IMP Landmarks

         The diaphragma sellae was an important landmark structure in pituitary surgery in the past - extension of a pituitary tumour above it was an indication for a subfrontal approach through a craniotomy. However, a transsphenoidal approach is currently the first preferred option, irrespective of whether there is suprasellar extension.

·         The anterior and posterior commissures are important neuroradiological landmarks. Prior to the introduction of modern imaging techniques the anterior and posterior commissures could be identified by ventriculography. This led to the use of these two landmarks as the markers of the baseline used for stereotaxic surgical procedures. This convention is now universal and the positions of the anterior and posterior commissures are used as the basic reference points for most surgical atlases of brain anatomy. The narrow interventricular foramen is located immediately posterior to the column of the fornix and separates the fornix from the anterior nucleus of the thalamus.

·         The floor of the temporal fossa is formed by the frontal and parietal bones, the greater wing of the sphenoid, and the squamous part of the temporal bones. All four bones meet on each side at an H-shaped junction of sutures termed the pterion. This is an important landmark on the side of the skull because it overlies both the anterior branch of the middle meningeal artery and the lateral (Sylvian) cerebral fissure intracranially (it is also known as the Sylvian point). The pterion corresponds to the site of the anterolateral (sphenoidal) fontanelle on the neonatal skull, which disappears about three months after birth.

·         The retromandibular vein, formed by the union of the maxillary and superficial temporal veins (which enter near the points of exit of the corresponding arteries), is superficial to the external carotid artery. It is invariably of a reasonable size and is an important landmark for the facial nerve.

·         Scalenus anterior forms an important landmark in the root of the neck, because the phrenic nerve passes above it, the subclavian artery below it, and the brachial plexus lies at its lateral border. The clavicle, subclavius, sternocleidomastoid and omohyoid, lateral part of the carotid sheath, transverse cervical, suprascapular and ascending cervical arteries, subclavian vein, prevertebral fascia and phrenic nerve are all anterior to scalenus anterior. Posteriorly are the suprapleural membrane, pleura, roots of the brachial plexus and the subclavian artery: the latter two separate scalenus anterior from scalenus medius.

·         Posteroinferior end of the superior turbinate is the most appropriate anatomic landmark for the identification of the natural ostium of the sphenoid sinus.

·         The pterygomandibular raphe - a tendinous band between buccinator and the superior constrictor - passes downwards and outwards from the hamulus to the posterior end of the mylohyoid line. When the mouth is opened wide, this raphe raises a fold of mucosa that marks internally the posterior boundary of the cheek, and is an important landmark for an inferior alveolar nerve block.

·         Superior petrosal triangle as anatomic landmark for subtemporal middle fossa orientation.

·         Histological studies have shown the radioscapholunate ligament is not a true ligament because it contains neurovascular structures which supply the scapholunate interosseous membrane and is covered by a thick synovial lining. However it a visible landmark inside the wrist joint when undertaking wrist arthroscopy.

·         A triangular zone ,the triangle of Koch is found between the attachment of the septal cusp of the tricuspid valve, the anteromedial margin of the ostium of the coronary sinus, and the round, collagenous, palpable, subendocardial tendon of Todaro. The triangle is a landmark of particular surgical importance, indicating the site of the atrioventricular node and its atrial connections.

·         The pubic tubercle is an important landmark in distinguishing inguinal from femoral hernias; the neck of the hernia is superomedial to it in inguinal hernia, but inferolateral in the femoral form.

·         The main trunk of the inferior mesenteric vein lies either posterior to the duodenojejunal flexure or beneath the adjacent peritoneal fold. The duodenojejunal flexure is a useful landmark to locate the vein radiologically or surgically.

·         The ligament of Treitz is an important landmark in the radiological diagnosis of incomplete rotation and malrotation of the small intestine.

·         The presacral fascia provides an important landmark because extension of rectal tumours through it signifiantly reduces the chance of curative resectional surgery being possible. Dissection in the plane posterior to it may result in bleeding from the presacral veins and, since the adventitia of the veins is partly attached to the posterior surface of the fascia, the haemorrhage may be severe because the veins are unable to contract down properly.

·         The rectosacral fascia, or Waldeyer's fascia, is a thick condensation of endopelvic fascia connecting the presacral fascia to the fascia propria at the level of S4 and extends to the anorectal ring. Waldeyer's fascia is an important surgical landmark, and its division during dissection from an abdominal approach provides entry to the deep retrorectal pelvis.

·         The common peroneal nerve is found emerging posterior to the biceps femoris tendon, which is thus a useful landmark to find the nerve and also to avoid injury to the nerve.

·         Flexor hallucis longus is an important surgical landmark at the ankle: staying lateral to it prevents injury to the neurovascular bundle.

·         The master knot of Henry is the anatomical landmark where the tendon of flexor hallucis longus crosses deep to the tendon of flexor digitorum longus, to reach its medial side in the sole of the foot. The medial plantar nerve can be irritated at the master knot of Henry: this is usually related to jogging.

·         intersegmental pulmonary veins form surgical landmarks; thus, a surgeon can remove a bronchopulmonary segment without seriously disrupting the surrounding lung tissue and major blood vessels.

·         Sacral cornu or horn: formed by the pedicles of the fifth sacral vertebra. It is an important landmark for locating the sacral hiatus.

·         Iliac crests: a horizontal line connecting the crests passes through the spinous process of L4 and the intervertebral disc of L4-5; a useful landmark for a lumbar puncture or epidural block

·         Umbilicus: site that marks the T10 dermatome, lying at the level of the intervertebral disc between L3 and L4

·         The pectoral neurovascular bundle is a good landmark in that it indicates the position of the axillary vein just above and deep (superior and posterior) to the bundle. This neurovascular bundle needs to be preserved during standard axillary dissection.

·         The left crus of the diaphragm is a useful landmark that leads the surgeon to the left inferior phrenic vein.

·         The inferior epigastric artery and vein are branches of the external iliac vessels and are important landmarks for laparoscopic hernia repair.

·         The white line of Toldt represents the fusion of the mesentery with the posterior peritoneum. This subtle peritoneal landmark serves the surgeon as a guide for mobilizing the colon and mesentery from the retroperitoneum.

·         The mesosigmoid is frequently attached to the left pelvic sidewall, producing a small recess in the mesentery known as the intersigmoid fossa. This mesenteric fold is a surgical landmark for the underlying left ureter.

·         The ureters lie on the psoas muscle, pass medially to the sacroiliac joints, and cross the iliac vessels anteriorly. An important anatomic landmark for easy identification of the ureters is at the site where the ureters cross anterior to the iliac vessels. After crossing the iliac vessels, the ureters swing laterally near the ischial spines before passing medially to penetrate the base of the bladder. In males, the vasa deferentia pass anterior to the ureters as they exit the internal inguinal ring. In females, the uterine arteries are closely related to the lower ureters.

·         The three taenia coli converge at the junction of the cecum with the appendix and can be a useful landmark to identify the appendix.

·         Descemet's membrane becomes continuous and uniform, and fuses with the trabecular beams. The fusion site, known as Schwalbe's line, is a gonioscopic landmark that defines the end of Descemet's membrane and the start of the trabecular meshwork.

·         The preaponeurotic fat pockets in the upper eyelid and the precapsulopalpebral fat pockets in the lower eyelid are anterior extensions of extraconal orbital fat. These eyelid fat pockets are surgically important landmarks and help identify a plane immediately anterior to the major eyelid retractors.

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·         The thyrohyoid membrane pierced by the superior laryngeal vessels and internal laryngeal nerves, and Laryngocele (external).

·         The orbital septum is pierced above by levator palpebrae superioris and below by the ligament from inferior rectus. The lacrimal, supratrochlear, infratrochlear and supraorbital nerves and vessels pass through the septum from the orbit on the way to the face and scalp.

·         The axillary fascia is pierced by the tail of the breast.

·         Branches of Musculo Cutaneous Nerve to biceps and brachialis leave after the musculocutaneous has pierced coracobrachialis, also pass between biceps and brachialis, the branch to brachialis also supplies the elbow joint.

·         The middle meningeal is the largest of the meningeal arteries. It passes between the roots of the auriculotemporal nerve and may lie lateral to the tensor veli palatini before entering the cranial cavity through the foramen spinosum.

·         The buccal branch of the mandibular nerve passes between the two heads of lateral pterygoid.

·         The mandibular part of maxillary artery passes between the neck of the mandible and the sphenomandibular ligament, parallel with and slightly below the auriculotemporal nerve.

The pterygopalatine part passes between the two heads of lateral pterygoid to reach the pterygomaxillary fissure before it passes into the pterygopalatine fossa.

·         The buccal nerve and maxillary artery passes between the two heads of lateral pterygoid.

·         The middle meningeal artery ascends between the sphenomandibular ligament and lateral pterygoid, passes between the two roots of the auriculotemporal nerve and leaves the infratemporal fossa through the foramen spinosum to enter the cranial cavity medial to the midpoint of the zygomatic bone.

·         The mandibular nerve immediately passes between tensor veli palatini, which is medial, and lateral pterygoid, which is lateral, and gives off a meningeal branch and the nerve to medial pterygoid from its medial side.

·         The auriculotemporal nerve usually has two roots which encircle the middle meningeal artery. It runs back under lateral pterygoid on the surface of tensor veli palatini, passes between the sphenomandibular ligament and the neck of the mandible, and then runs laterally behind the temporomandibular joint related to the upper part of the parotid gland.

·         The inferior alveolar nerve descends behind lateral pterygoid. At the lower border of the muscle the nerve passes between the sphenomandibular ligament and the mandibular ramus and enters the mandibular canal via the mandibular foramen.

·         Sometimes, when the right subclavian artery is the last aortic branch, it passes between the trachea and oesophagus.

·         GlossoPharyngeal Nerve curves forwards on stylopharyngeus and either pierces the lower fibres of the superior pharyngeal constrictor or passes between it and the middle constrictor to be distributed to the tonsil, the mucosae of the pharynx and postsulcal part of the tongue, the vallate papillae, and oral mucous glands.

·         The vagus descends vertically in the neck in the carotid sheath, between the internal jugular 

vein and the internal carotid artery, to the upper border of the thyroid cartilage, and then passes between the vein and the common carotid artery to the root of the neck.

·         The pharyngeal branch of the vagus passes between the external and internal carotid arteries to the upper border of the middle pharyngeal constrictor.

·         Lingual Artery passes between hyoglossus and the middle constrictor of the pharynx to reach the floor of the mouth accompanied by the lingual veins and the glossopharyngeal nerve.

·         Stylopharyngeus is a long slender muscle arises from the medial side of the base of the styloid process, descends along the side of the pharynx and passes between the superior and middle constrictors to spread out beneath the mucous membrane.

·         Posterior interosseous nerve PIN, which passes between the two heads of supinator and enters the extensor compartment of the forearm.

·         At the elbow the ulnar nerve is in a groove on the dorsum of the epicondyle. It enters the forearm between the two heads of flexor carpi ulnaris superficial to the posterior and oblique parts of the ulnar collateral ligament.

·         The median nerve enters the forearm between the two heads of pronator teres and gives off the anterior interosseous nerve, which supplies all the flexor muscles of the forearm apart from flexor carpi ulnaris and the ulnar half of flexor digitorum profundus.

·         The deep palmar arch and the deep branch of the ulnar nerve pass between the two heads of the adductor pollicis.

·         The superior thoracic artery  runs anteromedially above the medial border of pectoralis minor, then passes between it and pectoralis major to gain the thoracic wall.

·         The femoral nerve descends through psoas major and emerges low on its lateral border. It passes between psoas major and iliacus deep to the iliac fascia and runs posterior to the inguinal ligament into the thigh.

·         The inferior gluteal passes between the first and second or second and third sacral anterior spinal nerve rami, then between piriformis and ischiococcygeus.

·         The profunda femoris artery passes between pectineus and adductor longus, then between the latter and adductor brevis, before it descends between adductor longus and adductor magnus.

·         The anterior tibial artery is the terminal branch of the popliteal artery passes between the heads of tibialis posterior and through the oval aperture in the proximal part of the interosseous membrane to reach the extensor region.

·         the mediastinum is the partition between the lungs and includes the mediastinal pleura; however, the term is commonly applied to the region between the two pleural sacs.

·         There are four anatomical landmarks leading to the identification of the trunk of the facial nerve as it leaves the stylomastoid foramen.

1. The cartilaginous external auditory meatus forms a pointer’ at its anterior, inferior border indicating the direction of the nerve trunk.

2. Just deep to the cartilaginous pointer is a reliable bony landmark formed by the curve of the bony external meatus and its abutment with the mastoid process. This forms a palpable groove leading directly to the stylomastoid foramen. Unfortunately this groove is filled with fibrofatty lobules that often mimic the trunk of the facial nerve which can lie as much as 1 cm deep to this landmark.

3. The anterior, superior aspect of the posterior belly of the digastric muscle is inserted just behind the stylomastoid foramen.

4. The styloid process itself can be palpated superficial to the stylomastoid foramen and just superior to it. The nerve is always lateral to this plane and passes obliquely across the styloid process. A branch of the postauricular artery is usually encountered just lateral to the nerve.