Nanoscope-assisted release of the lacertus fibrosus: A cadaveric study of feasibility, visualization, and anatomical safety

Abstract

BACKGROUND

Compression of the proximal median nerve beneath the lacertus fibrosus is an increasingly recognized cause of activity-related forearm weakness and reduced grip endurance. Surgical decompression is typically performed using open or mini-open techniques. The anatomical feasibility and procedural safety of a minimally invasive nanoscope-assisted release have not been systematically evaluated.

AIM

To evaluate the anatomical feasibility, visualization quality, and procedural safety of nanoscope-assisted lacertus fibrosus release in a cadaveric model, with systematic open dissection used to confirm completeness of release and preservation of adjacent neurovascular structures.

METHODS

Twenty fresh-frozen cadaveric upper limbs underwent lacertus fibrosus release using a 1.9-mm 0° nanoscope system. Feasibility, operative time, and visualization quality were assessed. Visualization of the median nerve, lacertus fibrosus, and brachial artery was independently graded by two surgeons using a 5-point Likert scale. Systematic open dissection was subsequently performed to confirm completeness of release and assess for iatrogenic neurovascular injury.

RESULTS

Nanoscope-assisted release was successfully completed in all 20 specimens without conversion to an open procedure. Mean operative time was 6.9 ± 0.8 minutes. Mean visualization scores were 4.8 ± 0.3 for the median nerve, 4.9 ± 0.1 for the lacertus fibrosus, and 4.6 ± 0.4 for the brachial artery, with good interobserver agreement (intraclass correlation coefficient = 0.88). Open validation confirmed complete release in all specimens, with no residual constricting fibers or visible iatrogenic neurovascular injury.

CONCLUSION

Nanoscope-assisted lacertus fibrosus release was anatomically feasible and demonstrated a favorable procedural safety profile in this cadaveric model. Systematic open validation confirmed complete release and preservation of adjacent neurovascular structures in all specimens. These findings provide an anatomical foundation for prospective clinical evaluation.

Key Words: Lacertus fibrosus; Proximal median nerve compression; Nanoscope; Needle arthroscopy; Cadaveric study; Minimally invasive decompression

Core Tip: Compression of the median nerve beneath the lacertus fibrosus is an increasingly recognized but frequently underdiagnosed cause of proximal median nerve dysfunction. This cadaveric study demonstrates the anatomical feasibility of Nanoscope-assisted lacertus fibrosus release under direct visualization. Systematic post-procedural open dissection confirmed complete release and preservation of adjacent neurovascular structures in all specimens. By combining minimally invasive access with continuous visualization of critical anatomy, the Nanoscope-assisted approach represents a technically viable strategy for proximal median nerve decompression and provides an anatomical foundation for prospective clinical evaluation.

INTRODUCTION

Proximal median nerve compression at the elbow represents a heterogeneous group of entrapment neuropathies traditionally grouped under the concept of pronator teres syndrome. Although early descriptions emphasized compression between the humeral and ulnar heads of the pronator teres muscle, subsequent anatomical and clinical studies have demonstrated that several structures in the proximal forearm may contribute to median nerve compression, including the lacertus fibrosus, ligament of Struthers, and fibrous bands associated with muscular variations[1-5].

Among these potential compression sites, the lacertus fibrosus, also commonly referred to in anatomical terminology as the bicipital aponeurosis, has gained increasing attention as a distinct source of proximal median nerve dysfunction. The lacertus fibrosus is a broad fibrous expansion arising from the distal biceps tendon and extending medially across the antecubital region to blend with the deep fascia of the forearm. In this region, it forms part of the superficial boundary of the proximal median nerve passage, often referred to clinically as the lacertus tunnel, where the median nerve courses in close proximity to the brachial artery and adjacent muscular and fascial structures. This confined anatomical relationship provides a potential site of dynamic nerve compression and creates specific technical considerations during surgical decompression.

Unlike more static compressive neuropathies, lacertus-related compression is often dynamic and may be exacerbated by resisted elbow flexion and forearm supination. Clinically, patients frequently present with exertional forearm fatigue, reduced grip endurance, and subtle motor weakness rather than predominantly sensory symptoms, which may contribute to delayed or missed diagnosis[2,3,6,7]. The rationale for surgical release is to divide the constraining fibrous structure and restore unrestricted mobility of the median nerve within this proximal compression site. However, the close relationship between the lacertus fibrosus, median nerve, and adjacent vascular structures creates a potential risk of incomplete decompression or iatrogenic neurovascular injury, particularly when anatomical variation is present. Possible complications of decompression in this region include injury to the median nerve or its branches, vascular injury, incomplete release, and persistent or recurrent symptoms[8].

The diagnosis of proximal median nerve compression remains challenging because of symptom overlap with carpal tunnel syndrome and anterior interosseous nerve syndrome[9]. In addition, electrophysiological studies may be inconclusive, particularly in early-stage or activity-dependent compression syndromes[6,10]. Anatomical variability in the pronator teres and surrounding fibrous structures further complicates both diagnosis and surgical planning, highlighting the importance of precise anatomical understanding when approaching proximal median nerve decompression[4,5,11,12].

Surgical decompression is indicated in patients with persistent or function-limiting symptoms refractory to conservative management. Conventional open and mini-open techniques provide reliable decompression and direct anatomical exposure but require varying degrees of soft-tissue dissection[3,13]. Ultrasound-guided percutaneous approaches have been proposed as less invasive alternatives; however, they rely on image-based rather than direct endoscopic visualization and may be technically demanding in the presence of anatomical variation or unclear neurovascular relationships[14].

Needle arthroscopy, also referred to as nanoscopic surgery, uses small-bore arthroscopic systems to provide direct magnified visualization through minimally invasive access portals. The 1.9-mm 0° nanoscope system permits real-time visualization within narrow anatomical corridors and has expanded the potential role of minimally invasive endoscopic techniques beyond conventional joint inspection. Potential advantages include smaller access incisions, reduced soft-tissue disruption, and continuous direct visualization of critical structures. Conversely, limitations include a restricted field of view, constrained instrument maneuverability, dependence on precise portal positioning, equipment-related costs, and a potential procedural learning curve. In upper-limb surgery, nanoscopic techniques have been increasingly explored for diagnostic and therapeutic procedures involving the shoulder, elbow, wrist, and peri-tendinous structures, including distal biceps tendon repair[15].

Application of this technology to lacertus fibrosus release may permit controlled decompression through a limited surgical corridor while maintaining direct visualization of the median nerve and adjacent vascular structures. However, to our knowledge, the anatomical feasibility, visualization quality, and safety of nanoscope-assisted lacertus fibrosus release have not been systematically evaluated with mandatory post-procedural open anatomical validation. Therefore, the aim of this cadaveric study was to assess the feasibility, visualization quality, and anatomical safety of nanoscope-assisted lacertus fibrosus release, with systematic open dissection performed after each procedure to confirm completeness of decompression and preservation of adjacent neurovascular structures.

MATERIALS AND METHODS

Specimens

Twenty fresh-frozen human upper limbs were obtained through an institutional anatomical donation program. Specimens were eligible for inclusion if there was no evidence of previous elbow or forearm surgery, gross deformity, visible surgical scarring, or macroscopic disruption of the relevant neurovascular anatomy. The study included 12 male and 8 female specimens, with a mean specimen age of 72 years (range, 61-89 years). All specimens were thawed at room temperature for 24 hours before the procedure.

Each upper limb was treated as an individual anatomical specimen. All specimens were positioned supine with the elbow slightly extended and the forearm in full supination. Surface landmarks, including the antecubital crease and distal biceps tendon, were identified and marked before portal placement.

Surgical equipment

Nanoscope-assisted decompression was performed using a 1.9-mm 0° needle arthroscopy system (NanoScope™, Arthrex, Naples, FL, United States) in combination with a nanoneedle portal cannula and a Centerline™ cutting device. Standard microsurgical instruments were used for blunt dissection and subsequent open anatomical validation.

Surgical technique

A standardized surgical protocol was applied to all specimens (Table 1). A 1-cm transverse skin incision was made 2-3 cm distal to the antecubital crease. Blunt dissection was carried down to the deep fascia to establish a limited working corridor while minimizing disruption of the surrounding soft tissues. The nanoscope was then introduced through the nanoneedle portal, and further advancement was performed under direct visualization.

Table 1 Standardized NanoScope-assisted lacertus fibrosus release protocol.

Step	Description
1	Positioning of the specimen in supine position with the elbow slightly extended and the forearm in full supination
2	Identification and marking of surface landmarks, including the antecubital crease and distal biceps tendon
3	Creation of a 1-cm transverse skin incision 2-3 cm distal to the antecubital crease
4	Blunt dissection to the deep fascia to establish a limited working corridor
5	Introduction of the 19-mm 0° nanoscope through the nanoneedle portal
6	Nanoscopic identification of the lacertus fibrosus
7	Visualization of the median nerve and adjacent brachial artery
8	Introduction of the cutting device superficial to the median nerve under direct visualization
9	Controlled division of the lacertus fibrosus in a radial-to-ulnar direction
10	Nanoscopic confirmation of completion of the release

The lacertus fibrosus (bicipital aponeurosis) was identified as the fibrous expansion arising from the distal biceps tendon and extending medially across the proximal forearm. Its anatomical relationship to the underlying median nerve and adjacent brachial artery was established before release. The median nerve was visualized beneath the lacertus fibrosus and followed proximally and distally within the accessible field to confirm anatomical orientation and the relationship between the relevant neurovascular structures (Figure 1). Key procedural steps, including identification of the lacertus fibrosus, visualization of the median nerve, introduction of the cutting device, and completion of the release, are demonstrated in Figure 2.

Figure 1 Schematic illustration of the proximal forearm anatomy demonstrating the anatomical relationship between the lacertus fibrosus, median nerve, brachial artery, and pronator teres. The lacertus fibrosus is shown crossing superficially over the median nerve and adjacent vascular structures. BT: Biceps tendon; PT: Pronator teres; MN: Median nerve; BA: Brachial artery; LF: Lacertus fibrosus.

Figure 2 Sequential nanoscopic views during nanoscope-assisted lacertus fibrosus release. A: Identification of the lacertus fibrosus and underlying median nerve; B: Development of the working corridor with visualization of the lacertus fibrosus and median nerve; C: Introduction of the cutting device under direct visualization; D: Final nanoscopic view following controlled division of the lacertus fibrosus, demonstrating the underlying median nerve. LF: Lacertus fibrosus; MN: Median nerve; CD: Cutting device.

After identification of the relevant anatomical structures, the Centerline™ cutting device was introduced superficial to the median nerve. Under continuous direct nanoscope visualization, the lacertus fibrosus was divided in a controlled radial-to-ulnar direction. Nanoscopic release was considered complete when all visible fibers of the lacertus fibrosus had been divided and no residual constricting fibers were identified within the visualized operative field (Figure 3).

Figure 3 Final nanoscopic view following completion of lacertus fibrosus release, demonstrating the divided lacertus fibrosus and preserved continuity of the underlying median nerve. LF: Lacertus fibrosus; MN: Median nerve.

Immediately after completion of the nanoscope-assisted procedure, systematic open anatomical dissection was performed in every specimen. The lacertus fibrosus, median nerve, and adjacent vascular structures were exposed and inspected directly. Open validation was used to determine whether any residual intact fibers of the lacertus fibrosus remained and to assess the median nerve and adjacent vascular structures for evidence of iatrogenic injury (Figure 4).

Figure 4 Post-procedural open anatomical validation following nanoscope-assisted lacertus fibrosus release. The released lacertus fibrosus, decompressed median nerve, and preserved brachial artery are identified by orange arrows, confirming complete release and preservation of adjacent neurovascular structures. LF: Lacertus fibrosus; BA: Brachial artery; MN: Median nerve.

Outcome measures

Four predefined outcome domains were assessed: Feasibility was defined as successful completion of the nanoscope-assisted lacertus fibrosus release without conversion to an open procedure.

Procedural efficiency was assessed using operative time, measured from skin incision to nanoscopic confirmation of completion of the release.

Visualization quality was independently assessed by two fellowship-trained upper-extremity surgeons who were not involved in the dissection procedure. Visualization of the median nerve, lacertus fibrosus, and brachial artery was graded using a predefined 5-point Likert scale: 1 = poor visualization, 2 = limited visualization, 3 = acceptable visualization, 4 = good visualization, and 5 = excellent visualization. The complete scoring criteria are presented in Table 2.

Table 2 Visualization quality scores and interobserver agreement.

Structure evaluated	Visualization score, mean ± SD
Median nerve	4.8 ± 0.3
Lacertus fibrosus	4.9 ± 0.1
Brachial artery	4.6 ± 0.4

Anatomical safety and completeness of release were assessed by systematic post-procedural open dissection. Complete release was defined as full division of the lacertus fibrosus without residual intact constricting fibers. Anatomical safety was defined as preservation of the continuity of the median nerve and absence of visible iatrogenic injury to adjacent vascular structures.

Statistical analysis

Descriptive statistics were used to summarize procedural and anatomical outcomes. Continuous variables are presented as mean ± SD and range, where appropriate. Interobserver agreement for visualization quality scores was assessed using a two-way random-effects intraclass correlation coefficient (ICC) for consistency. ICC values were interpreted as follows: < 0.50, poor agreement; 0.50-0.75, moderate agreement; 0.75-0.90, good agreement; and > 0.90, excellent agreement. Statistical analyses were performed using SPSS Statistics, version 27 (IBM Corp., Armonk, NY, United States).

RESULTS

Feasibility

Nanoscope-assisted lacertus fibrosus release was successfully completed in all 20 specimens, corresponding to a feasibility rate of 100%. No conversion to an open procedure was required, and all specimens were included in the final analysis.

Procedural efficiency

The mean operative time was 6.9 ± 0.8 minutes, with a range of 5.8 minutes to 8.4 minutes.

Visualization quality

Visualization quality scores are summarized in Table 2. Mean scores were high for all evaluated anatomical structures: 4.8 ± 0.3 for the median nerve, 4.9 ± 0.1 for the lacertus fibrosus, and 4.6 ± 0.4 for the brachial artery.

Interobserver agreement for visualization quality was good, with an overall ICC of 0.88 using a two-way random-effects model for consistency.

Anatomical safety and completeness of release

Systematic open dissection performed after the nanoscope-assisted procedure confirmed complete division of the lacertus fibrosus in all 20 specimens (100%). No residual intact constricting fibers were identified.

The median nerve remained intact in all specimens, with no evidence of iatrogenic injury. No visible iatrogenic injury to the brachial artery or other adjacent vascular structures was identified during open anatomical validation.

Post-procedural open anatomical validation is demonstrated in Figure 4, showing the divided lacertus fibrosus and preserved adjacent neurovascular structures.

DISCUSSION

The principal finding of this cadaveric study is that nanoscope-assisted lacertus fibrosus release was technically feasible in all 20 specimens and enabled complete division of the lacertus fibrosus under direct visualization. Systematic post-procedural open dissection confirmed complete release in every specimen and demonstrated preservation of the median nerve and adjacent vascular structures. Visualization quality was consistently high, with good interobserver agreement. Importantly, the present study should be interpreted as an anatomical validation study rather than a comparative clinical effectiveness study. Its purpose was not to demonstrate superiority over established open, mini-open, or ultrasound-guided techniques, but to determine whether a minimally invasive nanoscope-assisted approach could provide adequate visualization, complete release, and preservation of critical neurovascular structures within a confined anatomical region.

Proximal median nerve compression represents a heterogeneous spectrum of entrapment neuropathies historically grouped under pronator teres syndrome[1-3]. Increasing anatomical and clinical evidence, however, suggests that compression beneath the lacertus fibrosus may represent a distinct clinical entity rather than simply a subtype of pronator syndrome[2-5,16-19]. In contrast to distal median nerve entrapment, lacertus-related compression may present predominantly with exertional weakness, reduced grip endurance, forearm fatigue, and motor dysfunction, while conventional electrophysiological findings may be inconclusive[6,10,20]. Clinical studies have reported meaningful improvement following isolated lacertus release in appropriately selected patients. Hagert and Lalonde reported outcomes from 275 minimally invasive decompressions performed over a 10-year period[17], while Ahmad et al[18] described favorable outcomes following lacertus release in a clinical cohort. Additional clinical reports have further supported symptomatic and functional improvement after decompression of the proximal median nerve at the lacertus fibrosus[16,19,21]. These studies address clinical effectiveness, whereas the present investigation addresses a different and preceding question: Whether complete decompression can be achieved through a nanoscopic corridor while maintaining direct visualization and preserving adjacent neurovascular structures.

The anatomical complexity of the proximal median nerve region remains an important surgical consideration. Multiple potential compression sites—including the pronator teres, lacertus fibrosus, ligament of Struthers, fibrous bands, and anatomical muscular variants-may coexist or contribute independently to nerve dysfunction[2-5,22,23]. Previous anatomical studies have demonstrated substantial variability in pronator teres morphology and its relationship to the median nerve[4,11,12], which may influence both clinical presentation and the technical difficulty of decompression. Uncommon causes of proximal median nerve compression, including schwannomas and anomalous fibrous or osseous structures, have also been described[22-25]. Moreover, the close relationship between the median nerve, lacertus fibrosus, and adjacent vascular structures creates specific technical risks during decompression. Eid et al[8] emphasized the potential complications associated with median nerve decompression at the lacertus tunnel, underscoring the importance of precise anatomical orientation in this region. In this context, a technique that permits continuous direct visualization may be advantageous when operating within a narrow and anatomically variable corridor.

Open and mini-open decompression remain established surgical approaches for proximal median nerve release and provide direct exposure of the relevant anatomy[3,13,17,18]. Their principal advantage lies in direct visualization and tactile confirmation of decompression. Less invasive approaches have also been investigated. Of particular relevance, Apard et al[26] evaluated percutaneous ultrasound-guided release of the lacertus fibrosus in a cadaveric model, demonstrating the feasibility of image-guided minimally invasive decompression. That study provides the closest methodological comparison with the present investigation. However, the two approaches differ fundamentally in visualization strategy: Ultrasound guidance provides real-time external image-based visualization, whereas the nanoscope-assisted technique provides direct intraprocedural visualization from within the working corridor. The present study additionally incorporated systematic post-procedural open dissection in every specimen to verify both completeness of release and neurovascular preservation. These differences should not be interpreted as evidence of superiority of one technique over another, because no direct comparative analysis was performed. Rather, they demonstrate distinct minimally invasive strategies that warrant future head-to-head evaluation.

One of the principal concerns regarding minimally invasive decompression is whether limited surgical exposure may compromise completeness of release. This issue is particularly relevant in dynamic compression beneath the lacertus fibrosus, because residual intact constricting fibers could theoretically contribute to persistent mechanical compression. In the present study, systematic open validation demonstrated complete division of the lacertus fibrosus in all 20 specimens, with no residual intact constricting fibers identified. The importance of this finding lies not simply in the 100% technical success rate, but in the method of confirmation: Completeness was determined by subsequent open anatomical inspection rather than by nanoscopic appearance alone. Thus, within the limitations of this cadaveric series, restricted access through a minimally invasive working corridor did not prevent complete anatomical release.

Neurovascular safety represents an equally important prerequisite for minimally invasive decompression in this region. The median nerve and brachial artery are closely related to the lacertus fibrosus, and anatomical variation may further alter their spatial relationships[4,5,8,12]. In the present study, no visible iatrogenic injury to the median nerve, brachial artery, or adjacent vascular structures was identified during systematic open validation. This finding supports the anatomical feasibility of introducing and using a cutting device under continuous nanoscope visualization. Nevertheless, the absence of injury in 20 cadaveric specimens should not be interpreted as evidence of zero procedural risk in clinical practice. Living tissue introduces additional factors-including bleeding, tissue deformation, patient movement, and variable tissue planes-that cannot be reproduced fully in a cadaveric model. Accordingly, the present safety findings should be considered preliminary anatomical evidence rather than proof of clinical safety.

Visualization quality was high for all predefined anatomical structures, with mean scores of 4.9 ± 0.1 for the lacertus fibrosus, 4.8 ± 0.3 for the median nerve, and 4.6 ± 0.4 for the brachial artery. Interobserver agreement was good, with an overall ICC of 0.88 using a two-way random-effects model for consistency. The slightly lower mean visualization score for the brachial artery compared with the lacertus fibrosus and median nerve may reflect the constraints of the narrow working corridor and the relative position of adjacent vascular structures. Although these findings suggest that the relevant anatomy could be visualized consistently in the cadaveric setting, visualization in living patients may be affected by bleeding and tissue response. This distinction is particularly important when considering translation of the technique into clinical practice.

The use of needle arthroscopy for procedures beyond conventional intra-articular assessment is evolving. Nanoscopic techniques have been described in upper-limb surgery, including distal biceps tendon repair, illustrating the potential extension of small-bore visualization systems into confined peri-tendinous anatomical regions[15]. The present study expands this concept to decompression of the proximal median nerve beneath the lacertus fibrosus. However, the nanoscope-assisted approach also introduces limitations inherent to the technology, including a restricted field of view, constrained instrument maneuverability, dependence on precise portal placement, additional equipment requirements, procedural costs, and a potential learning curve. These considerations are clinically relevant because conventional lacertus release is already effective in appropriately selected patients[13,17-19,21]. Therefore, the present findings do not support replacement of established techniques. Instead, they establish anatomical feasibility as a prerequisite for future evaluation of whether direct nanoscopic visualization offers any clinically meaningful advantage.

The present study has several limitations. First, the sample size was limited to 20 specimens and may not capture the full spectrum of anatomical variation encountered in clinical practice. Accordingly, the 100% feasibility and complete-release rates observed in this series should not be generalized to all anatomical configurations. Second, the cadaveric design precludes assessment of symptom relief, neurological recovery, postoperative pain, functional improvement, patient-reported outcomes, wound-related complications, procedure-associated weakness, or long-term safety in vivo. Third, although fresh-frozen specimens preserve anatomy and tissue handling more realistically than formalin-fixed material, cadaveric tissue differs from living tissue with respect to elasticity, perfusion, bleeding, tissue deformation, and biological response to instrumentation. These differences may affect both visualization and procedural safety.

Fourth, the mean specimen age was 72 years, with a range of 61 years to 89 years. Tissue characteristics in this relatively older cadaveric population may not fully reflect those of younger or more active patients presenting with dynamic lacertus-related compression. Fifth, although anatomical relationships were assessed during the procedure and subsequent open validation, the study was not designed or powered to systematically classify lacertus fibrosus morphology, pronator teres variants, vascular branching patterns, or other rare anatomical configurations. Therefore, the findings cannot establish procedural safety across the full spectrum of uncommon anatomical variants. Sixth, no formal learning-curve analysis was performed, and the relatively narrow range of procedural times should not be interpreted as evidence of absence of a learning curve. Finally, the study did not include direct comparison with open, mini-open, or ultrasound-guided decompression techniques, precluding conclusions regarding relative efficacy, safety, operative efficiency, cost-effectiveness, or clinical benefit.

Despite these limitations, the present study provides an anatomical foundation for further investigation of nanoscope-assisted lacertus fibrosus release. A logical next step would be a prospective first-in-human feasibility study in a small, carefully selected clinical cohort, with predefined assessment of neurological status, pain, grip strength, patient-reported outcomes, complications, and need for conversion or revision. If initial clinical feasibility and safety are demonstrated, subsequent comparative studies should evaluate nanoscope-assisted release against established mini-open and ultrasound-guided techniques. Such studies will be necessary to determine whether the theoretical advantages of reduced surgical exposure and continuous direct visualization translate into meaningful improvements in postoperative recovery, return to function, patient satisfaction, or complication rates.

CONCLUSION

Nanoscope-assisted lacertus fibrosus release was anatomically feasible and demonstrated a favorable procedural safety profile in this cadaveric model. The technique enabled direct visualization of the median nerve and adjacent vascular structures while allowing controlled division of the lacertus fibrosus through a minimally invasive working corridor. Systematic post-procedural open dissection confirmed complete release in all specimens, with no visible iatrogenic injury to the median nerve or adjacent vascular structures. These findings provide an anatomical foundation for prospective clinical evaluation of the feasibility, safety, functional outcomes, and comparative effectiveness of nanoscope-assisted release relative to established open, mini-open, and ultrasound-guided decompression techniques.