Sensitivity of the Cervical Disc Loads, Translations, Intradiscal Pressure, and Muscle Activity Due to Segmental Mass, Disc Stiffness, and Muscle Strength in an Upright Neutral Posture

Characterizing the Baseline Regional Biphasic Mechanical Properties of Cervical Intervertebral Discs

PURPOSE

While the regional viscoelastic biomechanical properties of lumbar intervertebral disc tissues are well documented, equivalent tissue-level characterizations for human cervical discs remain unexplored. This study aimed to quantify biphasic mechanical properties of the nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP) in cervical discs.

METHODS

A previously established confined compression testing technique was used to measure swelling pressure, equilibrium aggregate modulus, and hydraulic permeability in cervical NP, AF, and CEP tissues. Specimen-specific porosity was also assessed and correlated with these properties. A finite element model was used to simulate unconfined compression.

RESULTS

Swelling pressure (154.50 ± 89.47 kPa) and aggregate modulus (0.677 ± 0.671 MPa) were significantly higher in the CEP compared to the NP (p = 0.0308 and p = 0.0227, respectively) or AF (p = 0.0338 for aggregate modulus), with no significant differences observed between NP and AF. Permeability did not differ significantly among regions. Porosity showed negative correlations with both swelling pressure (r = - 0.55, p = 0.0006) and aggregate modulus (r = - 0.53, p = 0.001). Finite element analysis revealed a relatively uniform von Mises stress distribution between NP and AF, with higher magnitudes concentrated in the CEP.

CONCLUSION

Cervical NP and AF exhibit relatively homogeneous biomechanical properties, whereas the CEP is found to have greater stiffness and swelling pressure. These findings indicate unique tissue-level adaptations in cervical discs to support greater mobility. These data could also inform future studies investigating region-specific degeneration and aging effects on cartilaginous tissue function in cervical discs and enhance the representation of viscoelasticity in computational modeling of the IVD.

Finite element analysis of neck sports injury based on a whole cervical spine model with muscles

Cervical atlantoaxial subluxation injuries, often resulting from high-intensity external forces or improper posture during high-speed, rotational sports, pose significant risks to athletes' health and careers. This study aims to investigate the biomechanical effects of atlantoaxial subluxation on the cervical spine. Models representing Model 1 (healthy bone model), Model 2 (healthy muscle model), and atlantoaxial subluxation diseased model were developed using CT and MRI data. A 30 N gravitational force and a 1.5 Nm torque were applied to the C0 node. The study simulated changes in range of motion (ROM), disc stress, and muscle stress under six motion states-flexion-extension, lateral flexion, and axial rotation-to evaluate the post-injury movement limitations of the cervical spine. The validity and consistency of this study with cadaver data from the literature were verified through range of motion (ROM) comparison and Bland-Altman analysis. Compared to the healthy model, the diseased model showed a reduction in ROM, with a 10°-30° decrease in C0-C1 ROM across all six movements. The distribution of stress shifted from the bones to the damaged atlantoaxial joint and muscles, while the stress on the intervertebral discs decreased. This study, through the establishment of a finite element model of the cervical spine, reveals the biomechanical effects of atlantoaxial subluxation on the cervical spine, including reduced ROM, altered stress distribution, and increased muscle load. The findings provide a theoretical basis for the prevention of sports injuries, the development of rehabilitation programs, and personalized treatments, emphasizing the importance of muscle recovery and proper management of movement loads. Future work will further validate and expand the application by integrating clinical data.

Restoration of physiologic loading after engineered disc implantation mitigates immobilization-induced facet joint and paraspinal muscle degeneration

Intervertebral disc degeneration is commonly associated with back and neck pain, and standard surgical treatments do not restore spine function. Replacement of the degenerative disc with a living, tissue-engineered construct has the potential to restore normal structure and function to the spine. Toward this goal, our group developed endplate-modified disc-like angle-ply structures (eDAPS) that recapitulate the native structure and function of the disc. While our initial large animal studies utilized rigid internal fixation of the eDAPS implanted level to ensure retention of the eDAPS, chronic immobilization does not restore full function and is detrimental to the spinal motion segment. The purpose of this study was to utilize a goat cervical disc replacement model coupled with finite element modeling of goat cervical motion segments to investigate the effects of remobilization (removal of fixation) on the eDAPS, the facet joints and the adjacent paraspinal muscle. Our results demonstrated that chronic immobilization caused notable degeneration of the facet joints and paraspinal muscles adjacent to eDAPS implants. Remobilization improved eDAPS composition and integration and mitigated, but did not fully reverse, facet joint osteoarthritis and paraspinal muscle atrophy and fibrosis. Finite element modeling revealed that these changes were likely due to reduced range of motion and reduced facet loading, highlighting the importance of maintaining normal spine biomechanical function with any tissue engineered disc replacement. STATEMENT OF SIGNIFICANCE: Back and neck pain are ubiquitous in modern society, and the gold standard surgical treatment of spinal fusion limits patient function. This study advances our understanding of the response of the spinal motion segment to tissue engineered disc replacement with provisional fixation in a large animal model, further advancing the clinical translation of this technology.

Human head-neck model and its application thresholds: a narrative review

There have been many studies on human head-neck biomechanical models in the last two decades, and the associated modelling techniques were constantly evolving at the same time. Computational approaches have been widely leveraged, in parallel to conventional physical tests, to investigate biomechanics and injuries of the head-neck system in fields like the automotive industry, orthopedic, sports medicine, etc. The purpose of this manuscript is to provide a global review of the existing knowledge related to the modelling approaches, structural and biomechanical characteristics, validation, and application of the present head-neck models. This endeavor aims to support further enhancements and validations in modelling practices, particularly addressing the lack of data for model validation, as well as to prospect future advances in terms of the topics. Seventy-four models subject to the proposed selection criteria are considered. Based on previously established and validated head-neck computational models, most of the studies performed in-depth investigations of included cases, which revolved around four specific subjects: physiopathology, treatment evaluation, collision condition, and sports injury. Through the review of the recent 20 years of research, the summarized modelling information indicated existing deficiencies and future research topics, as well as provided references for subsequent head-neck model development and application.

Sensitivity of Thoracolumbar Spine Musculoskeletal Model Loading in Neutral Standing and Forward Flexion Static Postures to Thoracic Disc Stiffness

This study aimed to quantify the sensitivity of a thoracolumbar musculoskeletal model with a flexible thoracic spine and articulated ribcage to disc flexural stiffness variation inherent from in-vitro cadaveric data. The model was personalized to a normal weight subject, whose upper body segmental masses and centers of mass were computed using a body-shape-based approach. Joint flexural stiffness curves were defined based on in-vitro flexion-extension moment-rotation data from several specimens taken at various thoracic levels, with stiffness variation reaching 1.2 Nm/deg. Neutral standing and forward flexion postures were simulated using in-vivo measured spinal rhythm. The finding revealed negligeable sensitivity of joint reaction forces, less than 2.5% Body Weight (BW), to flexural stiffness in neutral standing posture. The sensitivity became more pronounced, especially at mid-level thoracic joints, with deviations reaching up to 14% BW and 26% BW for antero-posterior shear and compressive forces, respectively, in 60-degree forward flexion. Very low Root Mean Square Error (RMSE) and normalized RMSE values, calculated using intradiscal pressure based compressive forces, indicated no effects of flexural disc stiffness variation on model prediction validation. The findings underscored the importance of cautious consideration when utilizing flexural stiffness from a single cadaveric specimen.

Quantifying the Importance of Active Muscle Repositioning a Finite Element Neck Model in Flexion Using Kinematic, Kinetic, and Tissue-Level Responses

PURPOSE

Non-neutral neck positions are important initial conditions in impact scenarios, associated with a higher incidence of injury. Repositioning in finite element (FE) neck models is often achieved by applying external boundary conditions (BCs) to the head while constraining the first thoracic vertebra (T1). However, in vivo, neck muscles contract to achieve a desired head and neck position generating initial loads and deformations in the tissues. In the present study, a new muscle-based repositioning method was compared to traditional applied BCs using a contemporary FE neck model for forward head flexion of 30°.

METHODS

For the BC method, an external moment (2.6 Nm) was applied to the head with T1 fixed, while for the muscle-based method, the flexors and extensors were co-contracted under gravity loading to achieve the target flexion.

RESULTS

The kinematic response from muscle contraction was within 10% of the in vivo experimental data, while the BC method differed by 18%. The intervertebral disc forces from muscle contraction were agreeable with the literature (167 N compression, 12 N shear), while the BC methodology underpredicted the disc forces owing to the lack of spine compression. Correspondingly, the strains in the annulus fibrosus increased by an average of 60% across all levels due to muscle contraction compared to BC method.

CONCLUSION

The muscle repositioning method enhanced the kinetic response and subsequently led to differences in tissue-level responses compared to the conventional BC method. The improved kinematics and kinetics quantify the importance of repositioning FE neck models using active muscles to achieve non-neutral neck positions.

Rotation-traction manipulation induced intradiskal pressure changes in cervical spine-an in vitro study

Objective: Evaluate the effect of rotation-traction manipulation on intradiskal pressure in human cervical spine specimen with different force and duration parameters, and compare the intradiskal pressure changes between rotation-traction manipulation and traction. Methods: Seven human cervical spine specimens were included in this study. The intradiskal pressure was measured by miniature pressure sensor implanting in the nucleus pulposus. rotation-traction manipulation and cervical spine traction were simulated using the MTS biomechanical machine. Varied thrust forces (50N, 150N, and 250N) and durations (0.05 s, 0.1 s, and 0.15 s) were applied during rotation-traction manipulation with Intradiscal pressure recorded in the neutral position, rotation-anteflexion position, preloading, and thrusting phases. Futuremore, we documented changes in intradiscal pressure during cervical spine traction with different loading forces (50N, 150N, and 250N). And a comparative analysis was performed to discern the impact on intradiscal pressure between manipulation and traction. Results: Manipulation application induced a significant reduction in intradiscal pressure during preloading and thrusting phases for each cervical intervertebral disc (p < 0.05). When adjusting thrust parameters, a discernible decrease in intradiscal pressure was observed with increasing thrust force, and the variations between different thrust forces were statistically significant (p < 0.05). Conversely, changes in duration did not yield a significant impact on intradiscal pressure (p > 0.05). Additionally, after traction with varying loading forces (50N, 150N, 250N), a noteworthy decrease in intradiscal pressure was observed (p < 0.05). And a comparative analysis revealed that rotation-traction manipulation more markedly reduced intradiscal pressure compared to traction alone (p < 0.05). Conclusion: Both rotation-traction manipulation and cervical spine traction can reduce intradiscal pressure, exhibiting a positive correlation with force. Notably, manipulation elicits more pronounced and immediate decompression effect, contributing a potential biomechanical rationale for its therapeutic efficacy.

Tension-activated nanofiber patches delivering an anti-inflammatory drug improve repair in a goat intervertebral disc herniation model

Conventional microdiscectomy treatment for intervertebral disc herniation alleviates pain but does not repair the annulus fibrosus, resulting in a high incidence of recurrent herniation and persistent dysfunction. The lack of repair and the acute inflammation that arise after injury can further compromise the disc and result in disc-wide degeneration in the long term. To address this clinical need, we developed tension-activated repair patches (TARPs) for annulus fibrosus repair and local delivery of the anti-inflammatory factor anakinra (a recombinant interleukin-1 receptor antagonist). TARPs transmit physiologic strain to mechanically activated microcapsules embedded within the patch, which release encapsulated bioactive molecules in direct response to spinal loading. Mechanically activated microcapsules carrying anakinra were loaded into TARPs, and the effects of TARP-mediated annular repair and anakinra delivery were evaluated in a goat model of annular injury in the cervical spine. TARPs integrated with native tissue and provided structural reinforcement at the injury site that prevented aberrant disc-wide remodeling resulting from detensioning of the annular fibrosus. The delivery of anakinra by TARP implantation increased matrix deposition and retention at the injury site and improved maintenance of disc extracellular matrix. Anakinra delivery additionally attenuated the inflammatory response associated with TARP implantation, decreasing osteolysis in adjacent vertebrae and preserving disc cellularity and matrix organization throughout the annulus fibrosus. These results demonstrate the therapeutic potential of TARPs for the treatment of intervertebral disc herniation.

Relationship between Intervertebral Disc Compression Force and Sagittal Spinopelvic Lower Limb Alignment in Elderly Women in Standing Position with Patient-Specific Whole Body Musculoskeletal Model

The intervertebral disc loading based on compensated standing posture in patients with adult spinal deformity remains unclear. We analyzed the relationship between sagittal alignment and disc compression force (Fm). In 14 elderly women, the alignment of the sagittal spinopelvic and lower extremities was measured. Fm was calculated using the Anybody Modeling System. Patients were divided into low sagittal vertical axis (SVA) and high SVA groups. Comparisons between the two groups were performed and the relationship between the Fm and each parameter was examined using Spearman's correlation coefficient (r). The mean lumbar Fm in the high SVA group was 67.6%; significantly higher than that in the low SVA group (p = 0.046). There was a negative correlation between cervical Fm with T1 slope (r = -0.589, p = 0.034) and lumbar Fm with lumbar lordosis (r = -0.566, p = 0.035). Lumbar Fm was positively correlated with center of gravity-SVA (r = 0.615, p = 0.029), T1 slope (r = 0.613, p = 0.026), and SVA (r = 0.612, p = 0.020). The results suggested sagittal malalignment increased the load on the thoracolumbar and lower lumbar discs and was associated with cervical disc loading.

Temporomandibular Joint Disk Displacements in Class II Malocclusion and Cervical Spine Alterations: Systematic Review and Report of a Hypodivergent Case with MRI Bone and Soft Tissue Changes

(1) Background: This study aimed to perform a literature review related to disk displacement (DD) in class II malocclusion or cervical vertebrae position alterations and to report a hypodivergent case with cervical pain and right anterolateral DD with reduction, left anterolateral DD with reduction, and left joint effusion. (2) Methods: A structured electronic search was conducted between March 2022 and April 2022, without time limits, following PRISMA guidelines, in the following databases: PubMed, Scopus, Embase and Cochrane; the terms “disc displacement”, “disk displacement”, “temporomandibular joint”, “class II malocclusion” and “cervical vertebrae” are searched. (3) Results: the following thirteen publications are included in this review: two prospective studies and eleven cross-sectional studies; for evaluating disk position, eight included publications used magnetic resonance imaging (MRI), whilst six studies used lateral cephalogram to determine craniofacial morphology and relationships between the cranial base, vertical skeletal pattern, maxilla and mandible. (4) Conclusions: although the literature still shows contradictory opinions, a relationship between temporomandibular disorders and cervical posture has been shown in the presented case as well as in the literature review.