- Email: [email protected]
Force and Torque Measurements of Surgical Drilling Into Whole Bone
CHAPTER
6
Force and Torque Measurements of Surgical Drilling Into Whole Bone
Radovan Zdero1, Troy MacAvelia2, Farrokh Janabi-Sharifi2 Western University, London, ON, Canada1; Ryerson University, Toronto, ON, Canada2
1. BACKGROUND Repairing whole bone fractures requires surgical drilling to create pilot holes for easy insertion of cortical screws, cancellous screws, and locking bolts to align adjacent bone fragments, apply fracture plates, or insert fracture nails (Fig. 6.1).1e14 Although drilling may be done using CO2 pulsed lasers, haptic
A
B
C
D
FIGURE 6.1 Surgical drilling for fracture repair. (A) Surgical drill bit, (B) proximal femur, (C) proximal humerus, (D) humeral diaphysis.
Experimental Methods in Orthopaedic Biomechanics. http://dx.doi.org/10.1016/B978-0-12-803802-4.00006-8 Copyright © 2017 Elsevier Inc. All rights reserved.
85
86
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
systems that offer tactile force and torque feedback, and teleoperation in which a surgeon conducts the procedure off-site, electrically-powered hand drills remain the common clinical practice. Force and torque generated at the drill bite bone interface are influenced by feed rate (i.e., linear speed of the drill bit into bone), spindle speed (i.e., rotational speed of the drill bit), drill bit tip angle (i.e., angle of the pointed tip of the drill bit), drill bit size (i.e., outer diameter), and bone type (i.e., cortical vs. cancellous, normal vs. osteoporotic, human vs. animal, etc.). Nonoptimal force and torque can raise temperatures, causing bone necrosis, as well as poor pilot hole quality, causing poor screw fixation. Consequently, optimization of drilling force and torque is a clinically relevant topic. Therefore, this chapter explains how to measure force and torque during surgical drilling into whole bone, as well as how to analyze, present, and interpret the results.
2. RESEARCH QUESTIONS Typical research questions might include one or more of the following: • • • • • •
Do orthopaedic surgeons with varying experience generate different drilling force and torque? Do whole bone storage, thawing time, age, type, etc., affect drilling force and torque? Do drill bit feed rate, spindle speed, and geometry change drilling force and torque? Do drilling force and torque influence pilot hole quality and, thus, screw fixation strength? Do drilling force and torque alter bone temperature, potentially leading to bone necrosis? etc.
3. METHODOLOGY 3.1 GENERAL STRATEGY Human or animal whole bones or segments are secured by a vice directly under a surgical drill bit whose feed rate and spindle speed are computer controlled. The vice is equipped with a force and torque sensor, which actively records data during drilling. Raw force and torque data are then normalized by the surface area of the pilot hole to eliminate geometric effects. Statistical comparisons are made between the test groups for raw and normalized drilling measurements. Finally, correlation coefficients are computed for raw and normalized data vs. biological bone demographics (i.e., age, sex, limb side, bone mineral density (BMD), and clinical T-score) to determine which factors are important.
3. Methodology
GLOSSARY ✓ ✓ ✓ ✓
Feed rate. Linear speed of the drill bit along its long axis during drilling. Force. The linear load experienced by the bone along the long axis of the drill bit. Spindle speed. Rotational speed of the drill bit around its long axis during drilling. Torque. The angular load experienced by the bone around the long axis of the drill bit.
SAFETY FIRST ✓ ✓ ✓ ✓
Remember to always wear goggles and gloves for protection. Secure the vice and bone before using the drill press to conduct drilling tests. Use a fume hood or do tests in an open area, since bone may emit a burning odor when drilled. Clean the work area and all tools with bleach or disinfectant after testing bone specimens.
3.2 MATERIALS AND TOOLS LIST • • • • • • • • •
computer-controlled drill press force and torque sensors human or animal bone leveling gage surgical drill bits tape measure thickness gage Vernier calipers vice or clamp
3.3 SPECIMEN PREPARATION Step 1. Store the bone. Fresh or freshefrozen whole bones or segments initially need to be wrapped in plastic strips or vacuum-sealed in plastic bags for proper storage in a freezer prior to the study. The freezer should be held at 20 C or colder. Note that if using embalmed or dried/dehydrated bones, these do not need to be frozen and may simply be placed in a plastic box or bag for storage at room temperature prior to tests. Step 2. Thaw the bone. Bone specimens should be removed from the freezer, left in their plastic wrappings or vacuum-sealed bags, and placed on a surface at ambient room temperature or in a warm water bath to thaw for at least 12 h. Bone specimens are then removed from their plastic wrappings or bags and soaked or sprayed with saline water solution to prevent dehydration. Step 3. Determine the drilling site. Mount bone specimens into a vice or clamp them onto a table, make measurements for the drilling site, and mark the location
87
88
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
with a pen. Because of geometric variability among biological bone, the relative location of the drilling site with respect to a common reference dimension needs to be computed and used for all bone specimens (e.g., % ¼ relative dimension/reference dimension 100 ¼ distance from one end of the bone to the drilling site/total length of bone 100). TIPS AND TRICKS ✓ ✓ ✓ ✓
Whole bones can be cut into segments with a band saw for easier handling. Place a leveling gage on the bone surface to ensure drill direction is perpendicular. Insert a new drill bit when needed to avoid biasing data with dull drill bits. The same researcher should perform all tests for consistency.
THE “GOLD STANDARD” The International Organization for Standardization (ISO) provides guidelines for drill bit design factors, such as materials selection, mechanical properties, and labeling, in its document, ISO 9714-1 (Orthopaedic drilling instruments e Part 1: drill bits, taps and countersink cutters), but no standards exist for surgical bone drilling itself. Thus, researchers should use peer-reviewed journal articles and/or medical manufacturer’s surgical technique manuals as guidelines.
3.4 SPECIMEN TESTING Step 1. Assemble the test setup. An experimental test setup for surgical bone drilling research must first be assembled (Fig. 6.2). Several components are vital. A computer-controlled drill press will adjust drill bit feed rate and spindle speed over the typical ranges used in the research literature or clinically in orthopaedic surgery. A vice will rigidly hold the bone specimens in proper position. A biaxial force and torque sensor should be attached under the vice to monitor surgical drilling load, but it must be of sufficient resolution and accuracy. An LVDT (linear variable displacement transducer) can be attached to the drill bit chuck to measure drilling depth. Vibration isolation pads should be placed under the drill press and vice to minimize mechanical vibration. A heavy work bench will be used to mount the entire assembly, but will also further minimize mechanical vibration. Water, liquid coolant, or compressed air may be used to cool the drill bit and bone, as well as to unclog debris from the drill bit and clear away debris from the bone specimen. Orthopaedic surgical drill bits that are unused need to be procured in sufficient quantity. Step 2. Create a “pecking” hole. Create a preliminary pecking hole, which will later be used to guide the real surgical drill bit during the actual drilling test. To do this, place the bone into the vice under the center of the drill bit chuck. Use a leveling
3. Methodology
A
B
FIGURE 6.2 Experimental setup for drilling into bone. (A) Overall test setup, (B) close-up of the force and torque sensor.
gage to ensure the bone site is perpendicular to the drill bit direction. Insert a “dummy” center drill bit (e.g., size #3 with a 2.8-mm diameter) into the chuck, and center it about 20 mm above the bone drilling site. Set the desired spindle speed, but not the feed rate. Then, manually, slowly, and incrementally lower the spinning dummy center drill bit until the tip superficially punctures the bone to the correct pecking depth (e.g., 0.25e0.50 mm). Pecking depth can usually be detected by a grinding sound or slight vibration or visual observation, but the LVDT may be more reliable. Note that lowering the dummy center drill bit too quickly during pecking could cause the tip to slip overtop of a slippery bone surface, so the wrong location is punctured, as well as potentially bending or breaking the drill bit. Then, retract the dummy center drill bit and remove it from the chuck. Step 3. Choose the drill bit. The correct surgical drill bit for the real drilling test then needs to be chosen. This is based on the surgical procedure that will make use of the pilot hole (e.g., cortical or cancellous screw insertion during fracture plating, mounting an acetabular cup during total hip arthroplasty, etc.), the type of bone being drilled (e.g., cortical vs. cancellous, normal vs. osteoporotic, etc.), and/or the particular research question being asked about the drill bit (e.g., effects of cutting flute geometry, drill bit material, drill bit diameter, etc.). Typical drill bits used in the research literature and/or clinically have outer diameters of 1.98e4.76 mm and lengths of 60e200 mm.1,2,4,5,9,11,12,14 Step 4. Do the drilling test. Perform the real drilling test that will create the final pilot hole. To do this, insert the real unused surgical drill bit of appropriate size into the chuck and position it approximately 20 mm above the bone specimen. Then, start the automated computer-controlled drilling process by setting the desired feed rate
89
90
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
(e.g., 10e132 mm/min) and spindle speed (e.g., 40e3300 rpm) within typical ranges used in the research literature and/or clinically.1,2,5,9,11,12,16 Ensure that the drill and the force and torque sensors are started simultaneously for data collection. Allow drilling to continue until the drill bit tip reaches the desired depth. Then, return the drill bit to its home position, turn off the drill press, and remove the bone from the vice. Clear the drill bit flutes of clogged material in preparation for the next test. Cooling the bone can be done using water, saline solution, liquid coolant, or compressed air if the research question requires it; however, irrigation may not always be done during actual orthopaedic surgery. Step 5. Measure pilot hole length. Remove the bone from the drill press and hold it firmly on a work surface, whereas large specimens may be secured using a vice or clamp on a work surface. To measure cortical wall thickness at the drilling site, insert a clinical or mechanical depth gage that has a hooked end, pull the hooked end until it engages the intramedullary underside of the cortical wall, and then measure this length using a ruler or Vernier caliper (Fig. 6.3AeC). To measure cancellous pilot hole depth at the drilling site, insert a thin, rigid guide wire until it feels
A
B
C
D
E
F
FIGURE 6.3 Measurement of pilot hole geometry. (A) Cortical bone specimen with a pilot hole, (B) depth gage insertion into a cortical pilot hole, (C) depth measurement of a cortical pilot hole using a depth gage and Vernier calipers, (D) cancellous bone specimen with a pilot hole, (E) guide wire insertion into a cancellous pilot hole, (F) depth measurement of a cancellous pilot hole using a guide wire and ruler.
3. Methodology
like it has reached the bottom of the hole, use a pen to mark the guide wire’s surface that is at the entrance of the pilot hole, remove the guide wire, and then measure the length of the guide wire’s tip-to-pen mark using a ruler or Vernier calipers (Fig. 6.3DeF). Guide wires can also be used to clear away unwanted bone debris from the pilot hole. Step 6. Photograph or draw the specimen. Photograph or draw the pilot hole that was created, the bone debris that was generated during drilling, and the drill bit for later data analysis.
3.5 RAW DATA COLLECTION Step 1. Record bone characteristics. Enter human or animal demographic information (i.e., age, sex, and left or right limb) and quantitative bone properties (i.e., BMD and clinical T-score), which will help determine the factors that are statistically correlated with drilling force and torque (Table 6.1). Note that BMD is in 2D units (i.e., g/cm2 rather than g/cm3), since DEXA (dual-energy X-ray absorptiometry) bone density scans are only 2-D. DEXA scan reports often provide BMD values and corresponding clinical T-scores (i.e., normal Tscore 1, 1 > osteopenic T-score > 2.5, and osteoporotic T-score 2.5). When such analysis is overlooked or missing, computations can be made as Tscore ¼ (bone BMD e reference population mean BMD)/(1 standard deviation of reference population BMD), where T-score reference populations are young healthy adults of the same sex that are aged 20e40 years.15,16 Step 2. Record pilot hole geometry. Input the pilot hole diameter (i.e., drill bit diameter) into the summary table (Table 6.2). Enter the pilot hole length, which is the wall thickness for unicortical or bicortical specimens, whereas it is the pilot hole depth for cancellous specimens (Table 6.2). Step 3. Record drilling parameters. Feed rate, spindle speed, and drill bit tip angle should be noted (Table 6.2). Raw force and torque should have been recorded during drilling and then smoothed for noise artifacts using a running average function or other appropriate filtering algorithm. Then, peak force and torque can be identified (Table 6.2). Table 6.1 Bone characteristics.
Bone 1 2 3 etc. Avg SD
Age [years]
Sex [M, F]
Limb Side [L, R]
e e
e e
Avg, average; SD, standard deviation.
Bone Mineral Density [g/cm2]
T-score
91
92
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
Table 6.2 Drilling test parameters. Feed Rate f [mm/ min]
Spindle Speed u [rpm]
Drill Bit Tip Angle q [8]
Drill Bit Diameter D [mm]
Avg
e
e
e
e
e
SD
e
e
e
e
e
Bone
Pilot Hole Length L [mm]
Peak Force F [N]
Peak Torque T [N$mm]
Remarks
1 2 3 etc.
Avg, average; SD, standard deviation.
Step 4. Record visual observations. Inspect prior photographs or drawings and record remarks on the pilot hole (e.g., smooth? circular? surface cracking? etc.), the bone debris that has been removed (e.g., shape, size, texture, color), and the drill bit (e.g., wear, bending, breaking, etc.) (Table 6.2).
3.6 RAW DATA ANALYSIS Step 1. Normalize raw data. Raw peak forces and torques should be normalized by geometry by taking into account the surface area of the pilot hole that is in contact with the drill bit. This can be computed as FNORM ¼ FRAW/A ¼ FRAW/(pDL) and TNORM ¼ TRAW/A ¼ TRAW/(pDL), where F is force, T is torque, A is surface area, D is pilot hole diameter, and L is bone wall thickness (cortical specimen) or hole depth (cancellous specimen). Step 2. Calculate correlation coefficients. Plot the measured raw and normalized result for each individual specimen (i.e., peak force 1, 2, 3, etc.) (Table 6.2) vs. its corresponding specimen characteristic (i.e., age 1, 2, 3, etc.) (Table 6.1) to help visualize the interrelationship between measurements vs. characteristics. Then, calculate the correlation coefficient R for each measurementecharacteristic pair to determine which characteristic has an influence on measurements (e.g., R > 0.8 is a typical value considered to indicate a strong correlation). Step 3. Perform statistical comparisons. The criterion for statistical difference needs to be chosen (e.g., P <0.01 or <0.05). Then, data can be used to compare different patient groups (e.g., men 60 years old vs. men >60 years old, “normal” women vs. “osteoporotic” women, etc.), bone types (e.g., femur vs. tibia, left humerus vs. right humerus, etc.), drilling sites (e.g., anterior vs. lateral surface), drill bit designs (e.g., small vs. large outer diameter), etc. Various software programs exist for comparing two test groups (e.g., paired t-test) or two or more test groups influenced by multiple factors (e.g., analysis of variance, ANOVA). Step 4. Compute statistical power. Power analysis can be done after the study to ensure there were enough specimens per group to detect all statistical differences that were actually present (i.e., was type II statistical error avoided?). Statistical
4. Results
power >80% is usually considered to indicate there were enough specimens per test group. Note that if good predictions of averages and standard deviations are available from prior studies, then the number of specimens and/or tests can be chosen before the study begins to ensure a power >80%.
ENGINEER’S TOOLBOX Force and torque for bone drilling can be estimated using engineering formulas. Assume no effects from bone isotropy, homogeneity, and viscoelasticity, or from bone chip clogging of drill bit cutting flutes, bone temperature, and drill bit wear. So, for each revolution of the drill bit around its long axis, the force and torque , where F is force [N], T is torque U U [N$mm], U is ultimate tensile stress of bone [N/mm2], f is drill bit feed rate [mm/min], u is drill bit spindle speed [rpm or revolutions/min], D is drill bit outer diameter [mm], and q is drill bit tip angle [ ].
s
s
s
4. RESULTS Once all surgical drilling data collection and analysis have been performed, it is then important to communicate and present the primary results in an understandable and concise manner to the reader of a journal article, conference paper, technical report, or book chapter. Step 1. Show raw data profile. Begin with typical raw force and torque profiles vs. drilling depth or time for both unsmoothed and smoothed data (Fig. 6.4). Initially, force and torque are zero before the drill bit tip contacts the bone. Then, there is a small “step” in force and torque as the drill bit cutting flutes enter the “pecking” hole to make superficial contact with bone. Following this, there is a gradual increase in force and torque to some peak value as the drill bit tip becomes fully immersed into bone. Next, a gradual drop in force and torque occur as the drill bit exits through the other side of the bone wall (i.e., cortical drilling) or reaches the final drilling depth (i.e., cancellous drilling), but a small amount of torque is maintained as the drill bit
93
94
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
FIGURE 6.4 Raw data profiles of surgical drilling through one contiguous segment of cortical or cancellous bone. (A) Force (unsmoothed for noise), (B) torque (unsmoothed for noise), (C) force (smoothed for noise), (D) torque (smoothed for noise).
continues to spin and make superficial contact with surrounding bone. For the unsmoothed data, note that there are usually small-scale high-frequency fluctuations superimposed on top of large-scale low-frequency fluctuations in force and torque, which may be due a combination of factors: (1) bone anisotropy, porosity, and viscoelasticity; (2) mechanical flexing of the bone as the drill bit thrusts forward; (3) clogging of drill bit cutting flutes with bone debris; (4) drill bit vibration caused by drill bit bending or a loose bone specimen; and (5) load alterations with each circumferential pass of a cutting flute past a given point on the bone. Although noise smoothing algorithms eliminate these small-scale high-frequency fluctuations, some large-scale low-frequency fluctuations remain. Step 2. Show main results. The main numerical findings of the study can be presented, namely, raw and normalized peak force and torque (Fig. 6.5). For each of these parameters, all statistical pairwise comparisons should be made (i.e., bone type 1 vs. 2 vs. 3, drill bit type 1 vs. 2 vs. 3, etc.) to generate statistical P values
4. Results
FIGURE 6.5 Surgical drilling test results. (A) Force vs. test group, (B) torque vs. test group, where bar graphs show average 1 standard deviation and P is the statistical difference result for each pairwise comparison between groups which can be indicated using symbols like asterisks (*), pounds (£), etc. (C) Force vs. BMD, (D) torque vs. BMD, where BMD is bone mineral density, R is the linear correlation coefficient for each line of best fit, P is the statistical difference value ensuring the slope of the line of best fit (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0), and y ¼ mx þ b is the equation of each line.
(Fig. 6.5A and B). Any linear (or nonlinear) trends can be illustrated for force and torque vs. BMD (Fig. 6.5C and D). For each line of best fit, several items can be generated: an equation y ¼ mx þ b showing slope m and intercept b, a linear correlation coefficient R, and its own P value to ensure the slope of each line of best fit (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0). Also, if a substantially wide range of drilling parameters has been examined, then results can be presented as a family of curves that show how various parameters affect peak force and torque (Fig. 6.6). Step 3. Show specimen photos. Present photos to help visualize the quality of drilling, such as the pilot hole (e.g., circular? symmetric? surface cracking? etc.), bone debris (e.g., shape, size, texture, etc.), and drill bit cutting flutes (e.g., bending, breaking, wear, etc.) (Fig. 6.7).
95
96
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
FIGURE 6.6 is Typical family of curves for surgical drilling into bone. (A) Force, (B) torque. ultimate tensile stress of bone, f is drill bit feed rate, D is drill bit outer diameter, and q is drill bit tip angle.
FIGURE 6.7 Qualitative drilling test outcomes for the pilot hole, bone debris, and drill bit.
5. Discussion
ALTERNATIVES AND ADAPTATIONS ✓ Sawbone whole bones. Artificial whole bones, rather than biological bones, can be used as substitutes, since many commercially available sawbones mimic human bone geometry, they have been biomechanically validated, and they are increasingly used for research. 3-D density (i.e., g/cm3), rather than 2-D BMD (i.e., g/cm2), needs to be measured or obtained from the manufacturer, since DEXA scans for BMD are only suitable for biological bone. Note that sawbones have isotropic and homogeneous properties, unlike biological bone. ✓ Sawbone shapes. Artificial bones with standardized geometries and material properties may be useful when comparing different drill bit designs since bone parameters are fixed and known. These shapes are available as sheets, blocks, and cylinders. 3-D density (i.e., g/cm3), rather than 2-D BMD (i.e., g/cm2), needs to be measured or obtained from the manufacturer, since DEXA scans for BMD are only suitable for biological bone. Remember that sawbones have isotropic and homogeneous properties, unlike biological bone.
5. DISCUSSION After completing all surgical drilling tests, data collection, data analysis, and data presentation, then final results can be considered and interpreted in the broader context of some important clinical, biomechanical, and/or technological considerations, as follows. Drilling force and torque are not monitored during orthopaedic surgery. Instead, surgeons use an electrically-powered surgical hand drill to set the spindle speed, which depends on the surgical procedure being done. Moreover, the feed rate of the drill bit into the bone is controlled purely by “subjective feel.” Alternately, preoperative patient BMD scans could be done to permit engineering predictions of appropriate drilling force and torque, followed by the use of a surgical hand drill equipped with a digital force and torque display. Drilling force and torque in bone have a wide range of values since they can be affected by feed rate, spindle speed, drill bit tip angle, drill bit diameter, and bone type. Drilling force studies have yielded average peaks of 5e300 N (human cortical bone),9,10,13,14 1e1.5 N (human cancellous bone),13 2e50 N (porcine cortical bone),2,11,12 4e32 N (porcine cancellous bone),12 and 0e275 N (bovine cortical bone).1,3,5,8 Similarly, drilling torque studies have reported average peaks of 0e186 N$mm (human cortical bone),9,11,14 2e3 N$mm (human cancellous bone),13 55 N$mm (porcine cortical bone),2 and 0e400 N$mm (bovine cortical bone).1,3,5,8 Drilling force and torque can be estimated using engineering formulas, which are based on empirical research and mechanics theory.2,17 Assume there are no effects from bone isotropy, bone homogeneity, bone viscoelasticity, bone chip clogging of the drill bit cutting flutes, bone heating, and drill bit wear. Thus, for each revolution and the torque sU of the drill bit around its long axis, the force , where F is force [N], T is torque [N$mm], is ultimate tensU sile stress of bone [N/mm2], f is drill bit feed rate [mm/min], u is drill bit spindle speed [rpm], D is drill bit outer diameter [mm], and q is drill bit tip angle [ ].
97
98
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
Thermal generation during bone drilling is mainly caused by shear deformation of the bone, as well as friction between the drill bit, the bone chip being removed, and the underlying host bone.18,19 About 60% of heat is dissipated into bone chips, while 40% is absorbed by host bone.19 Heating could lead to bone necrosis, particularly if it is exposed to temperatures of 45 C for 5 h or more, 55 C for 30 s or more, or 70 C for even a moment.3,18 This can be minimized during surgery by irrigation using a saline solution, as well as the presence of blood and other biofluids. There is no consensus on the influence of drill bit force, feed rate, and spindle speed on bone peak temperature, perhaps due to the large variety of drill bit cutting flute geometries.18,19 Clogging of bone debris in the drill bit cutting flutes substantially raises drilling force and torque with increasing drilling depth.14,20 This occurs because bone chips lodged in the cutting flutes exert pressure against the inside surface of the hole that is being drilled. This, in turn, raises the friction at the interface between the bone chips and the host bone, thereby raising the temperature and creating greater axial and torsional resistance with each revolution of the cutting flutes. Bone chips that are generated during clogging will often be large, pellet-like, and burnt in appearance. Wear of the drill bit has a negative effect on the efficiency of the drilling process and the quality of the pilot hole.18,19 The main mechanisms of surface damage to the drill bit are abrasive wear, plastic deformation, cavity formation, and thermal load. Applying protective coatings to the drill bits has been tried, but this has not been particularly successful. Another consequence of drill bit wear is elevated bone temperatures, which can potentially cause bone necrosis.
6. SUMMARY • • • • • •
Clinically, orthopaedic surgeons use electric hand drills for whole bone. Clinically, surgeons do not monitor drilling force or torque for whole bone. Drilling force and torque tests are done using a computer-controlled drill press. Drilling force and torque can be estimated using engineering formulas. Feed rate, spindle speed, drill bit design, and bone type affect force and torque. Orthopaedic surgeons could monitor drilling force and torque during surgery.
7. QUIZ QUESTIONS 1. What is the definition of surgical drilling force and torque? 2. Does a surgeon’s experience level affect their use of an electric-powered hand drill? 3. How will the anisotropic nonhomogenous nature of biological bone affect force and torque? 4. How could orthopaedic surgeons control or monitor drilling force or torque during surgery?
References
5. Predict average force and torque for a unicortical drilling test. Assume ultimate tensile stress of cortical bone ¼ 107 MPa, feed rate ¼ 120 mm/min, spindle speed ¼ 1000 rpm, drill bit outer diameter ¼ 3.2 mm, and drill bit tip angle ¼ 90 (answer: 73 N and 82 N$mm).
REFERENCES 1. Alam K, Mitrofanov AV, Silberschmidt VV. Experimental investigations of forces and torque in conventional and ultrasonically-assisted drilling of cortical bone. Medical Engineering and Physics 2011;33(2):234e9. 2. Allotta B, Giacalone G, Rinaldi L. A hand-held drilling tool for orthopedic surgery. IEEE Transactions on Mechatronics 1997;2(4):218e29. 3. Hillery MT, Shuaib I. Temperature effects in the drilling of human and bovine bone. Journal of Materials Processing Technology 1999;92-93:302e8. 4. Hobkirk JA, Rusiniak K. Investigation of variable factors in drilling bone. Journal of Oral Surgery 1977;35(12):968e73. 5. Jacob CH, Berry JT, Pope MH, Hoaglund FT. A study of the bone machining process e drilling. Journal of Biomechanics 1976;9(5):343e9. 6. Karalis T, Galanos P. Research on the mechanical impedance of human bone by a drilling test. Journal of Biomechanics 1982;15(8):561e81. 7. Karmani S, Lam F. The design and function of surgical drills and K-wires. Current Orthopaedics 2004;18(6):484e90. 8. Lee J, Gozen BA, Ozdoganlar OB. Modeling and experimentation of bone drilling forces. Journal of Biomechanics 2012;45(6):1076e83. 9. MacAvelia T, Salahi M, Olsen M, Crookshank M, Schemitsch EH, Ghasempoor A, et al. Biomechanical measurements of surgical drilling force and torque in human versus artificial femurs. Journal of Biomechanical Engineering 2012;134(12):124503-1-9. 10. Natali C, Ingle P, Dowell J. Orthopaedic bone drills: can they be improved? Journal of Bone and Joint Surgery (British) 1996;78(3):357e62. 11. Ong FR, Bouazza-Marouf K. Drilling of bone: a robust automatic method for the detection of drill bit break-through. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 1998;212(3):209e21. 12. Ong FR, Bouazza-Marouf K. Evaluation of bone strength: correlation between measurements of bone mineral density and drilling force. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2000;214(4): 385e99. 13. Tsai M-D, Hsieh M-S, Tsai C-H. Bone drilling haptic interaction for orthopedic surgical simulator. Computers in Biology and Medicine 2007;37(12):1709e18. 14. Wiggins KL, Malkin S. Drilling of bone. Journal of Biomechanics 1976;9(9):553e9. 15. Lewiecki EM, Borges JLC. Bone density testing in clinical practice. Arquivos Brasileiros de Endocrinologia and Metabologia 2006;50(4):586e95. 16. Pettersson U, Nordstro¨m P, Lorentzon R. A comparison of bone mineral density and muscle strength in young male adults with different exercise level. Calcified Tissue International 1999;64(6):490e8.
99
100
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
17. Allotta B, Belmonte F, Bosio L, Dario P. Study on a mechatronic tool for drilling in the osteosynthesis of long bones: tool/bone interaction, modeling, and experiments. Mechatronics 1996;6(4):447e59. 18. Pandey RK, Panda SS. Drilling of bone: a comprehensive review. Journal of Clinical Orthopaedics and Trauma 2013;4(1):15e30. 19. Bertollo N, Walsh WR. Drilling of bone: practicality, limitations, and complications associated with surgical drill-bits. In: Klika V, editor. Biomechanics in applications. Rijeka (Croatia): InTech; 2011 (Chapter 3). Available free online at: http://cdn. intechopen.com/pdfs-wm/19652.pdf. 20. MacAvelia T, Ghasempoor A, Janabi-Sharifi F. Force and torque modelling of drilling simulation for orthopaedic surgery. Computer Methods in Biomechanics and Biomedical Engineering 2014;17(12):1285e94.
6
Force and Torque Measurements of Surgical Drilling Into Whole Bone
Radovan Zdero1, Troy MacAvelia2, Farrokh Janabi-Sharifi2 Western University, London, ON, Canada1; Ryerson University, Toronto, ON, Canada2
1. BACKGROUND Repairing whole bone fractures requires surgical drilling to create pilot holes for easy insertion of cortical screws, cancellous screws, and locking bolts to align adjacent bone fragments, apply fracture plates, or insert fracture nails (Fig. 6.1).1e14 Although drilling may be done using CO2 pulsed lasers, haptic
A
B
C
D
FIGURE 6.1 Surgical drilling for fracture repair. (A) Surgical drill bit, (B) proximal femur, (C) proximal humerus, (D) humeral diaphysis.
Experimental Methods in Orthopaedic Biomechanics. http://dx.doi.org/10.1016/B978-0-12-803802-4.00006-8 Copyright © 2017 Elsevier Inc. All rights reserved.
85
86
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
systems that offer tactile force and torque feedback, and teleoperation in which a surgeon conducts the procedure off-site, electrically-powered hand drills remain the common clinical practice. Force and torque generated at the drill bite bone interface are influenced by feed rate (i.e., linear speed of the drill bit into bone), spindle speed (i.e., rotational speed of the drill bit), drill bit tip angle (i.e., angle of the pointed tip of the drill bit), drill bit size (i.e., outer diameter), and bone type (i.e., cortical vs. cancellous, normal vs. osteoporotic, human vs. animal, etc.). Nonoptimal force and torque can raise temperatures, causing bone necrosis, as well as poor pilot hole quality, causing poor screw fixation. Consequently, optimization of drilling force and torque is a clinically relevant topic. Therefore, this chapter explains how to measure force and torque during surgical drilling into whole bone, as well as how to analyze, present, and interpret the results.
2. RESEARCH QUESTIONS Typical research questions might include one or more of the following: • • • • • •
Do orthopaedic surgeons with varying experience generate different drilling force and torque? Do whole bone storage, thawing time, age, type, etc., affect drilling force and torque? Do drill bit feed rate, spindle speed, and geometry change drilling force and torque? Do drilling force and torque influence pilot hole quality and, thus, screw fixation strength? Do drilling force and torque alter bone temperature, potentially leading to bone necrosis? etc.
3. METHODOLOGY 3.1 GENERAL STRATEGY Human or animal whole bones or segments are secured by a vice directly under a surgical drill bit whose feed rate and spindle speed are computer controlled. The vice is equipped with a force and torque sensor, which actively records data during drilling. Raw force and torque data are then normalized by the surface area of the pilot hole to eliminate geometric effects. Statistical comparisons are made between the test groups for raw and normalized drilling measurements. Finally, correlation coefficients are computed for raw and normalized data vs. biological bone demographics (i.e., age, sex, limb side, bone mineral density (BMD), and clinical T-score) to determine which factors are important.
3. Methodology
GLOSSARY ✓ ✓ ✓ ✓
Feed rate. Linear speed of the drill bit along its long axis during drilling. Force. The linear load experienced by the bone along the long axis of the drill bit. Spindle speed. Rotational speed of the drill bit around its long axis during drilling. Torque. The angular load experienced by the bone around the long axis of the drill bit.
SAFETY FIRST ✓ ✓ ✓ ✓
Remember to always wear goggles and gloves for protection. Secure the vice and bone before using the drill press to conduct drilling tests. Use a fume hood or do tests in an open area, since bone may emit a burning odor when drilled. Clean the work area and all tools with bleach or disinfectant after testing bone specimens.
3.2 MATERIALS AND TOOLS LIST • • • • • • • • •
computer-controlled drill press force and torque sensors human or animal bone leveling gage surgical drill bits tape measure thickness gage Vernier calipers vice or clamp
3.3 SPECIMEN PREPARATION Step 1. Store the bone. Fresh or freshefrozen whole bones or segments initially need to be wrapped in plastic strips or vacuum-sealed in plastic bags for proper storage in a freezer prior to the study. The freezer should be held at 20 C or colder. Note that if using embalmed or dried/dehydrated bones, these do not need to be frozen and may simply be placed in a plastic box or bag for storage at room temperature prior to tests. Step 2. Thaw the bone. Bone specimens should be removed from the freezer, left in their plastic wrappings or vacuum-sealed bags, and placed on a surface at ambient room temperature or in a warm water bath to thaw for at least 12 h. Bone specimens are then removed from their plastic wrappings or bags and soaked or sprayed with saline water solution to prevent dehydration. Step 3. Determine the drilling site. Mount bone specimens into a vice or clamp them onto a table, make measurements for the drilling site, and mark the location
87
88
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
with a pen. Because of geometric variability among biological bone, the relative location of the drilling site with respect to a common reference dimension needs to be computed and used for all bone specimens (e.g., % ¼ relative dimension/reference dimension 100 ¼ distance from one end of the bone to the drilling site/total length of bone 100). TIPS AND TRICKS ✓ ✓ ✓ ✓
Whole bones can be cut into segments with a band saw for easier handling. Place a leveling gage on the bone surface to ensure drill direction is perpendicular. Insert a new drill bit when needed to avoid biasing data with dull drill bits. The same researcher should perform all tests for consistency.
THE “GOLD STANDARD” The International Organization for Standardization (ISO) provides guidelines for drill bit design factors, such as materials selection, mechanical properties, and labeling, in its document, ISO 9714-1 (Orthopaedic drilling instruments e Part 1: drill bits, taps and countersink cutters), but no standards exist for surgical bone drilling itself. Thus, researchers should use peer-reviewed journal articles and/or medical manufacturer’s surgical technique manuals as guidelines.
3.4 SPECIMEN TESTING Step 1. Assemble the test setup. An experimental test setup for surgical bone drilling research must first be assembled (Fig. 6.2). Several components are vital. A computer-controlled drill press will adjust drill bit feed rate and spindle speed over the typical ranges used in the research literature or clinically in orthopaedic surgery. A vice will rigidly hold the bone specimens in proper position. A biaxial force and torque sensor should be attached under the vice to monitor surgical drilling load, but it must be of sufficient resolution and accuracy. An LVDT (linear variable displacement transducer) can be attached to the drill bit chuck to measure drilling depth. Vibration isolation pads should be placed under the drill press and vice to minimize mechanical vibration. A heavy work bench will be used to mount the entire assembly, but will also further minimize mechanical vibration. Water, liquid coolant, or compressed air may be used to cool the drill bit and bone, as well as to unclog debris from the drill bit and clear away debris from the bone specimen. Orthopaedic surgical drill bits that are unused need to be procured in sufficient quantity. Step 2. Create a “pecking” hole. Create a preliminary pecking hole, which will later be used to guide the real surgical drill bit during the actual drilling test. To do this, place the bone into the vice under the center of the drill bit chuck. Use a leveling
3. Methodology
A
B
FIGURE 6.2 Experimental setup for drilling into bone. (A) Overall test setup, (B) close-up of the force and torque sensor.
gage to ensure the bone site is perpendicular to the drill bit direction. Insert a “dummy” center drill bit (e.g., size #3 with a 2.8-mm diameter) into the chuck, and center it about 20 mm above the bone drilling site. Set the desired spindle speed, but not the feed rate. Then, manually, slowly, and incrementally lower the spinning dummy center drill bit until the tip superficially punctures the bone to the correct pecking depth (e.g., 0.25e0.50 mm). Pecking depth can usually be detected by a grinding sound or slight vibration or visual observation, but the LVDT may be more reliable. Note that lowering the dummy center drill bit too quickly during pecking could cause the tip to slip overtop of a slippery bone surface, so the wrong location is punctured, as well as potentially bending or breaking the drill bit. Then, retract the dummy center drill bit and remove it from the chuck. Step 3. Choose the drill bit. The correct surgical drill bit for the real drilling test then needs to be chosen. This is based on the surgical procedure that will make use of the pilot hole (e.g., cortical or cancellous screw insertion during fracture plating, mounting an acetabular cup during total hip arthroplasty, etc.), the type of bone being drilled (e.g., cortical vs. cancellous, normal vs. osteoporotic, etc.), and/or the particular research question being asked about the drill bit (e.g., effects of cutting flute geometry, drill bit material, drill bit diameter, etc.). Typical drill bits used in the research literature and/or clinically have outer diameters of 1.98e4.76 mm and lengths of 60e200 mm.1,2,4,5,9,11,12,14 Step 4. Do the drilling test. Perform the real drilling test that will create the final pilot hole. To do this, insert the real unused surgical drill bit of appropriate size into the chuck and position it approximately 20 mm above the bone specimen. Then, start the automated computer-controlled drilling process by setting the desired feed rate
89
90
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
(e.g., 10e132 mm/min) and spindle speed (e.g., 40e3300 rpm) within typical ranges used in the research literature and/or clinically.1,2,5,9,11,12,16 Ensure that the drill and the force and torque sensors are started simultaneously for data collection. Allow drilling to continue until the drill bit tip reaches the desired depth. Then, return the drill bit to its home position, turn off the drill press, and remove the bone from the vice. Clear the drill bit flutes of clogged material in preparation for the next test. Cooling the bone can be done using water, saline solution, liquid coolant, or compressed air if the research question requires it; however, irrigation may not always be done during actual orthopaedic surgery. Step 5. Measure pilot hole length. Remove the bone from the drill press and hold it firmly on a work surface, whereas large specimens may be secured using a vice or clamp on a work surface. To measure cortical wall thickness at the drilling site, insert a clinical or mechanical depth gage that has a hooked end, pull the hooked end until it engages the intramedullary underside of the cortical wall, and then measure this length using a ruler or Vernier caliper (Fig. 6.3AeC). To measure cancellous pilot hole depth at the drilling site, insert a thin, rigid guide wire until it feels
A
B
C
D
E
F
FIGURE 6.3 Measurement of pilot hole geometry. (A) Cortical bone specimen with a pilot hole, (B) depth gage insertion into a cortical pilot hole, (C) depth measurement of a cortical pilot hole using a depth gage and Vernier calipers, (D) cancellous bone specimen with a pilot hole, (E) guide wire insertion into a cancellous pilot hole, (F) depth measurement of a cancellous pilot hole using a guide wire and ruler.
3. Methodology
like it has reached the bottom of the hole, use a pen to mark the guide wire’s surface that is at the entrance of the pilot hole, remove the guide wire, and then measure the length of the guide wire’s tip-to-pen mark using a ruler or Vernier calipers (Fig. 6.3DeF). Guide wires can also be used to clear away unwanted bone debris from the pilot hole. Step 6. Photograph or draw the specimen. Photograph or draw the pilot hole that was created, the bone debris that was generated during drilling, and the drill bit for later data analysis.
3.5 RAW DATA COLLECTION Step 1. Record bone characteristics. Enter human or animal demographic information (i.e., age, sex, and left or right limb) and quantitative bone properties (i.e., BMD and clinical T-score), which will help determine the factors that are statistically correlated with drilling force and torque (Table 6.1). Note that BMD is in 2D units (i.e., g/cm2 rather than g/cm3), since DEXA (dual-energy X-ray absorptiometry) bone density scans are only 2-D. DEXA scan reports often provide BMD values and corresponding clinical T-scores (i.e., normal Tscore 1, 1 > osteopenic T-score > 2.5, and osteoporotic T-score 2.5). When such analysis is overlooked or missing, computations can be made as Tscore ¼ (bone BMD e reference population mean BMD)/(1 standard deviation of reference population BMD), where T-score reference populations are young healthy adults of the same sex that are aged 20e40 years.15,16 Step 2. Record pilot hole geometry. Input the pilot hole diameter (i.e., drill bit diameter) into the summary table (Table 6.2). Enter the pilot hole length, which is the wall thickness for unicortical or bicortical specimens, whereas it is the pilot hole depth for cancellous specimens (Table 6.2). Step 3. Record drilling parameters. Feed rate, spindle speed, and drill bit tip angle should be noted (Table 6.2). Raw force and torque should have been recorded during drilling and then smoothed for noise artifacts using a running average function or other appropriate filtering algorithm. Then, peak force and torque can be identified (Table 6.2). Table 6.1 Bone characteristics.
Bone 1 2 3 etc. Avg SD
Age [years]
Sex [M, F]
Limb Side [L, R]
e e
e e
Avg, average; SD, standard deviation.
Bone Mineral Density [g/cm2]
T-score
91
92
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
Table 6.2 Drilling test parameters. Feed Rate f [mm/ min]
Spindle Speed u [rpm]
Drill Bit Tip Angle q [8]
Drill Bit Diameter D [mm]
Avg
e
e
e
e
e
SD
e
e
e
e
e
Bone
Pilot Hole Length L [mm]
Peak Force F [N]
Peak Torque T [N$mm]
Remarks
1 2 3 etc.
Avg, average; SD, standard deviation.
Step 4. Record visual observations. Inspect prior photographs or drawings and record remarks on the pilot hole (e.g., smooth? circular? surface cracking? etc.), the bone debris that has been removed (e.g., shape, size, texture, color), and the drill bit (e.g., wear, bending, breaking, etc.) (Table 6.2).
3.6 RAW DATA ANALYSIS Step 1. Normalize raw data. Raw peak forces and torques should be normalized by geometry by taking into account the surface area of the pilot hole that is in contact with the drill bit. This can be computed as FNORM ¼ FRAW/A ¼ FRAW/(pDL) and TNORM ¼ TRAW/A ¼ TRAW/(pDL), where F is force, T is torque, A is surface area, D is pilot hole diameter, and L is bone wall thickness (cortical specimen) or hole depth (cancellous specimen). Step 2. Calculate correlation coefficients. Plot the measured raw and normalized result for each individual specimen (i.e., peak force 1, 2, 3, etc.) (Table 6.2) vs. its corresponding specimen characteristic (i.e., age 1, 2, 3, etc.) (Table 6.1) to help visualize the interrelationship between measurements vs. characteristics. Then, calculate the correlation coefficient R for each measurementecharacteristic pair to determine which characteristic has an influence on measurements (e.g., R > 0.8 is a typical value considered to indicate a strong correlation). Step 3. Perform statistical comparisons. The criterion for statistical difference needs to be chosen (e.g., P <0.01 or <0.05). Then, data can be used to compare different patient groups (e.g., men 60 years old vs. men >60 years old, “normal” women vs. “osteoporotic” women, etc.), bone types (e.g., femur vs. tibia, left humerus vs. right humerus, etc.), drilling sites (e.g., anterior vs. lateral surface), drill bit designs (e.g., small vs. large outer diameter), etc. Various software programs exist for comparing two test groups (e.g., paired t-test) or two or more test groups influenced by multiple factors (e.g., analysis of variance, ANOVA). Step 4. Compute statistical power. Power analysis can be done after the study to ensure there were enough specimens per group to detect all statistical differences that were actually present (i.e., was type II statistical error avoided?). Statistical
4. Results
power >80% is usually considered to indicate there were enough specimens per test group. Note that if good predictions of averages and standard deviations are available from prior studies, then the number of specimens and/or tests can be chosen before the study begins to ensure a power >80%.
ENGINEER’S TOOLBOX Force and torque for bone drilling can be estimated using engineering formulas. Assume no effects from bone isotropy, homogeneity, and viscoelasticity, or from bone chip clogging of drill bit cutting flutes, bone temperature, and drill bit wear. So, for each revolution of the drill bit around its long axis, the force and torque , where F is force [N], T is torque U U [N$mm], U is ultimate tensile stress of bone [N/mm2], f is drill bit feed rate [mm/min], u is drill bit spindle speed [rpm or revolutions/min], D is drill bit outer diameter [mm], and q is drill bit tip angle [ ].
s
s
s
4. RESULTS Once all surgical drilling data collection and analysis have been performed, it is then important to communicate and present the primary results in an understandable and concise manner to the reader of a journal article, conference paper, technical report, or book chapter. Step 1. Show raw data profile. Begin with typical raw force and torque profiles vs. drilling depth or time for both unsmoothed and smoothed data (Fig. 6.4). Initially, force and torque are zero before the drill bit tip contacts the bone. Then, there is a small “step” in force and torque as the drill bit cutting flutes enter the “pecking” hole to make superficial contact with bone. Following this, there is a gradual increase in force and torque to some peak value as the drill bit tip becomes fully immersed into bone. Next, a gradual drop in force and torque occur as the drill bit exits through the other side of the bone wall (i.e., cortical drilling) or reaches the final drilling depth (i.e., cancellous drilling), but a small amount of torque is maintained as the drill bit
93
94
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
FIGURE 6.4 Raw data profiles of surgical drilling through one contiguous segment of cortical or cancellous bone. (A) Force (unsmoothed for noise), (B) torque (unsmoothed for noise), (C) force (smoothed for noise), (D) torque (smoothed for noise).
continues to spin and make superficial contact with surrounding bone. For the unsmoothed data, note that there are usually small-scale high-frequency fluctuations superimposed on top of large-scale low-frequency fluctuations in force and torque, which may be due a combination of factors: (1) bone anisotropy, porosity, and viscoelasticity; (2) mechanical flexing of the bone as the drill bit thrusts forward; (3) clogging of drill bit cutting flutes with bone debris; (4) drill bit vibration caused by drill bit bending or a loose bone specimen; and (5) load alterations with each circumferential pass of a cutting flute past a given point on the bone. Although noise smoothing algorithms eliminate these small-scale high-frequency fluctuations, some large-scale low-frequency fluctuations remain. Step 2. Show main results. The main numerical findings of the study can be presented, namely, raw and normalized peak force and torque (Fig. 6.5). For each of these parameters, all statistical pairwise comparisons should be made (i.e., bone type 1 vs. 2 vs. 3, drill bit type 1 vs. 2 vs. 3, etc.) to generate statistical P values
4. Results
FIGURE 6.5 Surgical drilling test results. (A) Force vs. test group, (B) torque vs. test group, where bar graphs show average 1 standard deviation and P is the statistical difference result for each pairwise comparison between groups which can be indicated using symbols like asterisks (*), pounds (£), etc. (C) Force vs. BMD, (D) torque vs. BMD, where BMD is bone mineral density, R is the linear correlation coefficient for each line of best fit, P is the statistical difference value ensuring the slope of the line of best fit (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0), and y ¼ mx þ b is the equation of each line.
(Fig. 6.5A and B). Any linear (or nonlinear) trends can be illustrated for force and torque vs. BMD (Fig. 6.5C and D). For each line of best fit, several items can be generated: an equation y ¼ mx þ b showing slope m and intercept b, a linear correlation coefficient R, and its own P value to ensure the slope of each line of best fit (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0). Also, if a substantially wide range of drilling parameters has been examined, then results can be presented as a family of curves that show how various parameters affect peak force and torque (Fig. 6.6). Step 3. Show specimen photos. Present photos to help visualize the quality of drilling, such as the pilot hole (e.g., circular? symmetric? surface cracking? etc.), bone debris (e.g., shape, size, texture, etc.), and drill bit cutting flutes (e.g., bending, breaking, wear, etc.) (Fig. 6.7).
95
96
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
FIGURE 6.6 is Typical family of curves for surgical drilling into bone. (A) Force, (B) torque. ultimate tensile stress of bone, f is drill bit feed rate, D is drill bit outer diameter, and q is drill bit tip angle.
FIGURE 6.7 Qualitative drilling test outcomes for the pilot hole, bone debris, and drill bit.
5. Discussion
ALTERNATIVES AND ADAPTATIONS ✓ Sawbone whole bones. Artificial whole bones, rather than biological bones, can be used as substitutes, since many commercially available sawbones mimic human bone geometry, they have been biomechanically validated, and they are increasingly used for research. 3-D density (i.e., g/cm3), rather than 2-D BMD (i.e., g/cm2), needs to be measured or obtained from the manufacturer, since DEXA scans for BMD are only suitable for biological bone. Note that sawbones have isotropic and homogeneous properties, unlike biological bone. ✓ Sawbone shapes. Artificial bones with standardized geometries and material properties may be useful when comparing different drill bit designs since bone parameters are fixed and known. These shapes are available as sheets, blocks, and cylinders. 3-D density (i.e., g/cm3), rather than 2-D BMD (i.e., g/cm2), needs to be measured or obtained from the manufacturer, since DEXA scans for BMD are only suitable for biological bone. Remember that sawbones have isotropic and homogeneous properties, unlike biological bone.
5. DISCUSSION After completing all surgical drilling tests, data collection, data analysis, and data presentation, then final results can be considered and interpreted in the broader context of some important clinical, biomechanical, and/or technological considerations, as follows. Drilling force and torque are not monitored during orthopaedic surgery. Instead, surgeons use an electrically-powered surgical hand drill to set the spindle speed, which depends on the surgical procedure being done. Moreover, the feed rate of the drill bit into the bone is controlled purely by “subjective feel.” Alternately, preoperative patient BMD scans could be done to permit engineering predictions of appropriate drilling force and torque, followed by the use of a surgical hand drill equipped with a digital force and torque display. Drilling force and torque in bone have a wide range of values since they can be affected by feed rate, spindle speed, drill bit tip angle, drill bit diameter, and bone type. Drilling force studies have yielded average peaks of 5e300 N (human cortical bone),9,10,13,14 1e1.5 N (human cancellous bone),13 2e50 N (porcine cortical bone),2,11,12 4e32 N (porcine cancellous bone),12 and 0e275 N (bovine cortical bone).1,3,5,8 Similarly, drilling torque studies have reported average peaks of 0e186 N$mm (human cortical bone),9,11,14 2e3 N$mm (human cancellous bone),13 55 N$mm (porcine cortical bone),2 and 0e400 N$mm (bovine cortical bone).1,3,5,8 Drilling force and torque can be estimated using engineering formulas, which are based on empirical research and mechanics theory.2,17 Assume there are no effects from bone isotropy, bone homogeneity, bone viscoelasticity, bone chip clogging of the drill bit cutting flutes, bone heating, and drill bit wear. Thus, for each revolution and the torque sU of the drill bit around its long axis, the force , where F is force [N], T is torque [N$mm], is ultimate tensU sile stress of bone [N/mm2], f is drill bit feed rate [mm/min], u is drill bit spindle speed [rpm], D is drill bit outer diameter [mm], and q is drill bit tip angle [ ].
97
98
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
Thermal generation during bone drilling is mainly caused by shear deformation of the bone, as well as friction between the drill bit, the bone chip being removed, and the underlying host bone.18,19 About 60% of heat is dissipated into bone chips, while 40% is absorbed by host bone.19 Heating could lead to bone necrosis, particularly if it is exposed to temperatures of 45 C for 5 h or more, 55 C for 30 s or more, or 70 C for even a moment.3,18 This can be minimized during surgery by irrigation using a saline solution, as well as the presence of blood and other biofluids. There is no consensus on the influence of drill bit force, feed rate, and spindle speed on bone peak temperature, perhaps due to the large variety of drill bit cutting flute geometries.18,19 Clogging of bone debris in the drill bit cutting flutes substantially raises drilling force and torque with increasing drilling depth.14,20 This occurs because bone chips lodged in the cutting flutes exert pressure against the inside surface of the hole that is being drilled. This, in turn, raises the friction at the interface between the bone chips and the host bone, thereby raising the temperature and creating greater axial and torsional resistance with each revolution of the cutting flutes. Bone chips that are generated during clogging will often be large, pellet-like, and burnt in appearance. Wear of the drill bit has a negative effect on the efficiency of the drilling process and the quality of the pilot hole.18,19 The main mechanisms of surface damage to the drill bit are abrasive wear, plastic deformation, cavity formation, and thermal load. Applying protective coatings to the drill bits has been tried, but this has not been particularly successful. Another consequence of drill bit wear is elevated bone temperatures, which can potentially cause bone necrosis.
6. SUMMARY • • • • • •
Clinically, orthopaedic surgeons use electric hand drills for whole bone. Clinically, surgeons do not monitor drilling force or torque for whole bone. Drilling force and torque tests are done using a computer-controlled drill press. Drilling force and torque can be estimated using engineering formulas. Feed rate, spindle speed, drill bit design, and bone type affect force and torque. Orthopaedic surgeons could monitor drilling force and torque during surgery.
7. QUIZ QUESTIONS 1. What is the definition of surgical drilling force and torque? 2. Does a surgeon’s experience level affect their use of an electric-powered hand drill? 3. How will the anisotropic nonhomogenous nature of biological bone affect force and torque? 4. How could orthopaedic surgeons control or monitor drilling force or torque during surgery?
References
5. Predict average force and torque for a unicortical drilling test. Assume ultimate tensile stress of cortical bone ¼ 107 MPa, feed rate ¼ 120 mm/min, spindle speed ¼ 1000 rpm, drill bit outer diameter ¼ 3.2 mm, and drill bit tip angle ¼ 90 (answer: 73 N and 82 N$mm).
REFERENCES 1. Alam K, Mitrofanov AV, Silberschmidt VV. Experimental investigations of forces and torque in conventional and ultrasonically-assisted drilling of cortical bone. Medical Engineering and Physics 2011;33(2):234e9. 2. Allotta B, Giacalone G, Rinaldi L. A hand-held drilling tool for orthopedic surgery. IEEE Transactions on Mechatronics 1997;2(4):218e29. 3. Hillery MT, Shuaib I. Temperature effects in the drilling of human and bovine bone. Journal of Materials Processing Technology 1999;92-93:302e8. 4. Hobkirk JA, Rusiniak K. Investigation of variable factors in drilling bone. Journal of Oral Surgery 1977;35(12):968e73. 5. Jacob CH, Berry JT, Pope MH, Hoaglund FT. A study of the bone machining process e drilling. Journal of Biomechanics 1976;9(5):343e9. 6. Karalis T, Galanos P. Research on the mechanical impedance of human bone by a drilling test. Journal of Biomechanics 1982;15(8):561e81. 7. Karmani S, Lam F. The design and function of surgical drills and K-wires. Current Orthopaedics 2004;18(6):484e90. 8. Lee J, Gozen BA, Ozdoganlar OB. Modeling and experimentation of bone drilling forces. Journal of Biomechanics 2012;45(6):1076e83. 9. MacAvelia T, Salahi M, Olsen M, Crookshank M, Schemitsch EH, Ghasempoor A, et al. Biomechanical measurements of surgical drilling force and torque in human versus artificial femurs. Journal of Biomechanical Engineering 2012;134(12):124503-1-9. 10. Natali C, Ingle P, Dowell J. Orthopaedic bone drills: can they be improved? Journal of Bone and Joint Surgery (British) 1996;78(3):357e62. 11. Ong FR, Bouazza-Marouf K. Drilling of bone: a robust automatic method for the detection of drill bit break-through. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 1998;212(3):209e21. 12. Ong FR, Bouazza-Marouf K. Evaluation of bone strength: correlation between measurements of bone mineral density and drilling force. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2000;214(4): 385e99. 13. Tsai M-D, Hsieh M-S, Tsai C-H. Bone drilling haptic interaction for orthopedic surgical simulator. Computers in Biology and Medicine 2007;37(12):1709e18. 14. Wiggins KL, Malkin S. Drilling of bone. Journal of Biomechanics 1976;9(9):553e9. 15. Lewiecki EM, Borges JLC. Bone density testing in clinical practice. Arquivos Brasileiros de Endocrinologia and Metabologia 2006;50(4):586e95. 16. Pettersson U, Nordstro¨m P, Lorentzon R. A comparison of bone mineral density and muscle strength in young male adults with different exercise level. Calcified Tissue International 1999;64(6):490e8.
99
100
CHAPTER 6
Measurements of Surgical Drilling Into Whole Bone
17. Allotta B, Belmonte F, Bosio L, Dario P. Study on a mechatronic tool for drilling in the osteosynthesis of long bones: tool/bone interaction, modeling, and experiments. Mechatronics 1996;6(4):447e59. 18. Pandey RK, Panda SS. Drilling of bone: a comprehensive review. Journal of Clinical Orthopaedics and Trauma 2013;4(1):15e30. 19. Bertollo N, Walsh WR. Drilling of bone: practicality, limitations, and complications associated with surgical drill-bits. In: Klika V, editor. Biomechanics in applications. Rijeka (Croatia): InTech; 2011 (Chapter 3). Available free online at: http://cdn. intechopen.com/pdfs-wm/19652.pdf. 20. MacAvelia T, Ghasempoor A, Janabi-Sharifi F. Force and torque modelling of drilling simulation for orthopaedic surgery. Computer Methods in Biomechanics and Biomedical Engineering 2014;17(12):1285e94.