Anatomy of the Sheep Spine and Its Comparison to the Human Spine
HANS-JOACHIM WILKE,* ANNETTE KETTLER, KARL HOWARD WENGER, ANDLUTZ EBERHARDT CLAES
Department Unfallchirurgische Forschung und Biomechanik, Universita¨t Ulm, Ulm, Germany
ABSTRACT
Background: The sheep spine is often used as a model for
the human spine, although the degree to which these spines are
anatomi-cally comparable has yet to be categorianatomi-cally established. The purpose of
this study was to investigate the characteristic anatomical dimensions of
the sheep spine and to compare these with existing human data.
Methods: Five complete spines were measured to determine 21
dimen-sions from the pedicles, spinal canal, transverse and spinous processes,
facets, endplates, and disc.
Results: The results showed that sheep and human vertebrae are most
similar in the thoracic and lumbar regions, although they show
substan-tial differences in certain dimensions. Morphological variations as a
function of spine level typically were well matched in the two species.
Conclusions: Sheep spine may be a useful model for experiments related
to the gross structure of the thoracic or lumbar spine, with certain
limitations for the cervical spine. A thorough database has been provided
for deciding the appropriateness of using the sheep spine as a model for
the human spine. Anat. Rec. 247:542–555, 1997.
r
1997 Wiley-Liss, Inc.Key words: spinal biomechanics; spine; comparative anatomy; human;
sheep; ovine; gross anatomy; in vitro testing
Fresh human specimens are increasingly difficult to
obtain for in vitro experiments, and when available
such specimens are required in large quantities to
overcome the wide scattering effect associated with
biological variability (Ashman et al., 1989). Specifically,
to provide a model for the human spine, animals such
as sheep, goat, pig, calf, and dog have been used. Such
animal specimens are more readily available
(Edmons-ton et al., 1994) and show much better homogeneity
than do human specimens when selected for breed, sex,
age, and weight (Eggli et al., 1992; Gurwitz et al., 1993).
Sheep in particular are often used as a model for in vivo
studies concerning, for instance, histomorphology of
the intervertebral disc (Osti et al., 1990; Moore et al.,
1992; Gunzburg et al., 1993) and biomechanical efficacy
of fusion techniques in the lumbar spine (Ahlgren et al.,
1994). Sheep spines have also been used in vitro to
study the initial stabilizing effect of spinal implants in
the lumbar (Slater et al., 1988; Yamamuro et al., 1990;
Nagel et al., 1991) and cervical regions
(Vazquez-Seonae et al., 1993).
Comprehensive, quantitative data on the
characteris-tic anatomy of the sheep spine, however, are lacking.
Knowledge of the similarities and differences between
sheep and human spines is essential for interpreting
results from studies using this model and is needed to
establish for which investigations the sheep model is
suitable. Thus, the purpose of this study was to provide
an anatomical database of the sheep spine and a
detailed comparison with the human spine to improve
its utility as a model for the human spine in in vivo and
in vitro experiments.
MATERIALS AND METHODS
Five spines from 3- to 4-year-old female merino sheep
with a weight of 72.1
6
7.3 (57–81) kg were harvested
for this study. After storage at
2
20°C in double-sealed
bags, the specimens were thawed and then directly
measured. The use of frozen stored specimens is
consis-tent with the procedures of some investigators
(Mor-oney et al., 1988; Green et al., 1993; Xu et al., 1995),
whereas others have used dried specimens (Francis,
1955; Berry et al., 1987; Scoles et al., 1988; Doherty and
Heggeness, 1994, 1995), but this technique and
forma-lin fixation tend to shrink the tissue.
In general, sheep spines consist of 7 cervical, 12–14
thoracic, and 6–7 lumbar vertebrae (Nickel et al., 1984).
For consistency, only sheep with the most common
number of vertebrae—7 cervical, 14 thoracic, and 7
lumbar—were selected. All muscles were dissected, and
at first the ligaments and intervertebral discs were
kept intact to maintain the physiological curvature of
Received 23 August 1996; accepted 3 October 1996.
*Correspondence to: Priv. Doz. Dr. Hans-Joachim Wilke, Depart-ment Unfallchirurgische Forschung und Biomechanik, Universita¨t Ulm, Helmholtzstra¬e 14, 89081 Ulm, Germany; E-mail: wilke@sirius. medizin.uni-ulm.de
Contract grant sponsor: University Hospital of Ulm; Contract grant number: P.272.
THE ANATOMICAL RECORD 247:542–555 (1997)
the spine. After measuring anterior disc height, the
ligaments and disc were removed to determine
verte-bral dimensions (Fig. 1). Table 1 lists the nomenclature
key for the various measurements.
Linear Dimensions
Lengths, heights, and widths were measured with a
hand-held micrometer. Accuracy of the measurements
was governed by the users’ definition of the anatomical
landmarks, generally yielding a repeatability of about
0.5 mm. Assuming structural symmetry, pedicle height
and width, and transverse process length were
mea-sured only on the right side. Facet height and width
refer to the cranial joint space and were measured on
the right side.
Angular Dimensions
Angles were measured with a three-dimensional
goniometric linkage system consisting of six
potentiom-eters connected by five rigid rods and yielding a verified
accuracy of 0.1° and 0.1 mm (Wilke et al., 1994). Assuming
symmetry, angles of the transverse processes and multiple
joint surfaces were measured only on the right cranial side.
Various measuring probes were fabricated to maintain
the reliability and repeatability of the apparatus in its
contact with the various bony surfaces.
Fig. 1.Anatomical definition of reported dimensions. Abbreviations are listed in Table 1. a: T6 of the sheep spine, lateral view. b: T6 of the sheep spine, cranial view. c: L4 of the sheep spine, dorsal view. d: L4 of
the sheep spine, cranial view. e: C4 of the sheep spine, dorsal view. f: Thoracic vertebra, oblique perspective (adapted from Panjabi et al., 1993).
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Fig. 14.Facet width (FCW) of the sheep spine from C2 to L7 (mean6S.D.) in comparison with reported values for the human spine from C2 to L5. Vertebrae images are at equal scale.
Fig. 15.Interfacet width (IDH) of the sheep spine from C2 to L7 (mean6S.D.) in comparison with reported values for the human spine from C2 to L5. Vertebrae images are at equal scale.
lumbar regions. The strongest difference in trend is in
vertebral body height, which is greatest in the cervical
spine in sheep but in the lumbar spine in humans.
Nevertheless, regional trends are similar in most
mea-surements.
These results provide data that may be helpful to
plan future studies that contemplate the use of sheep as
a model for the human spine. Regarding spinal implant
tests, these results suggest that the sheep may be a
reasonable anatomical model for instrumentation
affect-ing the thoracic and lumbar regions.
ACKNOWLEDGMENTS
We specially thank Herrn Albert Aigner for the
preparation of the sheep spines. The maceration of our
exemplar specimen was done by Gustav Reiter. This work
was supported by the University Hospital of Ulm (P.272).
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