.org/record/373650

Tyrannosaurus rex

Stemming from more than a century of investigation, considerable understanding of tyrannosaurid osteology 4, myology 5, neurology 6, behaviour 7, 8, physiology 3, 9, physical capabilities 10, 11 and phylogeny 12, 13 have been gained. Lacking are empirical data on tyrannosaurid life history such as growth rates, longevity and somatic maturity (adult size) from which the developmental possibilities for how T. rex attained gigantism can be formally tested. Recent advances in techniques for determining the ages at death of dinosaurs by using skeletal growth line counts 3, 14, coupled with developmental size estimates 3, make quantitative growth-curve reconstructions for dinosaurs feasible. These methods have been used to study growth rates in two small theropods, a small and a large ornithischian and a medium-sized and a gigantic sauropodomorph 3. These data were used to derive a regression of body mass against growth rate and to generalize broadly about non-avian dinosaur growth 3. However, because of the phylogenetically disparate nature of these data (that is, none are close outgroups to one another) it has not been possible to use them to infer how specific cases of size change occurred within dinosaurian sub-clades such as the Tyrannosauridae. Such an understanding requires multi-species sampling at low taxonomic levels (that is, among closely related species) and access to growth series spanning juvenile through adult stages, a rarity among extinct dinosaurs 15. Furthermore, it requires the capacity to account for growth line losses due to medullar cavity hollowing and cortical remodelling 16, two processes that are pervasive in the major weight-bearing bones from large theropods such as tyrannosaurids. The sampling problem has been overcome in North American tyrannosaurids. A flurry of recent discoveries has greatly increased the number of substantially complete specimens representing various growth stages available for study. For example, more than 30 T. rex specimens are known 4, 17, compared with only 11 reported in 1993 (ref. 18; see Supplementary Information). Recent work has broadened the developmental representation of these animals by showing that several purported ‘dwarf’ tyrannosaur species are juveniles of larger, previously recognized forms such as T. rex 12, 13, 19, 20. Finally, preliminary analyses for this research revealed that several non-weight-bearing bones in tyrannosaurids (for example pubes, fibulae, ribs, gastralia and postorbitals) did not develop hollow medullar cavities and showed negligible intracortical remodelling during their entire life history (Fig. 1). Like major long bones, these elements are effective for assessing longevity in living reptiles (Fig. 1) 21, 22 and hence provide a viable alternative method for determining the age at death of extinct reptiles such as tyrannosaurids. Here we exploit these findings to determine the pattern of growth in T. rex and three of its close tyrannosaurid relatives. We then use character optimization methods 23 to infer how T. rex attained giant proportions among tyrannosaurids. Finally, this new evidence is used to further our understanding of tyrannosaurid biology. In performing these analyses, we sampled several amedullar bones from adolescent, juvenile, sub-adult and adult representatives of the North American Late Cretaceous tyrannosaurids Albertosaurus sarcophagus, Gorgosaurus libratus, Daspletosaurus torosus and T. rex. Longevity in each of the 20 specimens was assessed from line counts in histological sections by using polarizing, dissecting and reflected-light microscopy (Fig. 1) 3, 14. Conservative estimates of body mass (see Supplementary Information) were made by using femoral circumference measures 24. Longevity and size data were plotted and least-squares regression was used to determine the first empirical growth curves for tyrannosaurids 3. The length and timing of the various developmental stages and the maximal growth rates for each taxon were compared 25. The results were examined in an evolutionary context 23 by using two competing phylogenetic hypotheses for the Tyrannosauridae 12, 13. Sampled longevities for T. rex ranged from 2 to 28 years and corresponding body mass estimates ranged from 29.9 to 5,654 kg (Table 1). The transition to somatic maturity in this taxon seems to have begun at about 18.5 years of age (Fig. 2). At least one individual (exemplified by FMNH (The Field Museum) PR 2081), showed evidence for prolonged senescence in the form of conspicuously narrow pericortical growth-line spacing (Fig. 1). Maximal growth rates in T. rex were 2.07 kg d ‾ 1 and such exponential rates were maintained for about 4 years (Fig. 2). The longevity estimates for T. rex outgroups ranged from 2 to 24 years and corresponding body sizes spanned from 50.3 to 1,791 kg (Table 1). Somatic maturity occurred at between 14 and 16 years in these taxa (Fig. 2). Like T. rex, at least some exceptionally large individuals of A. sarcophagus and D. torosus showed narrow pericortical growth-line spacing indicative of the onset of senescence. The maximal growth rates for the three smaller tyrannosaurid taxa ranged from 0.31 to 0.48 kg d ‾ 1 ; such exponential stage rates were also maintained for about 4 years (Fig. 2). Optimization of growth rates onto the two current phylogenetic hypotheses of tyrannosaurid relationships suggests that a 1.5-fold acceleration in maximal growth rate might diagnose Tyrannosaurinae (the clade comprising Daspletosaurus and Tyrannosaurus 13, 19, Fig. 2). A second substantial increase in growth rate optimizes as a physiological autapomorphy of Tyrannosaurus irrespective of phylogenetic hypothesis and optimization criterion. T. rex is notable for its great size, which is at least 15-fold greater than the largest living terrestrial carnivorous animals today and second only to Giganotosaurus 26 among theropod dinosaurs. How did it attain such great proportions within the Tyrannosauridae? From the two competing hypotheses of tyrannosaurid phylogeny it is most parsimonious to conclude that T. rex acquired the majority of its giant proportions after diverging from the common ancestor of itself and D. torosus, a species with an optimized body mass of about 1,800 kg. Direct comparison between the tyrannosaurid growth curves shows that the transition to the exponential and stationary phases of development occurred about 2–4 years later in T. rex (Fig. 2). However, such temporal post-displacement had little to do with the evolution of its gigantism because the exponential stage, during which most body size is accrued 25, was not extended beyond the ancestral, 4-year condition observed in other tyrannosaurids. Rather, the key developmental modification that propelled T. rex to giant proportions was primarily through evolutionary acceleration in the exponential stage growth rate and the transition zones bounding it. This is reflected in the regions of maximal slope on the growth curves depicted in Fig. 2 and holds true regardless of which evolutionary hypothesis is correct and how the maximum growth rates are optimized. Notably, this method of attaining gigantism contrasts with that in the largest crocodilians and lizards, where ancestral growth rates were retained and the exponential stages lengthened 27. How other dinosaurs attained gigantism within their respective sub-clades will serve as an interesting line of inquiry in the future. Does the same pattern of acceleratory growth seen here characterize the means by which all or most members of the Dinosauria attained great size? ................................................................................................................................................................................................................................................................................................................................................................... FMNH, The Field Museum; RTMP, Royal Tyrrell Museum of Palaeontology; ICM, Indianapolis Children’s Museum; LACM, Los Angeles County Museum; AMNH, American Museum of Natural History; USNM, United States National Museum. R, rib; G, gastralia; F, fibula; P, pubis; C, dermal skull bones; OLB, other long bones; est., estimated; EFS, external fundamental system 16. Besides revealing how the evolution of T. rex gigantism was obtained, the data garnered here provide for a more comprehensive understanding of tyrannosaurid biology. For instance the presence of thin, tightly packed growth lines late in development (Fig. 1) shows that these animals, like nearly all (if not all) dinosaurs, had determinate growth 3, 14. They would not have gained an appreciably greater size than the largest specimens studied here and could spend nearly 30% of their lives as full-grown adults (Fig. 2). In addition, the maximal growth rates for these tyrannosaurid species are only 33–52% of the rates expected for non-avian dinosaurs of their size when compared with the more broadly sampled data of Erickson et al. 3. This provides the first evidence of its kind pointing to major differences in whole body growth rates among a non-avian dinosaur sub-clade. Such findings are not unexpected because similar patterns (for example primates within Eutheria) occur within living vertebrate groups 28. Our findings also have a bearing on the biomechanical capacities of tyrannosaurids. T. rex ’s capacity for ‘fast running’ was biomechanially infeasible after a body mass of about 1,000 kg was attained 11. This corresponds to a juvenile-sized animal just 13 years of age on the basis of our longevity data and conservative estimates of body mass (Fig. 2). If we assume that the same relationship held true for the smaller tyrannosaurid species studied here, such locomotory limitations would not have emerged until these animals were much closer to adult size (Fig. 2). Finally, a glimpse into the potential population age structure for a dinosaur is also afforded from these data. Currie 7 has described a catastrophic death assemblage consisting of eight or nine A. sarcophagus specimens thought to represent an entire pack or a subset of one. On the basis of femoral lengths, the age and developmental stage of each animal can now be estimated. The group seems to have consisted of two or three older adults ~21 or more years of age, one ~17-year-old young adult, four ~12–17- year-old sub-adults that were undergoing exponential stage growth at the time of death, and one ~10-year-old juvenile that was beginning the transition to exponential stage growth. A reopening of the site has revealed at least one more specimen (RTMP (Royal Tyrrell Museum of Palaeontology) 2002.45.46) shown here to be only 2 years old (Table 1). This indicates that A. sarcophagus groups, whether temporary or permanent, might have been composed of individuals spanning the age spectrum from adolescents to very old, senescent adults, a finding consistent with trackway evidence for other theropod dinosaurs 7.Published as part of Gregory M. Erickson, Peter J. Makovicky, Philip J. Currie, Mark A. Norell, Scott A. Yerby & Christopher A. Brochu, 2004, Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs, pp. 772-775 in Nature 430 on pages 772-774, DOI: 10.1038/nature02699, http://zenodo.org/record/373647

The effect of implant placement on sacroiliac joint range of motion: Posterior versus transarticular

© 2015, Wolters Kluwer Health, Inc. Study Design. A human cadaveric biomechanical study of 2 sacroiliac (SI) joint fusion implant placement techniques. Objective. To evaluate and compare the biomechanical properties of 2 implant placement techniques for SI joint fusion. Summary of Background Data. Minimally invasive placement of SI joint fusion implants is a potential treatment of SI joint disruptions and degenerative sacroiliitis. Biomechanical studies of screw fixation within the sacrum have shown that placement and trajectory are important in the overall stability of the implant. Although clinical results have been promising, there is the possibility that a more optimal arrangement of implants may exist. Methods. Bilateral SI joints in 7 cadaveric lumbopelvic (L4-pelvis) specimens were tested using a single leg stance model. All joints were tested intact, pubic symphysis sectioned, and treated (3 SI joint fusion implants). The implants were laterally placed using either a posterior or transarticular placement technique. The posterior technique places the implants inline in the inlet view, parallel in the outlet view, and parallel to the posterior sacral body in the lateral view. The transarticular technique places all implants across the articular portion of the SI joint. For all conditions, the range of motion was tested in flexion-extension, lateral bending, and axial rotation. Results. The posterior technique significantly reduced the range of motion in flexion-extension, lateral bending, and axial rotation by 27% ± 24% (P = 0.024), 28% ± 26% (P = 0.028), and 32% ± 21% (P = 0.008), respectively. The transarticular technique significantly reduced the range of motion in flexion-extension, lateral bending, and axial rotation by 41% ± 31% (P = 0.013), 36% ± 38% (P = 0.049), and 36% ± 28% (P = 0.015), respectively. No significant differences were detected between the posterior and transarticular placement techniques (P \u3e 0.25). Conclusion. Posterior and transarticular placement of SI joint fusion implants stabilized the SI joint in flexion-extension, lateral bending, and axial rotation

Failure and fatigue characteristics of adhesive athletic tape

Comparaison de trois types de ruban adhésif, ( Zonas, Leukotape, Jaylastic), pour la prévention des blessures des articulations établie à l'aide de tests mécaniques

Effect of Sacropelvic Hardware on Axis and Center of Rotation of the Sacroiliac Joint: A Finite Element Study

Background: The sacroiliac joint (SIJ) transfers the load of the upper body to the lower extremities while allowing a variable physiological movement among individuals. The axis of rotation (AoR) and center of rotation (CoR) of the SIJ can be evaluated to analyze the stability of the SIJ, including when the sacrum is fixed. The purpose of this study was to determine how load intensity affects the SIJ for the intact model and to characterize how sacropelvic fixation performed with different techniques affects this joint. Methods: Five T10-pelvis models were used: (1) intact model; (2) pedicle screws and rods in T10-S1; (3)pedicle screws and rods in T10-S1, and bilateral S2 alar-iliac screws (S2AI); (4) pedicle screws and rods in T10-S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory ; and (5) pedicle screws and rods in T10-S1, bilateral S2AI screws, and 2 bilateral triangular implants inserted in a lateral trajectory. Outputs of these models under flexion-extension were compared: AoR and CoR of the SIJ at incremental steps from 0 to 7.5 Nm for the intact model and AoR and CoR of the SIJ for the instrumented models at 7.5 Nm. Results: The intact model was validated against an in vivo study by comparing range of motion and displacement of the sacrum. Increasing the load intensity for the intact model led to an increase of the rotation of the sacrum but did not change the CoR. Comparison among the instrumented models showed that sacropelvic fixation techniques reduced the rotation of the sacrum and stabilized the SIJ, in particular with triangular implants. Conclusion: The study outcomes suggest that increasing load intensity increases the rotation of the sacrum but does not influence the CoR, and use of sacropelvic fixation increases the stability of the SIJ, especially when triangular implants are employed. Clinical relevance: The choice of the instrumentation strategy for sacropelvic fixation affects the stability of the construct in terms of both range of motion and axes of rotation, with direct consequences on the risk of failure and mobilization. Clinical studies should be performed to confirm these biomechanical findings

Stability and Instrumentation Stresses Among Sacropelvic Fixation Techniques With Novel Porous Fusion/Fixation Implants: A Finite Element Study

Background: Sacropelvic fixation is frequently combined with thoracolumbar instrumentation for correcting spinal deformities. This study aimed to characterize sacropelvic fixation techniques using novel porous fusion/fixation implants (PFFI). Methods: Three T10-pelvis finite element models were created: (1) pedicle screws and rods in T10-S1, PFFI bilaterally in S2 alar-iliac (S2AI) trajectory; (2) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, triangular implants bilaterally above the PFFI in a sacro-alar-iliac trajectory (PFFI-IFSAI); and (3) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, PFFI in sacro-alar-iliac trajectory stacked cephalad to those in S2AI position (2-PFFI). Models were loaded with pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation. Outputs were compared against 2 baseline models: (1) pedicle screws and rods in T10-S1 (PED), and (2) pedicle screws and rods in T10-S1, and S2AI screws. Results: PFFI and S2AI resulted in similar L5-S1 motion; adding another PFFI per side (2-PFFI) further reduced this motion. Sacroiliac joint (SIJ) motion was also similar between PFFI and S2AI; PFFI-IFSAI and 2-PFFI demonstrated a further reduction in SIJ motion. Additionally, PFFI reduced max stresses on S1 pedicle screws and on implants in the S2AI position. Conclusion: The study shows that supplementing a long construct with PFFI increases the stability of the L5-S1 and SIJ and reduces stresses on the S1 pedicle screws and implants in the S2AI position. Clinical relevance: The findings suggest a reduced risk of pseudarthrosis at L5-S1 and screw breakage. Clinical studies may be performed to demonstrate applicability to patient outcomes. Level of evidence: Not applicable (basic science study)

Evaluation of a minimally invasive procedure for sacroiliac joint fusion - An in vitro biomechanical analysis of initial and cycled properties

Introduction: Sacroiliac (SI) joint pain has become a recognized factor in low back pain. The purpose of this study was to investigate the effect of a minimally invasive surgical SI joint fusion procedure on the in vitro biomechanics of the SI joint before and after cyclic loading. Methods: Seven cadaveric specimens were tested under the following conditions: intact, posterior ligaments (PL) and pubic symphysis (PS) cut, treated (three implants placed), and after 5,000 cycles of flexion-extension. The range of motion (ROM) in flexion-extension, lateral bending, and axial rotation was determined with an applied 7.5 N · m moment using an optoelectronic system. Results for each ROM were compared using a repeated measures analysis of variance (ANOVA) with a Holm-Šidák post-hoc test. Results: Placement of three fusion devices decreased the flexion-extension ROM. Lateral bending and axial rotation were not significantly altered. All PL/PS cut and post-cyclic ROMs were larger than in the intact condition. The 5,000 cycles of flexion-extension did not lead to a significant increase in any ROMs. Discussion: In the current model, placement of three 7.0 mm iFuse Implants significantly decreased the flexion-extension ROM. Joint ROM was not increased by 5,000 flexion-extension cycles. © 2014 Lindsey et al