The elbow is the load-bearing joint during arm swing.

BACKGROUND: Arm swing plays a role in gait by accommodating forward movement through trunk balance. This study evaluates the biomechanical characteristics of arm swing during gait. METHODS: The study performed computational musculoskeletal modeling based on motion tracking in 15 participants without musculoskeletal or gait disorder. A three-dimensional (3D) motion tracking system using three Azure Kinect (Microsoft) modules was used to obtain information in the 3D location of shoulder and elbow joints. Computational modeling using AnyBody Modeling System was performed to calculate the joint moment and range of motion (ROM) during arm swing. RESULTS: The mean ROM of the dominant elbow was 29.7°±10.2° and 14.2°±3.2° in flexion-extension and pronation-supination, respectively. The mean joint moment of the dominant elbow was 56.4±12.7 Nm, 25.6±5.2 Nm, and 19.8±4.6 Nm in flexion-extension, rotation, and abduction-adduction, respectively. CONCLUSIONS: The elbow bears the load created by gravity and muscle contracture in dynamic arm swing movement.

Bicortical Screw Purchase at Upper Instrumented Vertebra (UIV) Can Cause UIV Fracture After Adult Spinal Deformity Surgery: A Finite Element Analysis Study.

OBJECTIVE: To examine the biomechanical stress distribution at the upper instrumented vertebra (UIV) according to unicortical- and bicortical purchase model by finite element analysis (FEA). METHODS: A T8 to Sacrum with implant finite element model was developed and validated. The pedicle screws were unicortically or bicortically inserted from T10 to L5, and each model was compared and the von Mises (VM) yield stress of T10 was calculated. According to the motion (flexion, extension, lateral bending, and axial rotation) of spine, boundary condition values were set as 15°, 15°, 10°, 4°. RESULTS: Although the 2 stress values did not show a significant difference between the unicortical- and bicortical purchase models in the flexion and extension, bicortical purchase model showed a larger stress distribution. However, the asymmetric behavior was significantly greater in the case of lateral bending (0.802 MPa vs. 0.489 MPa) and the rotation (5.545 MPa vs. 4.905 MPa). The greater stress was observed on the spinal body surface abutting the implanted screw. Although the maximum stress was observed around the implanted screw in the bicortical purchase model under axial loading, the VM stress of both models was not significantly different. CONCLUSION: Bicortical purchase model showed a larger stress distribution than the unicortical model, especially in the case of lateral bending and the rotation behavior. Our biomechanical simulation by FEA indicates that bicortical fixation at UIV can be a risk factor for early UIV compression fracture after adult spinal deformity surgery.

Computational simulation study on ilio-sacral screw fixations for pelvic ring injuries and implications in Asian sacrum.

OBJECTIVES: Despite a high possibility of technique-related complications, ilio-sacral (IS) screw fixation is the mainstay of operative management in posterior pelvic ring injuries. We aimed to make IS screw trajectory with fully intraosseous path that was optimal and consistent, and confirm the possibility of transiliac-transsacral (TITS) screw fixation in Asian sacrum. METHODS: Eighty-two cadaveric sacra (42 males and 40 females) were enrolled and underwent continuous 1.0-mm slice computed tomography (CT) scans. CT images were imported into Mimics® software to reconstruct three-dimensional model of the pelvis. To simulate IS screws, we inserted 7.0-mm-sized TITS cylinder for first (S1) and second (S2) sacral segment and 7.0-mm oblique cylinder for S1. TITS cylinder could not be inserted into S1 of 14 models (sacral variation models) but could be inserted into the S2 of all models. The actual length of virtual IS screws was measured, and anatomic features of safe zone (SZS2) including the area, horizontal distance (HDS2), and vertical distance (VDS2) were evaluated by the possibility of TITS screw fixation in the S1. RESULTS: When the oblique cylinder was directed toward the opposite upper corner of S1 at the level of the first foramen, there was no cortical violation regardless of sacral variation. The average length of TITS cylinder was 152.3 mm (range 127.9-178.2 mm) in S1 and 136.0 mm (range 97.8-164.1 mm) in S2, and for oblique cylinder it was 99.2 mm (range 82.4-132.2 mm). The average VDS2, HDS2, and the area of SZS2 were 15.5 mm (range 8.7-24.4 mm), 18.3 mm (range 12.7-26.6 mm), and 221.1 mm2 (range 91.1-386.7 mm2), respectively. The VDS2 and SZS2 of sacral variation were significantly higher than those of normal (both p = 0.001). CONCLUSIONS: Considering the high variability of the S1, it is better to direct the IS screw trajectory toward the opposite upper corner of the S1 at the level of first sacral foramen. If a TITS screw is needed, the transverse fixation for the S2 could be performed alternatively due to its sufficient osseous site even in Asian sacrum.

Implications of three-dimensional modeling of the proximal femur for cephalomedullary nailing: An Asian cadaver study.

PURPOSE: To determine the variability in the ideal entry point of cephalomedullary (CM) nail around the greater trochanter (GT) and the consequent conformity with the proximal femur by analyzing three-dimensional (3D) modeling and virtual implantation MATERIALS AND METHODS: A total of 105 cadaveric femurs (50 males and 55 females) underwent continuous 1.0mm slice computed tomography (CT) scans. CT images imported into Mimics® software to reconstruct the 3D model of the proximal femur and medullary canal. PFNA-II® was processed into a 3D model using a 3D-sensor at the actual size and optimally implanted in the proximal femur model using Mimics® software. The ideal entry point, nail conformity with the proximal femur, and the relationship between the entry point and adjacent structures were assessed. RESULTS: The ideal entry point was located a mean of 2.38mm (SD, 3.53mm) medial to the tip of GT. No lateral cortex impingement of the proximal femur occurred in the coronal plane based on the recommended point. However, a disparity in the sagittal plane between the proximal shaft and nail curvature was found in 47 models (44.8%). Rotation and magnification of the 3D model exposed all nails above the surface of the medial side of the GT. The proximal nail end was contained entirely within bone and circumferential endosteal cortical contact was present at the nail-bone interface.