Shoulder Arthroplasty: Principles and Biomechanics

Fig. 6.1

The glenoid center line

The distance from the base of the coracoid to the greater tuberosity is called “lateral humeral offset” and generally measures about 57 mm. This reflects the size of the humeral head and the location of the joint line (the surface of the glenoid). The lateral humeral offset usually decreases in glenohumeral arthritis due to cartilage and bone loss on both sides of the joint. Shortening of the lateral humeral offset causes a decreased deltoid lever arm and a shortening of the resting length of the rotator cuff [4].

The physiological plane of elevation of the upper limb is situated on the plane of the scapula (anterior elevation) and not in the frontal plane (abduction) or in sagittal plane (flexion).

The biomechanics of the shoulder involves a complex variety of synchronous movements of the sternoclavicular, scapula-thoracic, and glenohumeral joints.

Anterior elevation of the glenohumeral joint is about 120°, combined with humerus lateral rotation.

In order to allow the arm to achieve full elevation (180°), a supplementary curve of 60° is needed and is possible because of scapula rotation.

6.3 Shoulder Stabilizers

The primary muscles and dynamic stabilizers of the shoulder can be divided into three primary groups. The scapulohumeral group includes the deltoid and rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis). The axioscapular group comprises muscles that act on the scapula and includes the rhomboids, trapezius, serratus anterior, and levator scapulae. The axiohumeral group includes the muscles that originate on the thorax and insert on the humerus and includes the latissimus dorsi and pectoralis major muscles.

Trapezius, rhomboids, serratus anterior, and levator scapulae are scapular rotators muscles. Scapula-thoracic joint is constituted by a sliding surface between anterior face of the scapula and thoracic cage. The coordinated movement between the scapula-thoracic joint and the glenohumeral joint has been defined by Codman as scapula-thoracic rhythm. The term scapula-humeral rhythm refers to the 2:1 ratio of glenohumeral to scapulothoracic motion. Full 180° elevation of the humerus cannot be achieved without 60° of upward rotation by the scapula on the thoracic spine [5].

The scapula-thoracic muscles transfer the potential energy of the trunk to kinetic energy in the shoulder. The kinetic train is a concept describing the transfer of energy from the trunk to the shoulder and arm. The scapula is a key link in the kinetic chain between the trunk and the shoulder. Any alteration in scapula-thoracic rhythm could predispose to shoulder joint modification. During an abduction movement of the arm, in the shoulder the glenoid (concave) is stable while the humerus (convex) abducts resulting in a sliding down or glide of the convex humerus on the concave glenoid surface.

The deltoid muscle is the primary abductor of the arm with supraspinatus contributing in the initiation of movement.

Biomechanically, during abduction of the arm at the shoulder, the supraspinatus muscle raises the arm during the first 15° of shoulder abduction. Then, from 15° to 90° of shoulder abduction, the medial deltoid assists to raise the arm biomechanically.

The rotator cuff muscles are important stabilizers of the glenohumeral joint during shoulder motion. They work in concert to elevate and rotate the arm, to compress and center the humeral head within the glenoid fossa, and to counteract antagonist moments from the three prime shoulder movers (deltoid, pectoralis major, and latissimus dorsi) at multiple shoulder angles.

Multiple muscles are activated synchronously to move the clavicle, scapula, and humerus to generate smooth movement of the arm.

The supraspinatus compresses, abducts, and generates a small external rotation torque peaking between 30° and 60° of elevation. In the absence of this check, the humeral head translates superiorly during humeral elevation resulting in subacromial impingement.

The infraspinatus and teres minor muscles provide glenohumeral compression, external rotation, and abduction. They also resist superior and anterior humeral head translation by exerting a posteroinferior force to the humeral head.

The subscapularis acts to produce glenohumeral compression, internal rotation, and abduction. Similar to infraspinatus, its muscle bellies generate their peak torque with the arm at 0° of abduction.

With rotator cuff pathology, altered kinematics and muscle activity are present, and superior humeral head translation increases and subacromial space decreases. In conditions such as osteoarthritis, cartilage degeneration and a collapsed head further alter the joint kinematics.

Retraction of the scapula is accomplished by the joint action of the trapezius and rhomboids. Upward rotation of the scapula is achieved by a force coupling of the upper trapezius, lower trapezius, and serratus anterior muscles. Scapular elevation is achieved through a force coupled action of the upper trapezius, elevator scapulae, and rhomboids. These force couples work together to rotate the scapula upward and contribute to the elevation of the arm.

The goal of conventional TSA is to restore stability, motion, strength, and smoothness—critical characteristics of a healthy shoulder joint. This is accomplished by replacing the humeral head and glenoid with prosthetic implants that are designed to recreate the original anatomy. In the presence of intact rotator cuff and extrinsic shoulder muscles, a TSA is successful in restoring motion and improving function (Fig. 6.2).

../images/429740_1_En_6_Chapter/429740_1_En_6_Fig2_HTML.jpg — Fig. 6.2

Anatomic resultant joint force

6.4 Prostheses Biomechanics

Conformity is the interrelationship of the articular surface of the glenoid to the humeral head (Fig. 6.3). Glenohumeral conformity has been reported to be one of the most critical implant-related features that may affect the occurrence of glenoid loosening.