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An anatomical study of the suprascapular nerve
Arthroscopy: The Journal of Arthroscopic and Related Surgery 6(4):301-305 Published by Raven Press, Ltd. 0 1990 Arthroscopy Association of North America
An Anatomical Study of the Suprascapular Nerve Louis U. Bigliani, M.D., Robert M. Dalsey, M.D., Peter D. McCann, M.D., and Ernest W. April, Ph.D.
Summary: The course of the suprascapular nerve and its distance from fixed scapular landmarks were measured in 90 cadaveric shoulders. In an additional 15 cadavers, three pins were passed at various angles in a general anteriorposterior direction through the middle of the glenoid neck just inferior and lateral to the base of the coracoid process. The distance between the exit site on the posterior glenoid neck and the suprascapular nerve at the base of the scapular spine was recorded for each pin. Inferiorly directed pins were the furthest from the suprascapular nerve and averaged 16 mm. On the basis of these data, a relative safe zone is described in the posterior glenoid neck.
Knowledge of the anatomic course of the suprascapular nerve may aid the physician in the diagnosis and treatment of suprascapular neuropathies. Appreciation of the safe zone may help the shoulder surgeon avoid iatrogenic injury to the suprascapular nerve during arthroscopic Bankart procedures and other open surgical procedures requiring dissection of the posterior glenoid neck. Key Words: Suprascapular nerve-Anatomy-Bankart repair-Safe zone.
sponse to traction (13), cutaneous sensory branch distribution (14), relative points of nerve fixation at the suprascapular notch and the scapular spine (15), and finally, the relationship of the suprascapular nerve to the spinal glenoid ligament at the base of the scapular spine (16). However, little data exist concerning the course of the suprascapular nerve, its levels of muscular innervation, and its relationship to anatomic surgical landmarks. The purpose of this anatomical study is to examine the course of the suprascapular nerve, measure its distance from fixed scapular landmarks, and determine the levels of innervation of the supraspinatus and infraspinatus muscles. With this information, a potential safe zone can be established for pin placement when performing blind drilling through the glenoid neck for arthroscopic repair of anterior Bankart lesions and for open procedures requiring dissection in the posterior region of the shoulder.
A thorough anatomical knowledge of the course of the suprascapular nerve is important in shoulder surgery. This information can be useful in preventing potential iatrogenic injury during open surgical procedures for posterior instability (l-4), repair of massive rotator cuff tears (5-8), decompression of suprascapular nerve entrapment @lo), as well as newer arthroscopic procedures for repair of anterior glenohumeral instability (11). Previous anatomical studies of the suprascapular nerve have concentrated on cervical root contribution (12), shape of the suprascapular notch, translational movement of the suprascapular nerve in re-
From the Department of Orthopaedic Surgery, Columbia Rresbyterian Medical Center (L.U.B., R.M.D.), New York, New York; the Department of Orthopaedic Surgery (P.D.M.), Helen Hayes Hospital, West Haverstraw, New York; and the Department of Anatomy and Cell Biology (E.W.A.), College of Physicians & Surgeons, Columbia University, New York, New York, U.S.A. Address correspondence and reprint requests to Dr. Louis U. Bigliani, c/o Dr. Peter D. McCann, Helen Hayes Hospital, Dept. of Orthop. Surgery, Route 9W, West Haverstraw, NY 10993, U.S.A.
MATERIALS AND METHODS Ninety cadaveric shoulders were dissected to study the course of the suprascapular nerve, the 301
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level of muscle innervations, and its distance from standard bony landmarks. There were 53 male and 37 female cadavers of an average age of 78 years (range 54-90 years). The distance from the suprascapular notch to the point of innervation of the supraspinatus muscle was measured, as well as the distance from the base of the scapular spine to the point of innervation of the infraspinatus muscle. Three distances between the suprascapular nerve and bony landmarks were measured (Fig. 1): supraglenoid tubercle (SGT) to the nerve at the suprascapular notch (SSN), supraglenoid tubercle to the nerve at the base of the spine of the scapula (BSS), the midline of the posterior glenoid rim (PGR) to the nerve at the base of the scapular spine. In an additional 15 cadavers, three pins (0.62-mm Kirschner wires) were passed at various angles in a general anterior-posterior direction through the middle of the glenoid neck just inferior and lateral to the base of the coracoid process and -1cm medial to the articular surface. One pin was passed parallel to the glenoid articular surface, one pin passed as medial as possible, and one pin aimed as inferior as possible. The distance between the exit site on the posterior glenoid neck and the suprascapular nerve at the base of the scapular spine was recorded for each pin.
RESULTS The suprascapular nerve passed through the scapular notch and inferior to the transverse scapular ligament in all specimens (90/90). The motor nerves to the supraspinatus muscle branched within 1 cm of the suprascapular notch in all specimens @O/90) (Fig. 1). From the suprascapular notch, the suprascapular nerve coursed obliquely and laterally in the supraspinatus fossa along the inferior surface of the supraspinatus muscle directly to the base of the scapular spine. The spinal glenoid ligament at the base of the scapular spine was difficult to identify in some of our cadaveric specimens, and therefore, could not be consistently recorded in this study. From the base of the scapular spine, the suprascapular nerve curved medially and innervated the infraspinatus muscle within 1 cm of the base of the scapular spine in 89% of specimens (80190). Distance A (Fig. l), from the supraglenoid tubercle to the nerve at the suprascapular notch, averaged 3.0 cm (range 2.5-3.9 cm). Distance B (Fig. I), from the supraglenoid tubercle to the nerve at the base of the spine at the scapula, averaged 2.5 cm (range 1.9-3.2 cm). Distance C (Fig. l), from the midline of the posterior glenoid rim to the nerve at the base of the scapular spine, averaged 1.8 cm (range 1.62.5 cm). These results are summarized in Table 1. In the 15 specimens with muiltiple pin placement, the distance from the posterior exit site to the suprascapular nerve at the base of the scapular spine for the pins directed medially averaged only 4 mm (range l-8 mm) (Table 2). The distance for the pins directed parallel to the articular surface averaged 6 mm (range 1-14 mm), and the distance for the inferiorly directed pins averaged 16 mm (range 4-35 mm). Among the inferiorly directed pins, only one (l/15) exited within 12 mm of the suprascapular nerve.
TABLE 1. Lengths of distances illustrated in Fig. 1 Distance
FIG. 1. Course of the suprascapular nerve. Diagram of posterior scapula illustrating passage of the suprascapular nerve inferior to the transverse scapular ligament and innervation of the spinati. Distances A, B, and C are described in Table 1.
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A, Supraglenoid tubercle to suprascapular notch B, Supraglenoid tubercle to base of scapular spine C, Midline posterior glenoid rim to base of scapular spine
Length (cm) (range) (2.E.9) (LE.2) (I.&)
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TABLE 2. Distance between the posterior exit site of pins and the suprascapular nerve at the base of the scapular spine Direction of pins (relative to glenoid surface)
Distance (mm) (range)
Medial (l-46) Parallel (l-614, Inferior $5)
DISCUSSION The findings of the course of the suprascapular nerve from the suprascapular notch to the innervation of the infraspinatus muscle determined in this study concur with the findings described in standard texts (17,18). To our knowledge, no complications involving the suprascapular nerve following shoulder surgery have been reported in the English literature. Nevertheless, the potential for iatrogenic suprascapular nerve injury does exist due to the proximity of the nerve to the operative field. Several authors have recently reported on arthroscopic techniques in the surgical management of anterior shoulder instability (11). The ‘‘Arthroscopic Bankart Procedure” is essentially a modification of an open technique first reported by Lucky in 1949 (19). This procedure was further modified by Viek and Bell in 1959 to include braided wire pull out sutures (20) and updated by Reider and Inglis (21) in 1982 to include the use of prolene pull-out sutures. The arthroscopic technique requires the placement of sutures in the detached anterior labrum and capsule and then the passage of the sutures posteriorly through drill holes placed in the glenoid neck. By pulling the sutures tightly posteriorly, the labrum is reduced to the anterior glenoid rim, and the sutures tied over the infraspinatus. This arthroscopic Bankart procedure requires blind drilling through the glenoid neck, which may place the suprascapular nerve at risk for injury posteriorly. The findings of this study showed that pins directed medially place the suprascapular nerve at greatest risk (average distance 4 mm; see Figs. 2-4). Several factors contribute to medial pin placement. First, the transverse axis of the scapula (i.e., scapula plane) lies -35” anterior to the mid-coronal plane of the body. An anterior-posterior directed pin beginning at the anterior glenoid rim and passed straight pos-
FIG. 2. A scapular model viewed from above showing the course of a medially directed K-wire.
terior will, by necessity, exit medially on the posterior glenoid neck. Second, the glenoid surface has a normal 7” of retroversion relative to the transverse axis of the scapula (22). Consequently, it is the natural tendency of the arthroscopic surgeon to aim these pins medially in an effort to avoid violating the articular surface of the glenoid. The results of this study confirm the findings of Morgan and Bodenstab (11): that anterior-posterior pins directed inferiorly are the least likely to injure the suprascapular nerve. For inferiorly directed pins, the average distance from the posterior exit site to the suprascapular nerve was 16 mm, and only one pin was < 12 mm from the nerve. Thus, a relative safe zone (Fig. 5) can be defined in the inferior and lateral portion of the posterior glenoid neck where anterior-posterior directed pins should exit in an effort to minimize the risk of injury to the suprascapular nerve.
FIG. 3. Posterior view of scapular model showing exit of a mediallydirected K-wire. Arrow points to line marking the distance from the posterior exit site of the pin to the base of the scapular spine.
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FIG. 4. Close-up view of posterior left shoulder of a cadaver specimen with infraspinatus muscle retracted inferiorly away from scapular spine. The exit sites of three medially directed pins are in close proximity to the suprascapular nerve (arrow) at it courses around the base of the spine of the scapula (nerve probe).
Mobilization of the supraspinatus and infraspinatus tendons is frequently required during surgical management of massive rotator cuff tears (S-8). Such mobilization may place the suprascapular nerve at risk for injury. The findings of this study showed that the distance of the suprascapular nerve from the supraglenoid tubercle averaged 3.0 cm at the suprascapular notch. The distance from the supraglenoid tubercle to the suprascapular nerve at
FIG. 5. Safe zone of the posterior glenoid neck. The highlighted region represents the safe zone of the posterior glenoid neck where surgical dissection will minimize risk of injury to the suprascapular nerve. The safe zone measures 2 cm at the level of the supraglenoid tubercle and decreases to 1 cm at the level of the scapular spine. The inferior shaded area represents the safe zone for the exit sites of anterior-posterior pins directed inferiorly during arthroscopic Bankart repairs.
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the base of the scapular spine averaged 2.5 cm. Therefore, the safe zone (Fig. 5) in the posterosuperior aspect of the glenoid lies within 2 cm of the glenoid rim. In this area the capsule and rotator cuff muscles can be safely mobilized with minimal risk of injuring the suprascapular nerve. Glenoid osteotomies have been described for the treatment of posterior instability (l-3). It is important to avoid articular damage during this procedure and it has been recommended to perform the osteotomy at least 1 cm medial to the posterior articular surface. In addition, the management of posterior instability may include soft tissue procedures that necessitate dissection of the infraspinatus muscle and posterior capsule medial to the glenoid rim (4). During these procedures knowledge of the location of the suprascapular nerve is important to minimize its risk of injury. In this study, the average distance of the suprascapular nerve between the midline of the posterior glenoid rim and the base of the spine of the scapula averaged only 1.8 cm, with a range of 1.4-2.5 cm. We conclude, therefore, that the safe zone at the level of the midline of the posterior glenoid decreases to 1 cm medial to the posterior artitular surface (see Fig. 5). The syndrome of suprascapular entrapment neuropathy is well described (23), and may require surgical decompression by either section of the transverse scapular ligament (24), or suprascapular notch resection (25). Furthermore, entrapment of the nerve secondary to a ganglionic cyst distal to the suprascapular notch has been reported at different locations including the suprascapular fossa (26) and the base of the scapular spine (27-29). Finally, entrapment of the suprascapular nerve has also been described at the spinoglenoid notch where the termination of the suprascapular nerve courses inferior to the spinoglenoid ligament, curves around the base of the scapular spine and innervates the infraspinatus muscles (30). Other causes of injury to the suprascapular nerve have been described including sports (3 1,32), upper extremity resistive exercises (33), blunt trauma to the shoulder (34), anterior dislocation of the shoulder (35), and finally, fractures of the suprascapular notch (36). In summary, we have recorded the anatomic course and muscular innervations of the suprascapular nerve. This information may aid the physician in the diagnosis and treatment of suprascapular neuropathies. In addition, a safe zone in the posterior glenoid neck has been described based on the distances of the suprascapular nerve from fixed surgi-
ANATOMY
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cal landmarks. An appreciation of this safe zone may help the shoulder surgeon avoid iatrogenic injury to the suprascapular nerve during arthroscopic Bankart procedures and during open surgical procedures requiring dissection of the posterior glenoid neck. REFERENCES 1. Scott DJ. Treatment of recurrent posterior dislocation of the shoulder by glenoplasty. J Bone Joint Surg 1%7;49A:471-6. 2. Hernandez A, Drez D. Operative treatment of posterior shoulder dislocation by posterior glenoidplasty, capsulorrhaphy, and infraspinatus advancement. Am J Sports Med 1986;14:187-91. 3. Brewer BJ, Webben RC, Carrera GF. Excessive retroversion of the glenoid cavity. .I Bone Joint Surg 1986;68A:72431. 4. Neer CS, II, Foster CP. Inferior capsular shift for involuntary inferior and multidirectional instability of the shoulder. A preliminary report. J Bone Joint Surg 1980;62A:897908. 5. Debeyre J, Patte D, Elmelik E. Repair of ruptures of the rotator cuff of the shoulder-with a note on advancement of the supraspinatus muscle. J Bone Joint Surg 1965;47B:3642. 6. Colield RH. Tears of the rotator cuff. In: AAOS Instructional Course Lectures XXX. St. Louis, MO: CV Mosby, 1981:25&73. 7. Colield RH. Rotator cuff disease of the shoulder. J Bone Joint Surg 1985;67A:974-9. 8. Neer CS. Impingement lesions. Clin Orthop 1983;173:7&7. 9. Rengachary SS, Neff JP, Singer PA, Brackett CF. Suprascapular entrapment neuropathy. A clinical, anatomical, and comparative study, part I. Neruosurgety 1979;4:441-6. 10. Post M, Mayer J. Suprascapular nerve entrapment, diagnosis and treatment. C/in Orthop Relat Res 1987;223:12636. 11. Morgan CD, Bodenstab AB. Arthroscopic Bankart suture repair. Technique and early results. Arthroscopy 1987;3: 11l-22 12. Kerr A. The brachial plexus of nerves in man. The variations in its form and branches. Am J Anat 1918;23:285-395. 13. Rengachary SS, Neff JP, Singer PA, Brackett CE. Suprascapular entrapment neuropathy: a clinical, anatomical and comparative study, part 2: anatomical study. Neurosurgery 1979;5:447-55. 14. Houguchi M. The cutaneous brance of some humerus suprascapular nerves. J Anat 1980;130:191-5. 15. Mestdagh M, Drizenko A, Ghestem P. Anatomical basis of suprascapular nerve syndrome. Anatomica Clinica 1981;3: 67-71.
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16. Kaspi A, Yanai J, Pick CG, Maun G. Entrapment of the distal suprascapular nerve-an anatomic study. Znt Orthop 1988;12:273-5. 17. Williams PC, Warwick R, eds. Gray’s Anatomy, 36th ed. Philadelphia: Saunders, 1980. 18. Sunderland S. Nerves and Nerve Injuries. 2nd ed. New York: Churchill Livingstone, 1978: 1015-6. 19. Lucky CA. Recurrent dislocation of the shoulder modification of Bankart capsulorrhaphy. Am J Surg 1949;77:22&22. 20. Viek P. Bell BT. The Bankart shoulder reconstruction-the use of pull-out. J Bone Joint Surg 1959;4lA:23ti2. 21. Reider B, Inglis AE. The Bankart procedure modified by the use of prolene pull-out sutures. J Bone Joint Surg 1982;64A: 628-9.
22. Saha AK. Dynamic stability of the glenohumeral joint. Acta Orthop Stand
1971;42:491-505.
23. Kopell HP, Thompson
WA. Suprascapular nerve. In: PeBaltimore: Williams L Wilkins, Co., 1963: 130-42. 24. Clein CJ. Suprascapular entrapment neuropathy. J Neuroripheral entrapment
neuropathies.
surg 1975;43:337-42.
25. Rask MB. Suprascapular nerve entrapment-a report of two cases treated with suprascapular notch resection. Clin Orthop Relat Res 1977;123:73-5.
26. Laulund T, Fedders 0, Sgaard I, Komum M. Suprascapular nerve compression syndrome. Surg Neurol 1984;22:308-12. 27. Neviaser TJ, Ain BR, Neviaser RJ. Suprascapular nerve degeneration secondary to attenuation by a ganglionic cyst. J Bone Joint Surg 1986;68A627-8. 28. Ganzhom RW, Hacker JT, Horotwitz M, Switzer H. Suprascapular nerve entrapment. J Bone Joint Surg 1981;63A: 492-4. 29. Thompson RC, Schneider W, Kennedy T. Entrapment neuropathy of the inferior branch of the suprascapular nerve by ganglia. CORR 1982;166:185-7. 30. Aiello I, Serra G, Traina GC, Tugnoli V. Entrapment of the suprascapular nerve at the spinoglenoid notch. Annals of Neurology 1982;12:314-6. 31. Bateman, JE. Nerve injuries about the shoulder in sports. J Bone Joint Surg 1967;49A:785-92.
32. Ferrett A, Cerullo G, Russo G. Suprascapular neuropathy in volleyball players. J Bone Joint Surg 1987;69A:26&3. 33. Agre JL, Ash N, Cameron MC, House J. Suprascapular neuropathy after intensive progressive resistive exercise: case report. Arch Phys Med Rehabil 1987;68:236-8. 34. Yoon TN, Gabrois M. Suprascapular nerve injury following trauma to the shoulder. J Trauma 1981;21:652-5. 35. Zoltan JD. Injury to the suprascapular nerve association with anterior dislocation of the shoulder: case report and review of the literature. J Trauma 1979;19:203-6. 36. Edeland HG, Zachrisson BE. Fracture of the scapular notch associated with lesion of the suprascapular nerve. Acta Orthop Stand
1975;46:758-63.
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An Anatomical Study of the Suprascapular Nerve Louis U. Bigliani, M.D., Robert M. Dalsey, M.D., Peter D. McCann, M.D., and Ernest W. April, Ph.D.
Summary: The course of the suprascapular nerve and its distance from fixed scapular landmarks were measured in 90 cadaveric shoulders. In an additional 15 cadavers, three pins were passed at various angles in a general anteriorposterior direction through the middle of the glenoid neck just inferior and lateral to the base of the coracoid process. The distance between the exit site on the posterior glenoid neck and the suprascapular nerve at the base of the scapular spine was recorded for each pin. Inferiorly directed pins were the furthest from the suprascapular nerve and averaged 16 mm. On the basis of these data, a relative safe zone is described in the posterior glenoid neck.
Knowledge of the anatomic course of the suprascapular nerve may aid the physician in the diagnosis and treatment of suprascapular neuropathies. Appreciation of the safe zone may help the shoulder surgeon avoid iatrogenic injury to the suprascapular nerve during arthroscopic Bankart procedures and other open surgical procedures requiring dissection of the posterior glenoid neck. Key Words: Suprascapular nerve-Anatomy-Bankart repair-Safe zone.
sponse to traction (13), cutaneous sensory branch distribution (14), relative points of nerve fixation at the suprascapular notch and the scapular spine (15), and finally, the relationship of the suprascapular nerve to the spinal glenoid ligament at the base of the scapular spine (16). However, little data exist concerning the course of the suprascapular nerve, its levels of muscular innervation, and its relationship to anatomic surgical landmarks. The purpose of this anatomical study is to examine the course of the suprascapular nerve, measure its distance from fixed scapular landmarks, and determine the levels of innervation of the supraspinatus and infraspinatus muscles. With this information, a potential safe zone can be established for pin placement when performing blind drilling through the glenoid neck for arthroscopic repair of anterior Bankart lesions and for open procedures requiring dissection in the posterior region of the shoulder.
A thorough anatomical knowledge of the course of the suprascapular nerve is important in shoulder surgery. This information can be useful in preventing potential iatrogenic injury during open surgical procedures for posterior instability (l-4), repair of massive rotator cuff tears (5-8), decompression of suprascapular nerve entrapment @lo), as well as newer arthroscopic procedures for repair of anterior glenohumeral instability (11). Previous anatomical studies of the suprascapular nerve have concentrated on cervical root contribution (12), shape of the suprascapular notch, translational movement of the suprascapular nerve in re-
From the Department of Orthopaedic Surgery, Columbia Rresbyterian Medical Center (L.U.B., R.M.D.), New York, New York; the Department of Orthopaedic Surgery (P.D.M.), Helen Hayes Hospital, West Haverstraw, New York; and the Department of Anatomy and Cell Biology (E.W.A.), College of Physicians & Surgeons, Columbia University, New York, New York, U.S.A. Address correspondence and reprint requests to Dr. Louis U. Bigliani, c/o Dr. Peter D. McCann, Helen Hayes Hospital, Dept. of Orthop. Surgery, Route 9W, West Haverstraw, NY 10993, U.S.A.
MATERIALS AND METHODS Ninety cadaveric shoulders were dissected to study the course of the suprascapular nerve, the 301
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level of muscle innervations, and its distance from standard bony landmarks. There were 53 male and 37 female cadavers of an average age of 78 years (range 54-90 years). The distance from the suprascapular notch to the point of innervation of the supraspinatus muscle was measured, as well as the distance from the base of the scapular spine to the point of innervation of the infraspinatus muscle. Three distances between the suprascapular nerve and bony landmarks were measured (Fig. 1): supraglenoid tubercle (SGT) to the nerve at the suprascapular notch (SSN), supraglenoid tubercle to the nerve at the base of the spine of the scapula (BSS), the midline of the posterior glenoid rim (PGR) to the nerve at the base of the scapular spine. In an additional 15 cadavers, three pins (0.62-mm Kirschner wires) were passed at various angles in a general anterior-posterior direction through the middle of the glenoid neck just inferior and lateral to the base of the coracoid process and -1cm medial to the articular surface. One pin was passed parallel to the glenoid articular surface, one pin passed as medial as possible, and one pin aimed as inferior as possible. The distance between the exit site on the posterior glenoid neck and the suprascapular nerve at the base of the scapular spine was recorded for each pin.
RESULTS The suprascapular nerve passed through the scapular notch and inferior to the transverse scapular ligament in all specimens (90/90). The motor nerves to the supraspinatus muscle branched within 1 cm of the suprascapular notch in all specimens @O/90) (Fig. 1). From the suprascapular notch, the suprascapular nerve coursed obliquely and laterally in the supraspinatus fossa along the inferior surface of the supraspinatus muscle directly to the base of the scapular spine. The spinal glenoid ligament at the base of the scapular spine was difficult to identify in some of our cadaveric specimens, and therefore, could not be consistently recorded in this study. From the base of the scapular spine, the suprascapular nerve curved medially and innervated the infraspinatus muscle within 1 cm of the base of the scapular spine in 89% of specimens (80190). Distance A (Fig. l), from the supraglenoid tubercle to the nerve at the suprascapular notch, averaged 3.0 cm (range 2.5-3.9 cm). Distance B (Fig. I), from the supraglenoid tubercle to the nerve at the base of the spine at the scapula, averaged 2.5 cm (range 1.9-3.2 cm). Distance C (Fig. l), from the midline of the posterior glenoid rim to the nerve at the base of the scapular spine, averaged 1.8 cm (range 1.62.5 cm). These results are summarized in Table 1. In the 15 specimens with muiltiple pin placement, the distance from the posterior exit site to the suprascapular nerve at the base of the scapular spine for the pins directed medially averaged only 4 mm (range l-8 mm) (Table 2). The distance for the pins directed parallel to the articular surface averaged 6 mm (range 1-14 mm), and the distance for the inferiorly directed pins averaged 16 mm (range 4-35 mm). Among the inferiorly directed pins, only one (l/15) exited within 12 mm of the suprascapular nerve.
TABLE 1. Lengths of distances illustrated in Fig. 1 Distance
FIG. 1. Course of the suprascapular nerve. Diagram of posterior scapula illustrating passage of the suprascapular nerve inferior to the transverse scapular ligament and innervation of the spinati. Distances A, B, and C are described in Table 1.
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A, Supraglenoid tubercle to suprascapular notch B, Supraglenoid tubercle to base of scapular spine C, Midline posterior glenoid rim to base of scapular spine
Length (cm) (range) (2.E.9) (LE.2) (I.&)
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TABLE 2. Distance between the posterior exit site of pins and the suprascapular nerve at the base of the scapular spine Direction of pins (relative to glenoid surface)
Distance (mm) (range)
Medial (l-46) Parallel (l-614, Inferior $5)
DISCUSSION The findings of the course of the suprascapular nerve from the suprascapular notch to the innervation of the infraspinatus muscle determined in this study concur with the findings described in standard texts (17,18). To our knowledge, no complications involving the suprascapular nerve following shoulder surgery have been reported in the English literature. Nevertheless, the potential for iatrogenic suprascapular nerve injury does exist due to the proximity of the nerve to the operative field. Several authors have recently reported on arthroscopic techniques in the surgical management of anterior shoulder instability (11). The ‘‘Arthroscopic Bankart Procedure” is essentially a modification of an open technique first reported by Lucky in 1949 (19). This procedure was further modified by Viek and Bell in 1959 to include braided wire pull out sutures (20) and updated by Reider and Inglis (21) in 1982 to include the use of prolene pull-out sutures. The arthroscopic technique requires the placement of sutures in the detached anterior labrum and capsule and then the passage of the sutures posteriorly through drill holes placed in the glenoid neck. By pulling the sutures tightly posteriorly, the labrum is reduced to the anterior glenoid rim, and the sutures tied over the infraspinatus. This arthroscopic Bankart procedure requires blind drilling through the glenoid neck, which may place the suprascapular nerve at risk for injury posteriorly. The findings of this study showed that pins directed medially place the suprascapular nerve at greatest risk (average distance 4 mm; see Figs. 2-4). Several factors contribute to medial pin placement. First, the transverse axis of the scapula (i.e., scapula plane) lies -35” anterior to the mid-coronal plane of the body. An anterior-posterior directed pin beginning at the anterior glenoid rim and passed straight pos-
FIG. 2. A scapular model viewed from above showing the course of a medially directed K-wire.
terior will, by necessity, exit medially on the posterior glenoid neck. Second, the glenoid surface has a normal 7” of retroversion relative to the transverse axis of the scapula (22). Consequently, it is the natural tendency of the arthroscopic surgeon to aim these pins medially in an effort to avoid violating the articular surface of the glenoid. The results of this study confirm the findings of Morgan and Bodenstab (11): that anterior-posterior pins directed inferiorly are the least likely to injure the suprascapular nerve. For inferiorly directed pins, the average distance from the posterior exit site to the suprascapular nerve was 16 mm, and only one pin was < 12 mm from the nerve. Thus, a relative safe zone (Fig. 5) can be defined in the inferior and lateral portion of the posterior glenoid neck where anterior-posterior directed pins should exit in an effort to minimize the risk of injury to the suprascapular nerve.
FIG. 3. Posterior view of scapular model showing exit of a mediallydirected K-wire. Arrow points to line marking the distance from the posterior exit site of the pin to the base of the scapular spine.
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FIG. 4. Close-up view of posterior left shoulder of a cadaver specimen with infraspinatus muscle retracted inferiorly away from scapular spine. The exit sites of three medially directed pins are in close proximity to the suprascapular nerve (arrow) at it courses around the base of the spine of the scapula (nerve probe).
Mobilization of the supraspinatus and infraspinatus tendons is frequently required during surgical management of massive rotator cuff tears (S-8). Such mobilization may place the suprascapular nerve at risk for injury. The findings of this study showed that the distance of the suprascapular nerve from the supraglenoid tubercle averaged 3.0 cm at the suprascapular notch. The distance from the supraglenoid tubercle to the suprascapular nerve at
FIG. 5. Safe zone of the posterior glenoid neck. The highlighted region represents the safe zone of the posterior glenoid neck where surgical dissection will minimize risk of injury to the suprascapular nerve. The safe zone measures 2 cm at the level of the supraglenoid tubercle and decreases to 1 cm at the level of the scapular spine. The inferior shaded area represents the safe zone for the exit sites of anterior-posterior pins directed inferiorly during arthroscopic Bankart repairs.
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the base of the scapular spine averaged 2.5 cm. Therefore, the safe zone (Fig. 5) in the posterosuperior aspect of the glenoid lies within 2 cm of the glenoid rim. In this area the capsule and rotator cuff muscles can be safely mobilized with minimal risk of injuring the suprascapular nerve. Glenoid osteotomies have been described for the treatment of posterior instability (l-3). It is important to avoid articular damage during this procedure and it has been recommended to perform the osteotomy at least 1 cm medial to the posterior articular surface. In addition, the management of posterior instability may include soft tissue procedures that necessitate dissection of the infraspinatus muscle and posterior capsule medial to the glenoid rim (4). During these procedures knowledge of the location of the suprascapular nerve is important to minimize its risk of injury. In this study, the average distance of the suprascapular nerve between the midline of the posterior glenoid rim and the base of the spine of the scapula averaged only 1.8 cm, with a range of 1.4-2.5 cm. We conclude, therefore, that the safe zone at the level of the midline of the posterior glenoid decreases to 1 cm medial to the posterior artitular surface (see Fig. 5). The syndrome of suprascapular entrapment neuropathy is well described (23), and may require surgical decompression by either section of the transverse scapular ligament (24), or suprascapular notch resection (25). Furthermore, entrapment of the nerve secondary to a ganglionic cyst distal to the suprascapular notch has been reported at different locations including the suprascapular fossa (26) and the base of the scapular spine (27-29). Finally, entrapment of the suprascapular nerve has also been described at the spinoglenoid notch where the termination of the suprascapular nerve courses inferior to the spinoglenoid ligament, curves around the base of the scapular spine and innervates the infraspinatus muscles (30). Other causes of injury to the suprascapular nerve have been described including sports (3 1,32), upper extremity resistive exercises (33), blunt trauma to the shoulder (34), anterior dislocation of the shoulder (35), and finally, fractures of the suprascapular notch (36). In summary, we have recorded the anatomic course and muscular innervations of the suprascapular nerve. This information may aid the physician in the diagnosis and treatment of suprascapular neuropathies. In addition, a safe zone in the posterior glenoid neck has been described based on the distances of the suprascapular nerve from fixed surgi-
ANATOMY
OF THE SUPRASCAPULAR
cal landmarks. An appreciation of this safe zone may help the shoulder surgeon avoid iatrogenic injury to the suprascapular nerve during arthroscopic Bankart procedures and during open surgical procedures requiring dissection of the posterior glenoid neck. REFERENCES 1. Scott DJ. Treatment of recurrent posterior dislocation of the shoulder by glenoplasty. J Bone Joint Surg 1%7;49A:471-6. 2. Hernandez A, Drez D. Operative treatment of posterior shoulder dislocation by posterior glenoidplasty, capsulorrhaphy, and infraspinatus advancement. Am J Sports Med 1986;14:187-91. 3. Brewer BJ, Webben RC, Carrera GF. Excessive retroversion of the glenoid cavity. .I Bone Joint Surg 1986;68A:72431. 4. Neer CS, II, Foster CP. Inferior capsular shift for involuntary inferior and multidirectional instability of the shoulder. A preliminary report. J Bone Joint Surg 1980;62A:897908. 5. Debeyre J, Patte D, Elmelik E. Repair of ruptures of the rotator cuff of the shoulder-with a note on advancement of the supraspinatus muscle. J Bone Joint Surg 1965;47B:3642. 6. Colield RH. Tears of the rotator cuff. In: AAOS Instructional Course Lectures XXX. St. Louis, MO: CV Mosby, 1981:25&73. 7. Colield RH. Rotator cuff disease of the shoulder. J Bone Joint Surg 1985;67A:974-9. 8. Neer CS. Impingement lesions. Clin Orthop 1983;173:7&7. 9. Rengachary SS, Neff JP, Singer PA, Brackett CF. Suprascapular entrapment neuropathy. A clinical, anatomical, and comparative study, part I. Neruosurgety 1979;4:441-6. 10. Post M, Mayer J. Suprascapular nerve entrapment, diagnosis and treatment. C/in Orthop Relat Res 1987;223:12636. 11. Morgan CD, Bodenstab AB. Arthroscopic Bankart suture repair. Technique and early results. Arthroscopy 1987;3: 11l-22 12. Kerr A. The brachial plexus of nerves in man. The variations in its form and branches. Am J Anat 1918;23:285-395. 13. Rengachary SS, Neff JP, Singer PA, Brackett CE. Suprascapular entrapment neuropathy: a clinical, anatomical and comparative study, part 2: anatomical study. Neurosurgery 1979;5:447-55. 14. Houguchi M. The cutaneous brance of some humerus suprascapular nerves. J Anat 1980;130:191-5. 15. Mestdagh M, Drizenko A, Ghestem P. Anatomical basis of suprascapular nerve syndrome. Anatomica Clinica 1981;3: 67-71.
NERVE
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