Spine update. The use of animal models to study spinal fusion.

The outcome of a spinal fusion depends upon a multifactorial process. Therefore, it is difficult and impractical to study individual variables clinically. Animal models have been useful in studying the parameters that influence the spinal fusion process. Despite the knowledge gained from previous animal studies, there exists a large void in our basic understanding of this complex process. The purpose of this update is to review the concept of animal models of spine fusion by summarizing previous models, identifying strengths and deficiencies, reviewing the strategy for selecting a model, and highlighting knowledge gaps suitable for future research. With recent advances in the knowledge regarding gene-specific therapies and the availability of osteoinductive proteins, we have entered a new era of biologic manipulation of bone formation. Animal data, however, are only as applicable as the model from which it is derived, and future animal models must overcome the deficiencies highlighted in this review. A valid animal model will prove essential in studying the use of growth factors for spinal fusion. The ultimate goal is that fusion nonunions will no longer be of clinical concern.