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Ossicle
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The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss. The term \"ossicle\" literally means \"tiny bone\". Though the term may refer to any small bone throughout the body, it typically refers to the malleus, incus, and stapes (hammer, anvil, and stirrup) of the middle ear.
As sound waves vibrate the tympanic membrane (eardrum), it in turn moves the nearest ossicle, the malleus, to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis (oval window), the opening to the vestibule of the inner ear.
Sound traveling through the air is mostly reflected when it comes into contact with a liquid medium; only about 1/30 of the sound energy moving through the air would be transferred into the liquid.[4] This is observed from the abrupt cessation of sound that occurs when the head is submerged underwater. This is because the relative incompressibility of a liquid presents resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum a mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations are stronger but don't move as far. This allows more efficient coupling than if the sound waves were transmitted directly from the outer ear to the oval window. This reduction in the area of force application allows a large enough increase in pressure to transfer most of the sound energy into the liquid. The increased pressure will compress the fluid found in the cochlea and transmit the stimulus. Thus, the lever action of the ossicles changes the vibrations so as to improve the transfer and reception of sound, and is a form of impedance matching.
However, the extent of the movements of the ossicles is controlled (and constricted) by two muscles attached to them (the tensor tympani and the stapedius). It is believed that these muscles can contract to dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise (theory 1) and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies (theory 2) (see acoustic reflex). These muscles are more highly developed in bats and serve to block outgoing cries of the bats during echolocation (SONAR).
Occasionally the joints between the ossicles become rigid. One condition, otosclerosis, results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically using a passive middle ear implant.[further explanation needed]
There is some doubt as to the discoverers of the auditory ossicles and several anatomists from the early 16th century have the discovery attributed to them with the two earliest being Alessandro Achillini and Jacopo Berengario da Carpi.[5] Several sources, including Eustachi and Casseri,[6] attribute the discovery of the malleus and incus to the anatomist and philosopher Achillini.[7] The first written description of the malleus and incus was by Berengario da Carpi in his Commentaria super anatomia Mundini (1521),[8] although he only briefly described two bones and noted their theoretical association with the transmission of sound.[9] Niccolo Massa's Liber introductorius anatomiae[10] described the same bones in slightly more detail and likened them both to little hammers.[9] A much more detailed description of the first two ossicles followed in Andreas Vesalius' De humani corporis fabrica[11] in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape.[12] The first published description of the stapes came in Pedro Jimeno's Dialogus de re medica (1549)[13] although it had been previously described in public lectures by Giovanni Filippo Ingrassia at the University of Naples as early as 1546.[14]
The etiology of a meniscal ossicle has not been definitively established, and congenital, traumatic, and degenerative origins have been suggested. Its association with the posterior horn of the medial meniscus may favor a traumatic origin 1.
If symptomatic, conservative noninterventional therapy is tried first. If this fails, arthroscopic resection can be considered. Interventional therapy may also be considered if the ossicle is associated with other meniscal pathology.
Meniscal ossicle, or bone within the substance of meniscus, is a rare entity and commonly confused with a loose body both clinically and radiologically. MRI is the modality that can definitely diagnose meniscal ossicle and avoid unnecessary diagnostic arthroscopy. Here we report one such case diagnosed using MRI; this patient is doing well without surgery one year after diagnosis.
Meniscal ossicle is mature lamellar and cancellous bone containing fatty bone marrow surrounded by hyaline cartilage within the substance of menicus. It is a rare entity and must be differentiated from various other entities like chondrocalcinosis, avulsion at the site of attachment of posterior cruciate ligament or the posterior horn of the meniscus, and avulsions of the semimembranosus tendon, as well as from a loose body, as it requires different management. MR is the modality of choice for conclusively diagnosing meniscal ossicles and thus avoiding unnecessary intervention, both diagnostic and therapeutic. We share our experience with one such symptomatic case of meniscal ossicle.
A 25-year-old male farmer presented with right knee pain and swelling for six months. The pain was intermittent in nature. The patient also had intermittent locking of the knee joint. There was no recent history of trauma or any other relevant past history. Clinical examination showed mild swelling without any restriction of movement. The patient was thought to have internal derangement of knee, and so an MRI of the knee was done on a 1.5T scanner (Philips Intera, Germany) using a dedicated knee coil. T1W, T2W and proton density weighted (PDW) sequences with and without fat suppression were used to image the knee in the sagittal and coronal planes. A well-defined lesion was identified in the substance of the posterior horn of the medial meniscus, isointense to bone marrow on all pulse sequences, and suppression of the signal on PDW fat suppressed images, with a complete hypointense rim (see Figs. 1 and 2). This represented a meniscal ossicle. There was no other associated finding in the knee. The articular cartilage was also normal. The patient was treated conservatively with analgesics, anti-inflammatory drugs and activity modification (restricting rigorous sports activities and discouraging a cross-legged sitting position). His symptoms subsequently disappeared and he is well after one year of follow-up. The patient provided his consent to the publication of this case report.
Meniscal ossicle was first reported in 1931 and later by Burrows [1] and by Watson-Jones and Roberts [2] in 1934. To the best of our knowledge, it has been reported 42 times so far [3, 4]. Most of the reported cases were located in the posterior horn of the medial meniscus [5, 6]. The meniscal ossicle in our case was also located in the posterior horn of the medial meniscus.
Ossicles are mature lamellar and cancellous bone containing fatty bone marrow surrounded by hyaline cartilage [7]. Many theories have been put forward regarding the etiology of meniscal ossicles. They may be vestigial structures, as they are common in rodents, domestic cats and Bengal tigers [2], or they may represent a degenerative phenomenon due to the ossification of mucoid degeneration [5]; however, this is unlikely, as ossicles occur mostly in younger men, before the onset of significant mucoid degeneration [8]. A traumatic etiology has also been put forth, suggesting that the ossicles represent heterotopic ossification [9]; or they may represent bone fragments arising from the tibial attachment of the meniscal root insertion. This theory is supported by the fact that the most common location for meniscal ossicles is in the posterior horn of the medial meniscus [1, 8], which shows a strong attachment to the tibia and reduced mobility and is thus more prone to an avulsion tear. The normal contour of the adjoining bone on MRI, however, argues against this theory [8]. In short, there is no definite consensus on the etiology of meniscal ossicles.
Most patients complain of intermittent pain, as in our case. According to Van Breuseghem et al. [3], a locking sensation is usually not experienced as expected with a free intra-articular body, but this is not always true, as our patient had intermittent locking, and the two patients reported by Schnarkowski et al. [8] also had locking. So this clinical sign is not a definite criterion for distinguishing a meniscal ossicle from a loose body.
Radiologically, the commonest misdiagnosis is a loose body. A lateral X-ray knee shows a triangular radiodense opacity projected over the posterior joint line. On fluoroscopy, the ossicle moves with the tibia during knee rotation [7]. USG can distinguish between loose bodies and ossicles, but it is operator dependent. This differentiation can also be made with arthrography and CT arthrography, but these are invasive procedures [10].
MRI can easily depict the location of ossicles within the substance of the meniscus [8, 11], thus distinguishing them from loose bodies, chondrocalcinosis, osteochondritis dessicans and semimembranosus and popliteal tendon avulsions [8]. Their characteristic isointensity to adjacent normal bone marrow, along with a hypointense rim, further distinguishes them from loose bodies and chondrocalcinosis, the latter being hypointense on T1W images. MRI also helps to identify associated abnormalities like meniscal tears, ligament tears and avulsions, cartilage damage, and synovial effusion, and hence is the modality of choice for assessing such cases. 59ce067264