This paper presents the analysis and comparison of the results of mechanical testing of dumbbell-shaped specimens and multifunctional fixation miniplates made via injection forming. Three types of materials were used: a) polylactic acid; b) a composite made of a polylactic acid matrix modified with tricalcium phosphate β-TCP; c) a composite made of a polylactic acid matrix modified with a mixture of bioceramic powders of tricalcium phosphate β-TCP and hydroxyapatite HAp. All the samples were stored in normal conditions, no special treatment was applied. Tests were conducted right after samples were prepared and they were repeated two and four years after preparation. The values of basic mechanical parameters and stress-strain curves were recorded and analyzed. The attention was focused on changes in time of tensile strength and stiffness of materials and implants. It was discovered that having been stored for four years in the open air, without sunlight, with no hermetic sealing, and no sterilization, all the materials (PL38, PL38/TCP, PL38/TCP/HAp) showed slight changes in mechanical characteristics when compared to the data of the initial samples tested after fabrication. These changes were not critical and did not adversely affect either tensile strength or Young’s modulus of the implants. All the analyzed miniplates maintained their mechanical properties at an acceptable level, fulfilling requirements for fixation devices for osteosynthesis. Therefore, it was proposed that the expiry date of these implants can be indirectly determined, based on long-term mechanical testing.