Purpose of study: The theoretical rationale for the disc replacement prosthesis as an alternative to arthrodesis is the avoidance of junctional degeneration by preservation of motion at the instrumented segment. The purpose of this study is to quantify radiographically the in vivo flexion-extension range of motion (ROM) of the Prodisc I disc prosthesis at long-term follow-up. In addition, the relationship between ROM of the prosthetic discs and junctional disc degeneration at the level above disc prostheses is examined.Methods used: This study is a retrospective review of active flexion-extension radiographs of 34 patients who had placement of 50 Prodisc I disc prostheses at an average follow-up of 8.6 years (7 to 10). All patients underwent disc replacement surgery for the treatment of chronic mechanical back pain secondary to degenerative disc disease. Flexion-extension ROM was measured by Cobb's method at every instrumented level, as well as at the cephalad junctional level. Junctional levels were examined for radiographic signs of disc degeneration, such as loss of disc space height or marginal osteophyte formation. Two-tailed unpaired Student's t testing was performed to examine the relationship between junctional degeneration and the ROM of disc prostheses below junctional levels.of findings: We found that ROM in the Prodisc I prosthesis is preserved in flexion and extension at long-term follow-up, although ROM is less than that reported in asymptomatic individuals. Mean flexion-extension ROM for disc prostheses was 3.5 degrees at L2–L3, 4.0 degrees at L3–L4, 4.5 degrees at L4–L5 and 3.2 degrees at L5–S1. Of the 34 junctional levels evaluated, 9 showed signs of disc degeneration. However, none of these required further surgery at last follow-up. The mean flexion-extension ROM of a prosthesis below a degenerated junctional disc was 1.4 degrees compared with 5.0 degrees for prostheses below a junctional disc without radiographic signs of degeneration (p=.015).Relationship between findings and existing knowledge: No previous studies have documented the in vivo ROM of the Prodisc I prosthesis at long-term follow-up. Furthermore, no studies have examined the relationship between the development of junctional degeneration and disc prosthesis ROM. It is not clear how much motion must be maintained to avoid junctional degeneration. Previous authors have found that flexion-extension ROM in asymptomatic individuals is so variable that norms of motion cannot be precisely defined [1].Overall significance of findings: The documentation of in vivo motion of disc prostheses at long-term follow-up is essential to justify their use. Without preservation of some motion, a disc prosthesis amounts to an expensive arthrodesis equivalent. It is evident from our study that there is a relationship between ROM of a disc prosthesis and the development of junctional disc degeneration. We believe that these data support the use of a disc prosthesis instead of arthrodesis in selected patients.Disclosures: Device or drug: Prodisc. Status: investigational.Conflict of interest: Federico Girardi, consultant; Federico Girardi, stockholder; Frank Cammisa, Jr., consultant; Frank Cammisa, Jr., stockholder; Thierry Marnay, consultant and stockholder: Spine Solutions Inc.