###### Abstract  
In minimally invasive bone fracture reduction surgery, broken femur bones are firmly fixed to a metallic intramedullary nail (IMN) after they are properly aligned. One of the greatest challenges of this process is that surgeons cannot directly see holes on the IMN, which increases the difficulty of the procedure and results in the requirement of taking a large number of X-ray images to find the location and direction of holes. We propose a novel distal interlocking screw guidance system that consists of a parallel guidance system using a laser pointer (PCGLP) and a mechanical fine-adjustment device (FAD). The PGSLP is used to make the planes of the C-arm and FAD parallel. The FAD is used to concentrically align the IMN hole with the guiding hole. The performance of the proposed device was evaluated by a series of experiments. The tilted angle error between the C-arm and FAD was measured to be 1.24 \(\pm\) 0.715\({}^{\circ}\). The translational error between the IMN hole and guiding hole was measured to be 0.378 \(\pm\) 0.120 mm. Since the proposed guiding system is simple, cost-effective, and accurate, we expect it will soon be used in real operations. b 2021 11 1109 1  
To solve this issue, various methods have recently been proposed. The first category includes mechanical devices [6; 7; 8; 9] for facilitating finding the hole location. [PERSON] et al. [6] used a grid marker and a flag to roughly predict the position of the IMN hole. Its effectiveness was confirmed through in vivo testing. However, it did not describe how to align the drilling direction to that of the IMN hole. [PERSON] et al. [7] developed a mechanical jig device to align to the IMN hole by translation in the directions of up, down, left, and right. The alignment of the screwing direction needed to be checked using X-ray images. [PERSON] et al. [8] proposed a mechanical jig to accurately modify the position as well. Recently, Distalock, LLC [9] released a new device that drills from the inside to the outside of the bone to find the screw position. The disadvantage of this is that it cannot make a hole perfectly perpendicular to the IMN hole, resulting in possible incorrect guiding of the screw. The second category includes methods that utilize image processing. With the help of fluoroscopy [10; 11], surgeons can check the position of IMN hole by taking X-ray images to find the proper position of screws. This technique does not eliminate X-ray exposure. They guide the direction of the drilling tool virtually not physically. The C-arm should be calibrated accurately, which is not a simple task due to the arm deformation. Finally, environmental conditions such as X-ray strength and the distance to the object can affect the targeting accuracy of the IMN hole. The third category includes methods that use magnetic fields [12; 13; 14] to eliminate the necessity of X-ray images. However, the accuracy is not sufficient to be used in real operations and it is easily affected by surrounding metal equipment. Different from all three categories, [PERSON] et al. [15] proposed a new approach to visualize the hole location by using optical fiber that delivers strong illumination into the bone cavity. However, the illumination could not penetrate the tissue, which resulted in a large incision on the patient. It also was not accurate enough to use in real operations. [PERSON] et al. [16] proposed a guiding method using a laser beam to visualize the hole locations but the drilling procedure needed to be performed with a free hand, which is unstable. Robotic guidance systems were  
Figure 1: Concept on distal screwing. (**a**) Window-making for insertion of a guide wire. (**b**) Distal fixation with screws. also proposed [17; 18]; however, using robotic systems is expected to be cumbersome and expensive. In addition, they require more evaluation to confirm their feasibility.