Objective To investigate stress distributions of the knee joint at 0 and 15th day after anterior cruciate ligament reconstruction(ACLR)under a compressive force through the axis of the femoral shaft onto the proximal femur.Methods A three-dimensional(3D)finite element model of the human knee joint incorporating viscoelastic material properties was developed.The one-dimensional(1D)Prony series viscoelastic constitutive model parameters for articular cartilage,menisci,ligaments,and anterior cruciate ligament(ACL)grafts were determined by fitting experimental creep curves.The viscoelastic parameters of ACL grafts at 15th day after ACLR surgery were extrapolated.Finite element simulations were then performed to analyze the von Mises stress distributions in knee ligaments,ACL grafts,articular cartilage,and menisci under 1.5 kN vertical downward compressive load applied to the femur,with loading durations of 1 second and 600 seconds.Results At 15th day after ACLR surgery,the initial relaxation modulus and equilibrium modulus of human ACL grafts remained elevated compared to native ACL tissues,resulting in a significantly higher stress concentration within the grafts relative to healthy ACL.Despite the compromised mechanical properties of the grafts after ACLR surgery,the vertical downward compressive force applied to the femur under both short-term(1s)and prolonged(600 s)loading durations,exhibited a minimal biomechanical impact on articular cartilage and meniscal structures.Conclusions Following ACLR,vertical compressive loads during weight-bearing rehabilitation exercises such as standing demonstrate minimal impact on articular cartilage and meniscus,while promoting fibrous regeneration of the graft.This renders such exercises a prudent early-stage rehabilitation strategy.Graft preparation requires balanced consideration of elastic and viscous properties,with grafts exhibiting higher relaxation modulus and viscosity coefficient than healthy ACL proving more effective in maintaining early postoperative knee stability.

Objective To investigate stress distributions of the knee joint at 0 and 15th day after anterior cruciate ligament reconstruction(ACLR)under a compressive force through the axis of the femoral shaft onto the proximal femur.Methods A three-dimensional(3D)finite element model of the human knee joint incorporating viscoelastic material properties was developed.The one-dimensional(1D)Prony series viscoelastic constitutive model parameters for articular cartilage,menisci,ligaments,and anterior cruciate ligament(ACL)grafts were determined by fitting experimental creep curves.The viscoelastic parameters of ACL grafts at 15th day after ACLR surgery were extrapolated.Finite element simulations were then performed to analyze the von Mises stress distributions in knee ligaments,ACL grafts,articular cartilage,and menisci under 1.5 kN vertical downward compressive load applied to the femur,with loading durations of 1 second and 600 seconds.Results At 15th day after ACLR surgery,the initial relaxation modulus and equilibrium modulus of human ACL grafts remained elevated compared to native ACL tissues,resulting in a significantly higher stress concentration within the grafts relative to healthy ACL.Despite the compromised mechanical properties of the grafts after ACLR surgery,the vertical downward compressive force applied to the femur under both short-term(1s)and prolonged(600 s)loading durations,exhibited a minimal biomechanical impact on articular cartilage and meniscal structures.Conclusions Following ACLR,vertical compressive loads during weight-bearing rehabilitation exercises such as standing demonstrate minimal impact on articular cartilage and meniscus,while promoting fibrous regeneration of the graft.This renders such exercises a prudent early-stage rehabilitation strategy.Graft preparation requires balanced consideration of elastic and viscous properties,with grafts exhibiting higher relaxation modulus and viscosity coefficient than healthy ACL proving more effective in maintaining early postoperative knee stability.

Objective To investigate the effect of sustained fatigue on the passive and active biomechanical characteristics of the knee.Methods Twenty-seven healthy university students were recruited to perform a fatigue experiment using an isokinetic dynamometer.The fatigue experiment included three fatigue cycles with more than 30 sustained quadriceps submaximal voluntary isometric contractions per cycle.The maximum passive torque,mean maximum voluntary contraction(MVC)torque,integrated electromyography of the vastus lateralis and biceps femoris,and co-contraction index(CI)during different fatigue cycles were compared.Results The maximum passive torque decreased significantly only after the 3rd fatigue cycle(P＜0.05).The mean MVC torque and integrated electromyography of the vastus lateralis and biceps femoris decreased significantly during the 1st,2nd,and 3rd fatigue cycles(P＜0.05).However,the Cl did not change significantly(P＞0.05).Conclusions Sustained fatigue intervention with 90-fold isometric contraction training of the quadriceps significantly affected the active and passive biomechanical properties of the knee.After fatigue intervention,the resistance of the knee joint to passive flexion and extension decreased.With fatigue intervention,the active contraction ability of the quadriceps decreased,and the activation of the quadriceps femoris and hamstring muscles also decreased;however,the co-contraction level of the two muscle groups remained unchanged.This mild muscle group co-contraction pattern is beneficial for maintaining a benign mechanical loading environment in the knee joints.These results help us understand the active and passive biomechanical properties of knee joints after fatigue.

Objective To study the short-term variation patterns of graft viscosity after anterior cruciate ligament reconstruction (ACLR) surgery. Methods Six male New Zealand rabbits were selected. The ACLR animal model of unilateral knee was made with Achilles tendon as the graft. The experimental rabbits were euthanized 15 days after ACLR surgery, with removal of the graft, healthy anterior cruciate ligament (ACL) and Achilles tendon. The cross-sectional area and viscosity coefficient of the graft were measured, and the creep experiments were carried out under equilibrium conditions of 0.1 MPa and 1 MPa, respectively. The viscosity coefficent was calculated. Variation patterns of graft viscosity were summarize. The grafts were compared with healthy ACL. Results The cross-sectional area of the graft increased slowly within 15 days after ACLR surgery. The viscosity of ACL and graft changed nonlinearly. The viscosity coefficient was quite different under different stresses. The viscosity coefficient of the graft decreased with the time after ACLR surgery, which was more obviously under the condition of low stress. Conclusions The results are helpful to guide the implementation of early postoperative rehabilitation plan after ACLR surgery .

Objective To study the effects of prosthetic alignment on internal contact mechanical characteristics of intact knee joint for transfemoral amputees. Methods The gait experiment of transfemoral amputees was performed under different alignment conditions, and the differences of lower limb motion and ground reaction force (GRF) were analyzed and compared with those of the non-amputees. The three-dimensional （30） finite element model of knee joint was build and used to analyze the effect of alignments of socket adduction and abduction on internal contact mechanical characteristics between femur cartilage, tibia cartilage and meniscus. Results For knee joint of the non-amputees, contact force was mainly concentrated on the medial sides at the moment of the first GRF peak, while contact force was mainly concentrated on the lateral sides at the moment of the second GRF peak. However, for intact knee joint of the transfemoral amputees, contact force was mainly concentrated on the medial side at the moment of two GRF peaks. The stress of the medial meniscus, contact force and contact area between the medial meniscus and cartilage all obviously increased under the alignment of 6° socket adduction. Conclusions Compared with non-amputees, the incidence of knee osteoarthritis (OA) in amputees was higher, which was related to the long-term overload of the medial knee joint. The alignment of socket adduction may increase the risk of knee OA in the intact side of transfemoral amputees. In clinic, excessive adduction of the socket should be avoided during prosthetic alignment.