Background: Successful internal fixation of fractures of the
surgical neck of the humerus can be difficult to achieve because of osteopenia
of the proximal aspect of the humerus. The purpose of this study was to
compare the biomechanical stability of a proximal humeral intramedullary nail
and a locking plate for the treatment of a comminuted two-part fracture of the
surgical neck in a human cadaver model.
Methods: Twenty-four cadaveric humeri were instrumented with use of
either a titanium proximal humeral nail (PHN) or a 3.5-mm locking compression
plate for the proximal part of the humerus (LCP-PH). The specimens were
matched by bone mineral density and were separated into four experimental
groups with six humeri in each: PHN bending, LCP-PH bending, PHN torsion, or
LCP-PH torsion. Comminuted fractures of the surgical neck were simulated by
excising a 10-mm wedge of bone. Bending specimens were cyclically loaded from
0 to 7.5 Nm of varus bending moment at the fracture site. Torsion specimens
were cyclically loaded to ±2 Nm of axial torque. The mean and maximum
displacement in bending, mean and maximum angular rotation in torsion, and
stiffness of the bone-implant constructs were compared.
Results: In bending, the LCP-PH group demonstrated significantly
less mean displacement of the distal fragment than did the PHN group over 5000
cycles (p = 0.002). In torsion, the LCP-PH group demonstrated significantly
less mean angular rotation than did the PHN group over 5000 cycles (p = 0.04).
A significant number of specimens in the PHN group failed prior to reaching
5000 cycles (p = 0.04). The LCP-PH implant created a significantly stiffer
bone-implant construct than did the PHN implant (p = 0.007).
Conclusions: The LCP proximal humeral plate demonstrated superior
biomechanical characteristics compared with the proximal humeral nail when
tested cyclically in both cantilevered varus bending and torsion. The rate of
early failure of the proximal humeral nail could reflect the high moment
transmitted to the locking proximal screw-bone interface in this implant.
Clinical Relevance: The high failure rate in torsion of the proximal
humeral nail-bone construct is concerning, and, with relatively osteoporotic
bone and early motion, the results could be poor.