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Orthopedic sawblades
Orthopedic Sawblades A Case Study
H. W . W e v e r s , P E n g , * E. E s p i n , M S c , * a n d T. D. V. C o o k e , / V I A , M B , BChir, FRCS(C)t
Abstract: Surgical stainless steel sawblades are frequently used with metal tem-
plates at implant surgery, hence there is a high potential for blade damage and suboptimal bone cuts. The authors appraised currently used operating room sawblades with a comparison of their cutting efficacy. There were numerous instances of damage to the cutting teeth surfaces. At a controlled cutting speed, such blades require significantly greater force than unused blades to cut cancellous and conical samples of bovine bone. Microscopic assessment of cancellous bone specimens cut with damaged blades revealed tom, irregular, debris: filled surfaces. Blades should be inspected for damage after each use and discarded if their condition is in doubt. Key w o r d s : osteotomy, sawblade, bone tool, wear, cutting, surface.
w h e n it is d r i v e n into the bone) was measured. The surface patterns of cut cancellous bone and cortical bone were observed. There were significant differences between cancellous bone surface cut with a sharp blade and that cut with a damaged blade.
Orthopedic reciprocating sawblades are made from stainless steel, which has a hardness of approximately 50 Rockwell C, well below that of metalor wood-cutting tools. In total knee replacement, powerful reciprocating saws are used to drive these blades for fashioning bone. Because the blades are frequently used more than once and in conjunction with metal templates as cutting guides, there is a high potential for blade damage and the production of suboptimal bone surfaces. Stand~-d reciprocating sawblades were examined for quality of manufacturing, sharpness of cutting edge, and tooth geometry. A r a n d o m sample of sawblades from an operating r o o m was examined and their condition described. The effect of tooth wear o n the thrust force (ie, the resistance on the blade
Sawblade Geometry and Nomenclature Reciprocating sawblades are designed to cut from side to side. This means that the tooth must be symmetric about the axis perpendicular to the cutting motiofl. As a result, the tooth face has a negative rake angle of approximately 30 ~ (Fig. 1). By comparison, the ordinary hacksaw is designed for cutting in only the forward direction, enabling it to have positive, rake angles. This is also the case with the bandsaw and the osteotome. The sawtooth has an outward set, to cut a wider path than the thickness of the blade. This improves the clearance of the b o n e
From the Departmentsof*Mechanical Engineering and'~ Orthopaedie Surgery, Queen's University, Kingston, Ontario, Canada.
Supported by a grant from Natural Science and Engineering Research Council. Reprint requests: Professor H. W. Wevers, Department of Mechanical Engineering, McLaughlin Hall, Queen's University, Kingston, Ontario K7L 3N6, Canada.
43
44
The Journal of Arthroplasty Vol. 2 No. 1 March 1987
1
E
Fig. 1. (A) Orthopedic sawblade, tooth geometry, and tool nomenclature. 1, primary cutting edge; 2, secondary cutting edge; 3, rake plane; 4, rake angle, , 30 ~ for cutting action to right; 5, wedge angle 60~ 6, rake angle, - 3 0 ~ for cutting action to left. (B) SEM photograph of a sawblade. chips or fragments, by allowing them to m o v e laterally, out of the way of the cutting edge. The sidecutting or secondary edges provide for better cutting action, compared with the main cutting edge, because their rake angle is 0 ~ (Fig. 1).
Materials and Methods The n e w blades used in this study, manufactured by 3M Company (catalog numbers L122, D391, D392, D401, D402) were obtained directly from the
distributor. Twelve blades in use longer than 1 m o n t h in an orthopedic operating room were inspected with the use of a Wild 3-D light microscope at 10 • and 20 • magnification. A Hitachi model 450 scanning electron microscope (SEM) was used for further observations and to photograph the characteristic aspects of the blades. The teeth of each blade were numbered for identification. A 25-ram square sample of bovine cancellous b o n e was cut from the distal femur and embedded in a holding block. A Stryker reciprocating saw was clamped in an Instron testing machine with the sawblade perpendicular to the long axis of the bone
Fig. 2. (A, B) Composite photographs of two osteotomy blades taken from a sample of blades in use at surgery.
Orthopedic Sawblades
block. The Instron provided a controlled advance of the saw at 100 ram/minute and recorded the thrust force graphically against time. The saw speed was adjusted to 10,000 cpm. Bone blocks were cut, 20 m m deep, perpendicular to the long axis of the bone. The samples were fixed and dehydrated, as described by Karnovsky (2), in preparation for scanning electron microscopy.
Results All of the 12 blades obtained from the operating r o o m had some degree of damage to the cutting surfaces. Major damage was observed in approximately one-half of the blades (Fig. 2).
9 Wevers et al.
Damage at the two corners of the sawblade probably occurred w h e n it impacted on metal objects. This damage is serious, because it restricts the sidecutting action in deep cuts, which in turn makes the cutting path narrower w h e n advancing. Eventually, the reciprocating action stops. Damage at the tip of tooth 5 (third from the left in Figure 2A) was caused by rubbing of the tooth against a steel object (eg, a bone screw or surgical wire embedded in the bone). The tooth set wears rapidly w h e n the side of a tooth rubs against the metal guide jigs used during arthroplasty. The teeth to the right of tooth 5 showed this effect. W h e n a blade such as that shown in Figure 2A was used, there was a statistically significant increase in thrust force required to cut the bone at a constant rate of advance and controlled speed, compared with
Fig. 3. (A) SEM photograph of bone cut with a sharp blade. (B) Bone cut with a used blade is tom and smeared across the surface.
'
i~ ~
~!i~ ~
. , -~
~,..~,
.....
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:
9
45
,-~.~.~ i ~.:;")~" ,":
L~," ':::"~
":"; "
46
The Journal of Arthroplasty Vol. 2 No. 1 March 1987 T a b l e 1. Thrust Force
Cancellous bone Cortical bone
Dull Blade
N e w Blade
(N)
(N)
3.15 __ 0.1l 32.08 _ 3.59
2.22 __ 0.74 6.75 _ 1.17
that required w h e n an unused blade was used (Table 1). In cancellous bone the differences were relatively small, but in cortical bone the differences were greatly magnified. Under SEM magnifications, strikingly different features were noted in bone samples cut with damaged blades versus new blades (Fig. 3). With a n e w blade, a flat and debris-flee surface was produced. In contrast, the surface formed by a dull blade was irregular, with trabecular space overshadowed by deformed and cut bone.
Discussion Osteotomy sawblades have negative rake angles, so that the teeth will cut during the left and right m o t i o n of the oscillating blade. The negative rake angle tends to push bone fragments into the uncut bone at the front of the tooth. Removal from the cutting zone must be achieved by the set of the teeth, which make a path larger than the sawblade thickness, thus providing r o o m for bone fragments to escape. More optimal blade designs are possible (3), but these have not materialized for general use. All sawblades taken from the operating r o o m during a 1-month period showed evidence of wear and damage, and approximately one-half of the blades were severely damaged. Because these blades are used primarily for total knee arthroplasty, it is probable that the damage occurred from direct contact of the cutting edges with metal templates or instruments used in the operation. This type of damage had a direct influence on the mechanical work needed to operate the saw. In cortical bone, the w o r n blades left a wavy surface; the new blades produced a similar p a t t e m but with smaller wave lengths, because all teeth participated in the cutting process. The damaged blades left marks in the form of larger grooves, spaced farther apart.
Direct evidence of these deleterious effects was obtained from the SEM examinations of cut surfaces. Those cut with the n e w blade were clean and debrisflee. In comparison, those cut with the damaged blades were irregular, with m a n y broken trabeculae, torn edges, and debris-filled crevices. These experiments did not define an increase in heat resulting from the damaged blades, which is likely to occur. Excessive heat induces thermal damage to osteosites and expands the zone of necrosis b e y o n d that shown microscopically. Smooth, accurately cut surfaces are recognized as an important factor for bone ingrowth into porouscoated prostheses. Such clean bone cuts enhance prosthetic fit and seating, therefore promoting an even load bearing to the bone, and improve alignment of the prostheses or osteotomies. These aspects were not defined in these experiments, but their potential detrimental influence due to p o o r bone cuts is clear. Damage to blade cutting surfaces due to inadvertent contact with templates and instruments may be unavoidable with currently available techniques. However, with the use of saws held in jigs, which eliminates the need for templates, such damage m a y be minimized. This has been our experience with the use of a total knee replacement saw jig (1). Blades regularly inspected after use in the saw jig showed little w e a r and no major damage. Say,blades should be inspected and replaced frequently. Examination without magnification is inadequate, but a 10 x magnifying hand lens will define damages such as loss of teeth, blunting of tips, scouring of surfaces, and loss of set.
References 1. Cooke TDV, Saunders G, Siu D et al: Universal bone cutting device for precision knee replacement arthroplasty and osteotomy. J Biomed Eng 7:45, 1985 2. Karnovsky M J: A formaldehyde-glutaraldehyde fixative of high osmality for use in electron microscopy. J Cell Biol 27N:I37A, 1965 3. Krause W: Mechanical Effects of Orthogonal Bone Cutting. Ph.D. Thesis. Clemson University, Clemson, SC, 1976
H. W . W e v e r s , P E n g , * E. E s p i n , M S c , * a n d T. D. V. C o o k e , / V I A , M B , BChir, FRCS(C)t
Abstract: Surgical stainless steel sawblades are frequently used with metal tem-
plates at implant surgery, hence there is a high potential for blade damage and suboptimal bone cuts. The authors appraised currently used operating room sawblades with a comparison of their cutting efficacy. There were numerous instances of damage to the cutting teeth surfaces. At a controlled cutting speed, such blades require significantly greater force than unused blades to cut cancellous and conical samples of bovine bone. Microscopic assessment of cancellous bone specimens cut with damaged blades revealed tom, irregular, debris: filled surfaces. Blades should be inspected for damage after each use and discarded if their condition is in doubt. Key w o r d s : osteotomy, sawblade, bone tool, wear, cutting, surface.
w h e n it is d r i v e n into the bone) was measured. The surface patterns of cut cancellous bone and cortical bone were observed. There were significant differences between cancellous bone surface cut with a sharp blade and that cut with a damaged blade.
Orthopedic reciprocating sawblades are made from stainless steel, which has a hardness of approximately 50 Rockwell C, well below that of metalor wood-cutting tools. In total knee replacement, powerful reciprocating saws are used to drive these blades for fashioning bone. Because the blades are frequently used more than once and in conjunction with metal templates as cutting guides, there is a high potential for blade damage and the production of suboptimal bone surfaces. Stand~-d reciprocating sawblades were examined for quality of manufacturing, sharpness of cutting edge, and tooth geometry. A r a n d o m sample of sawblades from an operating r o o m was examined and their condition described. The effect of tooth wear o n the thrust force (ie, the resistance on the blade
Sawblade Geometry and Nomenclature Reciprocating sawblades are designed to cut from side to side. This means that the tooth must be symmetric about the axis perpendicular to the cutting motiofl. As a result, the tooth face has a negative rake angle of approximately 30 ~ (Fig. 1). By comparison, the ordinary hacksaw is designed for cutting in only the forward direction, enabling it to have positive, rake angles. This is also the case with the bandsaw and the osteotome. The sawtooth has an outward set, to cut a wider path than the thickness of the blade. This improves the clearance of the b o n e
From the Departmentsof*Mechanical Engineering and'~ Orthopaedie Surgery, Queen's University, Kingston, Ontario, Canada.
Supported by a grant from Natural Science and Engineering Research Council. Reprint requests: Professor H. W. Wevers, Department of Mechanical Engineering, McLaughlin Hall, Queen's University, Kingston, Ontario K7L 3N6, Canada.
43
44
The Journal of Arthroplasty Vol. 2 No. 1 March 1987
1
E
Fig. 1. (A) Orthopedic sawblade, tooth geometry, and tool nomenclature. 1, primary cutting edge; 2, secondary cutting edge; 3, rake plane; 4, rake angle, , 30 ~ for cutting action to right; 5, wedge angle 60~ 6, rake angle, - 3 0 ~ for cutting action to left. (B) SEM photograph of a sawblade. chips or fragments, by allowing them to m o v e laterally, out of the way of the cutting edge. The sidecutting or secondary edges provide for better cutting action, compared with the main cutting edge, because their rake angle is 0 ~ (Fig. 1).
Materials and Methods The n e w blades used in this study, manufactured by 3M Company (catalog numbers L122, D391, D392, D401, D402) were obtained directly from the
distributor. Twelve blades in use longer than 1 m o n t h in an orthopedic operating room were inspected with the use of a Wild 3-D light microscope at 10 • and 20 • magnification. A Hitachi model 450 scanning electron microscope (SEM) was used for further observations and to photograph the characteristic aspects of the blades. The teeth of each blade were numbered for identification. A 25-ram square sample of bovine cancellous b o n e was cut from the distal femur and embedded in a holding block. A Stryker reciprocating saw was clamped in an Instron testing machine with the sawblade perpendicular to the long axis of the bone
Fig. 2. (A, B) Composite photographs of two osteotomy blades taken from a sample of blades in use at surgery.
Orthopedic Sawblades
block. The Instron provided a controlled advance of the saw at 100 ram/minute and recorded the thrust force graphically against time. The saw speed was adjusted to 10,000 cpm. Bone blocks were cut, 20 m m deep, perpendicular to the long axis of the bone. The samples were fixed and dehydrated, as described by Karnovsky (2), in preparation for scanning electron microscopy.
Results All of the 12 blades obtained from the operating r o o m had some degree of damage to the cutting surfaces. Major damage was observed in approximately one-half of the blades (Fig. 2).
9 Wevers et al.
Damage at the two corners of the sawblade probably occurred w h e n it impacted on metal objects. This damage is serious, because it restricts the sidecutting action in deep cuts, which in turn makes the cutting path narrower w h e n advancing. Eventually, the reciprocating action stops. Damage at the tip of tooth 5 (third from the left in Figure 2A) was caused by rubbing of the tooth against a steel object (eg, a bone screw or surgical wire embedded in the bone). The tooth set wears rapidly w h e n the side of a tooth rubs against the metal guide jigs used during arthroplasty. The teeth to the right of tooth 5 showed this effect. W h e n a blade such as that shown in Figure 2A was used, there was a statistically significant increase in thrust force required to cut the bone at a constant rate of advance and controlled speed, compared with
Fig. 3. (A) SEM photograph of bone cut with a sharp blade. (B) Bone cut with a used blade is tom and smeared across the surface.
'
i~ ~
~!i~ ~
. , -~
~,..~,
.....
[i'~ : ,.-~.-" ~.~ ~ U ~,
:
9
45
,-~.~.~ i ~.:;")~" ,":
L~," ':::"~
":"; "
46
The Journal of Arthroplasty Vol. 2 No. 1 March 1987 T a b l e 1. Thrust Force
Cancellous bone Cortical bone
Dull Blade
N e w Blade
(N)
(N)
3.15 __ 0.1l 32.08 _ 3.59
2.22 __ 0.74 6.75 _ 1.17
that required w h e n an unused blade was used (Table 1). In cancellous bone the differences were relatively small, but in cortical bone the differences were greatly magnified. Under SEM magnifications, strikingly different features were noted in bone samples cut with damaged blades versus new blades (Fig. 3). With a n e w blade, a flat and debris-flee surface was produced. In contrast, the surface formed by a dull blade was irregular, with trabecular space overshadowed by deformed and cut bone.
Discussion Osteotomy sawblades have negative rake angles, so that the teeth will cut during the left and right m o t i o n of the oscillating blade. The negative rake angle tends to push bone fragments into the uncut bone at the front of the tooth. Removal from the cutting zone must be achieved by the set of the teeth, which make a path larger than the sawblade thickness, thus providing r o o m for bone fragments to escape. More optimal blade designs are possible (3), but these have not materialized for general use. All sawblades taken from the operating r o o m during a 1-month period showed evidence of wear and damage, and approximately one-half of the blades were severely damaged. Because these blades are used primarily for total knee arthroplasty, it is probable that the damage occurred from direct contact of the cutting edges with metal templates or instruments used in the operation. This type of damage had a direct influence on the mechanical work needed to operate the saw. In cortical bone, the w o r n blades left a wavy surface; the new blades produced a similar p a t t e m but with smaller wave lengths, because all teeth participated in the cutting process. The damaged blades left marks in the form of larger grooves, spaced farther apart.
Direct evidence of these deleterious effects was obtained from the SEM examinations of cut surfaces. Those cut with the n e w blade were clean and debrisflee. In comparison, those cut with the damaged blades were irregular, with m a n y broken trabeculae, torn edges, and debris-filled crevices. These experiments did not define an increase in heat resulting from the damaged blades, which is likely to occur. Excessive heat induces thermal damage to osteosites and expands the zone of necrosis b e y o n d that shown microscopically. Smooth, accurately cut surfaces are recognized as an important factor for bone ingrowth into porouscoated prostheses. Such clean bone cuts enhance prosthetic fit and seating, therefore promoting an even load bearing to the bone, and improve alignment of the prostheses or osteotomies. These aspects were not defined in these experiments, but their potential detrimental influence due to p o o r bone cuts is clear. Damage to blade cutting surfaces due to inadvertent contact with templates and instruments may be unavoidable with currently available techniques. However, with the use of saws held in jigs, which eliminates the need for templates, such damage m a y be minimized. This has been our experience with the use of a total knee replacement saw jig (1). Blades regularly inspected after use in the saw jig showed little w e a r and no major damage. Say,blades should be inspected and replaced frequently. Examination without magnification is inadequate, but a 10 x magnifying hand lens will define damages such as loss of teeth, blunting of tips, scouring of surfaces, and loss of set.
References 1. Cooke TDV, Saunders G, Siu D et al: Universal bone cutting device for precision knee replacement arthroplasty and osteotomy. J Biomed Eng 7:45, 1985 2. Karnovsky M J: A formaldehyde-glutaraldehyde fixative of high osmality for use in electron microscopy. J Cell Biol 27N:I37A, 1965 3. Krause W: Mechanical Effects of Orthogonal Bone Cutting. Ph.D. Thesis. Clemson University, Clemson, SC, 1976