Biomechanically, obesity can overload intervertebral discs, accelerate cervical disc degeneration and lead to abnormal stress on surrounding small joints, spinal ligaments and muscles, causing ASD (14). However, the relationship between BMI and the development of ASD following fusion surgery remains debated. Studies by Wei et al (14) and Zhong et al (18) demonstrate that elevated BMI is a risk factor for ASD in patients undergoing cervical ACDF and minimally invasive lumbar interbody fusion for degenerative lumbar conditions. Additionally, some investigations have suggested that a BMI >34 is associated with increased risk following lumbar fusion procedures (19). Conversely, certain studies have reported no correlation between BMI and the risk of ASD following both cervical ACDF (20) and adult lumbar spondylolisthesis fusion procedures (21). Notably, this discrepancy is more pronounced in the studies addressing cervical fusion, without accounting for the potential effect of postoperative changes in BMI. Therefore, in clinical practice, in addition to emphasizing preoperative BMI control, it is also crucial to focus on maintaining BMI within a reasonable range following fusion surgery for different anatomical regions, which warrants further in-depth investigation.

Osteoporosis reduces vertebral hardness, alters stress distribution and induces significant biomechanical changes in adjacent segments (22). After fusion surgery, the BMD of adjacent segments decreases compared with preoperative levels (23). Biomechanical studies demonstrate that in lumbar posterior interbody fusion models with osteoporosis, there is reduced pressure within adjacent intervertebral discs, decreased shear stress on fibrous rings and limited range of motion, which negatively influences ASD progression (24). Studies indicate a higher prevalence of ASD in postmenopausal women (25), suggesting that osteoporosis is a risk factor and a predictor of reoperation (19,22). Patients with low BMD undergoing SF surgery may experience implant failure, resulting in poor fusion and heightened stress on nearby vertebrae, worsening ASD onset (26). Studies have demonstrated that osteoporosis (T-score <-2.5) is associated with the development of ASD following ACDF (27). Among patients undergoing fusion for lumbar degenerative conditions, those with osteoporosis show a higher incidence of ASD (28). However, the reoperation rate in osteoporotic patients is lower compared with non-osteoporotic patients (7.4 vs. 13.1%) (28). Additionally, animal experiments illustrate the effectiveness of anti-osteoporotic drug therapy in mitigating intervertebral disc degeneration (IDD) near the lumbar fusion site in rats (29). However, some studies indicate no direct association between osteoporosis and ASD, possibly due to significant BMD variations before and after studies or measurement inaccuracies (19,30). These conflicting findings underscore the importance of monitoring vertebral BMD pre-fusion, implementing timely postoperative anti-osteoporosis treatments and adopting precise BMD measurement methods, including CT scans alongside the potentially less accurate Dual X-ray Absorptiometry (DXA) (23).

Research indicates that diabetes alters the composition and biomechanical properties of intervertebral disc, contributing to degenerative changes and increased spinal instability (10). Beyond microvascular complications, diabetes affects osteoclast and osteoblast function, leading to the release of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin (IL) 6 and IL-18, which impair bone graft vascularization, formation and remodeling (10,31). Diabetic patients undergoing multilevel fusion surgery face higher complication rates, including non-union and pseudoarthrosis, compared with non-diabetic individuals (10). Consequently, the risk of revision surgery for ASD following pseudoarthrosis exceeds the risk of the initial procedure (10,31). However, the literature indicates that diabetes is not a risk factor for the development of ASD in patients undergoing ACDF (20). While diabetes has no effect on fusion rates in lumbar fusion surgeries using cellular bone allografts (32), other findings reveal that diabetes is a significant factor for reoperation following lumbar fusion due to ASD, with a 44% higher revision rate compared with non-diabetic patients (10). Therefore, it is important to note that current literature lacks comprehensive studies on diabetic patients, particularly regarding preoperative blood glucose levels and postoperative glycemic control. Further clinical and animal studies are needed to confirm these findings.

Patients with pre-existing degenerative changes in the spine, such as disc degeneration, facet joint arthritis, vertebral slippage and spinal stenosis, face a higher risk of developing ASD before fusion surgery (33). These degenerative changes reduce disc height, alter biomechanical and increase pressure on neighboring segments (34). Research by Kim et al (35) found that MRI-detected disc degeneration was a significant predictor of ASD development. Similarly, one study has suggested that patients with pre-existing degeneration in adjacent disc segments are more prone to ASD following SF (2). Degenerative changes in adjacent facet joints also affect spinal mobility, stability and ASD progression (2). Research stresses that preoperative facet joint degeneration is a risk factor for the development of ASD (2). A study by Tan et al (36) further supports this, demonstrating a strong association between facet joint degeneration and an increased incidence of ASD following lumbar fusion surgery. Patients with grade III degeneration [according to the Weishaupt classification (37)] showed a markedly higher risk of developing ASD compared with those with grade I or II degeneration (38). Vertebral slippage can also lead to instability and heightened pressure on neighboring segments (39). Research indicates that preoperative vertebral slippage (spondylolisthesis) and spinal stenosis increase the risk of developing ASD following lumbar fusion and ACDF (40). Moreover, as the severity of postoperative spinal stenosis worsens, patients with ASD show significantly higher Visual Analog Scale scores (41) for back pain and Oswestry Disability Index scores (42) during follow-up compared with those without ASD (43). The current absence of comparative clinical studies leaves uncertainty regarding whether degenerative-related ASD results from fusion surgery or natural degeneration, necessitating further confirmation through animal experimental studies.

Spinal and pelvic sagittal parameter imbalance can result in lumbar spine instability, widely recognized as a risk factor for postoperative ASD (44). Proper alignment of the spine and pelvis ensures that the load distribution across the spine is balanced, minimizing excessive stress on individual segments. When there is an imbalance in spinopelvic parameters, such as pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS) and lumbar lordosis (LL), this balance is disrupted, leading to abnormal biomechanical forces on the adjacent segments, which accelerates their degeneration (45). A mismatch between PI and LL-especially when the difference exceeds 10°-forces adjacent spinal segments to compensate by altering their alignment. This compensation increases shear forces and mechanical stress on the adjacent discs and facet joints, leading to premature degeneration and, ultimately, ASD (44,45). One study has shown that degenerative changes in neighboring intervertebral discs are associated with elevated PT and persistent pelvic tilt mismatch with the PI-LL change, particularly in lower lumbar fusions (L4-S1) (44). SS is crucial for sagittal alignment as a compensatory mechanism (45). Postoperative PI-LL mismatch increases the risk of ASD 10-fold compared with controls (34). As PT increases, the center of gravity shifts forward, raising the spine axis deviation (SVA) and increasing stress on adjacent segments (46). After posterior lumbar interbody fusion (PLIF) at L4/5, higher preoperative vertical SVA, vertical PT angle, reduced LL and PI-LL mismatch are closely associated with ASD (45). Maintaining a sacral tilt angle >20° at L4/5 is vital for postoperative ASD prevention (34,45). Longer fusion segments in patients with Cobb angle >25° (47) are more effective in correcting spinal issues, but higher preoperative Cobb angle may contribute to postoperative ASD (34). Moreover, individuals with a greater C2-C7 SVA following ACDF are at an increased risk of ASD (20). Despite conflicting evidence, orthopedic surgeons should carefully consider these parameters during surgery, especially when feasible intraoperative CT scanning and measurements are available.

The incidence of ASD following spinal surgery varies depending on the surgical approach; however, there is ongoing debate regarding which technique is most effective in minimizing postoperative ASD. PLIF, due to its increased rigidity, places greater mechanical stress on adjacent segments, resulting in a higher incidence of ASD compared with posterolateral fusion (PLF) (50). A retrospective study revealed that the incidence of ASD in PLIF patients is 3.4 times higher than in PLF patients, with the 10-year ASD-free survival rate being significantly lower in the PLIF group (51). By contrast, transforaminal lumbar interbody fusion (TLIF) causes less disruption to posterior spinal structures, reducing ASD occurrence compared with PLIF (52). Anterior lumbar interbody fusion, which accesses the spine through the abdomen, avoids posterior disruption and has been linked to a lower incidence of ASD compared with both PLIF and TLIF (53). Furthermore, minimally invasive approaches, such as percutaneous fixation, cause less damage and significantly reduce ASD risk (16). Total disc arthroplasty was developed as an alternative to fusion, aiming to preserve segmental motion and potentially delay or prevent adjacent-level degeneration (54). Studies suggest that patients undergoing ACDF with anterior revision surgeries have higher rates of recurrent radiculopathy and ASD compared with those undergoing posterior revisions (55). Additionally, patients who undergo posterior cervical fusion tend to exhibit higher rates of early ASD compared with those who opt for anterior approaches (56). While the literature presents mixed findings regarding the risk of ASD with different surgical techniques, numerous uncontrollable variables, such as the severity of the condition, complicate the conclusions. Therefore, careful consideration of the surgical approach is crucial in planning SF surgeries to minimize the risk of ASD.

Tissue disruption during surgery, particularly damage to the paraspinal musculature, ligaments and facet joints, plays a critical role in ASD development. Surgical techniques that cause extensive damage to the paraspinal muscles, ligaments and joints significantly alter spinal biomechanics, thereby increasing stress on adjacent segments (57). One study has demonstrated a strong correlation between extensive joint resection and a higher incidence of ASD (57). Moreover, ASD is more commonly observed in the intervertebral disc above the fused segment rather than the segment below, probably due to the biomechanical changes that increase stress on the superior adjacent segment (3). This stresses the importance of preserving the superior facet joint capsule during the initial surgery, as it plays a key role in maintaining the stability and biomechanics of the adjacent upper segment, potentially preventing the development of ASD. Additionally, damage to the spinous processes and surrounding muscles during open surgery can lead to scar formation and alterations in spinal mechanics, further contributing to the ASD development (58). Comparative studies have shown that traditional open surgeries, which involve extensive muscle dissection, result in higher rates of ASD compared with minimally invasive surgeries, which improve the preservation of the integrity of surrounding tissues (57). Furthermore, research has indicated that minimally invasive surgeries, which cause less disruption to the paraspinal muscles and ligaments, are associated with a lower incidence of ASD (59).

It is well known that the primary goal of fusion is to alleviate pain and prevent further spinal deformity by stabilizing the affected vertebral segments. Therefore, different fixation methods aim to achieve physiological spinal motion and a higher fusion rate, improving biomechanical compatibility and reducing ASD (60). One study has shown that the more rigid the instrumentation used, the earlier ASD tends to develop after fusion (61). Varol et al (60) and Hsiao et al (62) found that, compared with traditional rigid systems, the Dynesys dynamic system induced less range of motion in adjacent joints and preserved the intervertebral disc structure of adjacent segments, thereby reducing the incidence of ASD. Further biomechanical studies have shown that the hybrid Dynesys-Transition-Optima system, composed of both dynamic (flexible and non-fusion) and static (rigid and fusion) components, can significantly reduce the range of motion at the fusion level (L4-L5), while improve the preservation of the mobility of stable segments. This results in a reduced range of motion at the transitional segments, which may help prevent the occurrence of adjacent segment degeneration (62). Similarly, research by Guan et al (63) revealed that non-fusion techniques incorporating dynamic stabilization not only reduced the incidence of ASD but also maintained spinal stability.

Conservative treatments are often the first line of defense in managing patients with ASD, particularly in those with mild to moderate symptoms. These treatments aim to relieve symptoms, improve functional capacity and delay the need for surgical intervention by addressing the mechanical and inflammatory aspects of ASD.

Following SF surgery for degenerative spinal diseases, patients often experience a protective state in the spine, making it challenging to full return to normal function quickly (82). Appropriate postoperative rehabilitation therapy not only improves functional capacity but also strengthens the muscles supporting the spine, helping distribute mechanical loads more evenly and reducing stress on adjacent segments (83). Although there is no consensus on the optimal timing, intensity and duration of rehabilitation after fusion surgery (83), early endurance and muscle strengthening exercises to restore core balance significantly enhance back strength, alleviate pain and reduce disability (83). Changes in paravertebral muscle size following cervical, lumbar and thoracolumbar fusion surgeries are significant risk factors for ASD (58,84,85). For instance, research by Xu et al (58) demonstrate a reduction in the functional area of the multifidus and erector spinae muscles, along with an increase in the functional area of the psoas major muscle following L4-S1 PLIF. Furthermore, Zhou et al (82) reveal that gait alterations (such as stride length, speed and cadence) and post-minimally invasive transforaminal interbody fusion may affect spinal-pelvic and lower limb joint parameters, underscoring the importance of postoperative rehabilitation in preventing ASD. Research indicates that physical therapy methods such as flexion-distraction techniques, high-velocity low-amplitude adjustments and thermotherapy may effectively relieve pain following fusion surgery (9,86). Additionally, therapies such as myofascial release and acupuncture, commonly used by chiropractors, benefit postoperative care following lumbar fusion surgery (9). Gliedt et al (87) report that stimulating multiple acupuncture points and auricular therapy effectively reduces postoperative pain. Compared with traditional rehabilitation, electroacupuncture demonstrates significant improvements in functional recovery following lumbar fusion surgery (88). To address ASD risk factors, it is essential to develop a detailed postoperative rehabilitation treatment plan that involves collaboration orthopedic and rehabilitation physicians.

Similar to the initial treatment for degenerative disc disease and radiculopathy, ASD can be managed with physical therapy, rehabilitation therapies such as early back bracing and lifestyle modifications such as avoiding excessive weight and bending (89). Medication, including non-steroidal anti-inflammatory drugs, steroids and muscle relaxants, can relieve clinical symptoms (90). Recently, small molecule drugs such as naringin have shown promise in preventing further degeneration of disc cells and enhancing regeneration, but most are still in early stages and have not been applied in clinical studies (91). Non-surgical treatment should be the first choice for ASD, as long as significant clinical improvement is observed, regardless of imaging findings (89). However, research comparing non-surgical and surgical treatments for ASD is lacking, highlighting the need for more in-depth studies.

Research indicates that ~20% of patients experience pain following surgeries for spinal stenosis or herniated discs, necessitating additional measures to alleviate this pain (92). When conservative treatments such as rehabilitation techniques and pharmacotherapy prove ineffective, ESIs may be used. Corticosteroids reduce inflammatory and edema by inhibiting inflammatory mediators, resulting in pain relief (93). Interventional treatments via different approaches, such as interlaminar, transforaminal and caudal, demonstrate promising outcomes for various types of chronic back pain (93). For instance, Song et al demonstrated that both transforaminal and caudal ESIs effectively alleviated chronic pain and improved function following spinal surgery (92). Additionally, Park et al showed that nerve root blocks and interlaminar epidural steroid injections were effective in relieving cervical radicular pain and enhancing function (94). While the relief provided by nerve root blocks is typically temporary, they can be used as a diagnostic tool to confirm the source of a patient's symptoms and guide further treatment decisions. However, the efficacy of ESIs in the treatment of ASD warrants further clinical investigation to establish its benefits conclusively.

ASD following SF surgery often involves degeneration of the facet joints, which can cause referred or radicular pain (95). Direct injection of medications into affected facet joints or thermal ablation to target pain-transmitting nerve fibers are viable treatment options (95). Evidence-based guidelines for managing chronic spinal pain recommends various facet joint interventions, including corticosteroid injections, saline injections, facet joint nerve blocks and radiofrequency ablation, depending on the specific spinal segment involved (96). Recently, procedures guided by ultrasound or CT improved the accuracy, safety and efficacy of these interventions (97). For example, Wong and Rajarathinam (97) demonstrate that the accuracy of ultrasound-guided injections into the cervical facet joints and their innervating nerves ranged from 78–100%, while the accuracy for lumbar facet intra-articular injections was between 86–100%. Compared with fluoroscopy or CT-guided methods, ultrasound guidance resulted in shorter procedure times with comparable pain relief outcomes (97). Similarly, research by Suputtitada et al (98) demonstrate that intra-articular injections of saline, corticosteroids and anesthetics provide favorable long-term clinical outcomes for patients with chronic low back pain. For patients who do not achieve long-term relief from facet joint injections, radiofrequency ablation (RFA) is a minimally invasive procedure that targets the nerve fibers transmitting pain signals from the degenerated facet joints (99). By using heat to ablate these nerve fibers, RFA provides longer-lasting pain relief compared with injections alone (99). A study has shown that RFA can significantly reduce pain and improve function in patients with chronic low back pain-related facet joint degeneration, making it a valuable tool in the interventional management of ASD (99). However, these treatment modalities are insufficiently emphasized in the management of ASD, with limited related literature available. Orthopedic surgeons should not focus solely on surgical interventions for ASD.

In cases of ASD following SF, only 6% of patients with significant clinical symptoms require additional surgical intervention (90). Surgical interventions are a crucial treatment option for patients with ASD, especially when conservative therapies fail to provide adequate symptom relief or when significant spinal instability and neurological deficits are present (16). Spinal instability or structural deformities, such as kyphosis or spondylolisthesis, often require surgical correction to prevent further degeneration and potential damage to the spinal cord or nerve roots (16). However, as with any surgical procedure, there are risks involved, including the potential for further degeneration at other levels of the spine. Careful preoperative planning and postoperative rehabilitation are essential to optimize outcomes and minimize complications. The choice of surgical procedure depends on the specific pathology and the patient's overall health status.

TDR is an alternative to fusion that involves replacing the degenerated disc with an artificial disc. TDR has emerged as a significant alternative to traditional fusion techniques for the treatment of degenerative disc disease and ASD. The primary advantage of TDR is its ability to preserve motion at the affected segment, which is in contrast to fusion, where the vertebrae are permanently immobilized (100). By maintaining the natural biomechanics of the spine, TDR reduces the mechanical stress placed on adjacent segments, potentially decreasing the risk of degeneration at these levels (100). TDR shows favorable outcomes in selected patients, with improvements in pain, function and range of motion (100). Rajakumar et al (100) demonstrate that cervical TDR surgery effectively alleviates nerve compression-related symptoms caused by ASD following ACDF. Additionally, none of the patients required further surgery at the same vertebral level during the three-year follow-up period. A study demonstrated that in the treatment of ASD, TDR provided an improved range of motion at C2-C7 over a follow-up period of more than one year compared with ACDF (40.2 vs. 35.1°; P=0.001) (101). Additionally, TDR shows comparable outcomes in terms of improvement in the neck disability index, neck visual analog scale and upper limb function (101). However, TDR is not suitable for all patients, particularly those with pre-existing joint degeneration. One study showed no difference in the incidence of ASD between TDR and fusion surgery (102). This finding necessitates more rigorous evaluation of the efficacy of TDR for treating ASD post-fusion surgery, including larger sample sizes and longer follow-up periods. Therefore, careful patient selection is crucial to ensure the success of the procedure.

Extended fusion surgery has been extensively studied and widely applied as an effective intervention for treating ASD following SF. This approach aims to reduce degeneration caused by biomechanical stress transfer by extending the fusion to include affected adjacent segments (103). It is particularly suitable for patients with significant ASD and accompanying clinical symptoms, such as spinal instability (90). This method involves the addition of instrumentation and performing fusion at the affected segments to stabilize the spine (104). Research indicates that combining extended fusion with decompression surgery alleviates symptoms and reduces the need for subsequent surgeries during long-term follow-up (104). However, compared with traditional surgeries, extended fusion is associated with greater trauma, longer operative times and potential complications (105). To address these drawbacks, recent advancements introduced modified techniques, such as using connectors to extend fixation without removing the existing hardware, which was showed to significantly reduce surgical trauma and costs (106). Nonetheless, further randomized controlled trials are essential to determine the long-term outcomes of various surgical strategies, particularly in the context of advancing minimally invasive techniques.

Decompression surgery is recommended for patients with significant nerve root compression, leading to radiculopathy or myelopathy following SF. However, it is not ideal for patients with pre-existing spinal kyphosis or instability (90,107). Procedures, such as laminectomy, laminoplasty and foraminotomy, aim to relieve the spinal cord or nerve roots pressure, alleviating pain and improving neurological function (107). Yang et al (107) found that laminectomy with instrumentation effectively improves symptoms and function in ASD patients following anterior cervical corpectomy and fusion (ACCF), though lordosis gradually declined. He et al (108) report that in ASD patients following ACCF or ACDF, laminoplasty provides satisfactory clinical outcomes when cervical lordosis was <10° and spinal canal encroachment occupied <50% of the canal's cross-sectional area. Früh et al (109) showed that microscopic decompression reduced operative time and trauma in lumbar ASD. However, microscopic surgery significantly reduced operative time and surgical trauma.

MIS has become an essential approach in the management of ASD due to its numerous benefits over traditional open surgery (59). MIS techniques, such as endoscopic decompression, minimally invasive fusion and robot-assisted cortical bone trajectory (CBT) screw fixation, aim to minimize the damage to paraspinal muscles, ligaments and other soft tissues, which are often compromised in open procedures. The preservation of these structures is critical for maintaining spinal stability and reducing the risk of further degeneration in adjacent segments (59). Han et al (110) demonstrate that percutaneous endoscopic lumbar foraminotomy and interlaminar decompression effectively alleviate ASD symptoms following lumbar decompression surgery. Feng et al (111) report that for elderly patients with ASD following lumbar fusion, presenting with unilateral radiculopathy or intermittent claudication and showing radiographic stability, percutaneous full-endoscopic lumbar discectomy is a viable alternative. Additionally, one study showed favorable outcomes with the unilateral biportal endoscopic approach (112). Furthermore, minimally invasive discectomy, without fusion, proves effective in treating new-onset cervical disc herniation in patients with previous multilevel fusion (113). Robot-assisted CBT screw fixation is identified as an effective salvage strategy for ASD after lumbar fusion (114). Moreover, MIS techniques can be combined depending on the disease characteristics, such as integrating percutaneous spinal endoscopy with fusion techniques and CBT screw placement (59). However, no standardized criteria currently exist for selecting surgical methods and further studies with larger sample sizes and longer follow-up periods are needed to evaluate the efficacy of these revision surgeries.

The OLIF technique accesses the intervertebral disc via the natural corridor between the peritoneum and the psoas muscle, minimizing trauma to posterior muscles, ligaments and other structures (115). This approach reduces the risks of vascular and nerve plexus injuries, as well as postoperative low back pain (116). OLIF also facilitates the removal of substantial disc tissue, increasing the fusion surface area, which enhances fusion rates (115,116). OLIF alone is an effective option for symptomatic adult ASD (115). Compared with PLIF, OLIF demonstrates superior outcomes in terms of the operative time, blood loss, postoperative complications and restoration of disc height (116). However, OLIF provides only indirect decompression, which may be inadequate for patients with large disc herniations, ossifications, or spinal stenosis (115). One study indicated that complication rates after lateral approaches (lateral lumbar interbody fusion, LLIF or OLIF) and posterior approaches (PLIF or TLIF) are similar when treating ASD. Lateral approaches may increase the risk of radicular pain due to manipulation of the psoas muscle (117). Therefore, surgical decisions must be carefully tailored to the specific characteristics of the patient's ASD.

With the increasing clinical application of ACDF and ACCF and longer follow-up periods, ASD has gained attention from spine surgeons, with a number of cases requiring surgical treatment. Scar tissue and prior anterior fixation devices limit the space for revision surgeries, often necessitating hardware removal, which prolongs operative time and increases blood loss (118). The Zero-P system, a stand-alone device developed for ACDF, reduces complications such as dysphagia and esophageal injury while providing the benefits of fusion and anterior plating (119). However, one study suggested that the Zero-P system is not recommended for single-segment ASD cases with severe ossification of the posterior longitudinal ligament, osteoporosis, or vertebral fractures (120). Moreover, there is a lack of prospective studies and long-term follow-up observations on its use in ASD surgeries, raising concerns about its long-term efficacy and safety in these specific patient populations.