Ultrastructural Analysis of Infected Absorbable Bone Fixation Material Using Scanning Electron Microscopy After Zygomatic Bone Fracture Surgery

Summary:

Absorbable bone fixation materials are commonly used in facial bone fracture surgery. However, reports on the microstructure of infected implants are rare. This study presents a case involving a 74-year-old male patient who developed an α-Streptococcus infection 2.5 months after zygomatic bone fracture surgery and the ultrastructure of an infected absorbable plate observed under a scanning electron microscope (SEM). The patient sustained a right zygomatic bone fracture due to a fall and underwent open reduction and internal fixation with SonicWeld on the seventh day after the injury. The postoperative course was uneventful, but 2.5 months after the surgery, the patient presented with redness, swelling, and pain in the right lower eyelid, along with purulent discharge, leading to the removal of the bone fixation material. The SEM observation of the extracted implant revealed a biphasic pattern, with areas showing signs of infection and areas without such effects. The infected regions showed wide cracks, bacterial clusters, and biofilm formation, whereas noninfected areas exhibited narrow cracks and distinct geometric striations. These findings suggest a link between crack formation and bacterial proliferation, emphasizing the importance of proper handling and implantation techniques. Although the infection was controlled and bone healing achieved after the removal of infected implants, the patient experienced lower eyelid retraction due to adhesion between the skin and underlying bone, leading to persistent cosmetic issues. This case report presents a rare visualization of the ultrastructural characteristics of infected absorbable implants, offering insights into the potential role of SEM in understanding these changes and processes.

Absorbable bone fixation materials are often used to treat facial bone fractures, but reports on the microstructure of infected absorbable implants are rare. This report presents electron microscope images of a poly-d-lactic acid and l-lactic acid implant (SonicWeld Rx system, KLS Martin Co., Ltd., Tuttlingen, Germany) infected with α-Streptococcus.

CASE

A 74-year-old man sustained a right zygomatic bone fracture after a fall. Seven days postinjury, open reduction and internal fixation were performed under general anesthesia. Cefazolin 1 g was administered intravenously before surgery. Resorbable plates (SonicWeld) were placed at the infraorbital rim, frontozygomatic suture, and zygomaticomaxillary buttress through subciliary, lateral eyebrow, and upper gingivobuccal incisions. We did not soak or rinse the implants in an antibiotic solution before insertion, or irrigate with antibiotics after implantation. Postoperative treatment included oral cephalexin 250 mg 3 times a day for 3 days.

The patient initially recovered uneventfully. However, 2.5 months postoperatively, he developed redness, swelling, and pain in the right lower eyelid, suggesting an infection. Under general anesthesia, an incision in the lower eyelid revealed pus, leading to a diagnosis of plate infection. No abnormalities were observed at the lateral eyebrow or upper gingivobuccal incision sites. All plates and screws were removed, the affected areas were irrigated, and the wounds were closed. Postsurgical treatment included intravenous cefazolin 1 g twice daily for 1 week, followed by a 7-day course of oral cephalexin 250 mg 3 times a day. Cultures confirmed α-Streptococcus as the causative organism. Although the infection subsided and bone healing was achieved, adhesion between the skin and the underlying bone caused retraction of the lower eyelid. Although slight improvements were observed over the next 3 years, significant cosmetic issues persisted. (See figure, Supplemental Digital Content 1, which displays 6 months after the complete removal of the bone fixation material due to infection [left] and 3 years postoperatively [right]. At 6 months postoperatively, significant deformity persists, with recession due to adhesion between the skin and bone of the right lower eyelid and downward traction of the lower eyelid. Even after 3 years, the deformity remains evident, http://links.lww.com/PRSGO/D905.) Eyelid deformity correction was not performed per the patient’s request. SonicWeld samples from the infected site (the infraorbital rim) were fixed in 10% buffered formalin, washed with distilled water, and dried. For ultrastructural analysis, the fixed samples were rehydrated, stained with Methenamine Silver (Muto Pure Chemicals Co., Ltd., Tokyo, Japan), washed with distilled water, dried, and coated with osmium using Neoc-Pro/P (Meiwafosis Co., Ltd., Tokyo, Japan), as previously reported.¹ Surface morphological analysis was conducted using a low vacuum scanning electron microscopy (SEM, TM4000Plus; Hitachi High-Tech Corporation, Tokyo, Japan) at accelerating voltages of 5, 10, or 15 kV.

Ultrastructural analysis showed that the implant was divided into an infected area, where bacteria and debris had accumulated, and a noninfected area, where such elements were absent (Figs. 1A and B). In the infected area, wide cracks several micrometers in width were visible in the plate, corresponding to bacterial clusters and organic and bacterial debris forming biofilm on the SonicWeld. Bacterial clusters were highlighted by silver staining in regions with notable surface elevations, and debris showing weak reactivity to silver staining was detected nearby (Figs. 1C and D). No significant bacterial or debris adhesion was noted in 0.2-µm-wide cracks in the noninfected area. In the noninfected area, geometric striations approximately 1 µm wide were observed (Fig. 1B).

Fig. 1.

SEM images of the infected implants. A, The surface of the implants is shown with low magnification. A smooth surface area (lower left part of the image) and a rough surface area with significant irregularities (upper right part of the image) can be observed. These correspond to the noninfected and infected areas, respectively. Scale bar = 50 µm. B, Uninfected area. A smooth surface and striped patterns (red arrowhead) are observed. Cracks (blue arrowhead) with a width of approximately 0.2 µm are visible. Scale bar = 10 µm. C, The infected area exhibits an uneven surface with several-micrometer-wide cracks (green arrowhead) and deposits. Granular structures with a diameter of 1 µm are clustered together, forming a protrusion. Scale bar = 10 µm. D, Infected area with high magnification. The yellow arrow indicates α-Streptococcus. Scale bar = 5 µm.

DISCUSSION

This case report presents a rare visualization of the ultrastructural characteristics of infected absorbable implants using electron microscopy. Infections related to SonicWeld are exceedingly rare, with only one previous case reported.² Açil et al³ reported that bacterial colonization was frequently detected in titanium plates but rarely observed under SEM. Although infection rates for maxillofacial plates can be as low as 0.9%,⁴ SonicWeld’s rapid absorption (complete degradation within 6 mo in vivo) further complicates the availability of human samples for detailed microscopic analysis.^5,6

Our findings suggest a possible correlation between cracks in the implant material and bacterial proliferation. Infected areas displayed wide cracks alongside bacterial clusters and biofilm, whereas noninfected areas showed narrow cracks and unique geometric striations. Although it remains unclear whether cracks initiated bacterial growth or vice versa, these observations emphasize the importance of preventing cracks during implant handling.

To minimize crack formation, it is crucial to soften the plates by heating them in a water bath and shaping them using a closely fitting template. Excessive force after cooling should be avoided, as this may promote crack formation and increase infection risk. Although multiple factors likely contributed to the infection in this case, crack formation appears to have been a significant contributing factor.

Biofilm formation observed on the infected implant adds another layer of complexity. Biofilms protect bacteria from antibiotics, often necessitating implant removal for complete resolution. Antibiotic soaking of implants, as demonstrated in breast and orthopedic surgery,^7,8 has shown efficacy in reducing infection rates. However, such approaches have not been extensively studied in facial bone fractures. Further research is warranted to evaluate the role of preimplantation soaking and postimplantation irrigation in maxillofacial applications.

The geometric striations observed in noninfected areas are particularly intriguing because they were absent in unused SonicWeld products.⁵ These patterns may indicate degradation processes influenced by biological or mechanical factors. Their regularity suggests a repetitive mechanism at play, which warrants further investigation.

A key limitation of this study is that it is based on a single case. To establish a more comprehensive understanding of the relationship between implant cracks, bacterial infection, and biofilm formation, further research, including in vitro and animal studies, is essential. Electron microscopy remains a valuable tool for elucidating these processes.

CONCLUSIONS

This case highlights the structural changes observed in infected SonicWeld. Wide cracks were associated with bacterial clusters in infected areas, whereas narrow cracks and geometric striations were observed in noninfected areas. These findings suggest a potential correlation between cracks and bacterial proliferation. Although SonicWeld remains a viable option for bone fixation, proper handling to avoid crack formation is crucial to minimizing infection risks.

DISCLOSURE

The authors have no financial interest to declare in relation to the content of this article.