The Longissimus Capitis Insertion as a Superficial Landmark for the Sigmoid Sinus: An Anatomical Study

Abstract

Objective  Many external anatomical landmarks have been used for approximating deeper, intracranial structures. Herein, we evaluate the attachment of the longissimus capitis (LC) on the mastoid process as a landmark for the underlying sigmoid sinus.

Methods  Adult cadavers underwent dissection of the posterior occiput with special attention to the attachment of the LC muscle. Once the periphery of the muscle's tendon of attachment was determined, a burr hole was made in this area and evaluated internally for its relationship to the sigmoid sinus.

Results  From an intracranial view, burr holes on all sides were over the sigmoid sinus and just slightly lateral to the center of the sinus. The distance from the midline to the medial border of the insertion of the LC had a mean of 63.0 ± 7.2 mm. The width of the tendon of insertion of the LC on the mastoid process had a mean of 17.6 ± 5.7 mm. The length of the tendon insertion of the LC had a mean of 14.7 ± 4.7 mm. The distance from the inferior border of the insertion of the LC to the tip of the mastoid process had a mean of 6.2 ± 4.5 mm.

Conclusion  To our knowledge, use of the attachment site of the LC on the mastoid process as an external landmark for the underlying sigmoid sinus has not previously been reported. Based on our cadaveric findings, the sigmoid sinus is centered under the attachment of the LC regardless of the width of its tendon.

Introduction

Even in an era of intraoperative neuronavigation, external estimation of internal structures, such as the dural venous sinuses, using superficial anatomical landmarks is still important to neurosurgeons. Many external anatomical landmarks have been used for approximating deeper, intracranial structures. ^{1 2 3 4 5 6 7 8 9} Some of these landmarks have been osteological features and others have been soft tissue structures. ^{5 7 9} For example, Tubbs et al used the semispinalis capitis (SSC) and its attachment on the occipital bone as a landmark for the underlying transverse sinuses and torcular herophili. ⁶ Therefore, our goal was to evaluate the attachment of the longissimus capitis (LC) on the mastoid process as an external landmark for the underlying sigmoid sinus ( Fig. 1 ). Such anatomical information might be useful to neurosurgeons.

Fig. 1.

Schematic drawing of the posterior occipital musculature. Note the relationship of the left longissimus capitis muscle insertion onto the skull (arrow) to the sigmoid sinus and skull base. The right side is a more superficial dissection with the trapezius and sternocleidomastoid muscles intact. On the right, just deep to these two muscles, one sees the splenius capitis muscle, which attaches onto the mastoid process and superior nuchal line. One the left side, the trapezius, sternocleidomastoid, and splenius capitis muscles are removed to illustrate the more deeply lying longissimus capitis muscle.

Materials and Methods

Fourteen sides from seven, fresh-frozen, Caucasian, cadaveric heads derived from three females and four males were used in this study. The mean age at death was 78.1 ± 12.7 years (range: 63–98 years).

Preparation

In the prone position, a transverse skin incision was made in the occipital area, 2 cm above the inion. Subsequently, a vertical skin incision was made to the posterior edge of the auricle and then connected with the first incision. Skin, subcutaneous tissue, and the trapezius muscle were removed to identify the underlying sternocleidomastoid muscle (SCM). The SCM was excised at its insertion along the mastoid process and superior nuchal line to expose the splenius capitis (SC) and SSC muscles. Next, the SC was cut at its insertion with a scalpel and the LC and its insertion were defined ( Fig. 2 ).

Fig. 2.

Step-by-step photos showing the dissection of the left longissimus capitis (LC: posterolateral view). White circle pointing the inion. ( A ) Before dissection, ( B ) dissection of the sternocleidomastoid muscle (SCM), ( C ) after reflection of the SCM. Splenius capitis (SC) and semispinalis capitis (SSC) are shown. ( D ) After reflection of the SC. LC is shown inserted on the mastoid process (MP).

Measurements

The distance from the midline to the medial border of the insertion of the LC, width of the insertion of the LC tendon, length of the insertion of the tendon of the LC, width of the of the LC muscle, and distance from the inferior border of the insertion of the LC to the tip of the mastoid process (MP) were measured ( Fig. 3 ).

Fig. 3.

Measurements made for this study. Distance from the midline to the medial border of the insertion of the longissimus capitis (LC) (1), width of the insertion tendon of the LC (2), length of the tendon insertion of the LC (3), width of the LC muscle (4), and distance from the inferior border of the insertion of the LC to the tip of the mastoid process (MP) (5).

Burr Hole

After measurements were performed, the LC was cut at its insertion and retracted inferiorly to expose the area of the insertion on the mastoid process. Then a burr hole was made on the center of the insertion of the LC using a high-speed drill (Medtronic, Minneapolis, Minnesota, United States) to penetrate the skull ( Fig. 4 ). Once the burr hole was made, the calvaria was opened and the contents of the intracranium were removed to observe the positional relationship between the burr hole and sigmoid sinus.

Fig. 4.

Burr hole craniotomy deep to the insertion of the longissimus capitis (LC) tendon. ( A ) White dashed line indicating the insertion of the tendon of the LC. ( B ) Burr made deep to the insertion of the LC tendon. MP, mastoid process; pDM, posterior belly of the digastric muscle.

Two clinical anatomists (J.I. and R.S.T.) performed all the dissections and measurements. All measurements were performed with a digital microcalipers (Mitutoyo, Kanagawa, Japan). The present study protocol did not require approval, and was performed in accordance with the requirements of the Declaration of Helsinki (64th World Medical Association General Assembly, Fortaleza, Brazil, October 2013). Statistical analysis between sex and sides was performed using Student's t -test with significance set at p  < 0.05.

Results

Overall, 11 sides (five right sides and six left sides) were available for this study. Three sides (right side from a 90-year-old female, right side from a 70-year-old female, and left side from a 63-year-old male) were excluded due to preexisting damage in the posterior auricular area. An LC muscle was found on all sides and on all 11 sides, attached on the mastoid process of the temporal bone. The distance from the midline to the medial border of the insertion of the LC ranged from 52.4 to 75.4 with a mean of 63.0 ± 7.2 mm. The width of the tendon of insertion of the LC on the mastoid process ranged from 13.2 to 34.7 mm with a mean of 17.6 ± 5.7 mm. The length of the tendon insertion of the LC ranged from 6.8 to 24.2 mm with a mean of 14.7 ± 4.7 mm. The width of the LC muscle belly just prior to its bony attachment ranged from 5.8 to 20.1 mm with a mean of 12.9 ± 4.4 mm. The distance from the inferior border of the insertion of the LC to the tip of the mastoid process ranged from 0 to 12.8 mm with a mean of 6.2 ± 4.5 mm. From an intracranial view, burr holes on all 11 sides were over the sigmoid sinus and just slightly lateral to the center of the sinus ( Fig. 5 ). There was no statistically significant differences between sex or sides in distance from the midline to the medial border of the insertion of the LC, width of the tendon of insertion of the LC, length of the tendon of insertion of the LC, width of the LC muscle belly just prior to its bony attachment, or distance from the inferior border of the insertion of the LC to the tip of the mastoid process.

Fig. 5.

Burr holes made into the sigmoid sinus (arrows).

Discussion

External landmarks on the skull have been used since the inception of modern neurosurgery to estimate intracranial structures. ^{1 3 10} For intracranial dural venous sinus localization, extracranial features of the bony skull, such as the sagittal suture and asterion, have been used. The sagittal suture has been used to visualize the internal course of the superior sagittal sinus and the asterion has been used to localize the junction between the sigmoid and transverse sinuses. ^{2 5} However, one study ² found that the sagittal suture, especially posteriorly, is often not directly overlying the superior sagittal sinus, which often deviates from it to the right. Similarly, the asterion, which has been used by many neurosurgeons as an external landmark, has been found to often be unreliable in accurately localizing the junction between the transverse and sigmoid sinuses. ²

Previously, we localized the junction between the sigmoid and transverse sinuses for application in placing a “strategic” burr hole for posterior fossa craniotomy, that is, inferior and medial to the junction of these two venous sinuses. ⁷ A horizontal line parallel to the zygomatic arch and a vertical line through the mastoid notch were used. On the majority of sides, the strategic burr hole was located inferior to the zygomatic arch line and medial to the mastoid line.

Soft tissues overlying the skull, such as the pinnae or medial canthus, have also been used to estimate deeper lying intracranial structures. As another example, we evaluated the intracranial anatomical relationships of the superficial temporal artery. ⁸ In the present study, we found that the insertion of the LC muscle on the mastoid process was an excellent landmark for identifying the underlying sigmoid sinus. The distance from the midline to the medial border of the insertion of the LC ranged from 52.4 to 75.4 with a mean of 63.0 ± 7.2 mm. The LC is part of the erector spinae muscle and arises from the transverse processes of the lower three to four cervical vertebrae and inserts on the mastoid process just deep to the SC muscle. These two muscles along with sternocleidomastoid share an attachment on the mastoid process. Although the attachment of the LC might not be seen at surgery, when it is in the surgical field, and based on our study, the sigmoid sinus will lie deep to it. This landmark, combined with others, might be useful as an adjunct to image guidance during skull base surgery. Many approaches near the skull base might not encounter the proximal LC as this muscle is not always dissected separately from overlying muscles. Moreover, the tip of the mastoid may not be identified and exposure of this part of the mastoid must be performed cautiously to avoid injury to the facial nerve. Additionally, the midline might lie under surgical drapes thereby making the measurements, obtained in the present study, difficult. However, when the muscle is encountered during surgical approaches and these landmarks are palpated, estimated or seen, knowledge of the LC attachment overlying the sigmoid sinus might be of use. Confirmation of such anatomical results with clinical application are now necessary.

Conclusion

Additional extracranial landmarks for localizing intracranial structures can be of use to the neurosurgeon. In concert with digital localization or additional superficial landmarks, more precise neurosurgery with fewer complications can be performed. To our knowledge, use of the attachment site of the LC on the mastoid process, as an external landmark for the underlying sigmoid sinus, has not previously been reported. Based on our cadaveric findings, the sigmoid sinus is centered under the attachment of the LC regardless of the width of its tendon. Surgical use and validation of these anatomical findings are now needed.