1. Right Angle Screwdriver
Team Members: Scott Carpenter, Charles Donaldson, Nathan Retzlaff, John McGuire
Department of Biomedical Engineering
Advisor: Thomas Yen, PhD
Client: Ashish Mahajan, MD
Abstract
Alternative Designs
Testing
Worm and Wheel
One idea was the worm and wheel design. The worm gear has an angled thread which turns a perpendicular wheel gear. One full rotation of the worm causes reduced rotation of the wheel. This increases the applied torque, but it also increases the number of required turns to seat a screw. A casing would be placed around the outside of the gears in order to protect the device and the patient’s mouth.
Right Angle Gears
Another idea was the bevel gear design. This concept uses conical gears which allow for a 90-degree translation of rotational motion about the axes. The gears allow for a 1:1 torque ratio and can be modified based on the size of each gear. This design also requires a casing for protection to the mouth and device.
Required Torque
The device must supply enough torque to seat the surgical screws into the human mandible. The requirement was found using a torque-screwdriver. A torque of 26 inch-ounces or foot pounds seats the self-tapping screw.
Supplied Torque
The prototype obviously must supply at least foot pounds of torque on the surgical screw. By using a torque-screwdriver and the prototype in conjunction with a created adaptor, the device easily applied the required torque. The device maxed out the torque-screwdriver at 100 inch ounces ( foot pounds).
In the case of mandible fractures, titanium plates are fastened to the mandible with titanium screws to aid in the healing process. Current procedure requires an external incision on the cheek to gain access to these screws resulting in a scar following operation. A right angle screwdriver could eliminate this need by allowing access to the screws from inside the mouth. However, such a device would need to meet many specifications including size and safety restrictions. Three ideas were taken into consideration. Through research and an evaluation of the design ideas, a sprocket and chain concept was chosen as a promising solution to this problem.
Figure 2
Problem Statement
Figure 3
Motivation
Facial reconstructive surgeries often include the fastening of titanium plates onto the skull to fixate bones during the healing process. Incisions are generally made along the folds of the face to minimize visible post-operative scarring. However, reconstructive surgery of the mandible requires a visible incision on the exterior cheek for the screwdriver to gain access. The surgeon limits the size of the incision made to minimize scarring, which in turn makes it difficult to reach the screws effectively. The implementation of a right angle screwdriver that could fit though an incision of standard size and still effectively seat the screws into the bone would simplify the procedure for the surgeon by allowing better access to the screws without the need for an external incision.
Figure 6-Testing set up
Criteria
Possible Designs
Considerations
Weight
Sprocket and Chain
Worm and Wheel
Right Angle Gears
Safety 10
Ease of Use/Ergonomics 20 18 12 13
Size 35 32 15
1:1 Torque provided 25 23
Durability 8
Total
100 91 76 71
Future Work
Although the team has completed a functional prototype, specifications were not fully met; the overall width of the screwdriver head (from the bottom of the bit to the top frame plate) measures 1.75 cm, when the specifications stated a maximum width of 1.5 cm. Available components and fabrication complications limited the ability to meet this specification.
Other areas of potential future work deal with the casing. Although the easiest concept for casing was to completely enclose the entire device with stainless steel plates, this wouldn’t allow for any sort of maintenance or cleaning. The team decided on a wire meshing for enclosing the majority (lower 14.5 cm) of the device . A more ergonomic handle could also be an area where the design could be altered more to the liking of the surgeon.
In regards to future testing, the device would need to be tested to ensure that constant torque is applied over time, and that the structural integrity of the device holds up with repeated use.
Final Design
Current Apparatus
The current screwdriver is basically like any everyday screwdriver with a handle, straight metal shaft and screw head. The main difference in the existing screwdriver is the feature of a ball-bearing handle that rotates the metal shaft. The ball-bearing eliminates wrist twisting motion. The current apparatus also has interchangeable screw heads. If the surgeon changes screw size, the metal shaft containing the screw head simply pops off the handle and a new screw head is simply clicked in. The screw head is also specially designed to fit the driver bit, ensuring the driver doesn’t slip off the screw head.
Conceptual Operation
The final design operates on a wheel and sprocket concept. Two sprockets are connected by a chain which allows rotational motion to be transferred. One of these sprockets is mounted to the handle, which is turned by the surgeon. The other sprocket is attached to a bit which fits the heads of the screws used in the surgery. The device is made entirely of stainless steel, ensuring it is suitable for surgical safety standards.
Cost Analysis
Item
Cost
Rough prototype mock-up materials
$31.43
Screwdriver head and handle
$89.48
Frame materials
$57.76
Casing (wire mesh)
$7.64
Sprockets and chain
$124.67
TOTAL
$310.98
All materials used in the final device are stainless steel. The handle and head of the prototype were made from general use screwdriver parts, as procuring the surgery-specific screwdriver was cost-ineffective. Components were fastened with a cold-weld epoxy compound. It would be possible to increase cost-effectiveness if components were machined to specifications from scratch.
Figure 1
Client Requirements
Prototype must be in line with surgical safety standards: composed of non-toxic, non-corrosive material and withstand steam autoclaving for sterilization.
It should be able to apply proper torque without compromising the structural integrity even after repeated uses.
It should be durable enough to withstand multiple uses during the day for extended periods of time.
One surgeon should be able to easily operate the prototype with a 1:1 torque ratio
Device and its casing should fit through incision of 5 cm and screw head cannot be more than 1.5 cm wide.
Figure 4
Fabrication
Assembly of the prototype began once all of the materials arrived. To ensure reliable operation, some parts had to be machined in order to become compatible with each other. The width of the sprockets posed a problem with fastening the shaft; if the area of sprocket/shaft attachment was too small, slipping would occur. Sprocket height was therefore unable to be reduced, resulting in a larger overall prototype thickness. Combining the sprocket and shaft together and machining it from a single piece could address this issue.
References
Acknowledgements
Mahajan, A. (personal communication, September 9, 2009).
Synthes CMF. 2.0 mm Mandible Locking Plate System: Advanced plating system for trauma, microvascular reconstruction, and orthognathic surgery
Figure 5
Special thanks to: Professor Thomas Yen, Ashish Mahajan, Dr. Venkat Rao, and Curtis Laluzerne