Background/Purpose: Vitamin D is a steroid hormone metabolized to its active form in the presence of parathyroid hormone in the kidney. Vitamin D is essential in the process of bone formation which in turn is vital for achieving spinal arthrodesis. Previous studies have indicated that 43% of patients who undergo orthopedic surgery are vitamin D deficient (Bogunovic et al. 2010). Animal studies demonstrated that vitamin D modulates consolidation of bone after grafting for spinal posterolateral fusion in rats (Metzger et al 2015). Low vitamin D levels and increased inflammatory markers have been found following total knee arthroplasty (Henriksen et al 2013). Studies have suggested that spinal fusion patients presenting with vitamin D deficiency achieved lower fusion rates and had higher rates of recurrent persistent back pain compared with patients with normal vitamin D levels (Rodriguez and Gromelski 2013). If patients are deficient in vitamin D preoperatively and their levels decrease postoperatively, do they ever attain the preoperative baseline and, if not, does this have any bearing on successful spinal fusion? To our knowledge no large prospective study has been published that measures changes in vitamin D levels immediately after surgical spinal fusion. We share this data as part of an ongoing study investigating the relationship between serum 25-hydroxyvitamin D 25(OH)D levels and spinal fusion outcomes 1 year after surgery.

Methods: In total, 103 participants were enrolled in this prospective cohort study of patients undergoing one or two-level primary posterior lumbar spinal fusion with four surgeons between 2016 and 2018. The current study describes a preliminary analysis of all patients who had serum 25(OH)D measured at baseline, postoperative day 1 (POD-1) and at a follow-up visit within six weeks after surgery (n=70, mean follow-up time=31.9 ± 20.0 days). Average age and BMI of participants were 64.1 ± 10.7 years and 28.7 ± 6.2 kg/m², respectively. 54% of patients were taking vitamin D supplements at baseline, and were on average older (p=0.005) and more likely to be female (p=0.001) than those who did not take supplements. Median differences in 25(OH)D were calculated at each time point and tested by Wilcoxon signed rank test.

Results: Median 25(OH)D was 35.3 ng/mL at baseline (range, 9.5-95.0 ng/mL) and 27.8 ng/mL at POD-1 (range, 7.0-63.5 ng/mL). Median change in 25(OH)D from baseline to POD-1 was -7.1 ng/mL or -20.9% (p< .0001). The majority (63%) of patients who had been taking vitamin D supplements at baseline had normal ( >30ng/mL) levels at POD-1, while this was true for only 19% of patients who were not taking vitamin D (p=0.001).

Conclusion: The results of this study suggest that many patients who undergo posterior spinal fusion have inadequate 25(OH)D levels at the time of surgery. These results raise the question of whether or not ultimate success of fusion could be affected by low vitamin D levels. Given the high prevalence of vitamin D insufficiency and deficiency in patients not taking supplements as well as in those that do, we suggest that it might be beneficial to optimize 25(OH)D levels prior to surgery. Further research examining the effect of low 25(OH)D levels and 25(OH)D optimization on fusion success is recommended.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/depression-of-vitamin-d-levels-after-adult-primary-posterior-spinal-fusion-are-we-adding-insult-to-injury/