Does Iliac Crest Bone Graft Harvest Affect Iliac Screw Fixation-An In-Vitro Biomechanical Study?

• b, e - SpineVision, Inc.

Purpose: The current in-vitro biomechanical study was undertaken to determine if iliac crest bone graft (ICBG) harvest affects the fixation strength of iliac screws. Specifically, is there a critical bridge of bone graft that must remain adjacent to an iliac screw? Is screw tip penetration or the presence of a bone bridge more important in maintaining fixation?

Methods: Forty ilia were obtained from twenty fresh frozen human cadavers. Bone mineral densities (BMD) were obtained from the area deemed to be the most likely paths for iliac screws in each ilium (Figure 1). Iliac screws (7.5mm dia. x 70 mm) were implanted using standard technique, starting at the PSIS and aiming for the AIIS. The left hemipelvis specimens served as controls (n=20), and the right as experimental, which were randomized into four bone graft groups with n=5 per group: 1) 0x5 = no bone bridge adjacent to screw and 5cm depth (2 cm of screw tip remaining buried); 2) 1x5 = 1 cm bone bridge and 5 cm depth; 3) 1x7 = 1 cm bone bridge and 7 cm depth; 4) 2x7 = 2 cm bone bridge and 7 cm depth. Pullout to failure was performed at 2 mm/sec and a 22° angle off sagittal and transverse axes in order to simulate a flexion moment centered at the L5-S1 disc. Peak failure loads (Newtons) were normalized to the contralateral controls and compared using Tukey’s Test.

Results: BMD was consistently greatest in the suprasciatic region of interest and correlated with peak failure loads based on linear regression analysis (r=.85, p=0.000). Load to failure data represented as percent of the controls is as follows: 1) 0x5 Group = 55.2±24.0% of the control, 2) 1x5 Group was 102±37.0%, 3) 1x7 Group = 107±40.4 and 4) 2x7 Group = 89.7±28.4%. The 0x5 Group indicated significantly less fixation than 1x5 and control Groups (p<0 .05). No other comparisons were significant. Iliac screw failure patterns matched that observed in the clinical setting with fracture propagating through ilium a superior and lateral direction.

Discussion: DEXA revealed that the suprasciatic region has the highest BMD in the pelvis and remains a target for iliac screw fixation. By testing screws 22° off axis of the screw in the axial and sagittal planes, failure was reproducibly similar to clinical findings. Removal of bone directly adjacent to the iliac screw affected fixation more than the depth of the ICBG, and statistically reduced pull-out strength to 55% of the control value for the 0x5 group. All other ICBG groups demonstrated no significant differences from intact control specimens. The current study supports the following: 1) Greater than 1 cm bridge of bone should remain adjacent to the iliac screw, 2) Iliac screws should be directed into or through the suprasciatic bone and 3) The depth of the ICBG harvest does not appear to affect screw fixation as much as the proximity of the ICBG to the screw.

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