Ionizing radiation exposure and the development of intervertebral disc degeneration in humans: myth or reality

Ionizing Radiation Induces Disc Annulus Fibrosus Senescence and Matrix Catabolism via MMP-Mediated Pathways

Previous research has identified an association between external radiation and disc degeneration, but the mechanism was poorly understood. This study explores the effects of ionizing radiation (IR) on inducing cellular senescence of annulus fibrosus (AF) in cell culture and in an in vivo mouse model. Exposure of AF cell culture to 10–15 Gy IR for 5 min followed by 5 days of culture incubation resulted in almost complete senescence induction as evidenced by SA-βgal positive staining of cells and elevated mRNA expression of the p16 and p21 senescent markers. IR-induced senescent AF cells exhibited increased matrix catabolism, including elevated matrix metalloproteinase (MMP)-1 and -3 protein expression and aggrecanolysis. Analogous results were seen with whole body IR-exposed mice, demonstrating that genotoxic stress also drives disc cellular senescence and matrix catabolism in vivo. These results have important clinical implications in the potential adverse effects of ionizing radiation on spinal health.

Microgravity and Radiation Effects on Astronaut Intervertebral Disc Health

INTRODUCTION: The effects of spaceflight on the intervertebral disc (IVD) have not been thoroughly studied, despite the knowledge that spaceflight increases the risk of herniation of IVDs in astronauts upon return to Earth. However, as long duration missions become more common, fully characterizing the mechanisms behind space-induced IVD degeneration becomes increasingly imperative for mission success. This review therefore surveys current literature to outline the results of human, animal, and cell-level studies investigating the effect of microgravity and radiation exposure on IVD health. Overall, recurring study findings include increases in IVD height in microgravity conditions, upregulation of catabolic proteases leading to a weakening extracellular matrix (ECM), and both nucleus pulposus (NP) swelling and loss of annulus fibrosus (AF) fiber alignment which are hypothesized to contribute to the increased risk of herniation when reloading is experienced. However, the limitations of current studies are also discussed. For example, human studies do not allow for invasive measures of the underpinning biochemical mechanisms, correlating animal model results to the human condition may be difficult, and cellular studies lack incorporation of ECM and other complexities that mimic the native IVD microarchitecture and environment. Moving forward, the use of three-dimensional organoid culture models that incorporate IVD-specific human cells, ECM, and signals as well as the development of cell- and ECM-level computational models may further improve our understanding of the impacts that spaceflight has on astronaut IVD health.Smith K, Mercuri J. Microgravity and radiation effects on astronaut intervertebral disc health. Aerosp Med Hum Perform. 2021; 92(5):342352.

The age-dependent effect of high-dose X-ray radiation on NFκB signaling, structure, and mechanical behavior of the intervertebral disc

Purpose: Ionizing radiation damages tissue and provokes inflammatory responses in multiple organ systems. We investigated the effects of high-dose X-ray radiation on the molecular inflammation and mechanical function of the intervertebral disc (IVD).Methods: Functional spine units (FSUs) containing the vertebrae-IVDs-vertebrae structure extracted from 1-month, 6-month, and 16-month-old NFκB-luciferase reporter mice and from 6-month-old myeloid differentiation factor 88 (MyD88)-null mice. After a preconditioning period in culture, the FSUs were subjected a single dose of ionizing X-ray radiation at 20 Gys, and then NFκB expression was monitored. The IVDs were then subjected to mechanical testing using dynamic compression, glycosaminoglycan (GAG) quantification, and histological analyses.Results: In the 1-month-old FSUs, the NFκB-driven luciferase activity was significantly elevated for 1 day following the exposure to radiation. The 6-month-old FSUs showed increased NFκB activity for 3 days, while the 16-month-old FSUs sustained elevated levels of NFκB activity throughout the 10-day culture period. All irradiated groups showed significant loss of disc height, GAG content, mechanical function and changes in structure. Ablation of MyD88 blunted the radiation-mediated NFκB signaling, and preserved GAG content, and the IVDs' structure and mechanical performance.Conclusions: These results suggest that high-dose radiation affects the IVDs' NFκB-dependent inflammatory processes that subsequently lead to functional deterioration. Blocking the transactivation potential of NFκB via MyD88 ablation preserved the structure and mechanical function of the FSUs. The long-term effects of radiation on IVD homeostasis should be considered in individuals susceptible to occupational and medical exposure.

Comparison of Intended Lengthening of Magnetically Controlled Growing Rods: Ultrasound Versus X-Ray

BACKGROUND

In the treatment of early onset scoliosis (EOS), there has been a trend to use magnetically controlled growing rods (MCGR) in order to reduce the number of surgeries. To confirm the amount of lengthening, spine radiographs were required. Recently, ultrasound (US) has been added to monitor lengthening of MCGR to avoid radiation exposure. Our aim was to determine whether US is as accurate as plain radiography (x-ray) in determining the amount of length achieved at individual MCGR lengthening episodes.

METHODS

Retrospective study; inclusion criteria: EOS cases with dual MCGR with minimum 12 months follow-up. Intended lengthening IL (mm), lengthening on US (mm) and x-ray (mm) were documented from medical records for both right and left rods. Primary (no surgery before MCGR) and conversion (other types of instrumentation were replaced with MCGR) cases were reviewed separately. P-values determined with analysis of variance.

RESULTS

Sixteen cases with 100 lengthening episodes met the inclusion criteria. Eleven were primary MCGR cases with 67 episodes. Mean follow-up was 19±5 months. Significant differences were found between IL (3.4±1 mm), US (2.7±1.9 mm), and x-ray (4.1±2.2 mm) (P<0.001). The difference between IL and x-ray was minimal, but statistically significant (P=0.046). US showed statistically lower values than both IL (P=0.001) and x-ray (P<0.001). The mean ratio of x-ray/IL, US/IL, and US/x-ray were 1.1, 0.75, and 0.84, respectively. Five conversion cases had 33 episodes. Mean follow-up was 21±2 months. Significant differences were found between IL (3.4±0.8 mm), US (1.3±0.8 mm), and x-ray (1.7±0.9 mm) (P<0.001) but there was no significant difference between US and x-ray (P=0.283). IL was significantly higher than both US (P< 0.001) and x-ray (P<0.001). The mean ratio of x-ray/IL, US/IL, and US/x-ray were 0.64, 0.41, and 1.1, respectively.

CONCLUSIONS

US can provide confirmatory information of noninvasive lengthening of MCGR. However, US tended to underestimate the achieved length as measured by x-ray in primary cases. Conversion cases demonstrate better concordance between US and x-ray but in these cases less overall length was achieved at each lengthening episode.

LEVEL OF EVIDENCE

Level III.

Clinical utility of ultrasound to prospectively monitor distraction of magnetically controlled growing rods

BACKGROUND CONTEXT

Growing rods are commonly used for surgical treatment of skeletally immature patients with scoliosis, but require repeated surgeries for distractions and are fraught with complications. As an alternative, the use of magnetically controlled growing rods (MCGR) allows for more frequent non-invasive distractions to mimic normal growth. However, more plain radiographs are needed to monitor increased distraction frequency, thereby increasing ionizing radiation exposure to the developing child. The use of ultrasound, which emits no radiation, has been found in a cross-sectional study to be reliable in measuring MCGR distractions.

PURPOSE

The study aims to address the prospective clinical utility of ultrasound compared with plain radiographs for assessing MCGR distractions.

STUDY DESIGN

This is a prospective study.

PATIENT SAMPLE

The study includes patients with early-onset scoliosis undergoing distractions after MCGR implant.

OUTCOME MEASURES

The distraction length on plain radiographs and ultrasound was measured.

METHODS

This is a prospective study of patients treated with MCGR. Patients with both single- and dual-rod systems were included. Outpatient distractions were performed at monthly intervals, targeting 2 mm of distraction on each occasion. Assessment of distraction length was monitored by ultrasound at each visit; plain radiographs were taken every 6 months and were compared with ultrasound measurements.

RESULTS

Nine patients (5 female, 4 male), with a mean of 29 distractions (standard deviation [SD] ±14.3), were recruited. The mean distracted length per 6 months was 5.7 mm (SD ±3.6 mm) on plain radiographs and 5.2 mm (SD ±3.9 mm) on ultrasound for the concave rod, and 6.1 mm (SD ±3.6 mm) on plain radiographs and 5.9 mm (SD ±3.8 mm) on ultrasound for the convex rod. Excellent inter- and intra-rater reliabilities were observed for radiographic and ultrasound measurements. An excellent correlation was noted between the two imaging modalities (r=0.93; p<.0001).

CONCLUSIONS

This is the first prospective study to validate that ultrasound assessment of MCGR distraction lengths was highly comparable with that of plain radiographs. The present study has verified that ultrasound can be used to document length changes by distraction over time and that it had high clinical utility. Ultrasound can be a reliable alternative to plain radiographs, thereby avoiding radiation exposure and its potential detrimental sequelae in the developing child.

Ionizing radiation exposure and the development of intervertebral disc degeneration--no case to answer

In the following perspective article, Mellor and Breen provide a counterpoint to a previous perspective on the potential link between ionizing radiation exposure and intervertebral disc degeneration in humans [1]. The previous perspective asked, is this link a myth or reality? It suggested the potential for such a link. Mellor and Breen offer a drastically alternate view, in essence, that the question itself is flawed. To support their perspective, they explain the different units of radiation measurement and their conversion to risk in humans and how this impacts the previous perspective. They explain the variable sensitivity of different body tissues to radiation and highlight that neither human research nor any of the multiple international regulatory agencies have ever suggested that the intervertebral disc is sensitive to radiation. Finally, they claim that it is impossible to predict with any certainty the effects of low-level radiation on the intervertebral discs.

Ionizing radiation exposure and the development of soft-tissue sarcomas in atomic-bomb survivors

BACKGROUND

Very high levels of ionizing radiation exposure have been associated with the development of soft-tissue sarcoma. The effects of lower levels of ionizing radiation on sarcoma development are unknown. This study addressed the role of low to moderately high levels of ionizing radiation exposure in the development of soft-tissue sarcoma.

METHODS

Based on the Life Span Study cohort of Japanese atomic-bomb survivors, 80,180 individuals were prospectively assessed for the development of primary soft-tissue sarcoma. Colon dose in gray (Gy), the excess relative risk, and the excess absolute rate per Gy absorbed ionizing radiation dose were assessed. Subject demographic, age-specific, and survival parameters were evaluated.

RESULTS

One hundred and four soft-tissue sarcomas were identified (mean colon dose = 0.18 Gy), associated with a 39% five-year survival rate. Mean ages at the time of the bombings and sarcoma diagnosis were 26.8 and 63.6 years, respectively. A linear dose-response model with an excess relative risk of 1.01 per Gy (95% confidence interval [CI]: 0.13 to 2.46; p = 0.019) and an excess absolute risk per Gy of 4.3 per 100,000 persons per year (95% CI: 1.1 to 8.9; p = 0.001) were noted in the development of soft-tissue sarcoma.

CONCLUSIONS

This is one of the largest and longest studies (fifty-six years from the time of exposure to the time of follow-up) to assess ionizing radiation effects on the development of soft-tissue sarcoma. This is the first study to suggest that lower levels of ionizing radiation may be associated with the development of soft-tissue sarcoma, with exposure of 1 Gy doubling the risk of soft-tissue sarcoma development (linear dose-response). The five-year survival rate of patients with soft-tissue sarcoma in this population was much lower than that reported elsewhere.

Genotoxic stress accelerates age-associated degenerative changes in intervertebral discs

Intervertebral disc degeneration (IDD) is the leading cause of debilitating spinal disorders such as chronic lower back pain. Aging is the greatest risk factor for IDD. Previously, we demonstrated IDD in a murine model of a progeroid syndrome caused by reduced expression of a key DNA repair enzyme. This led us to hypothesize that DNA damage promotes IDD. To test our hypothesis, we chronically exposed adult wild-type (Wt) and DNA repair-deficient Ercc1(-/Δ) mice to the cancer therapeutic agent mechlorethamine (MEC) or ionization radiation (IR) to induce DNA damage and measured the impact on disc structure. Proteoglycan, a major structural matrix constituent of the disc, was reduced 3-5× in the discs of MEC- and IR-exposed animals compared to untreated controls. Expression of the protease ADAMTS4 and aggrecan proteolytic fragments was significantly increased. Additionally, new PG synthesis was reduced 2-3× in MEC- and IR-treated discs compared to untreated controls. Both cellular senescence and apoptosis were increased in discs of treated animals. The effects were more severe in the DNA repair-deficient Ercc1(-/Δ) mice than in Wt littermates. Local irradiation of the vertebra in Wt mice elicited a similar reduction in PG. These data demonstrate that genotoxic stress drives degenerative changes associated with IDD.