Assessment of the Breakaway Torque at the Posterior Pelvic Ring in Human Cadavers

Pelvic Ring Fractures: A Biomechanical Comparison of Sacral and Lumbopelvic Fixation Techniques

BACKGROUND CONTEXT

Pelvic ring fractures are becoming more common in the aging population and can prove to be fatal, having mortality rates between 10% and 16%. Stabilization of these fractures is challenging and often require immediate internal fixation. Therefore, it is necessary to have a biomechanical understanding of the different fixation techniques for pelvic ring fractures.

METHODS

A previously validated three-dimensional finite element model of the lumbar spine, pelvis, and femur was used for this study. A unilateral pelvic ring fracture was simulated by resecting the left side of the sacrum and pelvis. Five different fixation techniques were used to stabilize the fracture. A compressive follower load and pure moment was applied to compare different biomechanical parameters including range of motion (contralateral sacroiliac joint, L1-S1 segment, L5-S1 segment), and stresses (L5-S1 nucleus stresses, instrument stresses) between different fixation techniques.

RESULTS

Trans-iliac-trans-sacral screw fixation at S1 and S2 showed the highest stabilization for horizontal and vertical displacement at the sacral fracture site and reduction of contralateral sacroiliac joint for bending and flexion range of motion by 165% and 121%, respectively. DTSF (Double transiliac rod and screw fixation) model showed highest stabilization in horizontal displacement at the pubic rami fracture site, while the L5_PF_W_CC (L5-Ilium posterior screw fixation with cross connectors) and L5_PF_WO_CC (L5-Ilium posterior screw fixation without cross connectors) showed higher rod stresses, reduced L1-S1 (approximately 28%), and L5-S1 (approximately 90%) range of motion.

CONCLUSIONS

Longer sacral screw fixations were superior in stabilizing sacral and contralateral sacroiliac joint range of motion. Lumbopelvic fixations displayed a higher degree of stabilization in the horizontal displacement compared to vertical displacement of pubic rami fracture, while also indicating the highest rod stresses. When determining the surgical approach for pelvic ring fractures, patient-specific factors should be accounted for to weigh the advantages and disadvantages for each technique.

The ideal site of cement application in cement augmented sacroiliac screw fixation: the biomechanical perspective

Purpose

To compare construct stability of cement augmented sacroiliac screws using two different cementation sites in a biomechanical fragility fracture model of the pelvis.

Methods

A fracture model with an incomplete fracture of the sacral ala and complete fracture of the anterior pelvic ring mimicking a FFP IIB fragility fracture of the pelvis was established in five fresh frozen human cadaveric pelvises. Sacral fracture stabilization was achieved with bilateral 7.3 mm fully threaded sacroiliac screws. Cement augmentation was performed at the tip of the screw (body of S1; Group A) on one side, and at the midshaft of the screw (sacral ala; Group B) on the contralateral side. Biomechanical testing was conducted separately on both sides comprising cyclic loading of axial forces transferred through the tested hemipelvis from L5 to the ipsilateral acetabulum. Combined angular displacement in flexion and internal rotation (“gap angle”), angular displacement of the ilium in relation to the screw (“screw tilt ilium”), and screw tip cutout were evaluated.

Results

Relative interfragmentary movements were associated with significantly higher values in group A versus group B for “gap angle” (2.4° vs. 1.4°; p < 0.001), and for “screw tilt ilium” (3.3° vs. 1.4°; p < 0.001), respectively. No significant difference was indicated for screw tip cutout between the two groups (0.6 mm [Group A] vs. 0.8 mm [Group B]; p = 0.376).

Conclusion

The present study demonstrated less fragment and screw displacements in a FFP IIB fracture model under physiologic cyclic loading by cement augmentation of sacroiliac screws at the level of the lateral mass compared to the center of vertebral body of S1.

Trans-sacral screw fixation of posterior pelvic ring injuries: review and expert opinion

Posterior pelvic ring injuries (i.e., sacro-iliac joint dislocations, fracture-dislocations, sacral fractures, pelvic non-unions/malunions) are challenging injury patterns which require a significant level of surgical training and technical expertise. The modality of surgical management depends on the specific injury patterns, including the specific bony fracture pattern, ilio-sacral joint involvement, and the soft tissue injury pattern. The workhorse for posterior pelvic ring stabilization has been cannulated iliosacral screws, however, trans-sacral screws may impart increased fixation strength. Depending on injury pattern and sacral anatomy, trans-sacral screws can potentially be more beneficial than iliosacral screws. In this article, the authors will briefly review pelvic mechanics and discuss their rationale for ilio-sacral and/or trans-sacral screw fixation.

Bone cement augmentation of femoral nail head elements increases their cut-out resistance in poor bone quality- A biomechanical study

The aim of this study was to analyze biomechanically the impact of bone cement augmentation on the fixation strength and cut-out resistance of Proximal Femoral Nail Antirotation (PFNA) and Trochanteric Fixation Nail Advanced (TFNA) head elements within the femoral head in a human cadaveric model with poor bone quality. Methodology: Fifteen pairs of fresh-frozen human cadaveric femoral heads were randomized to three sets of five pairs each for center-center implantation of either TFNA blade, TFNA screw, or PFNA blade. By splitting the specimens of each pair for treatment with or without bone cement augmentation, six study groups were created. All specimens were biomechanically tested under progressively increasing cyclic loading featuring a physiologic loading trajectory in a setup simulating a reduced intertrochanteric fracture with lack of posteromedial support. Number of cycles to 5° varus collapse was evaluated together with the corresponding load at failure. Results: Compared to the non-augmented state, all types of implants demonstrated significantly higher numbers of cycles to failure and load at failure following augmentation, p ≤ 0.03. Augmented TFNA blades resulted in highest numbers of cycles to failure and loads at failure (30492; 4049 N) followed by augmented PFNA blades (30033; 4003 N) and augmented TFNA screws (19307; 2930 N), p = 0.11. Augmented TFNA screws showed similar numbers of cycles to failure and loads at failure compared to both non-augmented TFNA and PFNA blades, P = 0.98. From a biomechanical perspective, bone cement augmentation significantly increases the cut-out resistance of instrumented TFNA and PFNA head elements and is a valid supplementary treatment option to these nailing procedures in poor bone quality.

When and How to Operate Fragility Fractures of the Pelvis?

Fragility fractures of the pelvis (FFP) are an entity with an increasing frequency. The characteristics of these fractures are different from pelvic ring fractures in younger adults. There is a low energy instead of a high energy trauma mechanism. Due to a specific and consistent decrease of bone mineral density, typical fractures in the anterior and posterior pelvic ring occur. Bilateral sacral ala fractures are frequent. A new classification system distinguishes between four categories with increasing loss of stability. The subtypes represent different localizations of fractures. The primary goal of treatment is restoring mobility and independency. Depending on the amount of instability, conservative or surgical treatment is recommended. The operative technique should be as less invasive as possible. When the broken posterior pelvic ring is fixed operatively, a surgical fixation of the anterior pelvic ring should be considered as well. FFP Type I can be treated conservatively. In many cases, FFP Type II can also be treated conservatively. When conservative treatment fails, percutaneous fixation is performed. FFP Type III and FFP Type IV are treated operatively. The choice of the operation technique is depending on the localization of the fracture. Iliosacral screw osteosynthesis, transsacral bar osteosynthesis, transiliac internal fixation, and iliolumbar fixation are alternatives for stabilization of the posterior pelvic ring. Plate osteosynthesis, retrograde transpubic screw, and anterior internal fixation are alternatives for stabilization of the anterior pelvic ring. Postoperatively, early mobilization, with weight bearing as tolerated, is started. Simultaneously, bone metabolism is also analyzed and its defects compensated. Medical comorbidities should be identified and treated with the help of a multidisciplinary team.