Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

In the first several weeks after fracture, bony callus is deposited by intramembranous bone formation circumfer- entially around the periosteal and endosteal bone surfaces proximal and distal to the fracture. Initially, tissue strain is too high for cartilage or bone formation close to the fracture. Granulation tissue, which has a higher allowable strain, is deposited near the fracture site. The granulation tissue gradu- ally improves the mechanical stability of the fracture, allowing endochondral ossification to proceed by depositing cartilage along the intramedullary and extramedullary cortex near the fracture. This initial “soft callus” adds stability to the fracture but remains flexible and allows large deformations before fail- ure. As the callus volume increases, endochondral ossification proceeds and results in calcification of the soft callus, which further increases the stability of the fracture. 220 Hard callus formation marks the end of the healing process and the begin- ning of the remodeling phase. As healing progresses, fracture stability results from improv- ing material and structural properties of the callus. Progres- sive mineralization of the soft callus increases its stiffness and enhances the stability at the fracture site. The structural prop- erties improve as callus is deposited far away from the neutral axis of the bone, increasing the moment of inertia and stability at the fracture site (Fig. 1-13A,B). The extramedullary callus contributes far more to the strength of the fracture repair than

TABLE 1-6. Progressively Increasing Tissue

Properties of Fracture Healing Tissue

Ultimate Tensile Strength (N/mm 2 )

Tissue

Maximum Strain (%)

Hematoma

100

0.1

Soft callus

10–12.8

4–19

Hard callus

2

130

Adapted from Griffon DJ. Fracture healing. In: Johnson AL, ed. AO Principles of Fracture Management in the Dog and Cat . 1st ed. New York, NY: Thieme; 2007. Copyright © Georg Thieme Verlag KG.

tissues, each of which possesses two mechanical characteristics— the ability to stabilize the fracture and a strain beyond which the healing tissue will fail (Table 1-6). 104 Natural bone healing occurs by bridging the fracture with a callus envelope rather than direct remodeling across the frac- tured bone ends. 150 The earliest stage of fracture healing begins immediately after the injury, as the fracture site fills with hema- toma and forms a fibrin clot. This tissue provides little stability and can deform significantly, tolerating fracture strains up to 100%. 126 Most of the stability at this early healing stage is pro- vided by the soft tissues surrounding the fracture. 45

A

B

C

Figure 1-13.  A: Natural bone healing of a well-reduced transverse osteotomy under relative stabilization heals with abundant circumferential callus, as depicted on a cross-sectional image of a specimen that was loaded to failure through the osteotomy plane. B: Micro-CT shows cortical resorption, indicating that bridging does not occur between adjacent fracture surfaces, but in the periosteal and endosteal callus regions. C: A 5-mm-thick periosteal callus envelope surrounding the diaphysis provides over 12 times more structural stability ( I = 8.6 cm 4 ) than a 5-mm-thick endosteal callus ( I = 0.7 cm 4 ).

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