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Experimental analysis of insertion torques and forces of threaded and press-fit acetabular cups by means of ex vivo and in vivo measurements

Vogel D., Rathay A., Teufel S., Ellenrieder M., Zietz C., Sander M., Bader R.

Acta of Bioengineering and Biomechanics

2017

Vol. 19, no. 3

155--163

In THA sufficient primary implant stability is the precondition for successful secondary stability. Industrial foams of different densities have been used for primary stability investigations. The aim of this study was to analyse and compare the insertion behaviour of threaded and press-fit cups in vivo and ex vivo using bone substitutes with various densities. Methods: Two threaded (Bicon Plus®, Trident® TC) and one press-fit cup (Trident PSL®) were inserted by orthopaedic surgeons (S1, S2) into 10, 20 and 31 pcf blocks using modified surgical instruments allowing measurements of the insertion forces and torques. Furthermore, the insertion behaviour of two cups were analysed intraoperatively. Results:Torques for the threaded cups increased while bone substitute density increased. Maximum insertion torques were observed for S2 with 102 Nm for the Bicon Plus® in 20 pcf blocks and 77 Nm for the Trident® TC in 31 pcf blocks which compares to the in vivo measurement (85 Nm). The average insertion forces for the press-fit cup varied from 5.2-6.8 kN (S1) and 7.2-11.5 kN (S2) ex vivo. Intraoperatively an average insertion force of 8.0 kN was determined. Conclusions: Implantation behaviour was influenced by acetabular cup design, bone substitute and experience of the surgeon. No specific density of bone substitute could be favoured for ex vivo investigations on the implantation behaviour of acetabular cups. The synthetic bone blocks of high density (31 pcf) led to problems regarding cup orientation and seating. Therefore, bone substitutes used should be critically scrutinized in terms of the comparability to the in vivo situation.

Biomechanical simulation of needle insertion into cornea based on distortion energy failure criterion

Su P., Yang Y., Huang L.

Acta of Bioengineering and Biomechanics

2016

Vol. 18, no. 1

65--75

Purpose: This paper is mainly about biomechanical behavior of needle insertion into cornea, and proposes a failure criterion to simulate the insertion process which has attracted considerable attention due to its importance for the minimally invasive treatment. Methods: In the process of needle insertion into cornea, tiny and complex insertion force is generated due to contact between needle and soft tissue. Based on the distortion energy theory, there is proposed a failure criterion of corneal material that can solve contact problem between rigid body and biological tissue in insertion simulation, where Cauchy stress of corneal material is the key to numerical calculation. A finite element model of in vivo cornea is built, and the cornea constrained by sclera is simplified to two layers containing epithelium and stroma. Considering the hyper-viscoelastic property of corneal material, insertion simulation is carried out. Results: By insertion experiment, the insertion force increases with insertion depth accompanying obvious fluctuations. Different insertion forces are generated at different speeds. The punctured locations are obvious in the force-displacement curves. The results of insertion simulation are generally consistent with experimental data. Maps of von Mises stress reflect the tissue injury of the cornea during insertion process, and punctured status corresponds to the point in the curves. Conclusions: The ability of this study to reproduce the behavior of needle insertion into cornea opens a promising perspective for the control of robotic surgery operation as well as the real-time simulation of corneal suture surgery.