STRAIN FIELDS OF THE INFERIOR GLENOHUMERAL LIGAMENT SURFACE IN A SUBLUXED SHOULDER: A STEREORADIOGRAMMETRIC STUDY

Participants: D.M. Malicky, L.J. Soslowsky, M.J. Bey, J.E. Kuhn, C.M. Mouro, J. Frisancho, M.W. Stock, D. Kayner, M.A. Schork

Keywords: shoulder, ligament, strain

Introduction

Shoulder instability is believed to be associated with deformations of the shoulder ligaments and capsule, though the nature of this relationship is largely unknown. Instability is most common in the anteroinferior direction with the arm in 90° abduction and external rotation. In this position, the inferior glenohumeral ligament (IGHL) and its anterior band (AB-IGHL) have been shown to be important stabilizers. Previous studies have addressed one-dimensional strain in the IGHL regions based on isolated bone-ligament-bone mechanical tests. However, the IGHL is a membranous structure capable of supporting a two-dimensional strain field. A better understanding of the IGHL's complex mechanical environment will aid in guiding surgical reconstructive procedures in this often injured region. Therefore, the objective of the current study was to quantitate the strain field in the IGHL in an anteroinferiorly subluxed shoulder. We hypothesized that strains near the glenoid side would be greater than strains on the humeral side and that medial-lateral strains would be greater than circumferential (perpendicular) strains.

Materials and Methods

Specimens: Seven shoulders (average age: 55.4 years) were dissected retaining only the scapula, humerus, capsular and coracohumeral ligaments, and rotator cuff tendons. Meticulous dissection was performed to clean the extra-articular IGHL surface. Spherical lead object markers of 500µm diameter were attached to this surface in a 5x5mm grid pattern with a small amount of cyano-acrylate. Markers were applied from the most inferior portion of the capsule to the anterosuperior capsule. These markers were used to calculate AIC strains via a stereoradiogrammetric (SRG) procedure.

Testing Conditions: Specimens were mounted in 60° glenohumeral abduction (90û arm abduction) and external rotation (10° less than maximum) in a fixture to apply humeral rotations, subluxations and muscle loads. Three muscles were modeled: supraspinatus, subscapularis, and infraspinatus and teres minor (combined).Three capsular positions were used for strain field calculations: a nominal state, a subluxed state, and a post-subluxed state. For the nominal state, the capsule was inflated (pressure=1.0kPa (0.15psi)) in neutral rotation and minimal distraction for removal of capsular redundancy. Radiographs were taken at 30°-40° intervals for SRG analysis. For the subluxed state, 53N rotator cuff loads were applied and the

humerus was externally rotated and subluxed in an anteroinferior direction. Radiographs were repeated. The post-subluxed state was similar to the nominal, performed within 5 minutes. Post-sublux data was also taken 1 hour after the highest subluxation to investigate the potential for additional viscoelastic tissue recovery.

SRG and Strain Calculations: The three-dimensional coordinates of 14 calibration markers attached to a carbon-fiber-epoxy calibration frame were obtained using a coordinate measuring machine. Radiographs of the shoulder object markers and frame calibration markers

were digitized using an optical-mechanical digitizer. Three-dimensional coordinates of object markers were reconstructed with a direct linear transform program [6]. Coordinates of nominal/subluxed and nominal/post-subluxed capsule states were imported to FEM software to calculate total and non-recoverable strain fields, respectively. For hypothesis testing, the glenoid and humeral sides were defined as the tissue between the respective insertion zone and the mid-portion of the ligament. The peak and mean maximum principal strain of each side was computed, and data analyzed with paired t-tests and t-tests. Calibration studies showed positional accuracy of 35µm and strain value repeatability of 2.7% strain, each at 95% confidence.

Results

Strain fields of the AIC, including total (Fig. 1) and non-recoverable (Fig. 2) maximum principal strains were obtained in all specimens. No tearing was ever visible at any object marker site.

Total maximum principal strains: At 7mm subluxation, mean strains on the glenoid side of the glenohumeral capsule were greater than on the humeral side (p=.09). Peak strains were greater on the glenoid side at all subluxations examined: 7mm (p=.05), 12mm (p=.04), and 16mm (p=.09).

Non-recoverable maximum principal strains: The mean strains on the glenoid side were greater than on the humeral side following subluxations of 12mm (p=.07) and 16mm (p=.06). Non-recoverable strains were greater than zero at all subluxations (p<.05), with average strains of less than 5% following a 16mm subluxation. Overall, the 1 hour delay to allow for viscoelastic recovery had no effect on average strains (p=.96).

Discussion

This study reports a general method for measuring planar strains in a three-dimensional membranous structure. Peak strain values were often higher than those found in previous uniaxial ligament tests; the finer grid size and different loading conditions in the current study are two potential explanations for these differences. Generally, strains were not uniaxial, nor were they aligned with the anatomical ligaments.

This is the first study to show the existence of pre-failure, non-recoverable strain due to glenohumeral subluxation. After large subluxations, high non-recoverable strains often existed in isolated regions of the capsule, with remaining regions at low strain (Fig. 2). At lower subluxations, the distribution was more uniform. The largest total and non-recoverable strains were usually seen on the glenoid side of the AIC, consistent with injuries to this region. Clinically, large subluxations may produce non-recoverable, permanent, strain, which is likely to aggravate joint instability. Limitations of the study include measuring the surface strain field and testing quasi-statically.

Figures 1 & 2: Finite e11 strains of the IGHL in two specimens, from glenoid to humerus. Black dashed lines denote the borders of the AB-IGHL.