Be particularly vigilant if the patient has an impaired level of consciousness, decreased peripheral sensation (e.g., diabetic neuropathy), or has poor nutrition.
3. The risk of pressure sores can be reduced by appropriately padding all areas at risk.
4. Counterintuitively, total-contact casts are used in high-risk patients with less padding that allow less friction and a lower risk of wound development; however, this should only be applied by an experienced professional and not attempted by someone inexperienced.
Conclusion
Closed treatment can be applied to a wide variety of fractures with minimal risk to the patient. If a patient is not a good candidate for surgery, nonoperative methods can be attempted even in the most difficult cases. It is important for the surgeon to be aware of nonsurgical alternatives and methods of fracture treatment.
Suggested Readings
Court-Brown CM, Aitken S, Hamilton TW, Rennie L, Caesar B. Nonoperative fracture treatment in the modern era. J Trauma 2010;69(3):699–707
Egol KA, Walsh M, Romo-Cardoso S, Dorsky S, Paksima N. Distal radial fractures in the elderly: operative compared with nonoperative treatment. J Bone Joint Surg Am 2010;92(9):1851–1857
Gregson PA, Thomas PB. Tibial cast wedging: a simple and effective technique. J Bone Joint Surg Br 1994;76(3):496–497
Makwana NK, Bhowal B, Harper WM, Hui AW. Conservative versus operative treatment for displaced ankle fractures in patients over 55 years of age. A prospective, randomised study. J Bone Joint Surg Br 2001;83(4):525–529
McKee MD, Wild LM, Schemitsch EH. Midshaft malunions of the clavicle. J Bone Joint Surg Am 2003;85(5):790–797
Olerud P, Ahrengart L, Ponzer S, Saving J, Tidermark J. Internal fixation versus nonoperative treatment of displaced 3-part proximal humeral fractures in elderly patients: a randomized controlled trial. J Shoulder Elbow Surg 2011;20(5):747–755
Sarmiento A, Zagorski JB, Zych GA, Latta LL, Capps CA. Functional bracing for the treatment of fractures of the humeral diaphysis. J Bone Joint Surg Am 2000;82(4):478–486
Sarmiento A, Gersten LM, Sobol PA, Shankwiler JA, Vangsness CT. Tibial shaft fractures treated with functional braces. Experience with 780 fractures. J Bone Joint Surg Br 1989;71(4):602–609
4 Biomechanics of Internal Fracture Fixation
Jason A. Lowe, Hannah L. Dailey, and Jason Wild
Introduction
A proper discussion of biomechanics necessitates knowledge and understanding of key concepts. Since the definitions are complex, the concepts in this chapter will prove difficult without a command of the language of biomechanics.
Keywords: fracture biomechanics, bone healing, construct design, stress, strain, strength, implant, failure, elasticity, plasticity
I. Definitions
A. Stress is a force applied to an object distributed over the area that bears the load and is measured in Newtons per meter squared N/m2 (pascal).
B. Strain describes a change in shape in response to an applied stress. In axial loading, strain is the change in length of an object over the original length. In fracture management, multiplanar motion at the fracture site gives rise to complex three-dimensional strains.
C. A stress–strain curve is the experimentally observed relationship between applied load (stress) and deformation (strain) for a given material (see ▶Fig. 4.1). This curve also defines other material properties.
D. Young’s modulus of elasticity (E) describes the stiffness of a material and is defined by the slope of linear portion on a stress–strain curve. The modulus E is measured in megapascals (MPa) and is intrinsic to the material, so it does not depend on material geometry. The more stress it takes to deform an object, the steeper the curve (higher E).
Fig. 4.1 Representative material stress–strain graph showing Young’s modulus of elasticity (E), proportional limit (P), yield strength (σY)), and ultimate strength (σU)).
E. Proportional limit (P) of a material is the point on the stress–strain curve of a material after which any additional applied stress will cause nonelastic permanent deformation. For many materials, including implant-grade metals, the proportional limit is equal to the elastic limit.
F. Elastic deformation is the change in shape that is completely reversible when the applied stress is removed.
G. Plastic deformation or permanent set is deformation that does not completely resolve when stress is removed. When plastic deformation occurs, the object’s shape is permanently altered because of damage to the material microstructure.
H. Strength is the ability of a material to withstand applied loading without failure (breakage) or plastic deformation.
I. Yield strength (σY) is the stress at which a material starts to experience plastic deformation and it typically coincides with the proportional limit (P).
J. Ultimate strength (σU) is the stress in a material when catastrophic failure occurs following a one-time overloading event and corresponds to point UF of the stress–strain curve.
K. Fatigue strength (σN) is the maximum stress a material can withstand for N cycles of repeated loading. Trauma implant components are typically designed to withstand at least several hundred thousand cycles of weight bearing loading before fatigue failure (▶Fig. 4.2).
L. Endurance limit (σE) is the stress at which a ferrous material, such as stainless steel, can experience infinite cyclic loading without failing. Nonferrous metals including implant-grade titanium alloys have a fatigue limit, which is the stress corresponding to failure at a defined limit such as 500 million loading cycles.
M. Stiffness describes the ability of a material or of a manufactured part to resist deformation in response to an applied load. The material stiffness, or Young’s modulus E, does not depend on part geometry. The part stiffness is a function of the part geometry, the material’s Young’s modulus E, and the mode of loading (e.g., axial tension/compression, bending, or torsion). Choosing a larger part (e.g., thicker plate, larger-diameter nail or screw) always increases construct stiffness.
N. Flexural rigidity is a measure of the force required to bend an object and is the product of the material Young’s modulus of elasticity (E) and a geometric factor. For a plate with rectangular cross section, the flexural rigidity is proportional to the plate thickness to the third power (h3). For a screw or pin with a circular cross section, the flexural rigidity is proportional to the radius to the fourth power (r4).
Fig. 4.2 Representative fatigue life curves for metals. Higher applied loads (higher cycle stress amplitude, σa) are associated with earlier fatigue failure.