Background: Hinge fractures are considered risk factors for delayed or nonunion of the osteotomy gap in distal femoral osteotomies (DFOs). Limited evidence exists regarding the treatment of hinge fractures after DFO, which could improve stability and thus bone healing.
Purpose: To (1) examine the effect of hinge fractures on the biomechanical properties of the bone-implant construct, (2) evaluate the biomechanical advantages of an additional fixation of a hinge fracture, and (3) test the biomechanical properties of different types of varisation DFOs.
Study design: Controlled laboratory study.
Methods: A total of 32 fresh-frozen human distal femora equally underwent medial closing wedge DFO or lateral opening wedge DFO using a unilateral locking compression plate. The following conditions were serially tested: (1) preserved hinge; (2) hinge fracture along the osteotomy plane; (3) screw fixation of the hinge fracture; and (4) locking T-plate fixation of the hinge fracture. Using a servo-hydraulic materials testing machine, we subjected each construct to 15 cycles of axial compression (400 N; 20 N/s) and internal and external rotational loads (10 N·m; 0.5 N·m/s) to evaluate the stiffness. The axial and torsional hinge displacement was recorded using a 3-dimensional optical measuring system. Repeated-measures 1-way analysis of variance and post hoc Bonferroni correction were used for multiple comparisons. Statistical significance was set at P < .05.
Results: Independent from the type of osteotomy, a fractured hinge significantly (P < .001) increased rotational displacement and reduced stiffness of the bone-implant construct, resulting in ≥1.92 mm increased displacement and ≥70% reduced stiffness in each rotational direction, while the axial stiffness remained unchanged. For both procedures, neither a screw nor a plate could restore intact rotational stiffness (P < .01), while only the plate was able to restore intact rotational displacement. However, the plate always performed better compared with the screw, with significantly higher and lower values for stiffness (+38% to +53%; P < .05) and displacement (-55% to -72%; P < .01), respectively, in ≥1 rotational direction. At the same time, the type of osteotomy did not significantly affect axial and torsional stability.
Conclusion: Hinge fractures after medial closing wedge DFO and lateral opening wedge DFO caused decreased bone-implant construct rotational stiffness and increased fracture-site displacement. In contrast, the axial stiffness remained unchanged in the cadaveric model.
Clinical relevance: When considering an osteosynthesis of a hinge fracture in a DFO, an additional plate fixation was the construct with the highest stiffness and least displacement, which could restore intact hinge rotational displacement.
Keywords: biomechanics; hinge fixation; hinge fracture; torsional instability; varisation distal femur osteotomy.