Astudy that evaluated a novel intraoperative test for syndesmotic instability of the ankle involving use of a 3.5 mm blunt cortical tap found the technique served as a “simple, reliable, and accurate tool for diagnosis of coronal syndesmotic instability.”
Cesar de Cesar Netto, MD, PhD, offered that characterization as he presented the results of a cadaveric investigation during the American Orthopaedic Foot & Ankle Society 2018 Specialty Day.
“Precise diagnosis of distal tibiofibular syndesmotic injury is challenging, and a gold-standard diagnostic test has still not been established,” the authors of the study wrote, noting, “Increased tibiofibular clear space identified on radiographic imaging is considered the most reliable indicator of the injury, especially in the absence of a significant deltoid ligament injury.”
The Cotton, or hook, test is the most widely used intraoperative technique to evaluate the syndesmotic integrity, but it has limitations, Dr. de Cesar Netto said. In the hook test, he explained, after adequate bone fixation, the tibia is stabilized with one hand while a bone hook is used on the distal fibula to apply a lateral directed force, in an attempt to separate it from the distal tibia. Relative opening of the tibiofibular clear space (TFCS) greater than 2 mm or an absolute value greater than 6 mm is deemed a positive test. “When instability is identified, fixation with syndesmotic screws or suture buttons is needed,” he said.
In the hook test, he noted, “the amount of force applied is not controlled or reproducible in the clinical setting, and it is usually associated with traction movements of the lower extremity, making it challenging to evaluate the increased gapping of the TFCS using fluoroscopic imaging.”
A discussion with a mentor from Brazil, Alexandre Leme Godoy-Santos, led Dr. de Cesar Netto and his colleagues to perform a broad literature review to “come up with ideas for a better option.” They found a brief technical description, by Rajagopalan et al, of a new intraoperative technique for testing the distal tibiofibular syndesmosis. This “tap test” technique caught their attention, he said, “and we thought it would be exciting to investigate it in a cadaveric study.”
Testing the tap test
The investigators conducted their study using nine cadaveric specimens. Under fluoroscopic guidance and using a 2.5 drill bit, they percutaneously drilled a hole through both cortices of the distal fibula, in position for the potential placement of a syndesmotic screw or suture button, 2.5 cm proximally to the articular surface line of the distal tibia. A blunt-edged 3.5 mm cortical tap, with a 1.25 mm thread pitch, was then threaded into the hole until it reached the intact near tibial cortex. For each specimen, three sequential perfect fluoroscopic mortise images were taken (Fig. 1), with the ankle in neutral dorsiflexion.
The first image was performed with the syndesmotic ligaments intact and no force applied to the tap, which was left with its tip in contact with the undrilled near cortex of the distal tibia (intact, non-stressed) (Fig. 1A). In the second image, with the ligaments intact, the cortical tap was advanced so that its blunt tip began pushing against the undrilled lateral tibial surface, thus providing a tibiofibular separation force (intact, stressed) (Fig. 1B). After hitting the lateral tibial cortex, the tap was turned for a maximum of six full revolutions or until the surgeon achieved a torque that was close to the thread-stripping torque.
Following the second image, the tap was unthreaded and a 5 cm anterolateral approach was made over the level of the distal tibiofibular syndesmosis. The anterior inferior tibiofibular ligament (AITFL), interosseous tibiofibular ligament, and the posterior inferior tibiofibular ligament (PITFL) were sharply released with a number 10 scalpel. The third image was then acquired after the same stress was applied with the cortical tap, but all syndesmotic ligaments now released (injured, stressed) (Fig. 1C).
Radiographic tibiofibular clear space (TFCS) was measured 1 cm above the ankle joint line with the tibial plafond, by two independent and blinded readers using an available measurement software. One of the readers performed a second set of measurements 30 days after the initial assessment, respecting the wash-out period.
The authors summarized the results as follows: “We found excellent intra-observer (0.97) and inter-observer (0.98) agreement following the imaging assessment. Significant differences were found in the paired comparison between the groups (P < 0.05). When using an absolute value for TFCS > 6 mm as diagnostic for coronal syndesmotic instability, the tap test demonstrated 96.3 percent sensitivity and specificity, 96.3 percent positive and negative predictive values, and 96.3 percent accuracy in diagnosing coronal syndesmotic instability.” A clinical picture of a positive tap test is depicted in the photographs on page 1.
The very high reliability in performing the measurements of the TFCS was “an unexpected finding,” Dr. de Cesar Netto said. Another interesting finding, he said, “is the fact that before starting the study, we intended to release the syndesmotic ligaments progressively, from anterior to posterior (AITFL, intraosseous membrane [IO], and PITFL), and to evaluate the opening of the TFCS. Similar to what is found in the literature, in a pilot study, we noticed that releasing the AITFL and IO in the cadaveric specimens was not enough to destabilize the syndesmotic joint, and no significant changes in the TFCS could be noted. It would also jeopardize the amount of stress we would be able to apply with the tap with repeated stress considering the cadaveric bone quality, with the risk of thread stripping of the tap in the fibula. Because of that, we decided to do a complete injury to the ligaments in the real study, avoiding the progressive release and repetitive stress with the tap.”
The major clinical takeaway, Dr. de Cesar Netto said, is “that there are good options available for surgeons to assess the residual instability of the distal tibiofibular syndesmosis intraoperatively, following adequate fixation of associated fractures, when present. We believe that the traditional Cotton test is unreliable, with uncontrolled force applied, and almost impossible proper assessment of the TFCS in a dynamic fluoroscopic image, which needs to be taken while the surgeon is applying traction force and constantly moving the ankle.”Addressing limitations, he said, “Cadaveric studies are always hindered by the cadaveric rigidity. The accuracy of the test and the degree of the syndesmotic instability could be different in live tissue and real patients. Another significant limitation is that our study is not comparative, so no formal differentiation between the tap and Cotton test can be affirmed. Another fact that needs to be considered is the possibility of the blunt tap to penetrate the distal tibia cortex, especially in osteoporotic patients. If that happened, the accuracy of the tap test would be compromised, since no distraction force in the syndesmotic joint would be applied in that situation and one could fail to diagnose residual instability. It is important to emphasize that even though we acknowledge that this can potentially happen, we had no cases in the nine cadaveric specimens used.”
Dr. de Cesar Netto’s coauthors of “Intraoperative Tap Test for Coronal Syndesmotic Instability” are Alexandre Leme Godoy-Santos, MD, PhD; Ibukunoluwa Araoye, MS; Parke Hudson, BS; Martim Pinto, MD; Sierra Phillips, MD; Walter Smith, BS; Yvonne Chodaba, BS; and Ashish Shah, MD.
- Rajagopalan S, Upadhyay V, Taylor HP, Sangar A: New intra-operative technique for testing the distal tibiofibular syndesmosis. Ann R Coll Surg Engl 2010;92(3):258
- Jafet Massri-Pugin, MD, Bart Lubberts, MD, Bryan G. Vopat, MD, Daniel Guss, MD, MBA, Ali Hosseini, PhD, and Christopher W. DiGiovanni, MD.Effect of sequential sectioning of ligaments on syndesmotic instability in the coronal plane evaluated arthroscopically. Foot Ankle Int 38(12): 1387-1393.
Terry Stanton is the senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org.