Cadaveric Study Evaluating the Potential for Hindfoot Endoscopy and Flexor Hallucis Longus Tendoscopy Using a 1.9-mm Diameter Needle Arthroscope

Abstract

Background:

Hindfoot endoscopy is an effective treatment for posterior ankle impingement syndrome (PAIS) and flexor hallucis longus (FHL) tendon disorders. However, FHL tendoscopy, especially from the posteromedial portal, carries a risk of tibial nerve damage. A needle-arthroscopic system with a 1.9-mm-diameter arthroscope, a semirigid frame, and a 0-degree direction of view has been introduced. This study aimed to evaluate the efficacy and safety of this system in visualizing and reaching significant structures in hindfoot endoscopy and FHL tendoscopy through the posteromedial and posterolateral portals using a cadaveric model.

Methods:

The 1.9-mm-diameter arthroscopic system (NanoScope, Arthrex) was used to perform hindfoot endoscopy in 6 human donor ankles (3 pairs). The arthroscope tube is 9.5 cm long, semirigid, and has an outer diameter of 1.9 mm, a 0-degree direction of view, and a 120-degree field of view. Posteromedial and posterolateral portals were established. Visualization and operative reach were recorded, including the posterolateral talar process, posterior talofibular ligament, intermalleolar ligament, subtalar joint, and FHL tendon. The neurovascular bundle and FHL tendon were examined for kinks or damage.

Results:

All significant structures were successfully visualized in all specimens. The wide 120-degree field of view facilitated adequate visualization of all structures. In all specimens, the FHL tendon was visualized from the ankle joint to the knot of Henry (zones 1 and 2), and the flexor digitorum longus tendon was observed via both portals. There were no signs of neurovascular damage from either the posterolateral or posteromedial portals.

Conclusion:

In this cadaver experiment without known pathology, use of a 1.9-mm-diameter needle-arthroscopy with a 0-degree direction of view provided effective visualization of all significant structures in treating PAIS and FHL tendon disorders. It appears that FHL tendoscopy can be performed from the level of the ankle joint to the knot of Henry via both the posterolateral and posteromedial portals.

Introduction

Posterior ankle impingement syndrome (PAIS) is characterized by hindfoot pain caused by ankle plantar flexion. PAIS is a common injury in athletes who engage in repetitive ankle plantar flexion, particularly ballet dancers and soccer players. Since the development of 2-portal endoscopic surgery by van Dijk et al, ⁷ hindfoot endoscopy has been established as an effective and minimally invasive approach for the treatment of PAIS, yielding good clinical outcomes. ⁵ Given the frequent coexistence of PAIS and FHL tenosynovitis, it is crucial to thoroughly investigate the flexor hallucis longus (FHL) tendon disorders when addressing PAIS. FHL tendon disorders, including tenosynovitis and tendinopathy, are particularly common among ballet dancers and athletes who perform repetitive plantar flexion movements. Traditional treatment approaches through open surgery or conventional arthroscopy have shown variable outcomes, with the deep anatomical location and curved path of the FHL tendon presenting significant diagnostic and therapeutic challenges, particularly in visualizing and accessing the entire length of the tendon through conventional methods. However, conventional rigid arthroscopy from the posterolateral portal limits the visualization of the FHL tendon distal to the retinaculum through the tunnel underneath the sustentaculum tali because of the difference in the insertion angle of the arthroscope and the direction of the FHL tendon. Therefore, FHL tendoscopy can be performed from the posteromedial portal, although it carries the risk of tibial nerve damage.^{2 -4} Particularly, the rigid nature of conventional arthroscopes can cause kinking or mechanical stress on the neurovascular bundle when inserted through the posteromedial portal, potentially leading to nerve damage.

Recently, a 1.9-mm-diameter needle-arthroscopic system has been introduced. It uses a disposable chip-on-tip camera with a semirigid arthroscope, with a total diameter of just over 2 mm. Its small features can help reduce the risk of wound or neurovascular complications, potentially enabling earlier recovery owing to its minimally invasive nature compared with conventional arthroscopy. Needle arthroscopy may also be useful for FHL tendoscopy because of its smaller, semirigid design. Although cadaveric studies on the effectiveness of needle arthroscopy for the ankle joint and other tendons, such as the Achilles, tibialis posterior, and peroneus tendons, have been reported, ⁶ its application in hindfoot endoscopy has not been explored. In addition, it has a 0-degree direction of view and a 120-degree field of view, but it does not have an angled scope, which may reduce visualization.

This study aimed to evaluate whether 1.9-mm-diameter needle arthroscopy with a 0-degree direction of view can safely visualize and operatively reach all important structures in both hindfoot endoscopy and FHL tendoscopy, and to evaluate its safety in relation to the neurovascular bundle during FHL tendoscopy through the posteromedial and posterolateral portals using a cadaveric model.

Methods

Specimens

This study was approved by our institutional ethics committee. Thiel-embalmed cadavers from our institute’s anatomy department were used for this study. Six (three-paired) human lower leg specimens without a known or apparent history of ankle surgery were selected (1 male, 2 female; mean age 77.0 years, range 70-87). Specimens were amputated at the level of the proximal tibia for fixation.

Hindfoot Endoscopy

A needle arthroscopic system (NanoScope, Arthrex, Naples, FL, USA) was used for hindfoot endoscopy in six human donor ankles (3 pairs). The arthroscope tube, measuring 9.5 cm in length and semirigid, had an outer diameter of 1.9 mm with a 0-degree direction of view and a 120-degree field of view. We investigated whether the posterior structures of the ankle joint, typically observed with regular 30-degree oblique arthroscopy, can also be observed using direct-viewing needle arthroscopy.

Each leg was placed in the prone position for hindfoot endoscopic evaluation. Posteromedial and posterolateral portals were established as described by van Dijk et al. ⁷ Both the posterolateral and posteromedial portals were created using the sharp obturator provided with the needle arthroscopic system. Then, with the help of a 1.9-mm-diameter blunt obturator, a 2.26-mm-diameter cannula was inserted through the posterolateral portal. The obturator was removed and a 1.9-mm-diameter needle arthroscope was introduced through the cannula. Continuous irrigation was maintained using an arthroscopic pump system with normal saline at a pressure of 20 mm Hg to provide clear visualization of the posterior ankle structures. A 2.0-mm shaver was introduced into the posteromedial portal and careful debridement of the soft tissue was performed until the posterior structures were visualized. The combination of careful soft tissue debridement with the 2.0-mm shaver and continuous saline irrigation at 20 mm Hg helped maintain the working space. The FHL tendon was identified first by observing its movement during passive flexion and extension of the great toe. Once identified, the FHL tendon served as a crucial landmark, with all subsequent debridement and space creation performed posterior and lateral to it to protect the neurovascular bundle. Care was taken to identify the FHL tendon and maintain it in the posterior and lateral aspects. All arthroscopic procedures were performed by the same experienced foot and ankle fellowship-trained orthopaedic surgeon (Y.S.).

Visualization

Visualization and operative reach of specific structures with tailored arthroscopic instruments, including the posterolateral talar process, posterior talofibular ligament, intermalleolar ligament, subtalar joint, and FHL tendons, were recorded.

FHL tendoscopy

A needle arthroscope was further inserted into the FHL tendon sheath, and the extent to which the arthroscope could advance was evaluated. The FHL tendon is divided into 3 zones (Figure 1). Zone 1 is behind the ankle joint. Zone 2 is from the tunnel underneath the sustentaculum tali to the knot of Henry, which is the plantar crossing of the flexor digitorum longus (FDL) tendon obliquely over the FHL tendon in the midfoot, at the level of the navicular bone. Zone 3 is distal to the knot of Henry. ¹ FHL tendoscopy was first performed through the posterolateral portal and then through the posteromedial portal (Figure 2). Finally, after the arthroscopic procedure, the specimen was dissected and the neurovascular bundle and FHL tendon were examined for any kinks or damage. If the neurovascular bundle was kinked or damaged, its location was recorded.

Figure 1.

Running of the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) tendons and the knot of Henry, the site where the two cross. Three zones of the FHL tendon are shown.

Figure 2.

Magnetic resonance T1-weighted axial image showing the location of the posterolateral (PL) and posteromedial (PM) portals. The red circle indicates neurovascular bundles. FHL, flexor hallucis longus.

Results

In all specimens, all significant structures, such as the posterolateral talar process, posterior talofibular ligament, intermalleolar ligament, subtalar joint, ankle joint, and FHL tendon, were successfully visualized through the posterolateral portal (Figure 3). In addition, these structures were reached in all specimens using a probe and a shaver through the posteromedial portal. Owing to its wide 120-degree field of view, there was no difficulty in obtaining sufficient visualization of any structure. Even if the portals were switched and the needle arthroscopy was inserted through the posteromedial portal and the probe was inserted through the posterolateral portal, all significant structures could be observed and reached in all specimens.

Figure 3.

Viewing image from the posterolateral portal showing the talus, calcaneus, and FHL tendon. FHL, flexor hallucis longus.

In all specimens, through the posterolateral portal, the FHL tendon was visualized from the level of the ankle joint all the way to the knot of Henry (zones 1 and 2). The tendon sheath under the sustentaculum tali was passed through without any problems, and both the FHL and FDL tendons were well-visualized at their intersection (the knot of Henry) (Figure 4, A and B). Similarly, FHL tendoscopy could also be easily performed via the posteromedial portal, and the knot of Henry could be observed. After completion of the arthroscopic procedures, detailed examination revealed no evidence of kinking or damage to the FHL tendon in any of the specimens. Similarly, the tibial nerve and its branches showed no signs of iatrogenic damage in any specimen.

Figure 4.

(A) Viewing image of FHL tendoscopy showing the FHL tendon passing beneath the sustentaculum tali. (B) Viewing image of FHL tendoscopy showing both FHL and FDL tendons at the knot of Henry. FDL, flexor digitorum longus; FHL, flexor hallucis longus.

Discussion

The main finding of this study was that 1.9-mm-diameter needle arthroscopy with a 0-degree direction of view provided sufficient visualization of all the structures in the treatment of PAIS and FHL tendon disorders. Furthermore, it should be noted that 1.9-mm-diameter needle arthroscopy facilitated observation within the FHL tendon sheath and provided a smooth and sufficient field of view to the knot of Henry. Although FHL tendoscopy in zone 2 through the posterolateral portals is generally difficult using conventional arthroscopy and insertion through the posteromedial portal is recommended, FHL tendoscopy with needle arthroscopy was possible even from the posterolateral portal. As posteromedial portals carry the risk of nerve injury and posterolateral portals are safer, ⁴ FHL tendoscopy using needle arthroscopy through the posterolateral portal may be suggested as a safer method. Its small and semirigid features make FHL tendoscopy less invasive and more accessible than conventional rigid arthroscopy.

This needle arthroscopic system has a wide field of view of 120 degrees; however, the scope’s direction of view is 0 degrees. We investigated whether posterior structures of the ankle joint that can normally be observed with regular 30-degree oblique arthroscopy can also be observed without problems with this direct-viewing needle arthroscopy. Although cadaveric studies on the effectiveness and visualization of needle arthroscopy for the ankle joint and other tendons, such as the Achilles tendon, tibialis posterior tendon, and peroneus tendon, have been reported, ⁶ there are no reports on hindfoot endoscopy. In this study, we found that the main structural tissues of the hindfoot could be adequately observed with needle arthroscopy. Although the image quality (resolution, sharpness, and brightness) was lower compared with conventional arthroscopy and required positioning the scope closer to the target structures, the visualization was sufficient for identifying the structures and should be able to identify pathoanatomy.

Lui and Chan ⁴ reported a high risk of neurovascular bundle kinking when hindfoot endoscopy and FHL tendoscopy were performed from the posteromedial portal. This is because the rigid speculum and even larger arthroscope apply an excessive force. However, it is difficult to view the FHL tendon through the whole zone 2 from the posterolateral portal because of the angle problem, which is the difference in the insertion angle of the arthroscope and the direction of running of the FHL tendon. There have been no reports of FHL tendoscopy performed using the posterolateral portals. Even in a cadaveric study of FHL tendoscopy using a conventional arthroscope by Lui et al, they used a posteromedial portal and did not report the use of a posterolateral portal. ² In the present study, FHL tendoscopy using needle arthroscopy was possible without any problems with the posterolateral portal. This may be due not only to the smaller size of the needle arthroscope but also to its semirigid nature, which is expected to slightly bend and move through the tendon sheath even at the posterolateral portal. Needle arthroscopy suggests the possibility of performing FHL tendoscopy using a safer method involving the posterolateral portal rather than the posteromedial portal. Furthermore, in this study, no obvious neurovascular injury was observed when FHL tendoscopy was performed through the posteromedial portal using needle arthroscopy. FHL tendoscopy by using needle arthroscopy rather than conventional arthroscopy may reduce the risk of neurovascular injury, and may have merit.

The FHL tendon has been divided into 3 zones.^1,2 With conventional arthroscopes through the posterolateral portal, it is possible to see up to the middle of zone 2, below the sustenculum tali; however, there are no reports showing that it is possible to see down to the knot of Henry. As the FHL tendon runs in a curve, the rigid arthroscopy technique itself becomes difficult depending on the insertion angle. ² Needle arthroscopy, on the other hand, is first smaller and thus easier to insert and, second, semirigid, allowing the arthroscope to enter the FHL tendon along its path to the knot of Henry, even through the posterolateral portal. Further clinical studies are required to evaluate the potential of this approach to help address local pathologies.

This study has limitations. First, this study was conducted using a small number of donor ankles without known pathology. The small-diameter arthroscope allowed great mobility in the narrow tendon sheaths; however, the image quality, including resolution, sharpness, and brightness, was lower compared to conventional arthroscopy, requiring the operator to position the scope closer to the target structures. While these differences did not significantly impact the identification of key structures such as the posterior talar process or FHL tendon, surgeons accustomed to conventional arthroscopy may require an adjustment period, and this learning curve should be considered. Furthermore, the true diagnostic and therapeutic value of 1.9-mm-diameter operative tendoscopy cannot be derived from such a limited cadaveric setting. In clinical practice, visualization might be hampered because saline inflow is limited by the narrow scope and sheath, which could cause blood, debris, synovial tissue, and local pathology obscure vision. Future studies of effectiveness of needle arthroscopy in clinical settings are needed.

Conclusions

In the six cadaveric specimens without known pathology, we found that the 1.9-mm-diameter needle arthroscopy using either posterolateral and posteromedial portals provided visualization of significant structures involved in PAIS and FHL tendon disorders.