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ICVTS on-line discussion A Debonding of artificial hip joint

University Hospital Sestre Milosrdnice, Zagreb 10000, Croatia

Prosthetic hip dislocation following saphenous vein harvesting for coronary revascularisation

eComment: Al-Nammari et al. [1] stated that hip prosthesis stability depends on component design and alignment, soft tissue tensioning and soft tissue function. In my opinion at this point it would be useful to introduce some biomechanical statements on artificial hip joints. Besides the stated components (by authors), the method of continuum finite element models describes another important parameter affecting cemented implant interfaces which is the bonding between the implant and the cement [2]. There are two types of debonding that can occur. The first type of debonding is separation of the metal stem from the cement. If debonding occurs, the amount of shear stress that can be borne at the metal-cement interface must withstand the applied load through compressive forces. The second type of debonding that can occur is between the cement and the bone. This debonding is represented as a change in contact conditions. In addition to bonding effects, the stem material stiffness can have a significant affect on both interface motion and stress shielding, in contradictory ways. This is important since reduced interface motion will reduce cement stresses and metal debonding. In addition, higher friction also reduces interface motion. However, a disadvantage of a stiff stem is stress shielding. Namely, a stiffer implant to bone stiffness ratio means that the stem will carry more load. This means that the bone stresses will likely be much lower than stresses in the intact bone and will lead to bone resorption through excessive bone remodeling. In summary, the above mentioned statements suggest that successful total hip stem design is a tradeoff amongst many factors. Stiff stems will reduce interface motion, but will lead to more bone resorption. Increasing surface roughness of the metal will increase the friction coefficient of the metal cement interface, but this will also increase the localized stress concentrations within the cement, possibly wear debris. I hope that this additional information will reinforce and clarify the conclusions suggested by the authors.

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1. Al-Nammari SS, Saleh DB. Prosthetic hip dislocation following saphenous vein harvesting for coronary revascularization. Interact CardioVasc Thorac Surg 2007; 6:230–232.[Abstract/Free Full Text]
2. Warfield SK, Haker SJ, Talos IF, Kemper CA, Wiesenfeld N, Mewes UJ, Goldberg-Zimring D, Zou KH, Westin CF, Wells WM, Tempany CMC, Golby A, Black PM, Jolesz FA, Kikinis R. Capturing intraoperative deformations: research experience at Brigham and Womans hospital. Medical Image Analysis 2005; 9:145–162.[CrossRef][Medline]

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