Favorable Mechanical Environment Research Articles

Topology optimization of embracing fixator considering bone remodeling to mitigate stress shielding effect

The embracing fixator is one of the widely used internal fixation implants for bone fracture treatment. However, the stress shielding effect, a stress imbalance between the implant and bone caused by the mismatch in mechanical properties between them, is a significant and critical issue that may lead to treatment failure. Thus, it is of great importance to design the implant with an appropriate stiffness which can mitigate the stress shielding effect and provide the most favorable mechanical environment for bone healing and remodeling. To this end, a time-dependent topology optimization algorithm considering bone remodeling is proposed to optimize an embracing fixator used in the tibia fracture treatment. The change of callus density over time is simulated based on a bone remodeling model, and the callus density after a period of bone remodeling is selected to be the design objective to maximize. The design constraints include volume and the compliance of the whole fixation system. Meanwhile, the influence of the constraints on the regularity of material distribution of the optimized result is also studied. Besides, to test the effectiveness of the consideration of the bone remodeling in the embracing fixator design, a topology optimization concerning the minimization of the compliance of the entire system is also performed to make a comparison. Finally, the safety performance of optimized results considering bone remodeling is also verified by static analysis.

Anterior Closing-Wedge High Tibial Slope-Correcting Osteotomy Using Patient-Specific Preoperative Planning Software for Failed Anterior Cruciate Ligament Reconstruction

Failure of anterior cruciate ligament reconstruction (ACLR) remains a challenging problem. Recently, the effect of increased posterior tibial slope has been identified as a risk factor for ACLR failure. In cases with increased posterior tibial slope, an anterior closing wedge, slope-correcting high tibial osteotomy can be used as a robust adjunct to revision ACLR. In this Technical Note, we demonstrate our preferred method for isolated sagittal plane correction following multiple failed ACLRs with an anterior closing-wedge high tibial osteotomy technique using 3-dimensional patient-specific instrumentation. Through correction of the angular deformity and restoration of the defined sagittal slope via the use of advanced 3-dimensional patient-specific instrumentation, this technique fosters an accurate, favorable mechanical environment to prevent recurrent instability of the knee joint.

Enhancing the Efficiency of Distraction Osteogenesis through Rate-Varying Distraction: A Computational Study.

Distraction osteogenesis (DO) is a mechanobiological process of producing new bone and overlying soft tissues through the gradual and controlled distraction of surgically separated bone segments. The process of bone regeneration during DO is largely affected by distraction parameters. In the present study, a distraction strategy with varying distraction rates (i.e., “rate-varying distraction”) is proposed, with the aim of shortening the distraction time and improving the efficiency of DO. We hypothesized that faster and better healing can be achieved with rate-varying distractions, as compared with constant-rate distractions. A computational model incorporating the viscoelastic behaviors of the callus tissues and the mechano-regulatory tissue differentiation laws was developed and validated to predict the bone regeneration process during DO. The effect of rate-varying distraction on the healing outcomes (bony bridging time and bone formation) was examined. Compared to the constant low-rate distraction, a low-to-high rate-varying distraction provided a favorable mechanical environment for angiogenesis and bone tissue differentiation, throughout the distraction and consolidation phase, leading to an improved healing outcome with a shortened healing time. These results suggest that a rate-varying clinical strategy could reduce the overall treatment time of DO and decrease the risk of complications related to the external fixator.

Mechano-active biomaterials for tissue repair and regeneration

There is a lack of effective tissue repair and regeneration strategies in current clinical practices. Numerous studies have suggested that smart or responsive biomaterials possessing the ability to respond to endogenous stimuli in vivo may positively mediate the tissue micro-environment towards a tissue repair or regeneration. Mechanical stimuli, which constantly exist in a wide range of biological systems and are involved in almost all the physiological processes, belong to such stimuli to which responsive biomaterials can respond. In recent studies, a new type of smart biomaterials, which can dynamically adapt to the mechanical stimuli in vivo and thus has specific functionality consistently mediated by such mechanical stimuli, has emerged. In contrast to common biomaterials that passively react to the mechanical environment of an implantation site, such mechano-active biomaterials have enabled various active or automatic strategies for tissue repair or regeneration, such as providing precise spatial-temporal controls on delivery of drugs or cells in the organs of the musculoskeletal and the circulatory systems; in situ reconstructing the original or a favorable mechanical environment at a lesion site; and accelerating the tissue remodeling or healing process via a mechanobiological effect. This article elucidates a perspective of perfecting tissue repair or regeneration using mechano-active biomaterials, especially highlighting the rationale behind the concept of mechano-active biomaterials and their potential in the repair or regeneration of musculoskeletal and cardiovascular tissues. Albeit outstanding challenges and unknowns, the emergence of mechano-active biomaterials has become a new avenue for tissue engineering and regenerative medicine.

Toward a cure for lumbar spinal stenosis: The potential of interspinous process decompression

There is a growing impetus to treat aging as a disease in the quest to significantly extend the human life span through cellular regeneration methods. This approach, while promising, overlooks the fact that the evolutionary adaptation to bipedalism puts the human body in a distinctively vulnerable biomechanical and functional position. Orthograde human posture places unusually-high axial compressive loads on the weight-bearing joints of the skeleton, resulting in arthritic deterioration with aging. The effects are particularly robust in the lumbar spine were age-related degeneration, most commonly lumbar spinal stenosis (LSS), is ubiquitous among the elderly. It is postulated that re-establishing a favorable mechanical environment via interventions that unload the affected spinal joint complex may mitigate and potentially reverse the structural damage that is the cardinal pathoanatomical feature of this disease. The hypothesis of this paper is that a minimally-invasive surgical procedure, interspinous process decompression (IPD), which utilizes a stand-alone intervertebral spacer, effectively unloads the diseased spinal motion segment providing a healthy micro-environment to reverse and repair age-related and genetic deterioration of the spinal motion segment. Several lines of supporting evidence are provided from long-term follow-up results of a randomized controlled trial of IPD safety and effectiveness of the Superion® device including clinical outcomes, reoperation rates, opioid analgesic usage and advanced imaging utilization. All of these outcomes show uniquely-favorable trends with time that imply that the benefits of IPD are structural. The compendium of evidence suggests that IPD offers both a durable palliative effect due to direct blocking of back extension and a disease-modifying effect due to unloading of the spinal joint complex.

Impaired bone formation in ovariectomized mice reduces implant integration as indicated by longitudinal in vivo micro-computed tomography.

Although osteoporotic bone, with low bone mass and deteriorated bone architecture, provides a less favorable mechanical environment than healthy bone for implant fixation, there is no general agreement on the impact of osteoporosis on peri-implant bone (re)modeling, which is ultimately responsible for the long term stability of the bone-implant system. Here, we inserted an implant in a mouse model mimicking estrogen deficiency-induced bone loss and we monitored with longitudinal in vivo micro-computed tomography the spatio-temporal changes in bone (re)modeling and architecture, considering the separate contributions of trabecular, endocortical and periosteal surfaces. Specifically, 12 week-old C57BL/6J mice underwent OVX/SHM surgery; 9 weeks after we inserted special metal-ceramics implants into the 6th caudal vertebra and we measured bone response with in vivo micro-CT weekly for the following 6 weeks. Our results indicated that ovariectomized mice showed a reduced ability to increase the thickness of the cortical shell close to the implant because of impaired peri-implant bone formation, especially at the periosteal surface. Moreover, we observed that healthy mice had a significantly higher loss of trabecular bone far from the implant than estrogen depleted animals. Such behavior suggests that, in healthy mice, the substantial increase in peri-implant bone formation which rapidly thickened the cortex to secure the implant may raise bone resorption elsewhere and, specifically, in the trabecular network of the same bone but far from the implant. Considering the already deteriorated bone structure of estrogen depleted mice, further bone loss seemed to be hindered. The obtained knowledge on the dynamic response of diseased bone following implant insertion should provide useful guidelines to develop advanced treatments for osteoporotic fracture fixation based on local and selective manipulation of bone turnover in the peri-implant region.

Are Geometrical and Structural Variations Along the Length of the Aorta Governed by a Principle of “Optimal Mechanical Operation”?

It is well-documented that the geometrical dimensions, the longitudinal stretch ratio in situ, certain structural mechanical descriptors such as compliance and pressure-diameter moduli, as well as the mass fractions of structural constituents, vary along the length of the descending aorta. The origins of and possible interrelations among these observed variations remain open questions. The central premise of this study is that having considered the variation of the deformed inner diameter, axial stretch ratio, and area compliance along the aorta to be governed by the systemic requirements for flow distribution and reduction of cardiac preload, the zero-stress state geometry and mass fractions of the basic structural constituents of aortic tissue meet a principle of optimal mechanical operation. The principle manifests as a uniform distribution of the circumferential stress in the aortic wall that ensures effective bearing of the physiological load and a favorable mechanical environment for mechanosensitive vascular smooth muscle cells. A mathematical model is proposed and inverse boundary value problems are solved for the equations that follow from finite elasticity, structure-based constitutive modeling within constrained mixture theory, and stress-induced control of aortic homeostasis, mediated by the synthetic activity of vascular smooth muscle cells. Published experimental data are used to illustrate the predictive power of the proposed model. The results obtained are in agreement with published experimental data and support the proposed principle of optimal mechanical operation for the descending aorta.

Nonarthroplasty Hip Surgery for Early Osteoarthritis

The most favorable mechanical environment for the hip is one that is free of both instability and impingement, creating a concentric articulation with optimum femoral head coverage by the acetabulum. Anatomic variations such as acetabular dysplasia with associated instability, and femoroacetabular impingement with abnormal constraint, will lead to abnormal joint mechanics, articular damage, and osteoarthritis. Surgical techniques such as periacetabular osteotomies, and femoral and acetabular osteoplasties enable correction of anatomic variations that cause mechanical damage to the hip joint, thereby potentially preventing or delaying development of osteoarthritis and subsequent need for joint replacement.

Biomechanical and histological evaluation of the application of biodegradable poly- l-lactic cushion to the plate internal fixation for bone fracture healing

Background Internal plate fixation of fracture can provide favorable mechanical environment for fracture fragments. However, osteoporosis under the plate is often found, and refracture may occur after the plate is removed. There are two different opinions about the bone loss beneath the plate: first is the stress shielding effect brought by the rigid fixation, and the second is the insufficient blood supply of the bone caused by the placement of the plate. In this paper, we tried to achieve a favorable condition for the fracture healing by inserting a kind of biodegradable cushion, through which the stress shielding effect and the interruption of the bone blood supply could be relieved at the same time. Methods Animal models of internal fixation for tibia diaphyseal fracture with the placement of the poly- l-lactic (PLLA) cushion between the plate and the bone were established; a series of in vitro investigations and finite element (FE) analysis were performed to evaluate the effect of this new internal fixation system. Findings During both the initial and 50% healing periods, the extent of stress shielding of the fracture zone decreased due to the use of the PLLA cushion. Especially for the 50% healing stage, the insertion of the PLLA cushion on alleviating the stress shielding of the bone tissue between the inner two screws directly under the plate is more apparent than that at the initial healing period. Meanwhile, radiological and histological coloration results demonstrated sooner callus growth and better trabecular rearrangement of the fracture zone in the PLLA group with the degradation of the PLLA cushion during the healing periods. Interpretations This study showed that the use of the PLLA cushion at the initial period did not impair the stability of the whole system, which provides a favorable mechanical environment for the following fracture healing. On the other hand, its superiorities on alleviating stress shielding effect and interruption with the blood supply of the bone tissue beneath the plate contributed to the favorable fracture healing conditions in PLLA group with the degradation of the materials.

The physiology and degeneration of joints

There is a growing impetus to treat aging as a disease in the quest to significantly extend the human life span through cellular regeneration methods. This approach, while promising, overlooks the fact that the evolutionary adaptation to bipedalism puts the human body in a distinctively vulnerable biomechanical and functional position. Orthograde human posture places unusually-high axial compressive loads on the weight-bearing joints of the skeleton, resulting in arthritic deterioration with aging. The effects are particularly robust in the lumbar spine were age-related degeneration, most commonly lumbar spinal stenosis (LSS), is ubiquitous among the elderly. It is postulated that re-establishing a favorable mechanical environment via interventions that unload the affected spinal joint complex may mitigate and potentially reverse the structural damage that is the cardinal pathoanatomical feature of this disease. The hypothesis of this paper is that a minimally-invasive surgical procedure, interspinous process decompression (IPD), which utilizes a stand-alone intervertebral spacer, effectively unloads the diseased spinal motion segment providing a healthy micro-environment to reverse and repair age-related and genetic deterioration of the spinal motion segment. Several lines of supporting evidence are provided from long-term follow-up results of a randomized controlled trial of IPD safety and effectiveness of the Superion® device including clinical outcomes, reoperation rates, opioid analgesic usage and advanced imaging utilization. All of these outcomes show uniquely-favorable trends with time that imply that the benefits of IPD are structural. The compendium of evidence suggests that IPD offers both a durable palliative effect due to direct blocking of back extension and a disease-modifying effect due to unloading of the spinal joint complex.