Forty urethane foam hemi-pelvises (Pacific Research Laboratories, Vashon, Washington), each with a well defined cortical outer shell and cancellous inner matrix were randomly divided into four groups of ten. Urethane foam hemi pelvises were chosen to control for the variability in cadaveric specimens as well as for the large number of specimens needed based on our power analysis. An identical transtectal osteotomy using a hand held saw was performed on each of the specimens (Fig. ). The osteotomy began at the mid portion of the greater sciatic notch and traveled across the posterior column, through the roof of the acetabulum, exiting through the anterior column at the level of the iliopectineal eminence.
Figure 1 (A) A urethane foam pelvis used in this study is shown with a line demonstrating the location of the simulated transverse acetabular fracture. (B) A 10 hole 3.5 mm anterior column reconstruction plate with three bicortical screws on either side of the (more ...)
The osteotomy was reduced anatomically and fixed in one of four ways: 1) a 10 hole 3.5 mm anterior column reconstruction plate with three bicortical screws on either side of the osteotomy (ACP), 2) a 10 hole 3.5 mm anterior column locking reconstruction plate with three unicortical screws on either side of the osteotomy (LOCK), 3) a 10 hole 3.5 mm anterior column reconstruction plate with three bicortical screws on either side of the osteotomy and a 4.5 mm/120 mm posterior column lag screw (ACPLS), and finally 4) a 6 hole 3.5 mm posterior column reconstruction plate with three bicortical screws on either side of the osteotomy and a 4.5 mm/120 mm anterior column lag screw (PCPLS) (Fig ).
Each specimen was stabilized in a customized jig (Fig ). A PMMA mold stabilized the superior osteotomy fragment (i.e. the intact ilium) and was bolted to the testing table for stability. To enforce an anatomic boundary condition at the pubis, the pubic symphysis rested on a block of wood that was cut at an angle that matched the anatomical mid-sagittal plane. Thus, the only constraint to motion of the inferior portion of the pelvis was that the pubic symphysis portion of the hemipelvis could not cross the mid-sagittal plane of the body. It was otherwise free to translate and rotate in all other directions.
Figure 2 (A) The testing apparatus consists of a bipolar hemiarthroplasty attached to a servohydraulic materials testing machine (858 Mini Bionix, MTS, Eden Prairie, MN). The customized jig was oriented to allow femoral head loading to be oriented 45 degrees superomedially (more ...)
A bipolar hemiarthroplasty was attached to a servohydraulic material testing machine (858 Mini Bionix, MTS, Eden Prairie, MN) in order to load the construct. The customized jig was oriented to allow femoral head loading to be oriented 45 degrees superomedially (coronal plane) and 25 degrees posteriorly (sagittal plane) [1
Prior to specimen loading four markers were attached to each osteotomized urethane foam pelvis to allow measurement of the relative motion across the osteotomy (Fig. ). These markers were placed along the posterior column of the pelvis adjacent to the osteotomy gap. The markers were placed 5 mm from the gap on each side of the fracture line and were placed 2 cm apart. Two opposing markers (numbers 1 and 2) were in a more anterior position along the osteotomy line, while the other two opposing markers (numbers 3 and 4) were in a more posterior position. A photograph was taken with a digital camera (Coolpix 8700; Nikon) attached to a tripod in the unloaded state with a ruler in the field of view to allow for subsequent calibration. Specimens were then loaded at 0.2 mm/sec to 1000N and another photograph was taken. Lastly, the specimens were loaded up to 2000N while piston displacement and load were acquired and then a final photograph was taken. To avoid the effects of the toe region, stiffness of the construct was calculated between 1000 and 2000 N, where the load-displacement curve was most linear.
Marker positions from the three images were analyzed using ImageJ (http://rsb.info.nih.gov/ij/
; NIH, Bethesda, MD). The four markers were used to define how the gap opened at 2000N relative to 0N for the four plated constructs. Displacements at two points along the fracture line were defined. The anterior displacement was defined as the movement of pin 1 relative to pin 2, and the posterior displacement was defined as the movement of pin 3 relative to pin 4.
To represent overall motion of the fracture fragments at the fracture site, the average location of each pin in space for each fixation scheme was graphed. Visualizing the displacements in this way allows one to understand how the fracture displaced under load, either perpendicular to the fracture line and creating a gap, or parallel to the fracture line and generating shear.
Differences in stiffness of the various plating constructs were then analyzed using an ANOVA test. Assuming a stiffness standard deviation of 0.25 N/mm and a difference desired to detect of 0.5 N/mm, we calculated that 10 specimens per group would give a power of 0.9986. Standard deviation and mean values were based on a previous study [10