Abstract
To investigate the progression of adult spinal process impingement (SPI) and provide a staging system through a retrospective long-term MRI-based longitudinal study. From April 2005 to April 2025, 132 patients who underwent at least four lumbar MRI scans prior to index lumbar surgery were enrolled. The mean follow-up duration was 11.7 ± 3.2 years. Patients were divided into two groups: group 1 (no SPI; n = 70, mean age 60.0 ± 6.4 years, F:M = 45:25) and group 2 (with SPI; n = 62, mean age 67.2 ± 7.1 years, F:M = 43:19). A novel SPI staging system and grading system for herniation of the interspinal ligament (HISL) was established. The analysis focused on the L3/S1 levels. Radiographic assessment included standing anteroposterior and flexion-extension lumbar radiography. Osteoporosis was evaluated using DEXA scans. In group 1, two interspinal ligament patterns were observed: normal MRI (76.7%) and fatty degeneration (23.3%). Group 2 showed 107 SPI levels: 25 at L3/L4, 61 at L4/L5, and 21 at L5/S1 levels. The SPI progresses through four stages: early (I), soft tissue (II), bone (III), and spinal stenosis (IV), with variable intervals. Osteoporosis is a key factor in the development of SPI. HISL appeared in 28 segments of group 2 but none in group 1 (p < 0.001). Of the HISL cases, 12 were mild, 9 were moderate, and 7 were severe. Lumbar decompression was performed in four moderate and five severe cases. This long-term longitudinal MRI study delineated the stages and natural progression of SPI. HISL was strongly associated with the SPI, and its size was the principal factor contributing to spinal stenosis.