Abstract
Before an implantable medical device passes market clearance, a number of mechanical tests need to be conducted in order to verify the mechanical safety of the product. With regard to total hip replacement stems, these are subject to fatigue tests both in the proximal region of the neck as well as in the distal region of the shaft. Despite these tests, we experienced two clinical fractures of the same product in our clinic. It was shown that these fractures were initiated by a laser engraving of the manufacturer with subsequent crack growth. The basic question raised in the present work was to find out why fracture may not have occurred during experimental preclinical testing. Hence, a supplementary finite element analysis was conducted considering the same hip stem under the prescribed conditions of the test standard in different variations. It was found that the choice of orientation and side (left or right leg) strongly influences stress in the hip stem which has not yet been prescribed in test standards. Depending on the side, only compressive stress may be acting in the experimental situation on the locations under risk, thereby inhibiting clinically relevant crack growth and concealing an actual fracture risk.
Kurzfassung
Bevor ein implantierbares Medizinprodukt die Marktzulassung erhält, muss eine Reihe von mechanischen Tests durchgeführt werden, um die Sicherheit des Produktes nachzuweisen. Hüftendoprothesen-Stiele unterliegen unter anderem Ermüdungsversuchen sowohl im proximalen Bereich des Halses als auch im distalen Bereich des Schaftes. Trotz dieser erforderlichen Tests traten in unserer Klinik zwei Brüche desselben Produkts auf. Es zeigte sich, dass diese Brüche durch eine Lasergravur des Herstellerlogos mit anschließendem Risswachstum initiiert wurden. Die grundlegende Frage für die vorliegende Arbeit war, weshalb diese Brüche während der experimentellen präklinischen Testung möglicherweise nicht aufgetreten sind. Daher wurde eine Finite-Elemente-Analyse durchgeführt, in welcher der gleiche Hüftstiel unter den vorgeschriebenen Bedingungen des Teststandards mit unterschiedlichen Variationen untersucht wurde. Im Ergebnis zeigte sich, dass die Belastung des Hüftstiels stark von der Wahl der Orientierung und Seite (linkes oder rechtes Bein) abhängt, welche in den Testbedingungen nicht ausdrücklich vorgeschrieben ist. Abhängig von der betrachteten Seite können in der experimentellen Prüfung an den gefährdeten Stellen nur Druckspannungen wirken, wodurch das klinisch relevante Risswachstum gehemmt werden kann. Ein tatsächliches Bruchrisiko kann dadurch letztendlich unentdeckt bleiben.
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