Abstract
Background Minimally invasive surgical techniques have the potential to decrease the morbidity associated with traditional open surgery. However, surgeons may hesitate to implement minimally invasive techniques in their practice due to limited direct visualization and an arduous learning curve. Minimally invasive spine surgery requires precise docking of tubular or endoscopic retractors, which is difficult because without direct visualization of surrounding structures, surgeons may find themselves unable to orient themselves. Finding consistent and reproducible radiographic landmarks may decrease barriers to adoption of MISS techniques. Abd-El-Barr’s point is a novel radiographic landmark that is identified on lateral fluoroscopy to help with docking for both tubular and endoscopic spine procedures. The landmark is hypothesized to correlate closely with the caudal aspect of the lamina and serve as a reliable docking target.
Objective To validate Abd-El-Barr’s point as a radiographic landmark for safe, reliable, and consistent docking in minimally invasive lumbar surgery.
Methods A cadaveric study design: fluoroscopic localization of Abd-El-Barr’s point was performed bilaterally from L1 to S1 using a sharply pointed instrument that was inserted slightly into the lamina to mark it with a hole, followed by dissection to measure the distance between the mark and the caudal lamina.
Results A total of 5 cadaveric specimen data were analyzed. The mean distance from Abd-El-Barr’s point to the caudal aspect of the lamina across all measured levels and sides was 5.3 mm (bilaterally).
Conclusion Abd-El-Barr’s point is a reliable radiographic landmark that provides accurate and safe docking during minimally invasive lumbar decompression. Validated through anatomical dissection, it has the potential to standardize docking, provide efficient surgical workflow, and reduce variability across various surgeon experience levels.
Clinical Relevance Using this landmark as a docking point, it is hoped that surgeons can make mininally invasive spine surgery, whether tubular or endoscopic, safer and more efficient, thus helping patients recover faster.
Level of Evidence 5.
Introduction
Minimally invasive spine surgery (MISS) is a type of surgery that uses smaller incisions to approach and operate on spine-related pathologies as compared with traditional open methods.1–4 With the tremendous advancements in surgical techniques in recent decades, the popularity of MISS has increased dramatically.2–5 Evidence shows that compared with traditional open spine surgery, patients who undergo MISS experience fewer adverse effects intra- and postoperatively and faster return to work.6–11 While evidence shows great advantages of MISS, it is important to recognize its limitations.2 Direct visualization of anatomical landmarks is limited in minimally invasive or image-guided procedures, and current MISS techniques rely heavily on fluoroscopy to guide instrument placement, resulting in significant radiation exposure.12–14 Furthermore, there is a steep learning curve that requires mastery of specialized tools (eg, tubular retractors and endoscopes) and the ability to comfortably work through small corridors with limited visualization.14–16
As MISS techniques have developed over time, literature has been published to assist surgeons with overcoming the learning curve and avoiding complications. For example, Sharif et al list surgical pearls for multiple minimally invasive spine procedures, including the minimally invasive endoscopic interlaminar discectomy.15 One such surgical pearl is the importance of a good-quality lateral radiograph and for proper tubular retractor docking to avoid iatrogenic violation of structures.15 Having accurate intraoperative fluoroscopy is fundamental for the correct localization of important levels and parts of the spinal anatomy.17
In MISS, “docking” refers to the insertion and positioning of a tubular or endoscopic retractor through a small incision and surgical corridor to access a specific anatomical target of the spine. In many MISS cases, it is important to dock on the inferior edge of the lamina. Docking here will allow for easier entrance into the epidural space and also means that one can drill the lamina without too much maneuvering. One such target that we define in this study is Abd-El-Barr’s point, a novel radiographic landmark visible on lateral fluoroscopy at the caudal junction of the lamina and interlaminar ligament. Consistent landmarks reduce variability, improve reproducibility, help minimize iatrogenic injury to critical structures, and streamline the localization process to shorten the operative time. We hypothesize that this point corresponds reliably to the inferior edge of the lamina and can guide safe, consistent docking.
The purpose of this cadaveric study is to present a novel radiographic landmark that allows accurate and reproducible tubular and endoscopic docking in minimally invasive lumbar decompression surgery, supporting efficient decompression while minimizing surgical footprint and the risk of iatrogenic instability. The point is validated both radiographically and anatomically in cadaveric specimens. We also show a few clinical case examples of both microscopic and endoscopic spine surgeries and how this landing spot allows for accurate and reliable docking.
Methods
Study Design
In this cadaveric study, we used cadavers for anatomical validation to assess the radiographic-to-anatomical correlation of Abd-El-Barr’s point. The study was conducted at Duke University Hospital, and cadaveric work was performed in the cadaveric lab.
Specimen Selection
Freshly frozen human cadaver specimens were used. Specimens with deformity or severe degeneration were excluded. A total of 5 cadavers were included, examining bilateral L1 to S1 levels, depending on the condition of each cadaver. The specimens were stored at −20°C and were thawed at room temperature for approximately 3 days prior to marking and dissection. Levels that had a previous intervention at the laboratory were not measured.
Radiographic Localization
Under fluoroscopic guidance, on lateral fluoroscopy, Abd-El-Barr’s point is defined as the tip of the pyramid/mountain (Abd-El-Barr’s pyramid/mountain) shape representing the caudal junction of the lamina and the interlaminar ligament ( Figures 1 and 2 ). The middle point of the interlaminar window was marked on the anteroposterior (AP) view for the mediolateral direction ( Figure 3 ). A hard, radio-opaque metallic instrument with a pointed end was placed percutaneously at this point. The marker was advanced under fluoroscopic guidance until the tip of the marker reached the tip of the mountain. For marking purposes on the lamina, the metallic instrument was hammered slightly into the mountain top, leaving a hole (referred to as the mountain hole on Figure 4 ) to be used later after dissection and exposing. The process was repeated for all the levels bilaterally from L1 to S1 on the cadaver.
Illustration showing Abd-El-Barr’s point, which is on the tip of Abd-El-Barr’s mountain (green pyramid or mountain shape). The point correlates with the caudal border of the lamina of the same vertebra.
Abd-El-Barr’s mountain. (A) Lateral x-ray image showing an instrument at the top of Abd-El-Barr’s mountain. (B) Abd-El-Barr’s mountain highlighted (red dotted triangle). The tip of this mountain is Abd-El-Barr’s point.
Anteroposterior x-ray image showing an instrument pointing at the middle of the interlaminar window. Interlaminar window is defined as the horizontal distance between the lateral edge of the spinous process and the medial edge of the inferior facet.
Cadaver dissection. (A) Lateral x-ray image showing the metallic instrument at the tip of Abd-El-Barr’s mountain (red dotted highlight). (B) Showing the hole (white arrow) left by the metallic instrument (K-wire) hammered at the tip of Abd-El-Barr’s mountain on the lamina.
Surgical Exposure and Anatomical Validation
Following hole placement, a posterior approach was used to expose the spine through a paramedian dissection ( Figure 4 ). The accuracy of the radiographic point was validated by measuring the distance between the hole on the lamina and the caudal aspect of the corresponding lamina.
To test our hypothesis that Abd-El-Barr’s point corresponded to the point where the caudal lamina and intralaminar window joined, we undertook the following procedure: when the superior articular process was removed bilaterally, the previously identifiable tip of the “mountain” landmark disappeared. This finding supports the anatomical explanation of the mountain, confirming that the tip is primarily formed by the prominence of the superior articular process ( Figure 5 ). When the inferior articular process was removed bilaterally, the mountain did not have significant changes. These findings suggest that surgical removal of the process eliminates this radiographic and anatomical peak, further validating its anatomical basis and the correlation between imaging and underlying bony structures.
Removal of bilateral superior articular process. (A) Before the removal of the superior articular process. (B) After removal of the superior articular process. (C) Highlighted dotted illustration representing before the removal of the superior articular process. (D) Highlighted dotted illustration representing after the removal of the superior articular process.
Data Collection
Cadaver age, gender, and body mass index.
Lumbar level and side (right/left).
Distance from radiographic marker to caudal lamina (in millimeters).
All measurements were obtained using a standard surgical ruler with millimeter markings. Given the limitations of visual inspection and the measuring tool, distances were rounded to the nearest whole millimeter. For consistency, values are reported with 1 decimal place (eg, 5.0 mm), although submillimeter precision was not possible using this method.
Data Analysis
A descriptive statistics model and a paired t test were used to assess the consistency and reliability of Abd-El-Barr’s point as a landmark for docking. For each level (L1–S1) bilaterally, the distance in millimeters from the radiographic marker to the caudal border of the lamina was recorded.
Data analysis included the following:
Calculation of mean and SD for distances at each spinal level.
A paired t test was used to compare the right and left sides of each level.
Results were stratified by vertebral level (L1–S1) and by side (right vs left) to assess the reproducibility of the landmark.
Results
Cadaveric Findings
A total of 5 cadaveric specimen data were analyzed ( Table ). Four cadavers were men, and 1 was a woman. Ages range from 64 to 82 years (mean: 71.6). Body mass indices range from 14.9 to 29.6 (mean: 23.18).
Descriptive analysis by level (L1–S1) and side (left and right).
The mean distance from Abd-El-Barr’s point to the caudal aspect of the lamina across all measured levels and sides was similar bilaterally (approximately 5.3 mm; Figure 6 ). Cadaveric analysis showed that 67.5% (27 measurements out of 40 total valid measurements) of docking distances from Abd-El-Barr’s point to the caudal lamina were ≤6 mm, and 97.5% (39 measurements out of 40 total valid measurements) were ≤8 mm.
Box plot showing docking distances from Abd-El-Barr’s point to the caudal lamina of the same vertebra. X axis: Spinal levels (L1–L2 to L5–S1). Y axis: Distance in millimeters; red solid line: median; black diamonds = mean value.
Paired t test showed that none of the level comparisons reached statistical significance (P > 0.290), indicating no significant side-to-side difference in the measured distances from Abd-El-Barr’s point to the caudal lamina.
Discussion
One of the most important parts of MISS is docking in the “right spot” with minimal tissue disruption. The “interlaminar window” is the gap between the caudal edge of the upper lamina and the cranial edge of the lower lamina, and it is within this intralaminar window that many MISSs take place—whether it be decompressions or interbody fusions. Thus, docking close to this window is important. However, it should be noted that docking directly within this window can also be dangerous as it could lead to unintentional durotomies and/or nerve injuries. Thus, docking right at the interface between the caudal lamina and the ligamentum flavum is ideal. Currently, there are very few reliable radiographic landmarks to help with docking.
On AP fluoroscopy, the facet joint line and laminar outline are often used as targets. The medial borders of the facet joints and pedicles are also key landmarks. Surgeons use the medial edge of the pedicle (the “medial pedicle line”) as a reference to where the facet joint begins, and many surgeons stay just medial to the medial pedicle line during a decompression to preserve most of the facet joint. Existing literature on radiographic landmarks is limited.17–19 Kim et al described a location for docking on AP view as a “V” point corresponding to the spinolaminar junction—the spot where the lamina meets the facet—which is an ideal starting point for a laminotomy.18 Similarly, Derman et al described directing the initial dilator onto the inferior border of the lamina, just medial to the facet joint, which is essentially the same region and confirming this position on AP and lateral fluoroscopy.17
One difficulty of AP fluoroscopy is that it depends on the patients’ individual lordosis, and making the intralaminar window visible on AP fluoroscopy is not trivial. Moreover, many spine surgeons are more comfortable with lateral fluoroscopy. This reduced accuracy may increase risks of misplacement and associated complications such as incomplete decompression or compromised spinal stability.19,20 These potential risks underscore the limitations of relying solely on AP fluoroscopy, which Abd-El-Barr’s point on lateral imaging helps overcome. Overall, this study highlights the need for a more defined, reproducible target that correlates precisely with key anatomical structures. Abd-El-Barr’s point addresses this need by providing a validated landmark visible on fluoroscopy at the caudal border of the lamina that can serve as a critical starting point for safe decompression. Its consistency across levels and clarity on lateral fluoroscopy make it valuable to existing MISS docking strategies.
For many MISSs, the caudal junction of the lamina and the interlaminar ligament is an excellent starting point (Figure 1). We introduce a novel radiographic landmark—Abd-El-Barr’s mountain—with the goal of aiding surgeons in accurate and consistent docking on the caudal border of lumbar lamina during minimally invasive lumbar procedures involving a microscope or an endoscope. The name is in recognition of the senior author of this study, who was the first to observe and consistently utilize the point and mountain as a target in MISS. The pyramidal structure at the junction of the caudal lamina and the interlaminar ligament creates a distinct “mount-shaped” appearance, which is what we refer to as Abd-El-Barr’s mountain. The tip of this mountain is referred to as Abd-El-Barr’s point. Abd-El-Barr’s point showed consistent appearance across lumbar levels on lateral fluoroscopy. By targeting the tip of this mountain, surgeons can avoid caudal misplacement into surrounding structures, which can lead to dural tears. Similarly, docking too cranially will cause disorientation and increased time before finding appropriate landmarks. Additionally, this point is clear on fluoroscopy, which reduces the need for multiple imaging shots and minimizes radiation exposure.
Whether using a tubular retractor or performing endoscopic decompression, Abd-El-Barr’s point proves to be a dependable guide for docking. The practical versatility of Abd-El-Barr’s point is demonstrated by its successful use in both biportal endoscopic and microscopic minimally invasive lumbar decompression (Figures 7 and 8). In the biportal endoscopic case (Figure 7), the landmark was employed to guide the placement of the working and viewing portals, ensuring precise alignment over the caudal lamina and facilitating effective decompression through dual channels. Similarly, in the microscopic case (Figure 8), the point served as a docking reference for the tubular retractor system, enabling safe and efficient exposure of the target lamina for decompression. These demonstrations underscore the adaptability of Abd-El-Barr’s point across different minimally invasive surgical platforms, reinforcing its potential as a universal radiographic landmark to improve docking accuracy, minimize soft tissue disruption, and enhance reproducibility regardless of the surgeon’s chosen technique.
Biportal endoscopic case. Showing utilization of the biportal approach for Abd-El-Barr’s point.
Microscopic approach showing successful docking at the caudal border of the lamina utilizing Abd-El-Barr’s point. (A) Lateral x-ray image showing docking at Abd-El-Barr’s point. (B) Microscopic view of the docking at Abd-El-Barr’s point.
Cadaveric dissection revealed that the point consistently lay within a close margin (97.5% of the measurements were ≤8 mm) of the caudal lamina, confirming its reliability as a landmark for lumbar MISS docking. Since the majority of tubes are larger than 6 mm and most endoscopic dilators are close to 6 mm in diameter, this finding would ensure that minimal time and energy will be expended to find the most caudal aspect of the lamina, which is critical to identify in both tubular and endoscopic spine surgeries. This will help ensure that surgeons do not spend a lot of time trying to orient themselves and allow for faster, safer, and more efficient surgeries.
In the cadaver study, we observed variations in measurements. A potential explanation is that small deviations medially or laterally in the AP plane during needle placement could result in significant differences in the measured distance to the caudal border of the lamina. This finding suggests that even minor horizontal angulation during docking may contribute to variability. Given that the cadaver specimens were from elderly donors (ages ranged from 64 to 82 years), it is likely that age-related degeneration, including facet joint arthropathy and remodeling of posterior elements, contributed to alterations in the bony anatomy. These degenerative changes may affect the spinal anatomical relationships, leading to increased variability in the measurements from Abd-El-Barr’s point to the caudal aspect of the lamina.21,22 Eubanks et al demonstrated that older cadaveric specimens frequently show pronounced remodeling and hypertrophy of the facet joints, particularly at the lower lumbar levels, reflecting the cumulative effects of age-related degeneration.23
Another notable observation during the cadaver dissection was the horizontal variation in needle landing within the interlaminar window. The more medially the needle is positioned from the center of the window, the closer it gets to the caudal border—eventually reaching 0 mm. Moving laterally from the center still results in bony contact; however, this approach carries a high likelihood of contacting the pars interarticularis and facet joints.
A key strength of this study is the combined radiographic and anatomical validation of Abd-El-Barr’s point across multiple lumbar levels using human cadaveric specimens. By systematically correlating fluoroscopic localization with direct anatomical exposure, we were able to assess both the radiographic reliability and anatomic consistency of the landmark.
Abd-El-Barr’s point has been successfully utilized across a variety of minimally invasive lumbar procedures that require accurate docking at the caudal lamina. These include minimally invasive hemilaminectomy, minimally invasive transforaminal lumbar interbody fusion, uniportal intralaminar endoscopic discectomy, and biportal intralaminar endoscopic discectomy.
Building on these future directions, the identification and validation of Abd-El-Barr’s point as a reliable radiographic landmark have significant implications for MISS research and practice. Spine surgeons, with their unique access to intraoperative anatomy, are well positioned to develop standardized landmarks that improve communication between teams.20 Educational initiatives, such as cadaver laboratories and surgical simulators, could incorporate this landmark to help surgeons navigate the steep learning curve of MISS and improve surgical outcomes.24 Furthermore, Abd-El-Barr’s point could also enhance intraoperative performance through integration with advanced technologies. These include convolutional neural networks for vertebral anatomy recognition,25 landmark detection algorithms,26 and hybrid registration methods that combine both artificial and anatomical markers. Such technologies can improve workflow efficiency and reduce cognitive load during complex procedures.27 Notably, hybrid methods have already been shown to improve registration accuracy, suggesting that Abd-El-Barr’s point could serve as a valuable component within these systems.
Several limitations should be acknowledged. This study is limited by the small number of cadaveric specimens and the use of elderly donors, which may not reflect younger, nondegenerated spines. Degenerative changes may have influenced measurements, particularly at lower lumbar levels. A prospective clinical series would be needed to further validate its reproducibility and outcomes in live surgical settings.
Future studies should assess whether incorporating Abd-El-Barr’s point into navigation and robotic systems improves docking precision and reduces radiation exposure. Comparative studies with conventional docking methods would clarify its impact. In addition, training modules could facilitate widespread adoption. Image-guided navigation and robotic technology can enhance the precision of MISS by leveraging Abd-El-Barr’s point for docking.
Conclusion
This cadaveric study introduces and validates Abd-El-Barr’s point as a novel radiographic landmark for accurate and reproducible docking in minimally invasive lumbar decompression surgery. The landmark correlates closely with the caudal aspect of the lamina across lumbar levels and provides evidence for a successful surgical approach with no major complications. Future research should focus on multicenter validation, integration with emerging imaging modalities, and incorporation into surgical training programs to maximize its clinical utility.
Footnotes
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests The authors report no conflicts of interest in this work.
Disclosures Muhammad Abd-El-Barr reports consulting fees from BrainLab, Spinal Elements, Arthrex, Globus, and TrackX.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2025 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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