Fine Morphological Assessment of Stripped Screw Sites in Cortical Bone

Summary

Unintentional over-tightening of orthopaedic screws resulting in loss of screw purchase (“screw stripping”) is a potential complication during fracture fixation. This report describes the microstructural and radiographic effects on bone caused by stripping a 2.7 mm orthopaedic screw, and subsequent replacement in these stripped screw sites with either a 3.5 mm cortical or a 4.0 mm cancellous orthopaedic screws in both tapped and untapped holes. Over-tightening of a 2.7 mm screw resulted in shearing of the bone directly engaged by the screw threads. Additional fractures occurred in surrounding bone, primarily superficial subperiosteal saucer fractures in the cis cortex and to a lesser degree, in the subendosteum of the trans cortex. Bone damage was consistently more severe in the cis cortex. Salvaging stripped screw sites by insertion of a 3.5 mm or 4.0 mm screw resulted in additional fracturing and displacement, which was more severe in the untapped sites. Decreased holding power of screws placed in stripped screw sites could be attributed to collateral structural bone damage occurring during screw stripping and replacement screw application.

Over-tightening of orthopaedic screws until mechanical failure (“screw stripping”) results in damage to cortical bone which extends beyond the outer thread diameter of the screw. Damage, most often subperiosteal saucer fractures, was greater in the cis cortex. The subperiosteal saucer fractures decreased the functional length of screw engagement of the larger replacement screws in the cortical bone, which may account for decreases in expected holding power.

Keywords

Canine - screws - screw stripping - fracture repair

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