Does the Proximal Humerus Nail with 2 Distal Screws Provide Sufficient Rotational Stability?

Backgroud: Proximal humerus nails, frequently used for managing proximal humerus fractures, significantly enhance rotational stability and reinforce fractured fragments. Few research exists regarding the optimal number and positioning of distal screws. This study aimed to assess the stability of diverse screw configurations and scrutinize screw distribution and bone stress via finite element analysis.

Methods: The humerus intramedullary nail (Humerus Interlocking Nail System; TDM) underwent assessment using finite element analysis applied to a humerus model. Three groups were established based on varying distal screw numbers and locations: all 3 distal locking holes were used in group 1; 2 screws (dynamic hole and proximal static hole) in group 2, and 2 screws (dynamic hole and distal static hole) in group 3. Finite element analysis computed stress distribution within the implant and bone for each group. A 1-mm fracture gap was simulated at the surgical neck, and stress distributions were analyzed in both normal and osteoporotic bone models.

Results: Using two screws did not compromise rotational stability. Stress distribution analysis revealed stability across all groups without reaching failure strength. Group 3 exhibited a minor rise in component 11 (direct stress [force per unit area] acting on the positive and negative 1 faces in the 1-axis. direction) and component 22 (direct stress [force per unit area] acting on the positive and negative 2 faces in the 2-axis direction) stress, remaining below failure strength thresholds. Group 1 exhibited the lowest von Mises stress in the nail and screws, while groups 2 and 3 did not reach failure strength levels. Findings remained consistent in the osteoporotic model.

Conclusions: All 3 groups demonstrated rotational stability concerning stress distribution, indicating that using 2 screws for distal fixation does not adversely affect stability. This suggests the potential for saving surgical time and reducing radiation exposure without compromising stability.