Abstract
Objectives:
The primary aim of the current study was to assess the biomechanical utility of a posterior acromial bone block (PABB) in addressing posterior glenohumeral instability. Secondary goals included evaluating the biomechanical relationships between various posterior acromial morphology metrics such as posterior acromial height (PAH), posterior acromial coverage (PAC), and posterior acromial tilt (PAT) using a posterior glenohumeral instability model.
Our hypothesis was that the addition of a PABB would increase the force required to translate the humeral head posteroinferiorly when compared to cadaveric specimens with a simulated posterior capsulolabral lesion, both before and after capsulolabral repair. Secondarily, we hypothesized that the maximum force required to translate the humeral head 10 mm posteroinferiorly in both intact specimens and those augmented with a PABB will be inversely related to PAH (higher values denoting a higher, flatter acromion) but positively correlated with PAC (a metric of posterior acromial humeral head coverage).
Methods:
An a priori power analysis was performed based upon prior posterior glenohumeral instability biomechanics research performed at our institution, and determined that 10 specimens would be required to obtain a power of 0.84. Therefore, 10 cadaveric shoulder specimens were obtained for testing. These specimens were ≤ 65 years of age and free of known pre-existing shoulder pathology or surgeries as confirmed by Computed Tomography (CT). These specimens were dissected to preserve as much native anatomy as possible, and a 2-2.5 cm bone block from the scapular spine was harvested at its mid-aspect. These specimens were potted with the humerus in 30 degrees of abduction and 30 degrees of flexion. Biomechanical testing was performed using the KUKA industrial robot, which is a 6-axis robot containing integrated software and a multi-axis load cell. All testing was performed with a 50-N compressive force applied to the humeral head to maintain its compression in the glenoid, as per the joint forces in the native shoulder. The humeral head was translated 30 degrees posteroinferiorly from the center of the glenoid 10 mm at a rate of 1.0 mm/s, and the maximum force required to obtain this translation was recorded in several conditions. The conditions included: intact specimen, intact with PABB, posterior instability lesion (capsulolabral lesion from 6 to 11 o’clock on the glenoid face), addition of the PABB (oriented at the posterolateral border of the acromion, fixed with two, 3.5 mm cortical screws), the removal of the PABB and repair of the capsulolabral lesion, and finally the addition of the PABB with the capsulolabral repair (Figure 1). Each capsulolabral repair was performed with 4 transosseous, #2, high tensile suture. Fluoroscopic images were obtained to measure pre- and post-PABB posterior acromial morphology measurements. Paired T-tests and linear regression analyses were performed. A P-value of 0.05 was used to determine statistical significance.
Results:
10 cadavers with mean age 54.4 years (Standard Deviation (SD) ± 13.1 years) and mean glenoid version of 6.6 degrees of retroversion (SD ± 1.0 degrees) were studied. The PABB significantly increased the peak force required to translate the humeral head 10 mm posteroinferiorly in the setting of a posterior capsulolabral tear when compared to no PABB (PABB, 49.1 ± 21.5N ; posterior labral tear, 30.3 ± 15.7N; P = 0.0019) (Figure 2). Compared to a capsulolabral repair alone, the PABB again showed to have a significantly higher peak force (PABB, 49.1 ± 21.5N; repaired capsulolabral tear, 35.4 ± 21.2 N; P = 0.0024). The addition of the PABB to a labral repair did not significantly increase the peak force when compared to a PABB alone (PABB, 49.1 ± 21.5N ; PABB plus labral repair 54.7 ± 30.6 N P = 0.30).In both the intact specimens, as well as the specimens after undergoing the PABB, there were weak, negatively correlated relationships between the peak force during testing and PAH (Figure 3). On the contrary, there were weak, positively correlated relationships between the peak force and PAC (Figure 4).
Conclusions:
The addition of a posterior acromial bone block is biomechanically superior to capsulolabral repair alone in increasing peak forces required to translate the humeral head posteroinferiorly using a well-studied posterior glenohumeral instability model. Our findings also demonstrate that posterior acromial height was inversely correlated with peak forces, while posterior acromial coverage was positively correlated with peak forces. These results reinforce the emerging literature that suggests that posterior acromial morphology plays an important role in the biomechanics of glenohumeral instability. Our results support the consideration of this bone block technique to be used in all-comers with posterior glenohumeral instability, but this technique may be particularly useful in revision cases when repair of the posterior capsulolabral complex may be deemed insufficient or in cases with a patient who has a high, flat acromion. However, future research evaluating glenoid bone loss and bipolar lesions will be useful to elucidate the usefulness of this technique in these more complex pathologies.
