Perren's 1979 landmark paper establishes the biomechanical theory of fracture healing based on interfragmentary strain. It asks: why do different tissues form at different stages of healing, and what determines whether primary or secondary bone healing occurs? The answer centers on the concept that strain = displacement / gap width, and each tissue type can only survive below its own rupture elongation threshold.
Every time you choose between a plate, a nail, or non-operative management, you are making a decision about interfragmentary strain — even if you have never framed it that way.
Perren's strain theory explains why these decisions matter: if strain at the fracture gap exceeds 2%, bone cannot form there. If it stays below 100%, at least granulation tissue can survive. Everything in between is tissue differentiation driven by mechanics.
When you see callus on a post-op X-ray after plate fixation, do not reassure the patient that healing is progressing. Callus with a plate means your fixation was not truly rigid — investigate for instability or early infection.
When you see fragment end resorption on X-ray after intramedullary nailing, do not panic. The nail is a splint, not a compression device. Resorption widens the gap, lowers strain, and allows repair tissue to survive — the fragments will sinter together under load, and this is expected.
The direct clinical legacy of this paper is the AO principle of absolute stability for diaphyseal fractures requiring early mobilization, and the understanding that compression abolishes interfragmentary motion — not by making bone stronger, but by eliminating the strain that prevents bone from forming.
Perren's 1979 landmark paper establishes the biomechanical theory of fracture healing based on interfragmentary strain. It asks: why do different tissues form at different stages of healing, and what determines whether primary or secondary bone healing occurs? The answer centers on the concept that strain = displacement / gap width, and each tissue type can only survive below its own rupture elongation threshold.
Every time you choose between a plate, a nail, or non-operative management, you are making a decision about interfragmentary strain — even if you have never framed it that way.
Perren's strain theory explains why these decisions matter: if strain at the fracture gap exceeds 2%, bone cannot form there. If it stays below 100%, at least granulation tissue can survive. Everything in between is tissue differentiation driven by mechanics.
When you see callus on a post-op X-ray after plate fixation, do not reassure the patient that healing is progressing. Callus with a plate means your fixation was not truly rigid — investigate for instability or early infection.
When you see fragment end resorption on X-ray after intramedullary nailing, do not panic. The nail is a splint, not a compression device. Resorption widens the gap, lowers strain, and allows repair tissue to survive — the fragments will sinter together under load, and this is expected.
The direct clinical legacy of this paper is the AO principle of absolute stability for diaphyseal fractures requiring early mobilization, and the understanding that compression abolishes interfragmentary motion — not by making bone stronger, but by eliminating the strain that prevents bone from forming.