ISLD is performed on horses with impinging or overriding dorsal spinous processes, however, the mechanical effect of such surgery cannot be easily identified from rest radiographic appearance. This project investigates computed tomography measurements of seven equine thoracolumbar spine sections from T11 to L1 in seven normal horses prior to and following ISLD at eight interspinous levels, quantifying interspinous distance, total section length, L1 translation, and L1 rotation. The study aim is to identify the primary mechanical effect of ISLD through rest interspinous distance, loaded interspinous aperture, range of adjacent interspace motion, or L1 end position during loading. There were four Quarter Horses, one Thoroughbred, one Warmblood, and one Spotted Saddle Horse, including four mares and three geldings, aged 16.3 ± 3.3 years and weighing 461 ± 46.6 kg. Rest interspinous distance was not affected by ISLD surgery (39.89 ± 2.84 mm to 39.90 ± 2.84 mm, p = 0.98), but there was a significant increase in total rest length from 310.0 ± 13.9 mm to 313.9 ± 11.8 mm (p = 0.03). Loaded measurements were significantly more affected by ISLD surgery. Interspinous distance during flexion significantly increased from 34.3 ± 17.4 mm to 44.4 ± 10.6 mm (p = 0.046). The range of motion of adjacent interspaces during flexion also significantly increased from 17.9 ± 3.0 mm to 23.5 ± 4.0 mm (p = 0.02). In addition, L1 dorsoventral excursion significantly increased from 32.6 ± 10.8 mm to 48.0 ± 14.0 mm (p = 0.01), L1 craniocaudal translation significantly increased from 22.9 ± 10.8 mm to 29.8 ± 10.9 mm (p = 0.03), and median rotation of L1 during flexion significantly increased from 12.0∘ to 21.8∘(p = 0.01). Such observations indicate that ISLD should be considered a phase-related rise in thoracolumbar mechanical mobility, with its maximal manifestation observed during the terminal lumbar phase, whereas static interspinous distance does not make an adequate measure for estimating postoperative mobility.