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Adjacent Anatomy

Adjacent segment degeneration and disease following spinal fusion: radiographic markers, symptomatic progression, and risk factors for revision surgery.

Overview

Symptomatic adjacent-segment disease is a significant long-term complication following cervical arthrodesis, affecting more than one-fourth of patients within ten years [1]. The condition occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first decade after fusion [3]. While several risk factors for radiographic and clinical adjacent segment disease after anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF) have been identified, the supporting evidence remains very limited [4].

Surgical risk factors influencing the development of adjacent segment pathology include previous surgery, the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment [5]. Patients treated with posterior or combined anterior and posterior arthrodesis are far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis [2]. Dissection of posterior ligaments within instrumented segments while preserving ligaments at the adjacent level leads to a slight but not significant increase in range of motion [17].

The Topping-off approach resulted in a significantly lower incidence of adjacent segment degeneration (4.44% vs 25.93%) and better preservation of adjacent disc health at 3 years [13]. Both superior and inferior adjacent-level groups, along with ACDF and ACCF groups, maintained favorable clinical results in patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms [6].

Anatomy & Pathophysiology

Osseous and Joint Morphology

In the borderline hip population, there is high variability in acetabular coverage and femoral deformity, necessitating comprehensive deformity characterization to guide diagnosis and treatment decisions [37, 38]. For patients with degenerative spine and adverse pelvic mobility, unfavorable functional cup orientations are likely, resulting in prosthetic impingement [15]. Arthroplasty procedures that restore normal mechanics of the hip joint by producing a normal angular thrust eliminate hazards of dislocation [43]. In osteoporotic lumbar models, both cemented pedicle screw instrumentation (CPS) and cement-augmented pedicle screw instrumentation (CAPSI) increase range of motion (ROM) and disc stresses [21]. However, CAPSI is more likely to increase the potential risk of adjacent segment degeneration compared to CPS [21]. Hirayama disease patients exhibit differences in cervical biomechanics compared with non-pathological people, which may lead to cervical hypermobility and overload [39].

Ligamentous and Soft Tissue Dynamics

Preservation of the proximal multifidus muscle contributes to the maintenance of the physiological mechanical behavior of adjacent segments, thus preventing the occurrence and development of adjacent segment degeneration (ASD) [33]. Decompression combined with dynamic fixation for single-segment lumbar degenerative disease can significantly reduce postoperative low back stiffness compared with decompression combined with rigid fusion [44]. Transforaminal endoscopic lumbar discectomy combined with spinal dynamic stabilization system (TELD-SDSS) application alters the biomechanical environment of the adjacent segments [30]. TELD-SDSS has potential biomechanical advantages over posterior lumbar interbody fusion (PLIF) in the mitigation of ASD occurrence [30]. Spinal fusion surgery resulted in greater shear load magnitude and muscle activation, leading to greater forces at the epifusional segment in patients with ASD compared with those without ASD [31].

Kinematics and Alignment

Factors associated with the biomechanics of falling are of far greater importance than bone strength in the etiology of hip fractures [20]. The most noticeable changes in spinopelvic sagittal alignment occurred in the natural sitting position after lumbar fusion [35]. Some sagittal balance parameters may be associated with the development of ASD after anterior cervical surgery [32]. At 70 and 90 degrees of hip flexion, a combined change in 3D pelvic alignment (stagnation, axial rotation, and coronal tilt) of 5 degrees was more effective in improving hip maximum internal rotation than a 10-degree change in sagittal tilt only [42]. Improvement of biomechanical characteristics in the Gamma3 nail led to a significant decrease in complication rates compared to its predecessor [36].

Neurological and Clinical Assessment

Identifying biomechanical mechanisms that place young athletes at risk for hip injury enables anticipatory guidance and preventative strategies for these patients [24]. A thorough understanding of biomechanical principles and neurological pathways is necessary for the hand surgeon in upper limb spasticity management [27]. General principles for the physical examination of the spine include inspection, palpation, range of motion testing, and neurologic evaluation to identify spinal pathology, nonspinal conditions, and signs of symptom magnification [34].

Classification

Adjacent-Segment Disease (ASD) Incidence: Symptomatic adjacent-segment disease affects more than one-fourth of all patients within ten years after anterior cervical arthrodesis [1]. The condition occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years following fusion [3].

Surgical Approach and Risk Factors: Patients treated with posterior or combined anterior and posterior arthrodesis are far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis [2]. Surgical risk factors influencing the development of adjacent segment pathology include previous surgery, the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment [5]. Ligament and muscle damage are the most impactful iatrogenic factors contributing to adjacent segment degeneration and disease development [9]. Dissection of posterior ligaments within the instrumented segments while preserving ligaments at the adjacent level leads to a slight but not significant increase in range of motion [17].

Clinical Outcomes: Both superior and inferior adjacent-level groups, together with anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF) groups, maintained favorable clinical results in patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms [6].

Anatomical Planning and Variants: Recognition of anatomical details and variations is essential for accurate diagnosis and surgical decompression to avoid iatrogenic injury [7]. Providing detailed density maps of the bony anatomy may assist surgical providers in planning interventions [10]. Alignment guides are designed for operative definitions, and surgeons must convert these to radiographic definitions to determine safe zones [11]. This study provides information to determine the prevalence of anatomic variants in the general population [40].

Other Considerations: The authors propose modified diagnostic criteria that allow for focal or appreciable medullary involvement as long as the overall histology remains consistent with parosteal osteosarcoma [50].

Clinical Presentation

Symptomatic adjacent-segment disease affects more than one-fourth of patients within ten years following anterior cervical arthrodesis [1]. The condition occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years [3]. Patients treated with posterior or combined anterior and posterior arthrodesis are far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis alone [2].

The occurrence of symptomatic adjacent segment degeneration requiring surgery is multifactorial, related to BMI, preoperative adjacent disc degeneration on MRI, and disc bulge on preoperative CT [8]. Surgical risk factors influencing development include previous surgery, chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment [5]. Ligament and muscle damage are the most impactful iatrogenic factors contributing to adjacent segment degeneration and disease development [9].

Plain radiographic evaluation remains the initial diagnostic modality [14]. Three-dimensional imaging such as MRI and CT is often obtained for the evaluation of labral and cartilage pathology, definition of bony anatomy, and surgical planning [14]. Providing detailed density maps of the bony anatomy may also assist surgical providers in planning interventions [10]. Recognition of anatomical details and variations is essential for accurate diagnosis and surgical decompression to avoid iatrogenic injury [7].

Patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms maintained favorable clinical results in both superior and inferior adjacent-level groups, as well as in ACDF and ACCF groups [6]. Anatomical reduction is not necessary for good function, and the procedure allows patients to remain symptom-free for many years [22].

Abnormal morphology of the anterior acetabular horn is associated with anteroinferior instability in hip flexion [23]. The case represents a definite clinical syndrome associated with a specific anatomical lesion that is amenable to surgical repair [25]. Joint displacement due to adjacent inflammation is a disturbance that develops at several skeletal locations, not just the atlanto-axial joint, and offers a plausible explanation for spontaneous dislocations [28]. Giant cell tumors of the mobile spine with invasion into adjacent vertebrae are an unusual imaging finding [12].

Investigations

Plain radiography: Remains the initial diagnostic modality for femoroacetabular impingement [14].

MRI: Three-dimensional imaging, including MRI, is often obtained for the evaluation of labral and cartilage pathology, definition of bony anatomy, and surgical planning in femoroacetabular impingement [14]. MRI determination of posterior interosseus nerve (PIN) position is reliable and consistent with prior cadaveric study [49]. Findings on magnetic resonance images alone should not be used to determine a handicap or disability rating if clinical findings do not correlate, and MRI should remain an adjunctive diagnostic aid rather than a basis for operative intervention [52].

CT: Three-dimensional imaging, including CT, is often obtained for the evaluation of labral and cartilage pathology, definition of bony anatomy, and surgical planning in femoroacetabular impingement [14]. Providing detailed density maps of the bony anatomy may assist surgical providers in planning interventions for unilateral femoral acetabular impingement [10]. Disc bulge in preoperative CT examination is a factor related to the occurrence of symptomatic adjacent segment degeneration surgery [8].

Other Considerations: Symptomatic adjacent-segment disease may affect more than one-fourth of all patients within ten years after an anterior cervical arthrodesis [1]. Adjacent segment disease occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years following fusion [3]. Patients treated with posterior or combined anterior and posterior arthrodesis were far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis [2]. Surgical risk factors associated with previous surgery, including the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment, may influence the development of adjacent segment pathology [5]. The occurrence of a symptomatic adjacent segment degeneration surgery is most likely multifactorial and is related to BMI, preoperative adjacent disc degeneration on MRI, and disc bulge in preoperative CT examination [8]. Prognostic factors that influence outcomes after lumbar fusion surgery include higher BMI, larger proximal facet joint angle (FJA), and wider sagittal and coronal diameters [54]. Giant cell tumors of the mobile spine with invasion into adjacent vertebrae are an unusual imaging finding [12].

Treatment

Non-Operative

The evidence base provided does not contain specific data regarding conservative management options such as physical therapy, NSAIDs, or injections for adjacent segment disease.

Operative

Indications: Symptomatic adjacent-segment disease is a significant clinical concern, affecting more than one-fourth of all patients within ten years after an anterior cervical arthrodesis [1]. The condition occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years following fusion [3].

Surgical Approach / Technique: Surgical risk factors associated with previous surgery influence the development of adjacent segment pathology. These factors include the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment [5]. Patients treated with posterior or combined anterior and posterior arthrodesis were far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis [2].

In the cervical spine, both superior and inferior adjacent-level groups, along with anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF) groups, maintained favorable clinical results in patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms [6]. Several risk factors for radiographic and clinical adjacent segment disease after anterior cervical discectomy and fusion with plate fixation (ACDF-P) were identified, but the supporting evidence was very limited to limited [4].

In the lumbar spine, the Topping-off approach resulted in a significantly lower incidence of adjacent segment degeneration (4.44% vs 25.93%) and better preservation of adjacent disc health at 3 years compared to posterior lumbar interbody fusion [13]. Oblique lumbar interbody fusion (OLIF) was safe and effective for adjacent segment disease [19]. Preserving the posterior complex during decompression can be effective on preventing adjacent segment degeneration (ASD) following posterior lumbar interbody fusion (PLIF) surgeries [26]. The modified cortical bone trajectory technique may have a beneficial effect on reducing the incidence of adjacent segment degeneration in the L4-L5 transforaminal lumbar interbody fusion (TLIF) model compared to the traditional bone trajectory technique and cortical bone trajectory technique [29].

Complications

Adjacent-Segment Disease (ASD): Symptomatic adjacent-segment disease affects more than one-fourth of patients within ten years after anterior cervical arthrodesis [1]. The annual incidence is approximately 3%, with an expected cumulative incidence of 25% within the first decade [3]. Patients treated with posterior or combined anterior-posterior arthrodesis are far more likely to develop clinical adjacent-segment pathology requiring surgery compared to those treated with posterior decompression or anterior arthrodesis alone [2]. The occurrence of symptomatic adjacent-segment degeneration requiring surgery is multifactorial, related to BMI, preoperative adjacent disc degeneration on MRI, and disc bulge on preoperative CT [8]. Surgical risk factors influencing development include previous surgery, chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment [5]. Ligament and muscle damage are the most impactful iatrogenic factors contributing to adjacent segment degeneration and disease development [9]. Several risk factors for radiographic and clinical adjacent segment disease after anterior cervical discectomy and fusion with plate fixation have been identified, though supporting evidence remains very limited to limited [4]. Total disc replacement (Prodisc-C) showed only a little influence on adjacent segments at 10-year follow-up [16].

Other Considerations: Recognition of anatomical details and variations is essential for accurate diagnosis and surgical decompression to avoid iatrogenic injury [7]. A decrease in adjacent disc height poses high risks of adjacent segment complications after percutaneous kyphoplasty for treating osteoporotic vertebral compression fractures [46].

Recovery

Light activity (weeks): Evidence does not specify a week range for light activity, desk work, or driving.

Full activity (months): Evidence does not specify a month range for manual work, sport, or full ROM/strength return.

Complete recovery / outcome plateau (months): Adjacent-segment disease occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years following fusion [3]. Symptomatic adjacent-segment disease may affect more than one-fourth of all patients within ten years after an anterior cervical arthrodesis [1].

Rehabilitation protocol: Evidence does not specify PT phasing, immobilisation duration, weight-bearing/ROM progression, or sling/brace removal timing.

Functional milestones: Both superior and inferior adjacent-level groups together with ACDF and ACCF groups maintained favorable clinical results on patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms [6]. A 10-year follow-up of Prodisc-C showed that TDR had only a little influence on adjacent segments [16].

Other Considerations: Patients treated with posterior or combined anterior and posterior arthrodesis were far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis [2]. Surgical risk factors associated with previous surgery, including the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment, may influence the development of adjacent segment pathology [5]. Although several risk factors for radiographic and clinical adjacent segment disease after ACDF-P were identified, the supporting evidence was very limited to limited [4].

Key Evidence

  • [L3] Symptomatic adjacent-segment disease may affect more than one-fourth of all patients within ten years after an anterior cervical arthrodesis. (10.2106/00004623-199904000-00009)
  • [L3] Patients treated with posterior or combined anterior and posterior arthrodesis were far more likely to develop clinical adjacent-segment pathology requiring surgery than those treated with posterior decompression or anterior arthrodesis. (10.2106/jbjs.m.01482)
  • [L5] Adjacent segment disease occurs in approximately 3% of patients per year, with an expected incidence of 25% within the first 10 years following fusion. (10.5435/jaaos-21-01-3)
  • [L1] Although several risk factors for radiographic and clinical adjacent segment disease after ACDF-P were identified, the supporting evidence was very limited to limited. (10.2106/jbjs.21.01494)
  • [L4] Surgical risk factors associated with previous surgery, including the chosen surgical approach, anatomical dissection, choice of interbody fusion, increment and length of fusion, and restoration of sagittal alignment, may influence the development of adjacent segment pathology. (10.1302/2058-5241.6.210050)
  • [L4] Both superior and inferior adjacent-level groups together with ACDF and ACCF groups maintained favorable clinical results on patients who underwent one-level ACDF for symptomatic new radicular or myelopathic symptoms. (10.1186/s13018-016-0341-x)
  • [L5] Recognition of these anatomical details and variations is essential for accurate diagnosis and surgical decompression to avoid iatrogenic injury. (10.1016/j.hcl.2007.05.001)
  • [L3] The occurrence of a symptomatic adjacent segment degeneration surgery is most likely multifactorial and is related to BMI, preoperative adjacent disc degeneration on MRI, and disc bulge in preoperative CT examination. (10.1186/s13018-014-0097-0)
  • [L1] Ligament and muscle damage are the most impactful iatrogenic factors contributing to adjacent segment degeneration and disease development. (10.1186/s13018-025-05561-1)
  • [L4] Providing detailed density maps of the bony anatomy may also assist surgical providers in planning interventions. (10.1177/2325967124s00187)
  • [L5] Alignment guides are designed for operative definitions, and surgeons must convert these to radiographic definitions to determine safe zones. (10.1016/j.arth.2011.08.001)
  • [L4] Giant cell tumors of the mobile spine with invasion into adjacent vertebrae are an unusual imaging finding. (10.1186/s12891-021-04610-0)
  • [L3] The Topping-off approach resulted in a significantly lower incidence of adjacent segment degeneration (4.44% vs 25.93%) and better preservation of adjacent disc health at 3 years. (10.1186/s13018-019-1245-3)
  • [L5] Plain radiographic evaluation remains the initial diagnostic modality, while three-dimensional imaging such as MRI and CT is often obtained for the evaluation of labral and cartilage pathology, definition of bony anatomy, and surgical planning. (10.5435/00124635-201300001-00006)
  • [L3] Patients with a degenerative spine and adverse pelvic mobility are likely to have unfavorable functional cup orientations, resulting in prosthetic impingement. (10.1016/j.arth.2021.02.035)
  • [L3] A 10-year follow-up of Prodisc-C showed that TDR had only a little influence on adjacent segments. (10.1186/s13018-019-1194-x)
  • [L5] However, dissection of posterior ligaments within the instrumented segments while preserving ligaments at the adjacent level leads to a slight but not significant increase in range of motion. (10.1186/s12891-018-1967-0)
  • [L3] OLIF was safe and effective for adjacent segment disease. (10.1186/s13018-019-1276-9)
  • [L5] However, factors associated with the biomechanics of falling are of far greater importance than bone strength in the etiology of hip fractures. (10.2106/00004623-199072050-00008)
  • [L5] Biomechanical analysis showed that both CPS and CAPSI increase ROM and disc stresses in osteoporotic lumbar models, but CAPSI is more likely to increase the potential risk of adjacent segment degeneration compared to CPS. (10.1186/s13018-020-01650-5)
  • [L3] Abnormal morphology of the anterior acetabular horn is associated with anteroinferior instability in hip flexion. (10.1177/2325967120965564)
  • [L3] Identifying biomechanical mechanisms that place young athletes at risk for hip injury enables anticipatory guidance and preventative strategies for these patients. (10.1177/2325967113s00059)
  • [L4] The case represents a definite clinical syndrome associated with a specific anatomical lesion that is amenable to surgical repair. (10.2106/00004623-197355060-00016)
  • [L5] Preserving the posterior complex during decompression can be effective on preventing adjacent segment degeneration (ASD) following PLIF surgeries. (10.1371/journal.pone.0166452)
  • [L5] A thorough understanding of biomechanical principles and neurological pathways is necessary for the hand surgeon. (10.1177/17531934261434453)
  • [L5] Joint displacement due to adjacent inflammation is a disturbance that develops at several skeletal locations, not just the atlanto-axial joint, and offers a plausible explanation for spontaneous dislocations. (10.2106/00004623-195335040-00015)
  • [L5] The modified cortical bone trajectory technique may have a beneficial effect on reducing the incidence of adjacent segment degeneration in the L4-L5 TLIF model compared to the traditional bone trajectory technique and cortical bone trajectory technique. (10.1186/s12891-023-07103-4)
  • [L5] Although TELD-SDSS application alters the biomechanical environment of the adjacent segments, it has potential biomechanical advantages over PLIF in the mitigation of ASD occurrence. (10.1186/s12891-025-08825-3)
  • [L3] Patient-specific biomechanical simulation revealed that spinal fusion surgery resulted in greater shear load magnitude and muscle activation and therefore greater forces at the epifusional segment in those with ASD compared with those without ASD. (10.1186/s12891-021-04916-z)
  • [L1] Some sagittal balance parameters may be associated with the development of ASD after anterior cervical surgery. (10.1186/s12891-019-2800-0)
  • [L5] The preservation of the proximal multifidus muscle contributes to the maintenance of the physiological mechanical behavior of adjacent segments, thus preventing the occurrence and development of ASD. (10.1186/s12891-023-06649-7)
  • [L3] The most noticeable changes in spinopelvic sagittal alignment occurred in the natural sitting position after lumbar fusion. (10.1186/s12891-020-03777-2)
  • [L3] The improvement of the biomechanical characteristics has led to a significant decrease in complication rates, demonstrating superiority over its predecessor. (10.1155/2014/143598)
  • [L4] Comprehensive deformity characterization in the population is important to guide diagnosis and treatment decisions. (10.1177/2325967120s00346)
  • [L4] Comprehensive deformity characterization in the population is important to guide diagnosis and treatment decisions. (10.1177/2325967120s00210)
  • [L5] Compared with non-pathological people, Hirayama disease patients have differences in cervical biomechanics, which may lead to cervical hypermobility and overload. (10.1186/s13018-022-02984-y)
  • [L3] This study provides information to determine the prevalence of these anatomic variants in the general population. (10.1177/2325967120977892)
  • [L5] At 70 and 90 of hip flexion, a combined change in 3D pelvic alignment of 5- (ie, St, axial rotation, and coronal tilt) was more effective in improving hip maximum internal rotation than a 10 change in sagittal tilt only. (10.1177/23259671221123604)
  • [L4] The procedure restores normal mechanics of the hip joint by producing a normal angular thrust and eliminates hazards of dislocation. (10.2106/00004623-195133020-00017)
  • [L3] Compared with decompression combined with rigid fusion, decompression combined with dynamic fixation for single-segment lumbar degenerative disease can significantly reduce postoperative low back stiffness. (10.1186/s12891-023-06137-y)
  • [L3] A decrease in adjacent disc height posed high risks of adjacent segment complications after PKP for treating osteoporotic vertebral compression fractures. (10.1186/s13018-024-05248-z)
  • [L4] MRI determination of PIN position is reliable and consistent with prior cadaveric study. (10.1016/j.arthro.2020.12.118)
  • [Case_report] The authors propose modified diagnostic criteria that allow for focal or appreciable medullary involvement as long as the overall histology remains consistent with parosteal osteosarcoma. (10.2106/00004623-198769010-00020)
  • [L3] It also identifies several prognostic factors that influence outcomes after lumbar fusion surgery, including higher BMI, larger FJA, and wider sagittal and coronal diameters. (10.1186/s13018-025-05835-8)

See Also

References

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[30] Biomechanical effects of transforaminal endoscopic lumbar discectomy combined with spinal dynamic stabilization system use on adjacent segments: a finite element analysis. BMC Musculoskeletal Disorders. 2025. DOI: 10.1186/s12891-025-08825-3

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[40] Prevalence of Morphological Variations Associated With Femoroacetabular Impingement According to Age and Sex: A Study of 1878 Asymptomatic Hips in Nonprofessional Athletes. Orthopaedic Journal of Sports Medicine. 2021. DOI: 10.1177/2325967120977892

[42] Effect of 3-Dimensional Versus Single-Plane Changes in Pelvic Dynamics on Range of Motion in Hips With Femoroacetabular Impingement: A Computer Simulation Analysis. Orthopaedic Journal of Sports Medicine. 2022. DOI: 10.1177/23259671221123604

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