This paper offers first results using a novel semi-automated method to quantify tissue volumes, allowing for a time-effective approach applicable to the posterior sacroiliac joint region. This can be applied to research on the impact of certain tissue volumes (like fat) that may have an impact in the function or the potential development of a certain pathology at a specific location: e.g: sarcopenia, without needing to use cadaveric material. In the here given application, adipose tissue was targeted defining regions of interest with HU between − 150 and − 50 on regular CT scans of individuals considered representative of a generic adult population. The novelty of this method lies in its simplicity so that it does not require any previous training nor a license for its use. The program was developed in scope of introducing a novel friendly semi-automated method of volume quantification of tissues in CT. Utilising the exported numerical values of the ROIs originally computed by OSIRIX allows the Python algorithm to rely on dependable values. Compared to other available software, VolSEQ also allows for the quantification of tissues to a pixel level, revealing values that would be difficult, sometimes impossible to identify manually. The interface allows for the selection of the range of HU, so it can also be applied to other targeted tissues for volumetric analysis in CT scans in future studies. Additionally, the novelty of this study was that it quantifies the total volume of the PSIJ and the fat volume within a larger age span (16 to 82 years) than has been reported in the literature to this date. Its use could be valuable in the detection and quantification of fat content / infiltration in structures like the SIJ and aid in the potential diagnosis of related conditions.
Fat appears to be a relevant tissue type defining the ‘healthy’ sacroiliac joint
Fat is observed frequently within the sacroiliac joint ligaments, specifically in and around the interosseous and posterior ligaments 7,27. It has been hypothesised that fat provides two main functions within the SIJ. Firstly, it allows for the neurovascular structures to travel and supply the syndesmotic SIJ, essentially providing protection to these elements. Secondly, it serves as a mechanical damper within the PSIJ, where it effectively regulate forces transmitted in this region 27. As fat is largely incompressible but deformable, compressive forces applied to the posterior pelvis would cause the fat to deform within its given environment. This deformation of fat tissues is limited by the distensibility of the ligamentous meshwork surrounding the fat tissue (typically averaging 15%) 28,29. To illustrate the morpho-mechanical relationship between fat and ligaments would be to imagine a mattress where the foam compares to the fat and the surrounding fabric compares to the ligamentous tissue 27.
Previous analyses revealed an age-related correlation with fat volume 7. This finding was not been replicated in the present study reassessing this research question in a broader age-range. Considering this study has a more suitable age-range for such analyses, one may suggest the current result to be a more realistic reflection of the correlation of fat with age than our previous trials 7. The lack of correlation showed that age would not be related to fat volumes as previously reported. However, there is a poor correlation between fat volume and total PSIJ volume. Although lower than previously observed, this validates the finding that fat volume is directly proportional to the PSIJ size and is a physiological and functional element of the PSIJ region in general.
Fat volume remains largely unchanged throughout life span and PSIJ volume is sex-dependant
Analysis of both mean fat and total volume of the PSIJ over time revealed to be similar across the age. No evidence was found for fat volume decreasing with age therefore, our first hypothesis stating that fat content decreases with age within the SIJ can be rejected. However, it seems that, in general, older individuals tend to retain less fat than the other two younger age groups. As natural degeneration occurs due to age, older individuals may have an overall loss of this force-dampening structural adipose tissue. When separating sexes, it is revealed that there is a trend for an increase in total PSIJ volume with age in females and a decrease over time in males. This could be due to age-related changes that can occur at the SIJ typically involved in osteoarthritis such as ‘joint space narrowing’ 30,31. In this case, normal age-related factors such as osteoporosis, menopause and/or bony fusion of the anterior aspect after cartilage erosion may play a role in the alteration of PSIJ size after 65 years of age. This is sex dependent because of the influence of menopause on cartilage, synovium and bone density 32. Although difficult to measure because of the use of hormone replacement therapy, these physiological changes brought on by menopause may have a larger impact on the general bony architecture of the SIJ in females. For example, sacral displacement relative to the ilium could be instigated 7 disturbing the associated fat content within the PSIJ. This clearly differentiates the two sexes on how the PSIJ morphology might evolve in later life.
Furthermore, females have a higher PSIJ volume and total PSIJ fat volume than males. They also have a higher fat percentage as males which suggests that females have more fat locally regardless of their joint size, thus validating our second hypothesis. Considering fat has a load-bearing function it may further compensate for the higher forces and loads subdued by females during life due to their generally smaller articular joint surfaces, but proportionally speaking considering their joints are larger, higher fat content is also predictable.
Fat volume of the PSIJ is side-dependant
Side comparison revealed a higher fat volume on the right side as well as a significant fat percentage difference for the right side. Our third hypothesis suggesting that side has no influence on the fat content within the SIJ must therefore be rejected. Similar results were found previously 7, and it has been shown that the right leg is often the favoured limb regarding side dominance of the lower extremity in tasks such as kicking and jumping for example 33,34 but generally only 25–45% of the population demonstrate a right leg preference in lower extremity actions 35. Because of the higher use of one side over the other, we hypothesise that this may influence tendon and ligamentous morphology potentially impacting the ligament-fat ratio within the joint. If the right side is more often the dominant leg, it would typically contain more fat to compensate for the increase in forces and protect neurovascular structures of the joint in response to the increased ligament strain in that area because of the chronic ‘usage’ of the limb. However, the scenario in the scope of this study is arguable as limb dominance is highly task specific 34 so assuming that the right limb was more dominant than the left in this cohort is a stretch. Chronic limb usage and therefore limb dominance of the patient cohort of this study is unknown, therefore results cannot be reliably assessed and association between fat content in the PSIJ and limb dominance remains to be investigated further.
Limitations of the study include the sample size. Moreover, no paediatric cases were included, therefore this study is not applicable in individuals younger than 15 years which remains to be studied. In addition, the posterior sacroiliac ligament and the inferior boundary of the articular cartilage were sometimes difficult to determine and, in some instances, had to be separated manually, drawing a line between the ilium and the most posterior part of the sacrum acting as the posterior sacroiliac ligament biasing the annotation process. This approach may have altered the true shape of the structure. Furthermore, the program is not fully independent of manual input and requires the use of OSIRIX or similar software for the creation of the original ROIs. The BMIs of the patients were unknown; therefore, it was impossible to consider the impact of BMI on fat volume within the PSIJ. Further research should investigate this variable to understand its potential influence on altered loads and stresses on the joint.