The presence of long spinal muscles increases stiffness and hysteresis of the caprine spine in-vitro

An equine cadaver study investigating the relationship between cervical flexion, nuchal ligament elongation and pressure at the first and second cervical vertebra

Pressure in the atlanto-axial region due to hyperflexion ('rollkur') may influence the development of a nuchal bursa, as adventitious bursae may be caused by pressure. Investigating the pressure between the nuchal ligament and atlas/axis in a flexed position may provide information on the pathogenesis of nuchal bursitis. In this study, ten equine head and neck specimens with one side of the soft tissues over the cervical vertebral spine removed were placed in lateral recumbency on a table in neutral, mildly flexed, and hyperflexed head and neck positions. Angulations of the neck were measured using markers placed on the nuchal ligament and drilled into the skull, vertebrae and withers. In six specimens, the pressure between the nuchal ligament and the atlas and the axis was measured using an inflatable air pouch. Hyperflexion was associated with the highest nuchal ligament length and with the highest pressure values at the site of the nuchal bursa over the atlas (99±24mmHg, more than four times the pressure in the neutral position) and over the axis (77±30mmHg, more than twice the pressure values of the neutral position). Also, over the three head and neck positions, neck flexion angles were highly correlated with pressure values and with nuchal ligament length. This marked increase in pressure at the level of atlas and axis caused by head and neck hyperflexion should be considered during training of horses at risk of, or diagnosed with, nuchal bursitis.

A sphere fitting approach to determine the hip joint centre of the horse

Accurate identification of the hip joint centre (HJC) is crucial for the correct estimation of knee and hip joint loads and kinematics, which is particularly relevant in orthopaedic surgery and musculoskeletal modelling. Several methods have been described for calculation of the HJC in humans, however, no studies have used these methods in the horse despite a similar need for improved evaluation of hip joint biomechanics in rehabilitation and musculoskeletal modelling. This preliminary study uses the commonly used functional method (least-squares sphere fit) to determine the HJC in three equid cadavers. Bone pins with reflective markers attached were drilled into the tuber coxae (TC), tuber ischium (TI), tuber sacrale (TS), greater trochanter (GT), third trochanter (TT) and lateral femoral condyle (FC) of the uppermost limb of the cadavers positioned in lateral recumbency. Three repetitions of passive movements consisting of pro-and retraction, ab- and adduction and circumduction were performed. The HJC was calculated using a least-squares sphere fitting method and presented as a distance from the TC based on a percentage of the TC to TI vector magnitude. Mean (± standard deviation) of the HJC is located 52.4% (± 3.9) caudally, 0.2% (± 6.5) dorsally, and 19.8% (± 4.2) medially from the TC. This study is the first to quantify the HJC in horses ex vivo using a functional method. Further work (in vivo and imaging) is required to validate the findings of the present study.

In vivo MRI features of spinal muscles in the ovine model

Background

Muscle fatty infiltration (MFI) has been identified in patients with spinal pain using magnetic resonance imaging (MRI). Even though sheep are a commonly used animal model for the human spine, comparative sheep MFI data from MRI is not available. Determining MFI in sheep spinal muscles using acquisition protocols commonly used in man will identify the applicability of this approach in future sheep model studies, such that the effects of spinal interventions on muscle can be assessed prior to their use in a human (clinical) population.

Objective

To quantify ovine lumbar spine MFI using three-dimensional two-point Dixon and T1-weighted sequences.

Methods

T1-weighted and Dixon lumbar spine axial sequences were collected in 14 healthy Austrian mountain sheep using a 1.5-T MRI. At each vertebrae, the region of interest of psoas major and minor (PS), multifidus (M), and longissimus (L) were identified. To determine MFI from the T1-weighted images, the mean pixel intensity (MPI) was calculated as a percentage of subcutaneous or intermuscular fat. For the Dixon images, fat sequence MPI was calculated as a percentage of the summed fat and water sequence MPIs. Spinal degeneration was graded and correlated to MFI. Dixon MFI was compared to T1-weighted MFI obtained from subcutaneous and intermuscular fat.

Results

For every muscle, T1-weighted MFI calculated using subcutaneous fat scored significantly lower than Dixon MFI and T1-weighted MFI calculated using intermuscular fat (p < 0.001). There were no significant MFI differences between T1-weighted images calculated using intermuscular fat and Dixon images for M and L (p > 0.05), although significant differences were found for PS.

Conclusion

In sheep, Dixon sequences provide an acceptable comparison to T1-weighted sequences for lumbar extensor MFI based on intermuscular fat. However, compared to the human literature, ovine lumbar musculature contains greater MFI, making interspecies comparisons more complex.

Spinal Motion and Muscle Activity during Active Trunk Movements - Comparing Sheep and Humans Adopting Upright and Quadrupedal Postures

Sheep are used as models for the human spine, yet comparative in vivo data necessary for validation is limited. The purpose of this study was therefore to compare spinal motion and trunk muscle activity during active trunk movements in sheep and humans. Three-dimensional kinematic data as well as surface electromyography (sEMG) of spinal flexion and extension was compared in twenty-four humans in upright (UR) and 4-point kneeling (KN) postures and in 17 Austrian mountain sheep. Kinematic markers were attached over the sacrum, posterior iliac spines, and spinous and transverse processes of T5, T8, T11, L2 and L5 in humans and over the sacrum, tuber sacrale, T5, T8, T12, L3 and L7 in sheep. The activity of erector spinae (ES), rectus abdominis (RA), obliquus externus (OE), and obliquus internus (OI) were collected. Maximum sEMG (MOE) was identified for each muscle and trial, and reported as a percentage (MOE%) of the overall maximally observed sEMG from all trials. Spinal range of motion was significantly smaller in sheep compared to humans (UR / KN) during flexion (sheep: 6–11°; humans 12–34°) and extension (sheep: 4°; humans: 11–17°). During extension, MOE% of ES was greater in sheep (median: 77.37%) than UR humans (24.89%), and MOE% of OE and OI was greater in sheep (OE 76.20%; OI 67.31%) than KN humans (OE 21.45%; OI 19.34%), while MOE% of RA was lower in sheep (21.71%) than UR humans (82.69%). During flexion, MOE% of RA was greater in sheep (83.09%) than humans (KN 47.42%; UR 41.38%), and MOE% of ES in sheep (45.73%) was greater than KN humans (14.45%), but smaller than UR humans (72.36%). The differences in human and sheep spinal motion and muscle activity suggest that caution is warranted when ovine data are used to infer human spine biomechanics.

MRI-determined lumbar muscle morphometry in man and sheep: potential biomechanical implications for ovine model to human spine translation

The sheep is a commonly used animal model for human lumbar spine surgery, but only in vitro investigations comparing the human and ovine spine exist. Spinal musculature has previously not been compared between man and sheep. This additional knowledge could further indicate to what extent these species are biomechanically similar. Therefore, the purpose of the study was to investigate spinal muscle morphometric properties using magnetic resonance imaging (MRI) in different age groups of healthy human participants and sheep in vivo. Healthy human participants (n = 24) and sheep (n = 17) of different age groups underwent T1-weighted MRI of the lumbar spine. Regions of interest of the muscles erector spinae (ES), multifidus (M) and psoas (PS) were identified. The ratio of flexor to extensor volume, ratio of M to ES volume, and muscle fat relative to an area of intermuscular fat were calculated. Sheep M to ES ratio was significantly smaller than in the human participants (sheep 0.16 ± 0.02; human 0.37 ± 0.05; P < 0.001), although flexor to extensor ratio was not significantly different between species (human 0.39 ± 0.08; sheep 0.43 ± 0.05; P = 0.06). Age did not influence any muscle ratio outcome. Sheep had significantly greater extensor muscle fat compared with the human participants (M left human 40.64%, sheep 53.81%; M right human 39.17%, sheep 51.33%; ES left human 40.86%, sheep 51.29%; ES right human 35.93%, sheep 44.38%; all median values; all P < 0.001), although PS did not show any significant between-species differences (PS left human 36.89%, sheep 33.67%; PS right human 32.78%, sheep 30.09%; P < 0.05). The apparent differences in the size and shape of sheep and human lumbar spine muscles may indicate dissimilar biomechanical and functional demands, which is an important consideration when translating to human surgical models.

An experimental and morphometric test of the relationship between vertebral morphology and joint stiffness in Nile crocodiles (Crocodylus niloticus)

Despite their semi-aquatic mode of life, modern crocodylians use a wide range of terrestrial locomotor behaviours, including asymmetrical gaits otherwise only found in mammals. The key to these diverse abilities may lie in the axial skeleton. Correlations between vertebral morphology and both intervertebral joint stiffness and locomotor behaviour have been found in other animals, but the vertebral mechanics of crocodylians have not yet been experimentally and quantitatively tested. We measured the passive mechanics and morphology of the thoracolumbar vertebral column in Crocodylus niloticus in order to validate a method to infer intervertebral joint stiffness based on morphology. Passive stiffness of eight thoracic and lumbar joints was tested in dorsal extension, ventral flexion and mediolateral flexion using cadaveric specimens. Fifteen measurements that we deemed to be potential correlates of stiffness were taken from each vertebra and statistically tested for correlation with joint stiffness. We found that the vertebral column of C. niloticus is stiffer in dorsoventral flexion than in lateral flexion and, in contrast to that of many mammals, shows an increase in joint stiffness in the lumbar region. Our findings suggest that the role of the axial column in crocodylian locomotion may be functionally different from that in mammals, even during analogous gaits. A moderate proportion of variation in joint stiffness (R(2)=0.279-0.520) was predicted by centrum width and height, neural spine angle and lamina width. These results support the possible utility of some vertebral morphometrics in predicting mechanical properties of the vertebral column in crocodiles, which also should be useful for forming functional hypotheses of axial motion during locomotion in extinct archosaurs.

Results of cervical recapping laminoplasty: gross anatomical changes, biomechanical evaluation at different time points and degrees of level involvement

Background

Recapping laminoplasty has become the frequently-used approach to the spinal canal when bone decompression of the vertebral canal is not the goal. However, what changes will occur after surgery, and whether recapping laminoplasty can actually reduce the risk of delayed deformities remains unknown.

Methodology

We designed an animal experiment using a caprine model, and partitioned the animals into in vitro and in vivo surgical groups. We performed recapping laminoplasty on one group and laminectomy on another group. These animals were sacrificed six months after operating, cervical spines removed, biomechanically tested, and these data were compared to determine whether the recapping laminoplasty technique leads to subsequent differences in range of motion. Image data were also obtained before the surgery and when the animals were killed. Besides, we investigated the initial differences in kinetics between recapping laminoplasty and laminectomy. We did this by comparing data obtained from biomechanical testing of in vitro-performed recapping laminoplasty and laminectomy. Finally, we investigated the effect that longitudinal distance has on cervical mechanics. This was determined by performing a two-level recapping laminoplasty, and then extending the laminoplasty to the next level and repeating the mechanical testing at each step.

Principal Findings

There were three mainly morphological changes at the six months after laminoplasty: volume reduction and bone nonunion of the recapping laminae, irregular fibrosis formation around the facet joints and re-implanted lamina-ligamentous complex. In the biomechanical test, comparing with laminectomy, recapping laminoplasty didn’t show significant differences in the immediate postoperative comparison, while recapping laminoplasty demonstrated significantly decreased motion in flexion/extension six months later. Inclusion of additional levels in the laminotomy procedure didn’t lead to changes in immediate biomechanics.

Conclusions

Recapping laminoplasty can’t fully restore the posterior structure, but still reduced the risk of delayed cervical instability in a caprine model.

Shake a tail feather: the evolution of the theropod tail into a stiff aerodynamic surface

Theropod dinosaurs show striking morphological and functional tail variation; e.g., a long, robust, basal theropod tail used for counterbalance, or a short, modern avian tail used as an aerodynamic surface. We used a quantitative morphological and functional analysis to reconstruct intervertebral joint stiffness in the tail along the theropod lineage to extant birds. This provides new details of the tail's morphological transformation, and for the first time quantitatively evaluates its biomechanical consequences. We observe that both dorsoventral and lateral joint stiffness decreased along the non-avian theropod lineage (between nodes Theropoda and Paraves). Our results show how the tail structure of non-avian theropods was mechanically appropriate for holding itself up against gravity and maintaining passive balance. However, as dorsoventral and lateral joint stiffness decreased, the tail may have become more effective for dynamically maintaining balance. This supports our hypothesis of a reduction of dorsoventral and lateral joint stiffness in shorter tails. Along the avian theropod lineage (Avialae to crown group birds), dorsoventral and lateral joint stiffness increased overall, which appears to contradict our null expectation. We infer that this departure in joint stiffness is specific to the tail's aerodynamic role and the functional constraints imposed by it. Increased dorsoventral and lateral joint stiffness may have facilitated a gradually improved capacity to lift, depress, and swing the tail. The associated morphological changes should have resulted in a tail capable of producing larger muscular forces to utilise larger lift forces in flight. Improved joint mobility in neornithine birds potentially permitted an increase in the range of lift force vector orientations, which might have improved flight proficiency and manoeuvrability. The tail morphology of modern birds with tail fanning capabilities originated in early ornithuromorph birds. Hence, these capabilities should have been present in the early Cretaceous, with incipient tail-fanning capacity in the earliest pygostylian birds.