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Objective— To investigate the mode of failure of transcondylar screws retrieved from dogs previously treated for incomplete ossification of the humeral condyle (IOHC).

Study Design— Metallurgical study.

Sample Population— Broken screws (n=5) from dogs with IOHC.

Methods— Broken transcondylar bone screws were retrieved from dogs that had apparently successful surgery for IOHC. Screws were measured and examined using scanning electron microscopy (SEM) and energy-dispersive analysis by X-rays (EDX).

Results— EDX confirmed that screws were made from 316L stainless steel and there was no evidence of significant corrosion or manufacturing faults that might have contributed to implant failure. SEM of the fractured surface of screws revealed a similar and complex pattern of fatigue failure characterized by multidirectional fatigue cracks and the presence of several initiation sites. All screws had signs of fatigue failure because of multidirectional forces of variable magnitude.

Conclusions— Intercondylar instability is a feature of IOHC that persists despite placement of a transcondylar screw.

Clinical Relevance— The pathogenesis of IOHC involves chronic intercondylar instability and its treatment using even relatively large transcondylar screws has a risk of late implant failure.