Biomechanical comparison of an interspinous device and a rigid stabilization on lumbar adjacent segment range of motion.

PURPOSE OF THE STUDY: Decompression surgery with or without fusion is the gold standard treatment of lumbar spinal stenosis, but adjacent segment degeneration has been reported as a long-term complication after fusion. This led to the development of dynamic implants like the interspinous devices. They are supposed to limit extension and expand the spinal canal at the symptomatic level, but with reduced effect on the range of motion of the adjacent segments. The aim of the present study is the evaluation of the biomechanical effects on the range of motion (ROM) of adjacent lumbar segments after decompression and instrumentation with an interspinous device compared to a rigid posterior stabilization device. MATERIALS AND METHODS: 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. The preload was applied as a follower load of 400N along the curvature of the spine. The range of motion (ROM) of the adjacent segments L2/L3 and L4/L5 was measured with the intact segment L3/L4, after decompression, consisting of resection of the interspinous ligament, flavectomy and bilateral medial facetecomy, and insertion of the Coflex® (Paradigm Spine, Wurmlingen) and after instrumentation with Click X® (Synthes, Umkirch) as well. RESULTS: The interspinous and the rigid device caused a significant increase of ROM at both adjacent segments during all directions of motion and under follower load, without significant difference between these devices. The ROM of L2/L3 tends to increase more than the ROM of L4/L5 after instrumentation without statistical significance. DISCUSSION: The "dynamic" Coflex device caused a significant increase of ROM at both adjacent lumbar segments comparable to the increase of ROM after instrumentation with the rigid Click X device. Other in vitro studies observed comparable biomechanical effects on the adjacent segments after fusion, but biomechanical spacer studies concentrated on the "noncompressible" X-Stop® and could not demonstrate a significant adjacent segment effect of this device. CONCLUSIONS: The hypothesis, that an interspinous device would reduce the stress on adjacent segments compared to a rigid posterior stabilization device, could not be demonstrated with this biomechanical in vitro study. Therefore, the protection of adjacent segments after instrumentation with dynamic devices is still not completely achieved.

How does spinal canal decompression and dorsal stabilization affect segmental mobility? A biomechanical study.

INTRODUCTION: When decompression of the lumbar spinal canal is performed, segmental stability might be affected. Exactly which anatomical structures can thereby be resected without interfering with stability, and when, respectively how, additional stabilization is essential, has not been adequately investigated so far. The present investigation describes kinetic changes in a surgically treated motion segment as well as in its adjacent segments. MATERIAL AND METHODS: Segmental biomechanical examination of nine human lumbar cadaver spines (L1 to L5) was performed without preload in a spine-testing apparatus by means of a precise, ultrasound-guided measuring system. Thus, samples consisting of four free motion segments were made available. Besides measurements in the native (untreated) spine specimen further measurements were done after progressive resection of dorsal elements like lig. flavum, hemilaminectomy, laminectomy and facetectomy. The segment was then stabilised by means of a rigid system (ART((R))) and by means of a dynamic, transpedicularly fixed system (Dynesys((R))). RESULTS: For the analysis, range of motion (ROM) values and separately viewed data of the respective direction of motion were considered in equal measure. A very high reproducibility of the individual measurements could be verified. In the sagittal and frontal plane, flavectomy and hemilaminectomy did not achieve any relevant change in the ROM in both directions. This applies to the segment operated on as well as to the adjacent segments examined. Resection of the facet likewise does not lead to any distinct increase of mobility in the operated segment as far as flexion and right/left bending is concerned. In extension a striking increase in mobility of more than 1degree compared to the native value can be perceived in the operated segment. Stabilization with the rigid and dynamic system effect an almost equal reduction of flexion/extension and right/left bending. In the adjacent segments, a slightly higher mobility is to be noted for rigid stabilization than for dynamic stabilisation. A linear regression analysis shows that in flexion/extension monosegmental rigid stabilisation is compensated predominantly in the first cranial adjacent segment. In case of a dynamic stabilisation the compensation is distributed among the first and second cranial, and by 20% in the caudal adjacent segment. SUMMARY: Monosegmental decompression of the lumbar spinal canal does not essentially destabilise the motion segment during in vitro conditions. Regarding rigid or dynamic stabilisation, the ROM does not differ within the operated segment, but the distribution of the compensatory movement is different.

Friction of ceramic and metal hip hemi-endoprostheses against cadaveric acetabula.

INTRODUCTION: Studies of hip arthroplasty have dealt mainly with total endoprosthesis, while tribology measurement values of hemi-endoprosthetic implants are rare. The small amount of experimental tribological data concerning materials of hemi-endoprosthetic implants in the form of pendulum trials, animal experiments, in vivo measurements on human hip joints and pin on disc studies report friction coefficients between 0.014 and 0.57; the friction coefficients measured in fresh human cadaver hip joints were determined between 0.001 and 0.08. MATERIALS AND METHODS: The HEPFlEx-hip simulator was constructed to test the friction coefficients of unipolar femur head hemi-endoprostheses made of metal or ceramic against fresh cadaveric acetabula. Its plane of movement is uniaxial with a flexion-extension movement of +30/-18 degrees . The force is produced pneumatically dynamic with amounts of 2.5 kN. Newborn calf serum serves as a lubricant. We mounted 20 fresh porcine acetabula and 10 fresh human cadaver acetabula in the HEPFlEx-hip simulator and compared the two unipolar femur head hemi-endoprostheses (metal vs. ceramic). RESULTS: The mean friction coefficients against porcine acetabula were micro=0.017-0.082 for ceramic and micro=0.020-0.101 for metal; against human cadaver acetabula micro=0.017-0.083 for ceramic and micro=0.019-0.118 for metal. The frictional coefficient deltas (metal-ceramic) values of all measurements were Deltamicro=0.004 for porcine acetabula and Deltamicro=0.001 for cadaver acetabula. Box-plots graphics document significantly lower frictional coefficients of the ceramics. CONCLUSIONS: The lower frictional coefficients of ceramic compared to metal against fresh cadaveric acetabula may have a clinical impact on the process of the protrusion of the corresponding femoral head through the acetabulum.

[The HEPFIEx simulator, a device for determination of friction between femur head prosthesis and cartilage]. / Der HEPFIEx-Simulator, eine Apparatur zur Bestimmung der Reibzahlen zwischen Hüftkopf-Prothesen und Knorpel.

We describe a device designed to investigate friction between various femoral head prostheses and human acetabula. It enables not only the determination of friction and the relevance of the play between the femoral head and acetabulum, but also the evaluation of the kinematic behaviour of bipolar prostheses. In the simulator, the various femoral head prostheses are placed on a special cone and tested against a human cadaveric acetabulum. The swiveling range of the device is uniaxial, and the swiveling angle is +/- 35 degrees. The maximum force produced pneumatically is 5kN. Testing of the simulator with a TEP was successful and friction-coefficients of < 0.1 were measured, as are reported in the literature.

Friction in hip hemiendoprostheses. Review of literature and own model using cadaveric acetabula.

The science of tribology concerning hip arthroplasty has mainly dealt with total endoprosthesis, whereas measurement values of hemiendoprosthetic implants are rare. The small amount of experimental tribologic data concerning hemiendoprosthetic implants in the form of pendulum trials, animal experiments, in vivo measurements on human hip joints and pin on disc studies will be reviewed in the following work. The reported frictional coefficients in these studies were between 0.014-0.57. In order to test the friction coefficients of different femur head hemiendoprostheses (unipolar ceramic- and metal heads) against fresh cadaveric acetabula, the HEPFlEx-hip simulator (Hemi-EndoProsthesis Flexion Extension) was developed. In the simulator, the various hemiendoprosthetic heads are placed on a special cone and tested against a cadaver acetabulum cast in MCP 47 woodmetal. The plane of movement of the apparatus is uniaxial with a flexion-extension movement of 35 degrees. The force is produced pneumatically with amounts of up to 5 kN. Newborn calf serum serves as a lubricant. A PC collects the data from torque-, force-, and angle-sensors on-line and allows the simultaneous processing and visualization of the data. The frictional coefficients produced by the different head materials and the relevance of the play between the hemiendoprothesis head size and acetabulum can be determined. Preliminary results showed that the mean friction coefficient at 1 kN loading was =0.024-0.063 for ceramic against cartilage and =0.033-0.075 for metal against cartilage. (Hip International 2002; 2: 126-34).