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SECTION I SYMPOSIUM: Advances in Anatomy of the Upper Extremity

Osseous Anatomy of the Scapula

von Schroeder, Herbert P., MD, MSc*; Kuiper, Scott D., MD**; Botte, Michael J., MD†,‡,§

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Clinical Orthopaedics and Related Research®: February 2001 - Volume 383 - Issue - p 131-139
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Abstract

The scapula, a flat triangular bone (Fig 1), articulates with the distal clavicle and humeral head and has numerous muscular and ligamentous attachments. 1,10 The scapula may be subject to fracture, dislocation, arthritis, tumors, and developmental abnormalities, and anatomic considerations are central in understanding specific abnormalities such as rotator cuff injuries and glenohumeral dislocation. Numerous surgical procedures have been described for the scapula including arthroplasty and arthrodesis of the glenohumeral joint, internal fixation for fracture stabilization, acromioplasty or acromionectomy for rotator cuff disorders, and scapulothoracic tenodesis for winging. 3–6,9,12,16,21,22,25,30,31 Detailed knowledge of the anatomy is important for surgical procedures involving this bone, including arthroscopic procedures. Because of the unusual and complex morphologic features of the scapula, and the lack of complete quantitative anatomic studies, the purpose of the current study was to describe quantitatively the scapula’s exact osteologic features.

Fig 1.
Fig 1.:
An anterior view of the right scapula showing the standard terminology of its anatomic regions.

MATERIALS AND METHODS

Thirty scapulas from adult cadavers, consisting of 15 pairs (seven male, six female, two unknown), were dissected free of all soft tissues leaving the coracoacromial ligament intact. All scapulas were inspected and a generalized anatomic description was recorded (Fig 1) using standard nomenclature. 1,10 Linear measurements were made with a micrometer to 1/1000 mm and rounded to 1 mm. Measurements of thickness that could not be obtained by the micrometer, were obtained by drilling 2-mm diameter holes into the bone and measured using a depth gauge and rounded to 1 mm. Angular measurements were made using a goniometer to the nearest degree. For consistency, one micrometer, depth gauge and goniometer were used for all measurements and each measurement was made by one investigator. All measurements were made twice and averaged. The overall average, standard deviation, and range then were determined for each of the 26 measurements that are listed below. Dimensions of scapulas were compared between male and female specimens using Student’s t test (significance at α < 0.05).

General Measurements

Six general measurements were made and are shown in Figure 2. These measurements included the maximum length of the scapula from the tip of the superior angle to the tip of the inferior angle; the distance from the medial edge of the scapula where the scapular spine meets the scapular body to the posterior rim of the glenoid; the distance from the medial edge of the scapula where the scapular spine meets the scapular body to the anterior most rim of the glenoid (Fig 3); the inferior scapular angle; the anteroposterior (AP) thickness of the medial border of the scapula measured half way along the medial edge of the scapula and 1 cm lateral to the medial edge; and the distance from the superior rim of the glenoid to the base of the suprascapular notch.

Fig 2.
Fig 2.:
Posterior view of the right scapula showing terminology and general measurements taken. The measurements included (1) the maximum length of the scapula; (2) the width of the scapula measured to the posterior rim of the glenoid; (3) the width of the scapula measured to the anterior rim of the glenoid (also is shown in Fig 3); (4) the inferior scapular angle; (5) the anteroposterior thickness of the medial border of the scapula measured half way along the medial edge of the scapula and 1 cm from the edge; and (6) the distance from the superior rim of the glenoid to the base of the suprascapular notch. The measurement values are shown in Table 1.
Fig 3.
Fig 3.:
The right scapula (superior view as shown in the inset) showing the measurements of the spine. The measurements included (7) the length of scapular spine measured from the medial edge of the scapula where it meets with the scapular spine to the lateral edge of the acromion; (8) the distance from the medial edge of the scapula where it meets with the scapular spine to the edge of the spinoglenoid notch; (9) the anteroposterior width of the spine measured 1 cm from the medial edge of the scapula; (10) the anteroposterior width of the spine measured 4 cm from the medial edge of the scapula; (11) the anteroposterior width of the spine at the lateral edge (spinoglenoid notch); and (12) the anteroposterior thickness of the acromial neck at its thinnest diameter. Also shown is measurement Number 3, which is the width of the scapula measured on the anterior surface. The measurement values are shown in Table 1.
TABLE 1
TABLE 1:
Measurements of the Scapula

Scapular Spine

Six measurements were made of the spine (Fig 3). These measurements included the length of scapular spine measured from the medial edge of the scapula where it meets with the scapular spine to the lateral edge of the acromion at a point half way along the acromial length; the distance from the medial edge of the scapula where it meets with the scapular spine to the edge of the spinoglenoid notch; the AP width of the spine measured 1 cm lateral to the medial edge of the scapula; the AP width of the spine measured 4 cm lateral to the medial edge of the scapula; the AP width of the spine at the lateral edge (spinoglenoid notch); and the AP thickness of the acromial neck at its thinnest AP and superoinferior diameter.

Acromion

Acromial measurements are shown in Figures 4 and 5. These measurements included: the AP acromial length measured along middle of the acromion; the acromial width measured across the middle of the acromion from the lateral aspect of the acromion to the medial side at the middle of the acromioclavicular ligament insertion; the acromial thickness measured 1 cm posterior from the maximal anterior edge or point, and 1 cm from the lateral edge; the coracoacromial distance measured between the coracoid and the acromion along the middle of the substance of the coracoacromial ligament; and the minimum distance from the superior rim of the glenoid to the inferior surface of the acromion.

Fig 4.
Fig 4.:
Length (13), width (14) and thickness (15) measurements of the acromion, and coracoacromial distance (16) as seen from the superior view of the right scapula are shown. The measurement values are shown in Table 1.
Fig 5.
Fig 5.:
The right scapula is shown. This lateral view shows the coracoacromial distance (16), the minimal distance between coracoid and acromion (17), and the dimensions of the glenoid fossa (18–20). The measurement values are shown in Table 1.

Glenoid and Head of Scapula

Six measurements were made of the glenoid and head of the scapula and are shown in Figures 5 and 6. These measurements included the maximum AP diameter of the glenoid; the AP diameter of glenoid from the posterior rim to the anterior notch of the glenoid; the maximum superoinferior diameter of the glenoid; the thickness of the head in the AP direction parallel to the surface of the glenoid, 1 cm from the articular surface where the maximum AP diameter of the glenoid (above) was made; the thickness of the head in the AP direction parallel to the surface of the glenoid 2 cm from the surface and 0.5 cm inferior to the previous measurement; and the glenoid tilt angle (posterior tilt) which was the angle between a line along the anterior scapula and a line along the glenoid at the maximum diameter subtracted from 90°.

Fig 6.
Fig 6.:
Thickness measurements of the scapular head (22,22) and glenoid tilt angle (23) as seen from the inferior view of the right scapula are shown. The measurement values are shown in Table 1.

Coracoid Process

Three measurements of the coracoid are shown in Figure 7. These measurements were the length of the coracoid from the tip of the coracoid along its linear portion to the point at which the coracoid angulates inferiorly; the coracoid thickness measured in the superoinferior direction 1 cm posterior to the tip of the coracoid; and the distance from the tip of the coracoid to the base of the suprascapular notch.

Fig 7.
Fig 7.:
Dimensions of the coracoid process of the right scapula as seen from an anterior view are shown. The measurements were: the length of the coracoid from the tip of the coracoid to the point at which the coracoid angulates inferiorly (24); the coracoid thickness measured in the superoinferior direction 1 cm from the tip of the coracoid (25); and the distance from the tip of the coracoid to the base of the suprascapular notch (26). The measurement values are shown in Table 1.

RESULTS

The body of the scapula was concave anteriorly and had several inconsistent ridges for muscle attachment. All measurements are shown in Table 1. The average maximum length of the scapula from the tip of the superior angle to the tip of the inferior angle was 155 ± 16 mm (average ± standard deviation). The thickness of the medial border of the scapula half way along the medial edge of the scapula and 1 cm lateral to the medial edge was 4 ± 1 mm. Medial to this point, the scapular thickness transgressed to translucent thinness; lateral to this point the border was a thicker ridge. The superior angle could not be measured accurately because of the variable shape between the angle and the superior border. The superior border was sharp and thin, and the suprascapular (or scapular) notch was variable in size ranging from 1.7 mm to 1.4 cm wide and was covered by the suprascapular (transverse scapular) ligament that also was of variable thickness and strength. In two left scapulas (7%), the notch was present as a foramen (one male, one female). The distance from the base of the suprascapular notch to the superior rim of the glenoid (supraglenoid tubercle) was 32 ± 3 mm.

Posteriorly, the body was divided by the spine into the supraspinous and infraspinous fossae. The spine could be described as a triangle with its apex at the medial border of the scapula, its hypotenuse forming a posterior ridge, its base along the body of the scapula, and its lateral edge forming the spinoglenoid notch (Fig 2). The posterior ridge was only rarely straight, but more commonly had a bend between the medial ⅓ and the lateral ⅔. The posterior ridge continued laterally into the acromion, and the length of the spine from the medial apex to the lateral edge of the acromion was 134 ± 12 mm. The distance from the medial apex to the edge of the spinoglenoid notch was 86 ± 9 mm. The width of the spine in the AP direction at 1 cm lateral to the medial edge of the scapula was 7 ± 1 mm. As the spine flared out posteriorly and laterally, the width 4 cm lateral to the medial edge was 18 ± 3 mm, and the width at the lateral edge (spinoglenoid notch) was 46 ± 6 mm.

The AP length of the acromion was 48 ± 5 mm and the width from medial to lateral) was 22 ± 4 mm. The acromial thickness measured 1 cm posterior to the anterior edge and 1 cm medial to the lateral edge was 9 ± 1 mm. The acromial shape was classified 19 as flat Type I in 23%, curved Type II in 63%, and hooked Type III in 14%. The average minimal distance from the superior rim of the glenoid to the inferior surface of the acromion was 16 ± 2 mm. The AP thickness of the acromial neck at its thinnest AP and superoinferior diameter was 14 ± 2 mm.

The slightly concave glenoid cavity, which is the lateral portion of the head of the scapula, had the shape of teardrop with a notch anteriorly (Fig 5). The average glenoid dimensions were 29 ± 3 mm (AP) by 36 ± 4 mm (superoinferior). The glenoid faced slightly posteriorly with a tilt angle of 8° ± 4°. Supraglenoid and infraglenoid tubercles were present with varying degrees of definition. Thickness of the head of the scapula in the AP direction parallel to the surface of the glenoid, 1 cm medial to the surface was 22 ± 4 mm; whereas the thickness 2 cm medial to the surface and 0.5 cm inferior to the previous measurement was 13 ± 3 mm reflecting the conical nature of the head and neck.

The coracoid was located between the supraspinous notch and the head of the scapula. It projected anterosuperiorly and then curved in an anterolateral direction. The distal portion of the coracoid beyond the curve was relatively straight and measured 45 ± 5 mm. The superoinferior thickness of the coracoid 1 cm posterior to the tip was 11 ± 1 mm. The average length of the coracoacromial ligament was 27 ± 5 mm.

Scapulas from male cadavers were significantly (p < 0.05) larger than scapulas from female cadavers in all measurements with the exception of the following in which there were no differences: acromial width, glenoid tilt angle, width of the spine at 1 cm and 4 cm lateral to the medial edge, distance between the superior glenoid rim and the inferior surface of the acromion, and thickness of the scapula half way down the medial border (Table 1). There were no differences between left and right scapulas.

DISCUSSION

The shape and dimensions of the scapula have been described and the gender dimorphism confirmed with scapulas from male cadavers being larger than scapulas from female cadavers. 17,27 The body of the scapula was remarkably thin and was translucent in most cases, which would make fixation with pins or screws difficult. The medial and lateral borders of the body had sufficient mass within approximately 1 cm form the edge to support fixation, which is important in procedures such as scapulothoracic tenodesis or arthrodesis for scapular instability. 16

The scapular spine and acromion also had dimensions that should support screw, pin, or wire purchase for fracture fixation stabilization of the acromioclavicular joint 23 or arthrodesis of the glenohumeral joint. 21 Acromial shape and dimensions also are of particular importance in acromioplasty for rotator cuff impingement syndrome 5,9,22 and checkrein operations or tenosuspension of the humerus from the acromion. 11,12,26

Distances from palpable bony landmarks provide useful information for portal placement for shoulder arthroscopy. Of particular concern is the suprascapular nerve that may be injured through a superior portal medial to the acromion. The suprascapular (or scapular) notch, which contains the suprascapular nerve, was of variable size and was covered by an equally variable ligament, except in two cases in which a bony foramen was present instead of a notch. These features are important when considering suprascapular nerve entrapment as an etiology for supraspinatous and infraspinatous muscle wasting or shoulder pain. 7,20

Dimensions and standard deviations of the glenoid, and the amount of bone in the head of the scapula, are particularly important in the design and fitting of glenoid components for shoulder arthroplasty. The glenoid tilt angle of 8° posteriorly is in agreement with the reported glenoid tilt angle of 2° to 12°17,23,29 although some variation in angle may exist between the superior to the inferior poles. 13 An anterior tilt or anterior glenoid deficiency has been implicated in anterior dislocations of the glenohumeral joint, and osteotomies of the neck of the scapula are described to reestablish a posterior tilt. 29 Numerous other surgical procedures are available for treatment of anterior 3,4,30,31 or posterior 6,25 shoulder dislocations or subluxations that involve fixation to the glenoid rim, scapular head, or neck. Knowledge of the shape and dimensions of the scapula is particularly relevant for these procedures, as it is for other procedures such as fracture reconstruction and removal of tumor masses 2,8,14,15,18,24,28 or removal of portions of the scapula for mechanical reasons particularly when operating through limited incisions.

Acknowledgments

The authors thank Sean Devine, MD, and Mr. William Collins for their assistance in preparation of anatomic specimens.

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Section Description

Michael J. Botte, MD; and James R. Doyle, MD Guest Editors

© 2001 Lippincott Williams & Wilkins, Inc.