Objective: To investigate the biomechanical effects of different insertion angles of absorbable screws for the fixation of radial head fractures. Methods: The finite element models used to simulate the fractures were created based on CT scans. Two absorbable screws were used to fix and maintain the stability of the fracture, and the angles between the screws were set to 0 , 15 , 30 , 45 , 60 , 75 , and 90. A downward force of 100 N was applied at the stress point, which was coupled with the surface, and the distal radius was limited to six degrees of freedom. The direction and location of the applied force were the same in each model. The values of the von Mises stress and peak displacements were calculated. Results: Under the applied load and different screw angles, the maximum von Mises stress in the screws was concentrated on the surface contacting the fracture surfaces. The maximum von Mises equivalent stress in the screw decreased when the angle increased from 0 (19.54 MPa) to 45 (13.11 MPa) and increased when the angle further increased to 90 (24.63 MPa). The peak displacement decreased as the angle increased from 0 (0.19 mm) to 45 (0.15 mm) and increased when the angle further increased to 90 (0.25 mm). Conclusion: The computational stress distribution showed that fixation with absorbable screws is safe for patients. Moreover, the minimum von Mises stress and displacements were generated when the angle between the screws was 45 ; hence, this setting should be recommended for Mason type II radial fractures.

To investigate the biomechanical effects of different flexion angles of the annular ligament on elbow joint stability. Methods: Left elbow CT and MRI scans were chosen from a healthy volunteer, according to a previous research model. A cartilage and ligament model was constructed with SolidWorks software according to the MRI results to simulate the annular ligament during normal, loosen, and rupture conditions at different buckling angles (0,30, 60, 90, 120). In 15 elbow models, boundary conditions were set according to the literature. The different elbow 3D finite element models were imported into ABAQUS software to calculate and analyze the load, contact area, contact stress and stress of the medial collateral ligament of the olecranon cartilage. Results: 1. According to the analysis results, olecranon cartilage stress values when the annular ligament under different conditions(normal、loosened、ruptured)with elbow extension, were 2.1 ± 0.18, 2.4 ± 0.75, and 2.9 ± 0.94 MPa. As the buckling angle increased, the stress value decreased; with 120 degrees of elbow flexion, the minimum stress values were 0.9 ± 0.12, 1.1 ± 0.38, and 1.2 ± 0.29 MPa. 2. When the contact surface of the olecranon cartilage was flexed from 0 to 30 degrees, the olecranon cartilage contact area significantly increased, reaching a maximum value of 254±5.35 mm, and then the contact area gradually decreased, reaching a minimum value of 176±2.62 mm when the elbow joint was flexed to 120 degrees. The results when the annular ligament was loosened and ruptured were different from those of the normal annular ligament. The maximum values were 283±4.74and 312±5.49mm at 60 degrees of elbow flexion. The contact area gradually decreased with an increase in the angle, and the minimum values were 210±3.82 and 236±6.59 mm at 120 degrees of elbow flexion. 3. When the elbow joint was extended, the maximum stress of the medial collateral ligament was 6.5±0.23, 11.5±0.78 and 18.7±0.94 MPa under different states; as the stress decreased with an increase in the angle, the corresponding values were 2.8±0.18, 4.8±0.56 and 6.2±0.72 MPa at 120 degrees of elbow flexion. Conclusion:The annular ligament plays an important role in maintaining elbow joint stability. When the annular ligament ruptures, it should be reconstructed as much as possible to avoid the elevation of stress on the surface of the medial collateral ligament of the elbow and on the annular cartilage, which may cause clinical symptoms.

Background Cage retropulsion after transforaminal lumbar interbody fusion (TLIF) is a common complication that is more frequently detected in the early postoperative period. Revision in the early stages is relatively less difficult in symptomatic cases. However, cage retropulsion is quite rare for patients with intervertebral osseous fusion in the long term after TLIF, and there are no relevant reports related to the revision plan. Case presentation Here, we report a case of a patient who underwent L4‐S1 TLIF at another hospital 4 years ago, accompanied by recurrent pain and discomfort of the left lower limb after the operation. Due to recent condition aggravation, it was considered to be caused by compression of the nerve root due to cage retropulsion. Nerve root sealing and endoscopy surgery were performed on the operative segment. It was found that cage retropulsion at the L4/5 level was a suspicious focus according to careful analysis of the clinical manifestations of the patient. Selective block of the nerve root on the level resulted in relief of the patient's original symptoms. After the posterior edge of the cage was exposed under the endoscope through an intervertebral foramen approach, the posterior edge of the cage protruding into the spinal canal was removed by high‐speed burr grinding, working casing reduction and other methods. Postoperative symptoms of pain in the low back and lower limb were relieved completely. Conclusions It is feasible to use the power system to remove the retrograde cage under the endoscope through the intervertebral foramen approach for the revision of symptomatic polyether ether ketone (PEEK) cage retropulsion in the long term after TLIF.