The modeling of forces during needle insertion into soft tissue is important for accurate surgical simulation, preoperative planning, and intelligent robotic assistance for percutaneous therapies. We present a force model for needle insertion and experimental procedures for acquiring data from ex vivo tissue to populate that model. Data were collected from bovine livers using a one-degree-of-freedom robot equipped with a load cell and needle attachment. computed tomography imaging was used to segment the needle insertion process into phases identifying different relative velocities between the needle and tissue. The data were measured and modeled in three parts: 1) capsule stiffness, a nonlinear spring model; 2) friction, a modified Karnopp model; and 3) cutting, a constant for a given tissue. In addition, we characterized the effects of needle diameter and tip type on insertion force using a silicone rubber phantom. In comparison to triangular and diamond tips, a bevel tip causes more needle bending and is more easily affected by tissue density variations. Forces for larger diameter needles are higher due to increased cutting and friction forces.

Force information @om needle insertions was measured and modeled for use in robot-assisted percutaneous therapies. Data was collected on bovine livers using the Johns Hopkins University Steady Hand Robot, and modeled in three parts: force due to capsule stifiess, friction, and cutting. Capsule stifiess is modeled by a nonlinear spring model, friction by a mod$ed Karnopp model, and cutting by the remaining forces, which appear to be constant for a given tissue sample. During robot-assisted procedures, real-time force data can be compared to these models to control puncture of interior structures. Future work will combine imaging with force data to characterize tissue layers in spec$c applications, such as liver ablation and prostate brachytherapy.

We present the tip and friction forces acting on a needle during penetration into a canine prostate, independently measured by a 7-axis load cell newly developed for this purpose. This experimental apparatus clarifies the mechanics of needle penetration, potentially improving the development of surgical simulations. The behavior of both tip and friction forces can be used to determine the mechanical characteristics of the prostate tissue upon penetration, and the detection of the surface puncture, which appears in the friction force, makes it possible to estimate the true insertion depth of the needle in the tissue. The friction model caused by the clamping force on the needle can also be determined from the measured friction forces.

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