Fri., 10/17/08 Pelvis/Injury Prevention, Paper #28, 11:39 am OTA-2008
The Effect of Malreduction of the Sacroiliac Joint on the Stability of Pelvic Internal Fixation in Ligamentous APC3 Injuries
Jodi Siegel, MD1 (n); Paul Tornetta, III, MD1 (c-Lippincott; a,c,e-Smith + Nephew);
Marc C. Jacofsky, MD, PhD2,3 (a-Stryker);
David Jacofsky, MD2 (a-Biomet; a,c-Smith + Nephew, Stryker; a,c,e-Stryker);
John McCamley3 (a-Stryker); Andrew Jaczynski3 (a-Stryker); M. Wade Shrader, MD2 (a-Stryker);
1Boston University Medical Center, Boston, Massachusetts, USA;
2The CORE Institute, Sun City West, Arizona, USA;
3SHRI-CORE Orthopedic Research Labs, Sun City, Arizona, USA
Introduction: Type 3 sacroiliac (SI) joint injuries require reduction and fixation to obtain the best possible results. When combined with symphyseal dislocation, more surgeons have adopted a strategy of percutaneous fixation of the SI joint after anterior ring open reduction and internal fixation. To avoid the morbidity of opening the SI joint, some displacement may be accepted. However, the effect on the stiffness and strength of the posterior ring fixation with a nonanatomic reduction is unknown. The purpose of this study is to evaluate how an imperfect reduction of the SI joint affects the stiffness and ultimate yield strength of lag screw fixation of the SI joint.
Methods: Young and Burgess anteroposterior compression type 3 (APC3) pelvic injuries were created in 12 unembalmed cadaveric pelves (6 male and 6 female, age 83 ± 7 years) by sectioning the pubic symphysis and the anterior and posterior sacroiliac ligaments. The symphysis was then anatomically reduced and stabilized with a 3.5-mm reconstruction plate. The stiffness and ultimate yield of the posterior ring was tested with the SI joint anatomically reduced, and with it malpositioned (combined 10 mm posterior and 5 mm superior). The posterior ring was fixed with one 8.0-mm partially threaded screw low and anterior in S1, and one 6.5-mm partially threaded screw posterior and superior to the first, both in compression. A “dancing” pelvis model was created by potting the sacrum, applying loads through the femurs in a simulated standing orientation. The stiffness of both constructs was tested sequentially on each pelvis in alternating order to increase the power of the evaluation. Half of the constructs were tested in the reduced position first and the other half in the malaligned position first, with the same pathway and depth of the sacral screws using different starting points on the ilium. Each construct was preloaded at 200 N and cycled between 200 and 250 N at 1 Hz for 2 minutes. The construct was then loaded and unloaded 10 times at 1 Hz to generate stiffness and rebound curves. The load used was gender-specific and equivalent to half of the average body weight of the donors for each gender. The final construct for each pelvis was loaded to clinical (>3 mm) and catastrophic (loss of continuity) failure at a rate of 300 N/min. The position of the ilium with respect to the sacrum was monitored with an Optotrak marker system recording at 50 Hz. Dual-energy x-ray absorptiometry scans were performed on each specimen.
Results: Analysis of variance testing with repeated measures demonstrated no interaction for the order of testing, validating the sequential stiffness measurement technique. The malaligned SI joints demonstrated more displacement during preconditioning (0.13 mm vs 0.05 mm, P = 0.02) and lower stiffness in the load/displacement cyclic evaluation (695 N/mm vs 1881 N/mm, P = 0.03). The stiffness in the load-to-failure testing also favored the reduced SI joints (421 N/mm vs 156 N/mm, P = 0.03). The ultimate yield point for the reduced SI joints was 1415 N vs 912 N for the unreduced specimens (P = 0.08). Additionally, the displacement that occurred prior to catastrophic failure was greater in the unreduced than the reduced SI joints (>5 mm vs 3.15 mm, P = 0.01). The load to failure had a strong negative correlation with prefailure displacement for all specimens (r = –0.73, P = 0.006). Finally, the bone mineral density (BMD) of the specimens correlated with load to failure in the reduced speciments (r = 0.76, P = 0.04), but not in the unreduced specimens (r = –0.09).
Discussion: These data taken together reveal that more frictional stability can be generated by lag screw compression in a reduced SI joint than in an unreduced joint. While the absolute difference in displacements that occurred in the cyclic loading are small (0.41 mm vs 0.14 mm) and clinically insignificant, the load displacement data clearly demonstrate an advantage in the ability of the reduced joint to resist motion that is significant, even at one half body weight. This trend is further elucidated when the ultimate yield point and the displacement prior to catastrophic failure are taken together. The higher the load to failure, the smaller was the displacement prior to failure. These findings indicate that the fully reduced specimens are better able to resist displacement and failed at higher loads. Finally, the strong correlation of BMD with load to failure only in the reduced specimens supports the supposition that greater friction is generated in the reduced specimens.
Conclusion: Many factors may play a role in the ultimate strength of pelvic fixation, but in a well-controlled study, anatomic reduction of the SI joint allows for greater resistance to subsidence and increases the ultimate yield strength.
If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
• The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an “off label” use). ◆FDA information not available at time of printing. Δ OTA Grant.