The effects of ulnar axial malalignment on supination and pronation

Background: Forearm fractures are common injuries in both adults and childr en. Despite efforts to obtain anatomical alignment, axial rotational maluni ons occur, resulting in a decreased range of motion and a poor appearance. The objective of this study was to quantify loss of forearm rotation after simulation of ulnar malunions in supination and pronation. Methods: Six fresh-frozen cadaveric upper extremities (mean age at the time of death, 79.4 +/- 2.8 years) were used to quantify loss of forearm rotati on after simulation of axial rotational malunions of the ulna. First, maxim um forearm rotation in supination and pronation was measured at torques of 6.8, 13.6, and 20.4 kilograms-centimeter applied with use of a custom jig. Following a midshaft ulnar osteotomy, a custom adjustable internal fixation plate was used to simulate axial rotational malunions of the ulna of 0, 15 , 30, and 45 degrees in both directions. Measurements in supination and pro nation were then repeated at the prespecified torques. Analysis of variance , with a p value of 0.05, was used for statistical analysis. Results: In all instances, a decrease in forearm rotation after simulation of the ulnar rotational malunion was accompanied by an increase in rotation in the opposite direction. Supination and pronation were significantly inf luenced, whereas the total are of rotation was not affected by ulnar rotati onal malunion. At a torque of 20.4 kilograms-centimeter, pronation malunion s of 15, 30, and 45 degrees resulted in a mean loss of supination (and stan dard error of the mean) of 5 +/- 1, 11 +/- 1, and 20 +/- I degrees, respect ively; and supination malunions of 15, 30, and 45 degrees resulted in a mea n loss of pronation of 4 +/- 1, 10 +/- 2, and 18 +/- 4 degrees, respectivel y. The ratio of the simulated rotational malunion to the loss of motion was larger than one. Conclusions: Ulnar rotational malunions do not lead to a significant change in the total are of forearm rotation. Instead, loss of motion in one direc tion is accompanied by increased motion in the opposite direction. Even wit h a 45-degree ulnar rotational malunion, forearm rotation decreases no more than 20 degrees. Clinical Relevance: Ulnar rotational malunions have less effect on forearm rotation than that reported after radial malunions. This may be a considera tion when treating forearm fractures or correcting rotational malunion of t he forearm.