Fine morphological assessment of stripped screw sites in cortical bone

Unintentional over-tightening of orthopaedic screws resulting in loss of sc rew purchase ("screw stripping") is a potential complication during fractur e fixation. This report describes the microstructural and radiographic effe cts 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. O ver-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 corte x and to a lesser degree, in the subendosteum of the trans cortex. Bone dam age was consistently more severe in the cis cortex. Salvaging stripped scre w sites by insertion of a 3.5 mm or 4.0 mm screw resulted in additional fra cturing and displacement, which was more severe in the untapped sites. Decr eased holding power of screws placed in stripped screw sites could be attri buted to collateral structural bone damage occurring during screw stripping and replacement screw application.