Comparative and Functional Anatomy of the Mammalian Lumbar Spine

Boszcyk, Bronk M., Alexandra A. Boszczyk and Reinhard Putz. 2001. Comparative and Functional

Anatomy of the Mammalian Lumbar Spine. The Anatomical Record Part A 264 (2): 157-168.

Mammals Studied:

•Equus Przewalski (Przewalski’s Horse)

•Acinonyx jubatus (Cheetah)

•Lama vicugna (Vicugna)

•Capra ibex ibex (Alpine Ibex)

•Tursiops truncatus (Bottlenose Dolphin)

•Phoca vitulina (Harbour seal)

•Macropus giganteus (Kangaroo)

•Pongo pygmaeus (Orangutan)

•Pan troglodytes (Chimpanzee)

•Homo sapiens (Human)

Spine:

• Essential organ of both weight bearing and locomotion. It must maintain maximal stability while still maintaining crucial mobility as well as maintaining the integrity of neural structures.

• Composed of the Cervical, Thoracic, Lumbar, Sacrum and Coccyx sections of Vertebrae

Human Spine

Cat Spine

Lumbar Vertebrae and Sacrum:

Objective of the Study:

• To establish a detailed correlation between the varied morphological features of a diverse collection of mammalian lumbar vertebrae and the differing compositions of spinal loading patterns in locomotion.

• To determine if human low back problems are because of the imperfect morphological adaptation of the spine itself or lifestyle and environmental causes.

Materials:• Only clearly taxonomised, undamaged

adult mammalian spines without extensive degenerative changes from a zoological collection and human spines from an anatomical collection were selected.

• All specimens were dried and free of soft tissue, except for occasionally preserved spinal ligaments and intervertebral discs.

• Lumbar vertebrae and the first sacral vertebra of each species were studied.

Methods:• The ten mammalian species were chosen

based on their varying types of locomotion.

• Mathematical averages were determined for 14 measurements taken on lumbar vertebrae of the ten species studied.

• For most species four suitable lumbar spines were studied, however, in certain rarer species, only a smaller number could be obtained as was seen in Table 1.

14 Measurements taken:

Results:• The Vertebral Body Dimensions• The terrestrial species all display a

progressive sagittal narrowing of the vertebral bodies towards the sacrum.

• The marine mammals only display a slight decrease in sagittal diameter.

• The posterior height of the vertebral bodies lies constantly between 2 and 3 cm for humans and primates but is generally between 3 and 5 cm for the rest of the species.

Results:

• Superior Endplate Surface Area• Largest superior endplate surface area:

human fifth lumbar vertebra at 13 cm2

Results:

• Superior Endplate Angle

• Consistently parallel endplates are found only in the dolphin.

• A slight anterior wedge shape of the vertebral bodies was found in all other species, except the human fifth lumbar.

Results:

• Articular Process Prominance

• The orangutan, chimpanzee, and kangaroo possess more prominent superior then inferior articular processes whereas for the rest of the species the opposite is true.

Results:

• Transverse Distance Between the Inferior Articular Processes

• An increase towards the sacrum is found in the seal, Przewalski’s Horse, cheetah, ibex, kangaroo and humans.

• Humans possess the largest value at the fifth lumbar vertebra with 5.1 cm.

Results:

• Pedicle Length

• The pedicles tend to decrease in length towards the sacrum in all species, except the kangaroo.

• The kangaroo shows an increase in pedicle length of the sacrum compared to the LL vertebra.

Results:• Zygapophyseal Joint Profile

Results:• Joint Surface Area

• Humans possess the largest joint surface area at the lumbosacral junction with over 2 cm2.

• Przewalski’s Horse has the largest joint surface area overall.

Discussion:• The presented study is currently the

most comprehensive regarding the wide selection of ten mammalian species and the extent of the morphological criteria compared in the more than 200 lumbar vertebrae.

• The mammals were chosen based upon selecting a collective with highly divergent strain patterns of the lumbar spine in locomotion.

Discussion:

• Although the study was quite intensive, certain limitations applied:– Relatively low numbers of individuals were

examined from each of the species.– Results were based solely on the bony

structures on the vertebrae.– The zygapophyseal joints possess true-

dimensional surfaces and an in depth analysis is only possible through 3-D analysis.

Discussion:• The Vertebral Bodies• Marine Mammals:

– The dolphin and the seal both display a balance in sagittal and lateral flexion during propulsion and both possess round vertebral endplates. The dolphin furthermore, possesses parallel endplates throughout.

Seal Dolphin

Discussion:• The Vertebral Bodies• Terrestrial Quadrupedal Mammals:

– A slight anterior wedge shape and a decrease in sagittal diameter towards the sacrum is recognizable in the quadruped species, in whom the predominant sagittal flexion is seen in locomotion.

Przewalkski’s Horse Cheetah

Lama

Ibex

Discussion:

• The Vertebral Bodies• Terrestrial Bipedal Mammals and Primates

– The vertebral bodies of humans provide a structural adaptation to a high degree of axial loading through a remarkably enlarged cranial endplate surface area.

– A dorsally-tapering wedge shape is found only in the human LL vertebra which enhances lordosis.

Discussion:• The Vertebral Bodies• Terrestrial Bipedal Mammals and Primates

Human Chimpanzee Orangutan Kangaroo

Discussion:

• The Pedicles

• There is a remarkable similarity of lumbar pedicle length of approximately 1 cm in all species regardless of varying body size.

• Only Przewalski’s Horse shows a comparatively larger pedicle size in the caudal lumbar vertebrae.

Discussion:• The Articular Processes and Zygapophyseal

Joints• Marine Mammals• Both marine mammals are subjected to a lesser degree of

ventral and dorsal shear than the terrestrial species.

• Dolphin: significant torque only during steering with the pectoral fins in the area of the thoracic spine, with resistance provided by the medially-angled zygapophyseal joints and the wide transverse distance between the inferior articular processes.

• Seal: higher level of torsion in the lumbar spine finds a match in the essentially sagittal orientation of the zygapophyseal joints.

Discussion:• The Articular Processes and Zygapophyseal Joints• Marine Mammals

Dolphin Seal

Discussion:• The Articular Processes and Zygapophyseal

Joint• Terrestrial Quadrupedal Mammals• An increase was seen in the distance between the

inferior articular processes towards the sacrum in these species which is reflective of an increase in torsion of this region.

• These species experience dorsal as well as ventral shear within a motion-segment during locomotion and may possess encompassing zygapophyseal joints.

Discussion:• The Articular Processes and Zygapophyseal

Joint• Terrestrial Bipedal Mammals and Primates• Humans and primates possess a comparatively

short total axial span of the articular processes.• Torsion is similar to the other species in that it

creates a similar widening of the transverse distance between the inferior articular processes.

• Humans reveal the greatest distance at the lumbosacral junction.

• Primates contain the maximum values in the upper lumbar region and tapering towards the sacrum.

Discussion:• The Articular Processes and Zygapophyseal

Joint• Terrestrial Bipedal Mammals and Primates

Human

Chimpanzee

Orangutan

Kangaroo

Discussion:• The Articular Processes and Zygapophyseal

Joint• Terrestrial Bipedal Mammals and Primates cont’d.• The chimpanzee posses the greatest distance in all

vertebrae except the last lumbar.• Humans possess the greatest distance in the last

lumbar vertebra. • Rotation of the lumbar spine is particularly apparent

in the humans. • The kangaroo lumbar joints limit rotation however

the first tail vertebra reveals pronounced rotation.

Conclusion:• With respect to the spine, two principles of evolution

appear to compete:– Increasing structural support– Maintaining segmental mobility

• All mammals studied suggest an exact accommodation of the lumbar spine to the specific biomechanical demands sustained during the course of evolution.

Conclusion:• The functional individuality of mammalian species

should be appreciated when considering nonhuman spines as biomechanical models for humans.

• Humans reveal a highly advanced level of differentiation in lumbar spinal morphology.– Based on the differentiation observed, it may be

more reasonable to attribute the high incidence of low back problems to our evolving modern lifestyle rather than to imperfect morphological adaptation.