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Isolation of polyacetal wear particles from periprosthetic tissue of isoelastic femoral stems

Department of Physical and Organic Chemistry, Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia.

V. Pisot, MSc, MD, Orthopaedic Surgeon

Orthopaedic Hospital Valdoltra, Jadranska c. 31, 6280 Ankaran, Slovenia.

A. Cör, PhD, MD, Lecturer in Histology

Institute of Histology and Embryology, Faculty of Medicine, University of Ljubljana, Korytkova 2/1, 1000 Ljubljana, Slovenia.

V. Antolic, PhD, MD, Lecturer in Orthopaedic Surgery

Department of Orthopaedic Surgery, University Medical Centre, Zaloska 9, 1000 Ljubljana, Slovenia.

Correspondence should be sent to Dr I. Milossev.

We analysed revised Mathys isoelastic polyacetal femoral stems with stainless-steel heads and polyethylene acetabular cups from eight patients in order to differentiate various types of particle of wear debris. Loosening of isoelastic femoral stems is associated with the formation of polyacetal wear particles as well as those of polyethylene and metal. All three types of particle were isolated simultaneously by tissue digestion followed by sucrose gradient centrifugation. Polyacetal particles were either elongated, ranging from 10 to 150 µm in size, or shred-like and up to 100 µm in size. Polyethylene particles were elongated or granules, and were typically submicron or micronsized.

Polyacetal and polyethylene polymer particles were differentiated by the presence of BaSO₄, which is added as a radiopaque agent to polyacetal but not to polyethylene. This was easily detectable by back-scattered SEM analysis and verified by energy dispersive x-ray analysis.

Two types of foreign-body giant cell (FBGC) were recognised in the histological specimens. Extremely large FBGCs with irregular polygonal particles showing an uneven, spotty birefringence in polarised light were ascribed to polyacetal debris. Smaller FBGCs with slender elongated particles shining uniformly brightly in polarisation were related to polyethylene. Mononucleated histiocytes containing both types of particle were also present.

Our findings offer a better understanding of the processes involved in the loosening of polyacetal stems and indicate why the idea of ‘isoelasticity’ proved to be unsuccessful in clinical practice.