Firstly, medialization in the center of rotation doubles the lever arm of the deltoid muscle, allowing recruitment of more anterior and posterior deltoid fibers and therefore increasing deltoid abduction efficiency that compensate for a deficient rotator cuff. This phenomena is also responsible for impingement of the medial border of the humeral component on the scapular neck when the arm is adducted.57 Repetitive contact between polyethylene and bone may result in polyethylene wear debris, chronic inflammation and osteolysis,58,59 radiolucency around the glenoid component, loosening20 of the glenoid component,60 presence of an inferior bone spur and ossification in the glenohumeral space.6,57,61 Effectively, changes on the humeral side such as osteolysis, osteopenia, the development of medial and lateral cortical bone narrowing associated with osteopenia, condensation lines around the tip of the stem, and a spot weld between the cortical bone and the stem, radiolucent lines and loosening have also been reported.57,62 Therefore, the glenosphere has to be implanted on the lower part of the glenoid to avoid notching.57,58
Secondly, the fixed nature of the glenosphere places torsional forces on the humerus that may affect humeral component instability.57
Thirdly, the semi-constrained nature of the prosthesis created by the weight-bearing part being convex and the supported part concave (reversal of the ball and socket),57 restores glenohumeral stability. The lever of the deltoid muscle is almost doubled with a RSA, as is, therefore, the abduction efficiency of the deltoid. Under such tension, the deltoid provides the stable fulcrum essential for shoulder active anterior
elevation and prosthesis stability.7 The increase in compressive force component has also a stabilizing effect on the humeral head.63
Finally, and most relevant to this thesis, is lengthening of the arm which provides space for full range of motion of the proximal humerus, enhances stability, and retensions the deltoid (appendix 1). The type of glenosphere (size, eccentricity) allows for the adjustment of arm length by several millimeters (about 1% of arm length). Consequently, the only key factor for arm length is humerus length (including height of stem implantation, polyethylene thickness and spacer), as these factors permit the correction of arm length by several centimeters (about 10% of arm length).
Retension of the deltoid is critical due to the semi-constrained design of the prosthesis. The increase in compressive force between the humeral and glenoid components has a stabilizing effect.63 This tension is determined by arm length (illustration 4).
Illustration 4: Influence of glenosphere position in the vertical plane:
A: superior implantation of the baseplate or the use of a non-eccentric glenosphere does not allow proper deltoid tensioning. B: Use of an eccentric glenosphere or inferior positioning of the glenosphere in the vertical plane allows satisfactory deltoid tensioning. From Lädermann et al. 64
With the Grammont-type design, implantation of the humeral stem at the humeral cut then using the thickness of the polyethylene insert to obtain appropriate deltoid tension would seem to be a reasonable option.65,66 However, a variety of RSA designs are currently available and each design has its own anatomic characteristics (e.g. humeral cut angle and lateralizing designs) that may affect arm length (appendix 1 et 3).
Typical postoperative lengthening of the arm, humerus, and subacromial space (acromiohumeral) with a Grammont style prosthesis is summarized in Table 4.
Table 4: Average lengthening of arm, humerus and subacromial space in millimeters.64
Mean arm lengthening varies from 15 mm to 27 mm and mean humeral lengthening varies from -5 to 5 mm.64 The humeral cut is typically more aggressive when a transdeltoid surgical approach is performed but this is compensated for by an increase in thickness of the polyethylene liner.
Failure to adequately tension the deltoid may result in prosthetic instability, the most common clinically significant complication.66 Moreover, other complications following RSA, such as neurological lesions, fractures of the acromion, or permanent abduction of the arm,6,7,63,67,68, have also been described and could also be related to retensioning.66
Postoperatively, the arm is lengthened by approximately 24 to 27 mm beyond normal length.65,66,69,70 Biomechanically, the tension on the deltoid and the acromion is therefore greater due to arm lengthening and longer lever arm.
Adequate deltoid tension is accepted as a key to prosthetic function and stability.7,66,69 This tension is determined by arm length. The latter is dependent on 1) the position of the glenosphere in the frontal plane, 2) the size of the glenosphere, 3) the use of an eccentric or inferiorly tilted glenosphere, 4) the use of a spacer, 5) the thickness of the polyethylene, and 6) the height of humeral cut and stem implantation (Illustration 5).64
Illustration 5: Influence of humeral cut on arm length:
(A) Preoperative status with a lack of deltoid tension. (B-C) A low humeral cut induces a low implantation of the stem with a lack of deltoid retensioning. (D-E) A high humeral cut leads to a high implantation of the prosthetic stem with adequate deltoid retensioning.64
Different methods for measuring arm lengthening have been proposed (illustrations
6A
6B
Illustration 6: Technique proposed by Lädermann:
6A: Preoperative and postoperative true anteroposterior, bilateral, magnification-controlled radiographs of the humeri with neutral rotation and the patient standing.
6B: An epicondylar line (EL) defined as being between the most lateral part of the medial and lateral epicondyle. The diaphyseal axis (DI), is determined by a line drawn in the center of the proximal humeral medullary canal. The intersection between the epicondylar line and the diaphyseal axis represents the point C. The intersection between the diaphyseal axis and top of the humeral head is named H.
The point A is located at the intersection between the diaphyseal axis and a perpendicular line passing through the most lateral and inferior point of the acromion. A, C, and H are represented by small white points; large white points correspond to the magnification control marker adhered on the skin of the arm. A, acromion; C, condyles; H, head; EL, epicondylar line; DI, diaphyseal axis; preop, preoperative; contro, controlateral; EF, enlargement factor.64
Illustration 7: Technique proposed by Jobin:
Radiographic measurements of the deltoid length from the inferolateral acromion tip to the midpoint of the deltoid tuberosity are shown (Left) preoperatively (d) and (Right) postoperatively (d’). 71
Illustration 8: Technique proposed by Renaud et al.:
Two main lines are placed for measurement; an acromial line that represents the superior cortex of the acromion, and a tangent line to the center of the prosthetic epiphysis or to the center of rotation of the humeral head perpendicular to the first line. The two latter lines represent the acromio–epiphyseal distance and are compared to provide a ratio of lengthening. 72
Illustration 9: External measurement technique proposed by Boileau et al.
The distance between the acromion and olecranon with the elbow flexed is determined on non-operated (A) and operated (B) sides.7