Dohyung Lim

To develop a proof-of-concept digital twin framework for real-time prediction and visualization of lumbar spine biomechanics in both native and surgically treated models and to numerically verify its fidelity against coupled MBD-FEA simulations. A hybrid framework combining multibody dynamics (MBD) and finite element (FE) simulations with two sequential Kriging surrogate models was developed. Surrogate Model 1 predicted spinal loads from three-dimensional lumbar rotations; Surrogate Model 2 estimated von Mises stress and displacement from these loads for native and postoperative (TLIF + pedicle screw) spine models. Evaluation consisted of numerical verification against full-order MBD-FEA simulations and a plausibility check by benchmarking against literature-reported L4-L5 intradiscal pressure (IDP) ranges. For predefined compound 3-DOF (flexion/extension, lateral bending, and axial rotation) lumbar rotational motions, the mean absolute errors (MAEs) of nodal stress and displacement predicted by the digital twin relative to the coupled high-fidelity MBD-FEA outputs were ≤ 0.5% for the native spine and ≤ 0.2% for the postoperative spine. The L4-L5 IDP ranged from 108 to 513 kPa across all motions, indicating physiological levels overlapping with previously reported ranges (30-1530 kPa). However, the predicted values were lower than some reported maxima in flexion (approximately 1500 kPa). This framework demonstrated strong predictive performance in terms of numerical agreement, indicating how faithfully the surrogate model reproduces the high-fidelity coupled MBD-FEA reference solutions, and presented a proof-of-concept pipeline capable of real-time execution and visualization. However, experimental and clinical validation of the accuracy of IMU-derived segment kinematics and in vivo biomechanical outputs should be performed in future work.

As surgical practice has shifted from mechanical alignment toward kinematic alignment with growing interest in patient-specific soft-tissue laxity, we evaluated under dynamic, weight-bearing conditions-the relative impact of (1) soft-tissue laxity (modeled via MCL stiffness/compliance) and (2) the balanced medial collateral ligament (MCL) length achieved after soft-tissue balancing on tibiofemoral load sharing and kinematics in posterior-stabilized (PS) total knee arthroplasty (TKA). A validated, squat-based musculoskeletal model of mechanically aligned PS-TKA (0-120°) with anatomically segmented, nonlinear ligaments was used. Twenty-one simulations systematically varied soft-tissue laxity (± 20% change in MCL stiffness/compliance) and balanced MCL length (0-12% change in initial length). Outcomes included medial/lateral tibiofemoral contact forces, femoral internal-external and varus-valgus rotations, and compartmental rollback. Changes in balanced MCL length dominated knee mechanics. Lengthening the balanced MCL (greater final length/lower pre-tension) reduced medial contact force by up to 48% (0.76 BW), increased lateral loading, and increased femoral external rotation (+ 1.5°) and posterior rollback (up to 3.7 mm). Shortening (higher pre-tension) produced features of overconstraint with elevated medial pressures and diminished rollback. In contrast, modifying soft-tissue laxity alone (± 20% stiffness/compliance) had minimal effect on load sharing or kinematics unless the ligament was already tensioned. Across the flexion arc, the balanced MCL length (functional post-balancing length/pre-tension) has a substantially greater influence on tibiofemoral load distribution and kinematics than inherent soft-tissue laxity. Intraoperatively, prioritizing precise control of balanced MCL length-via selective release, preservation, or retensioning-may better normalize compartmental forces, mitigate midflexion instability, and avoid overconstraint than attempting to accommodate small variations in intrinsic laxity. These weight-bearing simulation data provide a practical baseline for integrating patient-specific laxity into alignment strategies and support targeting a physiologic, balanced MCL length as the primary means to achieve stability and near-native kinematics in PS-TKA.

This study aimed to investigate the biomechanical implications of Femoral Neck System (FNS) retention and removal in femoral neck fractures under varying bone mineral density (BMD) conditions, with a focus on structural stability and bone resorption risk. A finite element (FE) analysis was conducted using 3D models including intact femurs, FNS-retained (Group I), and FNS-removed (Group R) cases across three BMD levels of 0%, 20%, 40%, and 60%. Structural stability was assessed based on peak von Mises stress (PVMS) in proximal femur, while bone resorption risk was evaluated by analyzing maximum principal stress patterns and measuring strains below the critical bone-damage strain threshold (≤ 200 µstrain). Group I models exhibited localized stress concentrations around the FNS components, with cortical PVMS values of 30.9–34.0 MPa and cancellous PVMS 2.4–3.4 MPa, leading to a stress-shielding effect. The proportion of cancellous bone strains below the 200 µstrain threshold increased to 20.1–18.5% for Models I-0 to I-60, indicating a bone resorption risk. After FNS removal, cortical PVMS increased to 47.0–50.0 MPa, with cancellous bone PVMS returning to near-native levels (0.5–0.8 MPa). Strain analysis revealed a progressive decrease in sub-threshold strains from 3.7% to 0.1%, suggesting restored loading and reduced bone resorption risk. FNS removal can restore physiological stress distribution and reduce bone resorption risk, particularly in patients with low BMD. However, this benefit must be weighed against the potential for increased cortical bone stress, which could compromise structural stability in osteoporotic bone. Therefore, preoperative BMD assessment and postoperative monitoring should be considered to guide clinical decisions regarding FNS removal. the trade-off between restoring physiological load transfer and risking cortical overload must be carefully considered to achieve patient-specific benefit.

Cage subsidence is a common complication following transforaminal lumbar interbody fusion (TLIF) that can lead to poor clinical outcomes, including recurrent pain and segmental instability. Conventional TLIF cage designs often fail to distribute stress evenly, increasing the risk of endplate damage and subsequent subsidence. This study aims to evaluate the effect of a modified TLIF cage with upper and lower open windows (lattice structure) in reducing cage subsidence in patients with lumbar degenerative disc disease (LDDD). A finite element (FE) model of the lumbar spine was developed and validated. Three TLIF cage designs (Open, Lattice, Closed) were simulated under various loading conditions (flexion–extension, lateral bending, axial rotation), and von Mises stresses were analyzed within the TLIFs, endplates, and cancellous bone. The FE model demonstrated ROMs consistent with cadaveric studies. Elevated stresses were found in all cages, especially Open and Closed designs. The Lattice TLIF showed improved stress distribution, reducing peak stress on endplates. However, increased contact area had a limited effect on reducing subsidence under physiological loads. While contact area alone does not significantly mitigate subsidence risk, incorporating lattice structures may enhance resistance to physiological stress. These findings suggest that optimized TLIF designs integrating lattice structures can improve stability and reduce the likelihood of subsidence, leading to better clinical outcomes (e.g., reduced pain, improved fusion success, long-term stability) in LDDD patients.

To investigate the biomechanical effects of medial collateral ligament (MCL) stiffness adjustments on knee kinematics—medial femoral rollback, femoral rotation, and joint contact forces—in mechanically aligned posterior-substituting (PS) total knee arthroplasty (TKA). A musculoskeletal model simulating squatting was developed using the AnyBody modeling system. A PS-TKA prosthesis was implanted, and MCL stiffness was modified in 20% increments. The effects on femoral rollback, femoral rotation, and joint forces were evaluated. Medial femoral rollback was not significantly affected by changes in MCL stiffness. However, when MCL stiffness exceeded 20% above normal, the pattern and magnitude of lateral femoral rollback were altered compared to other conditions. Increased MCL stiffness also altered internal–external femoral rotation and raised joint contact forces in the medial compartment. Muscle activity was largely unaffected by changes in MCL stiffness, although hamstring activity increased slightly during early flexion (0°–5°) when MCL stiffness exceeded 20%. Excessive MCL stiffness (over 20% above normal) affects lateral femoral rollback and increases joint contact forces, potentially elevating the risk of prosthetic wear. Maintaining MCL stiffness within physiological limits is critical for optimizing outcomes in varus knee TKA.

This study aims to determine whether the removal of the femoral neck system (FNS) after bony union affects the biomechanical stability of the femur. Considering the technical challenges and potential complications, including screw stripping reported in recent studies, the study explores whether its removal impacts stress distribution within the femur and increases the risk of complications, such as screw stripping. The femurs were grouped into Intact, Group U (healed fractures with FNS in place), and Group R (healed fractures with FNS removed). Subgroup analysis was performed using Pauwels’ classification for fractures at 30, 50, and 70 degrees. Finite element analysis (FEA) was used to model and evaluate the biomechanical behavior. Material properties for the models were derived from the literature. No significant difference in biomechanical stability was observed between Group U and Group R across the fracture angles tested, indicating that removal of FNS does not compromise the structural integrity of the femur. However, the risk of screw stripping during removal requires consideration. Removing the femoral neck system (FNS) after fracture healing preserves the femur’s biomechanical stability, regardless of fracture angle. However, increased stress at the distal locking screw suggests caution to avoid complications such as screw stripping.

Background: This study aimed to evaluate the femoral tunnel position and fiber length of the anterolateral ligament (ALL) reconstruction compared with the natural anatomy of the ALL. We also evaluated whether the femoral tunnel position would affect residual pivot shift. Methods: This study was a retrospective review of 55 knees that underwent ALL reconstruction considering the anatomical and functional aspects, during primary anterior cruciate ligament (ACL) reconstruction in the presence of a high-grade pivot shift or revisional ACL reconstruction. We determined the position of the femoral tunnel and the length of graft using a three-dimensional (3D)-computed tomography (CT) model after ALL reconstruction. We also measured graft excursion during surgery and examined pivot shift 2 years after surgery. We conducted a subgroup analysis of femoral tunnel position, fiber length, isometricity, and residual pivot shift depending on whether the tunnel was anterior or posterior to the lateral epicondyle (LE). We also performed a subgroup analysis depending on whether the ACL reconstruction was primary or revisional. Results: The mean femoral tunnel position was 2.04 mm posterior and 14.5 mm proximal from the center of the LE. The mean lengths of the anterior and posterior fibers were 66.6 and 63.4 mm, respectively. The femoral tunnel was positioned more proximally than the anatomical position, and both anterior and posterior ALL fibers were longer than the natural anatomy. The anteroposterior femoral tunnel position was significantly correlated with anterior (p = 0.045) and posterior (p = 0.037) fiber excursion. In the subgroup analysis, there was no significant difference in the residual pivot shift between the posterior and anterior tunnel positions. However, there were significant differences for proximal position (p < 0.001) and fiber length (p = 0.006). There was no significant difference between primary and revisional ACL regarding femoral tunnel position and fiber lengths. Conclusion: It is challenging to reproduce both anatomical and functional aspects of ALL reconstruction in both primary and revision ACL reconstruction. Especially for functional reconstruction, the femoral tunnel tended to be positioned more proximally than the anatomical position. However, the femoral tunnel position did not affect functional clinical outcomes at the 2-year follow-up. Level of evidence: Level IV Case series.

Purpose: The purpose of this study was to assess whether kinematic alignment (KA) outperforms mechanical alignment (MA) in restoring patellar tracking to native patterns by using a clustering algorithm.Methods: Twenty cadavers (40 knees) were evaluated. For each cadaver, one knee was randomly assigned to KA and the other to MA. KA total knee arthroplasty (TKA) procedures were performed using a caliper‐verified technique, while MA TKA procedures utilized a measured resection technique. Subsequently, all specimens were mounted on a customized knee‐testing system, and patellar tracking was measured using a motion analysis system. All patellar tracking data were clustered using the density‐based spatial clustering of applications with noise algorithm. Differences in patellar tracking patterns and the restoration of native patellar tracking were compared between the two alignment strategies.Results: Patellar tracking patterns following KA were considerably different from MA. Pre‐ and post‐TKA patellar tracking patterns following MA were grouped into separate clusters, whereas a substantial proportion of patellar tracking patterns following KA were grouped into the pre‐TKA dominant cluster. Compared to MA, a greater proportion of patellar tracking patterns following KA showed similar patterns to native knees (p < 0.05) and more patellar tracking patterns following KA paired with preoperative patterns (p < 0.01).Conclusion: KA restored native patellar tracking patterns more closely compared to MA.Level of Evidence: Level I, therapeutic study.