Posterior Anulus Research Articles

Thursday, October 31, 2001 9:08–9:38 am Select Poster Presentations: Experimental intervertebral disc degeneration induced by a vertebral body defect

Purpose of study: To develop an experimental model of degenerative disc disease without direct intervention of the intervertebral disc (IVD). Our hypothesis was that inducing vertebral body damage at sites adjacent to the IVD would modify nutrient transport through the end plates and thus initiate degeneration in a manner relevant to degenerative disc disease in humans.Methods used: Vertebral body defects were created in lumbar motion segments of minipigs (10 experimental and 7 control levels, n=3). A high-speed drill with a diamond burr (3 to 5 mm diameter) was used to remove bone adjacent to the inferior and superior end plates of the experimental IVDs. A resorbable calcium phosphate bone cement was then packed into the defects. Spines were harvested at 3, 6 and 9 months and imaged in the sagittal plane with high-resolution magnetic resonance imaging (MRI; 7.1 T). Four blinded observers graded MRI appearance with the scheme of Thompson and coworkers (1990; 1 = nondegenerate, 5 = severely degenerate). In addition, MRI data sets were analyzed to determine changes in disc height and signal intensity for regions corresponding to the nucleus pulposus (NP) and anterior and posterior anulus fibrosus (AF). After imaging, individual motion segments were processed for routine histology (toluidine blue, hemotoxylin and eosin) and immunohistochemistry to detect matrix proteins including type II collagen and decorin.of findings: Higher MRI grades of degeneration were observed in all experimental IVD regions at 9 months after surgery. Reviewer grades were significantly higher for both NP (average of four graders = 1.8 vs. 3.8, control vs. experimental) and AF at 9 months after surgery (1.7 vs. 3.4). There was some evidence of end plate changes beginning at 3 months after surgery (1.5 vs. 2.3, control vs. experimental), which became quite dramatic at 9 months (1.4 vs. 4.5). No differences were noted in disc height between control and experimental IVDs at any region. However, decreases in MRI signal intensity (normalized to maximum values) were noted in experimental IVDs, particularly in the anterior AF (mean, 39%) and NP at 9 months (15%). Decreased signal intensity is a hallmark of IVD degeneration in the human, suggesting loss of hydration. Upon histological examination, there was evidence of inward bulging of the inner AF, end plate damage and decreased NP staining. Immunostaining demonstrated increased collagen type II in the experimental sections compared with controls, although few consistent changes were noted in the expression of decorin. Increases in the levels of both collagen type II and decorin have been demonstrated in other animal models of IVD degeneration and in human disease.Relationship between findings and existing knowledge: Overall, the changes observed in this experimental model confirm some key features of human IVD degeneration, including increased radiographic grade of degeneration, decreased MRI intensity and morphological and histological changes, such as increased staining for collagen type II and decreased staining of the NP. Holm and coworkers (1999) demonstrated that a controlled vertebral end plate defect will induce compositional and morphological evidence of disc degeneration in an animal model. Findings of the current study demonstrate that disruption of the vertebral body alone, without controlled violation of the end plate, may serve to initiate disc degeneration without direct, surgical injury of the IVD.Overall significance of findings: The mechanism for the onset of degeneration is not known but may relate to a role of vertebral body damage in altering nutrient transport across the end plate.Disclosures: No disclosures.Conflict of interest: William J. Richardson, grant research support, DePuy Orthobiologics.

Effect of tail suspension (or simulated weightlessness) on the lumbar intervertebral disc: study of proteoglycans and collagen.

An experiment to measure the proteoglycan and collagen content of the lumbar intervertebral discs of rats that had been tail-suspended for up to 4 weeks. To determine the effect of tensile force (or simulated weightlessness) on the intervertebral disc. During space flight the intervertebral disc experiences low compressive force (because of so-called "weightlessness"), which, in turn, produces, among other things, low hydrostatic pressure acting on the disc cells. Although disc cells respond (in vitro) to changes in hydrostatic pressure, it is unclear what effect low levels of hydrostatic pressure have in vivo and whether they lead to a degenerative catabolic process. The rat tail-suspension model is appropriate for studying the effects of tensile force on the disc. The disc (especially the anulus) is subjected to tension during various body movements (e.g., bending stretches the posterior anulus, and twisting tensions the whole anulus). Thirty-two Sprague-Dawley rats were tail-suspended for either 2 weeks (16 rats) or 4 weeks (16 rats). Sixteen other rats were left unsuspended for 4 weeks; these were used as controls. At the end of 2 or 4 weeks, as appropriate, the rats were killed and their lumbar spines were removed. In each rat the six lumbar discs were bisected and the discs (anulus and nucleus together) were carefully removed. The six lumbar discs from one rat were pooled with the six lumbar discs of a second matching rat (i.e., from the same group) to give one sample. The disc samples were then assessed using enzyme-linked immunosorbent assays. There was a 35% statistically significant decrease in proteoglycan content going from the control group down to the 4-week group, but no significant differences between the control group and the 2-week group or between the 2-week group and the 4-week group. There were no statistically significant differences between the three groups for collagen I or collagen II. These findings clearly establish a link between decreased proteoglycan content and tension on the disc, as modeled by the tail-suspended rat.

Discogenic low back pain and degenerative lumbar spinal stenosis - how appropriate is surgical treatment?

Surgery in acute and/or chronic low back pain is still a matter of intensive and controversial discussions. A vast number of minimally invasive or so called semi-invasive procedures have been published in the last 3 decades, but evidence-based data on efficacy and benefit of most of these techniques are still lacking. However, empirical data suggest good or at least satisfactory clinical results for a limited number of procedures if they are applied under restrictive indication criteria. Discogenic low back pain and lumbar spinal stenosis belong to the most frequent diagnoses associated with low back pain. This article gives a survey on definitions, indication criteria and modern surgical or semi- invasive techniques used for the treatment of these two pathologic entities. Discogenic low back pain: This clinical and morphological entity is defined as low back pain arising mainly from disc degeneration. Pain generators are usually nociceptors in the cartilaginous endplates, in the outer anulus fibrosus as well as in the periosteum of the vertebral bodies. Clinical symptoms correlate with morphologic changes detected with MR-imaging (modic type I) or with contained disc protrusions mainly without neurological symptoms. Surgery is rarely indicated, spontaneous remissions occur in more than 60% of all cases. Spinal fusion has been the only surgical option in cases which did not respond to conservative therapy. Recently, electro-thermal modulation of the posterior anulus fibrosus has been published as a semi- invasive technique to relieve low back pain generated by fissures in the outer anulus and ingrowing nociceptors (intradiscal electro-thermal therapy, IDET(TM)). First results are promising, however, prospective randomised studies comparing this technique with conservative therapy are still lacking. The same is true for artificial nucleus pulposus replacement using hydrogel cushions implanted in the intervertebral space after removal of the nucleus pulposus from posterior or through an anterior approach (PDN, prosthetic disc nucleus(TM)). In cases with severe disc degeneration total disc replacement is another innovative option (ProDisc(TM)). Two metal endplates with titanium surface coating are implanted through a minimal invasive anterior approach (mini-laparotomy). A polyethylene inlay anchored in the caudal endplate holds the distance between the endplates and preserves the physiological range of motion between the two vertebral bodies. Degenerative lumbar spinal stenosis: Narrowing of the spinal canal due to degenerative changes of the disc, the facet joints and thickening of the yellow ligament is a geriatric disease which is diagnosed in increasing numbers within the last 10 years. More than 80% of the patients present with low back pain in association with neurogenic claudication. Neurological symptoms at rest are less frequently found. The spontaneous course shows progressive symptoms in more than 50% of all patients. More than 35% of the patients have associated diseases which might influence the perioperative course, complication rates and outcomes of surgery. Surgery is indicated in patients with progressive neurological symptoms, unacceptable decrease of quality of life or progressive intractable pain. In patients with mainly "leg symptoms" microsurgical mono- or multisegmental decompression is the procedure of choice. If low back pain is predominant and associated with degenerative instability such as degenerative spondylolisthesis or lumbar scoliosis, decompression must be combined with instrumented spinal fusion. In general a restrictive indication for surgery must be recommended especially for spinal fusion procedures. Non-fusion techniques such as intradiscal electro thermal therapy or spine arthroplasty with replacement of nucleus pulposus or total disc show promising early results; however, little is known about the long-term effect. It should be a principle to apply surgery in the least invasive way.

Histology of intervertebral disc protrusion: an experimental study using an aged rat model.

In vitro experimental intervertebral disc ruptures of aged rats were examined histologically. To clarify the mechanism of intervertebral disc herniations by microscopic investigation of ruptured discs. Clinically, disc herniations have been classified into two types: extrusion and protrusion. However, the pathogenesis of protrusion type herniations has not yet been demonstrated by any studies. To clarify this issue, it is essential to establish an appropriate model producing disc herniations, and to examine the sequential changes in the structure of herniated discs. Lumbar discs of 2-year-old rats were examined histologically and compared with human lumbar discs. To examine structural changes in discs subjected to repetitive motion stress, 400 repetitions of a sequence of flexion (30 degrees ) and axial rotation (6 degrees ) were applied in vitro to the lumbar discs of the animals. The microstructure of normal lumbar discs in aged rats was similar in many ways to the human lumbar discs in a 20- to 40-year-old adult. Of 10 discs subjected to repetitive stress, 4 were ruptured at the junction between the posterior anulus fibrosus and the sacral cartilage endplate. One had an extruded nucleus pulposus, and three had a protruded anulus fibrosus, which displayed disorganized structure containing widened and flaccid lamellae. The results from this study indicate that disc protrusion can be caused by disorganization of the ruptured annular lamellae, not by focal compression of the nucleus pulposus.

Peak stresses observed in the posterior lateral anulus.

The stress distributions within cadaveric lumbar intervertebral discs were measured for a range of loading conditions. To examine the distribution of stress across the area of the intervertebral disc and to compare regional variations in peak stress during compression loading with various flexion angles. The rate of disc degeneration and the occurrence of low back disorders increase with higher mechanical loading of the spine. The largest peak stresses occur in the anulus. Human lumbar L2--L3 and L4--L5 cadaver functional spinal units were obtained and tested. The distribution of disc stress was measured using a pressure probe with loads applied, pure compression and compression with 5 degrees of either flexion or extension. Stress profiles were recorded across the intervertebral disc at a compressive force of 1000 N and each of the three flexion-extension angles. The highest values (2.99 +/- 1.31 MPa) were measured during extension-compression lateral to the midline of the disc in the posterior anulus. The pressure in the nucleus was relatively unchanged by flexion angle remaining about 1.00 MPa for a 1000-N compression. Pressure measurements of the cadaveric nucleus have been used to validate models of lumbar spine loading and to evaluate the risk of low back injury and disc herniation. Previous observations limited to midsagittal measurements of the nucleus did not identify the regions of highest stress. The highest values observed here within the posterolateral anulus correspond to common sites of disc degeneration and herniation.

Initial experience with a modified left atrial maze procedure concomitant to cardiac surgery

The excellent results of the Cox-Maze-III operation showed that a surgical treatment of atrial fibrillation is possible. The maze operation is surgically demanding and time-consuming. This fact led to the development of simplified modifications. Due to the good results of an exclusive left atrial maze procedure in combination with mitral valve surgery and new technologies of ablation, it seems reasonable to perform these techniques concomitant to cardiac surgery on a large number of patients.¶ From November 1999 until June 2000 a modified maze procedure concomitant to a cardiac surgical intervention was performed on 42 patients, with a mean age of 62.2 years, suffering from chronic or intermittent atrial fibrillation. Primary cardiac disease was mainly cardiac valve disease (mitral valve disease 19 patients, aortic valve disease 10 patients) or coronary heart disease (8 patients). After left atriotomy and with the use of the Thermaline probe (Boston Scientific, Corporation, San Jose, California, USA) a bilateral pulmonary veins isolation was carried out by radiofrequency ablation. An additional ablation line was directed from the left pulmonary veins to the posterior mitral valve anulus. Finally, the left atrial appendage was resected. An evaluation of the therapeutic success was carried out in the immediate postoperative course and 3 months after surgery, at that point of time operation-related influences on the refractory period of the atrial myocardium were negligible.¶ At the time of discharge 32 patients (76.2%) showed a regular supraventricular rhythm. Twenty-one patients have already undergone the 3-month follow-up examination. At the time of discharge 13 out of these 21 patients (61.9%) were in sinus rhythm and at 3 months after surgery 18 out of these 21 patients (85.7%) showed a sinus rhythm with a restoration of atrial transport function echocardiographically determined. Three patients were still receiving antiarrhythmic medication.¶ The modified maze procedure in combination with cardiac surgical interventions proved to be easy, quick and reliable to perform. The early results suggest a high success rate of this technique. The long-term results have to be verified with further, regular follow-up examinations of the patients.

Effects of backward bending on lumbar intervertebral discs. Relevance to physical therapy treatments for low back pain.

Mechanical testing of cadaveric motion segments. To test the hypothesis that backward bending of the lumbar spine can reduce compressive stresses within lumbar intervertebral discs. Lumbar extension affects the distribution of compressive stress inside normal cadaveric discs, but little is known about its effect on mechanically disrupted and degenerated discs. Nineteen lumbar motion segments (mean donor age, 48 years) were subjected to complex mechanical loading to simulate the following postures: moderate lumbar flexion, 2 degrees of extension, 4 degrees of extension, and the neutral position (no bending). The distribution of compressive stress within the disc matrix was measured in each posture by pulling a miniature pressure transducer along the midsagittal diameter of the disc. Stress profiles were repeated after a mechanical treatment that was intended to simulate severe disc degeneration in vivo. The "degeneration" treatment reduced pressure in the nucleus pulposus and generated stress concentrations within the anulus, in a manner similar to that found in severely degenerated discs in vivo. When all discs were considered together, 2 degrees of extension increased the maximum compressive stress within the posterior anulus by an average of 16%, compared with the neutral posture. The size of localized stress peaks within the posterior anulus was increased by 43% (P = 0.02). In 4 degrees of extension, changes observed between 0 degree and 2 degrees were usually exaggerated. In contrast, moderate flexion tended to equalize the distribution of compressive stress. In 7 of the 19 discs, 2 degrees of lumbar extension decreased maximum compressive stress in the posterior anulus relative to the neutral posture by up to 40%. Linear regression showed that lumbar extension tended to reduce stresses in the posterior anulus in those discs that exhibited the lowest compressive stresses in the neutral posture (P = 0.003; R2 = 41%). The posterior anulus can be stress shielded by the neural arch in extended postures, but the effect is variable. This may explain why extension exercises can relieve low back pain in some patients.

Potential mechanism of left ventricular outflow tract obstruction after mitral ring annuloplasty

Objectives: The purpose of this study was to explore whether geometric changes that predispose to left ventricular outflow tract obstruction after mitral ring annuloplasty are coupled to subvalvular apparatus disturbances. Methods: Radiopaque markers were implanted in sheep: 9 in the ventricle, 1 in the high interventricular septum, 1 on each papillary muscle tip, 8 around the mitral anulus, 4 on the anterior mitral leaflet, and 2 on the posterior leaflet. One group served as control (n = 5); the others were randomized to undergo annuloplasty with the Duran ring (n = 6; Medtronic, Inc, Minneapolis, Minn) or Carpentier-Edwards Physio ring (n = 6; Baxter Healthcare Corp, Irvine, Calif). After a 7- to 10-day recovery period, 3-dimensional marker coordinates were measured with biplane videofluoroscopy. Results: At the beginning of ejection, (1) the anterior leaflet was displaced toward the left ventricular outflow tract; (2) the normal atrially flexed anterior anulus was flattened into the left ventricular outflow tract; (3) the posterior anulus was displaced toward the left ventricular outflow tract; (4) the anterior papillary muscle was displaced septally; and (5) the posterior papillary muscle was dislocated inwardly toward the anterior papillary muscle in the Physio ring group compared with the control group. During ejection, all these structures moved septally, encroaching further on the left ventricular outflow tract. In the Duran ring group, only the posterior anulus was displaced toward the left ventricular outflow tract; the anterior leaflet was not displaced toward the left ventricular outflow tract, and it did not move septally during ejection. Conclusions: The semirigid Physio ring was associated with perturbations in annular dynamics that caused changes in papillary muscle geometry. We propose an integrated valvular–subvalvular mechanism to explain displacement of the anterior leaflet into the left ventricular outflow tract after mitral ring annuloplasty. (J Thorac Cardiovasc Surg 1999;117:472-80)

Groin pain associated with lower lumbar disc herniation.

Retrospective clinical and magnetic resonance imaging study of patients with groin pain associated with lower lumbar disc herniation. To demonstrate the clinical features and magnetic resonance imaging findings of these patients. Patients with lumbar disc herniation sometimes report groin pain. Little mention has been made, however, regarding the clinical features of groin pain stemmed from lower lumbar disc herniation until now, with only Murphey referring to groin pain in disc disease. A total of 512 patients were diagnosed with singular lower lumbar disc herniation (L4-L5 and L5-S1) at Kakegawa City General Hospital between July 1990 and December 1993. Of these patients, 21 (4.1%) reported groin pain. The characteristic clinical features and magnetic resonance imaging findings of the 21 patients were investigated and compared with the features and findings of patients with no groin pain. Patients with groin pain had a higher mean age and lower rate of low back pain, and L4-L5 discs were more likely to be involved than L5-S1 discs. In their magnetic resonance images, herniation tended to be more central than in patients with no groin pain. Elderly patients with L4-L5 protruding herniation of the anulus fibrosus were most likely to experience groin pain. The sinuvertebral nerve that innervates the posterior anulus fibrosus, the posterior longitudinal ligament, and the dura was indicated as the afferent nerve of groin pain.

Mechanical response of the lumbar spine to seated postural loads.

In vitro force and deformation measurements formed the basis for determinate, quasistatic analysis of principal forces in the seated lumbar spine. To explore the relationship between seated postures and the mechanical response in component tissues of lumbar intervertebral joints. Despite the high prevalence of low back pain syndrome, the precise mechanisms relating specific mechanical loads to spinal degeneration are not well understood. Simultaneous, time-dependent measurement of anterior column forces and articular facet forces has not been presented previously. consequently, a determinate analysis of principal component forces has not been possible. Twelve lumbar spines (L1-S1) were subjected to constant loading conditions while in flexed and extended seated postures. Time-dependent forces were measured in the anterior column at the L4 and L5 superior endplates and in the four facets of the L3-L4 and L4-L5 motion segments. A quasi-static analysis of sagittal plane forces was used to compute the remaining principal joint forces, including ligament, disc shear, and facet impingement forces. Component forces changed under static loading in both postures. There were significant differences between the mechanical responses of the two postures. Although the vertical creep displacement was greater in the extended seated posture (3.22 mm versus 2.11 mm), the escalation of forces was more severe in the flexed posture. The results suggest a mechanism of force balancing in lordotic postures under static loads, whereas flexed postures produce large increases to the tensile forces in the region of the posterior anulus.

An in vitro study of the biomechanical effects of flexible stabilization on the lumbar spine.

Lumbar motion segments were tested in vitro to examine biomechanical changes after posterior fixation by a flexible device. To assess changes in load distribution and conformation of vertebral structures after a flexible stabilization. This should provide the foundations for a scientific understanding of the immediate effects of this surgical procedure. Hooks were placed over the proximal spinous process and the distal laminas of a motion segment and connected by a polyester braid. Tension applied to the braid then generated a compression of the posterior elements. The force between the articular facets, the displacement of the posterior anulus fibrosus of the intervertebral disc, and the change in the relative position of the adjacent vertebrae were measured as the applied tension was increased. Facet joint force, disc bulge, and vertebral angulation increased with applied tension until a position of "locking" was achieved, apparently when the bony margin of the superior half of the facet joint contacted the inferior pars interarticularis. A tension of between 50 to 100 N in the braid was required for this. Facet joint force was less than 40% of this, and disc bulge was only 0.15 mm. The extension of the motion segment was between 2 degrees and 8 degrees. The results suggest that if such a system is applied surgically, stabilization is produced by compaction of the bony margins of the facet joints. Only a relatively small proportion of the posteriorly applied load is carried by the facet joints themselves, and little angulatory change is expected with minimal disc bulge.

Do bending, twisting, and diurnal fluid changes in the disc affect the propensity to prolapse? A viscoelastic finite element model.

A finite element model of a lumbar motion segment was constructed. The model was directed toward understanding the effect of compression, bending and twisting, and diurnal fluid changes in the disc on the propensity to disc prolapse. Tensile stresses in the anulus fibers were computed and used to determine the successive steps required to create a fissure in the disc. Disc prolapse is more likely under combined loading involving compression and bending and twisting. Changes in fluid content in the disc also affect the mechanical behavior of the disc. The three-dimensional model accounted for the viscoelastic material properties of the anulus fibers and ligaments. Diurnal fluid exchange was simulated by changing the fluid content in the nucleus of the disc. Combined with bending and twisting, a compressive load was applied at different loading rates. The maximum tensile stress in the anulus fibers always occurred in the fibers at the inner posterior anulus at the junction of the disc and the endplate. Of the three models tested, the "weakest" (or the first to fail) was the saturated disc subjected to compression and bending and twisting. As the loading rate increased, anulus fiber failure was initiated at a lower value of compressive load. An increasing compressive load applied to a flexed, twisted, and saturated disc resulted in progressive failure, or fissure propagation, starting at the posterior inner anulus at the junction of the disc and the endplate. The results from this study suggest that there are several key factors involved in the initiation and propagation of anulus failure: axial compressive load, bending and twisting, and disc saturation. If one of these is lacking, anulus failure is harder to achieve.

Extensive calcification of the mitral valve anulus: Pathology and surgical management

Extensive calcification of the mitral valve anulus is a pathologic entity frequently associated with degenerative valvular disease. The calcification process remains localized to the anulus in 77% of the cases. It may extend, however, to the underlying myocardium. Whenever an operation is necessary for an associated valve insufficiency, the question arises whether it is preferable to repair or to replace the valve and how to manage the calcification. In the first part of this paper the pathology of this disease is studied, and in the discussion a mechanism is proposed to explain the development of the process of calcification. In the second part, a new operation is described, which comprises the temporary detachment of the leaflets, en bloc resection of the calcium deposit, annular reconstruction, and valve repair. For patients in whom the calcification extends to the myocardium a “sliding atrioplasty” of the left atrium is described, which allows the area of exposed muscular fibers to be covered. Between 1986 and 1994, among 68 patients with extensive calcification of the anulus and severe mitral valve insufficiency, 67 benefited from these repair techniques. Ages ranged from 18 to 82 years (mean 62 years). Thirty-two patients had a billowing mitral valve (Barlow), 27 a fibroelastic deficiency, and two Marfan's disease. The calcification involved more than one third of the anulus in 88% of the patients, the posterior anulus in 10.5%, and the whole anulus in 1.5%. The calcification process extended to the myocardial wall in 12% of the patients and to the papillary muscles in 4.5%. In the group of 67 valve repairs, there were two hospital deaths (2.9%), no instances of anulus dehiscence, and no early reoperations. The follow-up period extended from 4 months to 8 years (mean 3 years 8 months). There were two late deaths, 2 and 17 months after the operation, for an actuarial survival of 93% at 7 years. Late reoperation (6 to 62 months) was necessary in four patients (6.4%) for residual mitral valve incompetence ( n = 2), hemolysis ( n = 1), or endocarditis ( n = 1). In one of these patients a new repair was possible, whereas the three other patients required a valve replacement. All patients but one survived the reoperation. Actuarial freedom from reoperation was 87% at 7 years. All 60 patients with valve repair were reviewed for this study by clinical examination and echocardiography. All but one were in functional class I or II. There was no incompetence or trivial residual mitral valve incompetence in 55 patients and moderate incompetence in five. Two thromboembolic events have been recorded for a linearized rate of 1%/pt-yr. This study shows that complete anulus decalcification and valve repair can be done safely in patients with mitral valve insufficiency and extensive calcification of the anulus, even when the calcification process deeply involves the myocardium. It also demonstrates that an initially good result remained stable up to 7 years. (J T HORAC C ARDIOVASC S URG 1996;111:718-30)

Effects of degeneration on the elastic modulus distribution in the lumbar intervertebral disc.

Local elastic moduli of sliced intervertebral disc specimens were studied after establishing the relation between the elastic modulus and indentation behaviors by model tests using polyurethane specimens. This study presents a method to quantify the distribution of compressive elastic moduli in the lumbar intervertebral disc and to clarify the effects of degeneration on the distribution. No study has been performed to evaluate the distribution of axial compressive elastic moduli, which is supposed to relate previous biomechanical, biological, and biochemical findings regarding the intervertebral disc. Local compressive elastic moduli of the intervertebral disc were estimated by indentation tests. To evaluate the distribution of elastic moduli, indentation tests were performed at nodal points of a 10 mm x 10 mm network on a specimen. Nine cadaveric lumbar discs (L3-L4 and L4-L5) with various degrees of degeneration were tested. The age of subjects ranged 39 to 90 years (mean, 58.4 years). The distribution of elastic moduli in normal discs was symmetric about the midsagittal plane. The mean elastic modulus in the nucleus pulposus was 5.8 kPa and those of the anterior and posterior anulus fibrosus were 110.7 and 75.8 kPa, respectively. The elastic moduli in the lateral portions were the lowest in the normal anulus, and were close to the values of the nucleus. Compared to normal discs, degenerated discs showed irregular distributions of elastic moduli. The elastic moduli of the degenerated nucleus were higher than those in normal discs. The distribution of elastic moduli is much different between discs with and without degeneration.

Sustained loading generates stress concentrations in lumbar intervertebral discs.

Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading. To show how sustained "creep" loading affects stress distributions inside intervertebral discs. The central region of an intervertebral disc acts like a hydrostatic "cushion" between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep. Twenty-seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2-6 hours. Creep reduced the hydrostatic pressure in the nucleus by 13-36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep. Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption, and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.

Mitral Value Replacement Using the Konsari Procedure: Unusual Two-Dimensional Echocardiography Findings

A 49-year-old woman with myxomatous mitral valve degeneration complicated by severe mitral regurgitation caused by endocarditis and left ventricular dilatation underwent successful mitral valve replacement using the Khonsari technique. Preoperative two-dimensional Doppler echocardiography showed severe mitral regurgitation, progressive left ventricular dilatation, left atrial enlargement, and marked calcification of the posterior anulus. Color-flow imaging revealed severe mitral regurgitation. Cardiac catheterization revealed normal coronary arteries. Postoperative findings detected by transthoracic two-dimensional and Doppler echocardiography are shown and a review of the procedure used in conjunction with mitral valve replacement is presented.

Pathogenesis of acute ischemic mitral regurgitation in three dimensions

Changes in the geometric and intravalvular relationships between subunits of the ovine mitral valve were measured before and after acute posterior wall myocardial infarction in three dimensions by means of sonomicrometry array localization. In 13 sheep, nine sonomicrometer transducers were attached around the mitral anulus and to the tip and base of each papillary muscle. Five additional transducers were placed on the epicardium. Snares were placed around three branches of the circumflex coronary artery. One to 2 weeks later, echocardiograms, dimension measurements, and left ventricular pressures were obtained before and after the coronary arteries were occluded. Data were obtained from seven sheep. Coronary occlusion infarcted 32% of the posterior left ventricle and produced 2 to 3+ mitral regurgitation by Doppler color flow mapping. Multidimensional scaling of dimension measurements obtained from sonomicrometry transducers produced three-dimensional spatial coordinates of each transducer location throughout the cardiac cycle before and after infarction and onset of mitral regurgitation. After posterior infarction, the mitral anulus enlarges asymmetrically along the posterior anulus, and the tip of the posterior papillary muscle moves 1.5 ± 0.3 mm closer to the posterior commissure at end-systole. The posterior papillary muscle also elongates 1.9 ± 0.3 mm at end-systole. The left ventricle enlarges asymmetrically and ventricular torsion along the long axis changes. The development of postinfarction mitral regurgitation appears to be the consequence of multiple small changes in ventricular shape and contractile deformation and in the spatial relationships of mitral valvular subunits. (J T HORAC C ARDIOVASC S URG 1995;109:684-93)

A correlation of cervical magnetic resonance imaging and discography/computed tomographic discograms.

The morphology of sagittal T2-weighted magnetic resonance imaging (MRI) of the cervical spine was correlated with provocative discography and subsequent computed tomography (CT) discograms in 52 patients with discogenic pain. The authors determined if the morphology of cervical spine discs, as seen on MRI, correlates with discography/CT discograms in patients with discogenic pathology. Several studies have demonstrated a correlation between MRI and discography in the lumbar spine. No studies have attempted to show if this relationship exists in the cervical spine region. The morphology of cervical T2-weighted MRI was characterized with regard to the disc nuclear signal and posterior anulus status. Provocative discography was evaluated with regard to positive or negative responses. Computed tomography was performed after discography on each patient. There was no correlation between pain response or morphology as seen on either discography or CT discography. A significant correlation was found between abnormality as seen on MRI and pain response on discography but the false-positive and false-negative rates were high. Our results suggest that several MRI patterns correlate well with positive or negative cervical discography responses while several other patterns are equivocal. Magnetic resonance imaging is a useful adjunct to cervical discography but there are some MRI patterns that cannot be considered pathologic, and discography is required to diagnose discogenic pain syndrome.

The strength in anterior bending of lumbar intervertebral discs.

This was a motion segment experiment. To determine the strength in bending of lumbar intervertebral discs. Eighteen "motion segments" consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate forward bending movements in life. The range of flexion and the resistance to bending of each specimen was compared before and after removal of the posterior elements. Nine of the discs were tested after the intradiscal pressure had been reduced by damaging the vertebral end plate in compression. The posterior elements restricted the disc to 80% of its full range of flexion. The strength in anterior bending of isolated discs decreased with age and was 33 Nm on average. Reducing intradiscal pressure did not affect the discs' strength in bending. Disc failure in bending occurs through overstretching of the outer anulus in the vertical direction. In life, the posterior elements may not adequately protect the posterior anulus from fatigue damage.

Can Intervertebral Disc Prolapse Be Predicted By Disc Mechanics?

The hypothesis was tested that stress concentrations in the posterior anulus of an intervertebral disc predispose it to prolapse under high compressive loads and anterolateral bending. The distribution of compressive stress inside the intervertebral discs of 22 cadaveric lumbar motion segments was measured with the specimens loaded in pure compression and in compression combined with anterolateral bending. Each motion segment was then loaded to failure in combined compression and anterolateral bending. Failure occurred in the vertebral body (n = 12) or posterolateral anulus (n = 10); the latter group showed a significantly greater incidence of stress concentrations (P<0.001) in the posterior anulus, when loaded in compression and bending. It was concluded that some discs are predisposed to prolapse because of damaging, localized concentrations of stress in the posterior anulus in combined anterolateral bending and compression.