Introduction: Total Hip Arthroplasty (THA) is one of the most frequent and successful procedures in orthopedics nowadays. Due to better flexibility to adjust specific patients characteristics, modular hip design in which femoral head is fitted on the stem neck has gained greater popularity. In theses cases, the clinical success depends on the rotational stability of the head and neck pair. Bad fitting ma impair the functional performance of the hip joint and promote fretting corrosion on the head-neck interface. According, there is published (ABNT NBR 14396-4)[1] and in development (ISO/DIS 7206-13)[2] technical standards to conduct laboratory testing for the determination of resistance to torque of head fixation of stemmed femoral components.
The present study aims to determine resistance to torque of head fixation for hip prosthesis from four different worldwide manufacturers. Five head-neck assemblies were tested for each brand (n = 5), in a total of twenty assemblies. For all tests, it was used femoral stem with offset of 37.5 mm and cone size of 12/14. All the femoral heads had diameter of 28 mm.
Materials and Methods: Torque tests were performed in a hydraulic biaxial universal testing machine (MTS model BIONIX, Eden Praire, USA) with a 15kN axial load transducer and 150 Nm torque transducer. The heads were manually positioned in the stem and the stems were immersed in acrylic resin for fixation. After this, the stem neck was positioned and fixed collinear to the machine load axis and an axial compression load of 500 N was applied to the head to fit it on the stem. The entire system was immersed in distilled water bath kept in 37°C ± 1°C. A propylene support was used to apply 1000 N of uniaxial static load at the head and maintain this load constant during the test. For the test a 0.5°/min rotation was applied to the head. The acquisition of torque, angle and load data was performed with frequency of 40 Hz. The torque strength was calculated through a parallel line of linear part of the graph made in a distance of 3 degrees from the origin.
Results: The Figures 1 and 2 show graphs of torque vs. angle and the average value of torque strength measured for each manufacturer respectively. From the graphs (Figures 1 and 2) it may be observed basically two different rotational resistance levels. The designs of brands A and B promote torsional strength all above 25 Nm while for brands C and D it is around 15 Nm.
Figure 1. Graphics of resistance to torque vs. angle for four different manufactures.
Figure 2. Resistence to torque of head fixation of stemmed femoral components for four different manufacturers.
Discussion: This demonstrates there is not a consensus between manufactures about what is the target value for the evaluated parameter. From a biomechanical point of view, Davy et al.,(1988)[3] measured maximum torsional moment of 22 Nm around a hip stem during daily tasks. Schneider et al.,(2001)[4] measured torsional moment around 10 Nm on femoral intramedullary nail during post-operative period. It is worth to note the greater repetibility of the torque vs. angle curves for the design of manufacturer D (Figures 1 and 2). It indicates a very precise manufacturing and finishing of the medical product with optimal matching.
Conclusion: The repetibility of the torque vs. angle curves indicates a precision machining of the hip prosthesis, that generate a fit between head and the stemmed that should be persecuted by all hip implant manufacturers.
References:
[1] ABNT NBR 14396-4, "Determinação da resistência ao toque da fixação de cabeça de hastes femorais", Dec. 2005.A
[2] ISO/DIS 7206-13, "Implants for surgery -- Partial and total hip joint prostheses -- Part 13: Determination of resistance to torque of head fixation of stemmed femoral components", Jan. 2015.
[3] D.T Davy, G.M. Kotzar, R.H. Brown, K.G. Heiple, V.M. Goldberg, K.G. Heiple Jr, J. Berilla and A.H. Burstein, "Telemetric Force Measurements across the Hip after Total Arthroplasty", The Journal of Bone and Joint Surgery. Vol. 70-A, 1988.
[4] E. Schneider, M. C. Michela, M. Genge, K. Zubera, R. Ganz, S.M. Perren, "Loads acting in an intramedullary nail during fracture healing in the human femur", Journal of Biomechanics 2001, Vol. 34. 2001.