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The spinal canal and neural foramina dimensions of cadaver lumbar spines were quantified during flexion and extension using magnetic resonance imaging before and after placement of an interspinous process implant.To quantify the effect of the implant on the dimensions of the spinal canal and neural foramina during flexion and extension.Lumbar neurogenic intermittent claudication symptoms are typically exacerbated during extension and relieved during flexion. It is understood that the dimensions of the spinal canal and neural foramen increase in flexion and decrease in extension. The authors hypothesized that an interspinous process implant would significantly prevent narrowing of the canal and foramina in extension and have no significant effect in flexion.Eight L2–L5 specimens were positioned to 15° of flexion and 15° of extension using a positioning frame. Each specimen was magnetic resonance imaged with and without an interspinous implant (X STOP) placed between the L3–L4 spinous processes. Canal and foramina dimensions were compared between the intact and implanted specimens using a repeated measures analysis of variance with a level of significance of 0.05.In extension, the implant significantly increased the canal area by 18% (231–273 mm2), the subarticular diameter by 50% (2.5–3.7 mm), the canal diameter by 10% (17.8–19.5 mm), the foraminal area by 25% (106–133 mm2), and the foraminal width by 41% (3.4–4.8 mm).The results of this study show that the X STOP interspinous process implant prevents narrowing of the spinal canal and foramina in extension.