Key points
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The opening of the eustachian tube, provided by the contraction of the tensor veli palatini muscle, is limited to Rüdinger safety canal.
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The contraction of the tensor veli palatini muscle is almost completely isometric; it depends on hypomochlia, which modulate the muscular force vectors.
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Due to their fibromuscular interconnections on both sides of the Weber-Liel fascia, tensor veli palatini muscle and medial pterygoid muscle form a functional unit.
The eustachian tube consists of 2 different portions: an osseous posterolateral and a fibrocartilaginous anteromedial portion. The osseous portion is grossly formed by the petrous part of the temporal bone, the flexible fibrocartilaginous portion by the tubal cartilage, and its surrounding tissue. The active eustachian tube function is located in the fibrocartilaginous portion. This portion is connected to the skull base by suspensory ligaments.
Spatial orientation
The longitudinal axis of the tube forms an angle with the mediosagittal plane as well as with the horizontal plane. The angle between the longitudinal axis of the cartilaginous part of the eustachian tube and the mediosagittal plane in adults is about 45° on average. In infants, it is only about 10°. The average angle between the Frankfurt horizontal plane (orbitomeatal plane) and the tubal longitudinal axis in adults is about 35°.
Spatial orientation
The longitudinal axis of the tube forms an angle with the mediosagittal plane as well as with the horizontal plane. The angle between the longitudinal axis of the cartilaginous part of the eustachian tube and the mediosagittal plane in adults is about 45° on average. In infants, it is only about 10°. The average angle between the Frankfurt horizontal plane (orbitomeatal plane) and the tubal longitudinal axis in adults is about 35°.
Length
The total length of the eustachian tube ranges between 31 and 44 cm in adults. Its length in newborns measures only about one-half of the adult’s. The length of the osseous part is about one-third; the length of the fibrocartilaginous part is about two-thirds of its total length. There is no sharp borderline between the osseous and the fibrocartilaginous portion for the cartilage extends into the roof of the osseous part. The tubal cartilage ends posterolaterally to the isthmus, which is the narrowest point in the tubal lumen. According to Zöllner, the distance between the pharyngeal orifice and the isthmus measures 24 to 28 mm. Rüdinger observed a fibrocartilaginous mass connecting the bone and the hyaline cartilage; this is the reason why there is a difference between the length of the cartilage, which is about 31.2 mm, and the fibrocartilaginous portion of the eustachian tube, which is about 26 mm. Pahnke even observed cartilage reaching the tympanic orifice of the tube.
Compartments
On a frontal 2-dimensional view, Fig. 1 depicts the main structures contributing to the functional eustachian tube anatomy.
The fibrocartilaginous portion of the eustachian tube is almost completely surrounded by the tubal cartilage and by the tensor veli palatini muscle. Both structures form the cartilaginous and the muscular wall of the eustachian tube.
Cartilage
With respect to the eustachian tube function, the cartilage is a very important structure, because it forms the luminal frame of the tube. Looking at the cross-section of the eustachian tube cartilage, its shape resembles a shepherd’s crook, consisting of a dome with a short lateral lamina and a long medial lamina. The lateral lamina has a mean height of 1.8 mm at its largest extension, and the medial lamina has a mean maximum height of 5.1 mm. This maximum height of both laminae can be found at about 6.6 mm behind the pharyngeal orifice. The size and shape of the lateral lamina are much more constant than that of the medial lamina. By means of MRI studies of the eustachian tube, Oshima and colleagues could demonstrate a wide individual variety especially of the medial lamina. They concluded that this could have potential implications for eustachian tube surgery. Pahnke also described this variety, which he found in an anatomic specimen. In about 25% of his specimen, the lower end of the medial lamina formed a hook around the lower portion of the eustachian tube lumen. The thickness of the medial and lateral lamina in the middle portion is approximately equal. In comparison to the medial lamina, the lateral lamina, however, becomes thinner toward both orifices.
According to Bluestone, the elasticity of the tubal cartilage is comparable to that of the pinna and the nasal cartilage. This elasticity, which is higher in adults than in infants, is crucial for the reset forces after the contraction of tensor veli palatini muscle.
Lumen
Fig. 2 is a copy of an original illustration by Rüdinger. It shows his cross-sectional view of the tubal lumen. Rüdinger distinguishes between 2 zones of the tubal lumen:
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A cranial half-cylindrical space, which today is called the “Rüdinger safety canal.” This space is situated between the lateral and the medial lamina of the cartilage and is filled with mucus or air. Its diameter is about 0.5 mm, and it is found in about 85% of the adults. Most probably this space is always open. The safety canal probably warrants pressure equalization and ventilation function of the eustachian tube.
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Under the safety canal, there is a gap that is mainly surrounded by the muscular or membranous wall of the eustachian tube and partly by the medial lamina of the cartilage. Rüdinger called this gap the “auxiliary gap.” The figure shows mucosal folds in the lower medial wall of the auxiliary gap. These inconstant folds were also described by Sando and colleagues and by Ozturk and colleagues. These so-called microturbinates seem to contribute to the mucociliary clearance and to the protection function of the eustachian tube.
The height of the lumen widely differs: in the petrous portion of the temporal bone, it is about 3.5 mm; 6 to 7 mm proximally to the pharyngeal orifice it is 6 to 10 mm.
Within the cartilaginous portion of the eustachian tube, there is mucosa-associated lymphatic tissue. Lymphatic tissue of the nasopharynx, on the other hand, does not extend into the tube.
Ligaments and Fasciae
Between the tubal cartilage and the skull base there is a system of superior tubal ligaments. It consists of the medial and the lateral tubal suspensory ligament, which are divided by a thin layer of fat tissue, the so-called medial Ostmann fat pad. These ligaments are tangentially connected to the medial and the lateral lamina of the cartilage. In a 3-dimensional (3D) view, these ligaments resemble a fibrous plate rather than a ligament. They arise from the temporal as well as the sphenoid bone. The most constant structure is the lateral suspensory ligament that is connected to the lateral lamina. This lateral suspensory ligament partially connects to the tendinous fibers of the lateral layer of the tensor veli palatini muscle.
There are 2 fasciae that cover the medial and the lateral surface of the tensor veli palatini muscle. Laterally, there is the Weber-Liel fascia, which separates the tensor veli palatine muscle from the medial pterygoid muscle. Medially, there is a fascia that runs from the lateral lamina of the tubal cartilage along the lateral surface of the so-called lateral Ostmann fat pad to the salpingopharyngeal fascia, which is also called the von Tröltsch fascia.
Thus, the lateral suspensory ligaments, the lateral lamina of the cartilage, as well as the Ostmann fat pad constitute a common functional unit.
Membranous Wall
Opposite to the tubal cartilage, there is the so-called membranous wall of the eustachian tube. This wall mainly consists of the tensor veli palatini muscle and of the levator veli palatini muscle; Rüdinger called it “muscular wall.” Nevertheless, there is a third anatomic compartment, which contributes to the muscular function: the lateral Ostmann fat pad. That is why the term “membranous wall” is more common. Additional to the constant “lateral Ostmann fat pad” between the lateral lamina and the lateral wall of the eustachian tube, other fatty tissue is found in characteristic locations, of which the medial Ostmann fat pad between the tubal suspensory ligaments is the most important.
The maximum average thickness of the lateral Ostmann fat pad is 2.4 mm. The position of this maximum is found about 20 mm proximal to the pharyngeal orifice. From this point, the Ostmann fat pad gradually decreases toward both orifices. Rüdinger estimated the average thickness of the lateral tubal wall, consisting of its mucous membrane and the Ostmann fat pad, to be 2 mm. According to Rüdinger, this fat pad does not correlate to the body weight. During childhood and adolescence, the thickness of Ostmann fat pad corresponds to its thickness in adults. However, its height increases during postnatal life, causing a growth in volume. This volume decreases again with advanced age. This physiologic decrease does not necessarily cause a patulous eustachian tube.
There are 2 different roles of the lateral Ostmann fat pad:
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First, the static pressure of the fat pad supports the passive closure of the eustachian tube after contraction of the tensor veli palatini muscle. This closing effect helps to prevent the ascension of fluids and acoustic noise from the nasopharynx toward the middle ear. This effect is nondirectional: at the same time the fat pad may prevent the evacuation of the middle ear due to a rapid decrease of nasopharyngeal pressure as postulated by the sniff theory.
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Second, the fat pad serves as a hypomochlion for the lateral layer of the tensor veli palatini muscle. It transfers the pressure of the almost isometric contraction of the muscle to the lower portion of the eustachian tube. Hence, the lateral Ostmann fat pad limits the eustachian tube opening to the Rüdinger safety canal.
Muscles
Although both the tensor and the levator veli palatini muscles are a part of the membranous wall of the eustachian tube, the muscles need to be depicted separately. Fig. 3 shows a methanal-fixated anatomic specimen of the muscles surrounding the eustachian tube.
