of the 1 5 mm pin of the invention was typically two to three times higher than that of the reference sample having the same diameter the test results demonstrate that the surface design of the pin of the invention reduces the risk of unstable fixation and gives more tolerance for instrumentation bone quality and surgical procedure than the reference sample the biomechanical rotation stability tests determined that the grooved surface design improves rotation stability of the pin of the invention when compared to the reference samples with a round and plain pin surface cylindrical pin geometry the tested rotation stability values for the pin of the invention were ca three times higher than the values for the reference sample biomechanical in vitro pull out tests determined that the maximum pull out force of pins of the invention gradually increases during the 63 hour test time this is a consequence of the hydrolytic self locking effect for the pin this behavior is an outcome of the dimensional changes of the pin of the invention during the hydrolysis the same dimensional change could be seen during the in vitro hydrolysis test series of the pin of the invention which were ongoing at the same time with the in vitro pull out tests during this test the pin of the invention gradually changed its dimensions after 2 weeks of hydrolysis the diameter of the pin was increased 0 5 1 and its length was decreased 0 5 1 when compared to the initial dimensions after 6 weeks of hydrolysis the diameter of the pin was increased 1 2 and its length was decreased 1 2 when compared to the initial dimensions these.