Thermal Imaging of Medical Saw Blades and Guides

R.B. Dinwiddie1 and T. E. Steffner2

  1. Oak Ridge National Laboratory, Oak Ridge, Tennessee
  2. Better Than New, LLC, 211 Healing Bluff Road, Chattanooga, Tennessee

ABSTRACT

Better Than New, LLC, has developed a surface treatment to reduce the friction and wear of orthopedic saw blades and guides. The medical saw blades were thermally imaged while sawing through fresh animal bone. An IR camera was used to measure the blade temperature as it exited the bone. The test compared the thermal performance of as-manufactured saw blades to surface-treated blades. A fresh blade was used for temperature calibration purposes in order to account for any emissivity changes due to organic transfer layers. Thermal imaging indicates that the treated saw blades cut faster and cooler than untreated blades. In orthopedic surgery, saw guides are used to perfectly size the bone to accept a prosthesis. However, binding can occur between the blade and guide because of misalignment. This condition increases the saw blade temperature and may result in tissue damage. Treated and untreated saw guides were also studied. The treated saw guide operated at a significantly lower temperature than the untreated guide. Saw blades and guides that operate at a cooler temperature are expected to reduce the amount of tissue damage (thermal necrosis) and may reduce the number of post-operative complications.

INTRODUCTION

Orthopedic surgeons use battery-operated oscillating Sagittal saws to cut bones during joint replacement procedures (see Fig. 1). The blades are cooled during operation through the use of copious amounts of saline solution sprayed on the blade. These saws have an elongated saw blade with a toothed segment on one end, while the other end of the blade is pivotally mounted to permit angular oscillation. The blades used in this test are approximately 25.4 mm wide at the cutting end and 111.1 mm long. Each blade has 12 teeth which are approximately 1.6 mm long and approximately 3.2 mm wide at their base. The blades are made of 303 stainless steel alloy. An example of a blade used in these tests is shown in Figure 2A below. If the cutting action heats surrounding tissue above a critical temperature, cell damage, referred to as thermal necrosis, may occur. Heat affects bone tissue by denaturation of enzymes and proteins leading to cell death. This may lead to post-operation complications and extended healing time.

In many instances, a metal saw guide is clamped to the bone to be cut, in order to assure a perpendicular cut to the axis of the bone. In addition, guides may be used to perfectly size the bone to accept a prosthesis. However, binding can occur between the blade and guide because of misalignment. This condition increases the saw blade temperature and may result in tissue damage. Saw blades that operate at a cooler temperature are expected to reduce the amount of tissue damage (thermal necrosis) and may reduce the number of postoperative complications. An example of a blade guide is shown in Figure 2B, below.

Figure 1. Photo of sagittal saw at work.

The purpose of this study is to determine if the heat of the cutting operation can be reduced through the use of a proprietary surface treatment, known as RF-85, developed by Better Than New, LLC, of Chattanooga, Tennessee. Previously unpublished tests have shown that the RF-85 surface treatment can reduce friction of ferrous alloys and help maintain a sharp cutting edge longer than untreated blades. This study will use a high-speed IR camera to measure the temperature during laboratory cutting of fresh swine bone. Treated and untreated blades will be studied in a double blind test. The saw operator and the IR camera operator did not know which blades were treated and which blades were untreated until after the temperature data was plotted. In the case of the saw guides, neither researcher knew which guide was treated and which guide was untreated until after the data was taken and plotted. Wood was used instead of bone during the saw guide tests due to the large variation observed in the thermal results of the bone cutting operations.

Figure 2. A) Photo of a sagittal saw blade. B) Photo of a saw blade in a guide.

DESIGN OF EXPERIMENTS

INFRARED IMAGING

The use of infrared imaging has proven to be a useful tool in measuring the temperature of oscillating saw blades. For this work an Indigo Phoenix mid-wave infrared (MWIR) camera was used. The specifications and settings for this study are:

SAW BLADES

In order to be able to convert the IR signal into a temperature, a calibration was performed by attaching Type-K thermocouples to the untreated and treated used saw blades. The blades were then placed on a hot plate. The hot plate was adjusted to different temperatures and IR images were acquired as a function of temperature. At each temperature, the IR signal was averaged over a large area on the blade. Then the average IR signal was plotted with temperature. A straight line was applied to the data and the equation of this line was used to convert the IR signal into temperature. There was no statistical difference between the untreated and treated saw blades. The calibration curve for the saw blades is shown in Figure 3 below.

Fresh swine bone was used for the testing of the treated versus untreated saw blades. It was expected that the swine bones would closely match the properties of human bone in hardness and diameter. Each section of swine bone to be cut was first mounted in a metal cup with a square cross section using epoxy. This arrangement allowed the bone to be held securely using a bench vice. Each cut was made manually, using a battery operated (12Volt) sagittal saw. A freshly-charged battery was inserted into the saw before each cut was made. Since saline solution would interfere with the IR imaging, no cooling was applied to the blades. Therefore, the temperatures recorded in this study are higher than in actual surgical practice.

The experiment was set-up as a double-blind test. The saw operator and the thermographer did not know which blades were treated and which blades were untreated until the data was analyzed. Figure 4 shows two IR images of the bone cutting tests. Figure 4A shows an untreated blade as it finishes the cut. Figure 4B shows a treated blade as the cut is finished. Notice the thermal profile on the cut-off section of bone as it drops away. The hottest area on the ejected bone section is the area near the final cut. The area near the initial part of the cut has already cooled significantly. Data analyzed included the maximum temperature of the saw blade versus time, the length of the cut (bone diameter), and the time required to complete the cut.

In addition to the saw blade data, a series of tests were performed to measure the temperature of the bone marrow as a function of distance away from the cut. In this set of experiments, the bone cross-section was imaged as shown in Figure 5. The saw was turned off prior to completing the cut in order to leave the bone in one piece. This allowed time for the heat wave to propagate along the bone and reach the viewable surface. The marrow had a significantly higher emittance so a separate calibration was made on the marrow. A thin section of bone was instrumented with a Type-K thermocouple in the marrow and the bone was place on a hot plate. A series of IR images were taken at different temperatures and the IR signal was correlated to the temperature of the marrow. Again, the calibration curve was found to be linear with temperature as was the case with the saw blades.

250

Untreated Blade

200

Treated Blade

150

100

50

0

Figure 3. Calibration for the saw blades.

Temperature, C

3000 3500 4000 4500 5000

IR Signal

Figure 4. View perpendicular to the bone axis: A) IR image showing a test of the untreated saw blade; B) IR image showing a test of the treated saw blade.

Figure 5. View along the bone axis: A) Visible light photo of bone mounted with epoxy in a square cup and clamped in a vice; B) IR image showing the blade heating at the start of a cut; C) The blade appears much hotter as it passes the halfway point through the bone.

SAW BLADE GUIDE BLOCKS

The MWIR camera was calibrated to measure the temperature of the saw blade guide blocks by attaching thermocouples to the guide blocks and resting them on a hot plate. The temperature of the hot plate was adjusted to allow a series of infrared images to be taken over a temperature range of approximately 45° to 200°C. The average IR signal for each block was recorded as a function of the thermocouple readout. From this data, a calibration curve was determined for each block (treated and untreated). The two calibration curves are virtually identical, so there appears to be no significant difference in emittance due to the surface treatment in the 3-5 µm region of the spectrum. The calibration curve for the guide blocks is shown in Figure

6. The IR signal was found to be linear with temperature over the range of the calibration. The temperature range of the calibration was chosen so that it would be unnecessary to extrapolate the calibration equation.

3500 4000 4500 5000 5500

IR Signal

Figure 6. Calibration curve for the guide blocks.

Wood was substituted for bone in the saw guide tests for two reasons. First, it was expected that there would be less variability in the hardness of the wood as compared to bone. The variability in the results, when bone was tested with the saw blades, depends on many factors, including bone diameter, bone wall thickness, and distance from a joint (bone is softer near the joint ends). The second reason for using wood is that the guide was easier to mount onto the wood plank using screws. The poplar wood planks were 31.75 mm by 31.75 mm in cross section and several feet long at the start of the testing. During each test, a 4 - 5 mm section was cut off using the sagittal saw with the blade being positioned through the guide block. In the case of the guide block testing, all cuts were made by an experienced surgeon. As in the case of the saw blades, all tests were conducted using a double-blind arrangement. The surgeon operating the saw and the thermographer did not know which blocks were treated and which blocks were untreated, until after the data was analyzed. A total of five tests were conducted for each of the guide blocks: four treated blades and one untreated blade.

RESULTS AND DISCUSSION

SAW BLADES

Ten sagittal saw blades were randomly selected from a batch of identical blades and treated with the RF-85 proprietary surface treatment at the Better Than New, LLC facility. Each of the 10 treated and 10 untreated blades were used to make one complete cut through a fresh swine bone. The IR camera captured the maximum temperature of each saw as it made the cut. In a preliminary test, it appeared that all of the bones could be cut in less than 6.5 seconds. So the IR image acquisition software was set-up to capture 6.5 seconds of images at 60 Hz. Unfortunately, during testing, half of the untreated saw blades were unable to complete their cuts in the allotted time; therefore we only compare results for 10 treated blades and five untreated blades in the thermal analysis. Cut times were measured using a stop watch, so time data is available for all 20 cuts.

There are several factors which can affect the time and temperature of a cutting operation, including bone diameter, bone hardness, saw feed-rate, and blade design. The issue of feed-rate will cause some variation in the data, due to the manual control of the feed-rate of the saw. However, the double-blind nature of the test should eliminate any systematic variation in feed-rate related to the surface treatment of the saw blades. Since all of the blades are of identical design, we only have to be concerned with the characteristics of the bone. The bone hardness tends to decrease near joints. So, to reduce this effect, all cuts were made several inches away from any joints. Unfortunately, there is no control over the diameter of the bones used in this study. Figure 7 shows the maximum temperature as a function of cut length (Bone diameter). There does not seem to be any indication that maximum temperature is a function of bone diameter. The average Tmax for the untreated blades is 160°C. The average Tmax of the treated blades is 17° cooler at 143°C.

Figure 8 shows the relationship between cut length (bone diameter) and cut time for treated and untreated blades. While there is significant scatter in the data, there is a clear trend of increasing time required to cut bones with bigger diameters. A linear function was fit to the data as a means of smoothing the data and comparing the cutting speed of the treated versus untreated blades. For a given cut length, the treated blades will complete the cut approximately 0.75 seconds faster. Cutting through a 25 mm diameter bone will require approximately 6.0 seconds for the untreated blade and 5.2 seconds for the treated blade. The treated blade cuts approximately 13% faster.

200 180 160 140 120 100

Treated Blade 80

Untreated

Blade 60 40 20

0

Maximum Temperature, C

19 2123 25 27 2931 33

Cut Length (mm)

Figure 7. Comparison of the maximum temperature achieved during cutting between treated (Green) and untreated (Red) saw blades as a function of the bone diameter (cut length). Five of the untreated blades are not plotted because they were unable to finish the cut in the allotted 6.5 seconds.

5 5.5 Faster
4.5 y = 0.1375x + 1.8 Untreated Blades Treated Blades
4 19 21 23 25 27 29 31 33
Cut Length (mm)

Cut Time (Seconds)

7

y = 0.1334x + 2.6895

6.5

Slower

6

Figure 8. Comparison of the cutting speed between treated (Green) and untreated (Red) saw blades. The large scatter in the data is due to the variability in the hardness of individual bones used.

Figure 9 shows the maximum temperature versus number of cuts for two treated and two untreated blades. The maximum temperature was not recorded for five of the untreated tests and one of the treated blade tests because the test ran longer than the 6.5 seconds of IR image acquisition. For each individual cut number, the average of the treated blade temperature is always lower than the average untreated blade temperature. The average for all of the cuts made by the untreated blades is approximately 10°C greater than the average of all of the cuts made by the treated blades. There does not seem to be a systematic trend in the maximum temperature versus cut number for the treated or untreated blades.

A series of experiments was set-up in order to study the distance along the bone affected by the temperature of cutting. The end-view of the bone was imaged so that the temperature of the bone marrow could be recorded by the IR camera. Four cuts were made at various distances from the end of the bone using a treated blade. Two cuts were made at different distances from the end of the bone using an untreated blade. The results are shown in Figure 10. Clearly, the temperature rise due to the untreated saw blade affects the bone marrow over a greater distance than a treated blade. At 4.2 mm away from the cut, the bone marrow temperature still rises above 50°C. However, the bone marrow temperature remains below 30°C even at half the distance. Thus, there is less tissue at risk of thermal necrosis when using the saw blades treated with the RF-85 technology.

200

180

160

140

Untreated 07 120

Untreated 08 100

Treated 06 80

Treated 09 60

40

20

0

Figure 9. Plot showing the effect of repeated cuts on the maximum saw blade temperature. Missing data is due to the time required for these cuts exceeding the 6.5 seconds of data acquisition.

SAW GUIDES

The maximum temperature of the saw guide was measured for 10 different blades. Eight of the blades and one of the guides were treated with the RF-85 proprietary surface treatment. The other guide and remaining two blades were untreated. Saw blades #2, #3, #4, #6, #8, #12, #13, and #14 were all treated. Blade number nine and number 11 were the untreated blades. The untreated blades were #2, #3, #6, #11, and #14. The untreated blade, #11, was the previously used blade. Blades #1, #5, #7, and #10 were not used in this test. The results from one of the treated blades (#3) in the untreated guide were lost due to a computer malfunction. The results are shown in Figure 11. The closed symbols represent the treated saw blades while the open symbols represent the untreated saw blades. In every case, the saw guide that was treated with the RF-85 technology ran cooler than the untreated guide. It is interesting to note that an untreated blade in a treated guide ran cooler than all of the treated blades in the untreated guide. The highest temperature test of the untreated guide resulted from the use of an untreated blade. The average temperature for all four tests of the untreated guide is 153°C. The average temperature of the 5 tests run with the treated guide is 63°C.

Maximum Temperature, C

Distance from Cut (mm)

Figure 10. Plot of the bone marrow temperature rise as a function of distance away from the cut.

234 5 67 8 910111213 14

Saw Blade Number

Figure 11. Comparison of the maximum temperature obtained between treated (Green) and untreated (Red) saw guides. The open symbols represent data using untreated blades, while the closed symbols are for treated blades.

SUMMARY

  1. Treated saw blades cut 13% faster than untreated blades.
  2. Nine out of 10 bones cut by treated blades in < 6 seconds.
  3. Three out of 10 bones cut by untreated blades in < 6 seconds.
  4. Two bones cut in < 5 seconds by treated blades.
  5. On average, treated blades ran at least 17ºC cooler than untreated blades.
  6. Untreated blades: Tmax =160ºC and treated blades: Tmax =143ºC.
  7. Unable to get maximum temperatures for cuts that lasted more than 6.5 seconds. Untreated: 5 out of 10 cuts Treated: 0 out of 10 cuts
  8. Used treated blades significantly outperform new untreated blades.
  9. Use of the treated blades result in significantly lower temperature rises in the bone marrow.
  10. The treated guide blocks allowed the un-cooled blade to run 90°C cooler than the untreated guides.
  11. Therefore, the region of thermal necrosis can be significantly reduced by using treated saw blades and guides.

REFERENCES

  1. Neal M. Blitz, DPM, and Ronald G. Ray, DPM, PT, “How To Handle Lapidus Complications,” Podiatry Today, ISSN: 1045-7860, Volume 16, Issue 8, August 2003, Pages: 38 – 50.
  2. Søren Toksvig-Larsen, MD, Leif Ryd, MD and Anders Lindstrand, MD, “Temperature Influence in Different Orthopedic Saw Blades,” The Journal of Arthroplasty, Vol. 7, No. 1, March 1992, Pages 2124
  3. William R. Krause, Douglas W. Bradbury, James E. Kelly and Emmet M. Lunceford, “Temperature Elevations in Orthopaedic Cutting Operations,” Journal of Biomechanics, Vol. 15, No. 4, 1982, Pages 267-75.

ACKNOWLEDGEMENTS

Research sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract number DE-AC05-00OR22725.

ABOUT THE AUTHOR

Ralph B. Dinwiddie obtained his M.S. and Ph.D. in Physics from the University of Delaware. He started working at Oak Ridge National Laboratory in 1989. Ralph is currently a senior research scientist, leading the Thermography and Thermophysical Properties User Center at the High Temperature Materials Laboratory. He has published more than 60 papers in the field of thermography and thermophysical properties and has edited/co-edited the proceedings from five international conferences, including Thermosense and the International Thermal Conductivity Conferences.