Design and Evaluation of a Percutaneous Fragment Manipulation Device for Minimally Invasive Fracture Surgery

Abstract

Reduction of fractures in the minimally invasive (MI) manner can avoid risks associated with open fracture surgery. The MI approach requires specialized tools called percutaneous fragment manipulation devices (PFMD) to enable surgeons to safely grasp and manipulate fragments. PFMDs developed for long-bone manipulation are not suitable for intra-articular fractures where small bone fragments are involved. With this study, we offer a solution to potentially move the current fracture management practice closer to the use of a MI approach. We investigate the design and testing of a new PFMD design for manual as well as robot-assisted manipulation of small bone fragments. This new PFMD design is simulated using FEA in three loading scenarios (force/torque: 0 N/2.6 Nm, 75.7 N/3.5 N, 147 N/6.8 Nm) assessing structural properties, breaking points, and maximum bending deformations. The PFMD is tested in a laboratory setting on Sawbones models (0 N/2.6 Nm), and on ex-vivo swine samples (F = 80 N ± 8 N, F = 150 ± 15 N). A commercial optical tracking system was used for measuring PFMD deformations under external loading and the results were verified with an electromagnetic tracking system. The average error difference between the tracking systems was 0.5 mm, being within their accuracy limits. Final results from reduction maneuvers performed both manually and with the robot assistance are obtained from 7 human cadavers with reduction forces in the range of (F = 80 N ± 8 N, F = 150 ± 15 N, respectively). The results show that structurally, the system performs as predicted by the simulation results. The PFMD did not break during ex-vivo and cadaveric trials. Simulation, laboratory, and cadaveric tests produced similar results regarding the PFMD bending. Specifically, for forces applied perpendicularly to the axis of the PFMD of 80 N ± 8 N deformations of 2.8, 2.97, and 3.06 mm are measured on the PFMD, while forces of 150 ± 15 N produced deformations of 5.8, 4.44, and 5.19 mm. This study has demonstrated that the proposed PFMD undergoes predictable deformations under typical bone manipulation loads. Testing of the device on human cadavers proved that these deformations do not affect the anatomic reduction quality. The PFMD is, therefore, suitable to reliably achieve and maintain fracture reductions, and to, consequently, allow external fracture fixation.

Keywords: biomechanical testing; cadaveric trials; fracture reduction; robot-assisted orthopedic surgery; surgical tracking.

Figure 1

Robot-bone fixation system. (A) CAD drawings of the Unique Geometry Pin (UGP) and its different cross-sections, (B₁) the anchoring system (AS), and (B₂) a detail of the Drilling Template (DT). (C) The UGP is secured in the Gripping System (GS) and connects the RFM end-effector with the bone fragment. Optical tools are placed on the UGP (OT_UGP) and the RFM (OT_RFM) allowing the measurement of their relative pose.

Figure 2

Laboratory testing experiments with (A) wood and (B) Sawbone model. The reference grid for the measurements can be seen and the angle between the UGP and one of the K-wires is indicated. The measurements were conducted in a series of still images with progressively increased torsional load, up to 2.6 Nm.

Figure 3

Swine trotter testing setup. (A) The elements of the electromagnetic tracking evaluation can be seen. The Instrumented handle (IH) and the Unique Geometry Pin (UGP) can be seen as well as the placement of the optical tools OT_UGP and OT_REF and the electromagnetic 6DOF sensor (EM6). (B) The key parts of the Robot-Bone Fixation System can be seen (GS, UGP, DT, K-wires), as well as the optical tools for the RFM (OT_RFM), the UGP (OT_UGP), and the optical tool used as reference for the calculations (OT_REF).

Figure 4

The setup for the cadaveric study. (A) The two types of generated 33-C1 fractures Y-shape and T-shape. (B) The stabilization of the femur with the Proximal and Distal Rings and the use of a 6 mm Long Drill Bit and a 3 mm K-wire. (C) The entire setup for one of the RFM, the UGP, and GS can be seen attached to the RFM end-effector (EE). The UGP is secured to the fragment with the use of the DT and a number of K-wires. The optical tools are also visible, the OT_RFM, the OT_UGP, and the OT_REF on the shaft of the femur proximal to the hip.

Figure 5

The manual cadaveric trial setup. View from the proximal side of the femur toward the distal. The UGP is visible as well as the instrumented handle (IH) The optical tools are also visible OT_UGP for the tracking of the PFMD and OT_REF as a reference. Feedback from the former is helping with the reduction.