Abstract
Background Current techniques for surgical correction of spinal deformities impart realigning the deviated vertebrae along a rigid rod. Solid rods are a major restricting factor to vertebral manipulation, leading to incomplete, imprecise, and less predictable 3-dimensional (3D) correction. Additionally, forceful manual nonquantifiable maneuvers may result in potential implant failures and increased incidence of complications. We are introducing a machine-operated device for digitized segmental 6 degrees of freedom (6 DOF) correction of individual vertebral deviations.
Methods We manufactured a 3D coupler incorporating multiple self-locking uniaxial joints. The device’s precision was tested by comparing targeted vs delivered motions. For functionality testing, we used computed tomography-based 3D-printed vertebral models to verify the device’s ability to manipulate the vertebra in each direction of motion.
Results In all tested motions, the coupler accurately and repeatedly delivered the predicted targeted motions. The device could mobilize 2 vertebral models relative to each other in 4 out of 6 DOF.
Conclusions The novel 3D coupler can deliver machine-driven, precise, and predictable multiplanar motion; it could manipulate the vertebral model in rotation and translation.
Clinical Relevance The novel device addresses a crucial unmet need in spinal surgery by offering digital precision, true 6 DOF correction, and supporting robotic execution of surgical actions.
Level of Evidence 5.
- spinal deformity
- segmental three-dimensional correction
- digitization
- automation
- robotic surgery
- 3D coupler
Introduction
Spinal deformities are mostly multiaxial; deviation of a vertebra from its normal position in any of the 6 degrees of freedom (6 DOF) can cause significant clinical consequences. The inability to achieve precise segmental 3-dimensional (3D) correction is a long-felt, but not yet solved, problem. Rigid rods used in spinal deformity correction are a major restricting factor to vertebral manipulation by limiting vertebral motion to rotation around and linear motion along the axis of the rod, leading to an incomplete, imprecise, and less predictable 3D correction of individual vertebral deviations. Forceful nonquantifiable maneuvers used with solid rods may result in potential implant failures and increase the incidence of complications. The current surgical techniques present a significant barrier to digitization and automation of the surgical workflow; accordingly, surgical robots in spine deformity surgery remain at level 1 autonomy, limited to navigation assistance without the capability to execute surgical tasks.1
Precision surgery and patient-specific realignment are evolving technologies with a promise of creating normalized individual patient anatomy and improved outcomes. Traditionally, restoration of global spinal alignment has been the primary objective in deformity correction. However, growing evidence suggests that achieving per-level accurate normalized segmental alignment is equally important for optimizing both clinical and biomechanical outcomes.2,3
To overcome the challenges associated with the lack of accuracy, predictability, and capability for segmental 3D correction, we designed a multiaxial self-locking segmental coupler. The device uses numerical data (degrees and millimeters) to correct vertebral deviations, thereby digitizing the correction process and paving the way for higher levels of robotic autonomy. Our first objective was to introduce a new concept and device for segmental 6 DOF correction of spinal deformities; the second was to verify the device’s precision of motion and assess its basic functionality.
Methods
The Concept
Spinal deformity occurs mainly as a result of individual vertebral deviations at the intervertebral junctions; analysis of these deviations is the key to segmental 3D correction. Correction aims at realigning each vertebra within the curve from its actual abnormal deviated position to its target normal anatomical position. Our novel concept of deformity correction is based on identifying each vertebra as an object having 6 DOF: 3 rotational (Rx, Ry, and Rz) and 3 translations (Tx, Ty, and Tz; Figure 1). The directions for rotation are designated as clockwise or counterclockwise, and for translation as positive or negative (+ or −). For any motion segment, individual vertebral deviations are measured as deviations of the more deviated vertebra relative to the less deviated vertebra. The position of each vertebra is identified as alphanumerical (digital) entries, including: (1) number of deviations (n); (2) axis: X, Y, or Z; (3) type: rotation (R) or translation (T); (4) direction: clockwise or counterclockwise or + or −; and (5) magnitude: angle (degrees) or distance (millimeters).
Deviations of the vertebra in 6 degrees of freedom. (a) Coronal Angulation, rotation around the X axis (Rx). (b) Anterior/posterior translation, linear motion along the X axis (Tx). (c) Sagittal angulation, rotation around the Y axis (Ry). (d). Right/left translation, linear motion along the Y axis (Ry). (e) Axial rotation, rotation around the Z axis (Rz). (f) Cephalic/caudal translation, linear motion along the Z axis (Tz).
The Device
We designed a multiaxial segmental coupler (3D coupler), the correction-fixation unit of the system, comprising an articulated bipartite body and 2 end-effector arms/rods. The 2 arms are to be attached to the cephalad and caudal pedicle screws of the level/s to be corrected. The body of the coupler incorporates multiple 1 degree of freedom self-locking revolute or linear “robotic joints” (Figure 2). Operating the joint mechanisms, the coupler is configured to manipulate a more deviated vertebra relative to a less deviated vertebra, realigning the deformed motion segment/s. The coupler unit can include 1 or more of 6 different types of uniplanar joint mechanisms, collectively capable of correcting deviations in the 6 DOF and realigning the motion segment/s. Each joint mechanism is controlled by a separate screw head/actuator. The number and type of joints included in the 3D coupler are chosen based on the number and directions of deviations it is intended to correct.
High-fidelity three-dimensional coupler prototype from the (a) top view and (b) side view.
The corrections, represented as digital entries, are calculated based on the difference between the actual deviated position and the target normal anatomical position. The worm gear joints within the coupler are selectively mobilized by rotating their corresponding calibrated screw head of the driving gear (input) to precisely deliver the required motion at the end-effector arm of the driven gear (output). Each uniaxial joint has a defined output/input or gear ratio that is fixed by the geometry of its internal actuation mechanism—specifically, the number of teeth on the worm wheel and the number of starts on the worm—and is therefore deterministic. The digital instructions to operate each joint mechanism include (1) direction and degree of screw head rotation and (2) torque. A software, incorporating gear ratio as an input, was developed to automatically generate the required magnitude and torque of the screw head rotation based on the targeted magnitude and force of motion that needs to be delivered at the end-effector arm.
In idiopathic scoliosis and adult spinal deformities, the coronal deformity is associated with intervertebral rotation and loss of lordosis.4,5 Accordingly, we designed and machined a high-fidelity stainless-steel prototype of a 3D coupler that includes 3 joint mechanisms capable of correcting these 3 rotational deviations (Figure 3 ; Online Supplemental Video 1).
Function of the 3-dimensional coupler’s joint mechanisms. (a) First joint mechanism, Z-axis rotation (Rz): controls axial rotation. (b) Second joint mechanism, Y-axis rotation (Ry): controls sagittal angulation. (c) Third joint mechanism, Z-axis translation (Tz): bilateral symmetrical motions control the disc height or cranial-caudal translation; compression on 1 side and distraction on the contralateral side controls coronal angulation (Rx).
Proof-of-Concept Precision Testing
The magnitude of screw head rotation was calibrated in a way that each marking delivers 1 degree of rotation at its corresponding rotational joint. For translation, each full turn (360°) of the screw head delivers 0.1 mm in linear motion. The motion of the device was also calibrated using a scale indicating the angle of rotation (in degrees) or translation (in millimeters; Figure 2). We rotated each screw head to a precalculated degree of rotation intended to deliver a targeted motion at its corresponding joint and compared the targeted vs delivered motion. We tested the revolute joints for targeted rotation of 5°, 10°, and 15° and the linear joint for targeted translation of 3, 6, and 9 mm.
Proof-of-Concept Functionality Testing
The device was attached using pedicle screws to computed tomography-based, 3D-printed, wire-embedded vertebral body models (L4 and L5). For each degree of freedom, we deviated L4 relative to L5 to deform the motion segment. Using a manual screwdriver, we realigned the motion segment in each of the 4 DOF, 1 at a time. The position of the vertebrae before and after manipulation was verified by fluoroscopy.
Results
Proof-of-Concept Precision Testing
The device accurately delivered the predetermined, targeted 5°, 10°, and 15° right and left axial rotations at the first arm (Figure 4a and 4b); 5°, 10°, and 15° lordosis and kyphosis at the coupler’s body articulation (Figure 4c and 4d); and 3-, 6-, and 9-mm cephalic-caudal linear displacement at the second arm (Figure 4e and 4f).
Proof-of-concept precision test. (a) Counterclockwise (CCW) rotation of the screw head of the first joint mechanism 5 markings results in a corresponding 5° right axial rotation. (b) Clockwise (CW) rotation of the screw head of the first joint mechanism 10 markings results in a corresponding 10° left axial rotation. (c) CCW rotation of the screw head of the second joint mechanism 15 markings results in a corresponding 15° lordosis. (d) CW rotation of the screw head of the second joint mechanism 5 markings results in a corresponding 5° kyphosis. (e) CW rotation of the screw head of the third joint mechanism 30 full turns (360°) results in a corresponding 3 mm cephalo-caudal distraction. (f) CW rotation of the screw-head of the third joint mechanism 60 full turns (360°) results in a corresponding 6-mm cephalo-caudal distraction.
Proof-of-Concept Functionality Testing
The 3-jointed 3D coupler manipulated the 2 vertebrae relative to each other in sagittal angulation, coronal angulation, axial rotation, and cranial-caudal translation (Figure 5). Across all trials, the delivered motions matched the targeted values within the visible scale resolution in degrees and millimeters, with no observable overshoot or undershoot.
Proof-of-concept functionality tests. Correction of the vertebral model of a 59-year-old man with unilateral 3-dimensional coupler configuration in sagittal angulation, axial rotation, coronal angulation, and cephalic-caudal translation (disc height restoration).
Discussion
Vertebral rotation or translation in any of the 6 DOF can lead to poor quality of life and disability; therefore, meticulous analysis and restoration of 3D spinal alignment are critical in achieving a satisfactory outcome. Historically, the coronal curve angle (Rx) has been considered the main parameter to describe the severity of scoliosis. The importance of analyzing the sagittal plane (Ry) has been emphasized, and its correlation with pain and disability is now well recognized.6 Patients with rotatory subluxation (Rz) report significantly more frequent low back pain and radiculopathy.7 The presence of retrolisthesis or anterolisthesis (Tx) was demonstrated to be associated with decreased health-related quality of life.8,9 Lumbar lateral listhesis (Ty) is associated with the presence of radiculopathy in adult scoliosis.10 Finally, restoring disc height (Tz) is 1 of the main goals of lumbar interbody fusion to improve canal and foraminal stenosis by indirect decompression.11
The precise restoration of individual motion segment alignment is increasingly recognized as critical for optimizing biomechanical and clinical outcomes. Accurate correction of segmental parameters, including disc height, foraminal height, and local vertebral alignment, is essential for effective indirect neural decompression, maintenance of physiological load distribution, and minimization of stress at adjacent levels.12–14 In long-segment fusion for adult spinal deformities, achieving optimal global alignment remains a primary surgical objective. However, growing evidence highlights the importance of evaluating individual lumbar segments relative to normative values to establish more personalized and anatomically precise alignment targets. The contribution of individual motion segments to lumbar lordosis must also reflect the patient’s specific spinal morphology in order to avoid proximal hypercorrection.15 Proportionate restoration of segmental alignment has been shown to be critical for preventing proximal junctional kyphosis, not only in the lumbar spine but also across upper thoracic segments.16 Furthermore, distinguishing the contributions of vertebral bodies and discs is crucial, as failing to account for vertebral kyphosis during multilevel correction can cause over- or undercorrection.2 Diebo et al extracted normative PI-adjusted segmental lordosis values and found that only 19% to 24% of patients achieved postoperative correction matching these norms at any segment; only 5.5% matched normative values at both T10 to L2 and L4 to S1. Although restoring normative segmental lordosis is associated with improved clinical and radiographic outcomes, most patients remain segmentally misaligned.3
Despite the latest technological developments, spinal surgeons are still not capable of precisely delivering the preplanned targeted 3D correction goals.17 A recent systematic review reported that while patient-specific rods may offer potential benefits, a lack of robust, high-quality evidence, combined with the considerable variation in study parameters, precludes definitive conclusions about their superiority over traditional rods.18 With the advent of newer imaging technologies, the data acquired allow for a highly precise 3D preoperative planning. These advancements need to be further translated to precision hardware that can deliver correction plans with a high degree of accuracy. The 3D coupler will, to our knowledge, be the first to allow a precise (in degrees and millimeters) intervertebral segmental 3D correction for each of the 6 DOF.
Safety in spinal deformity surgery is of utmost importance; connecting the pedicle screws to the prebent rod and performing the correction maneuvers requires a variable amount of force, which can cause screw loosening, breakage, and pullout, with the potential for associated neurological complications. Our coupler is configured to take the shape of the deformed spine 3-dimensionally, allowing its forceless attachment to the deviated pedicle screws. Using a torque limiter screwdriver, the device is subsequently reconfigured, driving the attached vertebrae to follow and realign the spine. This workflow will allow pedicle screws to be placed at their optimal location and ensure that the corrective forces applied are delivered within safe limits. For a safe correction of spinal deformities, the stiffness of the deformed motion segment/s may be addressed using suitable releases and/or osteotomies in order to reach a degree of flexibility that allows for correction of the deformity by using forces below the limits of pedicle screws pullout forces. A torque limiter will guarantee that the 3D coupler delivers precise forces to the pedicle screws within safe limits.
Automation in surgery offers a higher degree of accuracy and repeatability with less potential for human errors. Surgical automation is categorized into decision-making/planning and execution of surgical actions.19 Spine surgery robotic systems remained at the lowest level of autonomy (level 1 or robot assistance), offering only passive support and guidance.2 Robots will be used to operate the 3D coupler. They are equipped with an interface to program parameters like torque settings and movement sequences, as well as an integrated machine learning software to improve efficiency and make predictions. The operative instructions or digital commands will include the direction and magnitude of screw head rotation and the torque applied, paving the way for robotic spinal surgery with higher levels of autonomy.
Several critical challenges were identified during this study. The coupler must be sufficiently compact for segmental application, while being mechanically efficient to deliver corrective forces and structurally robust to withstand applied stresses. The friction-based self-locking joints provide mechanical advantage by enabling high force transmission within relatively small joint sizes. This can be further enhanced by increasing the gear ratio, employing multistart worms, selecting high-strength biocompatible materials, and minimizing housing volume through compact mounting designs. Although modern materials and manufacturing technologies allow for extreme miniaturization, the minimum feasible size remains constrained by the device’s force requirements. For reference, MAGEC rods employing similar self-locking mechanisms achieve forces over 250 N within a 9-mm housing.20,21 In anatomically restricted scenarios, including the concave side of thoracic deformities or in pediatric patients, it may not be feasible to place a coupler at each motion segment. In such cases, a single coupler may be configured to span and correct multiple adjacent levels (Online Supplemental Video 1), with the number of levels determined by local anatomy and the specific corrective strategy. Alternatively, smaller single- or double-jointed couplers may suffice at segments with predominantly unidirectional or bidirectional deviations, or the device can be positioned away from the spine and used temporarily during correction, after which the spine is fixed using rods or low-profile lockable connectors.
Another challenge lies in determining the corrective forces sufficient to realign the spine. These forces are highly variable and patient-specific, influenced by factors such as spinal level, curve rigidity, and the extent of intraoperative releases. In its initial clinical application, the device will be programmed to deliver gradually increasing corrective forces within predefined biomechanical safety limits, specifically below the pedicle screw pull-out threshold. Adjustments may include shifting to a different joint mechanism to target another DOF, pausing to allow for ligamentotaxis, or performing additional releases to reduce resistance. Real-time intraoperative feedback on applied force and achieved correction will guide further surgical decisions. Integrating machine learning techniques with intra- and postoperative feedback loops will enable more accurate prediction of level- and patient-specific force requirements.
The present study has several limitations, including the use of a freely mobile, nonconstrained single motion segment model for proof-of-concept testing. While this does not replicate the complexity of a stiff, multilevel deformed spine with bone and soft tissue constraints, the model was intentionally selected to isolate and demonstrate the device’s core functional ability to apply mechanically controlled vertebral manipulations across individual DOF. This simplified setup was a necessary first step before testing in more structurally constrained models. Another limitation of this study is the absence of biomechanical testing, which was beyond the scope of this work. Due to the device’s structural complexity, with 3 integrated self-locking joints, 2 independently articulated end-effector arms, and multiple enclosures, a comprehensive mechanical evaluation of force delivery and structural integrity under load requires a dedicated, thorough follow-up study. In addition, biomechanical evaluation is most informative only after the geometry, material selection, and dimensional tolerances of the clinical-grade design have been finalized; testing an early-stage prototype would not accurately reflect the performance of the definitive device.
The current study presents a novel concept for digitized, segmental correction of individual vertebral deviations across all 6 DOF, enabling controlled and reproducible deformity correction and force application. As a proof of concept, it demonstrates the precision and operational efficacy of a compact, machine-driven device in implementing this approach. This work establishes a firm foundation for the clinical translation of precision-based, robotically executed spinal deformity correction, paving the way for a new direction in spinal deformity surgery.
Conclusion
The novel 3D coupler can deliver precise and predictable multiplanar motion at the attached pedicle screws. The device could manipulate the vertebral model in specific directions in rotation and translation in 4 out of 6 DOF. Further in vitro functionality and biomechanical testing will be conducted to validate the feasibility and effectiveness of the device for clinical practice.
Supplementary material
Online Supplementary Video 1.
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.
Relevant Patents Hazem B. Elsebaie, Behrooz A. Akbarnia, and Gregory M. Mundis are inventors listed on patent application WO 2024/132079 A1.
Hazem B. Elsebaie, Gregory M. Mundis, Youssef H. Elsebaie, and Behrooz A. Akbarnia are inventors listed on patent application WO 2025/113763 A2.
Financial Disclosures Behrooz Akbarnia reports royalties from Stryker, DePuy Spine, and Globus. Robert Eastlack reports royalties or licenses from Aesculap, SI Bone, Seaspine-Orthofix, and Globus; consulting fees from Globus, SI-Bone, Seaspine-Orthofix, Medtronic, J and J, Curiteva, Spectrum, Neo, Silony, Osteocentric, Mainstay, Intrinsic Therapeutics, Amgen, Kuros, and Spinal Elements; participation on a data safety monitoring or advisory board for OcuDoc, San Diego Spine Foundation, Curiteva, and Spectrum; stock/stock options from Neo and Globus; private investments in SI Bone, Orthofix, Spine Innovation, Alphatec, and OnPoint; and fellowship support from AO, Orthofix, Alphatec, and Globus. Ron El-Hawary reports grants/contracts from the Scoliosis Research Society, the Pediatric Spine Foundation, and POSNA; consulting fees from DePuy Synthes Spine, Orthopediatrics, and Medtronic; payment/honoraria from Medtronic; leadership or fiduciary role for the Scoliosis Research Society and the Pediatric Spine Foundation; stock/stock options from Orthopediatrics; and fellowship support from DePuy Synthes and Medtronic (paid to institution). Darryl D’Lima reports grants/contracts from the NIH and CIRM (paid to institution); consulting fees from D’Lima Consulting; stock/stock options from Active4D; and additional research support from private foundations. Gregory Mundus reports grants/contracts from NuVasive, Medtronic, and Seaspine; royalties/licenses from Nuvasive, Orthofix, and Stryker; consulting fees from NuVasive, SI-Bone, Orthofix, and Theradaptive; leadership roles for San Diego Spine Foundation, Scoliosis Research Society, Global Spine Outreach, and San Diego Orthopedic Society; stock/stock options from Alphatec and Globus; and fellowship support from AO, NuVasive, Seaspine, and Medtronic (paid to institution).
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2026 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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