Modified Skin Incision and Location of Burr-Hole Surgery via a Retrosigmoid Approach: An Anatomical Study

Abstract

Objective  This study aims to reduce the tissue damage during craniotomy with retrosigmoid approach. A modified sickle-shaped skin incision was developed, and a new burr-hole positioning method was proposed.

Methods  Five adult cadaveric heads (10 sides) were used in this study. The sickle-shaped skin incision was performed during craniotomy. The nerves, blood vessels, and muscles were observed and measured under a microscope. Additionally, 62 dry adult skull specimens (left sided, n  = 35; right sided, n  = 27) were used to measure the distance between the most commonly used locating point (asterion [Ast] point) and the posteroinferior point of the transverse sigmoid sinus junction (PSTS) (Ast-PSTS), as well as the distance between the new locating O point and the PSTS (O-PSTS). Then, the reliability of the new locating O point was validated on the same five adult cadaveric heads (10 sides) used for the sickle-shaped skin incision.

Results  The sickle-shaped skin incision reduced the damage to the occipital nerves, blood vessels, and muscles during the surgery via a retrosigmoid approach. The dispersion and variability of O-PSTS were smaller than those of Ast-PSTS.

Conclusion  The sickle-shaped skin incision of the retrosigmoid approach can reduce the tissue damage and can completely expose the structures in the cerebellopontine angle. The modified O point is a more reliable locating point for a burr-hole surgery than the Ast point.

Introduction

The retrosigmoid approach is a multifunctional and safe surgical approach that can be used for resecting various lesions located in the cerebellopontine angle (CPA), such as vestibular schwannoma (acoustic neuroma) and meningioma. Since Krause developed the unilateral suboccipital approach at the beginning of the 20th century, ¹ this approach has been modified and popularized by Samii and many other scholars. ^{2 3 4} Therefore, the conventional retrosigmoid approach has become the most widely used method for the resection of CPA lesions, such as vestibular schwannoma (acoustic neuroma), meningioma, and microvascular decompression. The retrosigmoid approach uses the common skin incisions, including the “straight incision” and “lazy S-shaped incision.” ⁵ Intraoperatively, damage to the occipital nerves, blood vessels, and muscles is usually inevitable. The occipital artery (OA) provides blood supply to the occipital muscles, nerves, part of the dura mater in the posterior cranial fossa, and the occipital scalp. Injury of the OA can weaken the blood supply and delay healing of the surgical incision. ⁶ Additionally, postoperative head and neck pain is associated with occipital nerve cutting and scar compression of muscle healing following muscle incision. ⁶ Some scholars have highlighted the necessity of intraoperative protection of the lesser occipital nerve (LON), ^{7 8 9 10 11} and consequently, the “C-shaped incision,” “U-shaped incision,” and “crutchlike incision” were developed. ^{1 7 8 12}

The anatomical relationship of the occipital nerve and blood vessels, which is shown in Fig. 1 , is important. The greater occipital nerve (GON) originates from the posterior medial branch of the C2 nerve and connects with the posterior branch of the C3 nerve. The GON, located ∼5 cm laterally from the posterior median line, conducts the skin sensation in the medial side of the posterior neck and scalp. ^{13 14} The LON originates from the posterolateral branch of C2 and C3 nerves ^{8 15 16} and can be classified into two types. The type I nerve (60%) is close to the posterior edge of the sternocleidomastoid muscle (SCM), and part of the type I nerve crosses the anterior edge of the SCM near the mastoid process; the type I nerve is located 7 cm laterally to the posterior median line. Type II nerve (40%) is 4.5 cm posterior to the posterior edge of SCM. ¹² The LON comprises sensory and motor branches and conducts the skin sensation in the occipital, mastoid, and temporal regions. ^{10 15 17} The great auricular nerve (GAN) originates from the ventral branches of C2 and C3 nerves and rises on the anterior or posterior external surface of the SCM. The mastoid branch of the GAN is ∼9 cm laterally to the posterior median line, and the trunk of the GAN is located 1 cm above the mastoid tip. The GAN is a sensory nerve, mainly distributed in the auricle and parotid regions. ^{16 18} The OA originates from the posterior part of the external carotid artery, ascends to the mastoid area of the medial digastric muscle, and then ascends along the sulcus of the OA. The suboccipital end of the OA is always under the splenius capitis muscle (SpC) and continues to ascend along the upper neckline, accompanied by the GON and ∼4 to 5 cm away from the posterior median line; it is distributed in the superficial layer of galea aponeurotica. ^{14 19 20 21} Based on the above anatomical features, we proposed a modified sickle-shaped skin incision that may reduce the iatrogenic damage to the occipital structures.

Fig. 1.

Anatomy of the superficial muscles, nerves, and blood vessels in the occipital region. GAN, great auricular nerve; GON, greater occipital nerve; LON, lesser occipital nerve; OA, occipital artery; SCM, sternocleidomastoid muscle; SpC, splenius capitis muscle; trapezius, trapezius muscle.

In addition to reducing soft tissue injury during the retrosigmoid craniotomy, the location of the burr hole is also an important issue. Accurate localization not only reduces the damage to the venous sinus but also provides the maximum exposure range, reducing the operation time, and reduces the risk of complications.

The optimal location of the burr hole should be in the posteroinferior point of the transverse sigmoid sinus junction (PSTS). At present, the asterion (Ast) point is the most widely used landmark for locating the burr hole. However, the anatomical location of the Ast point has remarkable variability, and thus, it is difficult to accurately locate the burr hole intraoperatively. This study aimed to investigate the relevant anatomical characteristics and propose a new method for locating the PSTS.

Materials and Methods

Five wet cadaveric head specimens were fixed with 10% formalin. Red and blue silica gels were injected into the arteries and veins, respectively. The microanatomical observation was performed under a Leica microscope (Leica M320 F12 surgical microscope; Leica Microsystems Inc., Buffalo Grove, Illinois, United States). Wet specimens and 62 dry skull specimens were provided by the Anatomy Laboratory of Wannan Medical College. This study was approved by the Ethics Review Committee of Wannan Medical College.

Under the microscope, wet specimens were used for microsurgical simulation. A modified sickle-shaped skin incision was performed. The structures were dissected layer by layer and photographed. First, we located the anatomical landmarks, including the SCM, mastoid, digastric groove (DG), posterior median line, and upper edge of the zygomatic arch. Then, we confirmed the locations of the GON, LON, GAN, and OA. Next, we made a 10-cm sickle-shaped skin incision, the upper edge of the incision is located 1 cm distal to the superior margin of the auricle, and the lower edge of the incision is 1 cm parallel to the inferior mastoid. The lower boundary was ∼1 cm posterior to the trailing edge of the SCM. The entire skin incision was located within the hairline ( Fig. 2A ). This incision was, by our designed, to be located between the LON and GON to avoid potential damage to the LON, GON, and OA. Additionally, the risk of SCM cutting may be avoided.

Fig. 2.

Simulation of the intraoperative anatomy. ( A ) The sickle-shaped skin incision is ∼10 cm in length. The upper edge of the incision is located 1 cm distal to the superior margin of the auricle, and the lower edge of the incision is 1 cm parallel to the inferior mastoid, which is 1 cm lateral to the SCM. ( B ) The SCM is flipped to the ear, and the black arrow indicates the SpC fibers. ( C ) After flipping of the muscles to the neck, the mastoid and digastric sulcus can be completely exposed. ( D ) The O point is used as the burr hole for grinding off the bone flap. SCM, sternocleidomastoid muscle; SpC, splenius capitis muscle.

After the skin incision, the skull was easily exposed above the superior nuchal line, and the SpC was visible below the superior nuchal line. The operative procedure should be gentle and careful in this step. As the OA runs in the lower layer of the SpC, the correct localization of the SpC can avoid damage to the OA. We dissected the flap between SpC and SCM, flipped the flap with SCM, and then exposed SpC completely ( Fig. 2B ). The tendon at the superior nuchal line was retained with a width of 0.5 cm for postoperative suturing, and then all the remaining muscles were dissected. The SpC tendon at the root of the mastoid process was released ( Fig. 2C ). After the bone flap was ground and the mastoid and foramen magnum were peeled off, the surgical field was sufficiently exposed ( Fig. 2D ).

Furthermore, we measured the anatomical structures using dry skull specimens, and the corresponding points on the skull surface were located using a long-arm vernier caliper, which facilitated the localization of the projection point of the PSTS on the skull surface ²² ( Fig. 3A ). When the tip of one vernier caliper arm was placed on the PSTS on the interior surface of the skull ( Fig. 3B ), the landing point of the other vernier caliper arm reached the corresponding PSTS on the exterior surface of the skull, and the tip of the other vernier caliper arm was required to be perpendicular to the surface of the skull ( Fig. 3C ). The distance between the Ast point and PSTS (PSTS-Ast) was measured. The extension line of the superior margin of the zygomatic arch (zygomatic line) was taken as the X -axis, and the vertical line of the X -axis transiting the end point of the digastric groove (EPDG) was taken as the Y -axis. The intersection point was defined as the O point. The distance between the O point and PSTS projection (O-PSTS) was measured. The distance between the EPDG and PSTS (EPDG-PSTS) was also measured ( Fig. 3D ). After statistical analyses, the results were validated in wet head specimens.

Fig. 3.

Localization of the burr hole. ( A ) Long-arm vernier caliper and ordinary vernier caliper (precision 0.02 mm) were used. ( B ) The tip of one vernier caliper arm was placed on the PSTS on the interior surface of the skull. ( C ) The landing point of the other vernier caliper arm reached the corresponding PSTS on the exterior surface of the skull. ( D ) In a left skull specimen, PSTS-Ast was measured. The extension line of the superior margin of the zygomatic arch (zygomatic line) was taken as the X -axis, and the vertical line of the X -axis transiting the EPDG was taken as the Y -axis. The intersection point was defined as the O point. Ast, asterion; ASTS, anterosuperior point of the transverse sigmoid sinus junction; CSTS, central point of the transverse sigmoid sinus junction; EPDG, end point of the digastric groove; PSTS, posteroinferior point of the transverse sigmoid sinus junction; PSTS-Ast, distance between the PSTS and asterion; SS, sigmoid sinus; TS, transverse sinus.

The data were expressed as mean ± standard deviation. All the data were analyzed by Student's t -test. When p -value is less than 0.05, there is a statistically significant difference.

Results

Validation of the Skin Incision

The sickle-shaped skin incision was performed layer by layer. First, the body surface landmarks, including the SCM, mastoid, DG, posterior median line, and upper edge of the zygomatic arch were located, and the location of the surgical incision was determined ( Fig. 4A ). After flipping the scalp, the LON and GAN were located on the ventral side of the incision, while the GON and OA were located on the dorsal side of the incision, with the LON being near the posterior edge of the SCM ( Fig. 4B ). Then, we dissected the fascia between the SCM and SpC, and the SCM and occipitofrontal muscle were flipped to the ear. The SpC was completely exposed after these steps. Below the upper neck line, the SpC fibers running from the back to the front to the ear side were visible ( Fig. 4C ). When the SpC was flipped downward, we noted that the OA went through the longissimus capitis muscle and ran below the SpC ( Fig. 4D ). Then, when the longissimus capitis muscle was flipped downward, the OA penetrated behind the digastric muscle and the deep part of the mastoid process, and the mastoid branch of OA went into the skull ( Fig. 4E ). The SpC, longissimus capitis muscle, superior oblique muscle, digastric muscle, and OA were dissected along the periosteum and then turned to the cervical side. Tissue was retained with a width of ∼0.5 cm at the upper neckline for suturing after cranial closure, and the occipital foramen magnum, jugular foramen, and vertebral artery were sufficiently exposed ( Fig. 4F ). Given the route of the OA, the sickle-shaped skin incision avoided injury to the OA and vertebral artery while providing sufficient exposure.

Fig. 4.

Partial dissection of cadaver specimens. ( A ) The incision was simulated on cadaver specimens, and the distance from the incision to the external occipital eminence was ∼6 cm. ( B ) The LON and GAN were located on the ventral side of the incision, while the GON and OA were located on the dorsal side of the incision. ( C ) The SCM was flipped to the ear, and the SpC was exposed. ( D ) The SpC was flipped to the neck, and the OA was found to run below the longissimus capitis. ( E ) The longissimus capitis was flipped to the neck, and the occipital artery was found to pass below the mastoid and below the digastric muscle. ( F ) The digastric muscle, superior oblique muscle, and OA were flipped to the cervical side, and the vertebral artery was exposed. GAN, great auricular nerve; GON, greater occipital nerve; LON, lesser occipital nerve; OA, occipital artery; SCM, sternocleidomastoid muscle; SpC, splenius capitis muscle.

Localization of the Burr Hole

Among the 62 dry skull specimens, there were four types of digastric sulcus: long and narrow type ( Fig. 5A ), wide and large type ( Fig. 5B ), bifurcation type ( Fig. 5C ), and OA sulcus interference type ( Fig. 5D ). The EPDG was located at the top of the DG and near the root of the mastoid (as indicated by black dots in Fig. 5 ).

Fig. 5.

Variation types of the digastric groove (black dot indicating the EPDG close to the root of the mastoid process). ( A ) Long and narrow type; ( B ) wide and large type; ( C ) bifurcation type; and ( D ) occipital artery sulcus interference type. EPDG, end point of the digastric groove.

The measurement results for the skull surface were marked on the coordinate map using the scatter point method ( Fig. 6A ). The average PSTS-Ast distance was 1.655 ± 0.561 cm, with the greatest dispersion. The average O-PSTS distance was 0.576 ± 0.260 cm, with the smallest dispersion. The average distance of EPDG-PSTS distance was 1.595 ± 0.492 cm. Significant differences were observed between O-PSTS and the other two parameters ( p  < 0.05), whereas no statistical difference was found in O-PSTS between the left and right sides ( Fig. 6B ). Additionally, we found that the O point was closer to the PSTS in 43 cases (69.35%) and closer to the anterosuperior point of the transverse sigmoid sinus junction (ASTS) in 14 (22.58%) cases; the O point was in the central point of the transverse sigmoid sinus junction (CSTS) in 5 (8.06%) cases ( Fig. 6C ). The average distance between the CSTS and the O point was 0.393 ± 0.187 cm. The measured data are shown in Table 1 . There was no significant difference between the left and right sides of O-PSTS ( p  > 0 05).

Fig. 6.

Statistical results analysis of data of the measured distances. ( A ) Scatter plots of the measured distances. There were significant differences between O-PSTS and the other two parameters ( p  < 0.05). ( B ) There was no significant difference in the O-PSTS distance between the left-sided skull and right-sided skull. ( C ) Among the 62 specimens, the O point was closer to the PSTS in 69.35% specimens, closer to the ASTS in 22.58% specimens, and located in the CSTS in 8.06% specimens. ASTS, anterosuperior point of the transverse sigmoid sinus junction; CSTS, central point of the transverse sigmoid sinus junction; O-PSTS, distance between the O point and PSTS projection; PSTS, posteroinferior point of the transverse sigmoid sinus junction.

Table 1. The average distance (cm) between each positioning point and the PSTS on each side of the skull.

Groups	Right side ( n  = 27)	Left side ( n  = 35)	p -Value
O-PSTS	0.609 ± 0.272	0.551 ± 0.252	0.389 a
EPDG-PSTS	1.604 ± 0.499	1.658 ± 0.492	0.671 b
Ast-PSTS	1.666 ± 0.610	1.652 ± 0.528	0.928 c

Abbreviations: Ast, asterion; Ast-PSTS, distance between the Ast and PSTS; EPDG, end point of the digastric groove; EPDG-PSTS, distance between the EPDG and PSTS; O-PSTS, distance between the O point and PSTS projection; PSTS, posteroinferior point of the transverse sigmoid sinus junction.

Note: The distances of the left skull of O-PSTS ( p  = 0.389 ^a ), EPDG-PSTS ( p  = 0.671 ^b ), and Ast-PSTS (0.928 ^c ) were not statistically significantly different from those of the right.

Finally, the validation measurements were performed on the five wet specimens. Among the 10 sides of the five wet specimens, the O point was near the PSTS on the 8 sides of the specimens ( Fig. 7A ) and close to the ASTS on two specimens ( Fig. 7B ). When the O point was near the ASTS, during the process of drilling, the PSTS could be found backward and downward according to the actual situation.

Fig. 7.

Validation of burr holes in wet specimens. ( A ) The burr hole located in the O point was close to the PSTS. ( B ) The burr hole located in the O point was close to the ASTS. ASTS, anterosuperior point of the transverse sigmoid sinus junction; EPDG, end point of the digastric groove; PSTS, posteroinferior point of the transverse sigmoid sinus junction.

After grinding out the burr hole, we used a milling cutter to grind a 4 × 4 cm bone flap. After the bone flap was removed, CPA was sufficiently exposed, which was demonstrated by the exposure of the vagus nerve ( Fig. 8 ).

Fig. 8.

Exposure of the CPA area. ( A ) CPA area on the right. ( B ) CPA area on the left. CN V, trigeminal nerve; CN VIII, acoustic nerve; CN IX, glossopharyngeal nerve; CN X, vagus nerve; CPA, cerebellopontine angle; pet. V, petrosal vein; SS, sigmoid sinus; TS, transverse sinus.

Discussion

The conventional retrosigmoid approach applies a “straight incision” or “lazy S-shaped incision,” ^{5 7} which inevitably results in injury of the suboccipital nerve, blood vessels, and muscles during the operation. Specifically, cutting of the LON, GON, OA, and suboccipital muscles may be unavoidable. The OA provides blood supply to the occipital muscles, nerves, part of the dura mater of the posterior cranial fossa, and the occipital scalp. Injury of the OA can result in insufficient blood supply in the occipital area, which may lead to delayed healing of the incision. ⁶ The occipital nerve may be cut or muscle scarring may compress the occipital nerve, which may explain the occurrence of head and neck pain postoperatively. To date, some scholars have highlighted the importance of intraoperative protection of the LON and developed “C-shaped incisions,” “U-shaped incisions,” and “crutch-like incisions.” ^{1 7 8 12} However, these novel incisions have many limitations. Some incisions can only be used for microvascular decompression, and others can only be used for endoscopic surgery. Additionally, the risk of muscle injury cannot be avoided.

Our modified retrosigmoid approach may reduce postoperative complications. First, the skin incision is elaborately planned using multiple body-surface landmarks, which can avoid damage to the trunks of the GON, GAN, OA, and LON (injury of the occipital branch of the LON is unavoidable). During the anatomical dissection, we found a thick fascia between the SCM and SpC. ¹⁹ Through this fascia layer, the SCM can be completely separated from the SpC, and muscle cutting can be avoided. As the OA originates from the posterior part of the external carotid artery, ascends to the medial side of the digastric muscle, and then ascends from the sulcus of the OA, the suboccipital segment of the OA is always under the SpC and continues to ascend at the superior nuchal line. ^{19 20} After the scalp incision, the SpC running from the back to the front can be visible, which is the key feature for avoiding OA damage. ²¹ According to the route of the OA, it will not be hurt when the SpC is flipped laterally to the neck. ²³ When flipping the SpC, its tendon on the mastoid should be loosened to achieve greater exposure. Protection of the OA is beneficial to the healing of the incision and promotes postoperative recovery.

The accurate localization of the burr hole in the retrosigmoid approach is usually challenging, which may cause venous sinus injury, underexposure, and prolonged operation time. The optimal location of the burr hole should be in the PSTS. ²⁴ At present, most scholars use the Ast point as the superficial sign of the skull for locating the burr hole. ^{21 25} Rhoton placed a vertical incision on the Ast and found the PSTS on the upper lateral side of the incision. ²⁵ Ribas et al proposed that the PSTS is ∼1 cm in front of the Ast. ²⁵ However, Ast variability is so great that it is difficult to locate accurately during the operation.

In the measurement of 62 dry skulls, the distances of PSTS-Ast, O-PSTS, and PSTS-EPDG were measured and statistically analyzed. The distance between the PSTS and Ast showed the greatest dispersion and variability, and therefore, the localization of the burr hole via the Ast is difficult. In contrast, the dispersion and variability of the distance between the O point and PSTS were the smallest. Additionally, we analyzed the left-sided and right-sided measurements for O-PSTS distance and found no statistical difference of the O-PSTS distance between the left side and right side. The anatomical location of the O point is stable, and it may be used as a more accurate and convenient landmark for intraoperative localization of the PSTS.

During the actual skull measurement, we found that the O point was closer to the PSTS in 43 (69.35%) cases, closer to the ASTS in 14 (22.58%) cases, and located in the CSTS in 5 cases (8.06%). Then, we further measured the distances from the CSTS to the O point and compared them with O-PSTS. These results showed that the O point was closer to the PSTS. In the 62 skull specimens, 56 (90%) had an O-CSTS distance less than 0.6 cm. Moreover, care should be taken in procedures when the O point is taken as the location of the burr hole. Tearing of the venous sinus should be avoided. Finally, the validation in wet skull specimens demonstrated that this localization method can be used for localization of the burr hole.

At present, neuronavigation has been used to locate the burr hole in microsurgery via retrosigmoid approach in developed countries. ²⁶ But for hospitals in developing countries or lack of equipment, neuronavigation is almost impossible due to lack of equipment or because it will increase the financial burden on patients. Therefore, it is important to locate the burr hole by the body surface marks, which could not only reduce the surgical injury but also reduce the financial burden of the patients.

These are still some limitations to this surgical incision. The modified sickle-shaped skin incision can avoid injury to the trunk and auricular branch of the LON, but it cannot avoid the injury to the occipital branch of the LON. This incision is longer than the traditional straight incision, and surgeons need to be familiar with the occipital anatomy. The elasticity of skin and muscle of the cadaveric specimen is poor, which limited the maximum exposure range. Up to date, our innovative retrosigmoid approach has not been implemented in clinic.

Conclusion

The sickle-shaped skin incision of the retrosigmoid approach can reduce tissue injury and can completely expose the structures in the CPA. The innovative O point is a more reliable locating point for a burr hole used in the retrosigmoid approach than the Ast point.

Funding Statement

Funding The National Natural Science Foundation of China, 0601011501, Xiaochun Jiang; the Natural Science Foundation of Anhui Province, 060108021802 and 060108011802, Xiaochun Jiang.