A new analytical model to predict bone sawing forces is presented. Development of the model was based on the concept of a single tooth sawing at a depth of cut less than the cutting edge radius. A variable friction model was incorporated as well as elastic Hertzian contact stress to determine a lower bound for the integration limits. A new high speed linear apparatus was developed to simulate cutting edge speeds encountered with sagittal and reciprocating bone saws. Orthogonal cutting experiments in bovine cortical bone were conducted for comparison to the model. A design of the experiment's approach was utilized with linear cutting speeds between 2600 and 6200 mm/s for depths of cut between 2.5 and 10 μm. Resultant forces from the design of experiments were in the range of 8 to 11 N, with higher forces at greater depths of cut. Model predictions for resultant force magnitude were generally within one standard deviation of the measured force. However, the model consistently predicted a thrust to cutting force ratio that was greater than measured. Consequently, resultant force angles predicted by the model were generally 20 deg higher than calculated from experimental thrust and cutting force measurements.

No abstract

Sagittal bone saws are used by orthopedic surgeons for resection of bone; for example in total joint arthroplasty of the hip and knee. In order to prevent damage to surrounding tissue, sagittal saw blades typically oscillate through a small angle, resulting in reduced cutting rates due to short stroke lengths. To improve bone cutting efficiency, sagittal saws oscillate at high speeds, but this creates frictional heating that can harm bone cells. The focus of this research was to design a new sagittal sawing device for improved cutting efficiency. It was hypothesized that the addition of an impulsive thrust force during the cutting stroke would increase cutting rates in cortical bone. A cam-driven device was developed and tested in bovine cortical bone. The impulsive thrust force was achieved by creating a component of blade motion perpendicular to the cutting direction, i.e., orbital blade motion. At the start of each cutting stroke, the mechanism drove the saw blade into the surface of the bone, increasing the thrust force with the intention of increasing the depth of cut per tooth. As each cutting stroke was completed, the blade was retracted from the surface for the purpose of clearing bone chips. The design parameters investigated were cutting stroke length, thrust stroke length, and blade oscillation frequency. A three-factor, two-level design of experiments approach was used to simultaneously test for the effect of design parameters and their interactions on volumetric cutting rate (n = 32). The addition of orbital blade motion to the sagittal saw improved bone cutting rates over traditional oscillatory motion, especially at lower cutting stroke lengths and higher oscillation frequencies (p < 0.05). However, the magnitude of orbital blade motion based on thrust stroke length was limited by a threshold value of approximately 0.10 mm that when exceeded caused the sagittal saw to rebound from the surface of the bone causing erratic cutting conditions. The factor with the greatest positive effect on cutting rate was oscillation frequency. Cutting rates in cortical bone can be improved with the proposed orbital action sagittal saw.

Previous research into the cutting mechanics of bone sawing has been primarily approached from the perspective of orthogonal metal machining with a single edge cutting tool. This was a natural progression from the larger body of knowledge on the mechanics of metal cutting. However, there are significant differences between typical orthogonal metal cutting parameters and those encountered in bone sawing, such as anisotropic material behavior, depth of cut on the order of cutting edge radius, chip formation mechanism in the context of a saw blade kerf, non-orthogonal considerations of set saw blade teeth, and cutting speed to name a few. In the present study, an attempt is made to overcome these shortcomings by employing a unique sawing fixture, developed to establish cutting speeds equivalent to those of typical sagittal saws used in orthopaedic procedures. The apparatus was developed for research into bone sawing mechanics and is not intended to be a commercial sawing machine. The sawing fixture incorporates the cutting speed possible with lathe operations, as well as the linear cutting capabilities of a milling machine. Depths of cut are on the same order of magnitude as the cutting edge radius typical to saw blade teeth. Initial measurements of cutting and thrust force, obtained with this new experimental equipment, are compared to previous work.