Minimally invasive techniques and enabling technologies for adult spinal deformity: state of the art and future directions

Patient selection and corrective limitations

Surgical algorithms have been developed to standardize MIS for ASD. The original “minimally invasive spinal deformity surgery algorithm” (MISDEF) provided a reproducible framework for decision-making in surgery based on sagittal parameters and curve flexibility [14]. It was later updated to MISDEF-2 (Fig. 1) [8]. The revised framework expands the classification from three to four classes and adds crucial decision points, most notably whether the spine is “fused or rigid.” MISDEF-2 stratifies patients based on key radiographic parameters and curve flexibility.

Fig. 1

The MISDEF-2 (minimally invasive spinal deformity surgery algorithm-2) algorithm for guiding surgical decisions in minimally invasive (MIS) deformity surgerysurgery [8]. SVA, sagittal vertical axis; PT, pelvic tilt; PI–LL, pelvic incidence minus lumbar lordosis; ACR, anterior column realignment; PSO, pedicle subtraction osteotomy.

Class I comprises patients with flexible curves and minimal sagittal imbalance, characterized by sagittal vertical axis (SVA) <6 cm, pelvic tilt (PT) <25°, pelvic incidence (PI) minus lumbar lordosis (LL) mismatch <10°, and coronal Cobb angle <20°. These patients are typically indicated for simple MIS decompression or focal fusion. Class II includes patients with flexible curves but moderate deformity that exceeds the Class I criteria, such as PI–LL mismatch <30° and thoracic kyphosis <60°. Treatment involves multilevel MIS, often with interbody fusion. Class III designates patients with significant sagittal deformity (e.g., PI–LL mismatch >30°) or rigid curves that do not meet the criteria for Class IV. This class requires advanced MIS procedures, such as circumferential MIS with ACR, mini-open pedicle subtraction osteotomy (PSO), and hybrid approaches. Class IV is reserved for the most severe rigid deformities, including patients with prior long fusions (e.g., >5 levels, including L5–S1) or those requiring >10 segments of treatment. These patients are candidates for open surgery with osteotomies.

The necessity of a classification algorithm such as MISDEF-2, which carefully triages patients, highlights that MIS techniques possess inherent corrective limitations. Despite these advancements, MIS techniques have a “ceiling effect” for radiographic correction, meaning they are less effective for severe, fixed deformities compared to open surgery [15,16]. A retrospective study reported that circumferential MIS was limited to a maximum SVA correction of 89 mm and 61% correction of the coronal Cobb angle. In this series, successful restoration of the PI–LL mismatch (within 10°) was achieved only in patients with a preoperative mismatch ≤38° [17]. Other studies have confirmed that MIS has difficulties restoring sagittal balance in patients with severe sagittal deformity (SVA >9.5 cm) [18]. This “ceiling” may be due to anatomical and technical constraints of the procedure. Lateral and percutaneous approaches inherently limit the magnitude and location of osteotomies, reduce the ability to perform multilevel three-column releases, and rely on indirect decompression and cage lordosis rather than on aggressive posterior shortening. Consequently, circumferential MIS is often insufficient for patients with rigid, multiplanar deformities requiring large sagittal corrections over multiple segments in whom open osteotomies are necessary.

Advanced MIS techniques: anterior column realignment

Advanced techniques such as ACR have been developed to address the limited corrective power of basic MIS. ACR is a powerful MIS procedure that involves releasing the anterior longitudinal ligament and placing a hyperlordotic interbody cage, often using a lateral transpsoas approach [19]. This technique can achieve substantial segmental lordosis, reportedly up to a 15° per level [20]. ACR is particularly useful for correcting fixed sagittal imbalance. Retrospective studies comparing ACR to the more invasive PSO reported that ACR achieved similar radiographic sagittal correction, but with significantly less intraoperative blood loss [21].

Complications of ACR and risk mitigation

Although ACR can achieve radiographic corrections comparable to those achieved by PSO, their risk profiles and indications are distinct. ACR relies on the controlled release of the anterior longitudinal ligament and placement of large hyperlordotic cages, which raises concerns regarding mechanical failure [22,23]. A recent retrospective series of elderly patients with severe degenerative sagittal imbalance treated with multilevel ACR reported that approximately one-third developed mechanical failure, predominantly proximal junctional kyphosis and rod fracture, with osteoporosis and a greater number of ACR levels emerging as independent risk factors [22]. Moreover, segment-based analysis of long-segment fusion for ASD has shown that constructs incorporating ACR or posterior-only fusion at ≥L4–5 are associated with a higher risk of rod fracture than posterior lumbar interbody fusion at these segments [23]. In addition, these data highlight the importance of strategies for mitigating mechanical failure when performing ACR, including limiting the number of ACR levels in patients with osteoporosis, optimizing bone health, reinforcing high-stress segments with multi-rod constructs, and carefully selecting the lowest instrumented vertebra to avoid excessive junctional stress.

Comparative indications with PSO

PSO represents the traditional posterior-only option for notable three-column shortening. However, it is typically accompanied by greater blood loss, longer operative times, and higher rates of neurological complications and rod fractures compared to less invasive strategies [24]. Retrospective comparisons have demonstrated that ACR can provide sagittal correction comparable to PSO in patients with SVA >6 cm, particularly when the deformity remains relatively flexible rather than rigid [25]. Current evidence and prevailing practice patterns indicate that the ACR is typically used in patients with focal or relatively flexible sagittal imbalance in whom additional segmental lordosis can be obtained without resorting to a posterior three-column osteotomy. In contrast, PSO is more often selected for rigid, often multiplanar deformities requiring large angular correction over a limited number of segments.

Single-position surgery

A significant trend is the move toward single-position surgery (SPS) to improve operative efficiency. Traditional MIS often requires the repositioning of patients, for example, supine for anterior lumbar interbody fusion (ALIF), lateral for lateral lumbar interbody fusion (LLIF), and prone for posterior instrumentation. This staged approach increases anesthesia time and operating room usage. SPS protocols allow both anterior/lateral and posterior tasks to be completed without repositioning. For instance, the lateral decubitus position can be used for LLIF, followed by simultaneous robot-assisted percutaneous pedicle screw fixation [26,27]. This approach is associated with decreased blood loss and shorter hospital stays than traditional two-stage surgery [28]. Similarly, the prone transpsoas approach allows the surgeon to perform LLIF and posterior instrumentation, with the patient remaining in the prone position [29–31].

Despite these workflow advantages, SPS has specific technical challenges, including limited fluoroscopic windows, altered anatomical orientation, and constrained access for revision maneuvers. Early retrospective single-surgeon and multicenter series of prone transpsoas LLIF and prone single-position extreme lateral interbody fusion (XLIF) have shown that single-position approaches are feasible and can reduce overall operative time by approximately 50 min compared with lateral decubitus XLIF with separate posterior staging, while maintaining comparable blood loss, clinical improvement, and perioperative complication rates [30,32]. In a consecutive series of single-position prone LLIF, intraoperative lateral access-related complications, such as cage subsidence and inadvertent anterior longitudinal ligament rupture, occurred in approximately 10%–15% of patients. Moreover, intraoperative radiation exposure was substantial, indicating that prone LLIF should be considered a distinct procedure with its own learning curve, even for surgeons experienced in standard LLIF [33]. A pooled analysis of 10 studies, including 286 patients treated with prone LLIF, reported low rates of major intraoperative events (e.g., anterior longitudinal ligament rupture in approximately 2% and segmental artery injury in approximately 2%) and a pooled postoperative complication rate of 3%–4%. Nevertheless, it highlighted frequent transient psoas-related symptoms, such as hip flexor weakness and thigh or groin sensory changes, and emphasized the paucity of long-term data [34]. Therefore, surgeons adopting SPS should progress gradually from simpler to more complex constructs with careful anatomical considerations, ideally within a structured proctorship or team-based training environment that emphasizes position-specific anatomical assessment, careful retractor placement, and standardized bailout strategies for intraoperative complications [30,32]. Taken together, the current evidence indicates that SPS has been applied mainly to shorter, less complex constructs in well-selected patients, and its routine use for extensive ASD correction remains in its infancy. As positioning systems, retractors, and image-guidance workflows continue to evolve and become more widely used, SPS could become an attractive option for ASD surgery that allows circumferential correction without requiring positional changes, particularly in centers with established expertise in MIS for deformity.

Clinical outcomes versus radiographic correction

A significant finding in the recent MIS literature is the relationship between radiographic parameters and patient-reported outcomes. Although radiographic alignment is a key goal [5,35–40], achieving “ideal” numbers may not be the only factor for clinical success. A multicenter study comparing MIS, hybrid, and open surgery found no significant differences in the Oswestry Disability Index (ODI) or Visual Analog Scale (VAS) scores at the 1-year follow-up, although the open and hybrid groups had greater radiographic correction [41].

These findings suggest that MIS can achieve comparable clinical benefits with less aggressive radiographic correction. Further analysis showed that patients who were technically “misaligned” postoperatively (often those with more severe preoperative deformity) still achieved a minimum clinically important difference at rates similar to “aligned” patients [42]. This finding is supported by studies showing that circumferential MIS achieves 1-year clinical outcomes comparable to those of open surgery, but with significantly reduced blood loss and intensive care unit stays [9]. However, systematic reviews and meta-analyses have noted that while MIS provides clear perioperative advantages, open techniques still tend to show greater improvements in LL and PT [43,44].

Instrumentation accuracy of robotic surgery

The primary benefit of robotic guidance is to improve the accuracy of pedicle screw placement (Table 2), and multiple meta-analyses have confirmed this advantage [10,45]. Studies have reported optimal screw placement (Gertzbein-Robbins grade A or B) rates between 95.5% and 99% [11,46–49]. One retrospective comparison found that robot-assisted placement was significantly more accurate (94% grade A) than freehand techniques (87% grade A) [50].

Table 2

Accuracy of robot-assisted pedicle screw placement

Improved accuracy directly translates to enhanced safety by minimizing iatrogenic tissue damage. A significant clinical advantage is the reduction in facet joint violations (FJVs), which are associated with adjacent segment disease. Robot-guided screw placement has been shown to result in significantly fewer FJVs compared to fluoroscopy-guided and freehand techniques [50,51]. Furthermore, robotic assistance is associated with reduced radiation exposure to the surgeon and staff compared with conventional fluoroscopic methods [52–57].

This high accuracy extends to complex procedures, such as sacropelvic fixation. Robotic guidance has shown high utility in placing S2-alar-iliac (S2AI) screws, which are technically demanding [58,59]. Reported accuracy rates for robot-guided S2AI screw placement are excellent, ranging from 95.7% to 100% in ASD cohorts [11,60].

Clinical outcomes of robotic surgery

Although robot-assisted systems reliably increase the proportion of radiographically “optimal” pedicle screw trajectories, most comparative studies have not demonstrated a consistent advantage in patient-reported outcomes or reoperation rates over well-performed freehand or fluoroscopy-guided techniques [61,62]. Randomized controlled trials and meta-analyses have generally reported similar improvements in ODI and VAS scores between robotic and conventional cohorts [61–63].

However, more recent meta-analytic data suggest that robot-assisted pedicle screw placement provides modest short-term clinical benefits [64]. A systematic review and meta-analysis of eight comparative studies (n=508) found that the robotic assistance was associated with slightly greater improvements in ΔVAS and ΔODI, reduced intraoperative blood loss, and shorter hospital stays, without a significant increase in operative time, compared with the freehand technique. However, the absolute differences in VAS and ODI were small, and it remains uncertain whether they consistently exceed the minimal clinically important difference thresholds across diverse patient populations and indications.

When these findings are viewed alongside recent randomized trials and meta-analyses, a nuanced pattern emerges: robot-assisted pedicle screw placement consistently improves technical parameters and, in some contemporary series, is associated with modest short-term gains in pain and disability scores and perioperative recovery, whereas overall clinical efficacy, complication profiles, and reoperation rates are still broadly comparable to those of carefully performed conventional techniques. As robotic platforms, imaging workflows, and surgeon experience evolve, these incremental advantages in patient-reported outcomes may become more pronounced in selected indications. However, currently, robotics is best regarded as a technology that enhances precision and reproducibility rather than a standalone guarantor of superior long-term results, with durable outcomes in ASD still determined primarily by global alignment correction, construct design, fusion biology, and the effective prevention and management of complications.

Navigation technology and clinical outcomes

Intraoperative navigation technology, both integrated with robotic platforms and as a standalone system, has been increasingly adopted for complex deformity surgeries. Navigation provides real-time visualization of the instrument position relative to the patient’s anatomy, theoretically improving accuracy and safety while reducing radiation exposure. Although technical accuracy is clear, whether this translates to superior clinical outcomes remains uncertain, highlighting the complex relationship between technical precision and patient-centered outcomes [65–67].

A retrospective series of navigated OLIF at L1–L5 (214 patients, 350 levels) supports these theoretical advantages at the technical level, showing notable cage placement accuracy and acceptable efficiency and safety [67]. The mean follow-up duration was 17.4 months, the mean operative time was 211 minutes (129.0 minutes per level), and 94.86% of cages were positioned within anterior-to-posterior quartiles 1–3 on postoperative lateral radiographs. Only one patient (0.47%) required revision for cage malposition, transient neurological symptoms occurred in 10.28% of the patients, and no vascular injuries were observed, indicating that navigation can facilitate precise cage placement without excessive complications in appropriately selected patients. In contrast, a retrospective study found that navigation was associated with higher reoperation rates (4.8% vs. 3.1%, p=0.03), but propensity-score matching revealed that this finding reflected case complexity rather than technology-related harm [65]. In addition, navigation was associated with higher rates of infection and neurological and medical complications. This mixed evidence suggests that the benefits of navigation are not universal but are rather concentrated in specific high-risk scenarios. Therefore, future validation should shift from general efficacy to specific utility in cases with complex anatomy, revision surgery, or significant instability, wherein technical precision is hypothesized to provide a distinct clinical advantage.

Patient-specific rods

Patient-specific rods (PSRs) are designed to improve the execution of preoperative plans. Traditional intraoperative manual rod bending has variable outcomes and can lead to undercorrection or overcorrection. In contrast, PSRs are based on individualized surgical alignment plans derived from software such as UNID Adaptive Spine Intelligence or Surgimap (Medtronic, Minneapolis, MN, USA) [11,66]. This structure allows the rod to be pre-contoured to match the exact target sagittal curvature [68,69]. Studies have shown that using PSRs leads to a stronger correlation between planned and achieved alignment goals [70]. Patients receiving PSRs have been found to be 2.6 times more likely to achieve optimal PI–LL alignment compared with those receiving traditional instrumentation, highlighting the improved precision of this technology [71].

Notably, pre-contoured rods do not eliminate the risk of in vivo rod deformation following implantation and postoperative rod flattening [72,73], particularly in stiff spines or constructs with residual sagittal undercorrection. Most available PSR studies have reported improved alignment that matched the preoperative plan at short- to mid-term follow-up [13,70,74,75]; however, robust data linking PSR use to superior long-term clinical outcomes, lower rod fracture rates, and reduced revision surgery remain limited. Further prospective studies with standardized alignment targets and patient-reported outcomes are required before PSRs can be considered definitively superior to carefully contoured intraoperative rods.

Patient-specific interbody cages

Similar to PSRs, PSI implants are engineered to match a patient’s unique anatomy. These implants are created using computer-aided design and additive manufacturing (3D printing) based on high-resolution imaging. This customization allows for improved endplate conformity and an optimized contact area compared with standard off-the-shelf cages [12,76]. The primary goal is to reduce the risk of implant migration and subsidence, particularly in osteopenic bones. Biomechanical studies have supported this finding, demonstrating that 3D-printed patient-specific cages have a significantly higher peak failure force and greater construct stiffness, indicating a lower risk of subsidence under physiological loads [77].

Early clinical reports have extended these biomechanical observations to complex clinical scenarios. In patients with destructive spondylodiscitis and segmental deformity, LLIF using patient-specific cages combined with supplemental fixation achieved substantial improvements in lordosis and coronal alignment, with marked reductions in VAS and ODI scores and no reported cases of cage migration, subsidence, or nonunion at short-term follow-up [78]. Beyond simply filling irregular defects, PSI constructs could restore anterior column height and contribute to multiplanar correction in anatomically challenging settings.

Recent studies on degenerative and deformity populations have demonstrated that personalized interbody implants can reliably achieve surgeon-defined segmental lordosis targets, with intervertebral lordosis at the treated level typically within approximately 1° and 5° of the preoperative plan in >80% of levels across ALIF, LLIF, and TLIF [79]. Short-segment fusion studies further suggest that correcting the lordosis distribution toward a more physiological L4–S1 contribution using personalized cages can normalize the lordosis distribution index in a substantial proportion of patients with preoperative hypo-or hyperlordotic patterns, potentially reducing biomechanical overload at adjacent segments [80]. These early data indicate that PSIs are a promising adjunct for achieving precise segmental alignment and robust anterior column support, and they may ultimately play an important role in ASD surgery by helping surgeons implement patient-specific sagittal alignment strategies over long constructs.

Advanced rod materials and construct design

Innovations have been made in both material science and construct design to combat implant failure. Rod fractures remain a major complication, with rates as high as 22% in high-stress constructs such as PSOs [81]. In response, new materials such as Molybdenum-Rhenium (MoRe; MiRus, Marietta, GA, USA) superalloys have been introduced. MoRe rods have demonstrated significantly higher fatigue resistance than traditional cobalt-chromium or titanium rods [82]. Moreover, they produce lower cobalt, nickel, and chromium ion concentrations, which may reduce local inflammation and improve the biological environment for fusion.

In addition to materials, construction design has advanced. Multiple-rod constructs (MRCs), which use accessory rods to support areas of high stress, are used to enhance stability and rigidity [83]. Historically used in open surgery, recent reports have shown that preoperative robotic planning software facilitates multiple-rod implementation of complex MRCs in minimally invasive settings [84,85]. This combination allows for enhanced construct stability with the benefits of MIS, such as low blood loss and rapid rod passage times.

Augmented reality and virtual reality

Augmented reality (AR) and virtual reality (VR) are emerging as crucial tools for training and intraoperative execution. VR, which creates a fully simulated 3D environment, has shown significant value in surgical education. Studies, including randomized controlled trials, have demonstrated that VR training can improve the accuracy of pedicle screw placement for trainees [86,87]. In contrast, AR overlays digital information onto the surgeon’s view of the real-world surgical field. This technology, often through head-mounted displays, can project preoperative plans and navigation data directly onto a patient’s anatomy. This approach allows the surgeon to focus on the operative site rather than looking at a separate monitor [88–90]. However, it is important to note that these technologies are still in their nascent stages. High-level evidence demonstrating their direct impact on long-term clinical outcomes in complex ASD surgery is currently limited, and further validation is required before their widespread adoption.

Predictive analytics and adaptive surgery

The next frontier is the integration of machine learning (ML) and artificial intelligence to create data-driven adaptive surgical plans. Future systems may move beyond simply executing a preoperative plan to becoming “surgical co-pilots.” These ML models may be surgeon- and institution-specific, providing granular risk stratification and recommending personalized alignment goals based on the surgeon’s historical outcomes.