# The Biomechanical Importance of Bone Block Positioning in Glenoid Augmentation: Every Millimeter Matters

## TL;DR

This study shows that even small changes in bone block placement during shoulder surgery significantly affect joint stability and can lead to complications.

## Contribution

The study demonstrates that precise positioning of bone blocks is critical for restoring shoulder stability in cases of glenoid bone loss.

## Key findings

- Flush bone block placement significantly improves stability compared to bone loss.
- A 1-mm medialized bone block fails to restore stability, while a 1-mm lateralized block increases stability.
- Accurate bone block positioning is crucial to avoid complications like instability or osteoarthritis.

## Abstract

In the presence of anterior glenoid bone loss (aGBL), options for bony glenoid augmentation include Latarjet procedures and free bone block transfers. Bone graft placement is challenging, and malposition causes complications, such as recurrent instability or osteoarthritis.

With minimal changes in bone block positioning, osteochondral shoulder stability cannot be restored sufficiently.

Controlled laboratory study.

In a robotic test setup, 14 human cadaveric scapulae were included. Soft tissue was resected, and matching artificial humeri were selected for each specimen. Testing was performed in 60° of glenohumeral abduction with 50 N of glenohumeral compression and anterior-directed translational force to the humerus. Application of 20% aGBL and screw fixation of artificial bone blocks (artBBs) with different buildup shells allowed the following testing stages: (1) intact, (2) 20% aGBL, (3) flush artBB, (4) 1-mm medialized artBB, and (5) 1-mm lateralized artBB. The stability ratio (SR) and medial-lateral humeral head starting position were assessed.

Specimens with 20% aGBL provided lower mean SRs than native joints (20.6% [SD, 4.7%] vs 27.8% [SD, 6.7%]; P < .0001). Flush artBB placement (mean, 35.4%; SD, 7.7%) led to an increased SR compared with both native joints (P = .002) and 20% aGBL (P < .0001). The mean SR in 1-mm medialized artBBs (21.5%; SD, 5.7%) did not differ compared with that for 20% aGBL (P = .908). One-millimeter lateralized artBBs (mean, 40.8%; SD, 5%) provided higher SR and more lateral humeral head starting positions compared with flush artBB (P = .003 and P = .003, respectively).

In the presence of aGBL, flush bone block placement restores osteochondral glenohumeral stability, while a 1-mm medialized bone block fails to increase stability. Bone block lateralization of 1 mm provides higher stability but is associated with humeral head lateralization.

Glenoid bone block augmentations are established in patients with glenohumeral instability and aGBL. In the case of bone block malposition, complications like recurrent instability or the development of osteoarthritis can occur. This study underlines the importance of accurate bone block placement since only minimum bone block malposition relevantly affects osteochondral shoulder biomechanics.