Biomechanical analysis of pin placement for pediatric supracondylar humerus fractures:  does starting point, pin size, and number matter?
BACKGROUND
Several studies have examined the biomechanical stability of smooth wire fixation constructs used to stabilize pediatric supracondylar humerus fractures. An analysis of varying pin size, number, and lateral starting points has not been performed previously.


METHODS
Twenty synthetic humeri were sectioned in the midolecranon fossa to simulate a supracondylar humerus fracture. Specimens were all anatomically reduced and pinned with a lateral-entry configuration. There were 2 main groups based on specific lateral-entry starting point (direct lateral vs. capitellar). Within these groups pin size (1.6 vs. 2.0 mm) and number of pins (2 vs. 3) were varied and the specimens biomechanically tested. Each construct was tested in extension, varus, valgus, internal, and external rotation. Data for fragment stiffness (N/mm or N mm/degree) were analyzed with a multivariate analysis of variance and Bonferroni post hoc analysis (P<0.05).


RESULTS
The capitellar starting point provided for increased stiffness in internal and external rotation compared with a direct lateral starting point (P<0.05). Two 2.0-mm pins were statistically superior to two 1.6-mm pins in internal and external rotation. There was no significant difference found comparing two versus three 1.6-mm pins.


CONCLUSIONS
The best torsional resistances were found in the capitellar starting group along with increased pin diameter. The capitellar starting point enables the surgeon to engage sufficient bone of the distal fragment and maximizes pin separation at the fracture site. In our anatomically reduced fracture model, the addition of a third pin provided no biomechanical advantage.


CONCLUSIONS
Consider a capitellar starting point for the more distally placed pin in supracondylar humerus fractures, and if the patient's size allows, a larger pin construct will provide improved stiffness with regard to rotational stresses.
Department of Pediatric Orthopaedic Surgery, Rady Children's Hospital San Diego, San Diego, CA, USA.
Hilton Phillip
HP
Gottschalk
Daljeet
D
Sagoo
Diana
D
Glaser
Josh
J
Doan
Eric W
EW
Edmonds
John
J
Schlechter
D001696:Biomechanical Phenomena; D001858:Bone Nails; D002648:Child; D005592:Fracture Fixation / Q000379:methods*; D006801:Humans; D006810:Humeral Fractures / Q000601:surgery*; D008953:Models, Anatomic; D015999:Multivariate Analysis; D012189:Retrospective Studies; D012399:Rotation