Abstract
Background:
Endoscopic flexor hallucis longus (FHL) tendon transfer addresses chronic Achilles tendon ruptures. Traditional percutaneous plantar incisions gain FHL length but risk neurovascular injury. This study evaluated the feasibility of an all-inside endoscopic FHL zone 1 tenotomy and transfer technique, which avoids plantar incisions by using zone 1 of the FHL tendon. We hypothesized that this technique would (1) provide sufficient tendon length, (2) avoid neurovascular damage, and (3) securely anchor the FHL tendon without tunnel blowout.
Methods:
Ten cadaveric all-inside endoscopic zone 1 FHL tendon transfers were performed without fluoroscopic guidance. Specimens were independently assessed post hoc by 2 board-certified orthopaedic foot and ankle surgeons. Tendon length, anchor placement, tunnel integrity, and neurovascular bundle preservation were analyzed.
Results:
No neurovascular injuries or calcaneal tunnel blowouts occurred. Mean FHL tendon length for tunnel fixation was 17.5 (3.7) mm; 2 specimens had FiberLoop suture visible above the biotenodesis screw.
Conclusion:
Our study supports the feasibility, reproducibility and preliminary safety of an all-inside FHL zone 1 tenotomy and endoscopic transfer. These cadaveric findings support further clinical investigation of this technique for Achilles rupture repair.
Clinical Relevance:
This cadaveric study provides anatomical evidence that an all-inside endoscopic zone 1 FHL tendon transfer can be performed without neurovascular injury or tunnel blowout, offering preliminary support for future clinical investigation of this technique as a minimally invasive approach to Achilles tendon repair.
Introduction
Flexor hallucis longus tendon (FHL) transfer has become an increasingly used operative technique to address chronic and acute Achilles tendon ruptures.1-6 Indeed, FHL tendon transfer has demonstrated cost-effectiveness over both nonoperative and alternative surgical techniques for the management of Achilles tendon ruptures relative to both quality-adjusted life years and health-related quality of life metrics.4,6 Chronic Achilles tendon ruptures can result in a loss of plantarflexion strength and stability as well as diminished overall athletic capacity. These defects are often treated with a large open reconstruction and shortening, which are associated with high rates of complication, including large scar, calf atrophy, and reduced tendon strength when using V-Y advancement tendon flaps. 7 Despite these complications, the FHL serves as an attractive adjunct to the Achilles repair because of its similar force vector, in-phase neuromuscular control, and potentially additional vascularity from the posterior tibial artery. 8 The FHL also has the ability to hypertrophy 20.2% to 52% (based on MRI post-transfer) over several months providing additional strength and proprioception.9,10 However, although FHL tendon transfers offer an attractive repair construct with 93% success rate, there exist multiple techniques and harvest site candidates for conducting this reconstruction.4,5
Endoscopic minimally invasive approaches assisted by a percutaneous incision have been introduced to perform FHL tenotomy and transfer procedures with reduced operative trauma and wound healing complications that are often associated with open techniques.7,11-13 The FHL tendon is categorized into 3 zones, the first being just posterior to the ankle at the level of the subtalar joint, the second is from below the sustentaculum tali to the knot of Henry, and the third is from the knot of Henry to its insertion into the phalanges. 14 Although there are varying techniques among physicians, minimally invasive FHL tendon transfer is often accompanied with an arthroscope for visualization, and a percutaneous incision to perform the tenotomy. Each incision carries a risk of damage to the surrounding neurovascular structures, with the 2-incision method revealing a nerve injury rate of 33% in a cadaveric study. 15 Zone 2 harvest of the FHL yields an increased length of graft, but in addition to the incision for the arthroscope, 2 incisions are required, a medial midfoot incision to harvest the FHL and a posteromedial hindfoot incision on the ankle to anchor the FHL, increasing the risk of neurovascular damage.16,17 In addition to the increased risk of neurovascular damage from a medial plantar incision, screw fixation techniques are also associated with a risk of tunnel blowout, or unintentional violation of the calcaneal cortex along the drill tunnel dimensions.11,17 Accordingly, surgical technique that shifts the harvest area from zone 2 to zone 1 while maintaining structural integrity of the transfer construct may reduce risk of neurovascular damage while removing the need for one of the incisions, although cadaveric validation of this approach is limited.
Because of the proximal location of zone 1 relative to zones 2 and 3, using a zone 1 tenotomy limits the length of the FHL tendon harvested when compared to zone 2 but precludes the need for a second medial plantar incision by allowing the FHL to be harvested from the same hindfoot incision used for screw fixation. However, literature examining endoscopic zone 1 FHL tendon transfer is sparse. Previously, our team reported the use of an all-inside endoscopic FHL tendon zone 1 approach that required no fluoroscopy because of arthroscopic targeted tunnel placement.18,19 The goal of this study was to investigate the outcomes of an all-inside endoscopic FHL tendon zone 1 tenotomy and transfer. Our study aimed to validate the feasibility and safety of a zone 1 tenotomy and its ability to provide sufficient tendon length to an arthroscopically targeted insertion site. We hypothesized that this all-inside endoscopic FHL zone 1 tenotomy and transfer is reproduceable with consistent harvesting methods.
Methods
The all-inside endoscopic FHL zone 1 tenotomy and transfer surgical procedure was performed in the prone position on 10 lower extremity (midfemur to toe) cadaveric specimens (age: 52.3 ± 14.8 years, sex: 8 male, 2 female; leg: 7 right, 3 left). The surgical procedures were all performed by a fellowship-trained orthopaedic foot and ankle surgeon in order to standardize outcomes. Using the knick-and-spread technique, posteromedial and posterolateral portals were established. A shaver was used to debride soft tissue surrounding the calcaneus and FHL to allow for anatomic identification. Dorsiflexion and plantarflexion of the great toe were also used to help identify the FHL tendon. Anatomic structures and landmarks of the posterior ankle were observed and identified with a low-flow flexible, 1.9-mm NanoNeedle Scope (Arthrex, Inc). Throughout the procedure, an arthroscopic recording of the tenotomy was performed to visualize the safety and preservation of surrounding anatomic structures.
While in the neutral position, using an arthroscopic suture passer (Arthrex CPR Viper), a sliding suture knot was placed in the FHL tendon adjacent to the posteromedial aspect of the subtalar joint to establish a baseline position of measurement, and create a means of tension (Figure 1A). The ankle was placed in ~25° of plantarflexion depending on the allowances of the ankle, which was verified with a digital goniometer. The great toe was placed in maximum plantarflexion, and the FHL tendon was tensioned using the sliding suture knot previously noted to keep the FHL tendon taut while performing the tenotomy.
(A) Arthroscopic image of the FHL tendon with sliding suture knot prior to tenotomy. (B) Measurement of the FHL tendon pulled out of the anteromedial portal while in ~25° of plantarflexion. (C) Measurement of the FHL tendon prepared with FiberLoop while maintaining ~25° of plantarflexion. (D) Measurement of the FHL tendon diameter using a tendon sizer template. Note that anatomical measurements were taken after FHL tendon transfer was completed. Specimens were openly dissected to attain the most accurate anatomical measurements as indicated by the open images presented. FHL, flexor hallucis longus.
The tenotomy was performed using arthroscopic, small-joint scissors. The tendon was then pulled out through the posteromedial portal using the previously stated sliding suture knot. The distance from the suture knot to the distal cut end of the tendon was measured while using ~25° of plantarflexion (Figure 1B). The diameter of the FHL was measured at the approximate midpoint between the sliding suture knot and the distal end of the tenotomy using a tendon sizer template (Figure 1D). The distal end of the tendon was prepared utilizing a No. 2 nonabsorbable FiberLoop, No. 2 FiberWire made of polyester blend (Arthrex, Inc). The distance from the distal cut tendon to the most proximal edge of the FiberLoop was again measured with calipers for each sample to ensure a constant distance from the distal end of the FHL stump to the FiberLoop suture start (Figure 1C). Using a shaver, the dorsal aspect of the calcaneus was then prepared immediately anterior to the insertion of the Achilles to facilitate tendon placement. The anterior aspect of the Achilles insertion was then arthroscopically visualized, and the arthroscope was then advanced through the posteromedial portal into the desired location and held against the bony cortex. The arthroscope was then removed from its associated sheath and a 2.4-mm beeth pin was placed down the sheath and across the calcaneus from a posteromedial to a plantar lateral position. The sheath was then removed, and a 6.5-mm cannulated reamer was inserted over the pin to a depth of approximately 4 cm, under direct arthroscopic visualization. The arthroscope was then reinserted into the portal and a 360° rotational arthroscopic recording was achieved to visualize and verify if there was any blowout, or breaching of the cortical walls, within the calcaneal tunnel.
With the foot and ankle held in ~25° of plantarflexion, the beeth pin was used to shuttle the suture out of the plantar aspect of the foot and pull the FHL tendon into the reamed tunnel. While under arthroscopic visualization, a 6.25 mm biotenodesis screw (Arthrex) was inserted into the calcaneal tunnel adjacent to the tendon and was advanced into the tunnel until slightly recessed within the calcaneus. All of these actions were completed under arthroscopic visualization, which differentiates the current methodology from the approach previously published by Vernois et al 20 that implemented fluoroscopic guidance.
After the arthroscopic surgical procedure, specimen dissections were performed by 2 board-certified orthopaedic foot and ankle surgeons separate from the surgeon who performed the all-inside endoscopic FHL zone 1 tenotomy and transfer. Each observing surgeon took individual measurements that were compiled and averaged. An open hindfoot specimen dissection was performed to visualize and record any insult, such as laceration or tearing, to the neurovascular bundle located medial to the FHL tendon. At that time, it was also verified that each prepared FHL tendon was within the tunnel and that the screw was recessed below the cortical surface. Any FiberLoops that were visible outside the tunnel were noted and recorded, along with the distance of visible sutures. The distance between the calcaneus interface and the deep posterior (crural) fascia, between undissected neurovascular bundle and the drill tunnel, the bone and the FHL muscle, the Achilles tendon and the calcaneus drill tunnel, the width of the calcaneus, as well as the distance from the medial wall and lateral drill tunnel walls to their respective ends of the calcaneus were measured using both digital and manual graduated calipers. Utilizing video and images taken with the arthroscope, any drill tunnel blowouts that occurred during the procedure were also verified and recorded. All postoperative analysis was performed completely blinded from the operative surgeon to avoid learned behavior.
Similar specimen counts have previously been used in recent literature to validate lack of neurovascular damage surrounding surgical techniques in both the shoulder and foot.21,22 JMP software (version 18 Pro Student Edition, SAS institute, Cary, NC, USA) used for the statistical analysis. Descriptive statistics including means and standard deviations were used to characterize the previously discussed intraoperative and postoperative anatomical outcome measures.
Results
During the surgery, the average distance from the FHL tendon cut end to the reference marker stitch used to pull the FHL tendon out of the portal was 16.7 (3.4) mm, with measurements ranging from 11.0 to 23.0 mm. The mean distance of the FHL tendon cut end to the proximal FiberLoop was 17.5 (3.7) mm, with measurements ranging from 15.0 to 27.2 mm. The mean FHL tendon diameter was 6.2 (0.4) mm. Mean surgery time was 39.7(9.5) minutes. No injury or damage to the surrounding neurovascular structures was documented in any specimen (n = 10). All tendons were within the tunnel and the screws were recessed below the cortical surface. Two specimens had FHL tendons with FiberLoops visible. Further, on reviewing the intraoperative videos of the drill tunnels, no drill tunnel blowouts were noted.
Mean distance between the calcaneus interface to the deep posterior(crural) fascia, the neurovascular bundle to the drill tunnel, the bone to the FHL tendon muscle, Achilles Tendon to the calcaneus drill tunnel, the medial calcaneus edge to the medial drill tunnel wall location, and the lateral calcaneus edge to the lateral calcaneus wall location were 16.0 (3.7) mm (mean difference between raters 0.9 mm), 8.4 (3.3) mm (mean difference 0.9 mm), 11.6 (8.6) mm (mean difference 1.2 mm), 4.1 (1.5) mm (mean difference 1.2 mm), 4.2 (1.0) mm (mean difference between raters 0.9 mm), and 22.1 (2.6) mm (mean difference 2.4 mm), respectively. The mean distance of visible FiberLoop from the screw was 0.9 (2.1) mm (mean difference 0.2 mm). The mean width of the calcaneus measured was 28.5 (2.4) mm (mean difference 1.4 mm). The measures and their standard deviations are all displayed in Table 1.
Means and SDs of the Intraoperative and Postoperative Measure.
Abbreviation: FHLT, flexor hallucis longus tendon.
Discussion
Our study supports the feasibility, reproducibility, and preliminary safety of an all-inside FHL zone 1 tenotomy and endoscopic transfer. Similar studies have examined endoscopic FHL tendon transfers, but involving zone 2 transfers, which incorporate an additional medial plantar incision and an associated increased risk of damage to the medial plantar nerve, jeopardizing patient postsurgical sensation and strength. 23 The FHL tendon has been measured to be as close as 2 mm from the surrounding neurovascular structures. 23 Given this extreme proximity of the FHL tendon to the neurovascular structures, surgical complications when harvesting the tendon can occur very easily especially while utilizing additional percutaneous incisions. Zone 1 transfer has seen success in open FHL tendon transfer techniques despite the shorter length of FHL available compared to zone 2 tenotomy. 24 This success is exemplified in a case series of 56 patients with zone 1 transfer where, at 18 months postsurgery, patients achieved 95.1% of the dynamometry range and 99.5% of the plantar flexion strength of their contralateral limb without neurovascular deficits. 4
Based on the measurements of the FHL tendons from the FiberLoop, or the tendon to be introduced to the tunnel, zone 1 FHL tenotomy in this study was able to meet the 15 mm minimum proposed by Vega et al, 17 with measurements ranging from 15.0 to 27.2 mm. This suggests that using a sliding suture-knot as traction stitch with tension applied and plantarflexing the foot approximately 25° allowed for adequate length of tendon harvest. However, although all the FHL tendon measurements to be introduced to the tunnel were at least 15.0 mm in this study, it should be noted that 2 of our specimens had FiberLoop visible above the screw, meaning that the entire length of the FiberLoop was not pulled through the tunnel (Figure 2D). This indicates that although our prepared FHL lengths all met the minimum 15 mm required, 2 FHL tendons were not fully buried in the tunnel, potentially leading to a total buried FHL tendon length of less than 15 mm and a subsequent decrease in strength or fixation. Additional study would be relevant to assess the biomechanical pullout strength of these constructs at time zero after surgery.
(A) Arthroscopic image of the tunnel demonstrating absence of tunnel blowout or violation of tunnel geometry. (B) A located and dissected neurovascular bundle following all-inside endoscopic FHL zone 1 tenotomy and transfer. (C) Arthroscopic image providing visualization while the tenodesis screw was placed into the drill tunnel in the calcaneus. (D) Arthroscopic image of the screw placed in the drill tunnel. It can be noted that some of the FiberLoop is visible above the biotenodesis screw.
Accurately placed and structurally sound calcaneal tunnels are very important for adequate fixation and restoration of plantar flexion strength and anatomic alignment of the foot. Tunnel blowout would be suggestive of less anchor stability and therefore potentially putting the patient at risk of complication.11,25 This study used a 360° arthroscopic visualization of our tunnels and observed zero instances of tunnel blowout. Furthermore, although our measurements of the location suggest the medial side of the calcaneus was heavily favored, no fluoroscopy was used. The absence of fluoroscopic guidance in this study reduced radiation exposure to the surgeon and specimens; however, whether arthroscopic tunnel targeting achieves accuracy equivalent to fluoroscopic guidance was not directly assessed and warrants further study.
The mean distance of 11.6 (8.6) mm from the FHL muscle belly to the calcaneus bone suggests that, in some specimens, the muscle belly may lie in proximity to the watershed zone of the Achilles tendon—an area 3-6 cm above the calcaneus where as many as 97% of Achilles tendon ruptures occur 26 —although the large SD indicates considerable variability across specimens and proximity cannot be assumed in all cases. Notably, an area of the Achilles tendon 3 to 6 cm above the calcaneus where as many as 97% of Achilles tendon ruptures occur. 26
Our study had multiple limitations. As a cadaveric study, it was not possible to examine postoperative patient outcomes or healing, future work should seek to quantify this via imaging and longitudinal patient-reported outcomes. Another limitation to this design is that a single surgeon performed the technique on all our specimens, which inherently limits the generalizability of this study. Additional study across multiple surgical practitioners at various levels of training expertise as well as longitudinal follow-ups on patient functionality following this technique would be useful supplemental validation of this technique. Generalizing these results to a cohort of multiple surgeons would further strengthen the validity of the current results. Also, all instruments and implants used were manufactured by the study sponsor (Arthrex, Inc); whether equivalent results would be obtained with alternative equipment has not been assessed. An additional limitation of this study is the lack of a control comparison. Contralateral limbs performed under traditional zone 2 harvest would have allowed for statistical comparison of anatomical proximity measurements between the 2 techniques. Furthermore, future work could consider fluoroscopy-guided zone 1 harvest to see if additional distance from the neurovascular bundle is gained.
Conclusion
This cadaveric study revealed that the all-inside endoscopic FHL zone 1 tenotomy and transfer can be performed without violation of the neurovascular structures; however, further studies with larger samples and clinical evaluations are necessary. Adequate tendon length was accomplished with 25° of plantar flexion and the use of a traction stitch for additional tension. Arthroscopic targeted tunnel placement resulted in tunnels without blowout or iatrogenic Achilles tendon injury, without the use of fluoroscopy. However, 2 specimens had FiberLoop suture visible above the screw, indicating a narrow margin in some cases. These anatomic results support future studies to evaluate patient outcomes using the all-inside endoscopic FHL zone 1 tenotomy and transfer technique.
Supplemental Material
sj-pdf-1-fao-10.1177_24730114261442547 – Supplemental material for All-Inside Endoscopic Zone 1 Flexor Hallucis Longus Transfer: A Cadaveric Feasibility Study
Supplemental material, sj-pdf-1-fao-10.1177_24730114261442547 for All-Inside Endoscopic Zone 1 Flexor Hallucis Longus Transfer: A Cadaveric Feasibility Study by Kevin D. Martin, Yi Wei, Adam G. F. Smith, Franco Piscitani, Luke M. Martin, Brian Steginsky, Brian Tscholl and Nathaniel A. Bates in Foot & Ankle Orthopaedics
Footnotes
Acknowledgements
The investigators would like to acknowledge the Surgical Skills Laboratory at the Ohio State University.
Ethical Considerations
Ethical approval for this study was waived by the institutional review board of the Ohio State University as the study was performed on ethically sourced cadavers obtained from Anatomy Gifts Registry, Hanover, MD, USA.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Funding for this investigation was provided through a grant from Arthrex, Inc (N.A.B.), to the Ohio State University Wexner Medical Center. The principal investigator (N.A.B.) does not have any direct or intrinsic conflicts of interest or relationships with Arthrex, Inc. Kevin D. Martin, DO, is a paid consultant for Arthrex, Inc, but did not serve as one of the board-certified foot and ankle surgeon assessors. The funders had no role in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Declaration of Conflicting Interests
The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Kevin D. Martin, DO, is a paid consultant for Arthrex, Inc. Disclosure forms for all authors are available online.
References
Supplementary Material
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