gms | German Medical Science

Objectives: Comprising two to four percent of all carpal fractures, hamate hook fractures are rare injuries. Besides the common injury mechanism (direct impact to the hook) another theory has recently attracted attention. High load during different sports like gymnastics, weightlifting or climbing (wrist in ulnar deviation) is supposed to be transmitted to the hook of the hamate by tightened flexor tendons creating a high contact pressure to the ulnar margin of the carpal tunnel. Injuries of the hamate hook, caused by other than external impact but by contact pressure of the anatomical structures seem to be rare and have rarely been reported before. We now aimed to further investigate the pressure load on the hamate hook by a biomechanical cadaver study.

Methods: Eighteen fresh and unfixed forearms of body donators were used for the study after ethical board approval. After anatomical dissection of the palm and forearm, the specimen were placed with an ulnar deviated wrist in a self-designed testing device to analyze contact pressure of flexor tendons on the hamate hook. Finger flexion was then simulated by an engine pulling on the flexor tendons of the Dig. IV/V and forces were recorded by sensors. For identification of potential hamate hook fractures, all hands were then scanned by CT. Real time MRI sequences were used to visualize tendon sliding of the flexor tendons (Dig.IV/V) under high load as well as after hamate hook fracture or surgical resection, which is a commonly used therapeutical procedure.

Results and Conclusion: The self-designed testing device is suitable to precisely analyze forces of the forearm and wrist/hand. None of the tested hamate hooks showed a fracture after singular contact pressure strain. The average pulling force was 448(182-920) Newton (D.IV flexor tendon) and 345(185-486) Newton (D.V flexor tendon) and stop criteria were tendon rupture, bony avulsion or articular dislocation of the fingers. Real time MRI images revealed a clear change in tendon sliding behavior (lateralization of the tendons) after hamate hook fracture or surgical resection leading to a tendon dislocation.

Hamate hook fracture did not occur after singular max. tendon contact pressure in hands of body donators tested in this study. It must be assumed that fractures of the hook caused by high contact pressure rather result from repetitive pressure load than from singular strain. Tendon sliding seems to be significantly changed after complete fracture of the hamate hook or surgical resection and needs to be considered in clinical situations.