Biomechanical effects of malposition of tuberosity fragments on the humeral prosthetic reconstruction for four-part proximal humerus fractures

Variable outcomes in the prosthetic reconstruction of 4-part humerus fractu res often can be attributed to inconsistent and nonanatomic tuberosity plac ement. To compare the effects of anatomic (anterior fin) versus nonanatomic (lateral fin) tuberosity placement, we developed a dynamic cadaver model f or shoulder motion. With the use of a robotically driven, computer-controll ed articulator, we tested external rotation torque in 5 fresh human shoulde rs. After evaluation of the intact shoulders, we experimentally induced 4-p art humerus fractures in the specimens. These were then repaired by hemiart hroplasty, with the use of standard techniques to secure the greater and le sser tuberosities in either anatomic or nonanatomic positions; order was ra ndomized. Nonanatomic tuberosity reconstruction led to significant impairme nt in external rotation kinematics and an 8-fold increase in torque require ments (P = .001). In contrast, anatomic reconstruction produced results ind istinguishable from normal shoulder controls. This study underscores the im portance of rotational alignment of post-tuberosities during reconstruction . Failure to properly position tuberosity fragments in the horizontal plane may result in insurmountable postoperative motion restriction.