Anton Alexander Nolte Peterlin: Osteolysis in Bone Infection

BACKGROUND
Bone infections, such as fracture-related infections, chronic osteomyelitis, and diabetic foot osteomyelitis, are among the most difficult conditions in orthopaedic surgery. These infections can lead to progressive bone destruction (osteolysis), impaired healing, repeated surgeries, prolonged antibiotic treatment, and in severe cases amputation.

Traditionally, infection-induced bone loss has been explained by activation of osteoclasts through the RANKL pathway. However, much of this knowledge originates from rodent and laboratory studies and may not fully reflect human disease.

Recent evidence suggests that bacterial bone destruction is more complex and may involve inflammatory cells, proteolytic enzymes, and bacterial interactions within the bone microenvironment. Understanding these mechanisms is essential for improving diagnosis and treatment of bone infections.

This PhD project combined translational porcine models with clinical orthopaedic infection research to investigate the biological mechanisms behind bacterial-induced bone destruction and treatment outcomes.

PURPOSE 
The purpose of this PhD project was to investigate osteolysis in bacterial bone infections using both translational porcine models and clinical patient cohorts. The project specifically examined whether inhibition of the RANKL pathway reduces bone loss and whether alternative inflammatory and proteolytic mechanisms may play a more important role than previously assumed.

The thesis combined basic science and clinical orthopaedic infection research through four interconnected studies involving implant-associated osteomyelitis, fracture-related infections, chronic osteomyelitis, and diabetic foot osteomyelitis.

Particular focus was placed on inflammatory pathways, matrix-degrading enzymes, bacterial localization within bone tissue, radiographic progression of osteolysis, and the impact of host comorbidities on treatment outcomes.

Overall, the project aimed to improve understanding of infection-driven bone destruction and challenge the traditional paradigm that osteoclast activation alone is responsible for osteolysis in bone infections.

RESULTS
This PhD project demonstrated that bacterial-induced osteolysis is more complex than the traditional RANKL-driven osteoclast paradigm alone. In a translational porcine implant-associated osteomyelitis model, inhibition of RANKL with Denosumab did not reduce radiographic or macroscopic bone loss, and osteoclast numbers did not correlate with osteolysis severity.

In contrast, matrix metalloproteinase-1 (MMP1), a matrix-degrading enzyme, was associated with increased osteolysis in fracture-related infections. Clinical studies further showed that different bacterial species induce distinct osteolytic progression patterns. Gram-negative infections caused the most rapid early bone loss, while Staphylococcus aureus and polymicrobial infections also showed aggressive osteolytic behaviour.

In diabetic foot osteomyelitis, the CLOSE-UP limb-sparing surgical strategy showed favourable clinical outcomes and highlighted the importance of host factors such as vascular disease and comorbidities.

Finally, paired microbiological and histological analysis of intraoperative biopsies revealed that combined positive findings significantly increased the risk of clinical failure in chronic osteomyelitis and fracture-related infections. Histological analysis also demonstrated bacterial localization deep within bone microstructures, supporting mechanisms of immune evasion and persistent infection

Together, the findings challenge current paradigms of osteolysis and provide new translational insight into bacterial bone infections.

THE FUTURE
Should investigate alternative inflammatory and proteolytic pathways, including matrix metalloproteinases, neutrophils, complement activation, and bacterial-host interactions within bone tissue.

The translational porcine model developed in this project provides a valuable platform for future mechanistic and therapeutic studies. Compared with rodent models, pigs share closer anatomical and immunological similarities with humans, making them highly relevant for orthopaedic infection research.

Future work should also focus on Gram-negative infection models as well as genetically or metabolically modified porcine models incorporating conditions such as diabetes, immunosuppression, and vascular disease. Furthermore, implementing histological analysis as a standard diagnostic component in Denmark may improve diagnostic accuracy and strengthen treatment decision-making in bone infections.