Pars flaccida and pars densa
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Here is a comprehensive overview of the pars flaccida and pars tensa - the two parts of the tympanic membrane (TM).
Tympanic Membrane: Pars Flaccida vs. Pars Tensa
Endoscopic view of the right tympanic membrane (normal):
Normal tympanic membrane (labelled structures - short process of malleus, umbo, chorda tympani, long process of incus):
Overview
The TM lies at the medial end of the external auditory canal (EAC). It is slightly oval, ~9-10 mm in its longest diameter (posterosuperior to anteroinferior), and is oriented at ~55° with the floor of the meatus. It is divided into two distinct portions by the anterior and posterior malleolar folds (also called alveolar folds):
| Feature | Pars Tensa | Pars Flaccida (Shrapnell's Membrane) |
|---|---|---|
| Location | Majority of the TM (lower 4/5ths) | Small, superior triangular area above malleolar folds |
| Bony support | Attached to fibrous annulus in tympanic sulcus | Sits in the notch of Rivinus - no bony sulcus/annulus |
| Layers | Three layers (epithelium + lamina propria + mucosa) | Three layers (same) |
| Lamina propria | Well-organized: radial (outer) + circular/parabolic/transverse (inner) fibres; abundant type II collagen | Less marked, loosely and randomly arranged collagen; mainly type I collagen; more elastic |
| Rigidity | Firm and taut | Flaccid, more compliant |
| Thickness | Thinner, strong | Actually thicker than pars tensa, but with less-organized, looser collagen |
| Medial relation | Tympanic cavity (mesotympanum) | Prussak's space (epitympanum/attic) |
| Collagen density | Dense, well-organized | Sparse, loosely arranged in a vascular lamina propria |
Anatomy of the Pars Flaccida
- It occupies the notch of Rivinus, a region where the bony tympanic ring is deficient (formed by the squama of the temporal bone rather than tympanic bone)
- Bounded above by the notch, and below by the anterior and posterior malleolar folds which run to the lateral process of the malleus
- Medially it overlies Prussak's space - the most common site of primary acquired cholesteatoma formation
- Because it lacks the fibrous annulus and has disorganized collagen, it is inherently more prone to retraction under negative middle ear pressure
Anatomy of the Pars Tensa
- Forms the bulk of the TM
- Attached peripherally by the fibrocartilaginous tympanic annulus within the tympanic sulcus
- Concave toward the EAC; the tip of the malleus handle attaches centrally at the umbo
- The posterosuperior quadrant of the pars tensa is a weak point - it has thinner, more sparsely distributed collagen and increased vascularity, making it susceptible to retraction pockets
- Perforations of the pars tensa are always described as "central" (by definition, since they are surrounded by the annulus) - indicative of tubotympanic (safe/mucosal) disease
Clinical Relevance
Chronic Otitis Media (COM) Classification
| Type | TM Involvement |
|---|---|
| Tubotympanic (mucosal/safe) COM | Retraction/perforation of pars tensa only |
| Atticoantral (squamous/unsafe) COM | Retraction/involvement of pars flaccida (attic) |
- Pars flaccida retractions involve the attic and lead to atticoantral disease
- Pars tensa retractions are confined to the mesotympanum
Cholesteatoma
- The most susceptible area for retraction and cholesteatoma is the pars flaccida
- Retraction of the pars flaccida into Prussak's space forms primary acquired cholesteatoma
- Pars tensa retractions, especially posterosuperiorly, can also lead to cholesteatoma (secondary acquired)
Retraction Pockets
The pars flaccida and posterosuperior pars tensa are vulnerable because:
- Eustachian tube dysfunction generates negative middle ear pressure
- The pars flaccida has thicker but loosely organized, weakly supported collagen
- Inflammation/infection further damages collagenous support
- The result is progressive retraction, atelectasis, and potential cholesteatoma
Retraction of the pars tensa is graded by the Sade classification; retraction of the pars flaccida is graded by the Tos classification.
Innervation (both portions)
- Lateral surface: auriculotemporal nerve (Vc) posteroinferiorly; auricular branch of vagus (Arnold's nerve) anterosuperiorly; additional VII nerve fibres
- Medial surface: tympanic branch of glossopharyngeal nerve (Jacobson's nerve)
Sources: Shambaugh Surgery of the Ear; Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2; Cummings Otolaryngology Head and Neck Surgery
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