CHAPTER 1 Surface Anatomy of the Back and Vertebral Levels of Clinically Important Structures
Surface anatomy is defined as the configuration of the surface of the body, especially in relation to deeper parts. A thorough knowledge of surface anatomy is necessary for the proper performance of a physical examination. Information gathered by the eyes (inspection) and fingers (palpation) is often critical in the assessment of a patient. An understanding of the topography of the human body also allows the health care provider to locate the position of deep structures that may need further evaluation.
The locations of structures in reference to the surface of the body are always approximations, although it has been shown that reliability of locating spinal structures by palpation can be enhanced by training and experience (Byfield et al., 1992; Downey et al., 1999). Individual variations are common and are influenced by such factors as age, gender, posture, weight, and body type. Respiratory movements also can have marked effects on the locations of structures, especially those of the thorax. Determining the position of the contents of the abdomen can be particularly challenging, and the precise location of abdominal viscera can be established only by verification with appropriate diagnostic imaging procedures.
In keeping with the scope of this text, the surface anatomy included in this chapter is limited to the back. Spinous processes and posterior bony landmarks are used as points of reference in the first part of the chapter. One reason for the use of these as landmarks is to help clinicians with examination and treatment of the back and spine when the patient is in the prone position. In addition, the vertebral levels of structures of the anterior neck and trunk, which are either visible by means of advanced imaging procedures (magnetic resonance imaging [MRI] or computed tomography [CT]) or palpable during physical examination, are included. Knowledge of the normal relationships between the viscera and the spine is becoming increasingly important in clinical practice, as clinicians are asked with greater frequency to interpret or review studies employing these advanced imaging procedures. On a more practical level, knowledge of these relationships helps the clinician quickly become oriented with the vertebral level of diagnostic images taken in the horizontal plane.
The back, or dorsum, is the posterior part of the trunk and includes skin, muscles, vertebral column, spinal cord, and various nerves and blood vessels (Gardner, Gray, and O’Rahilly, 1975). The 24 movable vertebrae consist of, from superior to inferior, 7 cervical (C), 12 thoracic (or dorsal) (T), and 5 lumbar (L). Inferior to the lumbar vertebrae, five sacral vertebrae (S) fuse in the adult to form the sacrum. The lowermost three to five vertebrae fuse late in adult life to form the coccyx (Co).
Intervertebral discs are located between the anterior portions of the movable vertebrae and between L5 and the sacrum. There is no disc located between the occiput and C1 (atlas), or between C1 and C2 (axis). The discs are named for the vertebra located immediately above the disc; that is, the T6 disc is located between the T6 and T7 vertebrae.
Seven processes arise from the posterior portion of the typical vertebra. Several atypical vertebrae have variations in their anatomy and are discussed in Chapters 5, 6, and 7. The spinous process is a midline structure that is directed posteriorly and to a variable degree inferiorly. The transverse processes are a pair of lateral projections. The other four processes are articular, and each vertebra has a superior pair and an inferior pair. These processes are discussed in greater detail in Chapter 2.
The remainder of this chapter discusses visual landmarks of the back, palpatory landmarks of the back, spinal cord levels versus vertebral levels, and vertebral levels of structures in the anterior neck and trunk. This information enables the clinician to gain a thorough understanding of surface anatomy and serves as a reference for future patient assessment, both in the physical examination and through diagnostic imaging procedures, including x-ray examination, CT, and MRI.
In the midline of the back is a longitudinal groove known as the median furrow (or sulcus) (Fig. 1-1). Superiorly it begins at the external occipital protuberance (EOP) (see the following discussion) and continues inferiorly as the gluteal (anal, natal, or cluneal) cleft (or crena ani) to the level of the S3 spinous tubercle, the remnants of the spinous process of S3. It is shallow in the lower cervical region and deepest in the lumbar region. The median furrow widens inferiorly to form an isosceles triangle with a line connecting the posterior superior iliac spines (PSISs) forming the base above, and the gluteal cleft forming the apex of the triangle below. The PSISs are often visible as a pair of dimples located 3 cm lateral to the midline at the level of the S2 spinous tubercle. The gluteal fold (or sulcus) is a horizontal skin fold extending laterally from the midline and roughly corresponds with the inferior border of the gluteus maximus muscle. This fold marks the lower extent of the buttock.
Several muscles are commonly visible in the back region. The trapezius is a large, flat, triangular muscle that originates in the midline from the EOP to the spinous process of T12 and inserts laterally onto the spine of the scapula. Its upper fibers form the “top of the shoulder,” where the neck laterally blends into the thorax. The latissimus dorsi, extending from the region of the iliac crest to the posterior border of the axilla, forms the lateral border of the lower thoracic portion of the back. This muscle is especially noticeable when the upper extremity is adducted against resistance. Between the trapezius medially and the latissimus dorsi laterally, the inferior angle of the scapula may be seen at approximately the level of the T7 spinous process. The erector spinae muscles form two large longitudinal masses in the lumbar region that extend approximately a handbreadth (10 cm) laterally from the midline. These muscle masses are responsible for the deepening of the median furrow in this region.
Besides these muscles, several bony landmarks usually are visible in the region of the back. The spinous process of C7 (the vertebra prominens) usually is visible in the lower cervical region. The spinous process of T1 often is visible also and actually is the most prominent spinous process in 30% to 40% of the population. When the patient’s head is flexed, the spinous processes of C7 and T1, and often C6, usually are seen easily.
In the adult the vertebral column has several visible normal curves. In the cervical and lumbar regions the spine is anteriorly convex (lordotic), and in the thoracic and sacral areas it is posteriorly convex (kyphotic). Normally there is no lateral deviation of the spinal column, but such curvature is known as scoliosis when present. These curves are covered in more detail in Chapter 2.
The following structures usually are not visible, but can be located on palpation. Some of the structures in this discussion of palpable landmarks cannot normally be felt, but their relation to landmarks that can be localized is given.
The EOP (inion) is in the center of the occipital squama (Fig. 1-2). The superior nuchal line extends laterally from the EOP. The transverse process of the atlas may be found directly below and slightly anterior to the mastoid process of the temporal bone. Care must be taken when palpating this structure because of the relatively fragile styloid process of the temporal bone that lies just in front and the great auricular nerve that ascends in the fascia superficial to the C1 transverse process.
The spinous process of the axis is the first readily palpable bony structure in the posterior midline below the EOP (see Fig. 1-2), although according to Oliver and Middleditch (1991) the posterior tubercle of C1 may be palpable in some people between the EOP and the spinous process of C2. In the midline below the spinous process of the axis, the second prominent palpable structure is the spinous process of C7 or the vertebra prominens. In 60% to 70% of the population the vertebra prominens is the most prominent spinous process, whereas the spinous process of T1 is more evident in the other 30% to 40%. The other cervical spinous processes are variably more difficult to palpate. The spinous process of C3 is the smallest and can be found at the same horizontal plane as the greater cornua of the hyoid bone. The spinous process of C6 is the last freely movable spinous process with flexion and extension of the neck. It is usually readily palpable with full flexion of the neck.
The zygapophysial joints between the articular processes of the cervical vertebrae (collectively known as the left and right articular pillars) can be found 1.5 cm lateral of the midline in the posterior neck. With the exception of C1, the tips of the transverse processes of the cervical vertebrae are not individually palpable, but the posterior tubercles of these processes form a bony resistance that may be palpated along a line from the tip of the mastoid process to the root of the neck, approximately a thumb breadth (2.5 cm) lateral of the midline. The anterior aspects of the transverse processes of the cervical vertebrae may be found in the groove between the larynx and sternocleidomastoid muscle (SCM). It may be necessary to slightly retract the SCM laterally to palpate these structures. The anterior tubercles of the transverse processes of C6 are especially large and are known as the carotid tubercles (see Fig. 1-2). These may be palpated at the level of the cricoid cartilage. Care must be taken when locating the carotid tubercles (and the other cervical transverse processes), because they are in the proximity of the common carotid arteries, and they always should be palpated unilaterally.
Anteriorly, the superior border of the thyroid cartilage, forming the laryngeal prominence (Adam’s apple) in the midline, may be used to find the horizontal plane of the C4 disc. The body of C6 is located at the same horizontal level as the cricoid cartilage and the first tracheal ring.
The spinous process of T1 is the third prominent bony structure in the midline below the EOP; the spinous processes of C2 and C7 are the first and second, respectively (Fig. 1-3). The spinous process of T3 is located at the same horizontal plane as the root of the spine of the scapula. The spinous process of T4 is located at the extreme of the convexity of the thoracic kyphosis; therefore it is usually the most prominent spinous process below the root of the neck.
When patients are standing or sitting with their upper extremities resting along the sides of their trunk, the inferior scapular angle usually is at the horizontal level of the spinous process of T7. This changes when the patient is lying prone with his or her upper extremities resting toward the floor in a flexed position (the most common posture of the patient when this region of the back is palpated). In this position the scapulae are rotated so that the T6 spinous process is more commonly found at the level of the inferior scapular angle.
The spinous processes of T9 and T10 often are palpably closer together than other thoracic spinous processes, but this is not a consistent finding. Located roughly halfway between the level of the inferior angle of the scapula and the superior margin of the iliac crests is the spinous process of T12.
Because the spinous processes of the thoracic vertebrae project in an inferior direction to different degrees, the remainder of the vertebrae are located variably superior to the spinous process of the same vertebral segment (Keogh and Ebbs, 1984). The tips of the transverse processes of T1-4 and T10-12 are located one spinous interspace superior to the tip of the spinous process of the same segment. The tips of the transverse processes of T5-9 are located two spinous interspaces superior to the tips of their respective spinous processes because these spinous processes project inferiorly to a greater degree. For example, the tips of the transverse processes of T3 are located in the same horizontal plane as the inferior tip of the spinous process of T2, whereas the tips of the transverse processes of T8 are at the same horizontal plane as the inferior tip of the spinous process of T6. The transverse processes of the thoracic vertebrae progressively get shorter from superior to inferior, so that the tips of the transverse processes of T1 are located 3 cm lateral to the midline, although those of T12 are 2 cm. Sometimes the transverse processes of T12 are small and not readily palpable. The angles of the ribs may be palpated 4 cm lateral to the midline at the horizontal levels of their respective transverse processes.
The posterior aspects of the spinous processes of the lumbar vertebrae differ from the thoracic vertebrae in that they present more of a flat surface. The spinous processes of L4 and L5 are shorter than the other lumbar spinous processes and are difficult to palpate, especially the L5 spinous process. The spinous process of L4 is the most inferior spinous process that has palpable movement with flexion and extension of the trunk. Usually it is in a horizontal plane with the superior margin of the iliac crests, although in approximately 20% of the population the iliac crests are even with the spinous process of L5 (Oliver and Middleditch, 1991).
The tips of the transverse processes of the lumbar vertebrae are located approximately 5 cm lateral to the midline and usually are not palpable. The mamillary processes are small tubercles on the posterior-superior aspect of the superior articular processes of the lumbar vertebrae. They are located approximately a finger breadth (2 cm) lateral to the midline at the level of the spinous process of the vertebra above and are not readily palpable.