 So, when you talk about the anatomy of the spine, you must consider all these things. One is the vertebra, the discs between the vertebra, the pedicles, the lamina, the facet joints, the spinous processes and all the muscles and the ligaments which are attached over here. And these are the various ligaments which are important of great importance to us. Facet joint, capsule ligament very important and you will see why. Inter-transfer ligaments, all these are important believe me, the anterior longitudinal, the posterior longitudinal, the ligamentum flavum, the inter-transverse, the capsular ligament of the joint and the inter-spinous ligament. Now, let us look at the body, we will go one by one, first look at the body. If you see the width of the body is this and the depth is this and if you start from the cervical and go to the lumbar, the width of the body increases, then it is steady in the upper thoracic, then again it increases, then it is a little gentle slope in the lumbar region and the last three, the slope is even more gentle, L345. And if you see the largest vertebra, the widest vertebra is about 45 millimetres wide, so you have 4.5 centimetres of solid bone in the fifth lumbar vertebra or the fourth lumbar vertebra. The depth follows approximately the same curve, gradual increase, but the deepest vertebra again is in the lumbar region and usually it is the lumbar four and it is not more than 3 centimetres deep, so the body depth is about, so from ALL to PLL is about 3 centimetres in the biggest vertebra. Now, if you see the height of the vertebral body, the ventral and the dorsal and there is a difference between the two, especially in the lumbar region, because in the lumbar region the dorsal is a little bit less than the ventral. It is steady in the cervical 345 up to 6, both these are the same and they are about 12 to 13 millimetres in the cervical spine, then it rises gradually in the thoracic level up to the lumbar 3 and the dorsal height decreases in the last 3 lumbar vertebra. But still remember that that is important all at any point in space all these movements are possible. So, you must remember the vertebral body, any point in the vertebral body all these 6 movements are possible and there are some forces which resist the movement, some forces which are antagonists, some are agonists. Why is this important? In a normal anatomy that instantaneous axis of rotation of a vertebral body lies usually in the centre of the body. When it becomes kyphotic this instantaneous axis of rotation shifts back, remember that this is an important point in biomechanically it will help you make better constructs and more stable constructs if you know the basic biomechanics. In a kyphotic spine the instantaneous axis is nearer the canal and if it is a lorotic spine the instantaneous axis is nearer the anterior longitudinal ligament. Now the facets the angle of the facet jaw that is with respect to the midline what angle does the facet make with the midline? You see this facet the angle which it makes with the midline or a line drawn parallel to the midline. The lumbar vertebra have the about 25 to 30 degrees in l12, l23 it increases a little bit, l34 it is slightly more l5 and if you see l5s1 the angle is almost up to 50 degrees it can be up to 50 degrees that is a lot of angle. So to the midline there is always an angle at which the facets are attached and the orientation of the facets in the cervical spine they are facing forwards like that. In the thoracic spine they are facing inwards and in the lumbar spine they are facing outwards. So remember that that is what is responsible for the gliding movement which can happen in the cervical spine. The jumped facet happens because of this shape it does not resist while the facets do not jump because the orientation of the facet is different in the thoracic and lumbar spine. So in a cervical spine they support axial loads only in extension when it is opposed to each other. If it is flexed the facet joints become apart you can do that in the spines which you have try and flex the cervical spine and see what is happening to the facets they fall apart they go away from each other. Well if you make it lordotic then the facets come together. So the load bearing happens only in extension. So if your spine is flexed if you fuse the spine or fix with some plate or screws or rod or anything in flexion your facets are not going to take the load the whole load is going to be on your construct and your construct will fail. That is the most important reason why I am telling you this you have to get it in the normal position where the facets are taking the load only then your construct will be safe. If you do not make it load sharing the chances of failure are more and if your facets are not opposed to each other it is not sharing any load all the load is blown by your construct. Now movement in the thoracic spine is very little because of the rib cage and the sternum attach anteriorly you see it is only about allowed movement is about 5 degrees all kinds of movements flexion extension lateral bending axial rotation everything is around 5 to 7 degrees in the thoracic spine starting from T1 right up to T11, T12 it is much more in the cervical spine if you see the solid line the solid line is flexion extension and this occurs maximum at C0, C1 so about 25 degrees drops down to C2, C3 level to 10 degrees gradually rises again very little in the thoracic spine and a little bit more in the lumbar spine where the maximum flexion extension is about 15 degrees allowed in the lower lumbar spine. Lateral bending maximum at C1, C2 almost it is about 10 degrees lateral bending the dash dashes line is the lateral bending and axial rotation which is maximum at C1, C2 is about 40 to 42 degrees the rest of the spine there is hardly any axial rotation so as far as axial rotation is concerned maximum at C1, C2 about 40 degrees remember that C1, C2 40 degrees that is important so that is this movement the yes and no movement maximum at C0, 1 it is 25 degrees flexion extension so the pedicles as you know are the ones which connect the anterior column to the posterior column in the cervical spine they are short proportionally they have a greater diameter transverse width of the pedicle gradually decreases from cervical to mid thoracic and then increases again so it gradually decreases here like this and then it rises by transverse width I mean this width of the pedicle so in the lumbar spine it is much easier in the l4 and l5 you almost have 17 or 18 millimeters of solid bone available so your screw can be like this or it can be a little like this or it can be a little like this or it can be straight there is lot of room available but there is not so in the cervical spine if you see the width is only a few millimeters six millimeters so you have to be very accurate to put a pedicle screw in the cervical spine the height the height of the pedicle or the sagittal pedicle width again very little in the cervical spine seven millimeters average thoracic spine it gradually rises it is about 12 13 millimeters in the lower thoracic spine and in the lumbar it is the highest up to 16 to 17 millimeter that is the height so again it gives you a little bit of more leeway the entry point can shift a little bit in the lumbar spine but in the cervical spine you have to be absolutely accurate with the entry point because you have very little space the angle the angle at which the pedicle is attached cervical it is very medial it is almost 40 to 43 degrees so the pedicles are attached almost at a very acute angle 40 degree angle while it's negative in the lower thoracic spine so it's almost divergent the pedicle is divergent in the lower thoracic spine while in the lumbar spine again the obliquity occurs it's much more medial and in l4 l5 it can be as much as 25 degrees the angle the sagittal pedicle angle that is the angle at which the pedicle is attached to the body if you look at the spine you know in the thoracic spine it is very clear it's attached at an angle like this the pedicle this this and that's what this shows and how much is that angle it's negative in the cervical spine so this pedicle is attached vertically the pedicle goes vertically upwards in the cervical spine while in the thoracic spine it's about 15 to 18 degrees in the lumbar spine it's much closer to zero and again it can be a little bit vertical in the last lumbar vertebra so remember that in the thoracic it's downwards by positivity we mean it's downwards and by negativity we mean it's upwards so the cervical pedicle is attached upwards now the entry point for thoracic pedicle cervical pedicle i would advise you not to put unless you have seen assisted in some cases it's not right for beginners to put cervical pedicle that's what i believe clear lateral mass is much easier and it works very well so unless it's a very rare thing avoid putting a pedicle screw in the cervical spine at least in the beginning once you are an experienced surgeon maybe you can but thoracic spine sometimes you have to so you there are various ways of getting the midpoint but some ways are described here you can you can read it all but the best way i found is you actually see it for yourself you may nibble off a little bit of the the facet and actually see the pedicle and then put the screw so that's the safest disc you know nucleus palposes annulus fibrosis the way it shifts depending upon where the axial loading is happening the nucleus palposes shifts to the opposite side and that's how the disc relapses occur herniation because of axial loading or unequal loading on either side of the midline that's what usually causes and this may be because of more wear and tear on one side and one side the fibres being weak so transverse processes all paraspinal muscles are attached there so they're very important they allow leverage for the lateral bending they can be very easily fractured they are small with poor vascularity so this is important point small with poor vascularity because more often than not you end up doing a inter-transverse fusion you lay down bone graft between the transverse processes in the lumbar spine but it has it is small and with poor vascularity so that's not a very good fusion the inter-transverse fusion is not a good fusion they're better in the lateral mass they go upwards in the thoracic spine in lumbar they are much more ventral they're more far away from your operative field if the patient is prone so you have to be careful now we come to the ligaments now this shows the failure strength of all these ligaments at what tension these ligaments fail so this is for the ALL this is for the PLL this is for the ligamentum flavum this is for the capsular ligament and if you see in the cervical spine the strongest ligament is the capsular ligament so please do not damage the capsular ligament as far as possible preserve it it gives the maximum stability to the cervical spine even more than the ALL and the PLL so if you if you have nothing to do with that level please do not destroy please do not expose the whole facet with a coterie and rub away the capsular ligament don't do that because you'll make it unstable the capsule ligament is the one which gives maximum stability in the thoracic spine it's the ALL which gives the maximum stability again in the lumbar spine it's the ALL so if the ALL is intact a whole lot of stability is already there in the thoracic and lumbar spine but in the cervical spine remember it's the capsular ligament why is it so because it's the lever arm effect you know if this is the instantaneous axis of rotation the further away the ligament the longer the lever arm if you remember I think it was Archimedes who said give me a crowbar long enough and a place to stand and I can lift the earth so the longer your crowbar the more weight you can lift so that crowbar length is the length between the instantaneous axis of rotation and where you load the spine so the maximum length is the interspinus ligament so according to this principle the interspinus ligament becomes very important what we saw earlier was the failure strength that is when you pull when does it fail but this is physics principles and according to principles of physics the interspinus ligament because it acts the furthest away is the one contributing significantly to stability so please do not destroy the interspinus ligament as much as possible similarly the capsular ligament is also around here which is farther away than the PLL or the ALL and that is why it is responsible for stability so this gives you the physical explanation about why things are important so all ligaments have you know this neutral zone where it can keep on taking some load then an elastic zone where some deformity occurs and then finally it breaks because of excess stretch so as long as you are not reaching this point it's fine it's but it's better to stay in the neutral zone to preserve the sanctity of the ligament this is important because if you're fixing the spine in kyphosis then it's not good because all the stresses are being born not by the lateral masses but by the ligaments or by your construct all these muscles erectus spiny extension and lateral bending erectus spiny is the one paraspinal muscles so as is resolved for flexion rectus abdominis again anteriorly for flexion and it's a continuous influence the anterior and posterior muscles so it has to be maintained in balance in the thoracic spine the rib cage is a major spine stabilizer so inherently the thoracic spine is much more stable and can withstand a lot of insult even if you damage the facet or take off the facet still because of the rib cage attached inherently it is more stable the ratio of cortical to cancel is bone affects the load bearing so you where you're putting the screw is important and that's the reason why pedicle screws work very well because the ratio of cortical to cancel is bone is very good in the pedicles than in the body so the anterior fixations are not as good as the pedicle screw fixations because there's more cortical bone the threads catch in the cortical bone rather than the cancel is bone body most of it is cancel is bone so you're it doesn't catch it and that's why the anterior constructs are inherently weaker than the pedicle constructs and it also the bone density correlates with the screw pullouts as long as you it's in the cortical bone your pullout strength will be much more muscle mass and thoracic cage so a lot of energy is taken by these structures to make it more stable but it's the narrowest part of the canal so if the force overcomes all this stability then the injury to the thoracic spine is catastrophic because there's very little space so if the force is able to overcome all this it's going to cause paraplegia it's a dangerous thing while lumbar spine you know the spinal cord is ended so it can damage the roots at the most but no cord and cervical spine there is always more space there is a few millimeters available for some movement all transition points you must remember inherently more dangerous more prone to injury so either it's the sarahic or thoracic junction or the thoracolumbar junction where the lordosis turns to kyphosis that's a junctional area inherently more vulnerable or well the thoracic kyphosis changes to lumbar lordosis again junctional area so increased incidence of fractures at these places lumbar spine large bodies lordotic posture cord has ended there is no cord there you can the most important injuries are the one which affects the roots so catastrophic spinal injury is less likely in the lumbar spine lumbar sacral junction well it's difficult to obtain substantial points of fixation in sacrum you can get it in s1 maybe but below that the sacrum is not a very big bone so it's difficult to get fixation points in the sacrum so your construct if it's ends in s1 the s1 screw takes a lot of load so the x1 screw should be the biggest screw which is possible to put seven millimeters wide eight millimeters wide the longest length which takes both cortis you can take the anterior cortex of the s1 you must make it the strongest screw as possible s1 because in the upper spine you can have one level above you can go one level higher maybe have two more points adds to stability but in the sacrum there's hardly anything so the s1 screw is very crucial in any lumbar sacral fixation so there are a lot of issues with stability and instability this is a separate you can have a whole talk on stability and instability but basically it should be clinical you should concentrate more on the clinical part rather than just the bony part if an 80 year old comes with a neurology intact and there is some instability you can still wait so the clinical is more important than actual bone so column concept many people follow the three column or the two column depending upon whatever you follow but it helps the usefulness of the column concept is to understand the anatomy of the injury and devise a management way because people have managed ways based upon the number of columns injured with a certain reasonable amount of success so it helps you in anatomical definition of the injury and management so that's why you should follow one thing anyone whichever you want either the a o classification or two column or three column whatever but follow one thing then follow in all it will help you manage your patients better but if you want to exact quantitative analysis of instability the white and Punjabi thing which is too detailed it gives points for everything starting from loss of integrity each column to the movement on x-ray etc etc and it gives you points and then you can decide whether it is stable or unstable but sometimes there is acute overt instability that is inability of the spine to support torso during normal activity so if it's normal anatomical position maybe you don't see anything but the moment the patient flexes it may topple over so the dynamic x-rays especially in the cervical spine you need to look at that and then chronic glacial instability something like a parse defect or grade 1 l5 over s1 or l4 over l5 slip chronic it's glacial it goes very slowly many times you don't need to do anything except watch the patient get repeated x-rays done yearly or six monthly and just see if there is any neurology developing or the slip is increasing too fast then you may need to fix it otherwise usually nothing happens and number list thesis is a very good example of this all number list thesis do not need to be fixed remember that this is a new concept some people believe it or some people don't it's up to you if you remember this name you can read about it later I think that's a short overview of the important anatomical structures at what gives stability and what is the meaning of stability this will help you and the reference to this all this material is I think Benzel's book is a it's a very good book beautiful book it was published by the American Association of Neurological Surgeons it's the official publication of WANS and it talks a lot about all these concepts and how to make a good construct and to maintain it till the bone fuses thank you