Biomechanics of sacroiliac joint fixation using lag screws: a cadaveric study

BACKGROUND

Iliosacral screw placement is ubiquitous and now part of the surgeon's pelvic trauma armamentarium. More recent evidence supports sacroiliac arthrodesis for treating sacroiliac joint (SIJ) dysfunction in select patients. Regardless of the surgical indication, there are currently no studies examining lag screw compression biomechanics across the SIJ. The objective of this biomechanical investigation was to quantify iliosacral implant compressive loads and to examine the insertion torque and compressive load profile over time.

METHODS

Eight human cadaveric pelvic specimens underwent SIJ fixation at S1 and S2 using 11.5 and 10.0 mm iFuse-TORQ Lag implants, respectively, and standard 7.3 mm trauma lag screws. Load decay analysis was performed, and insertion and removal torques were measured.

RESULTS

For both implants at S1 and S2 levels, the load relaxed 50% in approximately 67 min. Compressive load decay was approximately 70% on average occurring approximately 15 h post-insertion. Average insertion torque for the 11.5 mm TORQ implant at S1 was significantly greater than the trauma lag screw. Similarly, at S2, insertion torque of the 10.0 mm TORQ implant was greater than the trauma lag screw. At S1, removal torque for the 11.5 mm TORQ implant was higher than the trauma lag screw; there was no significant difference in the removal torque at S2.

CONCLUSIONS

In this study, we found that a novel posterior pelvic implant with a larger diameter, roughened surface, and dual pitch threads achieved improved insertion and removal torques compared to a standard screw. Load relaxation characteristics were similar between all implants.

Biomechanical Stability of Primary and Revision Sacroiliac Joint Fusion Devices: A Cadaveric Study

STUDY DESIGN

An in vitro biomechanics study.

OBJECTIVE

To evaluate the efficacy of triangular titanium implants in providing mechanical stabilization to a sacroiliac joint with primary and revision sized implants.

METHODS

Ten lumbopelvic cadaveric specimens were tested in 4 stages: intact, pubic symphysis sectioned, primary, and simulated revision. Primary treatment was performed using 3 laterally placed triangular titanium implants. To simulate revision conditions before and after bone ingrowth and ongrowth on the implants, 7.5-mm and 10.75-mm implants were randomly assigned to one side of each specimen during the simulated revision stage. A 6 degrees of freedom spinal loading frame was used to load specimens in 4 directions: flexion extension, lateral bending, axial torsion, and axial compression. Biomechanical evaluation was based on measures of sacroiliac joint rotational and translational motion.

RESULTS

Both primary and revision implants showed the ability to reduce translational motion to a level significantly lower than the intact condition when loaded in axial compression. Simulated revision conditions showed no statistically significant differences compared with the primary implant condition, with the exception of flexion-extension range of motion where motions associated with the revised condition were significantly lower. Comparison of rotational and translation motions associated with the 7.5- and 10.75-mm implants showed no significant differences between the treatment conditions.

CONCLUSIONS

These results indicate that implantation of laterally placed triangular titanium implants significantly reduces the motion of a sacroiliac joint using either the primary and revision sized implants. No statistically significant differences were detected when comparing the efficacy of primary, 7.5-mm revision, or 10.75-mm revision implants.

Biomechanics of sacropelvic fixation: a comprehensive finite element comparison of three techniques

PURPOSE

Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for complex deformity correction and is commonly associated with pseudoarthrosis, implant failure and loosening. This study compared pedicle screw fixation (PED) with three different sacropelvic fixation techniques, namely iliac screws (IL), S2 alar-iliac screws (S2AI) and laterally placed triangular titanium implants (SI), all in combination with lumbosacral instrumentation, accounting for implant micromotion.

METHODS

Existing finite element models of pelvis-L5 of three patients including lumbopelvic instrumentation were utilized. Moments of 7.5 Nm in the three directions combined with a 500 N compressive load were simulated. Measured metrics included flexibility, instrumentation stresses and bone-implant interface loads.

RESULTS

Fixation effectively reduced the sacroiliac flexibility. Compared to PED, IL and S2AI induced a reduction in peak stresses in the S1 pedicle screws. Rod stresses were mostly unaffected by S2AI and SI, but IL demonstrated a stress increase. In comparison with a previous work depicting full osteointegration, SI was found to have similar instrumentation stresses as those due to PED.

CONCLUSIONS

Fixation with triangular implants did not result in stress increase on the lumbosacral instrumentation, likely due to the lack of connection with the posterior rods. IL and S2AI had a mild protective effect on S1 pedicle screws in terms of stresses and bone-implant loads. IL resulted in an increase in the rod stresses. A comparison between this study and previous work incorporating full osteointegration demonstrates how these results may be applied clinically to better understand the effects of different treatments on patient outcomes. These slides can be retrieved under Electronic Supplementary Material.

Postmarket surveillance of 3D-printed implants for sacroiliac joint fusion

Background

Government regulations require postmarket surveillance for cleared/approved medical devices. Trend analysis of newly marketed devices may help to confirm device-related safety or uncover other device or procedure-related problems.

Methods

Complaints related to the use of 3D-printed triangular titanium implants for sacroiliac joint (SIJ) fusion were compared with those of the prior machined version of the device manufactured with a titanium plasma spray (TPS) coating. Event rates were calculated either by dividing event counts by numbers of surgeries or, for late events, using Kaplan–Meier survival analysis.

Results

Three types of complaints with nontrial frequencies were identified. Issues in instruments occurred at a low and constant rate (1.3%). Using Kaplan–Meier analysis, pain-related complaints occurred at a low and similar rate in both groups (<0.5%). The 1-year cumulative probability of surgical revision was low in both the 3D and machined versions of the device (1.5% for machined and 1% for 3D printed, P=0.0408 for difference). No implant breakages or migrations were identified in either group, and overall rates were similar to a previously published report.

Conclusion

The 3D-printed version of triangular titanium implant was associated with complaint and adverse event rates similar to those for the prior machined version of the device.

Intermittent hydrostatic pressure inhibits matrix metalloproteinase and pro-inflammatory mediator release from human osteoarthritic chondrocytes in vitro

OBJECTIVE

This study tested the hypothesis that intermittent hydrostatic pressure applied to human osteoarthritic chondrocytes modulates matrix metalloproteinase and pro-inflammatory mediator release in vitro.

DESIGN

Human osteoarthritic articular chondrocytes were isolated and cultured as primary high-density monolayers. For testing, chondrocyte cultures were transferred to serum-free medium and maintained without loading or with exposure to intermittent hydrostatic pressure (IHP) at 10 MPa at a frequency of 1 Hz for periods of 6, 12 and 24 h. Levels of matrix metalloproteinase-2, -9 (MMP-2, -9), tissue inhibitor of metalloproteinase-1 (TIMP-1), and the pro-inflammatory mediators, interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1), released into the culture medium were assessed by ELISA. Matrix metalloproteinase activity was confirmed by zymographic analysis.

RESULTS

In the absence of IHP, levels of MMP-2, TIMP-1, IL-6, and MCP-1 in the chondrocyte culture medium increased in a time-dependent manner. Application of IHP decreased MMP-2 levels at all time periods tested, relative to unloaded control cultures maintained for the same time periods. Although 84/82 kDa bands were faintly detectable by zymography, MMP-9 levels were not quantifiable in medium from loaded or unloaded cultures by ELISA. TIMP-1 levels were not altered in response to IHP at any time period tested. IL-6 and MCP-1 levels decreased in cultures exposed to IHP at 12 and 24 h, relative to unloaded control cultures maintained for the same time periods.

CONCLUSION

IHP decreased release of MMP-2, IL-6 and MCP-1 by osteoarthritic chondrocytes in vitro suggesting that pressure influences cartilage stability by modulating chondrocyte expression of these degradative and pro-inflammatory proteins in vivo.

Effect of cutting flute design on cortical bone screw insertion torque and pullout strength

OBJECTIVE

To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws.

DESIGN

Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured.

SETTING

The study was conducted at an experimental biomechanics laboratory associated with a university medical center.

OUTCOME MEASUREMENTS

Insertion torque and pullout strength were normalized by the local bone mineral density.

RESULTS

The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05).

CONCLUSIONS

Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.

The relationship between mass and acceleration for impacts on padded surfaces

It is shown analytically that when an object impacts a deformable surface, the acceleration it experiences is inversely proportional to its mass. This is because acceleration is reduced by deformation of the surface, and more massive objects deform the surface more. Since head injury is thought to depend more on acceleration than applied force, this result implies that children may be at greater risk than adults when striking a padded surface. To test the theoretical prediction, 2.69 and 7.40 kg missiles were dropped 31.8 cm onto three deformable surfaces: dry sod, moist sod, and an artificial playing surface. Impact force was measured with a force plate, and divided by missile mass to obtain acceleration. For each test surface, the smaller mass produced a larger acceleration than the larger mass, confirming the theoretical result. Additionally, load-deformation characteristics of the three surfaces were measured, and the results used to predict the effects of mass on acceleration during impact. The predicted results agreed well with the experimental data for the artificial surface (2.3% difference) and moist sod (5.7% difference), but less well for dry sod (21% difference).