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Specific signs at the acetabulum of loosening in- clude the leakage of contrast at the cement-bone/metal- Bone-seeking 99mTc compounds demonstrate abnormal up- cement interface in 90% of loose replacements and ex- take for 9-12 months post-operatively order inderal 40mg mastercard arteria vesicalis. Thereafter discount inderal 40mg with visa blood pressure levels emergency, increased tension of contrast into pseudo-bursae in about 56%. Abnormal activi- Femoral loosening is confirmed in about 98% by contrast ty in the blood pool or perfusion phase should suggest in- medium tracking into the cement-bone interface below fection, particularly if the abnormality is diffuse. In the late the intertrochanteric line, or in the bone-metal interface phase, the classical 3-point scan suggests varus tilt and below the intertrochanteric line. Again, a diffuse increase in activity suggests in- stemmed devices, contrast medium below the level of fection; however, the ability to separate infection from asep- mid-component is abnormal. A normal bone scan has a hip include communication with greater trochanteric bur- strong negative predictive value. However, for reasons that sae (50%), supra-acetabular collections (33%) and filling have yet to be explained, bone scans are often abnormal in of the iliopsoas bursa (17%). Lymphatic filling remains a otherwise uncomplicated total knee replacements and thus controversial finding and is probably not significant, al- have a poorer positive predictive value. It had a high neg- At the knee, contrast under the tibial tray, or the cement ative predictive value but a poor positive predictive val- interface is abnormal. Component failure, especially the ue, making a negative result more useful. The finding of a 111Indium labeled white blood cells afford increased Baker’s cyst may explain a patient’s symptoms and signs, sensitivity and specificity when used in combination with but in most cases is not relevant. Aspiration was shown to 99mTc bone scans but also carry a significant false-nega- be 100% sensitive and specific for infection in the knee in tive rate. Suggestive signs of infec- Magnetic Resonance Imaging tion include the extension of abnormal activity outside the area of the synovial cavity and persistence of increased ac- MRI has received relatively little attention in the assess- tivity on 24-h images. The overall sensitivity is 86% with a ment of total hip and total knee replacements because of specificity of 78%. Generally, the artefacts produced by a prosthesis re- alignment pre-operatively especially, in patients with flect the orientation of the prosthesis relative to the main fixed flexion deformities and in prosthesis planning. The tibial landmarks for mea- MRI artefacts with orthopedic hardware include orientat- suring component rotation include the tibial tubercle or ing the frequency-encoding direction along the long axis posterior tibia. It is recommended that scans are obtained of the device and reducing voxel size (increasing the num- perpendicular to the femoral and tibial components in or- ber of pixels in the frequency-encoding direction does this der to assess rotation. However, it has been noted that the position of the strength to decrease misregistration artefact, the use of pelvis changes from the supine to the standing posture, lim- fast spin-echo imaging techniques, lower static magnetic iting the value of the measurement of component antever- field strength and a fast multiplanar inversion-recovery sion from CT scans. Acetabular anteversion, measured with sequence all assist in artefact reduction. The use of less relation to a horizontal baseline, is normally between 0 and ferromagnetic hardware (e. The inclination angle in the coronal plane nor- MRI scanning is of value in pre-operative planning and mally measures between 40 and 50°. Overhang of the is superior to CT in evaluating soft-tissue pathology after acetabular cup of greater than 12 mm has been seen in pa- a total hip replacement. Normal findings include that tients with iliopsoas impingement and irritation. Signs of the lat- shown by low-signal fluid collections adjacent to a com- ter include periostitis (100% specificity, but 16% speci- ponent, for example, paralleling the femoral stem. Poorly ficity for infection), soft-tissue infection (100% sensitiv- defined hyperintense areas suggest infection, with signal ity and 87% specificity) and the presence of fluid collec- intensity similar to fluid. CT may be used to signal distension of the joint pseudocapsule, may precede demonstrate and monitor developing bone radiolucen- granuloma formation. The latter is associated with focal cies, where it has been shown to be more sensitive than osteolysis and appears as discrete, well-demarcated inter- radiographs in detecting and quantifying acetabular mediate to slightly increased signal areas with low-signal small-particle disease. Granulomas may appear as focal periprosthetic in- What investigations Are The Most Useful? College of Roentgenology (see National Guideline Peripheral enhancement and some internal enhancement Clearing House) are as follows, graded (1=least useful, have been noted. Early work suggested that MRI allows good visualiza- For possible loosening, with or without infection: plain tion of femoral component complications (100% in a radiographs, with comparison to previous studies 9 (indi- study of 11 patients) but poorer visualization of the ac- cated), all other studies 1. For possible loosening, with or without infection, but The recent adaptations suggest that periprosthetic soft tis- radiographs normal: joint aspiration with or without an sues may be visualized better.
In addition buy discount inderal 80 mg pulse pressure explained, previous studies report- dural sac is reliable parameter for assessment of cervical ed a high rate of abnormal imaging findings in the lum- spine stenosis 40mg inderal with visa pulse pressure healthy range. A cross-sectional area of 60 mm2 has been bar spine of asymptomatic volunteers (Table 2) [15-22]. Since disk abnormalities, including disk bulging, disk Spinal canal stenosis may result in cervical myelopathy, protrusion and disk extrusion, are common in asympto- which presents as high signal intensity on T2-weighted matic volunteers, they cannot be used easily as parame- images (Fig. Only the presence of a disk ligament (OPLL) often results in a central cervical spinal extrusion may represent a clinical significant finding if stenosis. OPLL is more frequently present in men than in the symptoms of the patient correspond to the imaging women and typically manifests in the fifth to seventh findings. The diagnosis of OPLL is established by were matched according to age, sex and occupational risk its characteristic appearance on conventional radiography factors to patients with symptoms of disk herniation, or CT imaging. Thus, neural compromise over multiple levels, but also can be segmental. The pathophysiologic mechanisms that cause nerve- Myelography has for many years been the method of root symptoms are still not completely understood. For Currently, two concepts are discussed: mechanical nerve- clinical purposes, an anteroposterior diameter of the dur- root compression and chemically induced nerve-root in- al sac of 10 mm is indicative of absolute stenosis and 12 flammation caused by the nucleus pulposus. Using CT and MRI, mea- ter concept may explain why patients suffer from radicu- surement of the cross-sectional area of the dural sac is lopathy even though nerve root compression is not visi- probably the most reliable technique for assessment of ble on MRI. Prevalence of disk abnormalities in healthy volunteers Author, year, test Age HNP Bulging Degenerative Stenosis Annular tear Other group (n) disk disk (high signal intensity zone) Weishaupt (1998), 20-50 years, 60%c 20% 72% – 33% Nerve-root contact or MRI mean=35, n=60 deviation, 26%; nerve-root compression, 2%; thecal sac impression, 17% Stadnik, (1998), 17-71 years, 33% 81% 72% – 56% – MRI median 42; n=36a Savage (1997), 20-30 years, – – – – – Any abnormality 32% MRI n=78; 31-58 years, n=71 Burns (1996), 21-31 yreas, – – – – – Any abnormality 85% MRI n=41 Boos (1995), 20-50 years, 76% 51% 85% – – No sequestered disks; nerve- MRI mean=36 years, of disks root contact or deviation, 22% n=46b Jensen (1994), 20-80 years, 28% 52% – 7% 14% 64% had disk bulge, MRI mean=42, n=98 protrusion or extension Boden (1990), <60 years, n=53; 22%, 54%, 46%, 1%, –, – – MRI >60 years, n=14 36% 79% 93% 21% Weinreb (1989), Women 19-24 years, 9% 44% – – – – MRI mean=28, n=86 HNP, herniated nucleus polposus a Referred for head or neck imaging b Patients matched for age and occupational exposure c Numbers represent prevalence per subject Another important MRI finding that is probably relat- Another controversial issue in imaging of the degen- ed to discogenic pain is endplate abnormalities. Our own erative spine is the influence of the body position on study in 50 patients with discogenic pain has shown MRI findings. Several cadaver studies have shown that that type I and II endplate abnormalities are highly pre- flexion and extension, lateral bending, and axial rotation dictive for the presence of painful disk when compared to significantly change the anatomic relationships of the discography. A recent longitudinal study has shown that ligamentum flavum and intervertebral disk to the spinal type I endplate changes are dynamic lesions that either nerve roots in the lumbar spine. The authors refer to increase in size or convert to type II, and that if the type compression of the spinal canal or spinal nerve roots oc- I lesion does convert to type II, it starts to do so within 2 curring exclusively during axial loading and/or spinal years in most cases. Based on these data, at a statistically significant level, that conversion from clinically relevant spinal canal and foraminal stenosis, as type 1 to type 2 is related to an improvement in the pa- well as the degree of nerve-root compression, may not be tient’s back pain. Several attempts have been results of two studies [11, 26] have shown a high corre- undertaken in order to overcome the limitations of con- lation between the presence of HIZ and pain concordant ventional MRI. Willen and coworkers have de- with the usual symptoms at discography. However, the scribed a portable device allowing for axially loaded high prevalence of HIZ in asymptomatic volunteers, as lumbar-spine imaging using conventional MR scanners reported by three studies [16-18], indicates that these re- and with the patient in supine position. A recent study of MRI with the patient in an upright position can be per- the natural history has shown that HIZ often remains un- formed in vertically open-configuration MR systems, changed for several years, and there was no correlation which allow imaging in seated or even in standing posi- between resolution or increased severity of HIZ and tions. Preliminary experience has shown that, al- changes in symptoms. Weishaupt D, Zanetti M, Hodler J, Boos N (1998) MR imag- of axially loaded MRI should be reserved for selected ing of the lumbar spine: prevalence of intervertebral disk ex- trusion and sequestration, nerve root compression, end plate patients with low back pain. Stadnik TW, Lee RR, Coen HL, Neirynck EC, Buisseret TS, References Osteaux MJ (1998) Annular tears and disk herniation: preva- lence and contrast enhancement on MR images in the absence 1. Kent DL, Haynor DR, Longstreth WT Jr, Larson EB (1994) of low back pain or sciatica. Radiology 206(1):49-55 The clinical efficacy of magnetic resonance imaging in neu- 18. Ann Intern Med 120:856-871 Malkasian D, Ross JS (1994) Magnetic resonance imaging of 2. Modic MT, Masaryk TJ, Ross JS, Carter JR (1988) Imaging of the lumbar spine in people without back pain. Boos N, Rieder R, Schade V, Spratt KF, Semmer N, Aebi M (1988) Progressive and regressive changes in the nucleus pul- (1995) Volvo Award in clinical sciences. Radiology 169(1):93-97 racy of magnetic resonance imaging, work perception, and 4. Pearce RH, Thompson JP, Bebault GM, Flak B (1991) psychosocial factors in identifying symptomatic disc hernia- Magnetic resonance imaging reflects the chemical changes of tions. Burns JW, Loecker TH, Fischer JR Jr, Bauer DH (1996) Rheumatol Suppl 27:42-43 Prevalence and significance of spinal disc abnormalities in an 5. Pfirrmann CW, Metzdorf A, Zanetti M, Hodler J, Boos N asymptomatic acceleration subject panel.
In this case purchase inderal 80mg overnight delivery hypertension and pregnancy, the load pre- of drugs that cause relaxation of the muscle in the walls of sented to the heart by the blood pressure is too high (rel- the venous system safe 40mg inderal arteria hepatica propia. Lowering the blood volume also acts ative to the weakened condition of the muscle), and (as to decrease the over-distension of the heart during dias- the force-velocity curve describes) the rate of shortening tole. While it would seem that an increase in the resting (velocity) of the muscle will be reduced. The length-ten- muscle length would have a beneficial effect on the sion curve indicates that the larger the load, the less the strength of contraction, geometric factors in the intact shortening (see Fig. Therefore, less blood will be heart place the overstretched muscle at a mechanical dis- pumped with each beat. Therapy for this type of failure in- advantage that the length-tension curve cannot ade- volves improving the contractility of the muscle and/or re- quately overcome. Heart failure involves numerous interacting organ sys- Heart failure can also be of the diastolic type (and may tems. The mechanical behavior of the heart muscle, as un- occur along with systolic failure). Here the relaxation is derstood in the context of the length-tension and force-ve- impaired, and the muscle is resistant to the stretch that locity curves, is only a part of the problem. Some types of therapy must also consider factors external to the heart hypertrophy or connective tissue fibrosis also may con- muscle itself. The amount of cal- This mechanism is part of a coupled transport system in which cium in the cytoplasm, the cytosolic calcium pool, deter- three sodium ions, entering the cell down their electrochemi- mines the magnitude of the myofilament activation and, cal gradient, are exchanged for the ejection of one calcium ion. Proper function of this exchange mechanism requires a steep During relaxation, the cytoplasmic calcium concentration sodium concentration gradient, maintained by the membrane is rapidly lowered through several pathways. Because the 2 2 brane contains a vigorous Ca -ATPase (8) that runs continu- Na /Ca exchange mechanism derives its energy from the ously and is further activated, through a protein phosphoryla- sodium gradient, any reduction in the pumping action of the tion mechanism, by high levels of cytoplasmic calcium. Under level of the sarcolemma, two additional mechanisms work to normal conditions, these mechanisms can maintain a 10,000- 2 rid the cell of the calcium that entered via previous action po- fold Ca concentration difference between the inside and 2 tentials. Since a cardiac cell contracts repeatedly cium, ejecting one calcium ion for each ATP molecule con- many times per minute with each beat being accompanied by 2 sumed. Additional calcium is removed by a Na /Ca an influx of calcium, the extrusion mechanisms must also work exchange mechanism (10), also located in the cell membrane. The mito- CHAPTER 10 Cardiac Muscle 185 indicators of contractility) increases. This is the basis of the force-frequency relationship, one of the principal means of changing myocardial contractility. Cardiac glycosides are an important class of therapeutic agents used to increase the contractility of failing hearts. The drug digitalis, used for centuries for its effects on the circulation, is typical of these agents. While some details of its action are obscure, the drug has been shown to work by inhibiting the membrane Na /K -ATPase. This allows the cell to gain sodium and reduces the steepness of the sodium 2 gradient. This makes the Na /Ca exchange mechanism less effective, and the cell gains calcium. Since more cal- cium is available to activate the myofilaments, contractility increases. These effects, however, can lead to digitalis tox- icity when the cell gains so much calcium that the capacity of the sarcoplasmic and sarcolemmal binding sites is ex- ceeded. At this point, the mitochondria begin to take up the excess calcium; however, too much mitochondrial cal- FIGURE 10. When con- ATP needs already increased by enhanced contractility, is tractility is increased, the rate of rise of force is increased, the time less able to pump out accumulated calcium, and the final re- to afterload force is decreased, and potential force is increased. The muscle shortens faster and further (A) while isometric relaxation sult is a lowering of metabolic energy stores and a reduction (B) and relengthening (C) are minimally affected (D). Some changes in the contractility of car- diac muscle may be permanent and life threatening.
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