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It is often assumed that the sacrum is perfectly aligned with the line through the ischial tuberosities (ITs) but evidence suggests that this may not always be the case. This article discusses the potential impact of this on the management of pelvic obliquity and scoliosis in sitting. The discussion relies heavily on the doctoral thesis of Dr John Dulhunty The Measurement and Correction of Sacral Obliquity which can be found via TinyURL link https://tinyurl.com/y5fb5cox, and contains excellent x-rays, diagrams and images, and deeper reading.

The Sacrum (Os Sacrum. Latin: The Holy Bone)

The understanding of the sacrum's function and evolution has a long and complex history and has apparently been the cause of much controversy and division (Vleeming et al, 2012). Most investigations of the sacrum's shape and angulation as part of the pelvis have been focused on anisomelia, i.e. leg length discrepancy in ambulant patients rather than in sitting. It has been argued that anisomelia is common (although differences are usually small) and the formation of the sacrum and pelvis will compensate this by aligning S1 to the horizontal in standing and walking, and thus it is not clinically significant. Others argue that this can be the cause of acute back pain and uneven weight bearing. Sacral deformities seem to be less well studied in complex disability, though some investigations have looked at spina bifida and cerebral palsy.

The pelvis is a complex structure comprising 15 bones: the fused innominate bones (the ilia , ischia and pubes); five fused sacral bones and four fused coccyx bones. It has six articulations comprising the two acetabula, sacro-coccyx, lumbosacral and two sacroiliac (SI) joints, or seven if symphysis pubis is included. It articulates passively at the sacroiliac joints which are responsible for the movements of nutation and counternutation; an anterior/posterior rotation of just a few, but important, degrees. The movement of nutation is essential in the formation and support of normal lumbar curvature and alignment for sitting and standing, and when walking. The sacrum also articulates by just a few degrees with the lumbar spine at the L5-S1 lumbosacral joint, and forms the sacral plateau. The sacral plateau is typically horizontal to maintain postural equilibrium under the force of gravity in sitting and standing.

The sacrum distributes bodyweight through the sacroiliac joints - to the legs in standing and ischial tuberosities when sitting. The sacral bones begin to fuse at around 17 years and are completely fused around the age of 26. Without leg length discrepancy or other factors, the lumbosacral joint is laterally horizontal to gravity in sitting and standing. During bone development and in the presence of mitigating factors however, the plane of the sacral plateau relative to a line passing through the ITs can be affected such that these are no longer parallel. Dulhunty (2015) refers to this as a sacral obliquity as opposed to pelvic obliquity.

Sacral Obliquity

The sacrum is vulnerable to a variety of deformities during childhood skeletal development, as well as from injury or degenerative pathologies, that can affect its shape and function at one or more of its articulations. Dulhunty's particular focus was on people with leg length discrepancies in both sitting and standing.

The sacroiliac ligaments and tendons can become ruptured or slackened, resulting in instability and migration of the joint upwards or downwards: a phenomenon referred to as up-slip or down-slip by Dulhunty. Up-slip may happen as a result of an injury such as landing on one leg from a height, whereas down-slip may happen when a horse rider is dragged along by one stirrup following a fall, for example.

The sacroiliac joint can also become fused or otherwise immobilised and hence unable to enable or limit the movement of nutation, either bilaterally or unilaterally. Imbalanced nutation could also affect the ability of the sacral ligaments to optimally support the lumbar spine to that side. A bilateral limit to nutation may prevent a natural and balanced lumbar curvature by collapsing it.

Dulhunty, using radiographic evidence, looked at the relationship between the lateral angle of the S1 lumbosacral joint against the force of gravity and adjacent structures or reference points, and offers two definitions. Absolute sacral obliquity (ASO) and relative sacral obliquity (RSO):

RSO is defined as the

 “ ...angulation of the sacral base in the frontal plane in relation to other structures, primarily the ilia, femur heads or lumbar spine and generally represents internal pelvic or sacral asymmetry. This relationship would remain unchanged with various orientations of the structures to gravity e.g. standing, sitting or lying down.”

and ASO

“...refers to the angulation of the sacral base in the frontal plane for a specified orientation of the body in a global reference frame or in relation to gravity for a specific posture such as standing or sitting.”

Therefore, the absolute sacral obliquity angle (SOA) of someone in sitting is a function of the RSO and the angle across the ITs (ITA) to the horizontal seating surface, or

RSO + ITA = SOA

Dulhunty identified 6 patterns of obliquity to each side from this basic formula, depending upon which angle was greater: the SOA or the ITA (figure 1). Pattern 1 is a neutral or 'normal' position, and in only one of the six patterns of obliquity as defined by Dulhunty (pattern 4) is the ASO the same as the angle of pelvic rotation in the coronal plane i.e. the often represented image of pelvic obliquity.

It is worthwhile noting that Dulhunty’s images of obliquity in standing do not look unfamiliar compared to presentations often seen in sitting during seating clinics. The images and radiographs in his thesis are very worthwhile and unfortunately cannot be reproduced here.

Measurement

Dulhunty found that accurate measurement of sacral obliquity was only possible with radiography, and suggests the methodology and recommendations for an appropriate technique. He found in his research that studies looking at manual palpation of anatomical landmarks determined that these methods were unreliable in accurately determining the true status of the pelvis, sacrum and lumbar spine. The sacrum was particularly difficult to assess due to its location and overlying structures, and he concluded that only x-ray could reveal the true shape and angulation. Vleeming (2012) et al concur, also noting the unreliability of manual movement tests in their research.

Discussion

“...in severely disabled non-ambulatory patients – the pelvis, the lumbosacral junction and their interplay with the hip joints need to provide a solid fundament for unsupported sitting and balanced positioning of the head, ideally on the upper extension of the central sacral vertical line. Again, stability improves the functionality of arms and hands.” (Hasler et al, 2020) 

The movements, shape and function of the sacrum are important factors that seem to have been generally overlooked in wheelchair and postural seating provision and assessments. It is recognised that there are a multitude of considerations to be made with regard to successful seating. The sacrum can only be considered as one element of someone's postural patterns amongst other important factors that determine the requirements for good seating, such as spinal deformities, contractures, ligaments, pressure care, lifestyle, disability, prognosis, diagnosis etc. However, as Hasler et al (2020) note above, the pelvis and lumbosacral junction (sacral plateau) are an important part of  providing a solid fundament for posture.

Good clinic and plinth assessments will reveal what works and is balancing for the patient in their seating, and is a good practical approach to treatment. However, the awareness of the status of a patient's sacrum may guide better decision making when added to the complexity of variables that affect posture over time, e.g. pressure care, muscle tone, fixed deformities, contractures, centres of gravity etc.

The technique of palpating ASIS and PSIS seems to give only part of the picture as to what the real situation is. Ombregt (2013) describes a variety of sacral assessments but notes some are of little or no use in the presence of scoliosis. Whereas these assessments might indicate issues over a sacroiliac joint, they can only give limited information. Indeed he concludes the chapter with, “Dysfunction and subluxation are difficult to prove, remain obscure, and may not exist”. Dulhunty also states that we are unable to rely on pelvic palpation alone to understand what is happening to the pelvis and lumbar spine when, on page 29 of his thesis, he writes: 

“...Static palpation tests used to assess asymmetry and dysfunction of the pelvic region were evaluated by Holmgren et al .They concluded that for clinical practice, the tests used as methods for detecting pelvic asymmetry were of doubtful utility.” 

Vleeming et al (2012) also cast doubt over the effectiveness of physical assessments of the sacrum. 

As previously noted, Dulhunty's pattern 4 is frequently presented as being the only pelvic obliquity. It is often taught that in the case of pattern 4, if the pelvis is correctable then a raise should be placed under the lower side to correct it, bring it back to a midline and redistribute weight loading through the ITs to promote spinal alignment etc. Dulhunty's work implies that this may not be the case. Indeed in some patterns such as 7, the correction required to achieve a horizontal sacral plateau would be to raise the elevated IT even further. 

One implication of this, perhaps, is that cushions with IT build-ups should only be used where an ASO is present in sitting (other factors such as pressure needs excluded) and may not necessarily be to the expected side based on manual palpation of ASIS/PSIS alone. In the case of pattern 4, it implies that there should be no firm block under one IT, and that the corrected sitting is achieved on a level surface. If a pelvis, in which SOA=ITA, is placed on an uneven surface, SOA can never be levelled. It may explain why, on review, some provided with an IT build-up in a cushion seem to have experienced mixed successes. 

A further consequence could be that, if relying on foam to provide an upwards corrective force under one IT, both sides of the cushion would need to be level under compression in order to be balanced. This would therefore need to be carefully tuned to the individual client's weight and morphology. 

Dulhunty's representations of sacral obliquity in standing seem also to reflect the presentations often seen in complex seating clinic, i.e. with one iliac crest higher than the other, and complex rotations and scoliosis patterns. We may be able to determine if the sacrum is likely to be parallel or not to the iliac crests or ITs, and in which direction. Distinguishing between patterns 2 and 3, as well as between patterns 5 and 6, would likely be very challenging without radiography, though Dulhunty's corrected sitting theory suggests a similar treatment would be used.

It also needs to be acknowledged that Dulhunty did not consider the orthopaedic complexity of people with disabilities. However, Cho et al (2017) note that sacral obliquity is a “...frequent finding in patients with adolescent idiopathic scoliosis”. Cho et al also concur that up to 15% of the general population has anisomelia as found in previous studies.  Although not recorded, it would be reasonable to expect the incidence in complex disabilities will be at least as frequent, and is indicated to be much higher. Vleeming et al (2012) note that “Narrowing or obliteration of sacroiliac joints has been commonly reported in paraplegics”, and also with ankylosis. 

Conclusion

In the traditional depiction of pelvic obliquity (pattern 4), a build-up is not indicated by Dulhunty (download diagrams below). A normally formed and parallel pelvis and sacrum cannot be brought to level on an uneven surface. However, an asymmetrical cushion may be appropriate if the pelvis and/or sacrum are asymmetrical too. 

Palpation alone does not give us accurate or reliable information concerning the sacrum, yet the sacrum seems to be a principal agent affecting the curvatures of the lumbar spine and postural balance. 

With a better awareness of the 6 patterns of obliquity, but with a paucity of reliable assessments to objectively measure sacral obliquity, more deductive and functional methods may need to be devised to identify the type of obliquity and sacral involvement.

The sacrum is an elegant and important structure. Its intrinsic and complex movements may be small and nuanced but the effects could be profound and far reaching. 

This article has only touched on the issues arising from consideration of the relative alignment of the sacrum within the pelvis; and only in light of lateral obliquities, and without regard for many other factors such as different patterns of scoliosis, contractures, pressure care etc. It has also not taken account of the sacrum and coccyx, which together have a role in the formation of spinal kyphosis and lumbar hyper-lordosis. Nonetheless, it is recognised that the sacrum has a lot more to teach us about posture and seating, and perhaps deserves to be the subject of more debate amongst posture and seating specialists. 

Images displayed top right

1. Pelvis x-ray showing sacral obliquity 

2. Stephen Kebbell, author

References and Reading

Cho, J. H., Lee, C. S., Joo, Y. S., Park, J., Hwang, C. J., & Lee, D. H. (2017). ‘Association between Sacral Slanting and Adjacent Structures in Patients with Adolescent Idiopathic Scoliosis’, Clinics in orthopedic surgery, 9(1)57–62.

Dulhunty, J. (2015). ‘The measurement and correction of sacral obliquity’, (PhD Thesis) Macquarie University, Sydney, Australia.

Hasler C, Brunner R, Grundshtein A, Ovadia D (2020). ‘Spine deformities in patients with cerebral palsy; the role of the pelvi’s. Journal of Children's Orthopaedics 14(1)9-1.

Juhl JH, Ippolito Cremin TM, Russell G. (2004).’ Prevalence of frontal plane pelvic postural asymmetry--part 1’. J Am Osteopath Assoc. 104(10):411-21. Erratum in: J Am Osteopath Assoc. 2005 Jan;105(1):5.

Ombregt L (2013). Chapter 11,’ Applied anatomy of the sacroiliac joint’ in A System of Orthopaedic Medicine (Third Edition) Churchill Livingstone. e233-e238

Treble NJ, Owen R, Rickwood AMK (1988). ‘Classification of congenital abnormalities of the sacrum: Patterns of associated dysfunctions’, Acta Orthopaedica Scandinavica, 59(4)412-416.

Vleeming A, Schuenke MD, Masi AT, Carreiro JE, Danneels L, Willard FH (2012.) ‘The sacroiliac joint: an overview of its anatomy, function and potential clinical implications’. J Anat. 221(6)537-67.