Abstract
Computerized tomography (CT) and magnetic resonance (MR) imaging of the temporomandibular joint are often not a routine part of a dental patient's pain and clinical evaluation. As a result, the most poorly understood region within the masticatory system is the temporomandibular joint foundation. Unfortunately, patient care and occlusal management are often compromised because of a lack of insight into the relationship between the anatomy of the temporomandibular joints and the occlusion. This chapter's four distinct sections review the key concepts about the temporomandibular joint foundation anatomical structures, detail structurally intact and structurally altered temporomandibular joint anatomy, clarify how structurally altered temporomandibular joints influence occlusal function, and classify the stages of temporomandibular joint structural degeneration. The concept of joint-based malocclusion is explored with numerous temporomandibular joint foundation anomalous software renderings, and sample CT and MR images, which together illustrate in detail how soft tissue and bony abnormalities in a structurally altered temporomandibular joint can create distortions in the occlusion. Lastly, the chapter addresses the specific requirements a clinician must technically master to perform a comprehensive CT or MR examination.
TopIntroduction
The dental profession relies heavily upon diagnostic imaging in the evaluation of pathology of the dentition, the alveolar structures, and the cephalometric skeleton. However, most dentists do not understand the importance of computerized tomography (CT) and magnetic resonance (MR) imaging of Temporomandibular Joint structures. There are several reasons why the TMJ is not a part of the screening evaluation of most dentists. The dental profession frequently views “TMJ” or “TMD” as a pain condition, and as a result, the structural basis of the TM Joint foundation in the development of malocclusion and facial skeletal distortion is not appreciated (Schellhas, Piper, & Omlie, 1990).
Studies suggest that a clinical diagnosis of “absence of TMD” is associated with a high incidence of internal derangements, while the clinical variable of TMJ pain may have no effect on the prevalence of differing MR imaged TM Joint internal derangements. Further, clinical diagnostic criteria have been unreliable in predicting the MR imaged diagnosis of specific internal derangements (Emshoff et. al, 2002). Additionally, MR scans may be intimidating to dentists who are uncomfortable functioning in the medical arena, as dentists may not be oriented as to how and when to order these scans. Although CT is becoming more mainstream in the dental setting, dentists need guidance in knowing exactly how to generate and cut the CT, to maximize the diagnosis of the Temporomandibular Joint foundation. Therefore, the clinical value of careful scanning of joint structures often times is not understood, because dentists still have questions as to what to do with the images and how to apply the findings to daily practice.
Medical radiologists also face difficulties in offering insight back to the dental profession about Temporomandibular Joint abnormalities. Medical doctors are not familiar with the clinical needs of the practicing dentist and therefore feel inadequate in their interpretation of the information contained in TMJ scans. Knowledge of normal TMJ anatomy, appearance on cross-sectional imaging, pathologic appearance, and therapeutic technique is important for a radiologist to assist in the diagnoses and ultimate management of patients with TMJ dysfunction (Petscavage-Thomas & Walker, 2014). Likewise, the vast amount of information that is present in MR and CT scans can be intimidating to dentists, whereby clinicians may fear missing a diagnosis beyond the TMJ. As a result, TMJ scans are either not generated or the diagnostic information is not utilized. Most importantly, the value of CT and MR images is not applied back to patient care. As is the case most often in medicine, specialists drive the need for diagnostic imaging in clinical management, such that the radiologist then learns to function in unison with the specialist. This chapter covers the essential skills for dental and medical professionals to be able to utilize TMJ imaging to better serve the needs of patients. This teamwork between specialists serves to enhance communication of findings for the benefit of the patient.
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For CT and MR scans to enter the mainstream of dentistry, it is the responsibility of the twenty-first century dentist to know why scans are needed and how the diagnostic information in the scans is vital to their clinical practice. It is only logical that dentists own the responsibility for maximizing scan technology. This chapter is a starting point for dentists and radiologists to address their shortfalls.
Four Sections are included:
Key Terms in this Chapter
Magnetic Resonance (MR) Imaging: Magnetic resonance scans offer the clinician flexibility to look at both hard and soft tissues. MR is best for looking at the degree and direction of disc herniation. It also shows the general dimension and shape of the articular surfaces, and is very useful for diagnosing condylar bone marrow necrosis, and MR is a good technique to grade the degree and direction of discal displacement. MR is the only test that shows pseudodisc formation. Swelling or edema of soft tissues and bone marrow can also be imaged with MR, which assesses potential progressive structural TM joint instability. MR diagnostic projects future stability or instability of the bony structures within the temporomandibular joint.
MR Spin-Echo Sequences: MR image production selects the weighting to the contrast of lipid, water, or both, where T1-weighted sequences show lipid, T2-weighted sequences visualize water content, STIR visualizes water content, and proton-density scans improve the contrast between differing tissues.
Computerized Tomography (CT): CT is best for looking at bone dimensions and articular space, but the soft tissues, including the disc and the retrodiscal tissues, are not visualized. Temporomandibular joint CT is much like any other radiograph in that it records everything that has happened in the past within the region being studied. CT however, may not prove useful in projecting a future prognosis.
IEP (The Incisal Edge Position): The IEP is captured with MR imaging, using a bite registration that positions and aligns the maxillary and mandibular central incisors midlines while the patient holds their incisal edges in contact.
Mandibular Discal Complex: This complex includes the mandibular dentition, the mandible, and the TM Joint discs that are tethered to the mandibular condyle through the medial and lateral collateral ligaments.
Skull Base Parameters That Determine a Normal TM Joint Foundation: The skull base of the temporomandibular joint foundation is part of the cranial maxillary complex. The two parameters of the skull base determine the normal anatomy and function of the cranial maxillary complex health is determined from the depth and concavity of the articular fossa, and to the prominence and convexity of the articular eminence.
TCP (Translated Condylar Position): As the condyle moves away from the FSCP, it will re-posture into a translated condylar position with the condyle deflected inferiorly by the anatomical alignment the articular eminence, the condylar dimension, and the alignment of the disc between them. The inferior deflection of the condyle during translation is the condylar guidance, which combines with the anterior guidance tooth contacts to uncouple the posterior dentition in protrusion.
CT Slices: CT evaluation of the TM joint foundation must include sagittal and coronal slices, as well as sagittal slices from the tympanic fissure to the molars that assess the presence of a joint-based malocclusion.
FSCP (Fully Seated Condylar Position): The alignment of the disc is graded in the fully seated condylar position or FSCP, which is the most superior posture of the condylar head within the glenoid fossa. The posterior band of the disc is graded both at the medial and the lateral poles. Depending on the structural status of the TM joints, FSCP can correlate to centric relation (CR), adapted centric posture, or non-adapted centric posture.
Condylar Parameters That Determine a Normal TM Joint Foundation: The condyle must be evaluated with imaging to determine whether the foundation is normal as confirmed by three parameters that relate to the condylar assessment. The determination of a normal condyle is based upon the condyle’s overall growth, and size, relative to the overlying fossa. Ideally the condyle comprises 60-66% of the glenoid fossa dimension, it has 8 mm sagittal and 20 mm coronal dimensions, and a volume of 120/160 mm 2 axially, and it should demonstrate smoothly convex sagittal and coronal planes.
Cranial Maxillary Complex: It links the maxillary dentition to the TM joint articular eminence and fossa.
Anomalous Medical Models: Digital anatomical models of the human body that include the facial skeleton and associated structures. These models can be generated with and without structural deformities that exhibit both normal and compromised TM joint anatomy and occlusal disturbances. By programming these models with varying temporomandibular joint structural deformities, differing TM joint structural alterations can be readily visualized, along with how these alterations influence occlusal distortions.
Disc Parameters That Determine a Normal TM Joint Foundation: Seven associated anatomical relationships that determine health or pathology of the articular discs; the biconcave shape, the 1:00 o’clock posterior band, the anterior and posterior bands centering the condyles sagitally, the medial and lateral bearing zones bracing the condyle transversely, the superior bearing zone braces the condyles vertically, the posterior band deflects the condyle away from the ear canal, and posterior is 1:00 o’clock for both TMJs.
Temporomandibular Joint Foundation: The combined anatomy of the boney glenoid fossa, the articular eminence, the skull base, the mandibular condyles, the articular disks, the retrodiskal tissues, and the ligamentous attachments that hold the discs in place on top of the condyles.
Condylar Structural Alterations: There are 4 condylar structural alterations that can indicate the presence of boney disease; a disproportionate condyle to fossa ratio, a decreased condylar dimension secondary to a condylar growth center injury, a decreased condylar dimension secondary to degenerative changes, and when the condyle shows evidence of articular surface adaptation.
Piper’s Classification: The Piper classification subsets the spectrum of soft and hard tissue disorders and structural alterations that can occur within the temporomandibular joint into three components: staging the intra-articular soft tissue disorders, quantifying the degree and direction of disc displacements, and classifying the type of condylar head bony disorders. Piper’s classification places patients into diagnostic categories to help to determine the therapeutic options that would most suit the patient’s clinical and imaged findings.
Clock Face Orientation: The articular disc position as it sits on top of the mandibular condyle, can be categorized by the “hands on a clock face,” where 1:00 is a normal disc position. The disc position worsens as the disc displaces anteriorly towards 12:00, and 11:00, with severe disc displacement occurring at 10:00.
Joint-Based Malocclusion: A malocclusion occurs when structural alterations in the temporomandibular joint foundation influence changes within the dental occlusion. An intact condition of the TM joint foundation structures dictates the positioning of the mandibular discal complex relative to the cranial maxillary complex, which establishes a normal joint-based occlusion (JBO). However, when TM joint structural alterations misalign the mandibular discal complex, varying degrees of malocclusion develops at the tooth level.