Bearings help guide drilling machine in bone fracture repairs

To fix a fracture, especially in extreme circumstances where use of a splint or cast would be inadequate, osteosynthesis is performed. This surgical procedure aligns the bones and fixes the fracture internally using metal plates, pins, rods, wires or screws. To avoid injuries of the surrounding tissue structure and to ensure sufficient strength, the length of these screws needs to be precise. The bone diameter and depth of the hole can be determined in advance but only be measured during the operation using a gauge. This is currently dependent on the operator's experience and can be susceptible to human error.

To make this process safer, in a project called the Smart Drill, a student group from Bremen Institute of Measurement Technology, Automation and Quality Science (BIMAQ) developed an attachment for the existing drilling machine. During the drilling process, it measures the depth of the hole in order to determine the correct screw. Due to the special demands on the bearing points of the measuring system, such as resistance to chemicals and moisture, as well as a trouble-free mounting at high speeds, the use plastic self-lubricating bearings was explored.

The special requirements of the medical drilling machine were to develop an optimal bearing in different speed and vibration frequency ranges without a negative influence on the system. “The combination of rotary drive and axial vibration requires the use of high-quality plain bearings not only to implement freedom of movement in the axial direction, but also to achieve the most accurate signals possible at high vibration frequencies,” explained Michael Sorg of BIMAQ. To cover all requirements, various igus plain bearings were compared, and iglidur L500 selected as the best option.

The chosen plain bearing has low friction values and high wear resistance and allows both the rotary and the axial movement. It is also far from its load limit so can easily accommodate drill speeds of up to 900 revolutions per minute. In addition, the bearing withstands medical sterilisation at extreme temperatures. Moisture resistance also plays an important role – iglidur L500 absorbs a negligible amount of water. The lack of lubrication eliminates the risk of contamination of the wound or a premature bearing failure due to washout of the lubricant. Thus, the bearing meets the hygienic requirements of medical technology. The low installation sizes and light weight compared to metallic bearings were also decisive factors.