We simulate and theoretically analyze the properties of entangled polymer melts confined in thin film and cylindrical geometries. Macromolecular-scale conformational changes are observed in our simulations: the average end-to-end vector is reduced normal to the confining surfaces and slightly extended parallel to them, and we find that the orientational distribution of the chain end-to-end vectors is transmitted to the primitive path entanglement strand level. Treating the chains as ideal random walks and the surfaces via a reflecting boundary condition we are able to accurately theoretically predict the anisotropic global and primitive-path level conformational changes. Combining this result with a recently developed microscopic theory for the dependence of the tube diameter on orientational order allows a priori predictions of how the number of entanglements decreases with confinement in a geometry-dependent manner. The theoretical results are in excellent agreement with our simulations.

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AimsComputer-assisted 3D preoperative planning software has the potential to improve postoperative stability in total hip arthroplasty (THA). Commonly, preoperative protocols simulate two functional positions (standing and relaxed sitting) but do not consider other common positions that may increase postoperative impingement and possible dislocation. This study investigates the feasibility of simulating commonly encountered positions, and positions with an increased risk of impingement, to lower postoperative impingement risk in a CT-based 3D model.MethodsA robotic arm-assisted arthroplasty planning platform was used to investigate 11 patient positions. Data from 43 primary THAs were used for simulation. Sacral slope was retrieved from patient preoperative imaging, while angles of hip flexion/extension, hip external/internal rotation, and hip abduction/adduction for tested positions were derived from literature or estimated with a biomechanical model. The hip was placed in the described positions, and if impingement was detected by the software, inspection of the impingement type was performed.ResultsIn flexion, an overall impingement rate of 2.3% was detected for flexed-seated, squatting, forward-bending, and criss-cross-sitting positions, and 4.7% for the ankle-over-knee position. In extension, most hips (60.5%) were found to impinge at or prior to 50° of external rotation (pivoting). Many of these impingement events were due to a prominent ischium. The mean maximum external rotation prior to impingement was 45.9° (15° to 80°) and 57.9° (20° to 90°) prior to prosthetic impingement. No impingement was found in standing, sitting, crossing ankles, seiza, and downward dog.ConclusionThis study demonstrated that positions of daily living tested in a CT-based 3D model show high rates of impingement. Simulating additional positions through 3D modelling is a low-cost method of potentially improving outcomes without compromising patient safety. By incorporating CT-based 3D modelling of positions of daily living into routine preoperative protocols for THA, there is the potential to lower the risk of postoperative impingement events.Cite this article: Bone Jt Open 2023;4(6):416–423.

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