Objective : This multi-center clinical study was designed to determine the long-term results of patients who received a one-level posterior lumbar interbody fusion with expandable cage ($Tyche^{(R)}$ cage) for degenerative spinal diseases during the same period in each hospital. Methods : Fifty-seven patients with low back pain who had a one-level posterior lumbar interbody fusion using a newly designed expandable cage were enrolled in this study at five centers from June 2003 to December 2004 and followed up for 24 months. Pain improvement was checked with a Visual Analogue Scale (VAS) and their disability was evaluated with the Oswestry Disability Index. Radiographs were obtained before and after surgery. At the final follow-up, dynamic stability, quality of bone fusion, interveretebral disc height, and lumbar lordosis were assessed. In some cases, a lumbar computed tomography scan was also obtained. Results : The mean VAS score of back pain was improved from 6.44 points preoperatively to 0.44 at the final visit and the score of sciatica was reduced from 4.84 to 0.26. Also, the Oswestry Disability Index was improved from 32.62 points preoperatively to 18.25 at the final visit. The fusion rate was 92.5%. Intervertebral disc height, recorded as $9.94{pm}2.69;mm$ before surgery was increased to $12.23{pm}3.31;mm$ at postoperative 1 month and was stabilized at $11.43{pm}2.23;mm$ on final visit. The segmental angle of lordosis was changed significantly from $3.54{pm}3.70^{circ}$ before surgery to $6.37{pm}3.97^{circ}$ by 24 months postoperative, and total lumbar lordosis was $20.37{pm}11.30^{circ}$ preoperatively and $24.71{pm}11.70^{circ}$ at 24 months postoperative. Conclusion : There have been no special complications regarding the expandable cage during the follow-up period and the results of this study demonstrates a high fusion rate and clinical success.