Biomechanical effect of graded minimal-invasive decompression procedures on lumbar spinal stability.

INTRODUCTION: Decompression surgery represents the standard operative treatment for lumbar spinal stenosis, but this procedure is often combined with fusion surgery. It is still discussed whether minimal-invasive decompression procedures are sufficient and if they compromise spinal stability as well. The aim of this study was to analyze the effects of different minimal-invasive decompression procedures on the range of motion (ROM) of the decompressed and adjacent segments under preload conditions. METHODS: Fourteen fresh frozen human cadaver lumbar spines (L2-L5) were tested in a spinal testing device with a moment of 7.5 N m in flexion/extension, lateral bending and rotation with and without a preload. The ROM of the decompressed segment L3/4 and the adjacent segments L2/L3 and L4/L5 was measured intact and after creating a gradual defect with resection of the interspinous ligament (ISL), bilateral undercutting decompression, detachment of the supraspinous ligament (SSL) and bilateral medial facetectomy. RESULTS: The resection of the ISL had no significant effect on the ROM of all segments. Undercutting decompression showed a significant increase in the ROM of all segments during flexion/extension and lateral bending. The detachment of the SSL caused a significant increase of ROM during flexion/extension in the instrumented and adjacent segments. After bilateral medial facetectomy, a decrease of ROM was observed in all directions of motion except flexion/extension with preload. CONCLUSIONS: The results support minimal-invasive procedures for the preservation of spinal stability. Therefore, surgeons can determine which grade of decompression procedure can be performed in the individual patient without requiring additional fusion to maintain spinal stability.

Biomechanical effect of different interspinous devices on lumbar spinal range of motion under preload conditions.

INTRODUCTION: Interspinous devices are used as an alternative to the current gold standard treatment, decompressive surgery with or without fusion, for lumbar spinal stenosis. They are supposed to limit extension and expand the spinal canal and foramen at the symptomatic level, but still allow lateral bending and axial rotation in the motion segment. The aim of the present study is the biomechanical evaluation of the change in the range of motion of the affected and adjacent segments following implantation of different interspinous devices under load in all directions of motion. METHOD: Eight fresh frozen human cadaver lumbar spines (L2-L5) were tested in a spinal testing device with a moment of 7.5 nm in flexion/extension, lateral bending and rotation with and without a preload (follower load of 400 N). The ROM was measured after implantation of Aperius (Kyphon, Mannheim), In-Space (Synthes, Umkirch), X-Stop (Tikom, Fürth) and Coflex (Paradigm Spine, Wurmlingen) into the segment L3/L4. RESULTS: All interspinous devices caused a significant reduction in extension of the instrumented segment without significantly affecting the other directions of motion. The flexion was reduced by all implants only when the follower load was applied. All devices caused a higher ROM of the whole spine during lateral bending and rotation. CONCLUSION: The actual evaluated interspinous devices led to a significant reduction in ROM during flexion-extension, but to a significant increase in ROM for the whole specimen (L2-L5) during lateral bending and rotation, which could increase the risk of adjacent segment degeneration.

Impact of Different Screw Designs on Durability of Fracture Fixation: In Vitro Study with Cyclic Loading of Scaphoid Bones.

PURPOSE: The use of new headless compression screws (HCSs) for scaphoid fixation is growing, but the nonunion rate has remained constant. The aim of this study was to compare the stability of fixation resulting from four modern HCSs using a simulated fracture model to determine the optimal screw design(s). METHODS: We tested 40 fresh-frozen cadaver scaphoids treated with the Acumed Acutrak 2 mini (AA), the KLS Martin HBS2 midi (MH), the Stryker TwinFix (ST) and the Synthes HCS 3.0 with a long thread (SH). The bones with simulated fractures and implanted screws were loaded uniaxially into flexion for 2000 cycles with a constant bending moment of 800 Nmm. The angulation of the fracture fragments was measured continuously. Data were assessed statistically using the univariate ANOVA test and linear regression analysis, and the significance level was set at p < 0.05. RESULTS: The median angulation of bone fragments φ allowed by each screw was 0.89° for AA, 1.12° for ST, 1.44° for SH and 2.36° for MH. With regards to linear regression, the most reliable curve was achieved by MH, with a coefficient of determination of R2 = 0.827. This was followed by AA (R2 = 0.354), SH (R2 = 0.247) and ST (R2 = 0.019). Data assessed using an adapted ANOVA model showed no statistically significant difference (p = 0.291) between the screws. CONCLUSIONS: The continuous development of HCSs has resulted in very comparable implants, and thus, at this time, other factors, such as surgeons' experience, ease of handling and price, should be taken into consideration.

Are there any differences in various polyaxial locking systems? A mechanical study of different locking screws in multidirectional angular stable distal radius plates.

Numerous angular stable plates for the distal radius exist, and technically based comparisons of the polyaxial locking interfaces are lacking. The aim of this mechanical study was to investigate three different locking interfaces of angular stable volar plates by cantilever bending: VA-LCP Two-Column Distal Radius Plates 2.4 mm (Synthes® GmbH, Oberdorf, Switzerland), IXOS® P4 (Martin, Tuttlingen, Germany) and VariAX™ (Stryker®, Duisburg, Germany). We assessed the strength of 0°, 5°, 10° and 15° screw locking angles and tested the bending strength from 10° to 5° angles by cyclic loading until breakage. The final setup repeated the above assessments by inclusion of four locking screws. The single screw-plate interfaces of the VA-LCP showed the highest bending moment at an angle of 0° and 5°, the IXOS® P4 at an angle of 10° and 15° and the VariAX™ when changing the insertion angle from 10° into 5°. The strength of polyaxial locking interfaces and mechanism of failure proved to be different among the examined plates.

Development of a multisegmental test body to calibrate and validate studies with spinal testing devices with follower load.

INTRODUCTION: Spinal testing devices need a tool for calibration and comparison of accomplished studies. The objective of this study was the development of a polysegmental test body, which simulates several functional spinal motion units and provides a standard calibration. MATERIALS AND METHODS: The test body consists of aluminum moldings simulating the vertebral bodies, rubber discs simulating the intervertebral discs and cylindrical guide elements with ball joints simulating the vertebral joints. The test body was tested multidirectional with a moment of 7.5 Nm and follower loads up to 800 N. RESULTS: The results showed sigmoid load-deformation curves of the test body similar to a human lumbar cadaver spine, but with a higher range of motion (ROM). The follower load had no influence during extension-flexion, but an increasing follower load during lateral bending and rotation caused increased friction and restoring forces with a decrea-sed ROM. DISCUSSION: The polysegmental test body does not show the same mechanical behavior like a polysegmental human cadaver spine in all directions of motion, but it provides reproducible values without requiring preconditioning as proposed for human specimen. CONCLUSION: This test body could improve the comparability of cadaver studies performed with different spinal testing devices by standardization of the test set-ups.