Variation in the sacroiliac joint in Felidae

Felidae species show a great diversity in their diet, foraging and hunting strategies, from small to large prey. Whether they belong to solitary or group hunters, the behavior of cats to subdue resisting small or large prey presents crucial differences. It is assumed that pack hunting reduces the per capita risk of each individual. We hypothesize that the sacroiliac articulation plays a key role in stabilizing the predator while subduing and killing prey. Using CT-scan from 59 felid coxal bones, we calculated the angle between both iliac articular surfaces. Correlation of this inter-iliac angle with body size was calculated and ecological stressors were evaluated on inter-iliac angle. Body size significantly influences inter-iliac angle with small cats having a wider angle than big cats. Arboreal species have a significantly larger angle compared to cursorial felids with the smallest value, and to scansorial and terrestrial species with intermediate angles. Felids hunting large prey have a smaller angle than felids hunting small and mixed prey. Within the Panthera lineage, pack hunters (lions) have a larger angle than all other species using solitary hunting strategy. According to the inter-iliac angle, two main groups of felids are determined: (i) predators with an angle of around 40° include small cats (i.e., Felis silvestris, Leopardus wiedii, Leptailurus serval, Lynx Canadensis, L. rufus; median = 43.45°), the only pack-hunting species (i.e., Panthera leo; median = 37.90°), and arboreal cats (i.e., L. wiedii, Neofelis nebulosa; median = 49.05°), (ii) predators with an angle of around 30° include solitary-hunting big cats (i.e., Acinonyx jubatus, P. onca, P. pardus, P. tigris, P. uncia; median = 31.80°). We suggest different pressures of selection to interpret these results. The tightening of the iliac wings around the sacrum probably enhances big cats’ ability for high speed and large prey control. In contrast, pack hunting in lions reduced the selective pressure for large prey.

Normal Values and Variation of Radiographic and CT Infant Lateral Iliac Wall Angles in Normal and Dysplastic Hips

Purpose Indices from 3-D ultrasound may have a role in developmental dysplasia of the hip (DDH) assessment, but require a way to determine spatial orientation relative to body axes. The lateral iliac wall angle is a potentially suitable reference axis in 3-D ultrasound. We sought to quantify normal values and variations of the infant iliac wall angle on radiography, and compare with computed tomography (CT). Methods Acetabular and lateral iliac angles were measured on frontal pelvic radiographs of 200 patients (400 hips, 183 with DDH) and coronal CT of 20 patients (40 hips) aged 0-12 months. Relationships among morphologic indices and demographics were assessed using linear regression, Welch's t-test, Pearson's correlation coefficient (r) and coefficients of variance (CoV). Reliability was assessed using intra-class correlation coefficients (ICC). Results The radiographic iliac angle averaged 53.0° ± 7.7° (mean ± standard deviation; 95% CI, 38°-68°) in DDH vs. 56.2° ± 6.7° (95% CI, 43°-69°) in normal hips (p<0.001), correlated weakly with age (r = 0.25), and showed no inter-sex differences (p = 0.79). Inter-reader and intra-reader reliability were ICC = 0.946 and 0.965. CT iliac angle had mean difference 5.8° ± 6.2° (p<0.01), CoV = 10% and r = 0.68 vs. corresponding radiographs. Conclusions The radiographic infant lateral iliac wall angle has mean value 53-56° in dysplastic and normal hips with consistent range of variation approximately ±15°, was measured with high reliability, does not differ by sex, and is only slightly lower in the youngest infants. The iliac wall angle is an unbiased reference axis that may be suitable to establish general spatial orientation of 3-D hip ultrasound images.