What is the best way to determine humeral height in Total Shoulder Arthroplasty (TSA)?

Determining Humeral Height in Total Shoulder Arthroplasty

The best way to determine humeral height in Total Shoulder Arthroplasty (TSA) is to reduce the tuberosities to their anatomical place in the prosthesis.

Rationale for Anatomical Tuberosity Reduction

Proper humeral height determination is critical for optimal outcomes in TSA. The evidence strongly supports using anatomical landmarks rather than arbitrary measurements for several important reasons:

Anatomical Considerations

• Reducing tuberosities to their anatomical position provides a patient-specific reference that accounts for individual anatomical variation 1
• Using preserved nonarticular landmarks of the proximal humerus can accurately predict native humeral head radius of curvature and head height even in osteoarthritic, deformed humeral heads 1
• This method shows excellent reliability with intraobserver differences of only 0.5 mm for radius of curvature and 1.0 mm for head height 1

Consequences of Improper Height Placement

• Placing the humeral head prosthesis too high relative to the tuberosities impairs shoulder function through two mechanisms 2:

  1. The inferior capsule becomes tight at lower abduction angles, limiting range of motion
  2. The center of rotation is displaced upward, resulting in smaller moment arms and decreased abduction moments of rotator cuff muscles

• A high-positioned prosthesis can reduce maximum abduction angle by 10-16 degrees and decrease the moment arms of the infraspinatus and subscapularis by 4-10 mm 2

Problems with Alternative Methods

Fixed Measurements (5 cm proximal to Pectoralis Major)

• Using arbitrary measurements like 5 cm proximal to pectoralis major insertion fails to account for significant anatomical variation between patients
• Studies show the anatomy of the proximal humerus has a wide range of variables such as medial offset (3.9-8.6 mm) and greater tuberosity offset (2.5-9.2 mm) 3
• Fixed measurements don't account for individual differences in humeral length and anatomy

Acromion-Based Measurements (8-10 mm from acromion)

• Using the acromion as a reference point is problematic because:
  • The relationship between acromion and humeral head varies significantly between individuals
  • Acromial morphology itself varies considerably
  • This method doesn't account for potential acromial pathology or previous acromioplasty

Clinical Implications

• Improper humeral height determination can lead to:

  • Limited range of motion
  • Loss of abduction strength
  • Overload with long-term failure of the supraspinatus tendon 2
  • Altered biomechanics leading to accelerated component wear

• A study of 125 stemmed and 43 resurfacing shoulder arthroplasties found that 31.2% of stemmed cases and 65.1% of resurfacing cases had significant deviations from anatomic reconstruction, with improper humeral head size selection being a major factor 4

Practical Approach to Determining Humeral Height

1. Identify preserved nonarticular landmarks on the proximal humerus
2. Use these landmarks to map the native humeral head geometry
3. Reduce the tuberosities to their anatomical position relative to the prosthesis
4. Verify proper tension of the rotator cuff and deltoid muscles
5. Confirm appropriate range of motion without impingement

Common Pitfalls to Avoid

• Relying solely on fixed measurements that don't account for individual anatomy
• Failing to consider the three-dimensional relationship between the humeral head and tuberosities
• Not accounting for bone loss or deformity in the arthritic humeral head
• Overstuffing the joint by placing the prosthesis too high, which can lead to limited motion and accelerated wear

By prioritizing anatomical tuberosity reduction as the primary method for determining humeral height in TSA, surgeons can optimize biomechanics, maximize range of motion, and potentially improve long-term outcomes for patients undergoing shoulder arthroplasty.