DIURNAL CHANGES AND THEIR P. DOLAN, From Diurnal changes We have in the loads reviewed the SPINAL CLINICAL M. A. ADAMS, discs. IN acting effects SIGNIFICANCE W. C. HUTTON, the University on the spine of these MECHANICS affect changes R. W. PORTER of Bristol the water on spinal content mechanics, and height and of the intervertebral their possible clinical significance. Cadaveric lumbar spines subjected to periods of creep loading show a disc height change similar to the physiological change. As a result intervertebral discs bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content falls, and its affinity for water Disc prolapse becomes more difficult. The neural of the compressive and bending sfresses acting increases. proportion that these changes We conclude are not fully compensated for by modified muscle activity. that different spinal structures are more heavily loaded the time of onset of symptoms more about the pathophysiology Therefore, understand and signs, and any diurnal variation oflow back pain and sciatica. During the recumbency of sleep, the loading on the intervertebral discs is reduced, and their relatively unopposed swelling pressure results in them absorbing fluid and increasing in volume (Urban and McMullin 1988). The absorbed fluid is expelled during the day when the loading of the spine is increased. There is, thus, a diurnal variation in the fluid content and height of the discs which properties We these causes different We then a variation in of the spine. review the experimental changes, and spinal suggest then the changes at different time of onset variation M. A. Adams, PhD, Research Fellow P. Dolan, PhD, Research Fellow Comparative Orthopaedic Research Row, Bristol BS1 SLS, England. W. C. Hutton, DSc, Professor Department of Orthopaedics, USA. © should be sent in their in their CADAVERIC times severity, of the may day. help us SPINES Periods of creep loading of cadaveric lumbar spines cause a change in disc height similar to the diurnal change seen in vivo. Certain mechanical properties have been measured before and after the period of loading. Disc height. Constant loading at 1 000 N for six hours (simulating light manual labour: Nachemson 1981) causes disc height to decrease by 1 .53 ± 0.34 mm (Adams, Dolan and Hutton 1987). The height loss is rapid at first but much slower by the end of the six hours concerning in loading severity, at different of day. and might (Fig. 1). Similar results have been reported experiments (Adams and Hutton 1983 ; Koeller, and Hartmann 1984). Ifthe applied compressive increased and then at hourly intervals to 3 000 N in order in other Funke force is from 1 000 N to 2 000 N to simulate manual labour of increasing severity, then the height loss shows no sign of slowing down, and the cumulative loss after three hours is 2.13 ± 0.35 mm (Adams et al 1987, see Fig. 1). Unit, Emory to Dr M. 1990 British Editorial Society ofBone 0301-620X/90/2027 $2.00 J Bone Joint Surg [Br] 1990 ; 72-B :266-70. 266 mechanical times of the of symptoms University University, R. W. Porter, MD, FRCS, Consultant Orthopaedic Doncaster Royal Infirmary, Armthorpe Road, Yorkshire DN2 5LT, England. Correspondence the evidence discuss structures that the signs, and any diurnal be an aid to diagnosis. arch and associated ligaments resist an increasing on the spine. Observations on living people show of Bristol, Atlanta, Surgeon Doncaster, A. Adams. and Joint Surgery Park Georgia, South about mostly 1935). change The average diurnal variation in human stature is 19 mm (Tyrrell, Reilly and Troup 1985) which is attributable to changes in disc height (De Puky A 19 mm change in stature corresponds to a ofabout 1 .5 mm in the height ofeach lumbar disc (Adams et a! 1987), so the loading regimes above are sufficient to simulate physiological reduction in disc height. Changes in disc height are caused by fluid and creep deformation of the annulus fibrosus THE JOURNAL OF BONE AND JOINT discussed diurnal exchange (Koeller SURGERY DIURNAL et a! 1984). probably loading The relative CHANGES importance IN SPINAL of each depends upon the severity (Adams et a! 1987) and factors the degree of disc The diurnal similar magnitude MECHANICS mechanism and duration such as age degeneration. disc height change of to the normal narrowing 1.5 mm ofthe foramen averages and Goel 1983). only about 15 to 20 mm THEIR Fluid of and is of a lumbar discs expected with age (Koeller et a! 1986). It could have a significant effect when there is pathology in the nerve root canal since the total height of the lumbar intervertebral Takata AND CLINICAL loss dissipation 267 SIGNIFICANCE is accompanied during by a reduction a loading/unloading in energy cycle (Koeller et a! 1984). This means that the dehydrated disc behaves more like an elastic solid and less like a viscous fluid. Disc swelling pressure. Disc swelling pressure can be defined as that physical pressure which must be applied to the disc in order to prevent it from swelling up in saline (Urban and McMullin 1988). It is a measure of the tissue’s affinity for water. Swelling pressure can be measured by adjusting the (Panjabi, compressive force acting on a motion segment until there Disc water content. Creep loading reduces the water content of the discs. After four hours loading at about 700 N, when disc height is reduced by about 1.5 mm, the average fluid loss is 12% from the annulus and 5% from the nucleus (Adams and Hutton 1983). Discs from people under the age of 35 years lose almost twice this amount. is no detectable change in disc height. This force is then divided by the cross-sectional area of the disc. Swelling pressure increases rapidly during creep loading as shown in Figure 1, and its rise can be accelerated by more intense loading (unpublished results from our laboratory, 1988). The clinical significance of swelling pressure is Most hours that it determines the height (and mechanical of high loading. of the fluid loss probably occurs of loading because longer-term in the first few creep tests cause only a little more fluid loss : 24 hours loading at about 1 000 N reduces the fluid content of annulus and nucleus by 1 1% and Gowin 1985). 8% respectively (Kraemer, Kolditz Compressive stiffness. Creep loading increases the disc’s compressive stiffness. The increase is about 50% after two to three hours ofphysiological cyclic loading (Koeller et al 1984) and can rise to 100% after 28 hours (Smeathers 1984). Motion segment stiffness is ofclinical significance because it determines how much the disc and surrounding soft tissues deform during physiological dynamic loading and / 2 , Height loss ofthe spine. Disc bulging. (mm) 100 it may // ofthe disc has been observed after creep loading (Koeller et a! 1984). increase has not been measured directly, be inferred from the This suggests that the diurnal 1 .5 mm should be accompanied C 140 results The but of Brinckmann and reduction by an in disc height of increased radial bulge of about 0.5 mm. For comparison, the increased radial bulge caused by increasing the compressive force on the spine from 300 N (lying in bed) to 1 000 N (light manual work), is only about 0.2 mm (Brinckmann and Horst 1985). Compressive stiffness (%) 1 fl(l Diurnal disc bulging will have clinical implications when the central or root canal is stenotic. The width of the intervertebral foramen is normally about 8 to 10 mm (Panjabi et a! 1983) but it can be much less in the root 100 Resistance to flexion (%) 2 L.J 3 Hours Fig. of creep No. 2, MARCH 1990 4 5 6 loading I The effect ofcompressive creep loading on of lumbar motion segments. The solid compressive force of 1 000 N applied for refer to a compressive force of 1 000 N in second, and 3 000 N in the third. Resistance the physiological limit determined before pressure’ of an intervertebral disc is defined 72-B, bulging Horst (1985). They altered the volume of the disc, either by injecting fluid into it or by fracturing the vertebral body end-plate, and found that the change in radial bulging was about one-third of the change in disc height. / / (N/cm2) VOL. Radial to increase size of this C Swelling pressure rate at which a disc recovers lost properties) at the end of a period some mechanical properties lines refer to a constant six hours. The broken lines the first hour, 2 000 N in the to flexion was measured at creep loading. The ‘swelling in the text. canal and the lateral recess. Loading of the apophysial joints. The on the apophysia!joints segments were loaded measured a typical (lumbar posture spine (spine slightly in slight apophysialjoints either there simulated has been to simulate resist posture is little (Adams resistance standing flexed) and extension). little, if any, compressive when sitting an erect standing Before creep, the compressive force and Hutton 1980). After in the flexed spine, but posture the force motion posture apophysial joints in creep, in the resist 268 M. A. ADAMS, an average some cases, more result of 16% of the the proportion extended in high of the Hutton Forward applied can compressive be as high postures, compressive stress concentrations apophysial 1984) and bending force. as 70%. In In performed stresses Adams 1984). increases separately, and a and in the early morning in the osteoligamentous generate lumbar performed proportion much higher spine than do later in the day. ofthese increased 45% Backward bending properties. Creep loading reduces the disc’s resistance to backward bending by about 40% (Adams, Dolan and Hutton 1988). This is balanced by increased resistance from the apophysial joints and spinous processes, so that the resistance to backward bending of the movement, Prolapsed be induced combined whole motion are unaltered intervertebral to prolapse bending, shear segment, and Table I. Diurnal the lumbar spine variation Intervertebral disc Posterior longitudinal ligament Vertebral body end-plate Segmental nerve root Apophysial joint Articular surface Capsule and ligaments Supra/interspinous of maximal stress Period Comment AM AM AM AM PM Especially Especially in bending in flexion Increased Increased tension compression PM AM PM AM PM ligaments on the Flexion Extension Increased Increased extension various diurnal changes structures in in spinal mechanics segments prolapse repeated cadavers reduce content lowered fluid content is artificially (by injecting can probably of the nucleus raised (by chymopapain) (see Table I). loaded in this way, 26 failed by posterior (Adams and Hutton 1982). The experiment on a further group of 19 motion segments of a similar age range after they had been loaded (Adams prolapsed, and et they a! 1987). were both Only from two of the same disc was from creep these spine. Creep-loaded discs, in vivo, may also susceptible to prolapse, perhaps because of the fluid content of the nucleus pulposus and the flexion stresses in the posterior annulus. discs be less reduced reduced tension in flexion compression in CLINICAL posture the muscles effects of 2 Spinal These range PM Fig. showing the by creep loading. disc. Some cadaveric discs can posteriorly by loading them in and compression. Of61 motion AM Diagrams Also, the stresses in the morning. before creep to bending is for the disc is 85% R. W. PORTER similar movements discs resist a higher by 2#{176} or 3#{176} (Adams et to about 12.5#{176}extra lumbar spine. At the physiological limit of flexion (as determined loading) the motion segment’s resistance reduced by about 70% (Fig. 1). The reduction measured W. C. HUTFON, creep loading can on the lower margins joint surfaces (Dunlop, capsule (Yang and King properties. Creep loading motion segment’s range of flexion al 1987) which is equivalent movement for the whole of the and ligaments, respectively. P. DOLAN, be attributed pulposus. injecting then to the posture If the fluid saline) there or is a changes results corresponding increase or decrease in the disc’s resistance to bending (Andersson and Schultz 1979 ; Dolan, Adams and Hutton 1987). These results indicate that, inlife, flexion movements DIURNAL and of the in order mobifity. back VARIATION It is possible and abdomen to compensate in the underlying from our laboratory for spine. show may some that, modify of the in vivo, spinal diurnal However, unpublished that the lumbar lordosis increases by about 3#{176} during the day. This would increase the loading of the apophysial joints and compound the effects due to loss of disc height. It could be thought that the higher bending stiffness ofthe osteoligamentous spine in the early morning would THE JOURNAL OF BONE AND JOINT SURGERY DIURNAL be offset bending, CHANGES IN SPINAL MECHANICS by the trunk muscles restricting the range of so that the bending stresses on the spine remain AND THEIR We anics CLINICAL suggest SIGNIFICANCE that are ofclinical diurnal 269 variations significance. in spinal Since different mech- structures the same. However, the experimental evidence suggests that this does not happen to any significant extent. In vivo, the range of lumbar flexion is reduced by only 5#{176} in are more heavily loaded at different times of the day, the time of onset of a patient’s symptoms, and any diurnal changes in their degree of severity, might help us to the early morning (Adams et a! 1987) whereas the range of flexion of the underlying spine is reduced by about 12.5#{176}before creep loading (see above). Calculations comparing the in vivo and in vitro evidence suggest that, in life, bending stresses on the lumbar discs and ligaments understand can be increased the early morning The movement by about (Adams slight diurnal in vivo may ‘warm-up’ (Baxter observed in hip 300% and 80% et a! 1987). variation be partly 1987). movements though there is unlikely mechanical properties of normnal articular like the disc. cartilage has (Adams et a! 1987) to be any variation the underlying joints, swell up been even in the since overnight spinal injury little published information symptoms and signs, nor in pain. Varma (1987) recorded of back pain occurred in the day. This who found more agrees with that mine commonly pathophysiology of different back pain I lists some of the structures thought to be for low back pain and sciatica, and indicates are most heavily loaded. The disc resists all of the compressive force on the spine in the morning, and in addition, is much more highly stressed during flexion and extension movements. A herniated disc, however, may behave The posterior the morning a study workers differently from one with an intact annulus. longitudinal ligament is stretched more in because of the increased height of the disc, although reduced radial bulging ofthe disc will counteract this effect to some extent. Vertebral body end-plates can be ruptured by increasing the fluid content of the nucleus pulposus of the working et a! (1980) sustained effect not in of spinal to muscle similar does Low back pain. There is about diurnal variations in the time ofonset oflow back that 47% of ‘first episodes’ early part by Evans A Table responsible when they respectively in the range attributable the syndromes. (Jayson, Herbert The morning segmental because they are compressed bulging of the intervertebral in the morning. and Barks 1973) that the end-plates are more highly morning when the discs are swollen with so it is likely stressed fluid. nerve roots are stretched the spine is about 19 mm more disc, in the afternoon the reduced by foramen, and by more in the longer; but by extra radial height of the the buckling this timejoint the capsular variation nous ligaments will all be most stretched during forward bending movements performed in the early morning because the increased disc height allows them less slack. injuries. DISCUSSION The experimental evidence can be summarised as follows : with creep loading, the intervertebral discs lose height, bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content is reduced, and its affinity for water increases. Disc prolapse becomes less likely. The neural arch proportion acting Figure and associated ligaments resist an increasing of the compressive and bending stresses on the spine. These results are summarised 2. In life, these changes will occur mostly in the few hours of the ofthe changes day, will but the time depend upon scale the and throughout the spinal mechanics changes. VOL. 72-B. No. 2, MARCH 1990 day spinal in at pain may be expected to increase. However, ligaments and the supraspinous and interspi- we attempt clinical symptoms to understand studies into the and signs could more diurnal about spinal changes of be rewarding. REFERENCES first Adams minor diurnal MA, Hutton apophysial Bone of loading probably cause similar to the are especially bending, and This work was supported by the Arthritis and Rheumatism Council, Action Research for the Crippled Child and the Back Pain Association. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. the magnitude severity in the afternoon, during backward surfaces in on the spine : heavy labour will have a greater effect, and in less time, than sedentary activity. The swelling pressure results suggest that the effects of intense loading will be reversed more rapidly than the effects of less intense activity of longer duration. Alternating periods of rest and activity changes in As pathology, joint the trends due to changes in spinal mechanics and they must be taken into consideration in any surveys of diurnal closer together postures and apophysial of ligamentum pressed lordotic The the Accidents in general tend to become more frequent towards the end of a working shift, when people are tired and inattentive. 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