Design und Biomechanik inverser Schulterprothesen

Joint-line medialization after anatomical total shoulder replacement requires more rotator cuff activity to preserve joint stability

BACKGROUND

The biomechanical effects of joint-line medialization during shoulder surgery are poorly understood. It was therefore the purpose of this study to investigate whether medialization of the joint line especially associated with total shoulder arthroplasty leads to changes in the rotator cuff muscle forces required to stabilize the arm in space.

METHODS

A validated computational 3-D rigid body simulation model was used to calculate generated muscle forces, instability ratios, muscle-tendon lengths and moment arms during scapular plane elevation. Measurements took place with the anatomical and a 2 mm and 6 mm lateralized or medialized joint line.

RESULTS

When the joint line was medialized, increased deltoid muscle activity was recorded throughout glenohumeral joint elevation. The rotator cuff muscle forces increased with medialization of the joint line in the early phases of elevation. Lateralization of the joint line led to higher rotator cuff muscle forces after 52° of glenohumeral elevation and to higher absolute values in muscle activity. A maximum instability ratio of >0.6 was recorded with 6 mm of joint-line medialization.

CONCLUSION

In this biomechanical study, medialization and lateralization of the normal joint line during total shoulder arthroplasty led to substantial load changes on the shoulder muscles used for stabilizing the arm in space. Specifically, medialization does not only lead to muscular shortening but also to increased load on the supraspinatus tendon during early arm elevation, the position which is already most loaded in the native joint.

The risk of suprascapular and axillary nerve injury in reverse total shoulder arthroplasty: An anatomic study

PURPOSE

Implantation of a reverse total shoulder arthroplasty (rTSA) places the axillary and suprascapular nerves at risk. The aim of this anatomic study was to digitally analyse the location of these nerves in relation to bony landmarks in order to predict their path and thereby help to reduce the risk of neurological complications during the procedure.

METHODS

A total of 22 human cadaveric shoulder specimens were used in this study. The axillary and suprascapular nerves were dissected, and radiopaque threads were sutured onto the nerves without mobilizing the nerves from their native paths. Then, 3D X-ray scans of the specimens were performed, and the distance of the nerves to bony landmarks at the humerus and the glenoid were measured.

RESULTS

The distance of the inferior glenoid rim to the axillary nerve averaged 13.6mm (5.8-27.0mm, ±5.1mm). In the anteroposterior direction, the distance between the axillary nerve and the humeral metaphysis averaged 8.1mm (0.6-21.3mm, ±6.5mm). The distance of the glenoid centre to the suprascapular nerve passing point under the transverse scapular ligament measured 28.4mm (18.9-35.1mm, ±3.8mm) in the mediolateral direction and 10.8mm (-4.8 to 25.3mm, ±6.1mm) in the anteroposterior direction. The distance to the spinoglenoid notch was 16.6mm (11.1-24.9mm, ±3.4mm) in the mediolateral direction and -11.8mm posterior (-19.3 to -4.7mm, ±4.7mm) in the anteroposterior direction.

CONCLUSIONS

Implantation of rTSA components endangers the axillary nerve because of its proximity to the humeral metaphysis and the inferior glenoid rim. Posterior and superior drilling and extraosseous screw placement during glenoid baseplate implantation in rTSA place the suprascapular nerve at risk, with safe zones to the nerve passing the spinoglenoid notch of 11mm and to the suprascapular notch of 19mm.