BACKGROUND To compare the biomechanical properties and stability, using a finite element model, of four fixation constructs used for the treatment of anterior column and posterior hemi-transverse (ACPHT) acetabular fractures under two physiological loading conditions (standing and sitting). METHODS A finite element model simulating ACPHT acetabular fractures was created for four different scenarios: a suprapectineal plate combined with posterior column and infra-acetabular screws (SP-PS-IS); an infrapectineal plate combined with posterior column and infra-acetabular screws (IP-PS-IS); a special infrapectineal quadrilateral surface buttress plate (IQP); and a suprapectineal plate combined with a posterior column plate (SP-PP). Three-dimensional finite element stress analysis was performed on these models with a load of 700 N in standing and sitting positions. Biomechanical stress distributions and fracture displacements were analysed and compared between these fixation techniques. RESULTS In models simulating the standing position, high displacements and stress distributions were observed at the infra-acetabulum regions. The degree of these fracture displacements was low in the IQP (0.078 mm), as compared to either the IP-PS-IS (0.079 mm) or the SP & PP (0.413 mm) fixation constructs. However, the IP-PS-IS fixation construct had the highest effective stiffness. In models simulating the sitting position, high fracture displacements and stress distributions were observed at the regions of the anterior and posterior columns. The degree of these fracture displacements was low in the SP-PS-IS (0.101 mm), as compared to the IP-PS-IS (0.109 mm) and the SP-PP (0.196 mm) fixation constructs. CONCLUSIONS In both standing and sitting positions, the stability and stiffness index were comparable between the IQP, SP-PS-IS, and IP-PS-IS. These 3 fixation constructs had smaller fracture displacements than the SP-PP construct. The stress concentrations at the regions of quadrilateral surface and infra-acetabulum suggest that the buttressing fixation of quadrilateral plate was required for ACPHT fractures.