###### Abstract
These resonant frequency changes can be observed in the FRF evolution during the implant insertion. This increase in stiffness corresponds to an increase in primary stability. The principle of tracking a structure's resonant frequencies as an indication of changes in the structure's stiffness originates in structural engineering, where it is used for damage identification [11]. In the human body, the femoral bone is surrounded by soft tissue, which is a complex anatomical structure consisting of muscle, fat, ligaments, blood vessels, and nerves. The influence of the biological soft tissue elements, joints, and adjacent bones on the vibrational properties of the tibia was investigated in a cadaveric study by [PERSON] et al. [12]. This study demonstrated that the removal of the biological soft-tissue surrounding the tibia, the femur, and the foot did not substantially affect the resulting mode shapes of the tibia. An actual change in the boundary conditions of a structure would drastically affect the resulting mode shapes, suggesting that the boundary conditions, before and after the removal of the biological soft tissue, foot, and femur, are equivalent, given that the tibia mode shapes remained unaltered. As a result, the observed similarity of the mode shapes before and after the removal of the aforementioned anatomical elements suggests that the free-free boundary condition is the most appropriate to characterize the tibia inside the body. Many studies extend this principle to the femur, a bone significantly similar to the tibia, in terms of shape, size, and joints [4; 13]. The validity of this assumption, namely, considering the free-free condition appropriate for characterizing the tibia or femur in the body, can be carried over to the operating theatre during surgery, where the patient is lying down and is under anesthesia; a state similar to the one investigated in the study of [PERSON] et al. in terms of muscle loading and body position. In addition, the notion of using the free-free boundary condition to model the femur in the body is further supported by the fact that relative displacement between the femur and tibia at the knee joint is possible for all six degrees of freedom [14]. The free-free boundary condition is often practically achieved in vitro by suspending the structure using elastic bands or placing it on foam. Most studies on VA implant stability assessment techniques employ artificial bones [3; 4; 5; 6; 9] (Sawbones, Malmo Sweden), the material properties of which have been extensively validated [15; 16]. In contrast, studies considering the influence of surrounding soft tissue on these VA techniques are rare [10]. In order to reproduce the influence of biological soft tissue in an in-vitro setting, a phantom that mimics the dynamic properties of soft tissue can be used. Numerous specialized soft tissue phantom recipes are available in literature, each of whom refer to a specific tissue type and application [17]. With respect to structural dynamics applications, [PERSON] et al. proposes a gelatin-based soft tissue mimicking phantom recipe that is reasonably representative of fat and muscle tissue, simplified into a homogeneous medium, in terms of dynamic properties [18]. An overview of the phantom's properties and comparison to human fat and muscle tissue is included in Appendix A. Predicting the FRF of a patient's femoral bone-implant system at optimal implant stability would require a patient specific computational bone model as well as an in-depth knowledge of the patient specific soft tissue properties and forces exerted by muscles and ligaments. Obtaining and validating such a computational bone model poses a major challenge in the field of biomechanics with the currently available technologies. The specific technical challenges posed by these technologies are out of the scope of this article and will not be addressed. In addition, knowledge of the specific forces exerted by muscles and ligaments poses an equivalent scientific challenge [19].