Two-part surgical neck fractures of the humerus: Mechanical analysis of the fixation with four Shanz-type threaded pins in four different assemblies

The configuration of percutaneous pinning may affect the stability of the fixation of 2-part surgical neck fractures of the humerus. In this investigation, the mechanical properties of four different pin configurations were analyzed by use of 36 fresh-frozen swine femora as an experimental model. The bones were selected according to previously established morphologic parameters. A transverse osteotomy was made in the proximal metaphyseal region of the bone, and the pins were inserted according to the selected configuration (group 1, four ascending parallel pins; group 2, three ascending parallel pins and one descending intersecting pin; group 3, two ascending parallel pins and two descending parallel intersecting pins; and group 4, two ascending parallel pins and two descending converging pins, on different planes [two by two]). The assemblies were subjected to an axial load of up to 1500 N in a universal testing machine, simulating a varus stress. The parameters studied were the maximum load supported and corresponding deformation, load and deformation at the limit of proportionality, rigidity, and resilience. The results showed that all configurations were sufficiently strong, but the maximum load and load at the limit of proportionality were significantly higher (P < .05) in groups 3 and 4 (1314 and 1377 N and 950 and 923 N, respectively) than in groups 1 and 2 (1104 and 1033 N and 738 and 777 N, respectively). Rigidity was significantly higher in group 3 than in groups 1 and 2 but not in group 4. Resilience, maximum deformation, and deformation at the limit of proportionality were not significantly different between groups. We conclude that the experimental model used was reliable for the purpose of the study and that any of the configurations studied would be adequate for the percutaneous fixation of 2-part surgical neck fractures of the humerus but group 3 configuration would be the most efficient.