Guest Editorial

Review Article

J Biomech Eng. 2017;139(11):110801-110801-10. doi:10.1115/1.4037917.

Rotator cuff tears (RCTs) are one of the primary causes of shoulder pain and dysfunction in the upper extremity accounting over 4.5 million physician visits per year with 250,000 rotator cuff repairs being performed annually in the U.S. While the tear is often considered an injury to a specific tendon/tendons and consequently treated as such, there are secondary effects of RCTs that may have significant consequences for shoulder function. Specifically, RCTs have been shown to affect the joint cartilage, bone, the ligaments, as well as the remaining intact tendons of the shoulder joint. Injuries associated with the upper extremities account for the largest percent of workplace injuries. Unfortunately, the variable success rate related to RCTs motivates the need for a better understanding of the biomechanical consequences associated with the shoulder injuries. Understanding the timing of the injury and the secondary anatomic consequences that are likely to have occurred are also of great importance in treatment planning because the approach to the treatment algorithm is influenced by the functional and anatomic state of the rotator cuff and the shoulder complex in general. In this review, we summarized the contribution of RCTs to joint stability in terms of both primary (injured tendon) and secondary (remaining tissues) consequences including anatomic changes in the tissues surrounding the affected tendon/tendons. The mechanical basis of normal shoulder joint function depends on the balance between active muscle forces and passive stabilization from the joint surfaces, capsular ligaments, and labrum. Evaluating the role of all tissues working together as a system for maintaining joint stability during function is important to understand the effects of RCT, specifically in the working population, and may provide insight into root causes of shoulder injury.

Commentary by Dr. Valentin Fuster

Research Papers

J Biomech Eng. 2017;139(11):111001-111001-7. doi:10.1115/1.4037855.

Tears in the annulus fibrosus (AF) of the intervertebral disk can result in disk herniation and progressive degeneration. Understanding AF failure mechanics is important as research moves toward developing biological repair strategies for herniated disks. Unfortunately, failure mechanics of fiber-reinforced tissues, particularly tissues with fibers oriented off-axis from the applied load, is not well understood, partly due to the high variability in reported mechanical properties and a lack of standard techniques ensuring repeatable failure behavior. Therefore, the objective of this study was to investigate the effectiveness of midlength (ML) notch geometries in producing repeatable and consistent tissue failure within the gauge region of AF mechanical test specimens. Finite element models (FEMs) representing several notch geometries were created to predict the location of bulk tissue failure using a local strain-based criterion. FEM results were validated by experimentally testing a subset of the modeled specimen geometries. Mechanical testing data agreed with model predictions (∼90% agreement), validating the model's predictive power. Two of the modified dog-bone geometries (“half” and “quarter”) effectively ensured tissue failure at the ML for specimens oriented along the circumferential-radial and circumferential-axial directions. The variance of measured mechanical properties was significantly lower for notched samples that failed at the ML, suggesting that ML notch geometries result in more consistent and reliable data. In addition, the approach developed in this study provides a framework for evaluating failure properties of other fiber-reinforced tissues, such as tendons and meniscus.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111002-111002-7. doi:10.1115/1.4037549.

Intervertebral disc degeneration is a prevalent phenomenon associated with back pain. It is of critical clinical interest to discriminate disc health and identify early stages of degeneration. Traditional clinical T2-weighted magnetic resonance imaging (MRI), assessed using the Pfirrmann classification system, is subjective and fails to adequately capture initial degenerative changes. Emerging quantitative MRI techniques offer a solution. Specifically, T2* mapping images water mobility in the macromolecular network, and our preliminary ex vivo work shows high predictability of the disc's glycosaminoglycan content (s-GAG) and residual mechanics. The present study expands upon this work to predict the biochemical and biomechanical properties in vivo and assess their relationship with both age and Pfirrmann grade. Eleven asymptomatic subjects (range: 18–62 yrs) were enrolled and imaged using a 3T MRI scanner. T2-weighted images (Pfirrmann grade) and quantitative T2* maps (predict s-GAG and residual stress) were acquired. Surface maps based on the distribution of these properties were generated and integrated to quantify the surface volume. Correlational analyses were conducted to establish the relationship between each metric of disc health derived from the quantitative T2* maps with both age and Pfirrmann grade, where an inverse trend was observed. Furthermore, the nucleus pulposus (NP) signal in conjunction with volumetric surface maps provided the ability to discern differences during initial stages of disc degeneration. This study highlights the ability of T2* mapping to noninvasively assess the s-GAG content, residual stress, and distributions throughout the entire disc, which may provide a powerful diagnostic tool for disc health assessment.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111003-111003-9. doi:10.1115/1.4037932.

Elastic fibers are present in low quantities in tendon, where they are located both within fascicles near tenocytes and more broadly in the interfascicular matrix (IFM). While elastic fibers have long been known to be significant in the mechanics of elastin-rich tissue (i.e., vasculature, skin, lungs), recent studies have suggested a mechanical role for elastic fibers in tendons that is dependent on specific tendon function. However, the exact contribution of elastin to properties of different types of tendons (e.g., positional, energy-storing) remains unknown. Therefore, this study purposed to evaluate the role of elastin in the mechanical properties and collagen alignment of functionally distinct supraspinatus tendons (SSTs) and Achilles tendons (ATs) from elastin haploinsufficient (HET) and wild type (WT) mice. Despite the significant decrease in elastin in HET tendons, a slight increase in linear stiffness of both tendons was the only significant mechanical effect of elastin haploinsufficiency. Additionally, there were significant changes in collagen nanostructure and subtle alteration to collagen alignment in the AT but not the SST. Hence, elastin may play only a minor role in tendon mechanical properties. Alternatively, larger changes to tendon mechanics may have been mitigated by developmental compensation of HET tendons and/or the role of elastic fibers may be less prominent in smaller mouse tendons compared to the larger bovine and human tendons evaluated in previous studies. Further research will be necessary to fully elucidate the influence of various elastic fiber components on structure–function relationships in functionally distinct tendons.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111004-111004-8. doi:10.1115/1.4037937.

Bone development, maintenance, and regeneration are remarkably sensitive to mechanical cues. Consequently, mechanical stimulation has long been sought as a putative target to promote endogenous healing after fracture. Given the transient nature of bone repair, tissue-level mechanical cues evolve rapidly over time after injury and are challenging to measure noninvasively. The objective of this work was to develop and characterize an implantable strain sensor for noninvasive monitoring of axial strain across a rodent femoral defect during functional activity. Herein, we present the design, characterization, and in vivo demonstration of the device’s capabilities for quantitatively interrogating physiological dynamic strains during bone regeneration. Ex vivo experimental characterization of the device showed that it possessed promising sensitivity, signal resolution, and electromechanical stability for in vivo applications. The digital telemetry minimized power consumption, enabling extended intermittent data collection. Devices were implanted in a rat 6 mm femoral segmental defect model, and after three days, data were acquired wirelessly during ambulation and synchronized to corresponding radiographic videos, validating the ability of the sensor to noninvasively measure strain in real-time. Together, these data indicate the sensor is a promising technology to quantify tissue mechanics in a specimen specific manner, facilitating more detailed investigations into the role of the mechanical environment in dynamic bone healing and remodeling processes.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111005-111005-11. doi:10.1115/1.4037938.

Sloped walking is challenging for individuals with transtibial amputation (TTA) due to the functional loss of the ankle plantarflexors. Prostheses that actively generate ankle power may help to restore this lost function. The purpose of this study was to use musculoskeletal modeling and simulation to quantify the mechanical power delivered to body segments by passive and powered prostheses and the remaining muscles in the amputated and intact legs during sloped walking. We generated walking simulations from experimental kinematic and kinetic data on slopes of 0, ±3 deg and ±6 deg in eight people with a TTA using powered and passive prostheses and eight nonamputees. Consistent with our hypothesis, the amputated leg hamstrings generated more power to both legs on uphill slopes in comparison with nonamputees, which may have implications for fatigue or overuse injuries. The amputated leg knee extensors delivered less power to the trunk on downhill slopes (effect size (ES) ≥ 1.35, p ≤ 0.02), which may be due to muscle weakness or socket instability. The power delivered to the trunk from the powered and passive prostheses was not significantly different (p > 0.05), However, using the powered prosthesis on uphill slopes reduced the contributions from the amputated leg hamstrings in all segments (ES ≥ 0.46, p ≤ 0.003), suggesting that added ankle power reduces the need for the hamstrings to compensate for lost ankle muscle function. Neither prosthesis replaced gastrocnemius function to absorb power from the trunk and deliver it to the leg on all slopes.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111006-111006-10. doi:10.1115/1.4038110.

During pregnancy and lactation, the maternal skeleton provides calcium for fetal/infant growth, resulting in substantial bone loss, which partially recovers after weaning. However, the amount of bone that is lost and the extent of post-weaning recovery are highly variable among different skeletal sites, and, despite persistent alterations in bone structure at some locations, reproductive history does not increase postmenopausal fracture risk. To explain this phenomenon, we hypothesized that the degree of reproductive bone loss/recovery at trabecular sites may vary depending on the extent to which the trabecular compartment is involved in the bone’s load-bearing function. Using a rat model, we quantified the proportion of the load carried by the trabeculae, as well as the extent of reproductive bone loss and recovery, at two distinct skeletal sites: the tibia and lumbar vertebra. Both sites underwent significant bone loss during pregnancy and lactation, which was partially recovered post-weaning. However, the extent of the deterioration and the resumption of trabecular load-bearing capacity after weaning varied substantially. Tibial trabecular bone, which bore a low proportion of the total applied load, underwent dramatic and irreversible microstructural deterioration during reproduction. Meanwhile, vertebral trabecular bone bore a greater fraction of the load, underwent minimal deterioration in microarchitecture, and resumed its full load-bearing capacity after weaning. Because pregnancy and lactation are physiological processes, the distinctive responses to these natural events among different skeletal sites may help to elucidate the extent of the trabecular bone’s structural versus metabolic functions.

Topics: Bone , Stress
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2017;139(11):111007-111007-6. doi:10.1115/1.4038111.

This study aimed to experimentally track the tissue-scale strains of the tendon–bone attachment with and without a localized defect. We hypothesized that attachments with a localized defect would develop strain concentrations and would be weaker than intact attachments. Uniaxial tensile tests and digital image correlation were performed on rat infraspinatus tendon-to-bone attachments with defects (defect group) and without defects (intact group). Biomechanical properties were calculated, and tissue-scale strain distributions were quantified for superior and inferior fibrous and calcified regions. At the macroscale, the defect group exhibited reduced stiffness (31.3±3.7 N/mm), reduced ultimate load (24.7±3.8 N), and reduced area under the curve at ultimate stress (3.7±1.5 J/m2) compared to intact attachments (42.4±4.3 N/mm, 39.3±3.7 N, and 5.6±1.4 J/m2, respectively). Transverse strain increased with increasing axial load in the fibrous region of the defect group but did not change for the intact group. Shear strain of the superior fibrous region was significantly higher in the defect group compared to intact group near yield load. This work experimentally identified that attachments may resist failure by distributing strain across the interface and that strain concentrations develop near attachment defects. By establishing the tissue-scale deformation patterns of the attachment, we gained insight into the micromechanical behavior of this interfacial tissue and bolstered our understanding of the deformation mechanisms associated with its ability to resist failure.

Commentary by Dr. Valentin Fuster

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