reverse shoulder arthroplasty has been described as an effective treatment for several pathological conditions of the shoulder , such as cuff tear arthropathy , fracture sequelae , and revision shoulder arthroplasty . however , inferior scapular notching has been a frequently reported complication with an incidence ranging between 44% and 96% . caused by the reversed implant design concept , inferior impingement occurs between the humeral cup and the scapular neck during adduction of the arm and has been recognized as a mechanical factor of inferior scapular notching . furthermore , polyethylene wear may create a biologic reaction of the joint with inflammation and local osteolysis and progression of the notch . the clinical impact of inferior scapular notching remains unclear since controversial reports exist on this topic . in contrast , simovitch et al . , sirveaux et al . and wellmann et al . clinical studies have shown that an inferior position of the baseplate and glenosphere can reduce the incidence of notching . furthermore , the effect of implant placement on inferior impingement and adduction deficit has been investigated biomechanically . in concordance with the clinical observations , increased inferior overlap of the glenosphere reduced the adduction deficit . the reduction of the humeral neck - shaft angle and cup depth improved the impingent - free adduction and lateralization of the center of rotation resulted in an overall increased range of motion . de wilde et al . investigated the influence of the size and position of the glenosphere , the inclination angle of the humeral epiphysis and the depth of the humeral cup on inferior scapular impingement in a two - dimensional ( 2d ) computer model and concluded that increased inferior overhang of the glenosphere reduced the adduction deficit of the reverse prosthesis most effectively . the computerized scapula model in this study was based on an average anatomical shape using the data of 200 scapulae investigated by middernacht et al . the purpose of this biomechanical study was not only to investigate the influence of the glenosphere position and implant design features on the inferior contact between the humeral component and the scapula during adduction of the arm . furthermore , the simultaneous effect of the individual scapular anatomy on this conflict was studied . our hypothesis was that the size and position of the glenosphere , the humeral cup depth and the anatomy of the scapula had a combined influence on the inferior scapular impingement of the reverse shoulder arthroplasty . thirteen human cadaveric shoulder specimens with no radiographic evidence of glenohumeral osteoarthritis or cuff tear arthropathy were obtained for the study . prior to the preparation and biomechanical testing , all specimens were evaluated by computed tomography ( ct ) scans defining different parameters of scapula anatomy . only shoulders without any signs of fracture sequelae none of the specimens had a pathologic glenoid retroversion ( > 25 ) or a significant posterior subluxation of the humeral head . for further fixation of the shoulders in the testing apparatus , the soft tissue of the medial scapula margin to the scapula neck was dissected leaving the lateral portion of the rotator cuff muscles and the musculotendinous junction intact . briefly , the scapula was potted in a custom made box using a two - component polyurethane casting resin ( ureol fc 53 , vantico gmbh , wehr , germany ) . afterward , the scapula block was rigidly attached to the mounting tower using three threaded rods . neutral rotation of the humerus was defined by placing a k - wire in the humerus shaft , which was aligned to the axis of the forearm . the humerus was then transected approximately 20 cm distal to the center of the humeral head and potted in a brass cylinder . , the humeral head was centered in the glenoid the geometric center of the humeral head was defined to be zero point to measure anterior - posterior and superior - inferior translations . this zero point was saved for each specimen in the intact condition and re - established by the robot at the beginning of every following test condition . the setup consists of a guided industrial robot ( kuka gmbh , augsburg , germany ) , outfitted with a six - component force - moment sensor ( fms ) ( ipea , gmbh , berlin , germany ) to which the humerus is attached . the robot applies controlled motion and loading to the glenohumeral joint [ figure 1a ] . thereby the scapula is fixed to the mounting tower . the scapula is first aligned to the tower by placing the metaglene of the prosthesis exactly in the vertical plane . the specimens were adjusted by inserting a threaded rod in the central drill hole of the glenosphere . the direction of the rod was then brought into line with the horizontal axis using a water level bubble . testing setup : the humerus is attached to the arm of an industrial robot , outfitted with a force - moment sensor . the scapula body was potted using casting resin and rigidly fixed to the mounting tower using three threaded rods the robot is able to control motion by means of specially written control software and by the load and moment data provided by the fms . in the current study , a load controlled algorithm was utilized , the robot / fms system enables measurement of motion with a resolution of 0.02 mm and measurement of joint loading with a resolution of < 0.3 n force . for implantation of the reverse prosthesis ( delta xtend , depuysynthes ) an extended deltopectoral approach thereby the subscapularis tendon , supraspinatus tendon , and infraspinatus tendon were dissected in the course of soft tissue preparation . further , the pectoralis major tendon was released at the proximal 1/3 of its humeral insertion , and the long head of the triceps was released from the inferior scapula neck . the aim of the extensive soft tissue preparation was to get a clear view on the scapula neck for visual control of the testing procedure . the resection of the humeral head was performed with an inclination of 155 and with neutral version ( aligned to the k - wire placed in the humeral head collinear with forearm axis ) using a cutting guide . the preparation of the glenoid included a circumferential release of the glenoid labrum and a dissection of the remaining joint capsule to achieve a clear view of the entire glenoid fossa . the central peg of the metaglene was then placed at the center of the inferior circle of the glenoid so that its border followed the inferior edge of the glenoid . before fixation of the metaglene , a contact pressure film was interposed between the glenoid and the metaglene surface completely covering the inferior scapula neck ( tekscan 5051 , tekscan , inc . , south boston , ma ) [ figure 1a ] . the measurement range of the tekscan - film is given with 50 pounds force per square inch ( psi ) , and the resolution is defined by a number of 62 pressure sensors / cm . serial testing of the specimens was performed using a 38 and 42 mm glenosphere in a centric and an eccentric ( inferior overlap ) design . the testing sequence was set to be : 38 mm centric , 42 mm centric , 38 mm eccentric , 42 mm eccentric . further , each type of glenosphere was tested using a standard humeral onlay first and using a more shallow onlay ( high mobility onlay ) in a second step . the starting position was defined to be 30 of glenohumeral abduction in the neutral rotation of the arm . in this position n of lateral compression to the arm . in this position , the center of rotation of the shoulder was defined for each test condition using an algorithm published before . during the testing procedure , the arm of the specimen was adducted by the robot in the scapular plane with an angular velocity of 0.2/s . the arm movement was operated under force control with a maximum moment of 3 nm to avoid structural damage to the joint . thereby the contact force measured at the scapula neck was simultaneously monitored . with the appearance of contact pressure higher than 5 n / cm ( 7,25 psi ) the movement of the robot was stopped , and the scapula notching angle ( sna ) was measured [ figure 1b ] . the angle was defined to be positive if the arm was measured to be in an abduction position and it was defined to be negative , if the arm was measured to be in an adduction position . the threshold of 5 n / cm for the contact pressure measurements was defined during pretests to reflect an impingement of the prosthesis with scapula neck and to exclude any bias by soft tissue interposition . a positive sna displays a contact of the epiphysis at the inferior scapula neck in an abduction position of the arm . a negative sna displays a contact of the epiphysis at the inferior scapula neck in an adduction position of the arm computed tomography scans of the specimens were performed before and after reverse shoulder arthroplasty and biomechanical testing . the ct scans before implantation of the prosthesis were performed to exclude specimens with pathologic glenoid retroversion , pathologic glenoid inclination and with a significant glenoid defect or severe glenoid erosion . further , the humeral anatomy was investigated to exclude cases of fracture sequelae . for each specimen , three - dimensional ( 3d ) reconstructions of the glenoid with elimination of the humerus were performed . the following parameters were defined on the postoperative 3d reconstructions : implantation height of the metaglene : distance of the inferior aspect of metaglene to the most inferior aspect of the glenoid ( v ) , length of the scapula neck ( h ) , lateral pillar angle (  ) : angle between the margo lateralis scapulae and the inferior scapula neck [ figures 2a and b ] . parameters of the scapula anatomy and metagalene position , which were measured on three - dimensional - computed tomography  reconstructions : implantation height of metaglene : distance of the inferior aspect of metaglene to the most inferior aspect of the glenoid , ( v ) , length of the scapula neck including the width of the metaglene ( h ) , lateral pillar angle ( a ) : angle between the margo lateralis scapulae and the vertical to the inferior scapula neck exemplary three - dimensional - computed tomographyreconstruction with scapula measurements . the metaglene was placed 1.8 mm above the most inferior level of the glenoid ( v ) , a was defined to be 50.5 and the h was measured to be 13.6 mm statistical analysis of the data for the intact / vented joint and for all sequential conditions was performed using both unpaired and paired student 's t - tests ( spss inc . , thirteen human cadaveric shoulder specimens with no radiographic evidence of glenohumeral osteoarthritis or cuff tear arthropathy were obtained for the study . prior to the preparation and biomechanical testing , all specimens were evaluated by computed tomography ( ct ) scans defining different parameters of scapula anatomy . only shoulders without any signs of fracture sequelae none of the specimens had a pathologic glenoid retroversion ( > 25 ) or a significant posterior subluxation of the humeral head . for further fixation of the shoulders in the testing apparatus , the soft tissue of the medial scapula margin to the scapula neck was dissected leaving the lateral portion of the rotator cuff muscles and the musculotendinous junction intact . briefly , the scapula was potted in a custom made box using a two - component polyurethane casting resin ( ureol fc 53 , vantico gmbh , wehr , germany ) . afterward , the scapula block was rigidly attached to the mounting tower using three threaded rods . neutral rotation of the humerus was defined by placing a k - wire in the humerus shaft , which was aligned to the axis of the forearm . the humerus was then transected approximately 20 cm distal to the center of the humeral head and potted in a brass cylinder . , the humeral head was centered in the glenoid the geometric center of the humeral head was defined to be zero point to measure anterior - posterior and superior - inferior translations . this zero point was saved for each specimen in the intact condition and re - established by the robot at the beginning of every following test condition . the setup consists of a guided industrial robot ( kuka gmbh , augsburg , germany ) , outfitted with a six - component force - moment sensor ( fms ) ( ipea , gmbh , berlin , germany ) to which the humerus is attached . the robot applies controlled motion and loading to the glenohumeral joint [ figure 1a ] . thereby the scapula is fixed to the mounting tower . the scapula is first aligned to the tower by placing the metaglene of the prosthesis exactly in the vertical plane . the specimens were adjusted by inserting a threaded rod in the central drill hole of the glenosphere . the direction of the rod was then brought into line with the horizontal axis using a water level bubble . testing setup : the humerus is attached to the arm of an industrial robot , outfitted with a force - moment sensor . the scapula body was potted using casting resin and rigidly fixed to the mounting tower using three threaded rods the robot is able to control motion by means of specially written control software and by the load and moment data provided by the fms . in the current study , a load controlled algorithm was utilized , the robot / fms system enables measurement of motion with a resolution of 0.02 mm and measurement of joint loading with a resolution of < 0.3 n force . for implantation of the reverse prosthesis ( delta xtend , depuysynthes ) an extended deltopectoral approach thereby the subscapularis tendon , supraspinatus tendon , and infraspinatus tendon were dissected in the course of soft tissue preparation . further , the pectoralis major tendon was released at the proximal 1/3 of its humeral insertion , and the long head of the triceps was released from the inferior scapula neck . the aim of the extensive soft tissue preparation was to get a clear view on the scapula neck for visual control of the testing procedure . the resection of the humeral head was performed with an inclination of 155 and with neutral version ( aligned to the k - wire placed in the humeral head collinear with forearm axis ) using a cutting guide . the preparation of the glenoid included a circumferential release of the glenoid labrum and a dissection of the remaining joint capsule to achieve a clear view of the entire glenoid fossa . the central peg of the metaglene was then placed at the center of the inferior circle of the glenoid so that its border followed the inferior edge of the glenoid . before fixation of the metaglene , a contact pressure film was interposed between the glenoid and the metaglene surface completely covering the inferior scapula neck ( tekscan 5051 , tekscan , inc . , south boston , ma ) [ figure 1a ] . the measurement range of the tekscan - film is given with 50 pounds force per square inch ( psi ) , and the resolution is defined by a number of 62 pressure sensors / cm . serial testing of the specimens was performed using a 38 and 42 mm glenosphere in a centric and an eccentric ( inferior overlap ) design . the testing sequence was set to be : 38 mm centric , 42 mm centric , 38 mm eccentric , 42 mm eccentric . further , each type of glenosphere was tested using a standard humeral onlay first and using a more shallow onlay ( high mobility onlay ) in a second step . the starting position was defined to be 30 of glenohumeral abduction in the neutral rotation of the arm . in this position , the center of rotation of the shoulder was defined for each test condition using an algorithm published before . during the testing procedure , the arm of the specimen was adducted by the robot in the scapular plane with an angular velocity of 0.2/s . the arm movement was operated under force control with a maximum moment of 3 nm to avoid structural damage to the joint . thereby the contact force measured at the scapula neck was simultaneously monitored . with the appearance of contact pressure higher than 5 n / cm ( 7,25 psi ) the movement of the robot was stopped , and the scapula notching angle ( sna ) was measured [ figure 1b ] . the angle was defined to be positive if the arm was measured to be in an abduction position and it was defined to be negative , if the arm was measured to be in an adduction position . the threshold of 5 n / cm for the contact pressure measurements was defined during pretests to reflect an impingement of the prosthesis with scapula neck and to exclude any bias by soft tissue interposition . definition of the scapula notching angle ( sna ) . a positive sna displays a contact of the epiphysis at the inferior scapula neck in an abduction position of the arm . a negative sna displays a contact of the epiphysis at the inferior scapula neck in an adduction position of the arm computed tomography scans of the specimens were performed before and after reverse shoulder arthroplasty and biomechanical testing . the ct scans before implantation of the prosthesis were performed to exclude specimens with pathologic glenoid retroversion , pathologic glenoid inclination and with a significant glenoid defect or severe glenoid erosion . further , the humeral anatomy was investigated to exclude cases of fracture sequelae . for each specimen , three - dimensional ( 3d ) reconstructions of the glenoid with elimination of the humerus were performed . the following parameters were defined on the postoperative 3d reconstructions : implantation height of the metaglene : distance of the inferior aspect of metaglene to the most inferior aspect of the glenoid ( v ) , length of the scapula neck ( h ) , lateral pillar angle (  ) : angle between the margo lateralis scapulae and the inferior scapula neck [ figures 2a and b ] . parameters of the scapula anatomy and metagalene position , which were measured on three - dimensional - computed tomography  reconstructions : implantation height of metaglene : distance of the inferior aspect of metaglene to the most inferior aspect of the glenoid , ( v ) , length of the scapula neck including the width of the metaglene ( h ) , lateral pillar angle ( a ) : angle between the margo lateralis scapulae and the vertical to the inferior scapula neck exemplary three - dimensional - computed tomographyreconstruction with scapula measurements . the metaglene was placed 1.8 mm above the most inferior level of the glenoid ( v ) , a was defined to be 50.5 and the h was measured to be 13.6 mm statistical analysis of the data for the intact / vented joint and for all sequential conditions was performed using both unpaired and paired student 's t - tests ( spss inc . , we found a significant influence of the glenosphere size on the sna [ figure 3 ] . the sna was decreased by 5.8 when shifting from a 38 mm centric glenosphere to a 42 mm centric glenosphere ( p < 0.005 ) . comparing the 38 mm centric glenosphere with 38 mm eccentric glenosphere , the sna decreased by 8.9 ( p < 0.005 ) . comparing the 42 mm centric glenosphere with 42 mm eccentric glenosphere , the solitary change of the glenosphere geometry from a standard ( centric ) to an eccentric design significantly decreased the sna for each glenosphere size [ figure 4 ] . the increase was higher for the 38 mm glenosphere compared to the 42 mm glenosphere . the solitary replacement of the standard onlay to the more shallow hmo - onlay significantly decreased the sna depending on the glenosphere size tested [ figure 5 ] : 7.7 with a 38 mm centric glenosphere , 7.4 with a 38 mm eccentric glenosphere , 8.2 with a 42 mm centric glenosphere , 8.0 with a 42 mm eccentric glenosphere ( p = 0.001 ) . the simultaneous replacement of the onlay ( standard to hmo ) and the glenosphere from the standard design to the eccentric design resulted in the following improvements ( decrease ) of the sna : 16.3  5.8 , shifting from 38 mm centric to 38 mm eccentric glenosphere ; 19.0  7.3 , shifting from 32 mm centric to 42 mm eccentric glenosphere [ figure 6 ] . regarding the superior - inferior position of the metaglene the sna was found to significantly correlate with the implantation height of the metaglene [ figure 7 ] . the sna significantly correlated with the scapula neck length ( p = 0.029 ) and with the lateral pillar angle ( a ) [ figures 8 and 9 ] . a long scapula pillar length and a large lateral pillar angle were correlated with a lower sna . improvement of humerus adduction given by the scapula notching angle by changing the glenosphere size . the results are shown for the standard onlay and the more shallow high mobility onlay . all differences were found to be significant ( p < 0.05 ) improvement of the scapula notching angle by changing the geometry of the glenosphere from a centric to an eccentric design improvement of the scapula notching angle by a solitary change from the standard to the high mobility onlay scapula notching angle dependent on the glenosphere / onlay combination . positive values indicate a notching in an abduction position whereas negative values indicate a notching in an adduction position of the humerus . the boxplots display the median , the upper and lower quartile as well as the maximum and minimum value correlation of the scapula notching angle with the superior - inferior implantation height of the glenosphere correlation of the scapula notching angle with the scapula neck length correlation of the scapula notching angle with the lateral pillar angle in our study , an increased inferior overlap of the glenosphere resulted in a reduction of the adduction deficit of the grammont style reverse delta xtend prosthesis . this finding confirms the results of previous biomechanical investigations and the 2d computer model study by de wilde et al . in accordance , clinical studies have shown a reduced appearance of inferior scapular notching in the presence of an increased inferior overlap of the glenosphere . first , inferior baseplate placement flush to the inferior glenoid rim is an important precondition . our study has demonstrated a significant increase of the adduction deficit in the case of superior position of the metaglene baseplate . in concordance , previous authors recommended an inferior baseplate placement . however , in our study , the selection of a concentric 42 mm diameter implant showed only a small effect on the reduction of the adduction deficit compared to the 38 mm concentric glenosphere , which was previously been shown by gutirrez et al . the gain of adduction with the larger diameter implant previously was attributed to increased inferior overlap in the 2d computer model study by de wilde et al . another technical option to achieve our results confirmed this theory and reduced adduction deficits were observed in the presence of eccentric implants . interestingly , the eccentric 38 mm glenosphere showed better adduction angles than the 42 mm concentric implant , although the design given inferior distance from the central screw to the most inferior rim is identical for both glenospheres . this finding is explained by the smaller humeral diameter of the 38 mm cup compared to the larger 42 mm size , which results in later contact with the scapula during adduction . however , the 42 mm eccentric glenosphere allowed more adduction compared to the 38 mm eccentric glenosphere since the inferior overlap of the size 42 mm implant was 2 mm larger . eccentric glenospheres may increase rocking horse forces on the baseplate , potentially leading to implant loosening . this issue has not been investigated in this study , but in our own clinical experience , we have not seen this complication with the glenosphere design used here . in accordance with the 2d computer model study by de wilde et al . , the reduction of the humeral cup depth resulted in improved adduction angles , regardless the size of glenosphere . however , biomechanically , the increased range of motion with reduced humeral cup depth has the disadvantage of decreased stability , which must be considered in clinical practice . regarding a further parameter affecting scapula notching oh et al . found a significant increase of adduction with decreasing neck - shaft angles of the humeral prosthesis . in a biomechanical setup , the authors found 2 of abduction for a 155 neck - shaft angle , 7 of adduction for a 145 neck - shaft angle and 12 of adduction for a 135 neck - shaft angle . a maximum adduction of 7 - 12 was achievable using a 42 mm eccentric glenosphere in combination with a shallow onlay . a further biomechanical cadaver study by berhouet et al . confirmed an increased size of the glenosphere as well as a lateralization of the center of rotation to result in an improved adduction angle . moreover , the authors found that inferior scapula contact occurred earliest when the epiphysis was placed in 40 of retroversion and occurred later with 20 and 10 of humeral retroversion . de wilde et al . described the negative influence of a short horizontal scapular pillar on the adduction deficit of the reverse shoulder arthroplasty in a 2d computer model . in accordance , the length of the horizontal pillar of the scapula showed a significant impact on inferior impingement in our biomechanical study . in the presence of a short scapular pillar length significantly larger adduction deficits were observed . therefore , regarding the notching conflict , lateralization created by the glenosphere implant design or bio rsa might be beneficial in the presence of a short horizontal pillar of the scapula . biomechanically , however , lateralized glenosphere implant design may create rocking horse forces , potentially leading to implant loosening . furthermore , in our study , a small angle between the lateral angulated pillar of the scapula and the vertical plane increased the adduction deficit in the presence of a short horizontal pillar , and lacking inferior overlap of the glenosphere pronounced this effect . since only 13 shoulder specimens were investigated , further relevant anatomical variants of the scapula might not have been detected . glenoid bone loss and humeral pathologies , such as osteophytes and deformities could influence the inferior impingement of the reverse shoulder arthroplasty . during our biomechanical investigation , therefore , this model could not investigate the influence of the natural scapulothoracic motion and scapular tilt . furthermore , the position of the scapula on the thorax and muscle loading forces of the deltoid and rotator cuff muscles might influence the inferior impingement . our model did not consider the effect of glenosphere tilt in the coronal and horizontal plane . in the present study , the humeral component was implanted in 0 of retroversion in order to standardize the protocol , modification of the humeral version might influence inferior scapular impingement . however , our study confirms the importance of inferior placement of the glenosphere to decrease the adduction deficit of the reverse shoulder arthroplasty previously described in the literature . furthermore , critical preoperative radiological analysis of the scapular anatomy and good intraoperative exposure of the horizontal pillar of the scapula can be helpful to prevent inferior scapular impingement . this study demonstrates the importance of inferior glenosphere placement to decrease inferior scapular impingement of the reverse shoulder arthroplasty . the presence of a short horizontal pillar of the scapula can have a negative prognostic effect on inferior impingement during adduction of the arm .
purpose : the presence of inferior scapula notching is significantly affected by the anatomy the scapula and can be influenced by the glenosphere design and position and the onlay type.materials and methods : a biomechanical study was undertaken with 13 human shoulder specimens in a robot - assisted shoulder simulator . 
 inferior scapula contact during adduction of the humerus was detected using a contact pressure film . 
 computed tomography scans with three - dimensional reconstructions of each specimen were performed.results:the greatest improvement of the scapula notching angle ( sna ) was achieved by simultaneous implantation of a shallow humeral onlay and an eccentric glenosphere design : 16.3 - 19.0 ( p < 0.005 ) . 
 the sna was significantly decreased by 5.8 when shifting from a 38 mm centric glenosphere to a 42 mm centric glenosphere ( p < 0.005 ) and by 8.9 comparing the 38 mm centric glenosphere with 38 mm eccentric glenosphere ( p < 0.005 ) . 
 the solitary implantation of a shallow onlay significantly decreased the sna depending on the glenosphere size between 7.4 and 8.0 ( p = 0.001 ) . 
 a more inferior position of the metaglene as well as a long scapula neck ( p = 0.029 ) and a large lateral scapula pillar angle ( p = 0.033 ) were correlated with a lower sna.conclusion:this study demonstrates the importance of inferior glenosphere placement and the benefit of eccentric glenosphere and shallow humeral cup design to reduce the adduction deficit of the reverse shoulder . 
 the presence of a short neck of the scapula can have a negative prognostic effect on inferior impingement during adduction of the arm.level of evidence : basic science study