For over a century, since the first description of an orbital 'blowout' fracture, there has been debate and confusion regarding the mechanism of production of these fractures. An orbital blowout fracture throughout this paper refers to fractures exhibiting displacement of the orbital floor or walls without an associated fracture of the orbital rim. These are the so called 'pure' blowout fractures as described by Converse. Involvement of the orbit in a variety of facial fracture patterns is easily explained on anatomical grounds. The orbital blowout fracture and symptom complex are readily recognisable but explanation of the mechanisms involved is not easily apparent. Experimental and clinical studies have generally aimed to support one or other of two proposed mechanisms. The 'buckling' theory contends that the fracture is produced as a result of transmission of force to the orbital floor from a blow to the orbital rim (Fig. 1). The 'hydraulic' theory differs in suggesting the force is transmitted to the floor via a direct blow to the globe (Fig. 2). Review of the literature reveals that there are major flaws in the design and execution of previous experimental methods. Most studies have incorporated some or all of the following limitations: low numbers, unquantified forces, non human models, incomplete soft tissues, poor simulation of in vivo conditions and a failure to isolate the position of the striking force. No study has ever provided a direct comparison between the two mechanisms under identical conditions. We present the results of such a study undertaken on 47 fresh cadaver orbits using the same quantifiable force and under the same experimental conditions. The results demonstrate that the efforts to establish either mechanism as the primary aetiology have been misplaced. Both mechanisms produce orbital blowout fractures. The fractures produced, however, are fundamentally different in their size, position and likely clinical significance.
Copyright 1999 The British Association of Plastic Surgeons.