Rib fractures in infancy: establishing the mechanisms of cause from the injuries — a literature review

Abstract

In the absence of a history of a significant accidental event, the most likely diagnosis in an infant with rib fractures is non-accidental injury. Medical opinion is essential when deciding if child abuse has taken place or not and this requires a consideration of whether a proposed causal explanation fits with the observed injuries. To assist in the interpretation of injuries and inform medical practitioners, it is essential to develop a clear understanding of their biomechanical mechanism of causation. The types and 'likely' mechanisms of rib fracture are well-documented, however, what forces, deflections and loading rates are required to produce particular fractures are relatively unknown. This paper presents a review of the literature, from a biomechanical engineering perspective, to assess information regarding the various types of rib fracture and their likely mechanisms, paying particular attention to the likely forces, deflections and loading rates involved. The biomechanical parameters, applied during 'two finger' infant cardiopulmonary resuscitation (CPR), are identified and discussed, and are currently believed to be below the limit at which rib fracture may occur. However, a new 'two thumb' CPR technique is identified which involves a holding/gripping mechanism of the infant thorax, similar to that which is believed to occur in inflicted injury scenarios, such as shaking. The two thumb method may provide a greater magnitude of force and deflection, a greater rate of loading and may perhaps pose a greater risk of fracture, compared with the 'two finger' supine, anterior-posterior chest compression approach. It is proposed to introduce a force/deflection boundary condition into mechanical and computer/numeric infant models. Subject to the future inclusion of loading rate/response data, a three-dimensional (force/deflection/loading rate) boundary condition will be used to provide a minimum injury threshold for potentially injurious scenarios. A number of further studies are suggested, since this paper is intended to provide a first step to developing a more sophisticated understanding of the causes of infant rib fracture. Areas of further study include specific rib fracture mechanisms, as well as the effects of age dependent characteristics, positioning and area of force application. Such additional information would allow the proposed initial boundary condition to be further refined to provide an indication of the likelihood, type and number of fractures that might be expected.