Although composed of two words,tissue and engineering,the term tis- sue-engineering is an integrated research field which involves living cells,extra-cellular matrix, biomaterial and biomechanics etc.Cell biomechanics includes a series of research on the material properties,signal transduction,gene and protein expression after mechanical stimulation in cells. The ultimate purpose of tissue engineering is to build up tissues or organs in vitro that can replace corresponding human tissues and organs in vivo and work successfully.We hope this technology can help us resolve problems related to organic transplantation,such as rejection and unavailability of human organs in clinic.This special issue featuring tissue-engineering and cell biomechanics col- lected six papers from mainland China and four papers from Taiwan,which deal with application of stem cells,improvement of scaffolds,tissue-engineered cartilage and response of cells after force stimulation.We hope they can help readers realize the role and application of engineering in biology and clinical research.We also hope investigators in different fields can collaborate with each other.

Heartening advances continue to be made in numerous research areas of orthopedic biomechanics. Recently, orthopedic research has been greatly expanded, ranging from the cellular level to the whole body level. One of the most important research focuses is a probe into the relationship between biology and mechanics. Emerging information about genetic basis of bone and cartilage pathology is used to provide insight into the fundamental mechanism of common conditions, such as osteoporosis and degenerative arthritis. Other ardent areas of investigation include tissue engineering, biomaterials, tendon healing, and biomechanical properties of tissue at the microscopic level. Interdisciplinary research currently affords a partly unrealized potential to relate the mechanical environment to biological responsiveness. New approaches include iterative computational modeling allowing model parameters at the microscopic level, micro- mechanics, and relatively long- term in vivo manipulation of the mechanical environment. The field of orthopedic biomechanics is widely researched and has led to improved surgical procedures in such areas as bone grafting, joint replacement, and limb supports.

Objective To investigate the effect of unidirectional stretching on mechanical properties of different absorbable patches and evaluate its potential as a patch for rotator cuff repair. Methods The unidirectional stretching process was used to prepare absorbable patches with different polylactide based materials. Different unidirectional stretching temperatures (50-80 ℃) and stretching ratios (0.5-4.3) were set. The effects of different parameters on mechanical properties of the absorbable patches with different materials were studied. Their thermal properties, crystallization and surface morphology were characterized. Results The unidirectional stretching temperature and stretching ratio could adjust the tensile strength and strain, thermal property, crystalization and surface morphology of the absorbable patch. At directional stretching temperatures of 60, 70, 70 ℃ and stretching ratios of 3, 3, 4.3, respectively, the absorbable patches made of poly-L-lactide-co-glycolide (PLGA), poly-L-co-D, L-lactide (PLDLLA) and poly-L-lactide-co-ε-caprolactone (PLC) had the maximum tensile strength (74±7)，(97±6), (107±8) MPa, which were larger than the tensile strength for infraspinatus tendon of canine (40 MPa). However, only the strain of PLDLLA patch conformed to the flexibility of natural rotator cuff. Conclusions The unidirectional stretching process can improve mechanical properties of the absorbable patch. The absorbable patch made of PLDLLA has the potential to reinforce the rotator cuff tear.

BACKGROUNDA large amount of biomechanical and clinical evidence from previous studies suggest the efficiency of the two different posterior lumber non-fusion methods, interspinous distraction device (ISDD) and facet screw fixation system (FSS), but the biomechanical comparison of ISDD and FSS has not been thoroughly clarified.

METHODSIn the current study, finite element methods were used to investigate the biomechanical comparison of ISDD and FSS. The range of motion (ROM), intradiscal pressure (IDP) and the protective effects gained by maintaining disc heights were evaluated.

RESULTSThe ROM was similar between the two non-fusion methods under static standing, flexion and lateral bending. The FSS appeared to be more effective in resisting extension. At the implanted level L3/4, FSS displayed better results for maintaining and increasing posterior disc heights. At the L4/5 level in extension and lateral bending, FSS was better than ISDD, with comparable results observed in other motions. Comparing the posterior and lateral disc heights, FSS appeared to be more effective than ISDD. FSS also had a minor effect on the inferior adjacent segment than ISDD. FSS was more effective in reducing IDP than ISDD in extension.

CONCLUSIONThrough the finite element analysis study, it can be seen that FSS demonstrates more beneficial biomechanical outcomes than does ISDD, such as being more effective in resisting extension, maintaining and increasing lumbar disc heights and reducing the inferior adjacent IDP in extension.

Objective To measure and investigate the anatomical characteristics of the proximal femur canal curves in patients with developmental dysplasia of the hip (DDH), and to provide references for the design of femoral stem of DDH patients. Methods Three-dimensional reconstructions of the proximal femur were conducted from the CT data, and the anatomical morphology characteristics were obtained from 64 patients with DDH (74 hips, 12 males, 52 females, average age of 45.3 years) and 30 healthy controls (30 hips, 21 males, 9 females, average age of 29.4 years). Multiple anteroposterior diameter and mediolateral diameter around the lesser trochanter at different heights and the fitting curves of different groups were obtained. Results The femoral neck anteversion angle, neck-shaft angle, height of the isthmus, canal flare index (CFI), femoral head diameter and femoral offset of DDH patients and healthy controls were 26.39°±14.74° and 15.68°±7.95°, 125.65°±5.73° and 129.19°±5.80°, (99.14±14.62) mm and (110.13±11.73) mm, 3.63±0.77 and 4.45±0.79, (44.01±5.75) mm and (47.26±3.94) mm, (31.80±3.82) mm and (36.42±4.84) mm, respectively. DDH group had the significantly greater femoral neck anteversion angle, and other anatomical morphology characteristics were significantly smaller than the control group. The obtained anteroposterior diameter, mediolateral diameter and the fitting curves showed that DDH group had narrow medullary cavity and forward shifting anteroposterior diameter. In addition, there was a statistical significance between the Crowe Ⅳ DDH and other types of DDH, and deformation degree of the femur increased with the increase of DDH type. Conclusions The proximal femur of DDH patients was significantly different from the healthy controls. Moreover, there were significantly morphological differences between different types of DDH. The research findings can be used as quantitative references for understanding the morphological feature of DDH patients and the corresponding design of femoral stem for DDH patients.

Objective Aiming at solving the problems of pain on the anteromedial tibia, tibial component loosening and osteoarthritis progression after unicompartmental knee arthroplasty (UKA), the influence of different geometric shapes of tibial component pegs on stress distributions in tibia was analyzed by finite element method. Methods The finite element models with UKA were established and validated. Geometric shapes of tibial component were designed. Under the same loading condition, the tibial components with double-peg, single-keel, double-keel and cross-star were studied for finite element analysis and compared with intact model, so as to evaluate the influence of tibial component with different shapes on stresses of cortical bone in anteromedial tibia, cancellous bone under tibial component and cartilage in contralateral tibia. Results Compared with the intact model, the peak stress of cortical bone in anteromedial tibia with double-peg, single-keel, double-keel and cross-star tibial components increased by 56.1%, 55.9%, 54.5% and 68-4%, respectively. The peak stress of cancellous bone under tibial component with single-keel and double-keels decreased by 8.1% and 15.6% respectively, while the peak stress of cancellous bone under tibial component with double-peg and cross-star increased by 67-9% and 121-5%, which were higher than the fatigue yield stress of cancellous bone. The peak stress of cartilage in contralateral tibia with double-peg, single-keel, double-keel and cross-star tibial components decreased by 42.1%, 26.6%, 24.2% and 28.5%, respectively. ConclusionsThe load distribution of the medial and lateral tibia changed after UKA operation, and a greater load was observed on the replacement side. Single-keel and double-keel tibial components were more effective in reducing stresses on cortical bone in anteromedial tibia and cancellous bone, while the stress distribution in tibia with single-keel tibial component was closer to that of the intact tibia. The research findings can provide theoretical references for designing single-keel tibial component of unicompartmental knee prosthesis which conforms to better mechanical properties of the knee joint.

Objective By developing an automatic procedure for optimization of femoro-tibial contact area for knee prosthesis, to summarize the influence pattern of design parameters on contact area, and discover the relationship between the maximum contact stress and contact area. Methods A parametric finite element (FE) model was developed in the Isight software, which included three components: automatic parameter changes for the geometric model, automatic modeling in the FE software, and automatic FE calculation. The automatic workflow was realized, and then contact areas were statistically analyzed. Results The FE model was validated by using Tekscan pressure distribution system. When the femoral sagittal radius was gradually close to the tibial sagittal radius, the contact area gradually reached to the maximum 295 mm2. The femoral sagittal radius had a positive effect on contact area, while the tibial sagittal radius had a negative effect. The maximum contact stress had a linear relationship with contact area approximately. Conclusions This study analyzed the influence of femoro-tibial sagittal radius on contact stress and contact area, and the research findings would provide references for the design on reducing wear of tibial insert in clinic.