PROCEDURE
1.
PRE-WORK
(before lab) review the organization of the
jaw from your atlas or on an articulated skeleton in the lab.
(Netter022)
(Netter024)
(Netter011)
(Netter013)
Zygomatic arch (cheek bone) - The zygomatic arch is a transverse span of bone extending from temporal bone at the lateral side of the skull near the ear anteriorly to the rim of the orbit.
Temporal fossa - The space deep (medial) and superior to the zygomatic arch forms the temporal fossa, which is completely filled with the broad fan-shaped temporalis muscle.
Mandible - Posteriorly, the mandible turns sharply superiorly at the angle, with the superior extension forming the ramus. The ramus splits into two extensions, posteriorly the neck of the mandible which ends in the condylar process (head of the mandible) and anteriorly the coronoid process.
The condylar process of the mandible articulates with a depression in the temporal bone to form the temporomandibular joint (TMJ). The coronoid process of the mandible 'fits' just inferior to the zygomatic arch. The temporalis muscle projects through the arch to insert on the coronoid process.
PROCEDURE - THE FACIAL NERVE
2. Using fingers palpate the cartilage tube surrounding the
external auditory meatus and feel for the tragus cartilage oriented inferoanteriorly.
(Figure761) (Netter054)
The protrusion of tragus cartilage oriented anteriorly and inferiorly is referred to as the tragal pointer.
The main trunk of the facial nerve will be located approximately 1cm deep and slightly inferoanteriorly. Thus, the tragal pointer 'points' towards the location of the main trunk of the facial nerve.
There are several surgical approaches to the facial nerve using surface landmarks. The 'tragal pointer' is one of the most reliable and is commonly utilized in the surgery suite as it is a palpable landmark.
Electrical checkpoint nerve stimulators are highly effective during the surgery to observe muscle contraction when stimulating putative facial nerve bundles, but these are obviously not useful in donor dissection.
3. Follow the sternocleidomastoid muscle superiorly and
palpate the mastoid process of the skull.
(Figure761) (Netter054)
(NetterBP021)
If you have not already done so, remove enough skin and fascia to allow your fingers to palpate the position of the mastoid process.
4. Using scissors in a spreading motion, separate parotid
tissue and fascia in the region slightly anterior to the mastoid process and
slightly inferior to the tragal pointer.
(Figure761) (Netter054)
(NetterBP021)
(Photo7068)
Since the facial nerve course is from posterior to anterior into the parotid gland, use a spreading motion that is parallel to the axis of the nerve. This will minimize the chance that you will inadvertently cut the main trunk of the facial nerve.
The main trunk of the facial nerve will be located approximately 1cm deep at a position slightly inferoanterior to the tragal pointer and anterior to the mastoid.
In many cases the facial nerve will split immediately into several of the facial nerve branches, so you may find what looks like more than one nerve traversing this region.
The facial nerve emerges through the stylomastoid foramen which is located between the mastoid and styloid processes on the skull. The nerve turns sharply to an anterior orientation and enters the parotid gland.
5. Dissect away parotid gland tissue and follow the facial
nerve anteriorly to where it starts to form branches.
(Figure761) (Netter054)
(NetterBP021)
(Photo7069)
(Photo7041)
(Photo7042)
The point at which the main facial nerve begins to branch is the parotid plexus. It is beyond our scope to follow each of the individual branches out to their final target, but they can be reviewed on a photograph and in the description below.
Textbooks will talk about an upper and lower division in the plexus, but in reality the parotid plexus is highly variable, with nerves splitting apart and recombining, and rarely forms such neat divisions.
Typically five major divisions of the facial nerve radiate fan-like away from the parotid gland. Regardless of branching variations, these are named by the region they innervate. The five branches are: temporal, zygomatic, buccal, mandibular, and cervical ("To Zanzibar By Motor Car" mnemonic).
Temporal branch: crossing the zygomatic arch up to the temple to supply the frontalis and the superior part of orbicularis oculi muscles. This branch is vulnerable to injury or surgical damage as it crosses shallow to the skin over the bone of the zygomatic arch.
Zygomatic branch: crossing anteriorly towards the lateral angle of the orbit innervating the inferior part of orbicularis oculi. The zygomatic branch of the facial nerve is usually parallel and slightly superior to the parotid duct.
Buccal branch: typically larger than the other branches coursing horizontally to innervate the cheek and superior part of orbicularis oris mouth. There are commonly plexus cross connections between zygomatic and buccal branches.
Mandibular branch (also called the marginal mandibular branch): passing anteriorly parallel to the inferior margin of the mandible to innervate the muscles of facial expression of the inferior lip.
Cervical branch: crossing over the angle of the mandible to enter the neck over the suprahyoid region. It primarily innervates the platysma muscle.
A single small posteriorly directed branch, the posterior auricular nerve, courses from the main trunk posterior to the ear to innervate the occipitalis muscle and auricular muscles (vestigial ear muscles absent in most people, thus usually not present).
PROCEDURE - TEMPOROMANDIBULAR
6. On the donor
use your fingers to identify
the superior edge of the zygomatic arch.
(Figure763)
(Netter055)
The zygomatic arch spans from the temporal bone near the ear anteriorly to the rim of the orbit. The deep investing fascia over the temporalis muscle bonds tightly to the zygomatic arch where the muscle passes deep to the arch.
7. Working superiorly from the zygomatic arch, remove
connective tissue so that you can observe the temporalis muscle.
(Figure763)
(Netter055)
(Photo7043)
The temporalis muscle is a broad fan-shaped muscle that attaches to, and covers, most of the temporal bone on the lateral aspect of the skull.
The inferior attachment of the temporalis muscle is to the coronoid process of the mandible.
The temporalis muscle is one of the muscles of mastication and its contraction closes the jaw. The temporalis muscle, together with the masseter muscle (examined in later steps), can close the jaw with considerable force.
8. Using a scalpel make a horizontal cut to the bone
through the temporalis muscle along the entire length of the superior side of
the zygomatic arch.
(Figure763a,
cut #1) (Netter055)
(Photo7070)
The temporalis muscle is relatively thin at the posterior region of the arch. You will cut just a few millimeters deep before your scalpel reaches the temporal bone.
At the anterior aspect of the zygomatic arch, the temporalis muscle is quite thick. Your cut will need to be as much as several centimeters deep before your scalpel reaches temporal bone.
This cut should completely transect the temporalis muscle from the lateral orbital rim out to the posterior end of the zygomatic arch near the ear.
9. Push a blunt probe or scalpel handle into the posterior end of the incision and work the probe under the temporalis muscle.
10. Reflect the temporalis muscle superiorly and anteriorly,
making cuts along the posterior edge of the temporalis muscle as you
reflect it.
(Figure763a,
cut #2) (Netter055)
(Photo7070a)
As with all the muscles of mastication, the temporalis muscle is innervated by branches of the mandibular division of the trigeminal (V3) nerve. The branches going to the temporalis muscle are called the deep temporal nerves.
Arteries and nerves supplying the temporalis muscle enter the muscle from its deep side (typical of most muscles in the body).
11. At the anterior side of the temporalis muscle push
a blunt probe or scalpel handle into your horizontal incision and 'lever' the
temporalis muscle out of the temporal fossa.
(Figure763a,
cut #2) (Netter055)
(Photo7070a)
You may need to use a scalpel to cut connective tissue superiorly along the lateral orbital rim for several centimeters as the fascia holding the temporalis muscle in the temporal fossa can be very strong.
12. Clean connective tissue from the lateral and inferior
aspect of the zygomatic arch to identify the attachment of the masseter muscle to
the arch.
(Figure763)
(Netter055)
(Photo7071)
The masseter muscle is a thick, roughly quadrilateral muscle that spans from the zygomatic arch inferiorly to the posterior part of the mandible (attaching to the body of the mandible near the angle of the mandible).
The masseter muscle is one of the muscles of mastication. Contraction of the masseter muscle closes the jaw. Together with the temporalis muscle these powerful muscles close the jaw with considerable force.
13. Dissect inferiorly along the surface of the masseter muscle removing the parotid gland, buccal fat/fascia, and facial nerve branches
to completely expose the muscle.
(Figure763)
(Netter055)
(Photo7071)
Observe that anterior and deep to the anterior edge of the masseter muscle is the buccinator muscle.
The parotid duct enters the mouth by piercing through the buccinator muscle at approximately the location of the second upper (maxillary) molars.
14. Push a scalpel handle or probe deep to the zygomatic
arch from the superior side between the bone of the zygomatic arch and the
lateral surface of the temporalis muscle.
(Photo7072)
15. Work the handle or probe anteriorly and push the
instrument such that
the end of the instrument emerges from under the masseter muscle.
(Photo7072)
Leave the handle or probe in place as this will serve as a guide for cutting bone in the next steps.
16. Push a second probe deep to the zygomatic arch and work
the probe posteriorly to find the location at which the zygomatic arch ends at
the posterior temporal fossa.
(Photo7072)
Make a small mark on the bone at this location as a guide for cutting bone in the next steps.
17. Use a Stryker saw to cut completely through the
posterior end of the zygomatic arch as close to the end of the arch as possible (i.e. leave
as little zygomatic arch on the skull as possible).
(Figure764)
(Photo7073)
A straight edge Stryker saw blade will be more effective than a curved blade for this cut. If required change your Stryker blade to the straight edge.
18. Use a Stryker saw to cut completely through the
anterior end of the zygomatic arch as close to the orbital rim as possible,
using your inserted probe as a guide.
(Figure764)
(Photo7073a)
19. Use bone cutters/pliers to detach the zygomatic arch
and reflect it carefully in a lateral direction.
(Figure765)
(Photo7073b)
(Photo7073c)
(Photo7073d)
As you reflect the muscle you may observe a neurovascular bundle entering the deep side of the muscle.
The masseter muscle receives a moderately sized arterial supply from the masseter artery, which is a branch of the maxillary artery.
As with all the muscles of mastication, the masseter muscle is innervated by a branch of the mandibular division of the trigeminal (V3) nerve. The branch going to the masseter muscle is called the masseter nerve.
20. Transect the masseter neurovascular bundle and completely reflect the masseter muscle laterally
away from the mandible leaving it attached to the inferior aspect of the
mandible.
(Figure765)
(Photo7074)
The masseter muscle sits directly superficial to the mandible, with just investing connective tissue on the muscle and periosteum on the surface of the bone.
Innervation and blood supply, thus, must pass superiorly from the deeper infratemporal fossa and 'over' the mandibular notch. This notch is the broad groove in the mandible between the neck of the mandible and the coronoid process.
21. Clean connective tissue away from the lateral surface
of the coronoid process of the mandible so that you can observe the tendinous insertion of the
temporalis muscle to the coronoid process.
(Figure765)
(Figure766)
(Netter055)
(Photo7074)
22. Clean connective tissue from the lateral side of the
condylar process of the mandible where you will find dense tissue
of the temporomandibular joint capsule.
(Figure765)
(Figure766)
(Netter055)
(Photo7074)
If you move the mandible (open/close the donors mouth) you may be able to observe or palpate for the location of the temporomandibular joint.
23. Use a Stryker saw to cut partway through the bone at the
base of the coronoid process.
(Figure765b)
(Photo7074b)
In order to protect deeper structures, this initial cut should only go 50-75% of the way through the bone.
This will cut through the outer compact bone of the lateral side and into the internal spongy bone, while leaving the medial compact bone intact protecting deeper structures.
24. Using bone cutters 'snap' through the bone of the
mandible along the partial cut line made by the Stryker saw.
(Figure765b)
(Photo7074c)
If the bone has been cut sufficiently deep the cut line will snap easily. If it is difficult to snap, use the Stryker saw to deepen the cut (particularly on the anterior end of the cut) and try again with the bone cutters until they can snap cleanly through the cut line.
25. Remove the coronoid process along with the remaining
part of the temporalis muscle.
(Figure765b)
(Photo7074d)
(Photo7074e)
You may find branches of the temporalis neurovascular bundle innervating the deep surface of the muscle. These can be transected as needed to remove the muscle and coronoid process.
Sometimes you will leave fibers of the temporalis muscle behind which can be picked away with forceps.
26. Push a scalpel handle or probe deep to the upper third
of the ramus of the mandible until the handle emerges in the location of the coronoid
process.
(Photo7076)
Ensure that the scalpel handle slides along the bone as you pass it deep to the mandible, so that you will avoid damaging deeper structures.
27. Using a Stryker saw cut partway through the ramus of
the mandible just inferior to the temporomandibular joint at the condylar
process.
(Figure767b) (Photo7076a)
28. Using a Stryker saw cut partway through the ramus of
the mandible along an imaginary horizontal line slightly superior to the level
of the lower (mandibular) molars.
(Figure767)
(Figure767a)
(Photo7076b)
In order to protect deeper structures, this initial cut should only go 50-75% of the way through the bone.
The inferior alveolar nerve enters the mandible through the mandibular foramen, which is normally located on the ramus of the mandible in a line posterior to the last molar. Thus, cutting slightly superior to the level of the molars should position your bone cut where it will not sever the inferior alveolar nerve.
29. Position bone cutters into the groove of both cuts and use them to 'snap' through the remaining intact bone in the cut.
If the bone has been cut sufficiently deep, the cut line will snap easily.
If it is difficult to snap, use the Stryker saw to deepen the cut. Cutting completely through a few millimeters of the anterior and posterior end of your initial partial bone cut will make using the bone cutters easier.
Angle the saw to deepen the anterior and/or posterior end of the cut going completely through 2-3mm of the ramus. Then try again with the bone cutters until they can snap cleanly through the cut line.
30. Grasp the superior part of the isolated mandible ramus and lever the bone slowly laterally.
As you lever the isolated mandible ramus out of position, look along the deep surface to see if the inferior alveolar nerve has been 'trapped' with the excised bone.
If the mandibular foramen and inferior alveolar nerve entry are on the region of bone excised (visible as a distinct nerve bundle passing into the bone), stop levering the bone and use bone cutters or Stryker saw to free the nerve.
31. Grasp the remaining piece of the condylar process and manipulate the condylar process of the mandible to observe the wide motion possible around the temporomandibular joint.
32. Clean fascia from the area deep to the
excised mandible ramus to isolate several
nerves/vessels that course superior to inferior.
(Figure768)
(Netter057)
(Netter059)
(Netter082)
(Photo7079)
Most of the 'glassy' tissue layer present will be the periosteum of the mandible. In some cases this will have remained attached to the bone already exposing the neurovascular structures we will examine in the next steps.
From anterior to posterior the major structures are the lingual nerve, the inferior alveolar artery, the inferior alveolar nerve (and you may see the thin nerve to the mylohyoid).
33. As you clean fascia, the most anterior
of the nerves/vessels will be the lingual nerve.
(Figure768)
(Netter059)
(Photo7079)
Lingual nerve - The lingual nerve is a prominent nerve bundle that emerges from under the edge of the lateral pterygoid muscle (approximately half way between the ramus cut and the temporomandibular joint) to dive deep to the mandible and enter the body of the tongue.
The lingual nerve provides sensory innervation of the anterior 2/3rd of the tongue. The fibers in the nerve at this level are general somatosensory branches from V3 and taste fibers from the VII chorda tympani nerve, which joined to the lingual nerve superior to the point we are examining here (we will meet the chorda tympani nerve with the lecture on the ear).
34. The next structures are the inferior
alveolar nerve and artery.
(Figure768)
(Netter057)
(Netter059)
(Photo7079)
Inferior alveolar nerve - This nerve descends to enter the mandibular foramen and travel inside the bone of the mandible. Follow the nerve to its entrance into the mandibular foramen (you may need to pull the mandible laterally slightly to observe the entrance).
You can also examine the deep side of the dry mandible with your skull to observe the mandibular foramen opening.
The inferior alveolar nerve travels within the mandible, innervating the inferior teeth, before the terminal end of the nerve emerges from the mental foramen to become the mental nerve. It is accompanied by the inferior alveolar artery, a branch of the maxillary artery.
There is often a small nerve separating from the inferior alveolar nerve. This is the nerve to the mylohyoid muscle (which we will study in the Digestion and Hormones course).
35. Clear fascia from the region immediately deep to the
lingual/alveolar nerves to observe that the 'floor' over which those nerves run
consists of the muscle fibers of the pterygoid muscles.
(Figure768)
(Netter057)
(Netter056)
(Photo7079) (Photo7080)
We will not identify these muscles individually.
At this time only the sheet of muscle fibers deep to the nerves/vessels will be visible, the bony attachment points are too deep to see.
Medial pterygoid muscle - The medial pterygoid muscle courses from the medial aspect of the angle of the mandible in a roughly oblique anterosuperior direction to the lateral pterygoid plate on the skull.
Lateral pterygoid muscle - This muscle attaches to the neck of the mandible (as well as the capsule of the temporomandibular joint) and courses roughly anteriorly to the lateral pterygoid plate on the skull.
36. Check your tool container for any cutter batteries and return them to the charger stations (check if there is one in the cutting tool or in one of the flashlights) as there will be considerable power tool use in the next lab and fully charged batteries will be needed.
Post-Lab Daily Clinical Review Cases
Each lab has several multiple choice practice cases to reinforce understanding of the material within the lab and associated lectures. These are optional to review at home for consolidating and testing understanding.
CHECKLIST
Skeletal Structures
Mandible
Ramus
Coronoid process
Condylar process
Neck
Mandibular foramen
Temporomandibular joint
Skull
Zygomatic bone
Zygomatic arch
Temporal bone
Temporal fossa
Styloid process
Stylomastoid foramen
Soft Structures
Ear
Tragus
Facial nerve
Facial nerve trunk
Temporal branch
Zygomatic branch
Buccal branch
Mandibular
branch
Cervical
Temporalis muscle
Mandibular (V3) nerve
Inferior alveolar nerve
Lingual nerve