Amrit Sagar

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.

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.

Prediction of Cutting Time When Crosscutting Rounds, Pipe, and Rectangular Bar With a Gravity Fed Portable Bandsaw

Portable bandsaws are gaining in popularity for their use on remote jobsites to efficiently cut structural materials such as bar, pipe, and channel. Some of their increased popularity is due to the recent introduction of high watt-hour lithium ion batteries, which has further improved the portability of bandsaws by making them cordless. However, with cordless bandsaws, knowledge of cutting rates becomes more important as battery runtime limits productivity. Unlike industrial cutoff bandsaws that typically have feed rate control, the cutting rate of portable bandsaws is determined by operator applied pressure and gravity. While some research has highlighted the cutting mechanics of bandsaws and related wear processes, there is a lack of progress in the area of predicting cutoff time as a function of sawing parameters, such as applied thrust force, blade speed, workpiece material properties, and geometry of the cross section. Research was conducted to develop and experimentally verify a mechanistic model to predict cutting rates of various cross sectional geometries with a gravity fed portable bandsaw. The analytical model relies upon experimental determination of a cutting constant equation, which was developed for a low carbon steel workpiece cut with an 18 teeth per inch (TPI) blade. The model was employed to predict crosscutting times for steel rounds, squares, and tubes for several conditions of thrust force and blade speed. Model predictions of cutting time were in close agreement with experimental results.

Fabrication and Investigation of a Micro-Progressive Die Set for Microforming of Sheet Metals

Progressive microforming is an attractive option for manufacturing high aspect-ratio micro-parts out of sheet metal. It is a highly economical alternative to both MEMS processing and micro-machining due to the ability to produce near-net shape parts through parallel forming processes. Current limitations in the field include precise alignment of microforming tools and an understanding of forces encountered when scaling down traditional forming processes. A five stage micro-progressive aluminum die set consisting of four shearing stages and one bending stage was fabricated by micro-machining. The die set was designed to produce right angle micro-brackets from 25 um thick annealed copper foil, with a measured average grain size of 47 um. Die clearances were set at 3 um along shearing edges and 38 um along the bending edge, corresponding to 12% and 152% material thickness, respectively. The produced micro-brackets are intended to be used as electrical connectors and consist of a nominally 280 um by 260 um tab extending vertically from a 780 um by 260 um base. In order to implement and investigate the progressive microforming process, a novel micro-press system was constructed which allows for precision alignment of the die set and workpiece. Using a prepared workpiece, forces at the first stage of the die set were measured and compared to analytical predictions based on models from the literature.© 2014 ASME

Validation of a Conventional Finite Element Model for Simulation of a Micropunching Process

Finite element analysis (FEA) of metal microforming processes may require Crystal Plasticity Finite Element (CPFE) formulations to incorporate material inhomogeneity as feature size approaches grain size. Presently, it is unknown if the micropunching process, where holes are formed by shearing thin metal foils with a thickness on the same scale as grain size, can be accurately simulated by using the material’s bulk material properties or if CPFE is required. In the current research, validity of conventional FEA in simulating micropunching is investigated as CPFE formulations have yet to be integrated with most commercially available programs. Using DEFORM finite element software, strain hardening and strain rate hardening material models were employed to approximate flow stress when punching 200 μm diameter holes in 25 μm thick annealed copper foil. For validation of peak punching force, micro holes were fabricated with a nominal diameter of 200 μm for die clearances ranging from 7.6% to 48% of material thickness. The average grain size of the foil was determined to be approximately 47 μm. Therefore, micropunching was predominantly through a single grain across foil thickness and less than a grain in the direction of radial die clearance. Results indicate that the homogeneous material model in DEFORM is capable of predicting the maximum punching forces with reasonable accuracy, concluding that a CPFE model is not necessary for this category of micropunching. Regardless of die clearance, the maximum punching force was approximately 3 N.Copyright

Effect of Oscillation Speed and Thrust Force on Cortical Bone Temperature During Sagittal Sawing

Thermal necrosis of bone occurs at sustained temperatures above approximately 47°C. During joint replacement surgery, resection of bone by sawing can heat the bone above this necrotic threshold, thereby inducing cellular damage and negatively affecting surgical outcomes. The aim of this research was to investigate the effect of saw blade speed and applied thrust force on the heating of bone. A sagittal sawing fixture was used to make cuts in cortical bovine bone, while thermocouples were used to characterize the temperature profile from the cut surface. A full factorial Design of Experiments was performed to determine the relative effects of blade speed and applied thrust force on temperature. When comparing the effect of speed to force in the regression analysis, the effect of force on temperature (p 0.05). The results of this research can be used in the development of training simulators, where virtual surgeries with haptic feedback can be accompanied by the related temperatures in proximity to the cut. From a clinical perspective, the results indicate that aggressive cutting at higher blade speed and greater thrust force results in lower temperatures in the surrounding bone.Copyright

Deflection of Cancellous Bone Screws Under a Cantilever Bending Load

Surgical bone screws can be subjected to cyclic bending loads when plating constructs are used in the fixation of weight bearing members. While extensive research has been conducted on axial loading that leads to screw pull-out, there is a gap in our understanding of how asymmetric bending loads contribute to screw fracture. The focus of this research was to examine the effect of screw length (20 mm and 40 mm) and cancellous bone density (0.48 g/cm3 and 0.24 g/cm3) on the relative stiffness of 6.5 mm cancellous bone screws subjected to a cantilever bending load. It was hypothesized that longer screws in higher density cancellous bone would result in less screw deflection, supporting clinical practice. For testing, synthetic composite bone was used to simulate the characteristics of natural bone while subjecting screws to quasi-static loading with a universal testing machine. Contrary to the hypothesis, neither screw length nor cancellous bone density resulted in a statistically significant difference (p > 0.05) in deflection for loading up 450 N. The cortical shelf appeared to support the majority of the bending load through compression, rather than acting as a fulcrum. When the 3.0 mm cortex was removed, there was a significant difference in deflection due to both screw length and cancellous bone density.Copyright

Effect of Operating Parameters on the Removal of Bone Cement by a Sawing Process

Currently, there are approximately 33,000 total knee arthroplasty revision surgeries each year. This number is expected to increase with the aging population. During revision surgeries, metal implants are often secured to the tibial plateau by creating a mantel of polymethylmethacrylate (bone cement) that must be removed during revision, often times by sagittal sawing. 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, thrust force, and reciprocating speed on cutting rate. The results demonstrate a linear relationship between both force and cutting rate, and blade speed and cutting rate. A cutting rate model was developed by regression analysis of the experimental data. The model has applications in haptic feedback for surgical simulators. This study provides a basis for understanding the operational parameters of the bone saw, which is the first step in designing saw blades for cutting bone cement as these designs may differ significantly from those optimized to cut cortical bone.Copyright

Effect of Sagittal Sawing Parameters on Histopathology of Bone

A sagittal saw is used for resection of bone during joint replacement surgery. During sawing, tissue at the cut surface can be damaged by high temperatures, which may lead to aseptic loosening of implants. To date, there have been no studies relating sagittal sawing parameters to the level of tissue necrosis. The aim of this study was to determine the feasibility of using histopathological analysis in assessing the severity of thermal necrosis due to sawing. All sawing experiments were performed on cortical bone taken from fresh bovine femur. A two factor, two level design of experiments was performed looking at applied thrust force from 15 N to 30 N and blade oscillation speed from 12,000 cpm to 18,000 cpm. Each cut was subjected to standard histological preparation and the depth of empty lacunae was measured. Both experimental factors, force and speed, showed a statistically significant effect on the depth of thermal necrosis (p< 0.05). However, the interaction of speed and force did not prove to be statistically significant (p = 0.22). From a clinical perspective, the results indicate that choosing higher blade speeds and applying greater force can reduce the amount of thermal damage during sagittal sawing.Copyright