60 Canine Sports Medicine and Rehabilitation
Bone healing as it occurs in the absence of
surgical intervention is referred to as second
ary, or indirect healing, and occurs in several
overlapping phases. The initial injury causes
soft tissue damage and acute hemorrhage and
results in the formation of a fracture hematoma
that fills the fracture site and that may encom
pass displaced fragments. Within several hours,
an acute inflammatory response develops
and the area is infiltrated by neutrophils and
macrophages. Following the clearance of
necrotic tissue debris, the reparative phase
begins, marked by capillary outgrowth from
surrounding soft tissues and vascularization of
the fracture hematoma. Vascularization con
verts the fracture hematoma to granulation
tissue and forms the extraosseous blood supply
of healing bone. This is a critical step in the
initiation of secondary healing, and preserva
tion of this developing blood supply is a major
goal in the clinical management of fractures.
Proliferation and influx of progenitor cells from
the soft tissue envelope surrounding the frac
ture occurs. Periosteum and muscle are major
sources of mesenchymal stem cells that undergo
chondroblastic and osteoblastic differentiation
in response to the growth factor milieu within
the granulation bed at the fracture site.
Mechanical strains within the granulation tis
sue of the fracture site elicit the deposition of
chondroid matrix and woven bone in a disor
ganized fashion resulting in the formation of
soft callus. This occurs over several weeks.
Mineralization of this early matrix produces
hard callus, which is the earliest radiographi
cally detectable phase of secondary healing
(Figure 3.13). Bridging of the fracture gap with
mineralized hard callus re‐establishes bony
continuity and marks the transition from the
reparative phase to the remodeling phase.(A) (B)Figure 3.13 Radiographic appearance of a distal radial fracture in a dog (A), and mineralized callus of secondary bone
healing bridging the fracture site 7 weeks after repair (B).