Effects of Spina Bifida Occulta on Bone Union in Fifth Lumbar Spondylolysis.

Introduction: The effect of spina bifida occulta (SBO) on bone union in lumbar spondylolysis is controversial. The study aim was to assess the effect of SBO on bone union after conservative treatment of L5 spondylolysis, the most common level.

Methods: The study included 191 lesions in 145 patients (mean age, 14.0 years) with conservatively treated L5 spondylolysis. We examined the relationships between bone union after conservative treatment and the number of SBOs, levels, and single or multilevel status. Fisher's exact probability test, chi-square test, and Welch's T test were performed.

Results: The SBO incidence was 53%, with at least one SBO at any vertebral level. SBO at S1 (p=0.034) or S2 (p=0.0003), two SBOs (p=0.0018), and three SBOs (p=0.011) were associated with a lower bone union rate. The bone union rate was significantly lower for lesions with SBOs at both S1 and S2 than without (42% vs. 79%; p<0.0001).

Conclusions: The SBO incidence in L5 spondylolysis was 53%. SBO at S1 or S2 and a higher number of SBOs were associated with lower bone union rates. In particular, the bone union rate of lesions with SBOs at both S1 and S2 was <50%.