PSI - Issue 42

N.S. Hennicke et al. / Procedia Structural Integrity 42 (2022) 404–411 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The results show that the fracture load of femurs implanted with an artificial hip stem distinctively decreases with advancing age. The drop is especially dramatic for patients who already start out with a low BMD and Z-score at a young age as the observed reduction in fracture load by 42 % for femur 3L showed. With a very low Z-score of -1.97 the BMD of this femur was statistically located below the 5 th percentile of the density distribution in all age groups. The other femur 1L on the other hand had a relatively average Z-score at -0.553 and the fracture load only decreased by 10.8% over the lifetime of this patient. Fig. 4 displays the force-displacement curves for simulations of the femurs with different T-scores. For the over 80 years old femur 1L, the difference in fracture load between healthy bone stock and one with severe osteoporosis was 2.15 kN. For the younger femur 3L in the age group of 70 to 79, the difference was 5.73 kN.

Fig. 4. Force-displacement curves from simulations at different severity levels of osteoporosis (T-score values); A : femur 1L, B: femur 3L

It is apparent, that stiffness and fracture load are reduced with the progressing severity of osteoporosis (decreasing T-score). For the femur 3L the total difference of fracture load between normal and severely osteoporotic bone quality was 56.4 %. Although it is unlikely that a patient with a Z-score of -1.97 whose bones therefore already have to be diagnosed with osteopenia in his twenties will ever possess a normal bone quality with a high T-score in his eighties. Nevertheless, the decreasing T-score influences the load bearing properties of the implanted femurs considerably and early medical treatment of osteoporosis to reduce this effect would be advantageous for the prevention of PFF. For the femur 1L the total difference in fracture load for osteoporotic bone was 33%. It is noticeable, that the osteoporosis simulations for the two femurs showed different amounts of decrease in facture load and stiffness. And that, although the original Z-score of the femurs is irrelevant for the simulations of osteoporosis since the severity of osteoporosis is defined by the same T-score and BMD values. These numerical results reinforce the conclusion from many clinical studies (Bauer and Link, 2009; El Miedany, 2021; Osterhoff et al., 2016) that the BMD alone is an insufficient medical indicator for the severity of osteoporosis. Other influencing factors on the fracture risk in this numerical study are on one side the subject specific bone geometry and density distribution within the bone. These factors were also previously mentioned by Bauer and Link (Bauer and Link, 2009) to influence bone quality. On the other side, implant related factors like implant design or anchoring technique might also influence the development of osteoporotic PFF and need to be further investigated. Fig. 5 exemplarily shows the posterior fracture patterns of femurs 1L and 3L for simulations with a T-score of 0 (healthy bone stock) at the age of 81 and 77, respectively. The fracture patterns of the developing PFF in all aging and osteoporosis simulations remained overall the same compared to the original simulations for femurs 1L and 3L described by Hennicke et al. (Hennicke et al., 2022). For simulations with the chosen approach for damage modelling it follows, that the development of fracture pattern only depends on bone geometry, density distribution and implant characteristics. But not, on the overall BMD value and therefore neither on age or osteoporosis.