Screw fixation for syndesmotic injury is stronger and provides more contact area of the joint surface than TightRope ® : A biomechanical study

Abstract

BACKGROUND:

The rupture of syndesmotic ligaments is treated with a screw fixation as the gold standard. An alternative is the stabilization with a TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ . A couple of studies investigated the different clinical outcome and some even looked at the stability in the joint, but none of them examined the occurring pressure after fixation.

OBJECTIVE:

Is there a difference in pressure inside the distal tibiofibular joint between a screw fixation and a TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ ? Does the contact area differ in these two treatment options?

METHODS:

This biomechanical study aimed to investigate the differences in fixation of the injured syndesmotic ligaments by using a fixation with one quadricortical screw versus singular TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ both implanted 1 cm above the joint. By using 12 adult lower leg cadaveric specimens and pressure recording sensor, we recorded the pressure across the distal tibiofibular joint. Additionally we measured the contact surface area across the joint.

RESULTS:

The mean of the pressure across the distal tibiofibular joint from the start of the insertion of the fixation device to the complete fixation was 0.05 Pascal for the TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ and 0.1 for the screw ( $P=$ 0.016). The mean of the maximum pressure across the joint (after completion of fixation and releasing the reduction clamp) was 1.750 mega Pascal with the screw fixation and 0.540 mega Pascal with TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ ( $P=$ 0.008). The mean of the measured contact area of the distal tibiofibular joint after fixation was 250 mm ${}^{2}$ in the TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ group and of 355 mm ${}^{2}$ in the screw fixation ( $P=$ 0.123).

CONCLUSIONS:

The screw fixation is stronger and provides a larger surface contact area, which leads us to the conclusion that it provides a better stability in the joint. While previous clinical studies did not show significant clinical difference between the two methods of fixation, the biomechanical construct varied. Long term clinical studies are required to establish whether this biomechanical distinction will contribute to various clinical outcomes.

Keywords

Ankle syndesmosis TightRope®screw

1. Background

Isolated syndesmotic ligament rupture or in association with ankle fractures are very common injuries [1]. The gold standard of treatment is joint reduction and stabilization. However, there is a lack of agreement on several aspects of the surgical fixation including the choice of the implant [2, 1].

A recent meta-analysis investigated the difference between screw fixation versus suture-button TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ [3]. Their results showed no differences in the functional outcome or the complications rate between the two fixation methods. The only discrepancy found was the fact that patients with TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ fixation might have been able to return to work at an earlier date [3].

As mentioned before, the clinical outcome and the complications rate were comparable among the two methods, but biomechanics studies have shown differences between the two methods of fixation. However, most of these studies looked at stability of fixation by judging either through the required torque force prior to failure, or the degree of displacement post fixation [4, 5, 6].

We investigated the biomechanics strength of each fixation method in a different aspect. The primary aim of this study was to find out which method of fixation provides a stronger fixation by measuring the pressure forces across the distal tibiofibular joint. The secondary aim was to identify the fixation that provide more contact area of the joint surface. These parameters are necessary to provide a better stability for the ligaments to heal properly.

2. Methods

2.1 Experiment setting and cadaver preparation

The study was performed in collaboration with the anatomy department at the medical school of Hannover. We used 12 adult lower leg specimens (from 6 cadavers age 67–93, 3 males and 3 females). One right and one left leg of each deceased person. The corpses were prepared locally in this laboratory using a mixture with less formaldehyde and more glycerin which allows the specimens to remain relatively soft and close to fresh specimen [7].

To simulate the injury, lateral approach was performed into each specimen. The anterior and posterior inferior tibiofibular ligaments (AITFL, ITFL), superficial and deep posterior inferior tibiofibular ligaments PITFL and interosseous membrane were divided under direct view. In order to confirm that the dissection was successful to create the injury, we tested the instability using examination under image intensifier by manually enlarging the distal tibulofibular joint with a bone hook (II).

2.2 Measurement of the tibiofibular joint pressure

To identify the strength of fixation, we used the pressure sensor device Tekscan 4000 (Fig. 1) [8]. It allowed us to measure the pressure (compression) inside the distal tibiofibular joint. After we inserted the pressure sensor into the tibiofibular joint, we secured it in place using FiberWire ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ suture to avoid any variation of the readings due to misplacement during the test.

Figure 1.

Tekscan 4000 sensor was used to measure the joint pressure as well as contact surface area of the distal tibiofibular joint.

2.3 Fixation of the injured distal tibiofibular joint

The joint was reduced and hold temporarily in the reduced position using a King Tong clamp. The reduction was then checked using an image intensifier as well as manually by using the thumb technique [9]. Afterwards a 2.5 mm drill-bit guided with a 3.5 mm universal drill guide was used to establish a drill channel 1 cm above the joint with 45 ${}^{\circ}$ horizontal angle. The position was then checked using x-ray and the length was measured with a depth gauge. Then the appropriate 3.5 mm self-drilling quadricortical screw was inserted and subsequently the clamp was released. Now the screw was tightened manually with a small hexagonal screw driver. During this procedure, we measured the joint pressure (Fig. 2).

Figure 2.

Fixation with one 3.5 mm cortical screw. The Tekscan sensor can be seen on the right.

After removing the screw, we repeated the same maneuver of reduction, but the fixation was performed this time with the TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ . Therefore we used the drill sold with the TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ and widened the existing drill channel to an appropriate size. Not the TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ was inserted and tightened under similar monitoring of the pressure throughout the procedure. For every ankle we used a new sensor (Fig. 3).

2.4 Statistical analysis

We performed a $t$ -test as well as one way ANOVA with a Student-Newman-Keuls post-test as a pair testing with the data we collected to test the groups as well as the single data pairs against each other. Prior to the ANOVA a pre-test looking for Normality. The $P$ value was set to be statically significant at 0.05.

3. Results

Table 1
This graph compares the mean joint pressure between the two methods of fixation. The pressure in the group of the screw was significantly higher than the TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ ( $P<$ 0.05)

Figure 3.

Fixation with TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ and Tekscan sensor in place.

The mean of the maximum pressure across the joint (after completion of fixation and releasing the reduction clamp) was 1.750 mega Pascal with the screw fixation and 0.540 mega Pascal with TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ ( $P=$ 0.008) (Table 2).

Table 2

This graph shows the peak pressure in comparison between the screw fixation and the TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$ with a significantly higher pressure for the screw fixation ( $P<$ 0.05)

The mean of the measured contact surface area of the distal tibiofibular joint after fixation was 250 mm ${}^{2}$ in the TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ group and of 355 mm ${}^{2}$ in the screw fixation ( $P=$ 0.123) (Table 3). Even though there is no statistical significance there seems to be a difference in the areas.

Table 3

This bar chart shows the joint contact surface area in mm ${}^{2}$ of the group with the screw compared to the TightRope ${}^{\@setsize{\tiny}{7pt}{\vipt}{\@vipt}\textregistered}$

4. Discussions

Our results are showing that from a biomechanically point of view the screw fixation provides stronger fixation across the joint than using TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ and this was evident through higher pressure reading across the joint. But more importantly it provides a larger contact surface area of distal tibiofibular.

The previous biomechanical work on syndesmosis fixation were performed mainly to test either the degree of displacement or resistance to torsion. Clanton et al. used 24 lower leg specimens to compare the strength of fixation using one self-drilling 3.5-mm syndesmotic screw, one suture-button construct or two divergent suture-button constructs. In their work they used torsional cyclic loading to assess the strength of fixation. Using three-dimensional analysis of the sagittal fibular translation, their results revealed that the screw fixation had the smallest magnitude of posterior sagittal translation and a single suture-button construct demonstrated the largest magnitude of posterior sagittal translation. Therefore, they concluded that the repair with a single suture-button construct may not provide sufficient resistance to sagittal translation of the fibula [4].

Another similar biomechanical study used cadaveric specimens looked at the degree of the syndesmotic gap after cycling submaximal loading. In comparison to the screw fixation, the suture button group showed more movement of the fibula in the sagittal plane. The authors favored the suture button on the concept that provides less rigid fixation which may resemble the physiologic nature of the syndesmosis [6]. This is an interesting concept and the results of the mentioned both studies are correspondent with our results of a higher stress pressure across the joint when using the screw fixation in comparison to the TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ fixation. In other word, the screw fixation provides a more rigid fixation.

In addition to the findings above, our data showed that the contact surface are of the distal tibiofibular joint appears to be larger in the group of a screw fixation. This may have two significant impacts. First, it may indicate that the screw fixation allows to hold the reduction in a more stable position, hence the articulation between the distal tibia and fibula maintained a larger contact surface area in comparison with TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ group. However, we cannot confirm this hypothesis as our reduction was evaluated using x-ray images and not CT scan. Moreover, we know from recent work conducted on the anatomy of the distal tibiofibular joint that there are large variations in the shape of the tibial incisura and therefore the images of the reduced joint may vary based on the native shape of the joint of each individual [10]. The second point is that a decreased contact surface area at the distal tibiofibular joint may alter the ankle joint biomechanics and lead to long term ankle joint arthritis. To confirm the last statement, we will need prospective clinical studies with a long term follow up.

5. Conclusion

In summary, screw fixation and TightRope ${}^{\@setsize{\scriptsize}{9.5pt}{\viiipt}{\@viiipt}\textregistered}$ provide different biomechanical construct. Although previous clinical studies did not show significant clinical difference, there is no clear confirmation whether the biomechanical differences between the various methods of fixation may contribute to different long-term results. Hence, clinical studies with long term follow up are required.

A limitation of this work is that we do not know the normal contact pressure in the syndesmosis in the uninjured state during routine activity.

Footnotes

Acknowledgments

This study was funded by the Alwin Jäger Foundation.

Conflict of interest

The authors declare that there is no conflict of interest.

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