Osteotomy obliquity, contact area and working length in ulnar shortening osteotomy: a finite element analysis with a fixed implant

Abstract Background Oblique ulnar shortening osteotomy (USO) is widely preferred over a transverse cut, and its clinical performance has been attributed to a larger contact surface at the osteotomy. In every published comparison, however, osteotomy geometry is confounded with the fixation system, and the proposed mechanism has not been tested biomechanically. Obliquity also lengthens the axial span of the cut, forcing the innermost screws apart and increasing screw working length. Methods A three-dimensional model of the distal ulna was built from a publicly available, anonymised CT-derived surface. A 3 mm shortening was simulated at obliquities of 40, 45, 60 and 90 degrees and fixed with an identical generic six-hole 3.5 mm volar locking plate. In Arm 1 screw positions were identical across obliquities, holding working length at 24.0 mm; in Arm 2 the construct followed clinical practice, with working length varying from 20.0 to 34.5 mm. Each configuration was loaded in axial compression (100 N), flexion and extension bending and torsion (1 N·m). All obliquities were additionally analysed with a 300 N interfragmentary lag screw preload under axial load and torsion, giving 40 nonlinear frictional-contact analyses in total. Results Without a lag screw, axial interfragmentary shear motion fell from 23.3 µm at 40 degrees to 21.6 µm at 45, 12.0 µm at 60 and 0.01 µm at 90 degrees, while the surface loaded in compression rose from 16% to 70%; despite the larger geometric area of an oblique cut, the absolute loaded area was therefore smaller. Torsion reversed this ordering, the 45 degree construct permitting 34% less shear motion than the transverse construct. At matched obliquity, the clinically scaled construct increased axial shear motion by 80% at 40 degrees and 48% at 45 degrees, and at angles of 45 degrees or less only two distal screws could be accommodated in the distal fragment. Addition of the lag screw preload reduced axial shear motion by 90 to 95% in the oblique constructs and raised the loaded contact area at 45 degrees to 87%, exceeding the transverse construct (80%). Conclusions The mechanical behaviour of an oblique ulnar shortening osteotomy depended strongly on whether interfragmentary compression was present. Without a lag screw the oblique construct permitted substantially greater interfragmentary shear under axial load and engaged less than a third as much loaded contact area than a transverse cut; with a lag screw the disadvantage was largely abolished and the loaded contact area exceeded that of the transverse construct. The advantage of obliquity therefore appears to derive largely from the compression its geometry permits rather than from the geometry itself. Obliquity additionally imposes a longer working length and, at angles of 45 degrees or less, limits the distal fragment to two screws. Where reliable interfragmentary compression cannot be achieved, an oblique osteotomy cannot be assumed to retain the mechanical advantages usually attributed to it; where it can, the oblique configuration retains a torsional advantage that a transverse cut does not possess.