Objective To explore the clinical outcomes of posterior lumbar interbody fusion using BTwin expandable spinal spacer with microendoscopic discectomy (MED) for lumbar disc herniation accompanying degenerative instability.Methods From March 2006 to May 2010,87 patients with lumbar disc heniation (only one level) accompanying degenerative instability were managed with posterior lumbar interbody fusion using B-Twin with MED,includeing 49 males and 38 females with an average of 47.6 years (range,37-65).Objective level located in L3,4 in 2 cases,L4,5 in 43,and L5S1 in 41.The patients were treated with single BTwin (Single group,n=51) and double B-Twin (Double group,n=36).Clinical outcomes were evaluated with surgical time,blood loss,visual analogue scale (VAS) scores,Oswestry disability questionnaire (ODI),and the pre- and post-operative disk space heights.Results The patients were followed up for an average of 35.8months (range,12-46).All the patients felt the low back pain and radiation pain disappeared or relieved apparently.The mean preoperative ODI and VAS scores decreased from 78％±3％ to 18％±3％,and (8.70±11.3)to (0.65±10.48) at the final follow-up respectively.Disc space increased from a pre-operative height of (8.76±1.3) mm to a post-operative of (11.8±0.6) mm.ODI,VAS and the disk space heights in all patient showed statistical significance,which revealed no statistical significance between the two groups.However,the operation time,blood loss were statistical difference between the two groups.All the patients achieved solid union or probable union at a mean time of 5.6 months (range,3.9-8.6).Conclusion Posterior lumbar interbody fusion using B-Twin with MED can obtain satisfactory outcomes in the treatment of lumbar disc herniation accompanying degenerative instability.Single B-Twin can get similar clinical outcomes,but shorter surgical time,less blood loss,and less medical costs.