For this study, twenty-one NRs and twenty TRs were performed on Thiel-embalmed cadaveric specimens and compared regarding ultrasound visibility, outcome, and damage to adjacent structures. The high success rate and low rate of damage to adjacent structures demonstrated that both ultrasound-guided NR and ultrasound-guided TR are effective and safe techniques for the release of the A1P in an anatomical specimen model. Although the time for each procedure was not exactly documented, due to the nature of the intervention, NR was the less time-consuming technique and required less equipment than TR. Therefore, it is the potentially cheaper approach. We observed a steep learning curve for both approaches. We were able to decrease the time required for NR from about ten minutes (from the first placement of the probe to the finished dissection) at the first interventions to approximately four minutes in the final ones. The time required for TR decreased form about twenty- minutes to approximately seven minutes respectively. This time does not include the application of local anesthetic.

It should be noted that scores were given very precisely and only a perfect result led to a full score. Therefore, a partial transection might have also led to a sufficient decompression of the flexor tendon and symptom relief in patients. This was demonstrated by Lapègue et al. [9], whose NR did not achieve full transection of the A1P in any cadaveric specimen, but the same technique led to complete symptom relief in 96.8% of patients over six months. The same applies to the score for damage to adjacent structures. Very slight scores on tendons were already graded as a score of two. Similar findings have been previously described in several studies [13, 14, 26,27,28,29,30], but it appears to have no effects on clinical outcomes.[13, 29, 30]. As the lesions on the tendons occurred more often at TR and the morphology matches the thread, we believe that the thread scratched the surface of the tendons, probably due to insufficient hydrodissection. This effect can probably be reduced by more precise or additional hydrodissection directly before the cutting step. Furthermore, shortening the distance between the entry and exit point by using a less distal entry point may lead to shorter/less which may decrease the friction of the thread on the tendons. However, it would also probably increase the risk of incomplete transection. Considering the clinical insignificance of the superficial tendon lesions, we would rather favor them over the risk of incomplete transection. Notable were the first two NR cases, as they were the only cases that received a outcome score of one. This is probably a result of bias due to the sequence of interventions, as these two interventions were the first ones we performed. Incomplete transections occurred in some cases with both techniques. These instances were predominantly linked to diminished ultrasound visibility (score of 1 or 2), which hindered the intervention process. Specifically, delineating the borders of A1P was more challenging under these conditions, resulting in occasional difficulty visualizing clear borders and potentially leading to misplacement of needle insertions.

Our results for NRs align with previously published literature or exceed published data. Hoang et al. [13] achieved a full transection rate of 80% and a partial transection rate of 20% with a similar ultrasound-guided needle approach. They had one incidence of arterial damage, and longitudinal scoring of the flexor tendon in 23%. A study by Smith et al. [14] found a worse success rate of 32% for a needle technique. Slight damage to the flexor tendon occurred in one case of 25 cases. They reported no damage to neurovascular structures or the A2-pulley. Similar results were published by Yang et al. [15] in 2022, who reported a complete release in 36.7% of 30 cases and a partial release in 63.3% with minor scratches of flexor tendons in 50% of cases and tendon lacerations in 10%. Lapègue et al. [9] performed NR on 10 specimens in which the A1P was never fully transected, but there was no damage to any adjacent structure. Paulius et al. [26] reported a transection rate of the A1P of 15 of 18 and tendon lacerations in three of 18 cases in 2008. Compared to these studies, we achieved full transection in 71.5%, partial transection in 19%, and no transection in 9.5%.

For TR, the published literature is scarce and there is only one cadaveric [21] and one clinical [22] study thus far. Both were conducted by Guo et al., who developed this novel technique. They reported complete transection and no damage to neurovascular structures and the A2 pulley in all 18 cadaveric cases [21]. These results are slightly better than our data (85% success rate and very slight scoring on the flexor tendon in 25% of cases). However, these differences may arise from differences in the reporting of findings.

Our data suggests that the novel TR technique is on par with the clinically established NR with regard to effectiveness and safety and aligns well with previously published literature. It should also be noted that both approaches are probably even more effective and safer in patients, as ultrasound visibility is usually better compared to that in anatomical specimens. In addition, color Doppler can visualize the perfusion of vessels, enabling better navigation of the needle or the thread. Compared to classical open surgery, both techniques used in this study are less invasive and have the potential to decrease patient distress peri- and postsurgical. However, further clinical and comparative studies are required to prove this assumption.

Aside from advances in surgical release techniques, ultrasound systems are also constantly developing.

While we generally used a clinically established ultrasound frequency of 18–22 MHz, ultra-high frequency probes up to 70 MHz are being evaluated for musculoskeletal imaging [31]. At these ultra-high frequencies, very superficial subcutaneous structures, such as the A1P and small neurovascular branches could be visualized at a much higher resolution and precision. Consequently, we are almost certain that ultrasonic guidance will be more precise and easier to execute at ultra-high frequencies, resulting in safer and more effective intervention results.

Our study faces several limitations. First, due to the nature of cadaveric studies, we do not know how our results might translate into clinical practice, as the specimens did not display pathologic A1Ps. However, the visibility of the anatomical structures is better in living patients. Therefore, efficacy and safety should be even better in clinical practice. Second, the limited availability of anatomic specimens restricted us from performing a higher number of interventions and from comparisons with classic open surgery, which would have elevated the level of evidence. Third, we do not know the exact age and prior conditions of specimens due to privacy protection of the body donors. Therefore, we could not exclude specimens with non-assessable, potentially confounding comorbidities, such as chronic conditions during the lifespan. Fourth, even though Thiel-embalmed anatomical specimens have proven to be a well-suited model for ultrasound-guided interventions [24, 25], results in patients may differ. It should also be noted that we did not simulate the preoperative application of local anesthetic at the entry and exit points. Additionally, we also used saline for hydrodissection instead of saline with 1% lidocaine, as suggested by Guo et al. [20] for in vivo interventions.

In conclusion, both, ultrasound-guided needle release and ultrasound-guided thread release are effective and safe techniques for the release of the A1 pulley in the anatomical specimen model. The results align with previously published literature. Our study showed no significant differences in terms of outcome and damage to surrounding structures. It is important to note, however, that the applicability of these findings to clinical practice should be interpreted with caution, as the results in living patients may differ from those in a cadaveric model.