Microdamage at the pin-bone interface of external skeletal fixation pins after short-term cyclical loading ex-vivo

Abstract

SummaryThis study determined microdamage associated with external fixation pin insertion and short-term cyclical loading, using an ex vivo ovine tibial model. Orthofix tapered blunt-tipped 3.5/4.5 mm fixation pins and Apex® self-drilling self-tapping fixation pins, 4 mm in diameter, were used. After insertion, the constructs were either loaded in cantilever bending or not loaded. Constructs were then bulk-stained in basic fuchsin, and calcified sections were made. The sections were reviewed qualitatively and the microcrack surface density (Cr.S.Dn, μm/mm2) was quantified at the pin-bone interface. The pattern and quantity of microdamage induced was significantly influenced by fixation pin design and cortical region within the cisor transcortex, but not short-term cyclical loading. Overall, Cr.S.Dn was significantly increased with use of the Orthofix fixation pin (P < 0.01). Cr.S.Dn was also increased in the medial cis-cortex, compared with the lateral transcortex (P < 0.05). Diffuse damage within the medial cis-cortex was higher with the Orthofix pin. In contrast, the Howmedica Apex® fixation pin caused fracture of the periosteal region of the lateral transcortex, but relatively little microdamage within the medial cis-cortex. In this model as a consequence of pin insertion, fixation pin design had significant specific damage effects on both the medial cis-cortex and the lateral trans-cortex