Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

A B Figure 1-16.  Natural bone healing in presence and absence of a residual fracture gap. A: In presence of a 3-mm osteotomy gap, relative stabilization stimulates a stronger and circumferential callus healing response compared to rigid fixation with a locking plate, as seen on 9-week radiographs of a sheep tibia. (Reprinted with permission from Bottlang M, Tsai S, Bliven EK, et al. Dynamic Stabilization with Active Locking Plates Delivers Faster, Stronger, and More Symmetric Fracture-Healing. J Bone Joint Surg Am . 2016;98(6):466–474.) B: Similarly, relative stabilization of a well-reduced osteotomy stimulates a stronger callus healing response than rigid compression plating. (Reprinted with permission from Bottlang M, Tsai S, Bliven EK, et al. Dynamic Stabilization of Simple Fractures With Active Plates Delivers Stronger Healing Than Conventional Compression Plating. J Orthop Trauma . 2017;31(2):71–77.)

initial interfragmentary motion decrease the motion at a faster rate than fractures with less interfragmentary motion. 61 The presence of a residual gap after fracture reduction affects fracture healing in an otherwise relatively stable environment. Larger gaps heal more slowly, form less periosteal callus, and have lower BMD than smaller fracture gaps. 11,55,126 In an ovine model, gaps less than 2 mm result in more complete bridging than fractures with a 6-mm gap. 11 Natural bone healing pertains not only to comminuted fractures with a residual fracture gap (Fig. 1-16A), but also to simple, well-reduced fractures. In the case of a well-reduced fracture, absence of a residual fracture gap will limit interfrag- mentary axial motion. As noted previously, natural bone heal- ing does not occur inside the fracture gap, but rather around the periphery by the formation of periosteal callus. Even small residual motion in a well-reduced fracture will distort the peri- osteal envelope and induce callus formation. This has recently been documented in vivo in a series of ovine studies where a perfectly reduced, but not compressed fracture was stabilized with a flexible plating construct. In both oblique 171 and trans- verse 39 fracture models, healing occurred with periosteal callus. In the transverse fracture model, healing was over two times stronger than a compression plating construct (Fig. 1-16B). 39 The direction of interfragmentary motion affects healing. Pri- mary axial motion leads to robust callus formation and faster healing. 126 Multiple studies have investigated the effect of fracture site shear on callus healing and have shown that the presence of shear as the dominant motion prevents fracture healing in most animal studies. 9,226 However, shear can be compatible with heal- ing in situations where the shear motion was not dominant. 131,160 The timing of motion introduction also affects callus formation and fracture healing. The natural fracture healing process responds with a more robust callus formation when exposed to early motion relative to delayed motion. 46,131 In a ovine fracture healing study,

introducing motion in week 1 rather than week 6 yielded two times the bone mineral content when measured at 12 weeks. 103

PRIMARY BONE HEALING Direct or primary bone healing is the result of an induced mechan- ical environment achieved after anatomic fracture reduction with compression of the fracture ends. Primary bone healing requires interfragmentary motion to remain below 0.15 mm and fracture strains of less than 2% to 5%, without gaps at the fracture site. 61,166 It relies on direct remodeling of bone, whereby osteoclast cutting cones cross the adjacent fracture surfaces and osteoblasts form new bone. Because the healing process directly deposits bone and does not pass through intermediate stages of less organized tissues, the process is highly sensitive to fracture site motion and fracture site gaps. Frac- ture site motion that causes interfragmentary strain to exceed the 2% to 3% failure strain of bone will disrupt the healing process. Residual gaps at the fracture site will prevent osteoclast cutting cones from crossing the fracture. Because of the requirement for direct bony contact, primary bone healing is often referred to as “contact heal- ing.” Microscopically, even the most anatomic fracture reduction will result in small gaps between the bone ends. Since cutting cones can- not cross these residual gaps, a second mode of direct bone healing termed “gap healing” is recognized in these areas. In gap healing, bone is laid down directly as lamellar bone perpendicular to the long axis of the bone. This bone is then remodeled by cutting cones cross- ing the new bone parallel to the long axis. Because gap healed bone is not as well anchored to the fracture ends, it is not as strong as bone formed by contact healing. 130 The process of direct bone healing does not require the inter- mediate steps of natural bone healing and proceeds directly to the remodeling stage. This does not increase the speed of heal- ing; rather, the process of direct bone healing is recognized as a much slower process. 26 When loaded to failure, radiographically

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