CT appearance of incudomalleolar dislocation.

Skull fractures account for 23 to 66% of head traumas; between 18 and 22% of these fractures involve the temporal bone. (l-3) The clinical diagnosis of a temporal bone fracture is based on the presence of otorrhea, hemotympanum, and facial nerve palsy in a patient with head trauma.

The complications of temporal bone fractures include facial nerve injury, cerebrospinal fluid leakage, sensori-neural hearing loss, and conductive hearing loss. Initially, conductive hearing loss is common because of the presence of blood in the ear and/or perforation of the tympanic membrane, but in most patients, it resolves in a matter of weeks. In those few patients in whom the hearing loss does not resolve because of ossicular injury, surgical intervention is sometimes required. Reconstruction of the ossicular chain is considered when a patient has a hearing loss of more than 30 dB (mild to moderate) that persists for 6 months following the precipitating trauma. (4) Surgery involves reconstructing the ossicular chain either by reshaping the ossicles or by using a hydroxylapatite prosthesis and cartilage autografts. (5-7)

Disruption of the ossicles may be seen in longitudinal temporal fractures, which account for 70 to 90% of all temporal fractures. (3) These fractures have been defined as those that run parallel to the external auditory canal and course through the antrum and epitympanum, where they may lead to interruption of the ossicular chain. (3,8) They are usually the result of temporal or parietal bone trauma. In the normal anatomy, the incudomalleolar joint is a saddle-shaped diarthrosis surrounded by an articular capsule. The joint cavity is incompletely divided into two parts by a wedge-shaped articular disk, or meniscus. The incudostapedial joint is an enarthrosis surrounded by an articular capsule.

The ossicles are connected to the walls of the tympanic cavity by ligaments--three for the malleus (superior, anterior, and lateral ligaments) and one each for the incus and stapes (the posterior and annular ligaments, respectively). Dislocation of the malleus is rare because the malleus is firmly attached to the tympanic membrane and to the wall of the tympanic cavity by the strong anterior malleolar ligament. The most frequently dislocated ossicle is the incus because its attachments are easily torn. When the incudomalleolar joint is disrupted, the body of the incus is usually rotated and displaced superiorly, posteriorly, and laterally. Less often, the incus body is displaced inferolaterally and abuts the superior portion of the tympanic membrane.

High-resolution computed tomography (CT) with thin collimation is the imaging technique of choice for assessing temporal bone involvement in head trauma. The CT appearance of incudomalleolar dislocations is shown in 2 patients (figures 1 and 2). The diagnosis of incudomalleolar dislocation should be considered when conductive hearing loss persists after a temporal bone fracture.

[FIGURES 1-2 OMITTED]

References

(1.) Nosan DK, Benecke JE Jr., Murr AH. Current perspective on temporal bone trauma. Otolaryngol Head Neck Surg 1997;117:67-71.

(2.) Steadman JH, Graham JG. Head injuries: An analysis and follow-up study. Proc R Soc Med 1970;63:23-8.

(3.) Cannon CR, Jahrsdoerfer RA. Temporal bone fractures. Review of 90 cases. Arch Otolaryngol 1983; 109:285-8.

(4.) Lee D, Honrado C, Har-El G, Goldsmith A. Pediatric temporal bone fractures. Laryngoscope 1998; 108:816-21.

(5.) Babu S, Seidman MD. Ossicular reconstruction using bone cement. Otol Neurotol 2004;25:98-101.

(6.) Desaulty A, Shahsavari S, Pasquesoone X. [Ossicular reconstruction with autograft in type III]. Rev Laryngol Otol Rhinol (Bord) 2005;126:19-23.

(7.) Rondini-Gilli E, GrayeliAB, Borges Crosara PF, et al. Ossiculoplasty with total hydroxylapatite prostheses anatomical and functional outcomes. Otol Neurotol 2003;24:543-7.

(8.) Kelly KE, Tami TA. Temporal bone and skull base trauma. In: Jackler RK, Brackmann DE, eds. Neurotology. St. Louis: Mosby; 1994:1127-47.

Henriette E. Westerlaan, MD; Joost Gravendeel, MD

From the Department of Radiology, University Medical Center, Groningen, The Netherlands.