needed; and B) the instrument beak remains unchanged, which makes the new design fit in the current instrument palette and facilitates intuitive use.
Device to Assist Force Generation in High-load Orthopedic Actions Roel F.P. van Gorkum1 Jenny Dankelman1 Gabriëlle J.M. Tuijthof1,2 1
Department of Biomechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Delft, The Netherlands
bone cutter. Second, the amplification mode is enabled by engaging a pawl in a ratchet, which was integrated in one of the handles. The second lever is operated when reopening and closing the modified handle (Fig 1). A prototype of the new device was fabricated by modification of an existing bone cutter. The dimensions of the new lever arms were determined such that the number of squeezed to close the beak was minimized as well as the forces on the ratchet. The prototype was tested in a tensile testing machine to confirm the calculated additional force amplification for different positions of the handles. The handles were loaded with a mass of 2.12 kg, while the resulting force on the beak was measured simultaneously.
2
Orthopedic Research Center Amsterdam, Department of Orthopedic Surgery, Academic Medical Centre, Amsterdam, The Netherlands
1 Background The majority of orthopedic procedures requires bone cutting, such as in joint arthroplasty and spine surgery. The cutting mechanism consists of a single or double mechanical lever to enlarge the gripping force for cutting the bone (Fig. 1). Mechanical cutting of bone is useful in surgery, because it does not prevent bone regeneration and allows full surgical control by allowing cutting with one hand, verification of correct removal of tissue sample and force feedback during the entire cutting process. This contributes to patient safety. On the other hand, bone cutting requires significant hand force, and especially surgeons with small(er) hands and low gripping force sometimes cannot generate sufficient force [1]. In a survey amongst a group 31 surgeons, 60% indicated that they sometimes were unable to close the bone cutter with one hand leading to unnecessary delay of the operation and suboptimal performance. The goal was to develop and to evaluate a new device that maintains the strong characteristics of conventional bone cutters while simultaneously allows extra force generation in high-load orthopedic actions.
2 Methods The new device should enable all surgeons to cut through the toughest bone with one hand. Therefore, a worst case scenario was determined where the maximum gripping force of the weakest user was set at 200 N [2] and the required force to cut the toughest bone chip at once was experimentally measured to be 475 N. The user’s gripping force was amplified with a new second lever mechanism (Fig. 1), because of the following foreseen advantages: A) Patient safety is guaranteed, because the surgeon controls the cutting action from start to end and no additional energy source is needed; and B) the instrument beak remains unchanged, which makes the new design fit in the current instrument palette and facilitates intuitive use. The second lever mechanism causes a theoretical amplification of the user’s gripping force of 2.2 (Fig. 1). The concept has two operating modes. First, the traditional mode allows normal operation as if no changes were made to the
Figure 1. Prototype of modified bone cutter in used in amplification mode.
3 Results The prototype amplifies the user gripping force with at least 2.8 times more than operation in traditional mode, and closes the beak in a maximum of 3.3 squeezes.
4 Interpretation A new mechanism was developed to assist the surgeons when performing high-load orthopedic actions. The mechanism consists of a lever engaged by a ratchet and pawl to amplify the user gripping force when necessary. A prototype was integrated with a conventional bone cutter and proved to offer the required force amplification. Future work includes a usability test and a complete redesign of the entire cutter for sterilization.
References [1] Berguer, R. and Hreljac, A. 2004, "The relationship between hand size and difficulty using surgical instruments: a survey of 726 laparoscopic surgeons," Surg. Endosc., 18(3), pp. 508-512. [2] Bohannon, R., Peolsson, A., Massy-Westropp, N., Desrosiers, J., and Bear-Lehmann, J. 2006, "Reference values for adult grip strength measured with a Jamar dynamometer: a descriptive meta-analysis," Physiotherapy, 92(1), pp. 11-15.
