Since Osteoporosis is a pathological feature of ageing, how is the spine being affected? What are the changes in the vertebral bodies and intervertebral discs? [17]

The morphological changes of the osteoporotic spine include a gradual loss of vertical height because of micro-fractures of the vertebral bodies, and deformities might occur if more severe fractures occur, particularly when multiple levels are involved. The osteoporotic spine would not respond uniformly to the decline of mechanical strength of the supporting vertebral bodies, as bone mineral density (BMD) decreases. Instead, a number of risk factors would be jointly affecting different patterns of behaviour [18, 19].

Degeneration is a fact of life. The spine, being the major weight bearing pillar of the human body, naturally would give prominent manifestations of degeneration with aging. Loss of bone minerals is another sign of advancing age and slowing of physical activities. Epidemiological studies in the past years have shown the prevalence of lumbar spondylosis with increasing age, affecting particularly those with a high working demand on mechanical stress. The intervertebral discs are structurally designed as shock-absorbing units to maintain the weight bearing function and in so doing, would be very much affected by degeneration [20]. It has long been known that intervertebral discs lose their central fluid contents with aging, resulting in shrinkage and loss of height [17].

In a study looking at the prevalence of osteoporotic vertebral fractures (loss of height) among Asians which included Chinese, Koreans, Indonesians and Japanese, it was found that the risk factors were low BMD, maternal history of osteoporosis, older age, defective physical ability and the presence of cataracts [21]. In that study the intervertebral discs were not assessed. One additional interest could have been: whether the elderly people being studied had shrinking discs like the degenerative spine.

A number of studies aiming at the identification of risk factors in relation to osteoporotic fractures of the spine were available in current literature. None of them took special interests at the intervertebral discs [22–25].

In fact, it has appeared to the orthopaedic surgeon for a long time that although spondylosis and osteoporosis of the spine are both degenerative pathologies, they do not co-exist together in the same patient. This practical observation has not been taken seriously by other experts on osteoporosis, although as early as 1991 and 1996, reports on such relationship between lumbar spondylosis and osteoporosis were already available. In the recent few years, more clinical studies were completed on the investigation of the relationship between the degenerative and osteoporotic spine, and the morphology of the intervertebral discs.

Verstraeten and Van Ermen observed that osteoarthritis retarded the development of osteoporosis in 1991 [25]. Margulies and Payzer made the similar observations on the lumbar spine [26]. In 2003, Dequeker and Miyakoshi both reported clinical and research evidences of the inverse relationship between degenerative and osteoporotic spine [27, 28]. The BMD changes (increases) of the spine in relation to degenerative changes in elderly women were studied by Muraki and Yamamoto in Japan in 2004 [29] and the reverse relationship was again observed. There should be little doubt today that degenerative changes in the spine slows down the development of osteoporosis and the BMD is also better maintained.

Orthopaedic surgeons, for many years, have also observed that for the severely collapsed vertebral bodies caused by osteoporosis, the intervertebral discs adjacent to the affected vertebral bodies, tend to occupy a wider space radiologically.

In 2001, we completed a survey on the morphological changes in the spine of elderly Chinese women and found that shortening of the vertebral bodies co-existed with widening of the intervertebral discs.

While enlarged intervertebral discs assumingly, could be due to breakage of the end plates of the vertebral bodies, leaving vacant spaces to be filled, but why should the disc, which normally dry up and shrink with age, expand instead [30, 31]? This phenomenon has not been explained but increasing interests are mounting. In 1998, Harada studied the relationship between BMD and intervertebral discs and observed decreasing heights with increasing BMD [17]. In 2005, Baron reported disc changes in treated and untreated overweight post-menopausal women [32]. Pye and Reid did the same study on men and women with different values of BMD. The observation was: wider disc spaces accompanied low BMD [33]. In 2007, we reported a modified grading system for lumbar disc degeneration in elderly subjects [34]. We used this grading system to estimate, using MRI, the volume of vertebral bodies and discs, which were observed to decrease with decreasing BMD: the vertebral bodies strictly obeyed this principle while the intervertebral disc behaved less impressively. In fact, when marked decrease in vertebral heights occurred, the disc in-between would even expand [35].

We did another study using MRI to detect disc degeneration in relation to BMD in elderly subjects. Among 196 females and 163 males, females were found to have more severe disc degeneration than males; and lower BMD was associated with less severe disc degeneration [36].

Now that we are aware of the reverse relationship between osteoporosis and osteoarthritis of the spine, if the molecular causes leading to this reverse relationship were known, new treatment options for both conditions might emerge. Our earlier observations showed that decreasing spinal BMD could be related to a decline in the blood supply to the bone. Osteoporosis in the spine could be involving complex molecular changes [37]. The higher contents of fatty tissues within the vertebral bodies in the osteoporotic spine further supported the assumption and the need for further research [38].

With regard to the behaviour of the intervertebral disc in spinal osteoporosis, are there clinical implications? Firstly, to observe the progress of the osteoporotic spine, and to comment on the prognosis, it is important to include an analysis of the intervertebral discs. The more changes in the discs, the more collapses of the vertebral bodies may be expected. On the other hand, the expanded discs should have positive effects on the maintenance of the height of the individual and might also have symptom relieving effects.

Secondly, now that vertebroplasty and kyphoplasty are getting popular, surgeons should be aware that not only are the long-term effects of the minimally invasive procedures remain uncertain [39, 40] but the morphology of the intervertebral discs should have influences on the behaviour of the adjacent vertebral bodies which will be facing the stress-riser challenges of the hardened vertebrae after cement injection. This issue will be further discussed in the subsequent paragraphs.

Thirdly, surgeons attempting to perform open surgery on the osteoporotic spine should be aware of the bizarre morphology of the intervertebral discs: some of them undergoing the usual degenerative changes while other could be less affected and might be even expanded. Without the awareness, surgeons might not be able to put in implants accurately.

The vertebral bodies of the spine, because of their constant weight-bearing role, must be the most frequently affected skeletal units in osteoporosis associated with ageing. Fractures of the long bones resulting from bone loss have received a lot of attention because of the absolute need and sometimes, complexity of surgical management. When preventive treatment is administered for osteoporosis, the main justification lies on the reduction of fracture risks related to the hip region, while the fractures occurring in the vertebral bodies are not of equivalent concern [41]. As a matter of fact, both male and female, when reaching elderly age, commonly suffer from arthritic changes of the joints, of which the spine is also most frequently involved. Pain arising from degenerative changes of the spinal units cannot be differentiated from that initiated by the gradual collapse of the vertebral bodies as a result of loss of bone substance. Such back pain is usually taken as a fact of life among the elderlies, until an acute severe vertebral collapse initiates severe pain that demands an imaging investigation which will reveal the osteoporotic vertebral fracture.

An osteoporotic vertebral collapse might be the cause of severe back pain that would gradually go away automatically or, at its worst, takes a week or two to subside. Hence for the usual cases, no specific treatment plan is required apart from bed rest and simple pain control measures [42]. It is only in recent years, when interventional radiography becomes the fashion, that advocates on active treatment for osteoporotic vertebral bodies develop special techniques to achieve more effective pain control and even preservation of the vertebral height of the affected vertebra in operative procedures of vertebroplasty and kyphoplasty.

The rationale of either vertebroplasty or kyphoplasty is simple. Since vertebral bodies do not have a rich supply of sensory nerve fibres, the pain resulting from a osteoporotic collapse is due to a variable degree of instability arising from the lost height. Therefore preserving the vertebral height by using acrylic to fill up the empty spaces would preserve the stability, thence control the pain. Since the procedures are gaining higher and higher popularities, we would discuss the indications, procedures, justifications and adverse effects in detail in the following paragraphs.

Perfect facilities of radio-imaging and spinal injection instrumentations should be available. The technical team should be experienced with minimally invasive procedures, procedures related to spinal biopsy or intervertebral disc manoevres. The site of entry into the osteoporotic vertebral body is the spinal pedicle which can be entered unilaterally or bilaterally. The appropriate canulla should be chosen to allow sufficient amount of acryllic injection. Since the adequate amount of acryllic is essential for the success of the procedure, the fluidity of the acrylic is cruxial. Insufficient amount of acryllic fails to satisfy the filling effects to maintain stability. Too much amount predisposes to leakage around the vertebral body leading to spinal canal obstruction or might be even direct invasion of the paraspinal venous plexuses. Special acryllic bone cement has been created to facilitate vertebroplasty injections.

The choice of Kyphoplasty is based on the assumption that spacing up the fractured space in the vertebral body with an inflatable balloon would allow the appropriate amount of acryllic to be injected so that the preservation of vertebral height could be better achieved. This procedure is naturally more invasive and technically more demanding apart from being more costly [44].