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Spontaneous autoinflation of saline-filled breast implants is a rare phenomenon; only 20 cases have been reported in the world literature. Over the past seven years, three patients have presented with significant unilateral autoinflation of their smooth, single-lumen, round, saline-filled implants. This developed at various times: progressively over 23 years with a Simaplast implant; between the ninth and 10th year after augmentation with a leaflet valve implant; and slowly over the first four years with a leaflet valve implant. The etiology of the autoinflation was shown to be different for the two types of implants. The Simaplast implant had likely been injected with a hypertonic filling solution – one that was twice as concentrated as ‘normal saline’. This would have created an osmotic gradient, which would have facilitated autoexpansion by diffusion. The implant solution remained clear and transparent. In addition, there were no detectable levels of glucose, uric acid or albumin in the fluid. By contrast, auto-inflation of the leaflet valve implants likely resulted from mechanical alterations of the valve mechanism. This type of implant is known to have a high deflation rate, with frequent partial deflations. It is interesting that one of the patients with the leaflet valve implants presented with an autoexpansion on one side and a partial deflation on the other side. Both implants were from the same lot number. The same mechanism that caused partial deflations may have also allowed fluid from the implant pocket to pass through the valve into the lumen of these implants. This could allow glucose, protein and cellular elements to enter into the lumen (these would not pass through an intact elastomeric shell) which would create an osmotic gradient, allowing water to enter the elastomeric shell by diffusion. The fluid in these leaflet valve implants was brownish yellow, very viscous and turbid. It contained elevated levels of glucose and uric acid which would not have passed through the elastomeric shell. Over the past 10 years, four different theories have been proposed in an attempt to explain the etiology of autoinflation. However, the findings of the present study indicate that there are only two mechanisms – a hypertonic filling solution and alterations of the valve mechanism.