JBJS | Effects of the Glenoid Labrum and Glenohumeral Abduction on Stability of the Shoulder Joint Through Concavity-Compression : An in Vitro Study

The Journal of Bone & Joint Surgery, Volume 83, Issue 7

Articles | July 01, 2001

Effects of the Glenoid Labrum and Glenohumeral Abduction on Stability of the Shoulder Joint Through Concavity-Compression : An in Vitro Study

A. M. Halder, MD; S. G. Kuhl, BS; M. E. Zobitz, MS; D. Larson, MS; K. N. An, PhD

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Investigation performed at the Biomechanics Laboratory, Division of Orthopedic Surgery, Mayo Clinic and Mayo Foundation, Rochester, Minnesota
A.M. Halder, MD
S.G. Kuhl, BS
M.E. Zobitz, MS
D. Larson, MS
K.N. An, PhD
Biomechanics Laboratory, Division of Orthopedic Surgery, Mayo Clinic and Mayo Foundation, 200 First Street SW, Rochester, MN 55905

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were National Institutes of Health (National Institute of Arthritis and Musculoskeletal and Skin Diseases) Grant AR 41171 and the Max-Biedermann Institut, Berlin, Germany.

The Journal of Bone & Joint Surgery. 2001; 83:1062-1069

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Abstract

Background: Although the glenohumeral joint is the most mobile articulation of the human body, it is known to exhibit ball-and-socket kinematics. Compression into the glenoid labral concavity keeps the humeral head centered. The purpose of the present study was to determine the effects of joint position on glenohumeral stability through concavity-compression.

Methods: Ten cadaveric shoulders were tested. The glenoid was mounted horizontally onto a six-component load-cell while the humerus was clamped to a vertically unconstrained slide. An x-y stage translated the load-cell with the glenoid underneath the humeral head in eight different directions. Compressive loads of 20, 40, and 60 N were applied. The tests were repeated in 0°, 30°, 60°, and 90° of glenohumeral abduction with and without the labrum. Relative translations between the glenoid and the humeral head and the forces resisting translation were recorded. Then the stability ratio, defined as the peak translational force divided by the applied compressive force, was calculated.

Results: The average stability ratio was higher in the hanging-arm position than it was in glenohumeral abduction. The highest stability ratio was detected in the inferior direction (59.8% 7.7%) when the labrum was intact and in the superior direction (53.3% 7.9%) when the labrum had been resected. Under both conditions, the anterior direction was associated with the lowest stability ratio (32.0% 4.4% with the labrum and 30.4% 4.1% without the labrum). Resection of the glenoid labrum resulted in an average decrease in the stability ratio of 9.6% 1.7%. With increasing compressive load, the average stability ratio slightly decreased.

Conclusions: Glenohumeral stability through concavity-compression was greater in the hanging-arm position than it was in glenohumeral abduction. The average contribution of the labrum to glenohumeral stability through concavity-compression was approximately 10%, about one-half of the value previously reported. With the labrum intact, the glenohumeral joint was most stable in the inferior direction. Without the labrum, it was most stable in the superior direction. Under both conditions, it was least stable in the anterior direction. Glenohumeral joint stability through concavity-compression decreases with higher compressive loads.

Clinical Relevance: Anterior dislocation of the shoulder may be facilitated by the lower stability demonstrated in glenohumeral abduction. The labrum may not contribute to glenohumeral stability as much as was previously assumed. However, even moderate compressive forces are sufficient to provide stability through concavity-compression.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

The glenohumeral joint is the most mobile articulation of the human body because of its lack of osseous constraint. Despite its great mobility, the humeral head remains centered throughout the range of motion, exhibiting ball-and-socket kinematics^1,2.

Matsen et al. used the term glenohumeral joint stability to describe the ability to keep the humeral head centered³. Stability is achieved through compression of the humeral head into the glenoid labral concavity⁴. In the end-range of motion, the capsular ligaments passively tighten to center the humeral head^5-7. In the mid-range of motion, when the capsuloligamentous structures are lax^4,8, the rotator cuff muscles actively press the humeral head into the glenoid fossa⁹. Active joint compression through muscle contraction was found to be even more important than negative intra-articular pressure or ligament tension¹⁰.

The degree of stability of the glenohumeral joint depends on the perpendicular component of the rotator-cuff muscle forces as well as on the radius of the articular surface. A deeper glenoid labral concavity and a higher compressive load increase the resistance to joint subluxation¹¹. Howell and Galinat showed that the glenoid had an average depth of 9 mm in the superoinferior direction and 5 mm in the anteroposterior direction¹². They also found that the labrum contributes 50% of the socket depth. Soslowsky et al. showed that the humeral and glenoid radii of curvature approximated conforming spheres and that the difference in the radii was <2 mm in 88% of thirty-two shoulders¹³. Incongruence in roentgenographic measurements may be explained in part by cartilage thickness, which increases toward the center of the humeral head and the periphery of the glenoid fossa. Therefore, the relatively low degree of osseous stability is not caused by the shallowness or the lack of congruence but rather by the small surface area of the glenoid¹⁴.

Although the glenohumeral joint mainly shows ball-and-socket kinematics, some shoulder movements are coupled with translation of the humeral head on the glenoid^1,5,15. Therefore, we hypothesized that the degree of stability through concavity-compression is position-dependent. In previous studies of glenohumeral stability due to concavity-compression with an intact glenoid¹⁶ and with a chondral-labral defect⁴, compressive loads were applied manually and translations were performed manually in a single joint position. Other investigators have examined the effects of lesions of the labrum¹⁷, negative intra-articular pressure, and the glenohumeral ligaments¹⁰, but there is little information about the effects of joint position. The purpose of the present study was to determine the effects of different joint positions on glenohumeral stability through concavity-compression with precisely controlled translations and forces.

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Fig. 1:: Curves showing the vertical displacement, termed the effective depth, and the translational force versus the translation in inferior-superior (top) and posterior-anterior directions (bottom).

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Fig. 2:: The average stability ratios of the shoulders, with and without the labrum, in the hanging-arm position and in 30°, 60°, and 90° of glenohumeral abduction.

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Fig. 3:: The average stability ratios of the shoulders, with and without the labrum, in the eight tested directions. The values are given as the average (and the standard deviation).

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Fig. 4:: The average stability ratios, with and without the labrum, with 20, 40, and 60 N of compressive load.

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Fig. 5:: The average translation (in millimeters) from the center of the glenoid to maximum values of translational force and the effective depth of glenoid concavity. The values are given as the combined average of the results with and without the labrum.

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Fig. 6:: The average depth of glenoid concavity (in millimeters), with and without the labrum, in the eight tested directions. The values are given as the average (and the standard deviation).

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Ten fresh-frozen cadaveric shoulders were used after macroscopic evidence of glenohumeral osteoarthritis and degeneration or injury to the glenoid labrum had been ruled out. The average age of the donors of the specimens was 76.7 years (range, sixty-nine to ninety-eight years). During dissection, preparation, and testing, the specimens were moistened with physiological saline solution to prevent dehydration. All soft tissues including the joint capsule and the glenohumeral ligaments were removed, leaving the glenoid labrum attached. The glenoid was divided from the scapula about 5 cm medial to the articular surface and was fixed horizontally into a quadrangular block of polymethylmethacrylate bone cement. Both axes of the glenoid surface were carefully aligned parallel to the margins of the block of bone cement with the use of a guide. The humerus was transected about 8 cm distal to the center of the humeral head and was fixed in an aluminum sleeve with polymethylmethacrylate bone cement. The humeral shaft was carefully aligned in the center of the aluminum sleeve.

The block of bone cement with the glenoid was clamped onto a six-component load-cell (Advanced Medical Technologies, Watertown, Massachusetts). The load-cell was mounted on a motorized x-y stage (DCI Design Components, Franklin, Massachusetts). The x-y stage allowed motor-driven, computer-controlled movements in the plane of the glenoid surface. The aluminum sleeve containing the proximal part of the humerus was rigidly clamped above the glenoid onto a low-friction slide. The humeral head was fixed in neutral rotation, which was defined as the position in which the line bisecting the cartilage surface of the humeral head in the superior-inferior direction was aligned with the superior-inferior axis of the glenoid. The slide was unconstrained in the direction perpendicular to the glenoid surface and was counterbalanced to neutralize the gravitational force. Vertical displacement of the humeral head was measured with use of a linear potentiometer (Novotechnik, Stuttgart, Germany). The starting position, with the humeral head in contact with and centered on the glenoid, was defined as the position with zero force in the plane of the glenoid. In this position, the humeral head contacted the most inferior point of the concave glenoid with the most inferior point of its convex surface, and vertical displacement of the humeral head was at its minimum. Axial loads were applied through the center of the articulation by adding weights to the slide to which the clamped humerus was attached. The joint surfaces were lubricated by constant irrigation with bovine serum. Since Lazarus et al.⁴ did not find any differences in the zero position of the humeral head on the glenoid after thirty minutes of compression with a 50-N load, indicating an absence of chondral deformation, we tested immediately after centering the humeral head on the glenoid.

Under computer control, the glenoid underneath the humeral head was translated at a rate of 2 mm/sec starting from the zero position. After the defined end point of translation was reached, the movement was reversed and the humeral head was moved in the opposite direction, passing the center of the glenoid. To exclude the effect of initial static friction, only measurements made during dynamic movements were considered. The eight major directions of translation, which were 45° apart, were anterior, anteroinferior, inferior, posteroinferior, posterior, posterosuperior, superior, and anterosuperior. To measure what we termed the effective depth of glenoid labral concavity, we applied a compressive load of 10 N and translated the glenoid 25 mm from the center in each direction to a point beyond the maximum height of the glenoid labral socket. The resulting vertical displacement was measured and termed the effective depth. For translational force measurements, we applied compressive loads of 20, 40, and 60 N and translated the glenoid 10 mm from the center in each direction to a point past the force maximum. The tests were performed in 0°, 30°, 60°, and 90° of glenohumeral abduction (defined as the angle between the longitudinal axis of the humeral shaft and the superior-inferior axis of the glenoid) with and without the labrum.

The effective depth of the glenoid labral concavity was determined from measurement of the vertical displacements of the humeral head. Glenoid translations were recorded by the x-y stage, and translational forces in the plane of the glenoid and compressive forces to the glenoid were measured with the six-component load-cell. Force and displacement data were collected with a commercially available computer.

Force and displacement curves were plotted for each condition (Fig. 1). The maximum translational forces and the maximum effective depth of the glenoid labral concavity were derived from these curves in the eight directions of movement. The stability ratio, defined by Fukuda et al. as the peak translational force divided by the applied compressive force¹¹, was calculated. This measure indicates the stability through the joint-surface interaction, independent of the magnitude of the compressive force applied.

The end point of our analysis was the stability ratio, with the main factors of interest being the compression force, direction of movement, abduction angle, and labrum. The summary values are given as the average and the standard deviation. The data were first examined graphically to help to visualize the effects of the main factors and to look for interactions. In order to evaluate the effects of the factors of interest, the data were modeled with use of a four-factor analysis of variance with repeated measures on all four factors. However, significant interactions were identified between the factors in the model. Therefore, separate three-factor analysis-of-variance models with repeated measures on all three factors were run for the subset of data with the labrum intact and for the subset of data with the labrum removed. This eliminated the significant interactions and simplified the analysis. Significant main effects were then further analyzed with use of the Student-Newman-Keuls multiple-comparison procedure. Additionally, paired t tests and signed-rank tests were used to compare the average stability ratio with the labrum intact with the average stability ratio with the labrum removed, at each direction (averaged over force and angle). Because this resulted in multiple comparisons, the resulting p values were adjusted with use of the Bonferroni correction. All statistical tests were two-sided, and the threshold of significance was set at alpha = 0.05. The analysis was performed with use of SAS software (version 6.12; SAS Institute, Cary, North Carolina) and S-PLUS (MathSoft, Seattle, Washington).

Results

Introduction | Materials and Methods | Results | Discussion | References

The purpose of this study was to determine glenohumeral joint stability through concavity-compression, with and without the labrum, in 0°, 30°, 60°, and 90° of glenohumeral abduction. The results are reported as the average and the standard deviation of the calculated stability ratios, with all factors averaged except the one discussed.

The stability ratio was consistently higher in the hanging-arm position than it was in any of the abduction positions (Fig. 2). With the labrum intact, the average stability ratio was significantly (p < 0.05) higher in the hanging-arm position (49.4% 9.0%) than it was in 30° (46.4% 5.9%), 60° (45.3% 5.7%), or 90° (44.5% 5.6%) of glenohumeral abduction. Without the labrum, the difference between the ratio in the hanging-arm position (43.0% 7.3%) and that in 30° (41.8% 6.7%), 60° (40.8% 6.6%), and 90° (40.9% 6.8%) of glenohumeral abduction was not significant. The differences among the ratios at 30°, 60°, and 90° of glenohumeral abduction were not significant.

The stability ratios were substantially different among the tested directions (Fig. 3). With the labrum intact, the highest stability was detected in the inferior direction (59.8% 7.7%), followed by the superior (56.1% 6.5%), posteroinferior (50.4% 9.5%), and anteroinferior (48.2% 6.1%) directions. However, the stability ratio in the superior direction was not significantly (p > 0.05) different from that in the inferior direction.

Resection of the labrum changed the order of the stability ratios. Without the labrum, the joint was significantly (p < 0.001) more stable in the superior direction (53.3% 7.9%) than it was in the inferior (48.2% 7.3%), anteroinferior (42.1% 6.3%), and posteroinferior (41.0% 8.6%) directions. Under both conditions, with and without the labrum, the stability ratio in the posterior direction (36.6% 5.9% and 33.9% 6.2%, respectively) was higher than that in the anterior direction (32.0% 4.4% and 30.4% 4.1%, respectively), in which there was the least resistance to displacement of the humeral head. However, with the labrum intact, the stability ratios in the anterior and posterior directions were not significantly (p > 0.05) different.

Resection of the glenoid labrum resulted in an average decrease in the stability ratio of 9.6% 1.7%. The reduction in the stability ratio was significant in all directions (p < 0.05), except the posterior and posterosuperior directions, and it varied considerably among the directions (Fig. 3). The stability ratio decreased substantially in the inferior (19.2% 2.9%), posteroinferior (18.4% 4.9%), and anteroinferior (12.5% 1.5%) directions. It decreased less in the anterior (5.2% 1.8%), posterior (7.1% 6.2%), superior (5.0% 1.9%), and anterosuperior (8.7% 0.7%) directions. The smallest change occurred in the posterosuperior direction (0.4% 0.8%).

The stability ratio showed a constant decrease with increasing compressive load (Fig. 4). With and without the labrum, the stability ratio with 20 N of compressive load (49.1% 7.1% and 43.9% 7.7%, respectively) was significantly (p < 0.05) higher than that with a compressive load of 40 N (46.0% 6.4% and 41.1% 6.5%, respectively) or 60 N (44.0% 6.3% and 39.8% 6.3%, respectively). With and without the labrum, the stability ratio with a compressive load of 40 N was significantly (p < 0.05) higher than that with a compressive load of 60 N.

The average translation distance to the point of maximum translational force was 1.8 0.8 mm with the labrum and 1.7 0.7 mm without the labrum. The average translation distance to the point of maximum vertical displacement (maximum effective depth) of the glenoid labral concavity was 16.2 4.0 mm with the labrum and 15.1 3.2 mm without the labrum (Fig. 5). The distances between the points of maximum vertical displacement of the glenoid labral concavity were 36.4 5.3 mm in the superoinferior direction and 26.6 2.6 mm in the anteroposterior direction.

After resection of the labrum, the average maximum effective depth of the glenoid concavity decreased from 4.0 1.1 mm to 3.2 0.8 mm (Fig. 6). While the depth was substantially reduced in the inferior (5.1 to 3.9 mm) and superior (5.5 to 4.6 mm) directions after resection of the labrum, the decrease was less in the anterior (2.6 to 2.2 mm) and posterior (2.7 to 2.3 mm) directions. With the labrum removed, the distances between the points of maximum depth of the glenoid concavity were 32.7 4.3 mm in the superoinferior direction and 25.6 2.2 mm in the anteroposterior direction.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

We hypothesized that the degree of stability through concavity-compression is position-dependent. In our study, the stability of the glenohumeral joint through concavity-compression with an intact labrum was significantly (p < 0.05) greater in the hanging-arm position than it was in any of the abduction positions. Without the labrum, the difference was still obvious but not significant (p = 0.06, 16% power). It has been shown that the humeral and glenoid radii of curvature approximate conforming spheres¹³, exhibiting ball-and-socket kinematics^2,12. However, shoulder motion is coupled with translation of the humeral head on the glenoid^1,5,15, which requires a certain degree of mismatch of the articulating surfaces and leads to variations in joint-contact area. Although the maximum glenohumeral joint-contact area occurs at 120° of abduction and external rotation¹⁸, the resistance to translational forces is not necessarily at its peak in this position. The resistance to translational forces would be expected to increase with higher conformity in the radii of curvature between the humeral head and the glenoid. When the shoulder is adducted, the contact area of the humeral head on the glenoid is in the center of the glenoid, which is known as the bare area, but the fit is not perfectly congruent¹⁹ . Thus, the humeral head reaches the deepest point on the glenoid surface in the hanging-arm position, providing maximum resistance to translational forces. This requires a slightly smaller radius of curvature of the inferior aspect of the humeral head, which confirms earlier observations²⁰. In the hanging-arm position, the muscles generating joint-compression forces may be relaxed and thus maximum passive resistance to translational forces would be physiologically advantageous. Of course, scapular inclination angle and ligament tension also play important roles in providing passive stability in the hanging-arm position.

After resection of the labrum, we detected an average decrease in the stability ratio of approximately 10% throughout all loading conditions and test directions; this decrease was only one-half of the amount previously reported¹⁶. Howell and Galinat introduced the concept of a glenoid labral socket in which the fibrous labrum contributed 50% of the depth¹². Lippitt et al. manually applied 50 and 100 N of compressive force to test glenohumeral joint stability by concavity-compression in ten cadaveric joints in 45° of abduction and 35° of external rotation¹⁶. They reported an average decrease in the stability ratio of about 20% after resection of the glenoid labrum. Lazarus et al. used a similar methodology to test the effects of a large anteroinferior chondral-labral defect in five cadaveric glenohumeral joints and found a 65% decrease in the stability ratio⁴.

Resection of the labrum had the largest effect on stability in the inferior direction, which was consistent with previous results¹⁶ and corresponds to anatomical observations. The inferior part of the labrum, which has been described as being firmly attached to the glenoid, is a fibrous immobile extension of the cartilage^21,22 that enlarges the glenoid surface to enhance glenohumeral stability. Meanwhile, the superior and anterosuperior portions of the labrum are only loosely attached²¹. Consequently, resection of these portions resulted in a smaller decrease in the stability ratio. A similar amount of stability was lost in the anterior and posterior directions after resection of the labrum, corresponding to a minor change in the depth of the concavity.

We detected a stability ratio of approximately 60% in the superoinferior direction and only 30% in the anteroposterior direction. With the labrum intact, stability was maximum in the inferior direction; without the labrum, it was maximum in the superior direction. These results confirm those of previous studies¹⁶ and correspond to anatomical findings. The glenoid surface can be approximated by a section of a sphere with small deviations from sphericity¹³ and is shaped like an inverted comma with an anterior incision²³. Consequently, the glenoid labral concavity must be deeper in the superoinferior direction than it is in the anteroposterior direction. Howell and Galinat reported that the glenoid concavity was 9 mm deep in the superoinferior direction and 5 mm deep in the anteroposterior direction¹². In our study, we found a linear correlation (r² = 0.93) between the stability ratio and the effective depth of the glenoid labral socket. Without the labrum, the stability ratio was significantly higher in the posterior direction than it was in the anterior direction (p < 0.05); with the labrum intact, the difference was not significant. Previous investigators have reported an inverse trend¹⁶. Anatomically, the posterior aspect of the glenoid labral socket was only slightly deeper than the anterior aspect, but the anteriorly located incision may have played a role.

The amount of compressive force in the human glenohumeral joint is still unknown. Estimates of the compressive force in 90° of abduction have been ten times arm weight²⁴ and 0.89 times body weight⁹. In 60° of abduction, the compressive force was reported as reaching a peak value of 650 N²⁵. Warner et al. calculated the compressive force to be 90% of body weight on the basis of an estimated arm weight of 5% of body weight, but much lower forces were sufficient to stabilize the glenohumeral joint when subjected to an inferior force of up to 100 N¹⁰. In our study, the increase in the compressive force resulted in a decrease in the stability ratio, which is similar to trends observed in a previous study¹⁶.

The distance from the centralized position of the humeral head on the glenoid to the point of maximum translational force was about one-tenth of the distance to the point of maximum concavity depth. Resection of the labrum had a minor effect on both distances. The maximum translational force occurs with minimal movement of the humeral head, whereas additional movement until complete dislocation requires only a low force but a large translation. This is the mechanism by which the humeral head is maintained in the center of the glenoid without taking into account ligamentous, tendinous, or muscular effects.

The translation distances to the points of maximum glenoid labral concavity are similar to those in previous reports²³. As our measurements of the concavity depth were based on the vertical displacement of the humeral head with a compressive load of 10 N, we report the functional height. Consequently, the values are lower than previous anatomical measurements of the glenoid surface. Although we measured an average concavity depth of approximately 5 mm in the superoinferior direction and about one-half of it in the anteroposterior direction, Howell and Galinat, as mentioned, reported an average depth of 9 mm in the superoinferior direction and 5 mm in the anteroposterior direction¹². However, the contribution of the labrum to the total depth of the glenoid labral socket was 50% in both investigations.

As alignment of the glenoid is a decisive factor to ensure comparability of the results, we used a device that allowed fixation of the glenoid with its superior-inferior and anterior-posterior axes parallel to the margins of the block of bone cement. After the glenoid was mounted in the testing machine, the alignment of both axes of the glenoid parallel to the x-axis and the y-axis was checked. In addition to the alignment of the glenoid labral socket, the conditions of its surface and labrum were essential. The ages of the donors of the specimens ranged from sixty-nine to ninety-eight years, but we included only specimens with macroscopically intact cartilage surfaces. As the glenoid labrum undergoes degenerative changes with age²⁶, we included only specimens with a macroscopically intact labrum. Since older patients are rarely affected by labral detachments²⁷, the reported changes after resection of the labrum are less important for them. However, our results are meaningful for younger patients as the labrum might contribute even more to glenohumeral stability through concavity-compression.

In conclusion, glenohumeral stability through concavity-compression was greater in the hanging-arm position than it was in glenohumeral abduction, which may facilitate anterior dislocation of the shoulder. The average contribution of the labrum to glenohumeral stability through concavity-compression was approximately 10%, about one-half of the value previously reported¹⁶. With the labrum intact, the glenohumeral joint was most stable in the inferior direction. Without the labrum, it was most stable in the superior direction. Under both conditions, it was least stable in the anterior direction, which may also facilitate anterior dislocation of the shoulder. Glenohumeral joint stability through concavity-compression decreases with higher compressive loads. Even moderate compressive forces generated by the rotator cuff muscles may provide sufficient concavity-compression to stabilize the glenohumeral joint.

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