Abstract
Objective: To determine the best method for anterior internal fixation by comparing the biomechanical characteristics of various anterior fixation methods for vertically unstable sacroiliac dislocations.
Methods: Eight pelves with Tile C fracture with vertical loading were measured separately. Three‐ and two‐hole anterior double plates were fixed at different angles and compared with sacroiliac screws.
Results: For two‐hole anterior double plates, with increasing angle, axial stability increases while lateral stability decreases. For three‐hole plates with an angle of 60°, axial stability is clearly better when placed horizontally, while lateral stability shows no obvious differences.
Conclusion: For fixing sacroiliac dislocations, three‐hole anterior double plates placed at about 60° produce excellent stability, which would be greatly increased if screws were added on the iliac side.
Keywords: Biomechanics, Bone plates, Sacroiliac joint
Introduction
Because the posterior pelvic ring is an essential element for bearing body weight, a primary target of surgical treatments is to produce its continuity and stability to the maximal extent possible. Many fixation methods can provide strong stability after reduction 1 , 2 , 3 , 4 , 5 , and trans‐sacroiliac joint fixation with an anterior steel plate is a common one of these techniques. At present, both the four‐hole square plate and the double‐plate technique are frequently used. For the double‐plate technique, only one screw is recommended at the sacrum because of the presence of the sacral plexus, while two more screws are recommended at the ilium.
Common plates (non‐locked) attain stability through friction between the plates and bone surface. Therefore, if there are several screws at the ilium, the plates and screws acts as a lever system in which friction can not be achieved, and the extra screw strength can balance the pressure from the sacrum (Fig. 1). In addition, the method of anterior double‐plate fixation which is currently used usually involves placing two plates parallel to each other to form a parallelogram. It is well known that a parallelogram is not a stable configuration, and that its shape and area are variable when the side lengths are fixed 6 . Therefore, parallel placement of the anterior double‐plates will lack strength. A triangular configuration is a stable one, since it has only one shape when the side length is fixed. Accordingly, if two plates were placed with a definite angle between them to form a triangular configuration, we postulated that the stability would be increased.
Figure 1.
Forces at the sacrum (F1) and ilium (F2) maintain balance through a lever‐like mechanism, the middle screw acts as a fulcrum.
The main object of this study was to establish, through biomechanical research, the configuration for anterior double‐plate fixation which attains the greatest stability.
Materials and methods
The study utilized fresh frozen pelves (provided by the Anatomy Office of Zhejiang University), with an age range from 37 to 40 years (mean, 38.3 years), in order to minimize the difference caused by different ages. The sacroiliac, sacrospinous and sacrotuberous ligaments were preserved. All specimens were confirmed to be normal by X‐ray, but densitometry testing was not performed. Each pelvis was prepared by fixing the side of the would‐be unstable hemipelvis in polyester resin molded to a U‐shaped fixture, which could be attached to the load cell of the Materials Testing System (German Zwick/010 Electrical Universal Material Testing System, Ulm, Germany).
Materials and methods for internal fixation
All eight pelves were made to simulate single leg stance loading with Tile C pelvic dislocation, and divided into three groups: A, B and C, according to the following three different internal fixation methods. Group A: Two two‐hole reconstruction plates, one hole at the sacrum, and one hole at the ilium; Group B: Two three‐hole reconstruction plates, one hole at the sacrum and two holes at the ilium; Group C: Two sacroiliac screws, one in S1 and the other in S2.
Placement of the two plates in groups A and B: One plate was placed along the small pelvic ring, because there is greater bone density in this location. The second plate was firstly fixed with a screw (not tightened) on the sacrum, its position deviating upwards from the first plate on the iliac side with a certain angle D between the two plates (Fig. 2), then all screws were tightened. When the two plates were parallel, D equaled 0°. Not all of the angles needed to be measured, although there were many values for D. Therefore, this study measured only seven groups of angles precisely with a protractor: 0°, 15°, 30°, 45°, 60°, 75° and 90°. To attain stable fixation, cancellous screws longer than 3.5 cm were chosen and pierced through the two cortexes.
Figure 2.
The two plates were placed crossing the sacroiliac joint with various angles between them (D).
To achieve anatomical reduction: firstly, the internal fixation was installed on the pelvic specimens (with the screws not tightened); secondly, the internal fixation was removed, leaving the screw holes; thirdly, testing (as described in the next paragraph) was implemented with the pelvis complete; fourthly, a Tile C pelvic dislocation was created and the internal fixation installed again based on the existing screw holes. All the above operations were performed by the same surgeon (the 2nd author) in order to reduce experimental error.
Loading method and data collection
Vertical loading was adopted for all tests: the loading joint contacted the upper end plate of the first sacral vertebra directly with a displacement velocity of 0.05 mm/min, and the loading pressure was gradually increased from 0 to 300 N. The displacement reading was recorded when the pressure reached 300 N. Loading was cyclic, with a total of five cycles for each test run. The displacement values taken from the last five loading cycles of each test run were analyzed. An average value was calculated over the five cycles. The extent of both axial and lateral displacements was measured by a π‐shaped elastic element (extension arm) and electrical extension device (with a resolution of 0.01 mm of displacement).
In order to eliminate differences between the pelves in biomechanical characteristics, each pelvis underwent two tests: firstly, it was tested in the intact state and secondly, it was tested after fixation of the Tile C1 fracture. The displacement in the intact state and the state after fixation were both recorded; their ratio then functioned as a reference value (S) of internal fixation stability (the internal fixation stability was increased by the value of S). For each angle between the two plates, loading was cyclic for both axial and lateral stability, and an average displacement was calculated over the cycles.
Results
In the present study, we found that double three‐hole plates with angles from 60°–90° showed the greatest stability.
Axial stability
Among the seven groups of angles, for the two‐hole plates, axial stability was least when the plates were placed at 0° and greatest at 90° (P < 0.05), followed by 60° and 75°. For the three‐hole plates, axial stability was greatest at 60°, but the differences among the seven groups were not significant (P > 0.05, Fig. 3).
Figure 3.
The contribution to axial stability of different angle between plates. For two‐hole plates, axial stability was least when the two plates were placed at 0° and greatest at 90° (P < 0.05); For the three‐hole plates, the differences among the seven groups were not significant (P > 0.05). D, angle between plates; S, stability.
Among the three fixation methods (Group A, B and C), when three‐hole plates were placed parallel their axial stability was similar to that of other fixations, but the result was improved when the plates were placed at a 60° angle to one another. The two‐hole plates' axial stability was least when they were placed parallel, but at 90° their stability was not significantly different to the other fixations (Fig. 4). No method of internal fixation achieved the stability of the intact pelvis.
Figure 4.
The contribution to axial stability of different methods of fixation. The two‐hole plates' axial stability was weakest when they were placed parallel. No internal fixation achieved the stability of the intact pelvis.
Lateral stability
Among the seven angles, lateral stability was reduced with increasing angle. For the two‐hole plates, lateral stability clearly declined as the angle increased, especially at 75° and 90°; for the three‐hole plates, the difference among the seven angles was not significant (P > 0.05, Fig. 5).
Figure 5.
The contribution to lateral stability of different angles between plates. Lateral stability is reduced with increasing angle. For three‐hole plates, the differences between the seven angles was not significant (P > 0.05). D, angle between plates; S, stability.
Among the three fixation methods (group A, B and C), lateral stability with sacroiliac screws was slightly weaker than when the three‐ and two‐hole plates were placed parallel. Lateral stability of the two‐hole plates with an angle of 60° between them was weaker than that achieved with sacroiliac screws, but the three‐hole plates had stronger lateral stability than the sacroiliac screws (Fig. 6). None of the methods of internal fixation achieved the stability of the intact pelvis.
Figure 6.
The contribution of different methods of fixation on lateral stability. The difference between the three fixation methods was not significant.
Discussion
The anterior plate is a common technique for treating a vertically unstable sacroiliac joint dislocation, including four‐hole square plate, and anterior double‐plate, fixation 7 . For the double‐plate technique, only one screw is recommended at the sacrum because of the presence of the sacral plexus, while two more screws are recommended at the ilium. There are many choices for the angle between the two plates, but no universal standard for the angle of choice has been established clinically, and biomechanical research on this topic is inadequate. In this study,stability of these methods of anterior internal fixation was compared, and the influence of different angles between the anterior double‐plate was assessed.
In order to minimize differences in biomechanical characteristics, this study utilized fresh frozen pelves from a narrow age range and the ratio between injured and intact pelves was taken as the reference for stability.
Yinger et al. 7 investigated sacroiliac joint dislocations and anterior two‐hole double‐plate fixation utilizing an artificial pelvic model. The results of biomechanical research suggested that stability of this fixation method was weak. Leighton et al. 8 compared the mechanical stability of fixation of sacroiliac dislocations with anterior three‐hole double‐plate and sacroiliac screws in fresh frozen pelves, and found no difference between these two fixation methods when loading was 1000 N. Comstock et al. 9 evaluated six artificial pelvic models with similar fixation methods used by Yinger et al. 7 and Leighton et al. 8 and their results which suggested stability of fixation was similar with these studies. In the studies cited above, the anterior double‐plates were all placed parallel, but in this study, we placed the two plates at different angles to construct triangular configurations. Comparative biomechanical testing suggested that the greatest axial stability was attained by the anterior double‐plate when the two plates were placed at an angle between 60° to 90°, and that this stability was similar to that of sacroiliac screws.
Results of this study also showed the stability of three‐hole plates is better than that of two‐hole plates, we guess this is because of the lever formed by three screws and plate (Fig. 1): the middle screw serves as a fulcrum, and forces at the sacrum and ilium maintain balanced pressure on the lever. In such a balanced system, the middle screw bears the greatest strength and thus has the greatest possibility of loosening. In addition, according to the theories, the farther the extra iliac screw is away from the fulcrum screw, the less pressure it bears. Therefore, though no testing has been implemented, it can be inferred that greater axial stability can be attained by adding extra screws.
For two‐hole plates, the greatest lateral stability is attained when they are fixed parallel, at the same time this results in the lowest axial stability. When fixed at an angle of 60°–90°, the best axial stability is attained, but lateral stability definitely falls, thus the most appropriate angle is hard to determine. For three‐hole plates, lateral stability at 60° is not significantly different than when the plates are placed parallel, but the greatest axial stability is attained. Thus three‐hole plates at 60° have the greatest stability, better than that of two‐hole plates and sacroiliac screws.
All possible angles were not tested in this study, which leads to a certain bias in the results. However, it has been established that three‐hole plates do attain relatively ideal axial and lateral stability at an angle of 60°.
In the clinic, there are two main surgical approaches for Tile C pelvic dislocations: the anterior and the posterior approach. The posterior approach has a higher rate of infection, skin necrosis, nerve injury, lumbar pain and sexual dysfunction. The anterior approach is now preferred as it avoids the complication of skin necrosis and reduces the infection rate. In addition, fixation of the pelvic ring can be completed in one surgical position and via one surgical field. The drawback of this technique has been the relatively poor stability provided by anterior plates. This study has shown that the stability of the three‐hole double‐plate fixed at 60° is better than that of sacroiliac screws, and it can be increased by adding screws at the iliac side.
In conclusion, the stability of anterior double‐plate fixation is reduced by parallel placement. The three‐hole double‐plate placed at an angle of 60° has the greatest stability, which is better than two‐hole plates and sacroiliac screws, and would be greatly increased by adding screws at the iliac side.
Disclosure
There were no financial or personal relationships with any organization.
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