1) Reverse shoulder arthroplasty designs impact joint biomechanics by altering the deltoid moment arm and tension through variations in glenosphere medialization/lateralization and humeral component design. 2) Medializing the glenosphere increases the deltoid moment arm but can increase scapular notching and instability risks, while lateralizing the glenosphere reduces these risks but decreases deltoid efficiency. 3) Lateralizing the humeral component improves deltoid wrapping and compression at the joint while maintaining deltoid efficiency compared to more medial designs.

1. Reverse Shoulder Arthroplasty
The Impact of Prosthesis Design on
Joint Biomechanics
Moby Parsons, MD
The Knee Hip and Shoulder Center
Portsmouth New Hampshire

2. Disclosures
• Consultant/Med Ed – Exactech
• Design Surgeon - Exactech

3. Torque and why the Reverse works
• Force acting at a distance
(wrench)
• Torque = F x d
• 2x d  2x T
• Deltoid produces torque to
lift arm
• Magnitude of that torque is
related to dimensions and
design of the prosthesis

4. • Reverse shoulder arthroplasty developed in
the 1970’s to treat CTA.
History

5. Original Design Philosophy: lateralized glenoid

6. History of Early
Iterations
• Catastrophic failure of
glenoid >40%
• Initial problems
– Significant shear
– Insufficient fixation
– Too much constraint
Failure

7. Grammont’s Original Design
• Rationale
–Medialization of COR
–Constrain fulcrum
–Lower humerus to increase
deltoid tension

8. • The lever arm
distance (L) is
increased and deltoid
force (F) is increased
by lowering and
medializing the center
of rotation which is
now also fixed
• Torque (F x L) in
abducting the arm is
increased.
1987 Grammont Modifications

9. 0
50
100
150
200
250
300
350
0 20 40 60 80
DeltoidForce(N)
Abduction Angle (Degrees)
Deltoid Force vs Abduction
Angle
Intact Cuff

10. Effect of Medialization on Abduction
• Medializing the center of rotation recruits
more of the deltoid fibers for elevation and
abduction

11. Issues with Grammont Design
• Scapular Notching
• Instability
• Deltoid Failure

12. Notching Statistics with Medialization

13. Other Issues with Medialization
• Deltoid Flattening
– No compressive
vector or “wrap”
– In cases of medial
glenoid wear, vector
may actually
promote instability
by pulling deltoid
laterally

14. Effect of Medialization on Residual Cuff Fxn
• CoR medialization slackens ER tendons reducing
their ability to generate force
• Posterior deltoid does not act as external rotator

15. Glenoid Design Debate
Medial CoR Lateral CoR

16. Rationale For Lateralization CoR
• Reduced scapular notching
• Improved deltoid “wrap”
• Potentially improve ROM
• Middle deltoid wraps
around the greater
tuberosity and causes
compression and
stability of the
articulation

17. Deltoid Wrapping
• Middle deltoid wraps
around the greater
tuberosity and causes
compression and stability of
the articulation
• Lateralization or either
humerus or glenoid will
increase deltoid wrap.

18. Issues with Glenoid Lateralization
• Reduced deltoid moment arm = more force
required to do same amount of work
• Increased shear force on fixation
– Net vector is less compressive

19. Fixation
Mechanics
“The effects of progressive lateralization of the joint CoR of
reverse shoulder implants“ -
– Constantini JD et al: JSES Jan 2015
– Lateralization results in an increased shear vector and decreased
compressive vector
– Resultant force is more destabilizing than for medialized
glenosphere

20. Other Glenoid-sided Factors
• Eccentricity
• Diameter
• Lateral Offset (0, +2, +4)
• Tilt

21. Effect of Glenosphere Eccentricity in
Grammont Style RSA
• Improved adduction ROM
• Improved impingement free abduction
• Reduced scapular notching

22. Eccentricity: Know what your system
offers
• Some systems offer this
through variable offset
glenosphere (Biomet
Versa-dial)
• Others through baseplate
design (Exactech)
• Others with fixed
eccentric offset options
(Tornier)

23. The effect of glenosphere diameter in reverse
shoulder arthroplasty on muscle force, joint load
and range of motion
- Langohr GD et al: JSES Dec 2014
• Larger diameter sphere
– Increased joint loads
– Improved adduction ROM
– Decreased IR range of motion (increased tension
on posterior capsule)
– No effect on joint stability
– These effects were magnified by lateral offset of
the glenosphere

24. Rotation vs. Revolution
• Rotation: axis of rotation goes through center of
rotation (spin)
• Revolution: axis of rotation offset from center of
rotation (orbit)

25. Effect of Glenosphere Size on Abduction ROM
• Is bigger better?
• Larger sphere = larger orbit = may be more
likely to impinge on acromion, coracoid or
scapula

26. Inferior Glenosphere Inclination
• Some benefit on
improved ROM but not
better than eccentricity
• No benefit to scapular
notching as inferior
reaming reduces length of
scapular neck
• Whatever you do do not
place high or tilt superior

27. Cocktail Napkin Test
• Simple conceptual
drawing will tell you
that inferior tilt does
not impove notching
as once thought
• Eccentricity and
lateralization are the
best solutions

28. Humerus Design Debate
Inset Socket Stack-on Socket

29. Medial Humerus vs. Lateral Humerus
• Medial Humerus = inset
prosthesis
• Deepest portion of socket
close to axis of shaft
• Lateral Humerus = onlay
prosthesis
• Deepest portion of socket
displaced medially from
axis of shaft

30. 1. Keep COR on face of the glenoid
2. Lower neck angle than Grammont: 145 vs 155
3. Lateralization of the shaft provides improved
deltoid wrap
Lateral Humerus Design Philosophy

31. 1. Reduced risk of scapular notching
2. Maintain deltoid efficiency relative to Grammont
3. Restore anatomic Cuff tensioning
4. Minimize risk of baseplate loosening
Lateral Humerus Design Philosophy

32. Reverse Design Paradigms
Medial Glenoid
Medial Humerus
Lateral Glenoid
Medial Humerus
Medial Glenoid
Lateral Humerus
Lateral Glenoid
Lateral Humerus
Tornier Aequalis
Depuy Delta
Zimmer TM
Lima SMR
Biomet TESS
S&N Promos
DJO RSP Exactech Equinoxe
Tornier Ascend Flex
Zimmer Anatomical
Zimmer Fx Inverse
Stryker Reunion
Biomet
Comprehensive
MG/MH LG/MH MG/LH LG/LH

33. Many Options: Biomechanics Differ: Design Matters
Tornier Reverse DJO RSP
Smith & Nephew
Promos
Lima SMR
Biomet
TESS
Versa
Reverse
Delta
Reverse
Biomet
Comprehensive
Zimmer TM
Reverse
Zimmer
Inverse FX
Zimmer
Anatomical
Inverse

34. Glenoid COR Offset: 2mm
Humeral Offset: 20.8mm
Glenoid COR Offset: 10mm
Humeral Offset: 10.9mm
Glenoid COR Offset: 0mm
Humeral Offset: 9.8mm
Comparative Design Differences
MG/MH MG/LHLG/MH

35. Deltoid Moment Arm
• Abduction MA decreases with glenoid-side lateralization
(LG/MH: Encore)
• Abduction MA increases with humeral sided
lateralization (MG/LH: Equinoxe)
Lateral COR Medial COR
MG/MH MG/LHLG/MHAnatomic

36. Comparison of Deltoid Moment Arms
– Deltoid Efficiency at
Low Abduction Angles
– MG/LH: 350%
– MG/MH: 350%
– LG/MH: 270%
– Normal: 100%

37. Measuring Deltoid Wrapping
• Lateralizing the humerus
improves deltoid wrapping.
MG/LHMG/MH LG/MH
Anatomic
8°
48°
40°28°

38. Simulated Medial Wear
-1° 12° 18°
MG/LHMG/MH LG/MH

39. Hierarchy of Stability Factors
1. Compressive force
– Glenoid Lateralization: higher compressive force
– Humeral Lateralization: deltoid wrap
2. Socket depth
**Sphere size had
minimal impact

40. Rotator Cuff Muscles
Herrmann S. Journal of Ortho Surg Res 2011

41. Constants and Variables
• Muscle Origin and Insertion - Constant
• New Center of Rotation - Variable
• How does the NEW Center of Rotation
impact the function of the Cuff post-
RSA
– How do the different designs compare?

42. Reverse shoulder arthroplasty leads to
significant biomechanical changes in the
remaining rotator cuff
- Hermann S et al: J Orthop Surg Res Aug 2011
• Moment arms for humeral rotation are sig. smaller
for upper SSC and all segments of Tmin above 30
degrees

43. Residual Cuff Tension
MG/LHMG/MH LG/MHAnatomic

44. Optimal Humeral Component Version?
• Compromise between scapular impingement,
ER ROM and moment arm of IS and TM
• Several studies have looked at this
• Consensus: too much anteversion (-0+) and
too much retroversion (-40+) are bad
• Best compromise appears to be around 20
degrees or native version if close to this

45. Should we repair the
Subscapularis?
• Reverse shifts SC insertion below CoR

46. Biphasic Nature of the Subscapularis /
Infraspinatus

47. Subscapularis functions as ADDuctor at low abduction

48. Should the Subscap be Repaired?
• 7/76 patients with irreparable subscap dislocated
• All cases with Grammont-style MGMH prosthesis
– Limited deltoid wrapping

49. Clark et al JSES 2012
• 120 Shoulders 65 repaired / 55 unrepaired
– 6 months of f/u minimum
• No Difference in ROM, instability, pain, or
complications with or without a
subscapularis repair
• All cases using a lateralized prosthesis

50. Clinical Relevance: Subscapularis
• Intact Subscapularis will force the Deltoid to work
harder to achieve abduction
– At least for the first 80 degrees..
• Intact Subscapularis forces the posterior cuff to work
harder to avoid Hornblower’s Sign
• Not repairing the Subscapularis is associated with
instability in some designs
– Obligatory Subscap repair has a consequence

51. In Conclusion
• Not all systems are created equally
• Prosthesis design has mechanical implications
• Understand your choices