TMJ Imaging
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Direct answer
A click on opening, a grating on chewing, a jaw that locks shut — each maps to an imaging modality. Panoramic films screen condylar shape and gross disease; plain projections (transcranial, transpharyngeal, submentovertex, reverse Towne) once carried bony assessment and still appear in examinations; CT now owns bone — flattening, erosion, osteophytes, ankylosis and fractures, with coronal and 3D reformats; and MRI is the gold standard for the disc and soft tissues, using closed- and open-mouth sequences to separate anterior disc displacement with reduction (the click, as the disc recaptures) from displacement without reduction (the closed lock, with limited opening). Bone scintigraphy answers the activity question in condylar hyperplasia, and Wilkes staging grades internal derangement from early click to end-stage degeneration — the vocabulary every NEET-MDS answer on this topic needs.
What you must remember
- Modality matching: MRI for disc, effusion and marrow; CT for cortical bone, ankylosis and fracture; panoramic as the survey; arthrography and arthroscopy now largely diagnostic-historical and therapeutic respectively.
- The MRI protocol: closed- and open-mouth T1- and T2-weighted (with proton-density) sequences; a normal disc is biconcave and sits atop the condyle; in displacement it lies anterior, deformed (biconvex, folded), and open-mouth views show whether it reduces.
- Click versus closed lock: displacement with reduction produces the opening click and full excursion; displacement without reduction produces the closed lock — limited opening (commonly around 25-30 mm) with deflection to the affected side.
- Degenerative change checklist on CT or plain film: flattening, subchondral sclerosis, erosions, osteophyte formation and subchondral cysts — late-stage findings of internal derangement or osteoarthritis.
- Wilkes staging in brief: stage I-II early displacement with reciprocal clicking; III-IV locking and disc deformation; V degenerative bone change — a grading examiners quote as often as the imaging itself.
- Ankylosis imaging: CT (coronal reformats) defines bony union, medial pole involvement and the true ankylotic mass; true ankylosis (fibrous or bony, intra-articular) is separated from false ankylosis (extra-articular) and from coronoid hyperplasia.
- Condylar hyperplasia: CT shows the enlarged condyle, but 99mTc-MDP bone scintigraphy decides management — active uptake warrants high condylectomy; a quiet condyle means growth has ceased and deformity correction alone suffices.
From click to closed lock on one MRI
A 28-year-old woman reports two years of clicking on the right, now replaced by a jaw that will not open beyond two fingers, deviating right. The MRI is read in pairs. Closed-mouth T1: the disc lies anterior and deformed, folded between condyle and eminence, instead of capping the condyle. Open-mouth: the condyle translates but the disc never recaptures — displacement without reduction, the imaging counterpart of her closed lock, graded Wilkes III-IV. T2 adds effusion and early marrow oedema as markers of loading stress. Contrast her own history: in the click years, the open-mouth sequence would have shown the disc snapping back over the condyle — displacement with reduction, Wilkes I-II — the stage at which conservative therapy (soft diet, splint, physiotherapy) succeeds most often. Had the pain been crepitus with a 52-year history, the study would head toward degenerative bone change, stage V, and the CT would add flattening, sclerosis and osteophytes. One joint, one film pair, the whole natural history — which is precisely how the viva question walks.
Where students slip
The modality mismatch tops the error list: answering "CT" for disc position or "MRI" for ankylosis mapping inverts the physics — bone to CT, cartilage to MRI, activity to scintigraphy. The second slip is mechanism language: a click is reduction (the disc recaptures), a closed lock is failure of reduction (limited opening, deflection to the affected side) — candidates who reverse the deflection side lose a clean mark. The third is staging confidence: naming Wilkes staging with its logic (early clicking through locking to degeneration) rather than reciting numbers without meaning. A final favourite is condylar hyperplasia: the exam asks "how do you know it is still growing", and the expected answer is comparative bone scintigraphy, not another CT — morphology versus activity once again.
Frequently asked questions
Which imaging modality is the gold standard for TMJ disc position?
MRI with closed- and open-mouth sequences — the only modality that shows the disc, its deformity and whether it reduces on opening.
What distinguishes disc displacement with and without reduction?
With reduction the disc recaptures on opening, producing a click and full range; without reduction the disc stays anterior, giving a closed lock with limited opening deflected to the affected side.
How is bony ankylosis of the TMJ best imaged?
CT with coronal and 3D reconstruction — defining the ankylotic mass, medial pole extension and surgical anatomy; plain films and MRI are insufficient for operative planning.
What is the role of bone scintigraphy in condylar hyperplasia?
99mTc-MDP uptake compares condylar metabolic activity: active uptake supports early high condylectomy, while absent activity means growth has ceased.
List the CT or radiographic signs of TMJ osteoarthritis.
Condylar flattening, subchondral sclerosis, surface erosion, osteophyte formation and subchondral cysts.