Thomas P. James

Effect of applied force and blade speed on histopathology of bone during resection by sagittal saw

A sagittal saw is commonly used for resection of bone during joint replacement surgery. During sawing, heat is generated that can lead to an increase in temperature at the resected surface. The aim of this study was to determine the effect of applied thrust force and blade speed on generating heat. The effect of these factors and their interactions on cutting temperature and bone health were investigated with a full factorial Design of Experiments approach for two levels of thrust force, 15 N and 30 N, and for two levels of blade oscillation rate, 12,000 and 18,000 cycles per minute (cpm). In addition, a preliminary study was conducted to eliminate blade wear as a confounding factor. A custom sawing fixture was used to crosscut samples of fresh bovine cortical bone while temperature in the bone was measured by thermocouple (n=40), followed by measurements of the depth of thermal necrosis by histopathological analysis (n=200). An analysis of variance was used to determine the significance of the factor effects on necrotic depth as evidenced by empty lacunae. Both thrust force and blade speed demonstrated a statistically significant effect on the depth of osteonecrosis (p<0.05), while the interaction of thrust force with blade speed was not significant (p=0.22). The minimum necrotic depth observed was 0.50mm, corresponding to a higher level of force and blade speed (30 N, 18,000 cpm). Under these conditions, a maximum temperature of 93°C was measured at 0.3mm from the kerf. With a decrease in both thrust force and blade speed (15N, 12,000 cpm), the temperature in the bone increased to 109°C, corresponding to a nearly 50% increase in depth of the necrotic zone to 0.74 mm. A predictive equation for necrotic depth in terms of thrust force and blade speed was determined through regression analysis and validated by experiment. The histology results imply that an increase in applied thrust force is more effective in reducing the depth of thermal damage to surrounding bone than an increase in blade speed.

Predictive force model for haptic feedback in bone sawing

Bone sawing simulators with force feedback represent a cost effective means of training orthopedic surgeons in various surgical procedures, such as total knee arthroplasty. To develop a machine with accurate haptic feedback, giving a sensation of both cutting force and rate of material removal, algorithms are required to forecast bone sawing forces based on user input. Presently, studies on forces generated while machining bone are not representative of the high cutting speeds and low depths of cut common to the bone sawing process. The objective of this research was to quantify sawing forces in cortical bone as a function of blade speed and depth of cut. A fixture was developed to simulate linear bone sawing over a range of speeds comparable to surgical reciprocating and oscillating (sagittal) bone saws. A single saw blade tooth was isolated and used to create a slotted cut in bovine cortical bone. Over a range in linear sawing speed from 1700 to 7000 mm/s, a t-test (α=0.05) revealed there was no statistically significant effect of blade speed on either cutting or thrust force. However, an increase in depth of cut from 2 to 10 μm resulted in a 30% increase in thrust force, while cutting force remained constant. The increase in thrust force with depth of cut was relatively linear, R(2)=0.80. Using a two factor, two level design of experiments approach, regression equations were developed to relate sawing forces to changes in blade speed and depth of cut. These equations can be used to predict forces in a haptic feedback model.

Rounded Cutting Edge Model for the Prediction of Bone Sawing Forces

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 experiments 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.

Sagittal Bone Saw With Orbital Blade Motion for Improved Cutting Efficiency

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 ¼32). 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 0.10mm 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. [DOI: 10.1115/1.4023500]

A Cutting Rate Model for Reciprocating Sawing

In the present paper, the reciprocating sawing process is analyzed, and a model for linear cutting rate is developed. The resulting model is based on an orthogonal approximation of cutting at individual teeth and accounts for elastic and plastic indentation. Cutting rates obtained from an instrumented sawing fixture show good agreement with predicted results for the range of conditions considered. Cutting rate was found to be proportional to thrust force and reciprocating rate though this behavior is influenced by edge radius and flow stress at higher levels. While it was not possible to decouple the effect of pitch and blade set, it was confirmed that coarser pitch blades do provide higher cutting rates.

Is Synthetic Composite Bone a Substitute for Natural Bone in Screw Bending Tests

Composite replica bones have been used extensively for biomechanical studies. These studies normally rely upon the overall tensile, compressive, and bending strength of large replica bones, such as the tibia and femur. In this study, highly localized behavior of composite bone was scrutinized by examining the material’s response to cortical screws in bending. Of interest was localized deformation of the composite material as compared to the response of natural bone under similar loading conditions. Cortical screw deflection in a laminated composite bone was compared to deflection in a bovine bone under quasi-static loading. The laminated composite bone consisted of short glass fiber reinforced epoxy as a cortical bone substitute, while polyurethane foam was used as a cancellous bone substitute. A new laser projection method was used to make comparative measurements of the slope of the screw head near to the applied load. Initial results indicate that composite bone is a reliable substitute for natural bone in quasi-static studies of cortical screw deflection.© 2011 ASME

Linear Sawing Apparatus and its Evaluation for Research Into High Speed Bone Sawing

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.Copyright

Reciprocating Bone Saw: Effect of Blade Speed on Cutting Rate

Power reciprocating saws are used in surgical procedures to cut bone. Improved cutting rates are desirable in order to reduce operative time and improve patient outcome. A fixture was developed to test the effect of blade speed on cutting rate of bovine cortical bone. It was hypothesized that the volumetric cutting rate would increase in a linear manner for a fixed stroke length and a constant thrust force. A 7.0 N thrust force was applied. The reciprocating stroke length was held constant at 3.0 mm. Using an 18 TPI blade, cutting rate was determined to increase in a slightly non-linear manner, with disproportionately higher cutting rate at higher blade speeds. The data implies that a higher reciprocating frequency may invoke more efficient cutting.Copyright

Effect of operating parameters on the removal of bone cement by a sawing process.

The number of total knee arthroplasty revision surgeries is increasing each year, driven by the wide availability and general acceptance of the procedure accompanied by an aging population of implants. Metal implants are often secured to the tibial plateau by a mantle of poly(methyl methacrylate) bone cement. During revision surgery, a power oscillating saw is used to remove bone cement while preparing the boney bed. Presently, there are no published studies on the mechanics of bone cement removal by a sawing process. The aim of this research was to quantify the effect of blade speed and applied thrust force on the volumetric cutting rate of bone cement. A custom reciprocating saw with variable stroke length was used to conduct a three-factor design of experiments. Two levels, without center-points, were sufficient to model the effect of stroke length (6.75, 10.13 mm), thrust force (11, 19 N), and reciprocating speed in strokes per minute (6000, 8000 SPM) on cutting rate. The results indicate that each of the three parameters had a significant impact on cutting rate (p < 0.001), with a linear relationship between both force and cutting rate (r = 0.98) and blade speed and cutting rate (r = 0.98). For the parameters considered, increasing the reciprocating speed had the most significant effect on cutting rate. For example, while holding force and stroke length constant (11 N, 10.13 mm), an increase in speed from 6000 to 8000 SPM nearly doubled the cutting rate of bone cement. A cutting rate model was developed by regression analysis of the experimental data and validated through additional experiments. The model has applications in haptic feedback for surgical simulators to differentiate between the cutting rates of bone and bone cement during virtual training of resident surgeons.

Evaluation of Cutting Forces During Single Pass Microtrenching of Poly (Methyl Methacrylate)

Research was conducted to evaluate a microtrenching process to create microchannels on the surface of poly (methyl methacrylate) (PMMA) for applications in tissue engineering. Experiments with a trenching tool included an exaggerated cutting edge radius (48 μm) to study the impact of a highly negative effective rake angle on forces during single pass microtrenching at subradius cutting conditions. During microtrenching, forces were measured by dynamometer and compared to a finite element (FE) model using an elastic-perfectly plastic material model for an undeformed chip thickness from 9 to 64 μm. During experiments, cutting was first observed when the ratio of undeformed chip thickness to cutting edge radius was 0.33. Measured and predicted values of thrust force exceeded cutting force up to an undeformed chip thickness equivalent to the cutting edge radius. The FE model predicted a linear trend in cutting force with feed (r = 0.99) and was substantiated by linear regression of experimental data (r = 0.99). However, at lower values of feed the model overestimated force, with a maximum difference of 42% at a feed of 22 μm. Thrust force was also predicted to be linear (r = 0.99), but at greater feed the experiments indicated a nonlinear decline in thrust force, resulting in a maximum difference of 27% at 64 μm. Finally, an analysis of nodal velocity plots from the FE model revealed a material stagnation zone developed along the cutting edge, rising from the workpiece surface in proportion to feed and then remaining fixed at 63 deg (stagnation angle) for all feeds greater than 35 μm. While the application of an elastic-perfectly plastic material model for PMMA was sufficient to predict microtrenching forces by the FE method, differences between predicted and measured thrust forces at greater undeformed chip thickness implies a more complex rheological model may add value.