Differentiation of stem cells, a crucial step in the process of tissue development, repair and regeneration, can be regulated by a variety of mechanical factors such as the stiffness of extracellular matrix. In this review article, the effects of stiffness on the differentiation of stem cells, including bone marrow-derived stem cells, adipose-derived stem cells and neural stem cells, are briefly summarized. Compared to two-dimensional (2D) surfaces, three-dimensional (3D) hydrogel systems better resemble the native environment in the body. Hence, the studies which explore the effects of stiffness on stem cell differentiation in 3D environments are specifically introduced. Integrin is a well-known transmembrane molecule, which plays an important role in the mechanotransduction process. In this review, several integrin-associated signaling molecules, including caveolin, piezo and Yes-associated protein (YAP), are also introduced. In addition, as stiffness-mediated cell differentiation may be affected by other factors, the combined effects of matrix stiffness and viscoelasticity, surface topography, chemical composition, and external mechanical stimuli on cell differentiation are also summarized.

Tissue engineering of the annulus fibrosus (AF) shows promise as a treatment for patients with degenerative disc disease (DDD). However, it remains challenging due to the intrinsic heterogeneity of AF tissue. Fabrication of scaffolds recapitulating the specific cellular, componential, and microstructural features of AF, therefore, is critical to successful AF tissue regeneration. Poly-L-lactic acid (PLLA) fibrous scaffolds with various fiber diameters and orientation were prepared to mimic the microstructural characteristics of AF tissue using electrospinning technique. AF-derived stem cells (AFSCs) were cultured on the PLLA fibrous scaffolds for 7 days. The morphology of AFSCs significantly varied when cultured on the scaffolds with various fiber diameters and orientation. AFSCs were nearly round on scaffolds with small fibers. However, they became spindle-shaped on scaffolds with large fibers. Meanwhile, upregulated expression of collagen-I gene happened in cells cultured on scaffolds with large fibers, while enhanced expression of collagen-II and aggrecan genes was seen on scaffolds with small fibers. The production of related proteins also showed similar trends. Further, culturing AFSCs on a heterogeneous scaffold by overlaying membranes with different fiber sizes led to the formation of a hierarchical structure approximating native AF tissue. Findings from this study demonstrate that fibrous scaffolds with different fiber sizes effectively promoted the differentiation of AFSCs into specific cells similar to the types of cells at various AF zones. It also provides a valuable reference for regulation of cell differentiation and fabrication of engineered tissues with complex hierarchical structures using the physical cues of scaffolds. Effective AF repair is an essential need for treating degenerative disc disease. Tissue engineering is a promising approach to achieving tissue regeneration and restoring normal functions of tissues. By mimicking the key structural features of native AF tissue, including fiber size and alignment, this study deciphered the effect of scaffold materials on the cell differentiation and extracellular matrix deposition, which provides a solid basis for designing new strategies toward more effective AF repair and regeneration.

Objective: The aim of this study was to verify the biomechanical properties of a newly designed angulated lateral plate (mini-LP) suited for two-level oblique lumbar interbody fusion (OLIF). The mini-LP is placed through the lateral ante-psoas surgical corridor, which reduces the operative time and complications associated with prolonged anesthesia and placement in the prone position.Methods: A three-dimensional nonlinear finite element (FE) model of an intact L1–L5 lumbar spine was constructed and validated. The intact model was modified to generate a two-level OLIF surgery model augmented with three types of lateral fixation (stand-alone, SA; lateral rod screw, LRS; miniature lateral plate, mini-LP); the operative segments were L2–L3 and L3–L4. By applying a 500 N follower load and 7.5 Nm directional moment (flexion-extension, lateral bending, and axial rotation), all models were used to simulate human spine movement. Then, we extracted the range of motion (ROM), peak contact force of the bony endplate (PCFBE), peak equivalent stress of the cage (PESC), peak equivalent stress of fixation (PESF), and stress contour plots.Results: When compared with the intact model, the SA model achieved the least reduction in ROM to surgical segments in all motions. The ROM of the mini-LP model was slightly smaller than that of the LRS model. There were no significant differences in surgical segments (L1–L2, L4–L5) between all surgical models and the intact model. The PCFBE and PESC of the LRS and the mini-LP fixation models were lower than those of the SA model. However, the differences in PCFBE or PESC between the LRS- and mini-LP-based models were not significant. The fixation stress of the LRS- and mini-LP-based models was significantly lower than the yield strength under all loading conditions. In addition, the variances in the PESF in the LRS- and mini-LP-based models were not obvious.Conclusion: Our biomechanical FE analysis indicated that LRS or mini-LP fixation can both provide adequate biomechanical stability for two-level OLIF through a single incision. The newly designed mini-LP model seemed to be superior in installation convenience, and equally good outcomes were achieved with both LRS and mini-LP for two-level OLIF.

Study Design: Retrospective study. Objectives: To demonstrate that repeat Percutaneous vertebroplasty (PVP) performed for the same cemented vertebrae in Kümmell’s disease can offer therapeutic benefit for patients with recurrent symptoms after initial percutaneous kyphoplasty (PKP) treatment. Methods: From January 2019 to December 2021, we investigated 2932 patients with PKP. Among them, 191 patients were diagnosed Kümmell’s disease. 33 patients upon presentation of recurrent symptoms underwent repeat PVP procedure. Radiologic outcomes and clinic indices were investigated. Results: Bone cement reperfusion surgery was successfully completed in 33 patients. The average age was 73.5 ± 8.2 years old. The kyphosis angle showed significant correction from pre-operation to the final follow-up, descending from pre-operation (20.6 ± 11.1°) to final follow-up (15.4 ± 7.9°). The vertebral heights at different follow-up appointments were significantly higher than the pre-operative appointments. The VAS and ODI scores at final follow-up were respectively 1.2 ± .8 and 27.3 ± 5.4%, which were both significantly lower than those before operation. No complications such as cement leakage into the spinal canal or cement displacement occurred during follow-up. Conclusions: Bone cement reperfusion surgery can ameliorate kyphosis and restore vertebral height to some extent. Repeat PVP is a minimally invasive surgery that provides superior long-term results in clinical and radiological outcomes but is technically more difficult to perform.