Response of the Equine Metacarpus to Conventional and Ion‐Beam Textured Lag Screws

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Abstract
Four pairs of ASIF MP35N stainless steel screws, one conventional smooth surfaced and one that had its surface modified by texturing with an ion‐beam, were inserted in the third metacarpus of six horses. The pairs of screws were placed from dorsal to palmar in the metaphyses and diaphysis of the bone using the lag‐screw principle. When the screws were examined at 90 and 150 days after insertion, micromovement was detected histologically but not radiographically around the screw in the dorsal cortex but not the palmar cortex. The removal torque of all screws 90 days after insertion was reduced compared to the insertion torque, and no difference was found between the rough and smooth screws. At 150 days after insertion, the removal torque of the smooth screws remained low, but the removal torque of the rough screws was increased significantly (p < 0.001). The removal torque of both smooth and rough screws was significantly higher in the diaphysis than in the metaphysis (p < 0.02). Since the need for a screw's holding ability is greatest early after fracture repair, it is doubtful that ion‐beam textured screws have any advantage over smooth screws in the repair of metacarpal fractures. The response of the dorsal and palmar cortices indicated that the equine metacarpus undergoes bending during loading and that the dorsal and palmar cortices of the bone move relative to each other.

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