J Anat. Soc. India 49(2) 191-197 (2000)
Anatomy Of Temporomandibular Joint?A Review
1Patnaik V.V.G., 3Bala Sanju; 2Singla Rajan K. Department of Anatomy, Govt. Medical College, 1Patiala, 2Amritsar, 3Department of Oral & Maxillofacial Surgery, Pb. Govt. Dental College, Amritsar
Abstract : Anatomy of TMJ has been long luring the anatomists & oral surgeons alike. Without an exact knowledge of anatomy of the region, surgery can�ft be attempted because of dangerous structures present in the vicinity of the joint like maxillary artery, facial nerve & so on. In this article, literature on the topic has been reviewed and an attempt has been made to compile it so as to draw a clear image of the anatomy of the region.
Keywords : Temporo mandibular joint, anatomy
Temporomandibular joint also known as jaw joint or mandibular joint is an ellipsoid variety of synovial joints, right and left joints forming a bicondylar articulation (Williams et al, 1999). The common features of synovial joints exhibited by this joint include a fibrous capsule, synovial membrane et fluid and tough adjacent ligaments etc. However the features which differentiate and make it unique in itself are (i) Articular surfaces being covered by fibrocartilage instead of hyaline cartilage, (ii) movements are not only guided by shape of bones, muscles and ligaments but also by occlusion of teeth, (iii) since both joints are joined by a single bone i.e. mandible, these can�ft move independently of each other (Moore). Elaborating the last point, Dubrul (1996) emphasizes that it is not only the mandible which is a single bone but the other one i.e. cranium also is mechanically a single stable component. So he necessitates the usage of correct terminology for the joint as craniomandibular articulation. He further adds that the label temporomandibular joint constantly reinforces the tendency to think of only one side when speeking about the joint function. This habit is unwittingly implanted in beginning anatomy courses when joint dissections are carried out by different students on opposite sides of a sagittally sectioned head.
(i) Mandibular component
It consists of an ovoid condylar process seated atop a narrow mandibular neck. It is 15-20 mm side to side and 8-10 mm from front to back with its long axis being at right angle to plane of ramus. It thus doesn�ft lie in frontal plane of skull since 2 sides of mandible spread widely posteriorly. Thus if long axes of 2 condyles are extending medially, then they meet approximately at basion on the anterior limit of foramen magnum. This forms an angle open towards the front, varying from 145�‹?160�‹. The lateral pole of condyle is rough, bluntly pointed and projects only moderately from plane of ramus while medial pole extends strongly inwards from plane of ramus. The articular surface lies on its antero superior aspect thus facing the posterior slope of articular eminence of temporal bone. It further continues medially down and around the medial pole of condyle and faces entoglenoid process of temporal bone when the jaw is held in occluded position (Dubrul, 1996).
The adult condyle varies considerably in form from that found in the young child. In the former, the neck is thin and elongated and is readily fractured. Although an apparent weakness, this feature gives some protection to the delicate roof of glenoid fossa. In the infant the condyle is short and stubby having a copious blood supply (Blackwood, 1965), but avascular necrosis may occur following trauma, because the neck resists fracture and the crushing force is transmitted to its superior surface.
ii) Cranial component : (Fig 1)
The articular surface of the temporal bone or facies articularis is more complicated than that of mandible. It is situated on the inferior aspect of temporal squama anterior to tympanic plate. Various anatomical terms used in discussion of the joint are elaborated below :?
Fig. 1. Cranial component of TNJ. T-Articular tubercle: F-Mandibular fossa
(a) Articular eminence : is the entire transverse bony bar that forms anterior root of zygoma. This articular surface is most heavily travelled by condyle and dise as they ride forwards and backwards in normal jaw function.
(b) Articular tubercle : is a small rough bony knob raised on the outer end of articular eminence. It projects below the level of articular surface and serves for attachment of lateral collateral ligament of joint. Though termed as articular tubercle, it is non articulating.
(c) Preglenoid plane : is the slightly hollowed, almost horizontal articular surface continuing anteriorly from the height of eminence.
(d) Posterior articular ridge (lip) and postglenoid process : The tympanosquamosal suture is divided by protruding inferior edge of tegmen tympani into an anterior petrosquamosal and a posterior petrotympanic fissure. The posterior part of mandibular fossa i.e. posterior border of squamous temporal (or anterior margin of petrosquamous suture) is elevated to form a ridge known as posterior articular ridge or lip. This ridge increases in height laterally to form a thickened cone shaped prominence called post glenoid process immediately anterior to external acoustic meatus.
(e) Lateral border of mandibular fossa is usually raised to form a slight crest joining articular tubercle in front with postglenoid process behind.
(f) Medially the fossa narrows considerably and bounded by a bony wall the entoglenoid process which passes slightly medially as medial glenoid plane.
The roof of the mandibular fossa separating it from middle cranial fossa is always thin and even in heavy skull, translucent. This is a clear evidence that the articular fossa although containing the posterior rim of disc and the condyle is not a functionally stress bearing part of the craniomandibular articulation. This function is normally always between the condyle and disc on one hand and the disc and articular eminence with its extended planes (vide supra) on the other (Dubrul, 1996).
Articular coverings :
The smooth slippery, pressure bearing tissue carpeting, the surfaces of the bones varies in thickness across different articular areas. It is essentially a bed of tough collagen fibres bound by special glycoproteins. On the condyle, the tissue is thickest in anteroposterior direction and thickness is greater medially (average measurements 0.37 mm laterally and 0.48 mm medially). On temporal component, it is thickest along articular eminence and preglenoid plane. The thickness is less medially (0.49 mm and 0.36 mm for eminence and plane respectively laterally and 0.45 mm and 0.34 mm medially). In the depth of the mandibular fossa the thickness of periosteum is merely 0.07 mm.
Histologically, the articular tissue is arranged in successive layers. (Fig. 2) The surface layer is composed of thick, strong, tightly packed collagen fibres running parallel to surface forming an interlaced firm fabric which resists destructive torsion under heavy loading. A narrow transitional layer of less compactly packed fibres slanting obliquely lies underneath. Below this oblique layer a zone of strong vertical fibres is found which contains scattered cartilage cells in between fibres. Deepest layer is calcified cartilage lying on subjacent cortical bone. All these tissues including articular disc are saturated with water containing aggregates of protoglycans which makes for a tough, resilient and flexible construct that can sustain considerable loading forces.
Fig. 2. Functional arrangement of the articular tissues covering the bearing areas of the jaw joint. (1) horizontal, thick, tighlty packed, collagen fibers continuous with (2) oblique fibers leading into (3) strong vertical fibers with scattered cartilage cells emerging from (4) calcified cartilage overlying thin layer of cortical bone
It is a thin sleeve of tissue investing the joint completely. It extends from the circumference of the cranial articular surface to neck of the mandible. Outline of capsular attachment on cranial base can be followed from anterolaterally to articular tubercle, laterally to lateral rim of mandibular fossa, posterolaterally to postglenoid process, posteriorly posterior articular ridge, medially medial margin of temporal bone at its suture with greater wing of sphenoid and anteriorly it is attached anterior to preglenoid plane so as to enclose the same within the joint cavity. It is to be noted that posteriorly it is not merely inserted to crest of post glenoid process and ridge but also to their entire anterior surface so there can be no contact between this bony rim and condyle because both capsule and retrodiscal pad of soft tissue intervene.
The outline of attachment on the mandibular neck lies a short distance below the edge of articular surface in front and a considerable distance below the articular margin behind. Laterally it is attached to lateral condylar pole but medially it dips below the medial pole (Dubrul, 1996).
Anteriorly in the region of anterior extension of the articular disc, it is deficient, where it is pierced by anterior extension to give attachment to the lateral pterygoid muscle and that is the weak point in it. The capsule is loose above the attachment of articular disc and tight below it (Kreutziger and Mahan, 1975).
Since the articular disc is attached to inner surface of the capsule, dividing the joint cavity into two compartments, Fennol et al (1992) considered the capsule in two parts i.e. the fibres extending from condyle to disc and from disc to temporal bone thus forming two joint capsules. The longer bands extending from condyle to the temporal bone are regarded as reinforcing fibres by them. According to Schmolke (1994), true capsular fibres passing between mandibular condyle and temporal bone are present only on the lateral side of the joint while on the other three sides i.e. posteriorly, medially and anteriorly, the upper and lower laminae of articular disc are attached separately either to temporal bone or mandibular condyle.
The synovial membrane lining the capsule covers all the intra articular surfaces except the pressure bearing fibrocartilage. Thus it covers the upper and lower suface of soft retrodiscal pad. In lower compartment the lining continues on the inner surface of the capsule and then reflected on the condylar neck to a short distance to articular margin on the anterior aspect of the neck and upto articular margin on its posterior aspect. This forms fluid filled folds (sulci) in marginal gutters of joint cavity. Similarly in upper compartment the membrane lines the innersurface of the capsule only and not the articular surface of temporal bone or superior surface of disc (excepting both surfaces of bilaminar region) (Dubrul, 1996).
In this way, situated at the posterior and anterior ends of upper and lower compartments are four capsular or synovial sulci. These are upper anterior et posterior and lower anterior et posterior synovial sulci. These change shape during translatory movements which requires that the synovial membrane should be flexible (Toller, 1974).
Articular disc :
Articular disc is a roughly oval, firm, fibrous plate with its long axis directed transversely. It is shaped like a peaked cap which divides the joint in a larger upper compartment and a smaller lower compartment (Williams et al, 1999). Most of the authorities consider that rotatory movements take place in lower compartment and translatory movements in the upper, however Hilloowala, (1975) suggests that rotational movement involves both compartments and the joint should be termed diginglymous rather than gingylimo arthrodial. Superior surface of the disc is said to be saddle shape to fit into cranial contour while inferior surface is concave to fit against mandibular condyle. The question of whether the shape of disc is genetically determined, produced mainly by biomechanical constraints or both is still being discussed (Williams et al, 1999).
The disc is thick all around its rim, and thin in the centre. From anterior to posterior it shows - anterior extension, thick anterior band (2.0mm thick), intermediate thin zone (1.0 mm thick), thick posterior band (3.0 mm thick) and posterior most bilaminar region (Dubrul, 1996; Williams et al, 1999). The disc is attached all around the joint capsule except the strong straps those fix the disc directly to the medial and lateral condylar poles which ensures that the disc and condyle move together in protraction and retraction (Choukas and Sicher, 1960; Williams et al, 1999). The anterior extension of the disc is attached to fibrous capsule superiorly and inferiorly and through that to temporal bone and the mandibular neck respectively. In between it gives insertion to lateral pterygoid muscle where the fibrous capsule is lacking and synovial membrane is supported only by loose areolar tissue. In the opinion of Kreutziger and Mahan (1975), this deficiency anteriorly is the week point since there is no fibrous resistance to hypertranslation. Apart from lateral pterygoid, anteromedially, there are attached some fibres of masseter and temporalis laterally. Although more than one muscle is inserted into the disc, majority of the interest has been focussed on lateral pterygoid, whose deep position, unfortunately makes it difficult to investigate under natural conditions (Moore).
The anterior and posterior bands have predominantly transversly running fibres while thin intermediate zone has anteroposteriorly oriented fibres. Posteriorly the bilaminar region consists of 2 layers of fibres with loose connective tissue separating these. The upper layer has been shown by Griffin and Sharpe (1960) to be composed of elastin and is attached to postglenoid process and its medially extended ridge which is the true posterior boundary of the joint. It prevents slipping of the disc while yawning (Dubrul, 1996). The inferior layer of the fibres curve down behind the condyle to fuse with capsule and back of condylar neck at the lowest limit of the joint space. It prevents excessive rotation of the disc over the condyle. In between the two layers is sandwitched an expansile, soft pad of blood vessels and nerves wrapped in elastic fibres which aid in contracting vessels and retracting disc in recoil of closing movements (Dubrul, 1996).
Osborn (1985) views the disc as a viscoelastic deformable pad coextensive with cranial articular surface (i.e. much more extensive than mandibular condyle). Compression forces (increased during chewing biting and grinding) thin out an area of disc between condyle and posterior slope of articular eminence, thereby squeezing out material to form a thickened zone, �gthe annulus of osborne�f�f which surrounds the thin area - a recess for mandibular condyle.
The disc is primarily cartilagenous but Mathews and Moffet (1974) report the presence of chondroid cells. Thilander et al (1976) also noted that at birth the cells are mainly fibroblasts but with increasing age, chondroid cells are present. They also found that disc is vascular at birth but later on the central region becomes avascular. Earlier Blackwood (1965) had reported that condyle is also vascular at birth with vessels anastomosing over the articular surface but these disappear by age of three years. Wright and Moffet (1974) studied the post natal development of disc upto 21 years and found it to be flat at birth, developing its sigmoid profile as the articular eminence enlarges. Williams et al (1999) considered the macroscopic evidence of degeneration (fraying, thinning and perforation) found from fifth decade onwards as normal ageing. They further quote Weisengreen (1975) who found such degeneration in 40% of 183 individuals between 40-90 years of age. Blackwood (1963) noted that when the disc was perforated the degenerative changes were detected in articular eminence suggesting that disc and condyle may well suffer the initial phase of degenerative changes and upper compartment is affected only when the disc is perforated.
Various functions attributed to the disc are (i) it acts as a buffer and shock absorber; (ii) makes the articulation between saddle shaped glenoid cavity above and convex head of mandible below more harmonious; (iii) strengthens joint comprising one of accessory ligaments. However none of these is confirmed experimently. Although the joint can function without a disc, as shown by menisectomy operations, it is not long before it shows signs of excessive wear.
Temporomandibular ligament Complex :
On each side of the skull are, the collateral ligaments of the bilateral jaw joints. The ligament of each side is designed in two distinct layers, a wide outer or superficial layer and a narrow inner or deep band. The outer layer is usually fan shaped and arises from outer surface of articular tubercle and posterior most part of zygomatic arch. There is often a raised, roughened bony ridge of attachment on this area. The ligamentous fascicles run obliquely downwards and backwards to insert on the back of mandibular neck behind and below the lateral condylar pole. Immediately medial to this layer, a narrow ligamentous band arises from crest of articular tubercle continous with attachment of outer portion at this site. This inner band runs horizontally back as a flap strap to lateral pole of condyle. An upper part of this band continues on to attach to the back of disc (see figure 3). There is no comparable
Fig. 3. Attachments of lateral ligament of TMJ. (1) Outer, oblique lighament running from articular tubercle to condylar neck. (2) Inner, horizontal ligament running from tubercle to lateral pole of condyle with extension above to back of disc. (3) Level of articular surface; disc has been eliminated for clarity
double layered reinforcement on the medial side of condyle, but a medial horizontal band is present at a lower level. Medial slippage of condyle is prevented medially by entoglenoid process and laterally by temporo mandibular ligament. This arrangement is remarkably similar to that of knee joints which has two femoral condyles curbed centrally by raised intercondylar eminence of tibia and restrained peripherally by collateral ligaments (Dubrul, 1996). Nell et al (1996) have described various macroscopic and microscopic variations in the ligament. The outer oblique band gets taut in that protraction of condyle which accompanies opening of jaw, thus limiting the inferior distraction of condyle in forward gliding and rotational movements, while the inner horizontal band gets tightened in retraction of head of mandible thus limiting posterior movement of condyle (McMinn, 1994).
An additional band of fibrous tissue usually described as one of the accessory ligaments of joint and lying at a deeper plane; this ligament is a remnant of Meckel�fs cartilage. It arrises from angular spine of sphenoid and petrotympanic fissure and then runs downwards and outwards to insert on lingula of mandible. Some fibres of ligament may extend through medial end of petrotympanic fissure to attach to anterior process of malleus as a vestige (the anterior ligament of malleus) of dorsal end of Meckel�fs cartilage. As the ligament approaches the mandible inferiorly, it spreads fan like to extend its insertion posteriorly below the mandibular foramen and lower limit of the groove running down the mandibular neck. In most of the cases ligament is a thin compact layer of sturdy fibrous tissue with indistict anterior and posterior margins. This ligament is passive during jaw movements maintaining relatively the same degree of tension during opening and closing of the mouth.
The ligament is related laterally to lateral pterygoid muscle, auriculotemporal nerve going posteriorly, maxillary artery going anteriorly, inferior alveolar nerve and vessels going inferiorly and entering mandibular foramen and a lobule of parotid gland; medially to medial pterygoid, chorda tympani nerve and wall of pharynx with fat and pharyngeal veins intervening. The ligament is pierced by myelohyoid nerve and vessels.
(i) The role of this ligament in mandibular mechanics is neglible (vide supra).
(ii) It is a critical structure in block anaesthesia of inferior alveolar nerve as it forms a broad impenetrable wall medial to mandibular foramen. This holds the local anaesthesia concentrated against the inferior alveolar nerve as the later enters the mandibular foramen and prevents the fluid from dissipating into adjacent soft tissue.
(iii) During depression of mandible (opening the jaw), the axis of movements passes through the mandibular foramen, the sphenomandibular ligaments act like the ropes of a swing keeping the lingulae at entrance to mandibular foramina at a constant distance from the base of skull. This is what happens when opening the mough wide. But in an unconcious patient, mouth may drop passively open by hinge movement only in lower compartment of jaw, the tongue swings back, narrows the oropharynx and obstructs the airway with snoring or choking (McMinn, 1995).
It is a specialized, dense, local concentration of deep cervical fascia. It extends from apex and adjacent anterior aspect of styloid process and stylohyoid ligament to mandible�fs angle and posterior border. It then extends forwards as a broad fascial layer covering the inner surface of the medial pterygoid muscle. The anterior edge of ligament is thickened and sharply defined. This ligament is lax when the jaws are closed and slackens noticeably when the mouth is opened because angle of mandible swings up and back while condyle slides downward and forwards. The ligament becomes tense only in extreme protrusive movements. Thus it can be considered only accessory ligament and of uncertain function.
(i) it provides a sharp surgical landmark for locating, exposing and ligating external carotid artery in retromandibular fossa approach.
(ii) While excising the submandibular gland, the facial artery should be secured before dividing it, otherwise it may retract through the stylomandibular ligament and cause serious bleeding.
Lubrication of the joint
Since there is a synovial lining to temporomandibular joint, it can be assumed that its lubrication is by synovial fluid as in other joints. Unlike other synovial joints, there is no hyaline cartilage on articulating surfaces which may modify lubricating mechanism. The synovial fluid comes from 2 sources - first from plasma by dialysis and second by secretion from synoviocytes type A and B. Toller (1961) assessed its volume to be not more than 0.05 ml. However the same author, in 1974, using contrast radiography studies estimated that the upper compartment could accomodate approximately 1.2 ml of fluid without undue pressure being created, while the lower had a capacity of approximately 0.9 ml. The synovial fluid consists of hyaluronic acid and a protein, lubricin. Previously the former was considered to be the main luricating factor but recently Jay et al (1992) have clearly shown that it is primarily lubricin which is the lubricating factor while hyaluronic acid amplifies its boundary lubrication. Earlier Swan (1978) had found that hyaluronic acid may be an efficient lubricant of soft connective tissue including synovial membrane but it is inefficient on articular cartilage where the protein moiety appears to be uniquely involved. Phosphatidylcholine, the major lipid component of the synovial fluid has also been purposed as a boundary lubricant for articular cartilage as well as protector to joint surface (Williams et al, 1993).
Until 1960s the concept of surface of articular cartilage was that of a smooth surface but Gardner and Woodward (1969) by electron microscopy showed that the surface shows various irregularities grouped as - primary anatomical contours (1-5 mm diameter); secondary irregularities (0.4-0.5 mm diameter); tertiary undulations (20-40 micron) and quarternary fibre bundles (0.5-1.0 micrometer). Under load, primary and secondary irregularities may distort, but tertiary and quarternary undulation persist (Gardner, 1972). He points out that changing contours during loading provide low friction. All these studies have been conducted on the hyaline cartilage of synovial joints. Wilson (1978) shows that in guinea pigs, the fibrocartilagenous covering of TMJ condyle exhibits similar irregularities as above. Studies of human condyles by Longmore et al (1973) and Jagger and Whittaker, (1977) have also shown that ultrastructure of this cartilage is similar to that of hyaline cartilage, making it probable that any mechanism of lubrication found in other synovial joints also applies to TMJ.
The vascular supply to the joint is profuse. Every named vessel within a radius of some three centimeters contributes branches to the joint capsule. Major arterial donations come from the large superficial temporal and maxillary arteries posteriorly, and the smaller posterior deep temporal, masseteric and lateral pterygoid terminals anteriorly. The venous pattern is more diffuse forming a plentiful plexus all around the capsule. Posteriorly, the retrodiscal pad is copiously riddled with wide venous channels. These cavernous spaces fill and empty as the condyle rocks rhythmically forward and backward providing for unhampered, nimble movement in normal joint action. A similar venous feature is also seen anteriorly but a lesser degree.
Nerve supply :
The mandibular nerve, the third and major division of the fifth cranial nerve, innervates the jaw joint. Three branches from the mandibular are found sending terminals to the joint capsule. The largest is the auriculotemporal nerve which supplies the posterior, medial and lateral parts of the joint. The next in size is a branch from the masseteric nerve. It, and a more variable branch from the posterior deep temporal nerve, supply the anterior parts of the joint. No other nerves seem to be involved in joint innervation.
The auriculotemporal nerve on reaching the capsule, branches to spread in a fan of twigs which disperse medially and laterally in the retrodiscal pad. Several of the larger twigs swing laterally to the temporomandibular ligaments.
The masseteric nerve sends off a branch to the anteromedial part of the capsule at the sphenosquamosal suture just in front of the articular eminence and preglenoid plane. The posterior deep temporal nerve sends a branch to the capsule farther laterally as the nerve turns upward around the infratemporal crest. This branch runs backward a few millimeters to innervate the anterolateral parts of the capsule anterior to the preglenoid plane. In this way the posterior portion of the joint is innervated from the below, while the anterior portion innervated from above. Posteriorly, nerves accompany blood vessels into the retrodiscal pad.
Free nerve endings are found everywhere in the joint capsule. Ruffini-like endings are found in the lateral part of the capsule. In the outer layers of the joint ligaments end organs are found that resemble Golgi tendon organs and modified pacini an corpuscles are those in other joints of the body.
It is quite clear that the detail of craniomandibular joint innervation is clinically highly significant. Apart from diagnosing joint problems, understanding the neural pattern is essential when contemplating joint surgery. In any surgical procedures - such as removing the disc, etc - some nerves are inevitably cut. While this may eliminate pain for a while, it may in no way alleviate the pathological problem.
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