Biomechanics of Anatomic Total Shoulder Arthroplasty

Biomechanics of Anatomic Total Shoulder Arthroplasty

Christopher P. Roche, MSE, MBA

Matthew L. Hansen, MD


The motion of the shoulder is complex and created by the simultaneous action of three different joints and one articulation: the glenohumeral, sternoclavicular, and acromioclavicular joints and the scapulothoracic articulation (FIGURE 3.1). To perform shoulder elevation, each is required to elevate and rotate the scapula while the humerus simultaneously rotates and lifts the arm in coordinated motion termed scapulohumeral rhythm (FIGURE 3.2). As described by Codman et al and Inman, the approximate ratio of humeral to scapular rotation is 2:1, with scapulothoracic motion accounting for a maximum of 60° and humeral motion accounting for a maximum of 120° though most activities of daily living (ADLs) are completed with the scapula rotating no more than 30° and the humerus rotating no more than 60°.1,2

Inman et al described shoulder motion as being generated by three general muscle groupings: (1) scapulohumeral (muscles originating on the scapula and inserting on the humerus), (2) axiohumeral (muscles originating on the torso and inserting on the humerus), and (3) axioscapular muscles (muscles originating on the torso and inserting on the scapula).2 The scapulohumeral muscles are the supraspinatus, infraspinatus, teres minor, subscapularis (collectively known as the rotator cuff), as well as the deltoid, and teres major. The scapulohumeral muscles work together to elevate and rotate the arm. The axiohumeral muscles are the latissimus dorsi and the pectoralis major (with a portion of the pectoralis originating on the clavicle), which act to internally rotate and elevate the arm. The axioscapular muscles are the trapezius, rhomboids, serratus anterior, levator scapulae, and the pectoralis minor, which all act to rotate the scapula (it should also be noted that the biceps brachii and the long head of the triceps originate from the scapula and cross the elbow joint to insert on the proximal radius and ulna, acting as shoulder stabilizers) (FIGURE 3.3).


Joint Form and Function

The form of each diarthrodial joint has evolved to carefully balance stability and motion for its particular function. Joints requiring more constraint to perform ADLs typically have more conforming and congruent articular geometries to ensure that the joint reaction force is directed toward the center of the articulation to maintain stability, even at the extremes of the range of motion (ROM). Conversely, joints that require greater mobility typically have less articular constraint to minimize bony impingement at the end points of motion. For these highly mobile joints, stability at the end points of motion is achieved by both static (ligamentous and capsular constraint) and by dynamic (coordinated contraction of the surrounding musculature) stabilizers.

The relationship between articular joint geometry and the surrounding musculature is highly refined, with the number, size, position, and orientation of muscles crossing a particular joint designed to both generate the torque necessary for a particular motion but also to constrain and control that motion. Agonist and antagonist muscle groupings are necessary for finely controlled motions, particularly in highly mobile joints like the shoulder complex. These muscle group pairs coordinate contraction to dynamically stabilize the joint at various positions in order to direct the joint reaction force toward the joint center despite large joint rotations and translations.3,4

The glenohumeral joint is the largest joint in the shoulder complex and is composed of the articulation of the proximal humerus (ie, the humeral head) and the scapula (ie, the glenoid) (FIGURE 3.4). The concave humeral head articular geometry is large relative to the convex glenoid articular surface; and as the glenoid is approximately 1/4 of the size of the humeral head, it provides little to no intrinsic osseous stability. Furthermore, since the bony joint curvatures are nonconforming, with the glenoid bone flatter than the relatively spherical humeral head, the glenohumeral joint osseous morphology has little intrinsic constraint or congruity. With these parameters, the analogy of a golf ball on a tee is often invoked to describe the humeral head and glenoid size relationship. However, glenohumeral joint conformity is increased by the presence of the glenoid labrum and variable thickness of the glenoid articular cartilage.5,6,7 It should be noted that the glenoid is nonuniform and thicker on the periphery than in the glenoid center which increases joint congruency. Glenohumeral joint stability
changes with loading direction due to the varying glenoid morphology. As the glenoid is longer in the superior/inferior (S/I) direction than anterior/posterior (A/P) direction, the S/I glenoid is associated with additional depth as compared to the A/P glenoid.8,9,10,11,12 As a result, the humeral head is more constrained in the S/I direction than in the A/P direction.

Glenohumeral Joint Motion

Due to this nonconforming geometry and lack of osseous constraint, the glenohumeral joint is the most mobile joint in the human body and requires dynamic and static soft-tissue action for both joint motion and stability. Specifically, glenohumeral joint motion and stability are assisted throughout the ROM by the coordinated action of muscle contractions, which vary according to joint position and the different types of motion.3 The end points of motion are controlled by ligament and capsular tightening. Furthermore, due to both the minimal osseous constraint and the viscoelastic nature of the conforming articular cartilage and labrum, the glenohumeral joint center of rotation (CoR) can translate during motions. As such, glenohumeral joint motion has been described as (1) spinning (rotation only), (2) sliding (translation only), and (3) rolling (rotation + translation)13,14 (FIGURE 3.5).

There are three general types of arm motion enabled by these humeral head articulations against the glenoid, which are defined relative to the anatomic planes (FIGURE 3.6). Abduction/adduction is defined as the motion of the humeral head that results in arm elevation/de-elevation in the scapular and/or coronal plane, and forward/backward in the transverse plane. Flexion/extension is defined as the motion of the humeral head that results in arm forward/backward elevation in the sagittal plane. Finally, internal/external rotation is defined as the motion of the humeral head around its longitudinal axis, with arm rotation toward the midline termed internal rotation and arm rotation away from the midline termed external rotation.


Muscles generate straight line forces that are converted to torques in proportion to their perpendicular distance between the joint CoR and the muscle’s line of action.3,4 This perpendicular distance is termed the muscle’s moment arm. Muscle moment arms typically increase from deep to superficial. The torque generated by a particular muscle during contraction to either facilitate or stabilize a particular type of motion is determined by its moment arm position and orientation relative to the joint CoR. In general, shoulder muscles located anterior/posterior to the CoR internally/externally rotate the arm and muscles located superior/inferior to the CoR raise/lower the arm, respectively.

There are three types of muscle contraction. Concentric contractions generate submaximal tension during muscle shortening; eccentric contractions generate tension while the muscle elongates due to a larger opposing force; and isometric contractions generate muscle tension without a change in muscle length. Muscle contraction contributes to joint motion, typically in rotation, and may contribute to joint stability, depending on its line of action relative to the joint CoR. For finely controlled joint motions, muscles often function in groups as agonists (generating torque to cause joint rotation) or antagonists (contractions that oppose rotation). Agonists and antagonists typically mirror one another (A/P or S/I) for a particular type of motion and may work as force couples to increase the joint reaction force, thereby acting also to stabilize the joint. Individual muscles may also function in a biphasic manner, acting as agonists during one segment of motion and then acting as antagonists during another segment of motion because the muscle line of action crosses from one side of the CoR to the other as a result of joint motion.3,4 The joint motion produced by a given shoulder muscle depends on the position of its origin and insertion relative to the CoR; thus, a shoulder
muscle’s function may change depending on arm position for each of the different motions.

The larger the muscle’s moment arm, the greater its capacity to generate the torque required for motion and to support external loads. Thus, a larger moment arm may result in greater muscle efficiency, assuming the muscle mechanics inherent to a given muscle’s architecture (eg, pennation angle, cross-sectional area, sarcomere structure) can accommodate greater excursion.3 Muscles tend to specialize in varying degrees of force production or excursion. This is relevant for shoulder surgery and for shoulder prosthesis designs that alter the demands placed on shoulder muscles. Neurological status is also an important factor in muscle performance, as a poorly innervated muscle will not contract as well as a properly innervated muscle.

Most ADLs are performed with the arm internally rotated and elevated. As such, the largest muscles in the shoulder complex act to either elevate the arm and/or internally rotate the arm and to also stabilize the arm in those positions. External rotator muscles are also critical as they are necessary to resist internal rotation torque created by external loads as a result of elbow flexion. The deltoid is the largest muscle in the shoulder complex, and it is the primary elevator in the arm. The deltoid consists of three distinct heads which originate on the scapula and clavicle and wrap around the lateral proximal humerus as it inserts midway down the humeral shaft. The three heads of the deltoid are as follows: (1) anterior deltoid (originating from the anterior acromion and clavicle); (2) middle deltoid (originating from the lateral margin of the acromion); and (3) the posterior deltoid (originating from the scapular spine). At low levels of abduction, the wrapping of the middle deltoid around the lateral proximal humerus generates a stabilizing compressive force (FIGURE 3.7); however, this compressive force is small relative to that generated by the rotator cuff.15,16,17,18

The rotator cuff muscles generate the torque necessary for rotation of the humerus about the glenoid fossa while also compressing the humeral head into the glenoid concavity.19 The rotator cuff muscles are aligned around the proximal humerus for effective joint compression at all glenohumeral joint positions, allowing it to balance the joint by creating a dynamic fulcrum. This enables arm elevation while inhibiting superior migration of the humeral head and acromial impingement. In doing so, the rotator cuff effectively compensates for the lack of osseous constraint in the glenohumeral joint.20,21,22 Specifically, the anatomic arrangement of the anterior (subscapularis) and posterior (infraspinatus and teres minor) rotator cuff muscles creates a transverse force couple that centers the humeral head on the glenoid fossa in the A/P directions for all joint positions23,24,25 (FIGURE 3.8). The superior rotator cuff muscle (supraspinatus) is an abductor, which facilitates arm elevation, particularly at the initiation of motion. It also coordinates with the other rotator cuff muscles to generate a stabilizing compressive force to counteract the superiorly directed force generated by the deltoid, allowing for joint rotation as opposed to translation. This stabilizing mechanism of the rotator cuff interacting with the geometry of the glenohumeral joint has been termed concavity-compression.19 Concavity-compression is the concept of stability achieved by pressing a convex surface into a concave surface, with additional resistance to shear forces achieved with greater compression. The stabilizing effect of concavity compression is illustrated by the difference in humeral head translation observed between active and
passive motion, where humeral head translations during active motion are reported to be 1 to 4 mm in the S/I and A/P directions. During passive motion, humeral head translations can approach as much as 8 mm.26,27,28

Forces in the Shoulder

As shoulder muscle forces are applied between the CoR and the external load, the shoulder functions as a class three lever. While class one and class two levers amplify the input force at the expense of motion, class three levers place the input force at a mechanical disadvantage, requiring greater input force than the applied load. As such, the force exerted by the shoulder muscles must exceed that of the applied load (FIGURE 3.9). Furthermore, as the shoulder muscles act to dynamically stabilize the joint by co-contraction and the aforementioned concavity-compression mechanism, the summation of these muscle forces further increases the overall joint reaction force. Assuming that the arm weighs 5% body weight (BW), Poppen and Walker demonstrated that the maximum total joint reaction force in the shoulder occurs at 90° abduction and is approximately 0.89% BW, while the maximum shear force in the shoulder occurs at 60° abduction and is approximately 0.42% BW, based just upon the weight of the extremity29 (FIGURE 3.10). Poppen and Walker also reported that when 1 kg is held in the hand, the forces were increased by 60%.29 Westerhoff et al reported joint reaction forces of up to 1700 N and 238% BW using an instrumented anatomic total shoulder arthroplasty (ATSA) prosthesis in patients performing common ADLs.30,31,32,33 Thus, even though the shoulder joint is not “weight bearing,” it can still experience significant loads.

The relative size and cross-sectional areas of the muscles in the shoulder can be indicative of their use and function. Basset et al reported that the three heads of the deltoid account for 24% of the total muscle volume in the shoulder, the pectoralis major accounts for 17%, the latissimus dorsi accounts for 13.5%, the subscapularis
accounts for 10%, the combined infraspinatus and teres minor account for 9%, the biceps and triceps account for 7% each, the teres major accounts for 6.5%, the supraspinatus accounts for 3.5%, and finally the coracobrachialis accounts for 2% of the total muscle volume in the shoulder.34 Basset et al also reported that the physiological cross-sectional area of the deltoid was 23 cm2, the subscapularis was 16.3 cm2, the combined infraspinatus and teres minor were 13.7 cm2, the pectoralis major was 13.3 cm2, the latissimus dorsi was 12 cm2, the teres major was 8.8 cm2, the supraspinatus was 5.7 cm2, the biceps and triceps were 4 cm2 each, and finally the coracobrachialis was 1.6 cm2.34 Similarly, Veeger et al reported that the physiological cross-sectional area of the deltoid was 26 cm2, the pectoralis major was 13.7 cm2, the subscapularis was 13.5 cm2, the combined infraspinatus and teres minor were 12.4 cm2, the teres major was 10.0 cm2, the latissimus dorsi was 8.6 cm2, and the supraspinatus was 5.2 cm2.35 The muscle volume, cross-sectional area, and moment arms all influence the magnitude of torque generated by each muscle for a given motion. As these muscles work together in agonist and antagonist groups, injury or impairment to just one muscle can reduce ROM and/or result in joint instability.

Glenohumeral Instability

Glenohumeral joint stability is achieved by the coordinated function of both the dynamic (muscles) and static (ligaments, capsule, and articular constraint) stabilizers to orient and position the joint reaction force toward the glenoid fossa at all joint positions. Specifically, in the shoulder, muscles can stabilize the glenohumeral joint by the following five mechanisms: (1) passive muscle tension, (2) contraction causing compression across the joint, (3) joint motion that secondarily tightens passive ligamentous constraints, (4) barrier effect of a contracted muscle, and (5) redirection of the joint reaction force to the center of the glenoid articular surface.36

In contrast, glenohumeral joint instability occurs when the joint reaction force is directed outside the glenoid fossa, which is typically associated with (1) muscle imbalance, (2) glenoid or humeral wear/deformity, and/or (3) soft-tissue injury caused by trauma, joint overloading, or congenital deficiencies. Due to the complexity of this static/dynamic interaction, injury to just one or more tissues can dramatically impact stability and result in humeral head subluxation, humeral head dislocations, and/or compromised ability to recenter the humeral head on the glenoid during active motion19 (FIGURE 3.11). The inability to dynamically recenter the humeral head can lead to eccentric glenoid loading, reduced surface contact area, and a joint load concentration that can induce degenerative changes in the glenoid and/or humeral head.37,38,39,40,41

One of the most common soft-tissue injuries in the shoulder is a rotator cuff tear. Disruption of rotator cuff integrity, most commonly by a tear in one or more of the rotator cuff tendons, can have a significant impact on glenohumeral joint stability, with larger size and full-thickness tears resulting in greater loss of concavity-compression. As the rotator cuff fails to achieve concavity-compression and create a fulcrum that balances the superiorly directed deltoid force during arm elevation, the humeral head tends to migrate superiorly and impinge on the undersurface of the acromion (FIGURE 3.12). This impingement can lead to further tearing of the rotator cuff, typically starting with the supraspinatus and propagating to the superior portions of the subscapularis and infraspinatus. An unrepaired rotator cuff tendon will have compromised blood flow and reduced function which eventually leads to irreversible fatty atrophy of the rotator cuff muscle. This further reduces its function and results in the onset of arthritic changes that are secondary to increased friction and a lack of nutrients supplied to the cartilage. Continued rotator cuff tearing propagates further impingement and results in humeral head collapse, biceps tendon dislocation, and erosion of the superior glenoid, acromion, and coracoid.42,43 The resulting glenoid erosion can occur centrally or eccentrically; multiple classification systems44,45,46 have been developed to define the location and magnitude of glenoid erosion.

Anatomic Total Shoulder Arthroplasty

The bony geometry of the glenohumeral joint is highly variable. As described in TABLE 3.1, numerous anatomic studies have demonstrated that the morphology of the
humeral head in particular varies significantly relative to the intramedullary canal.10,47,48,49,50,51,52 These anatomic findings are relevant to hemiarthroplasty and ATSA because the intramedullary axis is coincident with the axis of the humeral stem prosthesis, also known as the orthopedic axis. As such, the intramedullary canal establishes the position of the humeral head for the majority of first- and second-generation shoulder arthroplasty systems when attempting to reconstruct the joint for treatment of glenohumeral arthritis. Failure to restore the patient’s original anatomy and joint CoR is believed to result in poorer clinical outcomes.53,54,55,56,57,58 Implanting too small a humeral head can result in excessive joint laxity and instability; whereas, implanting too large a humeral head can result in joint overstuffing and rotator cuff failure.

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Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on Biomechanics of Anatomic Total Shoulder Arthroplasty
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