"Three-dimensional anatomy of the equine sternum"
C. Eydt, C. Schröck, F. Geburek, K. Rohn, C. Staszyk, C. Pfarrer
Anatomia Histologia Embryologia
Vol. 44, Issue 2, 99 - 106
Publiziert am: 09.04.2014
Three-dimensional anatomy of the equine sternum
Carina Eydt a, Carmen Schröck b, Florian Geburek c, Karl Rohn d, Carsten Staszyk b and Christiane Pfarrer a
a Institute of Anatomy, University of Veterinary Medicine Hannover, Foundation, Bischofsholer Damm 15, D-30173 Hannover, Germany
b Department of Veterinary Anatomy, -Histology and Embryology, Faculty of
Veterinary Medicine, Justus-Liebig-University Giessen, Frankfurter Str. 98, D-35392 Giessen, Germany
c Equine Clinic, University of Veterinary Medicine Hannover, Foundation, Buenteweg 9, D-30559 Hannover, Germany
d Institute of Biometry and Information Processing, University of Veterinary Medicine Hannover, Foundation, Buenteweg 2, D-30559 Hannover, Germany
*Corresponding author: Tel.: +49 511 856 7471
Email address: carina.eydt@tiho-hannover.de Carmen Schröck and Florian Geburek contributed equally to this work.
Carsten Staszyk and Christiane Pfarrer contributed equally to this work.
Email addresses: Carina Eydt - carina.eydt@tiho-hannover.de
Carmen Schröck - carmen.schroeck@vetmed.uni-giessen.de Florian Geburek - florian.geburek@tiho-hannover.de
Karl Rohn - karl.rohn@tiho-hannover.de
Carsten Staszyk - carsten.staszyk@vetmed.uni-giessen.de Christiane Pfarrer - christiane.pfarrer@tiho-hannover.de
Summary
The sternum is a frequently used anatomical site to obtain bone marrow for diagnostic and therapeutic purposes in equine medicine and surgery. For a safe and reproducible aspiration of sternal bone marrow, a reliable anatomical description of the sternum is mandatory. However, the anatomical literature provides very heterogeneous information concerning the structure and number of sternebrae.
Isolated sterna (horses of different ages) underwent clinical computed tomography and single sternebrae were scanned by microcomputed tomography. Data sets were analysed in detail, the dimensions of each sternebra were determined, and correlations to the age and weight were generated. A uniform arrangement of seven sternebrae within the equine sternum was obtained, whereas the 6th and 7th sternebrae were fused in all sterna. The cranial sternebrae (sternebrae 1-3) had a lentiform shape with flattened lateral sides, while the caudal sternebrae (6 and 7) were flattened dorso-ventrally. In contrast, sternebrae 4 and 5 were spherical. The single sternebrae were well demarcated to the chondral sternum and showed two different zones. The periphery consisted of radiodense woven tissue, while in the centre the radiodense tissue was loosely arranged and contained large cavities with radiolucent tissue. A thin lamina (substantia corticalis) of <1 mm was arranged around the peripheral zone. There was no correlation between the body weight and the dimensions of the sternebrae, but there was a positive correlation to the age of the horses. The obtained data provide a sufficient basis to establish a standard nomenclature of the equine sternum.
Introduction
According to the present literature, the equine sternum is composed of three segments: The praesternum (cranial), the mesosternum and the caudal xiphosternum (Schwarze, 1960; Kovács and Fehér, 1961; Loeffler, 1970; Nickel et al., 2004;
Wissdorf et al. 2010). The praesternum consists of two parts: The bony manubrium sterni and the prominent cartilago manubrii, which possesses a ventrally convex contour in the horse (Nickel et al., 2004; König and Liebich, 2007; Wissdorf et al., 2010). The articular facets for the first pair of ribs are located at the dorsal aspect of the cartilago manubrii (Nickel et al. 2004). The manubrium sterni is the portion, which is located cranially from the 2nd pair of ribs. The mesosternum (corpus sterni) is composed of bony elements (sternebrae) that are connected to each other by hyaline cartilage forming synchondroses sternales (Koch and Berg. 1992; Nickel et al., 2004). Ventrally the cartilaginous mass forms a crest, crista sterni (Nickel et al., 2004;
Wissdorf et al., 2010). The lateral sides of the corpus sterni possess incisurae costales for the articulation with corresponding ribs. It has been documented that in aged horses, the synchondroses sternales ossify and the sternebrae fuse by synostoses (Schwarze, 1960; Koch and Berg, 1992; Nickel et al. 2004). However, no data exist concerning the number and/or position of ossifying synchondroses. The xiphosternum of the horse usually lacks a bony structure (Kovács and Fehér, 1961;
Koch and Berg. 1992; Nickel et al. 2004; Wissdorf et al. 2010). It only consists of a cartilaginous structure (cartilago xiphoidea) and has a flat shape expanding laterally in a caudal direction (König and Liebich, 2007).
Although this gross anatomical description of the equine sternum is generally accepted, the anatomical literature provides very heterogeneous information concerning the number and the denomination of the individual sternebrae. Several authors determine six individual bony elements within the equine sternum. In accordance with the Nomina Anatomica Veterinaria (2012), the first bony element is referred to as manubrium sterni and the bony elements 2-6 are referred to as sternebrae 1-5 (Schwarze, 1960; Loeffler, 1970; Koch and Berg, 1992; Nickel et al., 2004). Other authors identify one additional bony element and therefore divide the sternum into manubrium sterni and sternebrae 1-6 (Wissdorf et al., 2010). In a recent study, also seven bony elements were identified and referred to as sternebrae 1-7, omitting the term manubrium sterni (Kasashima et al., 2011).
In human anatomy the term manubrium sterni is clearly defined by three criteria.
First, the manubrium sterni is the most superior portion of the sternum and contains the most superior ossification centre (Ogden et al. 1979; Twietmeyer and McCracken, 2001; Platzer, 2009). Second, the human manubrium sterni is connected to the corpus sterni by fibrocartilage, called symphysis manubriosternalis (Benninghoff and Drenckhahn, 2008). Third, the human manubrium sterni has a distinct shape which differs from the other osseous components of the sternum (Standring et al., 2005). Comparing the human sternum with the equine sternum, the question arises whether adapting terms from human anatomy describes the equine anatomy appropriately.
In modern equine surgery and medicine, the aspiration of sternal bone gains many attention. Sternal bone marrow is widely used as a source of multipotent mesenchymal stromal cells (MSCs) to treat orthopaedic diseases (Smith et al., 2003;
Fortier and Smith, 2008; Kasashima et al., 2011). A sternal puncture is also suitable for cancellous bone biopsy, which is used for autologous cancellous bone grafts (Richardson et al., 1986; Désévaux et al., 2000), or for diagnosis and prognosis of abnormalities of blood cells (Russell et al., 1994; Sellon, 2006)
The technical aspect of sternal bone marrow aspiration requires a distinct and detailed anatomical description of the equine sternum for at least two reasons. First, although the technique of bone marrow aspiration from the sternum is a routine procedure, fatal thoracic and cardiac punctures (Jacobs et al., 1983) and a case of pneumopericardium (Durando et al., 2006) have been described. New data concerning the dimensions and the topographical relations of the individual sternebrae might help to identify the most suitable positions for sternal puncture and might help to avoid risks.
Second, to provide a clear and unambiguous description of surgical techniques related to the equine sternum, a revision and standardization of the anatomical nomenclature is mandatory. Therefore, this study investigated the gross anatomy and the morphometric characteristics of the equine sternum using modern imaging techniques such as clinical- and microcomputed tomography (cCT, µCT) and morphometric analyses on computerized 3-D models.
Materials and Methods
Material
This study was approved by the Ethics Committee of the University of Veterinary Medicine, Foundation, Hannover, Germany, and by the responsible German federal state authority (Lower Saxony State Office for Consumer Protection and Food Safety, 33.9-42502-04-11/0572). Nineteen warmblood horses (aged 2-28 years, median 14.77) were euthanized for other reasons than for this study in the Equine Clinic of the University of Veterinary Medicine Hannover. Sterna were collected in the Institute of Anatomy at the University of Veterinary Medicine Hannover, Foundation. After removal, the sterna were deep frozen and stored until further use.
Creation of 3-D models
After being placed with its dorsal aspect facing to the table, each sternum was scanned helically using clinical computed tomography (BrillianceTM CT – Big Bore Oncology Scanner, Philips Medical Systems, Best, The Netherlands). The following parameters were applied: slice thickness, 3 mm; rotation time 1,5 s, helical pitch 0,813; table-speed, 9 mm/s; X-ray tube potential, 140 kV; X-ray tube current x exposure time, 500 mAs. For bony details, a series with an edge-enhancing filter was reconstructed (1024 image matrix). For evaluation, a longitudinal, a transverse and a coronal series with a slice thickness of 2 mm were generated using multiplanar reformatting: WC, 70 Hounsfield Units (HU); WW, 2400 HU.
By use of a µCT-system (XTremeCT, Scanco Medical AG, Brüttisellen, Switzerland) with an isotropic spatial resolution of 82 µm, single sternebrae were scanned.
The obtained Digital Imaging and Communications in Medicine (DICOM) data sets were imported to the computer program AMIRA (version 5.2.0, Visage Imaging GmbH, Berlin, Germany). For each sternum, 500-1000 2-D cCT were created. Micro-CT data sets of individual sternebrae comprised about 1500 2-D μMicro-CT images. On the basis of the material-specific grey scales (Hounsfield units), sternal bone, including medullary cavities, and sternal cartilage were identified and labelled in 2-D images (Fig. 1). Subsequently, three-dimensional models were calculated and visualized.
Most structures were generated under visual control because of the limitation of automatic algorithms.
Measurements
The combined use of cCT-Data, µCT-Data and 3-D models allowed multiple morphological analyses. Measurements were performed using cCT-data sets.
Measurement accuracy was exemplarily checked in high-resolution µCT-data sets.
Using 3-D models, the position and shape of individual sternebrae were determined.
By means of multiplanar reconstructions, exact median, transversal and horizontal planes were visualized. Subsequently, the following measurements were conducted using a three-dimensional measuring tool in the program AMIRA:
1 cranio-caudal distance from sternebrae 1 to 7 (Fig. 1) 2 cranio-caudal distance of each sternebra (Fig. 1) 3 dorso-ventral distance of each sternebra (Fig. 1) 4 latero-lateral distance of each sternebra (Fig. 1)
The volumes (mm3) of the sternebrae were determined with the module MaterialStatistics (AMIRA) which calculates the volume of a selected region. The measurements were tested for significant correlations between the age/weight and the total volumes/centre volumes of the sternebrae.
Statistics
Differences between the distances of seven sternebrae and the volumes of 19 sterna from horses were calculated by one-way analysis of variance with repeated measurements and post hoc Tukey test, considering experiment-wise error rate.
Normal distribution of model residuals was confirmed by the Kolmogorov-Smirnov test and visual assessment of qq-plots. Resulting P-values of P < 0.05 were regarded as statistically significant. All analyses were performed with the statistics program SAS (Version 9.3, SAS Institute, Cary, NC, USA).
Results cCT- images
The typical cartilaginous structures (Cartilago manubrii, Crista sterni, Cartilago xiphoidea) were visible and identified in all examined specimens. A uniform arrangement of seven bony elements was detected. In the following, these bony elements are referred to as sternebrae 1-7, omitting the term manubrium sterni. In most cases, the sternebrae were separated from each other by a cartilaginous mass.
The most caudal sternebrae (6th and 7th) were fused in 18 of 19 sterna, only the youngest horse (2 years) showed an incomplete fusion. In 10 of 19 sterna (52.63 %) sternebra 5 was fused with the 6thone. In horses older than 15 years, sternebrae 5, 6 and 7 were nearly completely fused and the 1st and 2nd sternebrae were partly fused (Fig. 2).
The crista sterni is most prominent and radiodense at the ventral aspect of sternebrae 1-3. In contrast, the ventral aspect of sternebrae 4-7 is covered by much more radiolucent and thinner masses of cartilage (Figs. 1-3).
3-D reconstructions elucidated the shape of the individual sternebrae. Sternebrae 1-3 were lentiform with flattened lateral sides, sternebrae 4 and 5 had a spherical shape and sternebrae 6 and 7 were lentiform with flattened dorso-ventral sides (Fig. 3).
Each sternebra possessed a radiodense peripheral zone and a radiolucent centre (Fig. 4). These features were further analysed using high-resolution µCT data sets.
µCT- images
The total volume of the sternebrae consists of a meshwork of mineralized trabeculae resembling spongy bone. However, the mineralized trabeculae are aligned in a much denser arrangement in the periphery compared with the centre of the sternebra.
These features reflect the radiodense peripheral zone and the radiolucent centre described in the µCT images. The most peripheral outline of the sternebrae is not composed of a stratum compactum, but features a thin bony lamella (corresponding to a substantia corticalis) measuring <1mm (Fig. 4).
Measurements of sterna and sternebrae
The distance from sternebrae 1-7 was calculated in every sternum, and an arithmetic mean length of 381.93 mm was generated (Table 1). There was no correlation between the body weight and the length of the sternum (R2 = 0.0542) However, there was a low positive correlation between the age and the total length of the sternum (R2 = 0.2235).
The volumes of the total sternebrae and their centres are shown in Fig. 5. The total volumes of sternebrae 4 and 5 are significantly larger than those of other sternebrae.
The three most caudal sternebrae (sternebrae 5-7) possess very similar total volumes with no significant differences. There was no correlation between the weight of the horses and the total volume and centre volume (R2 = 0.0019-0.1564).
However, there was a positive correlation between the total volume of sternebrae 2-7 and the age (R2 = 0.1842-0.4711) with sternebra 5 showing the largest positive correlation (R2 = 0.4711). The volume of the centres of sternebrae 2, 3 and 7 showed a positive correlation to the age of the horses, too (R2 = 0.2353-0.4353).
The measurement analyses of the median, transversal and horizontal planes are shown in Fig. 6. The dorso-ventral extension increases from sternebrae 1 to 3 and then decreases from sternebrae 4 to 7. Concomitantly, the extension increases from sternebrae 1 to 7. Additionally, they become dorso-ventrally flattened caudally, which is depicted in the latero-lateral diagram (Fig. 6c). The cranio-caudal extension does not differ between the sternebrae. The shape of the sternebrae is best described by two geometric bodies. Sternebrae 1-3 resemble a latero-lateral biconvex lens, sternebrae 4 and 5 have a spherical shape and sternebrae 6 and 7 resemble a dorso-ventral biconvex lens.
Discussion
The term manubrium sterni has been adopted from human anatomy to describe the most cranial ossified structure in the equine sternum. The origin of the term manubrium sterni (Latin: manubrium = hilt, handle) is attributed to the distinct quadrangular shape of the human most superior osseous structure in the sternum, which resembles a hilt of a Roman sword, gladius (Nickel et al., 2004). This particular shape is understood as an adaption to functional requirements. The broad superior part of the human manubrium sterni provides articular surfaces for the clavicles and the first pair of ribs. In inferior direction, the human manubrium sterni narrows to its junction with the corpus of the sternum, the symphysis manubriosternalis (Standring et al., 2005). The prominent shape distinguishes the human manubrium sterni from all other osseous components of the human sternum and justifies its denomination. In contrast, the most cranial osseous structure in the equine sternum does not possess a distinct shape but is very similar to the next two following sternebrae. Furthermore, the most cranial osseous structure in the equine sternum does not provide articular surfaces for ribs, like the human manubrium sternum does. The first pair of ribs in horses articulates with the cartilago manubrii and the second pair of ribs articulates with the first incisura costalis placed in between the first two osseous structures of the equine sternum. Regarding the differences in shape and topographical position, the equine most cranial osseous structure of the sternum seems not to be homologous with the human manubrium sterni. In horses, the most cranial ossified component of the sternum should be considered as the first of a row of similar sternebrae. Therefore, we recommend the use of the term sternebra 1 to name the most cranial osseous structure of the equine sternum in accordance with Kasashima et al (2011). The simple adaption of the term manubrium sterni from human anatomy seems to be inappropriate. The following bony elements should be referred to as sternebra 2 to sternebra 7. Especially, the invasive technique of bone marrow aspiration from the equine sternum requires an exact and unambiguous methodical description to avoid fatal complications, for example penetration of the dorsal lamina of sternebrae. Therefore, the suggested nomenclature might contribute to avoiding heterogeneous and inconsistent descriptions of the equine sternum as present in the older literature.
Apart from a heterogeneous nomenclature, the number of bony elements has been determined controversially in the related literature. In contrast to previous reports describing the presence of either six or seven sternebrae (Schwarze, 1960; Loeffler, 1970; Koch and Berg, 1992; Nickel et al., 2004; Wissdorf et al., 2010), sterna in this study were consistently composed of seven separate sternebrae. Using cCT-scans, individual sternebrae were clearly distinguishable, also in cases of fusion of previously separated sternebrae. Fusion of sternebrae 6 and 7 frequently occurred in horses >2 years (Fig. 2). Such fused sternebrae might be misinterpreted as a singular sternebra based on the limitation of illustration facilities in former times.
However, it should be emphasized that in the current study, only warmblood horses were examined. Therefore, it cannot be ruled out that there may be differences in conducted to obtain important information concerning quantitative or qualitative abnormalities of blood cells, such as unexplained prolonged anaemia, polycythaemia, pancytopaenia, leucocytosis, thrombocytopaenia or thrombocytosis (Russell et al., 1994; Sellon, 2006). Most authors recommend aspirating bone marrow from sternebrae 4, 5, or 6 (Désévaux et al., 2000; Goodrich et al, 2008;
Kasashima et al., 2011; Kisiday et al., 2013). However, due to the use of different nomenclatures, it is not unambiguously clear which specific sternebra was punctured.
Considering the risks of fatal punctures of the thoracic cavity, it has been suggested to limit the insertion depth of the puncture needle to 20 mm (Goodrich et al, 2008;
Kasashima et al., 2011).
On the basis of the obtained morphometric results, sternebrae 4 and 5 appear to be most suitable for aspiration of bone marrow with minimized risks for complications for at least three reasons:
First, sternebrae 4 and 5 are the largest sternebrae according to their volume, which suggests a high yield of bone marrow aspirate.
Second, sternebrae 4 and 5 are spherically shaped and possess a dorso-ventral extension of at least 52 mm. These morphological features allow an appropriate range of movement when inserting the puncture needle, which reduces the risk of fatal transsternal penetrations of vital structures.
Third, the ventral aspect of the sternebrae 4 and 5 is not covered by a prominent crista sterni, which alleviates the surgical access by reducing the risk of lateral slipping of the puncture needle away from the median plane. Furthermore, the absence of a prominent crista sterni makes pre-operative ultrasonographic visualization of the ventral midline contour of sternebrae 4 and 5 easier.
For optimal positioning of the puncture needle, correct identification of the individual sternebrae is crucial. This may be achieved with the aid of anatomical landmarks like the olecranon tuber or the xiphoid process (Durando et al.; 2006, Adams et al., 2012;
Delling et al., 2012; Kisiday et al., 2013). However, to unambiguously identify an optimal puncture site targeting the centre of the sternebrae, direct visualization of the ventral outline of the sternebrae using ultrasound has been suggested (Désévaux et al., 2000; Smith et al., 2003; Arnhold et al., 2007; Kasashima et al., 2011).
Conclusion
The obtained results provide a basis for a revised and clear denomination of the bony elements of the equine sternum. The morphometric data (shape and volume) suggests the use of sternebrae 4 and 5 for optimized bone marrow aspiration and minimized the risk for fatal side effects.
Acknowledgements
The authors would like to thank Dr. M. Hellige for her support during cCT imaging, M.
Kielhorn for her assistance during µCT imaging, O. Stünkel for his excellent technical assistance and P. Schrock for her perfect support with AMIRA. The authors wish to thank Mrs. F. Sherwood-Brock for proofreading the manuscript. This work was supported by a grant from the Federal Ministry for Economic Affairs and Energy, AiF Project GmbH.
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