RESUMEN
STUDY DESIGN: Case report OBJECTIVES: To report a case of motor weakness caused by the increasing size of a sacroiliac joint cyst after spinal fusion. SUMMARY OF LITERATURE REVIEW: There have been no reports on the increased size of a sacroiliac joint cyst and motor weakness after spinal fusion. MATERIALS AND METHODS: A 63-year-old female was admitted with low back pain and right sciatica. Magnetic resonance imaging (MRI) findings showed the spinal canal narrowing at L4-5 and a cystic lesion on the right sacroiliac joint. After surgery, the symptoms were relieved. RESULTS: One month after the operation, motor function had worsened to grade 4. Follow-up MRI revealed an increase in the size of the cystic lesion. Selective nerve root blocks were performed. There was gradual improvement, and the motor grade reached grade 5 seven months after the operation. CONCLUSIONS: We recommend that surgeons evaluate the adjacent segmental lesion by MRI before performing spinal fusion.