The modular endoprosthesis for mandibular body replacement. Part 1: Mechanical testing of the reconstruction

INTRODUCTION: In this paper we present the results of the mechanical testing of a new generation modular endoprosthesis, which has been designed to improve the results of mandibular reconstruction. MATERIALS AND METHODS: The new cementless endoprosthesis consists of a male part, a female part (both with screws on the stems), connected via a dove-tailed connection and secured with a coronal screw. The endoprosthesis was fitted into standardized blocks of synthetic bone (Synbone AG, Malans, Switzerland). The set-up was fixed to an ElectroPuls testing machine at one end and loaded at the other end 25mm away. Three specimens were loaded continuously until failure to determine the average load to failure of the construct. Five specimens were then loaded cyclically between 10 and 150N until either failure or 500,000 cycles. A finite element analysis was also performed on the set-up. RESULTS: Of the five specimens in the fatigue testing, only one survived while the other four either were bent or fractured at the stem of the clamped portion. The specimen that survived had very good bony contact with the prosthesis at the lower border. The connection of the modules via the dove-tailed design did not show any loosening. Finite element analysis showed areas of stress concentration at the superior surface of the stems to 188.8MPa. This was well below the yield strength of titanium alloy of 897MPa. Statistical analysis performed for specimens 1 to 4 to calculated lower tolerance bounds on cycles to failure, representing the estimated minimum achievable cycles to failure at 90, 95, and 99% of the population at 90 and 95% confidence levels, showed that the estimated mean cycles to failure was 10,132 cycles at the mean, minimum and maximum loads of 120N and 18.4N respectively. CONCLUSION: Good bony contact seems to be essential at the lower border for long-term survival of the reconstruction. Small gaps increase the bending forces and thus shear stresses at the stem. The new design of the modular endoprosthesis is prone to stress concentrations at the superior surface of the stems. This is accentuated by the sharp screw threads of the stems. The loosening of the module connection seemed to have been stopped with the dove-tailed design