Purpose Interference screw fixation of tendon grafts is a common practice in ACL surgery. Tissues like tendons and bone show a complex viscoelastic behaviour, which could affect the contact forces in interference screw fixation. These effects are not well investigated, especially over prolonged periods of time. The time‐dependent behaviour of tendons was thus the subject of the study. It was our hypothesis that a substantial decrease of the contact forces will be observed. Methods Using an artificial bone surrogate and pig tendons, the time‐dependent behaviour of the contact force in a simulated interference screw fixation was investigated over a 12‐h period. As the screwing in can have deleterious effects on tendons, two scenarios, screwing in (A) and pressing the screw against the tendon (B), were investigated. Results The initial contact forces showed a magnitude of 1,299 N (A) and 2,156 N (B), respectively. A significant decrease to 530 N (A) and 461 N (B) after 12 h was observed (p < 0.001). However, more than 80 % of the relaxation occurred during the first hour. Conclusion Significant relaxation of the contact force in interference screw fixation of ACL grafts occurs, which has implications for research and clinical practice. The strength of ACL fixation methods should not be assessed immediately after assembly of the specimens and comparisons should be done only when the same time intervals from assembly to testing were used. Also, in clinical practice, the initial fixation strength will probably not be maintained. Therefore, a backup fixation might be advisable.

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