Heterotopic bone formation about the hip undergoes endochondral ossification: a rabbit model

Abstract

Background: Heterotopic ossification (HO) occurs most commonly after trauma and surgery about the hip and may compromise subsequent function. Currently available animal models describing the cellular progression of HO are based on exogenous osteogenic induction agents and may not reflect the processes following trauma.

Questions/purposes: We therefore sought to characterize the histologic progression of heterotopic bone formation in an animal model that recapitulates the human condition without the addition of exogenous osteogenic material.

Methods: We used a rabbit model that included intramedullary instrumentation of the upper femur and ischemic crush injury of the gluteal muscle. Bilateral surgical induction procedures were performed on 30 animals with the intention of inciting the process of HO; no supplemental osteogenic stimulants were used. Three animals were sacrificed at each of 10 predetermined times between 1 day and 26 weeks postoperatively and the progression of tissue maturation was graded histologically using a five-item scale.

Results: Heterotopic bone reliably formed de novo and consistently followed a pathway of endochondral ossification. Chondroid elements were found in juxtaposition with immature woven bone in all sections that contained mature osseous elements.

Conclusions: These results establish that HO occurs in an animal model mimicking the human condition following surgical trauma about the hip; it is predictable in its histologic progression and follows a pathway of endochondral bone formation.

Clinical relevance: By showing a consistent pathway of endochondral ossification leading to ectopic bone formation, this study provides a basis for understanding the mechanisms by which HO might be mitigated by interventions.

Fig. 1

A graphic representation of chronologic progression of tissue types as identified in serial specimens with a grading system to assess maturation of HO is shown.

Fig. 2

A high-magnification photomicrograph at 10 days after the induction procedure shows loose fibroconnective tissue (collagen fibers stain blue) is admixed with atrophic skeletal muscle fibers (red) (scale bar = 50 μm) (Stain, Masson’s trichrome; original magnification, ×40).

Fig. 3

An intermediate-magnification photomicrograph shows skeletal muscle 3 weeks after the induction procedure. Dense fibrous tissue can be seen with collagen deposition in continuity with hypertrophic chondrocytes, calcifying cartilage, and trabecular bone (scale bar = 200 μm) (Stain, hematoxylin and eosin; original magnification, ×10).

Fig. 4

A high-magnification photomicrograph shows skeletal muscle 4 weeks after the induction procedure. Dense fibrous tissue can be seen with collagen deposition in continuity with palisading chondrocytes, evolving through stages of calcification and ossification reminiscent of the growth plate (scale bar = 100 μm) (Stain, hematoxylin and eosin; original magnification, ×20).

Fig. 5

An intermediate-magnification photomicrograph shows skeletal muscle 12 weeks after the induction procedure. Typical endochondral ossification is apparent, with capillary ingrowth of cartilaginous matrix around hypertrophic chondrocytes and ongoing matrix mineralization (scale bar = 100 μm) (Stain, hematoxylin and eosin; original magnification, ×20).

Fig. 6A–B

(A) A low-magnification photomicrograph shows skeletal muscle 12 weeks after the induction procedure. The orientation provides observation of the head of the femur (FH) and greater trochanter (GT). A mass of heterotopic bone (arrowhead) contiguous with the femur, including marrow elements arising from hypertrophic chondrocytes in a subperiosteal location can be seen (scale bar = 1 mm) (Stain, hematoxylin and eosin; original magnification, ×1). (B) An AP radiograph of the left hip in a rabbit obtained 6 months after surgery shows HO around the hip in characteristic location.