Possible cause of failure of the spinal implant

Pedicle screws are often used to secure surgically implanted equipment to the spine in patients with spinal cord disease or spinal cord injury. In some cases, these screws loosen over time, resulting in instability of the spine and consequent pain. This is a common complication of spine surgery. One of the suggested reasons for loosening the pedicle screw is implant infection, but so far little clinical evidence has been put forward to support this theory.

In the article "The high frequency of weakly virulent microorganisms detected by pedicle screw sonication: a potential cause of implant failure", published today in the Journal of Neurosurgery: Spine, Vincent Prinz, MD, and colleagues report having detected microbial colonization of surgical implant material in patients without apparent clinical signs of infection.

The study involved 82 patients who underwent surgery during which a material implant of the spine had to be removed. None of these patients had any clinical signs or laboratory evidence of active infection and there was no evidence of obvious infection during surgery. After surgery, sonication was used to detect the presence of microbial biofilms on the extracted material. This application of sound energy breaks any sticky biofilm covering the surgical implants and separating the microorganisms that inhabit it. The microorganisms that had colonized the implants were then examined to specify the types present.

The authors found signs of loosening of the pedicle screw in 54 of the 82 patients; the other 28 patients formed a control group. Sonication verified microbial colonization of material extracted with one or more bacteria in 22 (40.7%) of 54 patients with loosening of the pedicle screw. Sonication did not reveal any microbial colonization in the material removed from the 28 patients in whom there was no loosening of the pedicle.

The most frequently encountered microorganisms have been identified as coagulase-negative staphylococci (in particular: Staphylococcus Epidermis, S. hominis, S. lugdunensis, S. haemolyticus, S. cohnii, and S. saprophyticus). These commensal bacteria are part of the normal flora covering the skin and mucous membranes of humans, where they rarely cause damage to their hosts. The second most common microorganism encountered was Cutibacterium acnes, which, as its name suggests, is at the origin of acne and is also commonly found on human skin. All these bacteria have a low level of virulence. When they group together on medical devices implanted in a patient, they form biofilms and can cause infections with no clinical symptoms and are not easily detected.

The authors reviewed patient charts to determine the duration of initial surgeries in which spinal material was implanted for the first time in the spine. They found a significant difference between the duration of surgery in patients for whom microbial colonization of the material was detected (mean of 288 ± 147 minutes) and those in which no microbial colonization was detected (mean of 201 ± 103 minutes).

Based on the relationship between loosening pedicle screws and microbial colonization, the authors suggest that patients in whom infected material is replaced would benefit from a "systemic biofilm-active antibiotic treatment to prevent further loosening of the screws. ". "Alternatively," they add, "local antibiotics may be applied, such as antimicrobial screw coating or the application of a local antimicrobial hydrogel or bone substitutes".

The authors summarize their conclusions in four points:

• Standard preoperative tests did not reveal possible infection at the low virulence implant.
• There was a significant badociation between the loosening of the pedicle screw and the presence of weakly virulent pathogens on the spinal implants.
• Coagulase-negative staphylococci were the most common pathogens found colonizing implants.
• There was a significant badociation between long surgical durations and a higher rate of implant infection.

Asked about the importance of the study, Dr. Vajkoczy said, "The results of our study will contribute to a paradigm shift in spinal surgery, if not more generally, the routine treatment of implant implants. Our growing awareness of slow virulence infections as a cause of late material failure has altered our concept of prevention and treatment of this unfortunate complication of spine surgery.The good news is that the infection is a complication that can be successfully treated if properly diagnosed. "

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Journal of Neurosurgery