Unfortunately, the biomechanical characteristics of these fixed-angle locking devices are not very well known. Assessing their biomechanical properties, developing and improving their design, and thus their performance in the internal fixation of proximal humerus fractures, are major challenges.

Accurate biomechanical models can be used to predict the mechanical failure of orthopaedic implants [1], for their pre-clinical testing [2], and in the diagnosis of many complex pathologies [3,4,5]. Studies of bone remodelling [6], or simply its ageing [7], can also make use of these models. In addition, three-dimensional visualisation has been playing a major role in the pre-surgery planning and preparation of operations, in terms of which type of surgical techniques should be used [8,9].

This study thus set out to develop finite element models to analyze the biomechanical behaviour of PHILOS titanium plates and PHN intramedullary titanium nails as osteosynthesis material in the treatment of a proximal humerus fracture. To validate the numerical results, an experimental setup was first developed, where six pairs of embalmed cadaveric humerus were submitted to a set of torsional loads up to fracture.

Based on experimental results it can be concluded that PHILOS plates provide some biomechanical advantages over PHN intramedullary nails, such as higher torsional load capacity, greater angular displacement and a higher energy absorption capacity, and so greater plastic deformation capacity, before failure. These characteristics are crucial, particularly if the bones present osteoporotic problems [10]. Maximum stresses obtained with the finite element method at experimental failure torques were relatively low compared with the bone's failure stress. The mismatch can be explained by the occurrence of failure mechanisms, i.e. fracture, which are not being modelled numerically. However, the numerical simulation also reveals the superiority of biomechanical behaviour exhibited by the PHILOS plate relative to PHN nail.

1

G.K. Knopf, R. Al-Naji, "Adaptive reconstruction of bone geometry from serial cross-sections, Artificial Intelligence in Engineering", 15, 227-239, 2001. doi:10.1016/S0954-1810(01)00006-1

2

R. Stolk, N. Verdonschot, L. Cristofelini, A. Toni, R. Huiskes, "Finite Element and Experimental Models of Cemented Hip Joint Reconstructions Can Produce Similar Bone and Cement Strains in Pre-Clinical Tests", Journal of Biomechanics 35, 499-510, 2002. doi:10.1016/S0021-9290(01)00213-5

3

M. Viceconti, M. Davinelli, F. Taddei, A. Cappello, "Automatic generation of accurate subject-specific bone finite element models to be used in clinical studies", Journal of Biomechanics 37(10), 1597-1605, 2004. doi:10.1016/j.jbiomech.2003.12.030

4

B.V. Rietbergen, S. Majumdar, D. Newitt, B. MacDonald, "High-resolution MRI and micro-FE for the evaluation of changes in bone mechanical properties during longitudinal clinical trials: application to calcaneal bone in post-menopausal women after one year of idoxifene treatment", Clinical Biomechanics 17, 81-88, 2002. doi:10.1016/S0268-0033(01)00110-3

5

D.L. Kopperdahl, E.F. Morgan, T.M. Keaveny, "Quantitative computed tomography estimates of the mechanical properties of human vertebral trabecular bone", Journal of Orthopaedic Research 20, 801-805, 2002. doi:10.1016/S0736-0266(01)00185-1

6

H. Weinans, R. Huiskes, H. J. Grootenboer, "Quantitative analysis of bone reactions to relative motions at implant-bone interfaces", Journal of Biomechanics, 26(11), 1271-1277, 1993. doi:10.1016/0021-9290(93)90351-E

7

D. Testi, M. Viceconti, F. Baruffaldi, A. Cappello, "Risk of fracture in elderly patients: a new predictive index based on bone mineral density and finite element analysis", Computer Methods and Programs in Biomedicine 60, 23-33, 1999. doi:10.1016/S0169-2607(99)00007-3

8

C. Zannoni, M. Vicenconti, R. Lattanzi, M. Petrone, "Efficacy of Fully 3D Monomodal Interface in Pre-Operative Planning o Total Hip Replacement", International Congress Series, 1256, 648-653, 2003. doi:10.1016/S0531-5131(03)00279-6

9

R. Lattanzi, R. Barufaldy, C. Zannoni, M. Vicenconti, "Specialized CT Scan Protocols for 3-D Pre-Operative Planning of Total Hip Replacement", Medical Engineering & Physics, 26, 237-245, 2004. doi:10.1016/j.medengphy.2003.11.008

10

D.M. Weinstein, D.R. Bratton, W.J. Ciccone II, J.J. Elias, "Locking plates improve torsional resistance in the stabilization of three-part proximal humeral fractures density", Journal of Shoulder and Elbow Surgery, 15(2), 239-243, 2006. doi:10.1016/j.jse.2005.08.006

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