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PubMed2000

The bursal and articular sides of the supraspinatus tendon have a different compressive stiffness.

OBJECTIVE: To measure the compressive stiffness of the supraspinatus tendon and to determine whether regional difference exists in the bursal and articular side of the tendon. DESIGN: Indentation testing was performed on both the bursal and articular sides of the supraspinatus tendon, focused on the 'critical area', where rotator cuff tears often occur. BACKGROUND: When the supraspinatus tendon wraps around the humeral head or is under impingement condition, compressive force on the tendon surface is expected. Therefore, compressive stress has been recently considered to be one of the important factors associated with the cuff tear. The mechanical properties would be essential for analytic modeling of stress distribution. METHODS: Indentation tests were performed at 15 locations on the bursal and articular surfaces of the supraspinatus tendon. A mathematical model with exponential relationship was used to describe the measured force-deformation relationship and to calculate the compressive stiffness of the supraspinatus tendon. RESULTS: The over-all initial stiffness on the bursal and articular sides of the tendon was significantly different. On the bursal side, the anterior third had a significantly higher initial stiffness than the other thirds on average. On the articular side, initial stiffness at location 10 mm proximal to the greater tuberosity was significantly higher than the rest on average. CONCLUSIONS: The compressive stiffness of the supraspinatus tendon was found to be non-homogenous throughout the structure.RelevanceNon-homogenous compressive stiffness of the supraspinatus tendon would affect the load transmission within the tendon, which might be associated with the potential mechanism of tear. Such characteristics needs to be considered when performing finite element modeling of stress fields in the tendon.

Aged↗

PubMed2001

Effect of mixing methods on the compressive strength of glass ionomer cements.

OBJECTIVE: The purpose of the study was to evaluate the effect of the mixing method on the compressive strength and porosity of dental glass ionomer cements. METHOD: Five glass ionomer cements were chosen for use in the study. Two were hand mixed and three were encapsulated. The latter were mixed either by shaking or rotating. Following mixing by rotation some samples were centrifuged before use. The 24h compressive strength was determined for each cement/mixing regime combination and fracture surfaces were examined using SEM. RESULTS: The mixing method had a significant effect on compressive strength (P<0.05). For the luting/lining cement, hand mixing produced a significantly greater compressive strength (P<0.05). For the restorative cement, there were only small differences between specimens mixed by different methods and hand mixing gave a significantly lower compressive strength than mixing by rotation followed by centrifuging (P<0.05). Porosity was incorporated in all samples and low values of compressive strength were associated with larger pores. SIGNIFICANCE: The strength of glass ionomer cements is affected by incorporated porosity and this is dependent on the method of mixing. For some cements hand mixing is favoured in order to reduce porosity and increase strength but this is not generally applicable to all cements.

Capsules↗

PubMed2000

Diagnosis of vertebral metastasis, epidural metastasis, and malignant spinal cord compression: are T(1)-weighted sagittal images sufficient?

The objective of this study was to determine whether T(1)-weighted sagittal images alone are adequate in the diagnosis of vertebral metastasis, epidural metastasis, and malignant spinal cord compression. Ninety-four complete magnetic resonance (MR) studies of the spinal column (a complete study consisting of T(1)-weighted sagittal images, T(2)-weighted sagittal images, and T(1)- and/or T(2)-weighted axial images) and 94 T(1)-weighted sagittal images alone (a subset of the complete studies) from 57 consecutive cancer patients over the last 2 years with clinically suspected cord compression were blindly and independently evaluated by four radiologists. The complete MR studies were used as the standard. Overall, the sensitivity of T(1)-weighted sagittal images alone to vertebral metastasis (87%) was statistically greater than cord compression (70%) (p = 0.05), and statistically greater than epidural metastasis (46%) (p </= 0.02). The specificity for cord compression (97%) was greater than the specificity for epidural metastasis (89%) (p = 0.03), and greater than the specificity for vertebral metastasis (83%) (p </= 0.02). There was a strong trend for better detection of cord compression overall and better detection of vertebral metastasis in the cervical spine by the most experienced radiologist. Complete studies of the spine are necessary in the diagnosis of vertebral metastasis, epidural metastasis, and cord compression, particularly with less experienced radiologists.

Adult↗

PubMed2001

Effect of intraocular lens haptic compressibility on the posterior lens capsule after cataract surgery.

PURPOSE: To evaluate the effect of intraocular lens (IOL) haptic compressibility on the posterior capsule after cataract surgery. SETTING: Teaching hospital, London, United Kingdom. METHODS: In this randomized prospective study, 60 patients had standardized phacoemulsification with in-the-bag placement of a poly(methyl methacrylate) (PMMA) (Storz P497UV) or hydrogel (Storz Hydroview H60M) IOL. Both IOLs had PMMA haptics of identical configuration and length. The IOL haptic compressibility was measured in air and then during incubation in saline at 37 degrees C over 1 month. Digital retroillumination imaging was performed 1, 7, 28, 90, 180, 360, and 720 days postoperatively. The presence and duration of postoperative capsule folds were recorded and correlated with the haptic compressibility measurements, lens epithelial cell (LEC) growth patterns on the posterior capsule at 6 months, and the extent of posterior capsule opacification. RESULTS: On the first postoperative day, 21 patients (88%) in the Hydroview group had posterior capsule folds that persisted in 12 patients (50%) for 2 years. Nineteen patients (68%) in the PMMA group had folds at day 1 (P =.01), with 1 patient (3%) still having folds at 1 month (P =.0002) and no patient having folds at 3 months. At 6 months, 11 patients (46%) in the Hydroview group and no patient in the PMMA group had LEC growth in the direction of the folds. The PMMA IOLs showed a greater decrease in haptic compressibility during incubation. CONCLUSIONS: Haptic compressibility should be an important consideration in IOL design. The results suggest that to avoid posterior capsule folds, the compressibility should be less than 2.5 mN.

Aged↗

PubMed1998

Magnetic resonance imaging of the whole spine in suspected malignant spinal cord compression: impact on management.

Patients with suspected malignant spinal cord compression may present with a misleading sensory level or have multiple levels of compression that are not apparent clinically or on imaging of a limited area of the spine. To estimate how often this occurs and to evaluate a policy of magnetic resonance imaging (MRI) of the whole spine for any patient with suspected cord compression, data from 127 patients who had undergone MRI scans of the whole spine were reviewed. In 85 of 127 scans, there was evidence of compression of or impingement upon the spinal cord. A sensory level was present in 47 of these 85 patients, but in 12/47 (26%) the sensory level was four or more segments below or three or more segments above the actual lesion. Multiple levels of compression or impingement were found in 33 of 85 (39%) patients; in 24 of these, more than one region (cervical/thoracic/lumbar) of the cord was involved. For 32 patients who commenced radiotherapy to a treatment volume based on clinical criteria before the MRI scan was available, the radiotherapy fields needed modification in 16 (50%) as a result of the MRI findings. The results support a policy of MRI of the whole spine in any patient with suspected malignant spinal cord compression.

Female↗

PubMed2003

Myoelectric signal compression using zero-trees of wavelet coefficients.

Recent progress in the diagnostic use of the myoelectric signal for neuromuscular diseases, coupled with increasing interests in telemedicine applications, mandate the need for an effective compression technique. The efficacy of the embedded zero-tree wavelet compression algorithm is examined with respect to some important analysis parameters (the length of the analysis segment and wavelet type) and measurement conditions (muscle type and contraction type). It is shown that compression performance improves with segment length, and that good choices of wavelet type include the Meyer wavelet and the fifth order biorthogonal wavelet. The effects of different muscle sites and contraction types on compression performance are less conclusive.A comparison of a number of lossy compression techniques has revealed that the EZW algorithm exhibits superior performance to a hard thresholding wavelet approach, but falls short of adaptive differential pulse code modulation. The bit prioritization capability of the EZW algorithm allows one to specify the compression factor online, making it an appealing technique for streaming data applications, as often encountered in telemedicine.

Adult↗

PubMed1999

Abnormally large evoked potentials arising from dorsal column fibers in the region of chronically compressed spinal cord.

OBJECTIVE: To understand the origin and mechanism of the large evoked potentials known to occur in chronically compressed spinal cord. METHODS: Electrophysiological analysis was made of evoked potentials produced in the cervical cord compressed for more than 1 year by rotating a screw in the vertebral body, in 4 adult cats. RESULTS: In all cats, manifesting no motor disturbance, large positive or negative evoked potentials were recorded from the cord dorsum at the compressed region following lumbar cord stimulation. These potentials followed high-frequency stimulation and were judged as conducting volleys of directly activated fibers. Intraspinally, single fiber spikes, normally barely recordable with the glass microelectrodes, were recorded extracellularly from localized areas within the dorsal column restricted to the compressed region. Such single fibers showed slowed conduction at the compressed region without a conduction block. Histologically, demyelinated fibers were observed in localized areas in the dorsal column of the compressed region. CONCLUSION: We concluded that the large abnormal surface potentials originated from the dorsal column and were attributed to an increase in action current in affected fibers, which presumably occurred associated with demyelination.

Animals↗

PubMed2006

Cell mechanics using atomic force microscopy-based single-cell compression.

We report herein the establishment of a single-cell compression method based on force measurements in atomic force microscopy (AFM). The high-resolution bright-field or confocal laser scanning microscopy guides the location of the AFM probe and then monitors the deformation of cell shape, while microsphere-modified AFM probes compress the cell and measure the force. Force and deformation profiles of living cells reveal a cubic relationship at small deformation (<30%), multiple peaks at 30-70% compression, and a rapid increase at over 80% deformation. The initial compression may be described qualitatively and quantitatively using a simple model of a nonpermeable balloon filled with incompressible fluid. Stress peaks reflect cell membrane rupture, followed by the deformation and rupture of intracellular components, beyond which the cell responses become irreversible. The Young's modulus and bending constant of living cell membranes are extracted from the balloon models, with 10-30 MPa and 17-52 kT, respectively. The initial compression of dead and fixed cells is modeled using Hertzian contact theory, assuming that the cell is a homogeneous sphere. Dead cells exhibit a cytoskeleton elasticity of 4-7.5 kPa, while fixation treatment leads to a dramatic increase in the cytoskeletal Young's modulus (150-230 kPa) due to protein cross-linking by imine bonds. These results demonstrate the high sensitivity of the single-cell compression method to the molecular-level structural changes of cells, which suggests a new generic platform for investigating cell mechanics in tissue engineering and cancer research.

Cell Shape↗

PubMed1987

Decompression of the spinal cord improves recovery after acute experimental spinal cord compression injury.

The value of decompression after spinal cord injury in patients is still an unresolved issue. It has previously been shown in our laboratory that functional recovery in rats after cord compression varied with both the force and time until decompression. However, the longest duration studied was only 15 minutes, which is far less than that usually encountered in clinical practice, and therefore, the present study was undertaken to determine the value of decompression after more prolonged periods of compression. A factorially designed experiment with five rats per cell was used with the clip compression injury model. Forces of 2.3, 16.9 or 53.0 gms were applied at C7-T1 until decompression was performed after 15, 60, 120, or 240 minutes of compression. Functional recovery was assessed weekly for 8 weeks using the inclined plane technique. Maximum and minimum performance limits were established in normal rats and rats with cord transection, respectively. Univariate analysis and multiple comparison tests were used to analyse the data. The major determinant of recovery was the force of the injury. For example, the animals injured by the 2.3 gm clip performed significantly better than those injured at higher forces for all times until decompression (p less than 0.0001), and there was a significant difference in recovery between the groups injured by the 16.9 and 53.0 gm clips, although only for the 15 minutes until decompression group (p less than 0.05). The time until decompression also affected recovery, but only for the lighter compression forces (2.3 and 16.9 gm). For example, animals decompressed after 60 minutes of 2.3 gm compression recovered significantly better than those decompressed after 240 minutes (p less than 0.05). Thus, if the initial injury force is small, decompression is beneficial even after prolonged injury.

Animals↗

PubMed2000

Comparison of spinal cord evoked potentials and peripheral nerve evoked potentials by electric stimulation of the spinal cord under acute spinal cord compression in cats.

STUDY DESIGN: Spinal cord evoked potentials and peripheral nerve evoked potentials after spinal cord stimulation were recorded under acute spinal cord compression in 19 cats. OBJECTIVES: To investigate the effects of acute compression upon grey matter and white matter by comparing both potentials. METHODS: We compared peripheral nerve evoked potentials, recorded at the biceps brachii branch of the musculocutaneous nerve, with descending spinal cord evoked potentials, recorded from the lumbar spinal cord, by stimulation to the C2 level, under compression of the C6 segment. RESULTS: The amplitude of both potentials decreased with increased compression. The second wave of peripheral nerve evoked potentials, which are motor fibre action potentials, decreased sooner than those of the spinal cord evoked potentials. CONCLUSION: These findings indicate that peripheral nerve evoked potentials are sensitive to acute damage of the segmented compression. This suggests that grey matter is more vulnerable to compression than white matter.

Animals↗

PubMed2003

Paradoxical enhancement of spinal-cord-evoked potentials rostral and caudal to the site of progressive cord compression in the cat.

STUDY DESIGN: Analysis of the sequential waveform changes of the spinal-cord-evoked potentials (SCEPs) associated with progressive cord compression in the cat. OBJECTIVES: To document the phenomenon of paradoxical enhancement of SCEPs despite conduction abnormalities and to evaluate its possible significance. SETTING: Kochi Medical School, Kochi, Japan. METHODS: SCEPs were recorded simultaneously at four serial intervertebral levels, from T6-7 to T9-10 caudal to, and at three serial levels from T2-3 to T4-5 rostral to the compression site at T5-6 following epidural stimulation at L6 in 14 cats. RESULTS: Caudal to the compression site, the area of negative peak significantly increased toward maximal values of 277+/-36 (mean+/-SE), 151+/-9 and 110+/-4% as compared to the baseline precompression values (100%) at T6-7, T7-8, and T8-9, respectively. Rostral to the compression site, the area of negative peak significantly increased before subsequent deterioration and reached 105+/-2, 106+/-2, and 104+/-2% at T4-5, T3-4, and T2-3, respectively. The onset of negative peak enhancement, recorded either caudal or rostral to the compression site, showed a close temporal correlation (r>0.8, P&<0.001) with that of the prolongation in latency of SCEPs at T2-3. CONCLUSIONS: A progressive focal conduction block induced by compression of the spinal cord can paradoxically enhance the ascending SCEPs both caudally and, though less consistently, rostrally, representing a warning of the impending risk of paraplegia.

Animals↗

PubMed2001

In situ compressive stiffness, biochemical composition, and structural integrity of articular cartilage of the human knee joint.

OBJECTIVE: Reduction of compressive stiffness of articular cartilage has been reported as one of the first signs of cartilage degeneration. For the measurement of in situ compressive stiffness, a hand-held indentation probe has recently been developed and baseline data for macroscopically normal knee joint cartilage were provided. However, the histological stage of degeneration of the measured cartilage was not known. The purpose of this study was to investigate whether there is a relationship between the in situ measured compressive stiffness, the histological stage of degeneration, and the biochemical composition of articular cartilage. DESIGN: Instantaneous compressive stiffness was measured for the articular cartilage of 24 human cadaver knees. Additionally, biochemical composition (total proteoglycan and collagen content) and histological appearance (according to the Mankin score) were assessed for each measurement location. RESULTS: Despite visually normal surfaces, various histological signs of degeneration were present. A high correlation between Mankin score and cartilage stiffness was observed for the lateral patellar groove (R(2)=0.81), the medial (R(2)=0.83) and the lateral femoral condyle (R(2)=0.71), whereas a moderate correlation was found for the medial patellar groove (R(2)=0.44). No correlation was observed between biochemical composition and cartilage compressive stiffness. CONCLUSIONS: Our results are in agreement with others and show that the instantaneous compressive stiffness is primarily dependent on the integrity of the extracellular matrix, and not on the content of the major cartilage constituents. The high correlation between stiffness and Mankin score in mild osteoarthrosis suggests that the stage of cartilage degeneration can be assessed quantitatively with the hand-held indentation probe. Moderate and severe case of osteoarthrosis remains to be investigated.

Adult↗

PubMed2001

Depth-dependent compressive properties of normal aged human femoral head articular cartilage: relationship to fixed charge density.

OBJECTIVES: Determine the depth-varying confined and osmotic compression moduli of normal human articular cartilage from the femoral head, and test whether these moduli are dependent on fixed charge density. METHODS AND RESULTS: Using an automated instrument to allow epifluorescence microscopy analysis during confined compression testing on cartilage samples, the equilibrium confined compression modulus (H(A 0)) was found to vary markedly with depth (z=0-1500 microm) from the articular surface. H(A 0) increased from 1.16+/-0.20 MPa in the superficial (0-125 microm) layer to 7.75+/-1.45 MPa in the deepest (1250-1500 microm) layer tested, and was fit by the expression, H(A 0)(z) [MPa]=1.44 exp(0.0012.z [microm]). Also, in successive slices of cartilage extending from the articular surface to the middle-deep regions, the bulk modulus (K(0)) and fixed charge density (FCD) increased, consistent with previous findings. While H(A 0), K(0), and FCD each varied with depth from the articular surface, the dependence of H(A 0) and K(0) on depth did not appear to be completely related to variations in FCD. CONCLUSIONS: The confined compression modulus of normal aged human femoral head articular cartilage increases markedly with depth from the articular surface, a trend similar to that observed for articular cartilage from other joints in animals but with an absolute amplitude that is several-fold higher. The compressive properties were not simply related to FCD at different depths from the articular surface, suggesting that other as yet undefined factors also contribute to compressive properties.

Aged↗

PubMed1985

A primate model for chronic nerve compression.

Peripheral compression neuropathy is a common clinical entity but little human nerve material has been available for study. It has been difficult to develop an experimental model of chronic nerve compression which is reliable and reproducible, making it difficult to assess the various treatment modalities currently in use to manage this problem. In a recent study using a rat model, connective tissue and nerve fiber changes associated with chronic nerve compression were described by the authors. The present study expands this same model of chronic nerve compression to the multifascicular median nerve of the cynomolgus monkey, using this model to test whether or not current surgical treatment modalities for nerve entrapment will alter the course of chronic nerve compression. Using histologic and morphometric parameters, there did not appear to be a difference between nerves treated by decompression alone or by decompression and internal neurolysis. Total number of nerve fibers remained constant, as did axon size, but a demyelinating process was demonstrated which is a significant component of chronic nerve compression. Although the small number of experimental animals (7) makes precise evaluation of the clinical modalities impossible, the authors expect that further study of the multifascicular median nerve of the cynomolgus monkey will prove to be a useful source of future experimental evidence and will help to interpret the diverse therapeutic and operative techniques currently in use.

Animals↗

PubMed2006

Gadolinium-enhanced magnetic resonance imaging after percutaneous vertebroplasty does not improve the short-term prediction of new compression fractures.

PURPOSE: To investigate the relationships between contrast-enhanced magnetic resonance imaging (MRI) findings and new compression fractures occurring after percutaneous vertebroplasty (PVP) in order to determine the necessity of contrast-enhanced MRI. MATERIAL AND METHODS: The material comprised 13 consecutive patients (27 vertebrae) with compression fractures who underwent vertebroplasty. Twenty-nine adjacent vertebrae were monitored for new compression fractures. We performed contrast-enhanced MRI within the 5 days following PVP. RESULTS: The 29 adjacent vertebrae displayed 10 enhanced lesions and 19 vertebral bodies without enhancement-unenhanced lesions on MRI within the 5 days after PVP. In 4 out of 10 vertebrae, enhanced lesions were seen within the 5 days after PVP, but no abnormalities were seen on preoperative MRI. In these 4 vertebrae, new compression fractures were seen within the 1 month following PVP. In the other 6 vertebrae, enhanced lesions were seen within the 5 days after PVP, and these signal changes were detected by preoperative MRI in the same area. In 3 of these 6 vertebrae, new compression fractures occurred at 4, 8, and 9 months after PVP, respectively. However, all of the enhanced lesions were detectable on unenhanced MRI, which was conducted at the same time as enhanced MRI. CONCLUSION: New compression fractures after PVP may be predicted with early postoperative MRI. Contrast-enhanced MRI does not improve detection of the new lesions.

Aged↗

PubMed2004

Mechanical characterization of biocompatible microspheres and microcapsules by direct compression.

The mechanical properties of biocompatible microparticles including alginate microspheres and alginate-chitosan microcapsules with different wall thickness were determined using a micromanipulation technique. Single microparticles with diameters of 20-60 microm were compressed to a given deformation and held, and compressed to rupture at different speeds. The corresponding force imposed on them was measured simultaneously by a force transducer. Results showed that the force imposed on these particles increased when they were compressed, but relaxed significantly when they were held. For alginate microspheres, the faster the compression speed was, the greater the force being imposed on them at a given deformation. Alginate-chitosan microcapsules showed less force relaxation when they were held, compared with alginate microspheres. The thicker their wall was, the less significant force relaxation the microcapsules exhibited. The mean rupture force of alginate microspheres increased with the compression speed, but this effect in general became less for alginate-chitosan microcapsules, which depended on their wall thickness. However, the deformation at rupture for all three samples was independent of the compression speed. On average, the alginate-chitosan microcapsules were bigger than alginate microspheres and had a greater rupture force.

Alginates↗

PubMed2003

A novel approach to derive a compression parameter indicating effective particle deformability.

In this article, a compression parameter is derived and its physical significance discussed. Tablets were formed from two sodium chloride powders and from two sucrose powders over a wide range of compaction pressures and the tensile strength and the porosity of the tablets were determined. From these data, the novel compression parameter and the Heckel parameter were calculated. Above a lower pressure threshold, the tablet strength increased relatively linear with compaction pressure up to 300-500 MPa and thereafter, the tablet strength leveled off. From the linear region, the compression parameter was derived. The tablet porosity data obeyed reasonably the Heckel function and from the linear region, the yield strength was calculated. For all four powders, a ratio between the compression parameter and the Heckel yield strength of about 3 was obtained. It is concluded that the suggested compression parameter represents an indication of the effective deformability of particles during compression which is suggested to correlate with the hardness of the particles.

Compressive Strength↗

PubMed1987

Conductivity of dorsal column fibers during experimental spinal cord compression and after decompression at various stimulus frequencies.

The effects of spinal cord compression on conduction of dorsal column fibers at various stimulus frequencies were analyzed in pentobarbital anesthetized cats. The responses to L6 dorsal root stimulation at 1 to 500 Hz were recorded from the L2 cord dorsum. The L4 cord segment was compressed gradually until the compound action potential (CAP) at 1 Hz was flat. There was no significant change of CAP at any frequency during the first part of compression, but there was progressive conduction failure, which was more severe with increased stimulus frequency, at a later stage. After decompression, the CAPs at all frequencies recovered progressively for 1 hour but slowly thereafter. However, marked differences were observed in recovery rate at different stimulus frequencies. The recovery rate at 500 Hz was much slower than that at 1 Hz, whereas the recovery rate at 100 Hz exceeded those at 1 Hz. Serial analysis of a train of high frequency impulses revealed the following different response patterns with stimulus frequencies after decompression. At 333-500 Hz the amplitude of CAPs decreased progressively, whereas at 33-125 Hz it increased up to 110-134% of the first CAP and then reached an almost steady level. At 200-250 Hz the amplitude increased transiently and then decreased progressively. The latency increased with decreased amplitude, and decreased with increased amplitude. Conduction failure at a high stimulus frequency (500 Hz) was observed at the compression site. In contrast, augmentation of CAPs at moderately high stimulus frequency (100 Hz) was observed rostral to the compressing site. The conduction failure at high stimulus frequency indicates incomplete impairment of spike generation in axons injured by mechanical compression and that these axons can transmit impulses at a low stimulus frequency. High frequency stimulation may be useful for monitoring of the function of the CNS axons. The mechanism underlying the augmentation of CAPs at moderately high stimulus frequency is briefly discussed.

Animals↗