Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

STIFFNESS OF INTRAMEDULLARY NAIL CONSTRUCTS The stiffness of intramedullary nail constructs depends on the nail diameter, the nail material, the interlocking technique, the anatomy of the bone, and the fracture location relative to the long axis of the bone. The change in treatment strategy from traditional large-diameter slotted nails for press-fit insertion into a reamed marrow cavity to small-diameter unreamed nail- ing significantly altered the stiffness of intramedullary fixation. Biomechanical in vitro studies on transverse and multifragmen- tary tibia fracture models stabilized by unreamed nails demon- strated that fractures without contact or compression between the fragments have a very low shear stiffness. 165 For unreamed nailing of an intramedullary canal of 12-mm diameter, a 9-mm diameter nail yielded a shear stiffness of 131 N/mm, and a canal filling 11-mm nail had almost twice the shear stiffness (224 N/ mm). 165 The axial stiffness was 723 N/mm for the 9-mm nail and 1039 N/mm for the 11-mm nail. 165 Another study reported axial stiffness in the range of 1,500 to 2,300 N/mm for 8- to 12-mm diameter nails. 186 For reamed nailing, far greater axial stiffness results of 4,000 N/mm were reported for an 11-mm nail. 114 The translational shear stiffness is especially critical for transverse and multifragmentary fractures, because with low shear stiffness, even small forces can shift a small-diameter unreamed nail within the larger diameter of the intramedullary canal until the nail contacts endosteal surface (Fig. 1-25A). This effect is even more pronounced when the fracture is not located at the isthmus but is more proximal or distal where the intra- medullary canal widens. Unreamed nails not only exhibit low translational shear stiff- ness, but also have a low torsional stiffness that increases the shear motion in the fracture plane. Under physiologic torsional moments, rotational angles of 6 to 47 degrees between fracture surfaces have been reported, depending on nail type and diam- eter. 165,186 This torsion-induced rotational shear motion occurs in addition to the translational shear motion (Fig. 1-25B),

stiffness. 114 Therefore, in vitro determined stiffness results are valid only for those clinical scenarios that resemble the construct configuration and loading scenario as the laboratory model. However, because construct configurations and postoperative activity levels differ greatly between patients, published stiffness results for various fixation constructs are only estimates of the actual construct stiffness present clinically. Figure 1-24.  Clinical assessment of interfragmentary motion under loading of the lower leg. A six-degree-of-freedom goniometer system is used to measure the motion at the fracture gap and a six-degree-of- freedom load cell determines the load applied to the foot. (Reprinted from Wehner T, Claes L, Niemeyer F, et al. Influence of the fixation sta- bility on the healing time—a numerical study of a patient-specific frac- ture healing process. Clin Biomech (Bristol, Avon) . 2010;25(6):606–612. Copyright © 2010 Elsevier Ltd.)

A

B Figure 1-25.  A: Shear stiffness of unreamed intramedullary nails is low because only small forces ( F ) are necessary to translate the nail relative to the bone (interfragmentary motion [IFM]). This translational IFM depends on the circular gap between the inner diameter of the bone and the nail diameter. B: For a reamed slotted intramedullary nail ( left ) fitted tight into the inner diameter of the bone, IFM results mainly from the rotation (R). An unreamed intramedullary nail ( right ) with a diameter smaller than the inner diameter of the bone allows transverse motion (TM) that adds to the shear motion caused by the rotation motion (RM). (Reprinted with permission from Claes L. Biologie und Biomechanik der Osteosynthese und Frak- turheilung. Orthop Unfallchirurg . 2006;1:329–341.)

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