Musculoskeletal System

Bone - Callus

Narrative
Comment:
Shortly after fracture and resolution of hemorrhage, fibroblasts adjacent to the fracture site begin to replicate and form loose granulation tissue (Figure 1, Figure 2, and Figure 3). Following granulation tissue formation, fibroblasts within the fracture site, as well as along the periosteum, differentiate into chondroblasts to form hyaline cartilage, while those distal to the fracture site develop into osteoblasts to form woven bone. As these populations of tissue form a new mass of connective tissue bridging the fracture site, a callus is formed. Following callus formation, hyaline cartilage and fibrous tissue are replaced by trabecular bone through mineralization of existing collagenous matrix (Figure 4). During a remodeling phase, the callus and lamellar bone deposition is contoured to the original bone structure. Generally, fracture callus is incomplete or disordered in cases of secondary fracture due to neoplasia, ongoing inflammatory or infectious disease, or metabolic bone disease.
Recommendations:
Bone - callus, when observed, should be diagnosed but not graded. When observed in conjunction with a fracture secondary to neoplasia, ongoing inflammation or infection, or metabolic disease, fracture callus should not be diagnosed but should be discussed in the narrative. Associated lesions, such as inflammation or fibrosis, should not be diagnosed separately unless warranted by their severity.
References:

Brighton CT, Hunt RM. 1991. Early histologic and ultrastructural changes in medullary fracture callus. J Bone Joint Surg 73:832-847.

Abstract: http://www.ncbi.nlm.nih.gov/pubmed/2071617

Leininger JR, Riley MGI. 1990. Bones, joints, and synovia. In: Pathology of the Fischer Rat: Reference and Atlas (Boorman G, Eustis SL, Elwell MR, Montgomery CA, MacKenzie WF, eds). Academic Press, San Diego, 209-226.

Long PH, Leininger JR. 1999. Bones, joints, and synovia. In: Pathology of the Mouse (Maronpot R, Boorman G, Gaul BW, eds). Cache River Press, St Louis, 645-678.

  • Image of callus in the bone from a male Wistar Han rat in a chronic study

    Bone - Callus in a male Wistar rat from a chronic study. There is callus formation surrounding a rib, characterized by a marked proliferative fibrous connective tissue response and bony remodeling.

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<p>Bone - Callus in a male Wistar rat from a chronic study. There is callus formation surrounding a rib, characterized by a marked proliferative fibrous connective tissue response and bony remodeling.</p>

Bone - Callus in a male Wistar rat from a chronic study. There is callus formation surrounding a rib, characterized by a marked proliferative fibrous connective tissue response and bony remodeling.
<p>Bone - callus in a male Wistar rat from a chronic study (higher magnification of figure 1). Section of a bone callus, characterized by a marked proliferative fibrous connective tissue response and bony remodeling.</p>

Bone - callus in a male Wistar rat from a chronic study (higher magnification of figure 1). Section of a bone callus, characterized by a marked proliferative fibrous connective tissue response and bony remodeling.
<p>Bone - callus in a male F344/N rat from a chronic study. This section of long bone with a callus is characterized by a focal fibroblastic response (arrowhead).</p>

Bone - callus in a male F344/N rat from a chronic study. This section of long bone with a callus is characterized by a focal fibroblastic response (arrowhead).
<p>Bone - callus in a male F344/N rat from a chronic study (higher magnification of figure 3). This bone callus has a focal fibroblastic response and associated osteoid deposition and mineralization (arrowheads).</p>

Bone - callus in a male F344/N rat from a chronic study (higher magnification of figure 3). This bone callus has a focal fibroblastic response and associated osteoid deposition and mineralization (arrowheads).

Authors

Mark J. Hoenerhoff, DVM, PhD, DACVP
Associate Professor
In Vivo Animal Core,Unit for Laboratory Animal Medicine
University of Michigan
Ann Arbor, MI

Amy Brix, DVM, PhD, DACVP
Senior Pathologist
Experimental Pathology Laboratories, Inc.
Research Triangle Park, NC

Reviewers

Philip H. Long, DVM, PhD, DACVP
Presiden and Senior Pathologist
Vet Path Services, Inc.
Mason, OH

Joel R. Leininger, DVM, PhD, DACVP
JRL Consulting, LLC
Chapel Hill, NC