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THE EFFECTS OF OVER-DISTRACTION ON ADJACENT SEGMENT DISEASE
AND CAGE SUBSIDENCE IN ANTERIOR CERVICAL DISCECTOMY AND FUSION
Azimov Ulugbek Mehriddinovich
Neurosurgeon at the Bukhara Regional Multidisciplinary Hospital
Email: neuro1086@gmail.com
https://doi.org/10.5281/zenodo.13923629
Abstract
Over-distraction has been identified as a risk factor for cage subsidence and postoperative
neck pain following anterior cervical discectomy and fusion (ACDF). Biomechanical studies
have also shown increased intradiscal pressure at adjacent segments after ACDF. This study
aims to investigate whether over-distraction of the index disc affects adjacent segment
pathology. A retrospective review was conducted on 145 patients who underwent primary
ACDF for cervical degenerative conditions between January 2016 and December 2023. Patients
were categorized into two groups: (1) Over-distraction group (postoperative–preoperative
index disc height difference ≥ 2 mm), and (2) No-distraction group (postoperative–
preoperative index disc height difference < 2 mm). Outcome measures included radiographic
parameters, Japanese Orthopaedic Association (JOA) score, and the incidences of cage
subsidence, as well as radiological and clinical adjacent segment pathologies (RASP and CASP),
which were assessed preoperatively, postoperatively, and at final follow-up.
Both groups were similar in terms of age, length of follow-up, JOA score, CASP incidence,
and radiographic parameters. However, the Over-distraction group (83 patients, 115 levels)
had a lower preoperative index disc height (4.5 mm vs. 5.2 mm, p < 0.001) and a greater
postoperative index disc height (7.7 mm vs. 6.6 mm, p < 0.001) compared to the No-distraction
group (62 patients, 90 levels). The Over-distraction group also exhibited significantly higher
rates of cage subsidence (47% vs. 31%, p = 0.04) and RASP (any progression: 48% vs. 15%, p <
0.001; progression ≥ 2 grades: 25% vs. 7%, p = 0.001). Multivariate analysis revealed that both
over-distraction and multilevel fusion were independent risk factors for RASP. Despite these
findings, no differences in clinical outcomes were observed between the two groups. The study
concludes that over-distraction of the index level by ≥ 2 mm should be avoided, as it
significantly increases the risk of RASP and cage subsidence.
Keywords:
Discectomy and fusion (ACDF), polyetheretherketone (PEEK), adjacent
segment pathology (ASP), radiological adjacent segment pathology (RASP).
Introduction
Since its introduction by Smith and Robinson, anterior cervical discectomy and fusion
(ACDF) has been established as a safe and effective treatment for cervical degenerative
conditions. The procedure involves the complete removal of disc material and protruding
osteophytes, followed by the placement of autologous bone grafts, allografts, interdiv spacers,
or combinations thereof. A variety of materials are used as interdiv spacers, with
polyetheretherketone (PEEK) cages being widely favored due to their excellent
biocompatibility and radiolucency, which allows for easier assessment of fusion. Porous
tantalum trabecular metal (TM) cages are also notable for their high porosity and
biomechanical properties, which closely mimic bone. Other materials, such as carbon fiber
composite cages, offer an elastic modulus similar to bone, reducing stress shielding and
promoting bone fusion, as described by Wolff's law. However, titanium cages have been
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associated with subsidence, leading to disc height collapse and kyphotic deformity in previous
studies.
One major concern following ACDF is adjacent segment pathology (ASP), which occurs at
an annual incidence of 2.9%. ASP can be classified into clinical adjacent segment pathology
(CASP) and radiological adjacent segment pathology (RASP), depending on whether it presents
with clinical symptoms. CASP typically manifests as radiculopathy, myelopathy, or axial neck
pain. Various surgical techniques have been suggested to reduce ASP risk, including motion-
preserving total disc arthroplasty, limiting damage to adjacent discs and the anterior
longitudinal ligament, and restoring normal cervical sagittal alignment.
Cage subsidence is another complication that can negatively affect surgical outcomes.
Subsidence leads to decreased foraminal height, local kyphosis, and disrupted cervical sagittal
alignment as the cage sinks into the vertebral div. Although anterior plating is sometimes
used to enhance stability and fusion rates, it has been associated with complications such as
postoperative dysphagia, ASP due to anterior longitudinal ligament damage, and reduced
micromotion at the bone-graft interface due to rigid fixation.
To restore disc height, surgeons often select larger-sized grafts or cages during ACDF.
However, larger cages (greater than 5.5 mm) have been linked to a higher risk of subsidence.
Additionally, studies have shown that patients with cage subsidence had significantly higher
distraction ratios than those without. The goal of our research is to explore how over-
distraction impacts radiographic parameters, cage subsidence, adjacent segment pathology,
and clinical outcomes in patients undergoing ACDF surgery.
Materials and Methods
This retrospective study reviewed a consecutive series of patients who presented with
myelopathy or radiculopathy caused by cervical degenerative pathologies and underwent
primary anterior cervical discectomy and fusion (ACDF) between January 2016 and December
2023 at our institute.
The inclusion criteria were: (1) Patients who underwent primary ACDF at 1 or 2 levels,
and (2) Patients with a follow-up period of at least 24 months. Exclusion criteria included: (1)
Surgery for non-degenerative conditions such as traumatic injury, malignant tumors, or
infection; (2) ACDF performed at more than 3 levels; (3) Patients who received cervical disc
prostheses; (4) Revisional cervical spine surgeries; (5) Patients who underwent anterior plate
augmentation, and (6) Follow-up period of less than 24 months.
Results
Comparisons of Radiographic Parameters Between the Two Groups
All measurements were taken by two spine fellows, and the interrater agreement was
excellent, with an ICC of 0.907 (95% CI 0.822–0.950). The Over-distraction group had a
significantly smaller preoperative index disc height (4.5 mm vs. 5.2 mm, p < 0.001), but a
greater postoperative index disc height (7.7 mm vs. 6.6 mm, p < 0.001) and local Cobb angle
(9.8° vs. 7.7°, p = 0.04) compared to the No-distraction group. However, by the final follow-up,
the index disc height (4.8 mm vs. 4.5 mm, p = 0.35) and local Cobb angle (5.2° vs. 5.0°, p = 0.86)
were similar between the two groups. All other radiographic parameters, except for
postoperative cSVA (25.8 mm vs. 19.8 mm, p = 0.03), were comparable between the groups at
the preoperative, postoperative, and final follow-up time points.
Risk Factors for RASP
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In the univariate analysis, significant risk factors included over-distraction, multilevel
fusion, TM cage implantation, smaller preoperative neck tilt, longer follow-up duration, and a
smaller preoperative local Cobb angle. Multivariate analysis identified over-distraction (Odds
Ratio [OR] = 7.52; 95% Confidence Interval [CI]: 2.82–20.04) and multilevel fusion (OR = 2.52;
95% CI: 1.12–5.68) as independent risk factors for RASP.
Discussion
Porous tantalum cages and polyetheretherketone (PEEK) cages are commonly used in
ACDF. Porous tantalum cages have biomechanical properties similar to cancellous bone,
allowing for efficient load transfer and minimizing the stress-shielding effect. PEEK, on the
other hand, is chemically inert and does not promote protein absorption or enhance cell
adhesion and bone contact when compared to titanium. Both TM cages and PEEK cages have
demonstrated high fusion rates in both the cervical and lumbar spine. However, previous
studies have shown that TM cages have a higher rate of subsidence compared to PEEK cages. In
this study, we included patients who underwent ACDF with either TM or PEEK cages, and the
overall fusion rate of 88% was consistent with prior reports.
Distraction of the intervertebral disc space helps improve visibility during disc material
removal and posterior osteophyte resection. However, excessive distraction force can cause
damage to the facet joints at the index level. Kirzner et al. reported that over-distraction of the
facet joint by 3 mm or more is associated with poorer functional outcomes and higher pain
scores in patients with traumatic cervical injuries treated with ACDF. Additionally, excessive
distraction may increase the pressure on adjacent discs and stress on surrounding structures.
Yuan et al. conducted a finite element study investigating the effect of disc arthroplasty height
on cervical biomechanics, showing that prostheses with a height increase of 2 mm or more
significantly raised intradiscal pressure, facet joint forces, and bone-implant interface stress
compared to prostheses with less than 2 mm of height increase.
Interestingly, regardless of the degree of distraction, our results showed that the index
disc heights in both groups collapsed to a similar extent at the final follow-up. The physiological
tension of surrounding muscles and ligaments may play a role in determining the final disc
height after ACDF with grafts or cages. Aryan et al. suggested relaxing the distraction force after
discectomy, demonstrating that the same level of distraction could be achieved with 20N less
force after the removal of intervertebral discs. Given that excessive distraction force may
interfere with physiological tension and influence the selection of graft or cage size, we
recommend releasing the distraction force during sizing trials to avoid selecting oversized
grafts or cages, as supported by the findings of this study.
Conclusions
In conclusion, over-distraction of the index level by 2 mm or more significantly increases
the risk of RASP, CASP, and cage subsidence. Based on these findings, we recommend releasing
the distraction force during graft/cage size selection in ACDF to prevent over-distraction of the
disc height, thereby reducing the incidence of RASP, CASP, and cage subsidence.
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