Lack of a central role for osteoprogenitor cells from the femoral canal in heterotopic ossification of the hip: an experimental study in a rat model

Heterotopic ossification in a patient with diffuse idiopathic skeletal hyperostosis: Input from histological findings

A high incidence of heterotopic ossification (HO) has been reported in patients with diffuse idiopathic skeletal hyperostosis (DISH), a metabolic disease characterized by calcifications of entheses at spine and peripheral sites. We performed histological and immunohistochemical analyses in five different HO sites in a patient with DISH to study a possible mutual interaction of bone morphogenetic protein 2 (BMP-2), transforming growth factor beta (TGF-β), and decorin, crucial for bone mass increasing, matrix calcification, and endochondral bone formation. We speculated that the surgical trauma triggered HO, inducing TGF-β release at the lesion site. TGF-β recruits osteoblast precursor cells and determines the overexpression of BMP-2 in the surrounding skeletal muscle, inducing a further osteogenic differentiation, contributing to HO onset.

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

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.