0195--5616/91 $0.00
Fracture Complications
+
.20
Biomechanics of Fracture Fixation Failure
Don Hulse, DVM,* and Bill Hyman, ScDt
PHYSIOLOGIC FORCES, STRESSES, AND STRAINS IN NORMAL BONE Long bones are subjected to both physiologic and nonphysiologic forces. Nonphysiologic forces occur in unusual situations such as automobile accidents, gunshot injuries, or falls. They can be transmitted to bone directly and may easily exceed the ultimate strength of bone, giving rise to a fracture. Physiologic forces are generated by weight bearing, muscle contraction, and associated physical activity. They are transmitted to the bone through the joint surfaces and muscle contraction and are applied as uniaxial tension or compression. Physiologic tension and compression also give rise to torsional and bending moments. These forces do not commonly exceed the ultimate strength of bone and are not responsible for bone fractures except in unusual cases. When forces and moments are applied to any structure, it deforms from its original shape, and local forces are produced within the structure. If the local force and deformations are determined for a specified area of the bone, the local force intensities are referred to as internal stresses and the deformations are referred to as internal strains. Internal stresses have the dimensions of force/cross-sectional area. Internal linear strains are equal to the change in length/original length. Internal shear strains are measured as the change in angle between two lines that initially were perpendicular. The four primary physiologic loads are: (1) axial compression, (2) axial tension, (3) bending, and (4) torsion. These loads, alone or in combination, result in a complex pattern of internal stresses and strains within the bone. Normal stresses are either tension or compression applied perpendicular to the surface of a cross-section of bone, whereas shear stress is directed obliquely or parallel to the surface. When a fracture occurs, stresses and From Texas A&M University, College Station, Texas
*Diplomate, American College ofVeterinary Surgeons; Professor, Department of Small Animal Medicine and Surgery, College of Veterinary Medicine t Professor, Section of Bioengineering Veterinary Clinics of North America: Small Animal Practice-Vol. 21, No. 4, July 1991
647
648
DON HULSE AND BILL HYMAN
strains are present at the fracture line interface. Most important are shear stress and tensile stress, because they can damage the fragile tissues crossing the fracture gap. It is important to understand the generation of internal stresses and strains to neutralize them with stabilizing devices. When axial compression of the bone occurs, the resulting internal stresses and strains are: (1) compression stress parallel to the column of bone, which causes shortening, (2) strain perpendicular to the column of bone, which causes expansion, and (3) shear stress oblique to the column of bone, which causes shortening and lateral displacement (Fig. 1). Clinically, the perpendicular strain arising from axial compression is not important, because expansion of the bone is unlikely. The compressive and shear stresses are significant, however, in that they cause collapse of a comminuted or oblique fracture if they are not resisted (Fig. 2). Axial tension is the direct result of muscle contraction acting at a point of insertion. The resulting internal tensile stress causes a tendency toward fracture displacement and must be resisted by orthopedic implants to prevent significant gapping of the fracture surface. Axial tension from muscle contraction is the only physiologic force of significance in certain fracture types, including (1) fractures of the greater trochanter, (2) fractures of the olecranon, and (3) fractures of the tibial crest (Fig. 3). When a structure such as bone undergoes bending, internal tensile stress is produced on the convex surface of the bone and internal compressive stress is produced on the concave surface of the bone (Fig. 4). The maximum tensile stress is present at the periosteal surface on the convex side of the bone, and maximal compressive stress is present at the periosteal
Figure l. Diagram showing the generation of tension, compression, and shear stresses associated with an axially applied compressive load. Note the change in relationship between the sides of the diamond box. Shear stress is produced at an oblique angle to the longitudinal axis of the bone.
BIOMECHANICS OF FRACTURE FIXATION FAILURE
649
Figure 2. Lateral radiograph of a collapsed comminuted tibial fracture. Axial compression causes shear stress oblique to the longitudinal axis of the bone. These stresses, which are then parallel to oblique fracture surfaces, are responsible for the bony collapse.
650
DON HULSE AND BILL HYMAN
Figure 3. Craniocaudal and mediolateral radiograph of a dog having an avulsion fracture of the tibial crest. The fracture is the result of axial tension forces exerted by contracture of the quadriceps muscle group.
Figure 4. Diagram representing physiologic bending. Note the compression stress on the concave surface and tensile stress on the convex surface. Maximum stress is present at the periosteal surface that is the greatest distance from the neutral axis. This point represents a point of zero stress because the stresses are changing from compression to tension.
BIOMECHANICS OF FRACTURE FIXATION F AlLURE
651
surface on the concave side of the bone. Because the stress varies from tension on one side to compression on the other, there is a point within the cross-section where the stress is equal to zero. This point is called the neutral axis. The surface of bone that is experiencing tension is referred to as the tension band side of the bone, whereas the surface experiencing primary compressive stress is referred to as the compression side of the bone. Because tensile stress pulls apart fracture surfaces, it is important to know the tension band surface of each long bone to resist the tension and prevent fracture gap widening. The tension band surface of the femur is the craniolateral surface. It is the craniolateral surface of the tibia and the humerus, the cranial surface of the radius, and the caudal surface of the ulna. Failure to adequately resist compressive stress on the concave surface of the bone results in collapse of the cortical surface. An example of this is collapse of the medial cortical surface (medial buttress) associated with a comminuted femoral fracture. Torsion causes internal shear stress perpendicular to the long axis of bone. The result is rotational deformation and lateral displacement of the fracture surface. Clinically, uncontrolled internal shear stress resulting from rotational instability can cause a delayed union or nonunion of a transverse fracture (Fig. 5). Although it is convenient to address each of these physiologic forces and subsequent internal stresses individually, clinically, bones experience a combination of compression, tension, bending, and torsional loading. An appreciation of the normal and shear stresses and strains generated by physiologic forces coupled with the knowledge of an implant's ability to resist these stresses is important for optimal fracture management. BIOMATERIALS
There are a variety of metals used in manufacturing implants for fracture fixation devices. Examples are iron-based alloys (stainless steels), cobalt-based alloys, and titanium-based alloys. The specifications for a variety of these alloys have been codified by the American Society for Testing and Materials (ASTM). Stainless steel specifications are codified by the American Iron and Steel Institute. Before an allov can be used for internal fracture stabilization, a number of requirement; must be satisfied. Among these are a suitable combination of properties that permit fabrication of properly sized and shaped implants, suitable mechanical properties relative to allowable sizes for implantation, and compatibility in vivo. Another factor that plays a significant role in veterinary orthopedics is the cost of the implant. Although the titanium- and cobalt-based alloys enjoy a degree of popularity in human orthopedics, economics favor the almost exclusive use of the stainless steel alloys in veterinary orthopedics. As such, this discussion is limited to fabrication principles and mechanical properties of stainless steel. Alloy fabrication begins with the extraction of metal ores from mineral deposits. For stainless steels, the base element is iron; other elements such as chromium, nickel, and molybdenum are added during fabrication. There
652
DON H ULSE AND BILL HYMA N
Figure 5. Craniocaudal radiograph of a hypertrophic nonunion of the femur in an ad ult dog. Rotation will cause internal shear stress perpendicular to the longitudin al axis of the bone . If a transverse fracture is present, shear stress will then be parallel to the fracture surface.
are four basic types of stainless steels. Two of these, martensitic and austenitic stainless steels, are commonly used in orthopedics. The martensitic steels are very hard and tough and therefore are favored for the manufacturing of surgical instruments. The superior corrosion resistance of austenitic alloys such as 316L stainless steel has led to its dominance in implant fabrication. 316L is a designation of the American Iron and Steel Institute. This alloy is equivalent to th e special quality stainless steel for surgical implants designated by the ASTM specifications for F138 and F139 alloys. The composition of austenitic 316L stainless steel is iron (62-72%), chromium (17-20%), nickel (13-16%), and molybdenum (2-3%). Carbon is limited to less than 0.3%, and the remainder of ingredients consists of small amounts of other elements. Each element has a specific purpose in the total composition of the alloy. Nickel is added to stabilize the alloy in the more corrosive-resistant austenitic microstructure. Molybdenum controls corrosion of stainless steel but can only be used in small amounts because it greatly harde ns the alloy, which makes it difficult to work with. Chromium is added to form a stable chromium-oxide surface layer, which helps to prevent corrosion. Carbon is important in some stainless steel applications
BIOMECHANICS OF FRACTURE FIXATION F AlLURE
653
because it forms carbide precipitates, which renders the surface very hard. This is advantageous in manufacturing surgical instruments such as osteotomes or carbide tip needle holders. Excess carbon is a disadvantage, however, in the fabrication of surgical implants because of the rapid corrosion of the carbide precipitates in vivo. To prevent formation of carbon precipitates and subsequent corrosion, the carbon content of surgical implants must be very low. The "L" designation in 316L stainless steels denotes low carbon levels (
Fracture Complications
+
.20
Biomechanics of Fracture Fixation Failure
Don Hulse, DVM,* and Bill Hyman, ScDt
PHYSIOLOGIC FORCES, STRESSES, AND STRAINS IN NORMAL BONE Long bones are subjected to both physiologic and nonphysiologic forces. Nonphysiologic forces occur in unusual situations such as automobile accidents, gunshot injuries, or falls. They can be transmitted to bone directly and may easily exceed the ultimate strength of bone, giving rise to a fracture. Physiologic forces are generated by weight bearing, muscle contraction, and associated physical activity. They are transmitted to the bone through the joint surfaces and muscle contraction and are applied as uniaxial tension or compression. Physiologic tension and compression also give rise to torsional and bending moments. These forces do not commonly exceed the ultimate strength of bone and are not responsible for bone fractures except in unusual cases. When forces and moments are applied to any structure, it deforms from its original shape, and local forces are produced within the structure. If the local force and deformations are determined for a specified area of the bone, the local force intensities are referred to as internal stresses and the deformations are referred to as internal strains. Internal stresses have the dimensions of force/cross-sectional area. Internal linear strains are equal to the change in length/original length. Internal shear strains are measured as the change in angle between two lines that initially were perpendicular. The four primary physiologic loads are: (1) axial compression, (2) axial tension, (3) bending, and (4) torsion. These loads, alone or in combination, result in a complex pattern of internal stresses and strains within the bone. Normal stresses are either tension or compression applied perpendicular to the surface of a cross-section of bone, whereas shear stress is directed obliquely or parallel to the surface. When a fracture occurs, stresses and From Texas A&M University, College Station, Texas
*Diplomate, American College ofVeterinary Surgeons; Professor, Department of Small Animal Medicine and Surgery, College of Veterinary Medicine t Professor, Section of Bioengineering Veterinary Clinics of North America: Small Animal Practice-Vol. 21, No. 4, July 1991
647
648
DON HULSE AND BILL HYMAN
strains are present at the fracture line interface. Most important are shear stress and tensile stress, because they can damage the fragile tissues crossing the fracture gap. It is important to understand the generation of internal stresses and strains to neutralize them with stabilizing devices. When axial compression of the bone occurs, the resulting internal stresses and strains are: (1) compression stress parallel to the column of bone, which causes shortening, (2) strain perpendicular to the column of bone, which causes expansion, and (3) shear stress oblique to the column of bone, which causes shortening and lateral displacement (Fig. 1). Clinically, the perpendicular strain arising from axial compression is not important, because expansion of the bone is unlikely. The compressive and shear stresses are significant, however, in that they cause collapse of a comminuted or oblique fracture if they are not resisted (Fig. 2). Axial tension is the direct result of muscle contraction acting at a point of insertion. The resulting internal tensile stress causes a tendency toward fracture displacement and must be resisted by orthopedic implants to prevent significant gapping of the fracture surface. Axial tension from muscle contraction is the only physiologic force of significance in certain fracture types, including (1) fractures of the greater trochanter, (2) fractures of the olecranon, and (3) fractures of the tibial crest (Fig. 3). When a structure such as bone undergoes bending, internal tensile stress is produced on the convex surface of the bone and internal compressive stress is produced on the concave surface of the bone (Fig. 4). The maximum tensile stress is present at the periosteal surface on the convex side of the bone, and maximal compressive stress is present at the periosteal
Figure l. Diagram showing the generation of tension, compression, and shear stresses associated with an axially applied compressive load. Note the change in relationship between the sides of the diamond box. Shear stress is produced at an oblique angle to the longitudinal axis of the bone.
BIOMECHANICS OF FRACTURE FIXATION FAILURE
649
Figure 2. Lateral radiograph of a collapsed comminuted tibial fracture. Axial compression causes shear stress oblique to the longitudinal axis of the bone. These stresses, which are then parallel to oblique fracture surfaces, are responsible for the bony collapse.
650
DON HULSE AND BILL HYMAN
Figure 3. Craniocaudal and mediolateral radiograph of a dog having an avulsion fracture of the tibial crest. The fracture is the result of axial tension forces exerted by contracture of the quadriceps muscle group.
Figure 4. Diagram representing physiologic bending. Note the compression stress on the concave surface and tensile stress on the convex surface. Maximum stress is present at the periosteal surface that is the greatest distance from the neutral axis. This point represents a point of zero stress because the stresses are changing from compression to tension.
BIOMECHANICS OF FRACTURE FIXATION F AlLURE
651
surface on the concave side of the bone. Because the stress varies from tension on one side to compression on the other, there is a point within the cross-section where the stress is equal to zero. This point is called the neutral axis. The surface of bone that is experiencing tension is referred to as the tension band side of the bone, whereas the surface experiencing primary compressive stress is referred to as the compression side of the bone. Because tensile stress pulls apart fracture surfaces, it is important to know the tension band surface of each long bone to resist the tension and prevent fracture gap widening. The tension band surface of the femur is the craniolateral surface. It is the craniolateral surface of the tibia and the humerus, the cranial surface of the radius, and the caudal surface of the ulna. Failure to adequately resist compressive stress on the concave surface of the bone results in collapse of the cortical surface. An example of this is collapse of the medial cortical surface (medial buttress) associated with a comminuted femoral fracture. Torsion causes internal shear stress perpendicular to the long axis of bone. The result is rotational deformation and lateral displacement of the fracture surface. Clinically, uncontrolled internal shear stress resulting from rotational instability can cause a delayed union or nonunion of a transverse fracture (Fig. 5). Although it is convenient to address each of these physiologic forces and subsequent internal stresses individually, clinically, bones experience a combination of compression, tension, bending, and torsional loading. An appreciation of the normal and shear stresses and strains generated by physiologic forces coupled with the knowledge of an implant's ability to resist these stresses is important for optimal fracture management. BIOMATERIALS
There are a variety of metals used in manufacturing implants for fracture fixation devices. Examples are iron-based alloys (stainless steels), cobalt-based alloys, and titanium-based alloys. The specifications for a variety of these alloys have been codified by the American Society for Testing and Materials (ASTM). Stainless steel specifications are codified by the American Iron and Steel Institute. Before an allov can be used for internal fracture stabilization, a number of requirement; must be satisfied. Among these are a suitable combination of properties that permit fabrication of properly sized and shaped implants, suitable mechanical properties relative to allowable sizes for implantation, and compatibility in vivo. Another factor that plays a significant role in veterinary orthopedics is the cost of the implant. Although the titanium- and cobalt-based alloys enjoy a degree of popularity in human orthopedics, economics favor the almost exclusive use of the stainless steel alloys in veterinary orthopedics. As such, this discussion is limited to fabrication principles and mechanical properties of stainless steel. Alloy fabrication begins with the extraction of metal ores from mineral deposits. For stainless steels, the base element is iron; other elements such as chromium, nickel, and molybdenum are added during fabrication. There
652
DON H ULSE AND BILL HYMA N
Figure 5. Craniocaudal radiograph of a hypertrophic nonunion of the femur in an ad ult dog. Rotation will cause internal shear stress perpendicular to the longitudin al axis of the bone . If a transverse fracture is present, shear stress will then be parallel to the fracture surface.
are four basic types of stainless steels. Two of these, martensitic and austenitic stainless steels, are commonly used in orthopedics. The martensitic steels are very hard and tough and therefore are favored for the manufacturing of surgical instruments. The superior corrosion resistance of austenitic alloys such as 316L stainless steel has led to its dominance in implant fabrication. 316L is a designation of the American Iron and Steel Institute. This alloy is equivalent to th e special quality stainless steel for surgical implants designated by the ASTM specifications for F138 and F139 alloys. The composition of austenitic 316L stainless steel is iron (62-72%), chromium (17-20%), nickel (13-16%), and molybdenum (2-3%). Carbon is limited to less than 0.3%, and the remainder of ingredients consists of small amounts of other elements. Each element has a specific purpose in the total composition of the alloy. Nickel is added to stabilize the alloy in the more corrosive-resistant austenitic microstructure. Molybdenum controls corrosion of stainless steel but can only be used in small amounts because it greatly harde ns the alloy, which makes it difficult to work with. Chromium is added to form a stable chromium-oxide surface layer, which helps to prevent corrosion. Carbon is important in some stainless steel applications
BIOMECHANICS OF FRACTURE FIXATION F AlLURE
653
because it forms carbide precipitates, which renders the surface very hard. This is advantageous in manufacturing surgical instruments such as osteotomes or carbide tip needle holders. Excess carbon is a disadvantage, however, in the fabrication of surgical implants because of the rapid corrosion of the carbide precipitates in vivo. To prevent formation of carbon precipitates and subsequent corrosion, the carbon content of surgical implants must be very low. The "L" designation in 316L stainless steels denotes low carbon levels (
