Anatomy of the Vertebral Pedicle

Fig. 1

(a) Schematic of a right-side view of a lumbar vertebra showing the position of the pedicle (P) between the vertebral body in front (1) and posterior arch. The posterior point of entry to the pedicle is located at the junction of the upper facet joint (2), the lamina (3) and the transverse process (4); it is separated from the lower facet joint (5) by the isthmus (6). We note the relationship of the pedicle with the exiting root from the intervertebral foraminal position (yellow circle). (b) Schematic oblique view of a lumbar vertebra showing the position of the pedicle (P) between the vertebral body anteriorly (1) and the posterior arch

Lateral or proximal escape of a screw during transpedicular instrumentation results in decreased mechanical strength, while a medial or distal breakage may be more likely to result in or spinal nerve root or cord injury, depending on the region.

The width of the pedicle is defined by the narrowest transverse diameter , and the height by the narrowest sagittal diameter. The transverse angle is defined by the obliquity in the transverse plane between the axis of the pedicle and the anteroposterior axis of the vertebra; this angle often has an anterior apex, thus convergent. The sagittal angle is defined by the obliquity in the sagittal plane between the axis of the pedicle and the superior endplate of the vertebra (Figs. 2 and 3). The transverse interpedicular distance is the distance between the centres of the pedicles of the same vertebra and the vertical interpedicular distance is the distance between the pedicles of two adjacent vertebrae.

../images/451372_1_En_10_Chapter/451372_1_En_10_Fig2_HTML.png — Fig. 2

Diagram of a vertebra, from above showing the transverse angle of the pedicle (between the two lines in black) and the transverse diameter of the pedicle (double red arrow) which is its smallest dimension

../images/451372_1_En_10_Chapter/451372_1_En_10_Fig3_HTML.png — Fig. 3

Diagram of a lateral view of a lumbar vertebra. The sagittal pedicle angle (between the two black lines) is almost nil. The double red arrow indicates the vertical diameter of the pedicle

The lateral and proximal cortices of the thoracic pedicles extend beyond the limits of the vertebral bodies and endplates, respectively, while at the lumbar level the vertebral bodies and endplates are beyond these same pedicular cortices [3].

Comparative Anatomy by Location

The height but especially the pedicle width are essential elements to know in order to adapt the diameter of the pedicle screw. Several cadaveric and radiological CT anatomical studies have been conducted to identify pedicle dimensions [3–5].

Pedicle width is the essential and determining element. It reaches its minimum at the level of T4 and T5 with an average pedicle diameter of 4.5 mm, sometimes with values <3 mm. This dimension increases by moving away from T4 and T5 to reach 8 mm at T1 and at T11.

At the lumbar level, the pedicle of L2 has the smallest width with an average of 6 mm but can sometimes fall well below this; the lumbar pedicle width increases progressively to reach 15 mm at L5. The pedicle height increases gradually from the T1 up to T12 where it reaches its maximum then decreases until L2 remaining similar through to L5. The height at T1 is on average 8 mm; the height at T12 is 17 mm; at the level of the lumbar region it is around 15 mm. Pedicle height is therefore not a limiting factor in pedicle screw instrumentation.

Orientation [6] (Figs. 2 and 3)

It is essential to know the orientation of the pedicle to optimize the quality of the pedicle screw.

In the sagittal plane, the sagittal pedicle angle is maximal at the thoracic level: it varies between 15 and 20° with a posterior and proximodistal (descending) orientation and then decreases very rapidly between T12 and L1 to reach 5° in L1 and a direction almost parallel with the endplate at L5. For an anatomic trajectory the thoracic pedicle screws are thus oriented more downward than lumbar screws.

In the transverse plane, the convergence is minimal at the level of the 12th thoracic vertebra with a transverse angle which is sometimes negative—a divergent pedicle; the average remains 5° of convergence nevertheless. This convergence increases progressively from T12 to reach 25° on average in T1 and L5. It should be noted that the pedicles between T10 and L1 have a mean convergence of <10°: the orientation when inserting a pedicle at these levels is “straight ahead”, without convergence.

Interpedicular Dimensions [3, 6]

The vertical interpedicular distance gradually increases from T1 to S1; this corresponds with the increase in the height of the vertebral body as well as the increase in the height of the intervertebral disc. This distance decreases with age especially at the lumbar level, due to natural disc narrowing.

The interpedicular transverse distance is minimal at the level of the mediothoracic region (T4 to T8) where it measures 20 mm on average. It increases gradually to T1 where it reaches 30 mm, and it also increases towards L5, where it reaches almost 40 mm.

To these quantitative parameters should be added qualitative parameters concerning the abundance of the cancellous bone within the pedicle; indeed in some cases the outer and inner cortices touch each other and there is no presence of cancellous bone between the two, which makes a classical pedicle target impossible (Fig. 4).

../images/451372_1_En_10_Chapter/451372_1_En_10_Fig4_HTML.jpg — Fig. 4

CT scan of a teenage girl with idiopathic scoliosis. We note the complete disappearance of the cancellous bone and the thin diameter of the right pedicle. This is the anatomical level where the pedicles are the thinnest; to this complexity must be added the effect of the scoliotic concavity. Classic pedicle screw insertion is impossible on the right whereas it is easy to access on the left

Radiological Assessment of Pedicle Dimensions

All the elements just mentioned and analysed vary according to each individual. This anatomical variability depends on the patient’s age, size and morphology.

In order to plan a surgical procedure in which a pedicle fixation is required, one should conduct a radiological estimation of these different dimensions and orientations. Standard radiography can be used which is sufficient in the majority of cases, especially when the pedicle width exceeds 5 mm. This corresponds with the anteroposterior radiographs that show well rounded pedicles with dense cortical and clearer cancellous bone, clearly visible inside. For this, radiography of good quality is necessary. EOS radiography meets these objectives. CT imaging allows a more accurate measurement of the pedicle dimensions but with much greater irradiation of the patient. It is useful in cases where standard radiography does not allow visualization of the pedicles. MRI with fine cuts, much more expensive, also allows morphological analysis of the pedicles without the drawbacks of the irradiation of the scanner. Up to date, the reference method remains CT. All these examinations will allow preoperative planning of the instrumentation and will determine the possibility of free hand screw insertion or with radiological or navigation assistance.

Applications to the Target Pedicle and Contribution of Navigation

The classic transpedicular trajectory is through the spongiosa or cancellous bone present in the pedicle canal. It requires the presence of this cancellous canal through which a perforator can find the path. The drill or probe will follow the path of least resistance and thus allows the cannulation of the pedicle without breaking the walls. When this channel is absent or when it is very thin, the classic direct path becomes impossible, and it may be necessary to replace it either by a hook at the level of the posterior arch or through an extrapedicular that will end in the vertebral body—the “in-out-in” technique. When the anatomical landmarks at the level of the posterior arc are intact and when the pedicle diameter permits, using a free hand technique, without the use of special technology, is very possible and gives satisfactory results with extremely low risk of screw mal-placement. Where this is not possible, the use of fluoroscopic guidance or even better with navigation guidance, it is possible to increase the precision of this technique.

Point of Entry to the Pedicle [7] (Fig. 5)

The posterior entrance of the pedicle lies at the junction of the superior articular facet, the transverse process and the lamina; its exact position is variable according to the vertebral level. The exact knowledge of the point of entry and the orientation of the pedicle is necessary.