Culture of Bone Biopsy Specimens Overestimates Rate of Residual Osteomyelitis After Toe or Forefoot Amputation

Abstract

Background: Guidelines recommend both histological analysis and culture for definite diagnosis of osteomyelitis. It is not clear if histological and culture criteria can be used interchangeably in the clinical scenario of toe amputation. We therefore prospectively compared the results of intraoperative culture and those of histological examination in this setting.

Methods: Consecutive patients requiring toe or forefoot amputation at the University Hospital Basel during a 2-year period were included in the study. Biopsy specimens from the residual bone were cultured according to microbiological standards. Histological analysis was performed using standardized criteria for osteomyelitis. Clinical outcomes were assessed retrospectively via chart review.

Results: Of 51 patients included in the study, 33 (65%) had a positive culture of residual bone and 14 (27%) showed histological signs of osteomyelitis. A negative histological result but a positive culture was found for 21 (41%) of the patients, suggesting that culture has a high false-positive rate if histological analysis is used as the reference to rule out osteomyelitis. The recommended criteria of both positive histological findings and positive culture were fulfilled by 12 (24%) of the 51 patients.

Conclusions: Positive cultures of residual bone after forefoot or toe amputation overestimate the true rate of osteomyelitis as defined by histological analysis, presumably because of contamination from soft tissue at the time of surgery. Additional studies are needed to evaluate the indications for, and the duration of, antibiotic treatment according to these findings.

Clinical relevance: Our results cast doubt on the strategy of relying solely on culture of bone biopsy specimens when deciding whether antibiotic treatment for osteomyelitis is necessary after toe or forefoot amputation.

Fig. 1

Technique for retrieving bone biopsy specimens from residual bone (“proximal biopsy”).

Fig. 2

Representative histological images of bone specimens taken from diabetic patients (hematoxylin and eosin). I = non-infectious changes in bone marrow of bone with perivascular fibrosis and plasmacytic infiltration in the absence of neutrophilic granulocytes (400×). II = possible osteomyelitis with <5 neutrophilic granulocytes (12.5× and 600×). III = definite osteomyelitis with ≥5 neutrophilic granulocytes, showing fatty marrow necrosis with loss of nuclear staining (image in the center) and microabscess-like infiltration accompanied by edema (image on the right) (20×, 200×, and 400×).