The effects of a new shape-memory alloy interspinous process device on the distribution of intervertebral disc pressures in vitro

This study was designed to measure the pressure distribution of the intervertebral disc under different degrees of distraction of the interspinous process, because of a suspicion that the degree of distraction of the spinous process may have a close relationship with the disc load share. Six human cadaver lumbar spine L2-L5 segments were loaded in flexion, neutral position, and extension. The L3-L4 disc load was measured at each position using pressure measuring films. Shape-memory interspinous process implants （SMID） with different spacer heights, ranging in size from 10 to 20 mm at 2 mm increments, were used. It was found that a SMID with a spacer height equal to the distance of the interspinous process in the neutral position can share the biomechanical disc load without a significant change of load in the anterior annulus. An interspinous process stabilizing device （IPD） would not be appropriate to use in those cases with serious spinal stenosis because the over-distraction of the interspinous process by the SMID would lead to overloading the anterior annulus which is a recognized cause of disc degeneration.

Interspinous Process Implants Causes Wear of the Spinous Processes in Patients Treated for Spinal Stenosis—An Experimental Biomechanical Study with Comparison to Clinical Cases

There are few biomechanical studies on Interspinous Process Implants (IPD);however none investigate the amount of wear on spinous processes. Therefore the objective of the present study was to investigate the effect of repetitive loading of the IPD Aperius on the spinous processes in a biomechanical porcine model. For comparison, three patients treated surgically with the same device have been followed for one to two years clinically and with image analyses (X-rays, MRI, CT-scans). Four lumbar spines from 6 months old porcine were divided into seven segments, which received IPD. The segments were exposed to 20,000 cyclical loads. Afterwards the deformation (wear) of the segments was registered. The wear of the spinous processes was measured in mm on a following CT-scan. Additionally, the wear of the ex-vivo was compared to that of the spinous processes investigated by CT-scans or X-ray in three patients treated surgically with the same interspinous implant. The mean maximal deformation of porcine specimens was 1.79 mm (SD 0.25) with the largest deformation occurring in the first quarter of the loading (<5000 cycles). The mean wear of the spinous processes after loading was 6.57 mm. A similar level of wear (mean 12.7 mm) of the spinous processes was detected in the patients. The Aperius IPD creates significant wear on the spinous processes in an experimental biomechanical study. Similar wear of the spinous processes is also present in patients treated with the same device post-operatively. How these findings influence the short and long term result of this implant device remains to be investigated in further biomechanical as well as clinical studies. For future development of this type of devices a proper selection of materials and design is essential to minimize wear effects on the spinous processes and thereby increases the possibilities for the devices to function as suggested.

Comparison of Topping-off and posterior lumbar interbody fusion surgery in lumbar degenerative disease： a retrospective study

Background Topping-off surgery is a newly-developed surgical technique which combines rigid fusion with an interspinous process device in the adjacent segment to prevent adjacent segment degeneration. There are few reports on Topping-off surgery and its rationality and indications remains highly controversial. Our study aims to investigate the short-term and mid-term clinical results of Topping-off surgery in preventing adjacent segment degeneration when mild or moderate adjacent segment degeneration existed before surgery. Methods The 25 cases that underwent L5-$1 posterior lumbar interbody fusion （PLIF） ＋ L4-L5 interspinous process surgeries between April 2008 and March 2010 formed Topping-off group. The 42 cases undergoing L5-$1 PLIF surgery formed PLIF group. Both groups matched in gender, age, body mass index and Pfirrmann grading （4 to 6）. The patients were evaluated with visual analogue scale （VAS） and Japanese orthopaedic association （JOA） scores before surgery and in the last follow-up. Modic changes of endplates were recorded. Results The follow-up averaged 24.8 and 23.7 months. No symptomatic or radiological adjacent segment degeneration was observed. There was no significant difference in intraoperative blood loss or postoperative drainage. VAS and lumbar JOA scores improved significantly in both groups （t=-12.1 and 13.5, P 〈0.05）. Neither anterior nor posterior disc height was significantly changed. Segmental lordosis of L4-L5 and total lordosis were all increased significantly （Topping-off group： t=-2.30 and -2.24, P 〈0.05; PLIF group： t=--2.76 and -1.83, P 〈0.01）. In the hyperextension and hyperflexion view, Topping-off group＇s range of motion （ROM） and olisthesis in the L4-L5 segment did not significantly change in flexion, but decreased in extension. In PLIF group, ROM （t=--7.82 and -4.90, P 〈0.01） and olisthesis （t=--15.67 and -18.58, P 〈0.01） both significantly increased in extension and flexion. Conclusions Compared with single segment PLIF surgery, Topping-off surgery can achieve similar symptomatic improvement in cases with pre-existing mild or moderate adjacent segment degeneration, restrict the adjacent segment＇s ROM in extension and prevent excessive olisthesis of adiacent seament in both extension and flexion