Absorbable plates are widely used for facial fractures. The main advantage of absorbable plates is that removal is unnecessary because they are completely decomposed by metabolism. The conventional absorbable plate is composed of polymers containing mainly PLLA. PLLA is semicrystalline and highly hydrophobic and requires 5–6 years to degrade (Xue et al. 2014; Eppley et al. 2004). The degradation of an absorbable plate is said to consist of two steps: The first step is hydrolysis, which breaks the chemical bonds of the polymer into a low molecular weight polymer. In the second step, these substances are further degraded and metabolized to CO2 and H2O (Jeon et al. 2016). During this degradation, a foreign body reaction may occur, causing, for example, redness and swelling. The crystallinity of the constituent substances is considered the most important factor in the foreign body reaction. The decomposition exceeds the permissible level of metabolism, and the crystalline substance as an intermediate product remains for a long time. This is main factor in the foreign body reaction. (Kim and Lee 2019).
The PDLLA absorbable plate is a new plate composed of PDLLA alone. Unlike PLLA, PDLLA is an amorphous substance, and therefore, no crystalline substance is formed as an intermediate product. In addition, PDLLA is absorbed in 72 weeks (Heidemann et al. 2001), so a foreign body reaction is less likely to occur than if a conventional absorbable plate is used. The foreign body reaction of the PDLLA absorbable plate was reported to be transient swelling, as it was for other absorbable plates (Aldanaa et al. 2011; Reichwein et al. 2009; Yukari et al. 2020; Itaru et al. 2020). However, as yet few reports have been published of a foreign body reaction requiring surgery: in one report, only a foreign body granuloma was formed in seven cases (Jeon et al. 2016) and in two other reports, the swelling remained without spontaneous regression in two cases (Hitoshi et al. 2019; Doh et al. 2018). Although the PDLLA absorbable plate is a new material with different features from those of the conventional product, further case study is necessary on the progress and frequency of the foreign body reaction in future.
In our case, the upper eyelid was highly swollen with redness and purulent drainage; thus, we considered the presence of severe inflammation. Plate fragments were observed at the wound, suggesting that degradation and absorption of the plate were stagnant. Furthermore, the bacterial culture results were negative, suggesting that the foreign body reaction accompanied by delayed degradation and absorption of the plates was the cause. The degradation process of an absorbable plates is histologically divided into five stages: protein adsorption, acute inflammation, chronic inflammation, foreign body giant cell formation, and fibrosis of the plate fragments, in which the fibrosis of the fragments is said to promote the foreign body reaction (Doh et al. 2018; Anderson et al. 2008). We thought that, because the plate was not completely decomposed, the inflammatory response to the fragments was prolonged and spread to the surrounding tissues, resulting in the formation of a sterile abscess. Although the bacterial culture results were negative, the possibility that bacterial infection caused the inflammation cannot to be completely ruled out.
In addition to the degree of crystallinity, factors that cause a foreign body reaction to an absorbable plate include the size, shape, and arrangement of the plate; the dead space around the indwelling plate; the thickness of the normal tissue covering the plate; and the proximity to the skin incision (Xue et al. 2014; Rha et al. 2015; Kim et al. 2018). In our case, the foreign body reaction was stronger at the frontozygomatic suture than at the infraorbital margin. Given aseptic inflammation rather than infection, the difference in the degree of the foreign body response may be related to periosteal repair after bone fixation. The periosteum at the infraorbital margin could be repaired after bone reduction, but that at the zygomaticofrontal suture could not be repaired because there was not enough periosteum to cover the thickness of the plate. In other words, whether hydrolysis of the plate placed on the bone surface occurs inside or outside the periosteum was important, and the periosteum possibly suppressed the spread of inflammation to the circumference. Therefore, periosteal repair sufficient to cover the plate was considered to be important for reducing the foreign body reaction to the plate. With regard to plate fixing, we should select a site that allows for not only bone fixation but also sufficient periosteum to cover the plate.
The foreign body reaction caused by an absorbable plate is often transient. However, if the abscess is formed with extensive and intense inflammation, as in our case, surgical intervention should be considered for distinguishing a foreign body reaction from a bacterial abscess. Although no clear criteria exist for the indication of surgical intervention for foreign body reactions, it has been suggested that surgical removal should be performed for swelling that remains 2 years after surgery because of the possibility of subsequent bone resorption (Mackool et al. 2006). PDLLA absorbable plates are relatively new products, and their hydrolysis progresses over a period of more than 1 year. Therefore, long-term follow-up for 1 year or more after surgery is necessary to confirm the presence and extent of a foreign body reactions. Furthermore, surgical intervention, such as drainage or removal, should be considered if the foreign body reaction symptoms are severe.