The Biomechanical Effects of Different Bag-Carrying Styles on Lumbar Spine and Paraspinal Muscles: A Combined Musculoskeletal and Finite Element Study

Walking Slope and Heavy Backpacks Affect Peak and Impulsive Lumbar Joint Contact Forces

Heavy load carriage is associated with musculoskeletal overuse injury, particularly in the lumbar spine. In addition, steep walking slopes and heavy backpacks separately require adaptation of torso kinematics, but the combined effect of sloped walking and heavy backpack loads on lumbar joint contact forces is unclear. Backpacks with hip belt attachments can reduce pressure under the shoulder straps; however, it is unknown if wearing a hip belt reduces lumbar spine forces. We used a musculoskeletal modeling and simulation approach to quantify peak and impulsive L1L2 and L4L5 lumbar joint contact forces in the anterior/posterior shear and compressive directions during walking on 0 deg and ±10 deg slopes, with no backpack and with 40% body weight backpack load using two different backpack configurations (hip belt assisted and shoulder-borne). Both walking slope and backpack load significantly affected shear and compressive peak and impulsive forces. The largest peak shear and compressive forces of 1.57 and 5.23 body weights, respectively, exceed recommended limits and were observed during uphill walking with shoulder-borne loads. However, only impulsive force results revealed differences due to the backpack configuration, and this effect depended on walking slope. During downhill walking only, the hip belt-assisted configuration resulted compressive impulses lower than during shoulder borne by 0.25 body weight seconds for both L1L2 and L4L5. These results indicate that walking uphill with heavy loads causes high shear and compressive lumbar forces that may increase overuse injury risk. In addition, our results suggest it is especially important to wear a hip belt when walking downhill.

Biomechanical Effects of Multi-segment Fixation on Lumbar Spine and Sacroiliac Joints: A Finite Element Analysis

OBJECTIVE

Spine fixation surgery affects the biomechanical environment in the sacroiliac joint (SIJ), which may lead to the SIJ pain or degeneration after surgery. The purpose of this study is to determine the impact of the number and position of fixed segments on the SIJs and provide references for surgeons to plan fixation levels and enhance surgical outcomes.

METHODS

The intact lumbar-pelvis finite element (FE) models and 11 fixation FE models with different number and position of fixed segments were developed based on CT images. A 400N follower load and 10° range of motion (ROM) of the spine were applied to the superior endplate of L1 to simulate the flexion, extension, bending and torsion motion after surgery. The peak stress on the SIJs, lumbar intervertebral discs, screws and rods were calculated to evaluate the biomechanical effects of fixation procedures.

RESULTS

With the lowermost instrumented vertebra (LIV) of L5 or S1, the peak stress on SIJs increased with the number of fixed segments increasing. The flexion motion led to the greater von Mises stress on SIJ compared with other load conditions. Compared with the intact model, peak stress on all fixed intervertebral discs was reduced in the models with less than three fixed segments, and it increased in the models with more than three fixed segments. The stress on the SIJ was extremely high in the models with all segments from L1 to L5 fixed, including L1-L5, L1-S1 and L1-S2 fixation models. The stress on the segment adjacent to the fixed segments was significant higher compared to that in the intact model. The peak stress on rods and screws also increased with the number of fixed segments increasing in the flexion, extension and bending motion, and the bending and flexion motions led to the greater von Mises stress on SIJs.

CONCLUSION

Short-term fixation (≤2 segments) did not increase the stress on the SIJs significantly, while long-term segment fixation (≥4 segments) led to greater stress on the SIJs especially when all the L1-L5 segments were fixed. Unfixed lumbar segments compensated the ROM loss of the fixed segments, and the preservation of lumbar spine mobility would reduce the risks of SIJ degeneration.

Finite Element Modeling of the Human Wrist: A Review

Background  Understanding wrist biomechanics is important to appreciate and treat the wrist joint. Numerical methods, specifically, finite element method (FEM), have been used to overcome experimental methods' limitations. Due to the complexity of the wrist and difficulty in modeling, there is heterogeneity and lack of consistent methodology in the published studies, challenging our ability to incorporate information gleaned from the various studies. Questions/Purposes  This study summarizes the use of FEM to study the wrist in the last decade. Methods  We included studies published from 2012 to 2022 from databases: EBSCO, Research4Life, ScienceDirect, and Scopus. Twenty-two studies were included. Results  FEM used to study wrist in general, pathology, and treatment include diverse topics and are difficult to compare directly. Most studies evaluate normal wrist mechanics, all modeling the bones, with fewer studies including cartilage and ligamentous structures in the model. The dynamic effect of the tendons on wrist mechanics is rarely accounted for. Conclusion  Due to the complexity of wrist mechanics, the current literature remains incomplete. Considering published strategies and modeling techniques may aid in the development of more comprehensive and improved wrist model fidelity.

Asymmetric Load Transmission Induces Facet Joint Subchondral Sclerosis and Hypertrophy in Patients with Idiopathic Adolescent Scoliosis: Evaluation Using Finite Element Model and Surgical Specimen

Adolescent idiopathic scoliosis (AIS) with thoracic curvature primarily progresses from the thoracolumbar region, causing abnormal twisting and rotation of the spinal column. This results in unbalanced, asymmetric loads on each vertebrae and increased demands on the thoracic facet joints to withstand rotational stress from adjacent vertebrae. However, no studies have focused on the stress distribution on the facet joints of the thoracic spine in patients with AIS. This study aimed to investigate the mechanical loading and its distribution on the thoracic facet joints of AIS patients using finite element (FE) analysis and surgical specimens. FE models of the thoracic spine were created from a total of 13 female AIS patients (Lenke type 1, n = 4; Lenke type 2, n = 4; Lenke type 3, n = 5). A load of 200 N on the T3 vertebrae and 30 N each on the bilateral superior articular processes were applied vertically to quantify the contact force on the facet joints from T3 to T11. In addition, morphological and histological analyses were performed on the inferior articular processes obtained during surgery. FE analysis demonstrated that contact forces of the facet joint progressively increased from the mid to lower thoracic spine of the concave side, reaching a maximum around the apex. More than 91% of the load was transmitted by the facet joints at the concave side, resulting in facet joint subchondral sclerosis and hypertrophy. The apical facet joint in AIS helps counteract rotational stress between vertebrae and transfers most stress through the concave side. In conclusion, this study found that asymmetric load transfer in the facet joints leads to subchondral sclerosis and hypertrophy. These findings can enhance our understanding of the stress loading on facet joints and the resulting biological changes and help clarify the mechanisms involved in scoliosis progression. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.