The Journal of Bone and Joint Surgery 78:94-102 (1996)
© 1996 The Journal of Bone and Joint Surgery, Inc.
Effect of a Chondral-Labral Defect on Glenoid Concavity and Glenohumeral Stability. A Cadaveric Model*
MARK D. LAZARUS, M.D. ,
JOHN A. SIDLES, PH.D. ,
DOUGLAS T. HARRYMAN II, M.D. and
FREDERICK A. MATSEN III, M.D., SEATTLE, WASHINGTON
Investigation performed at the Department of Orthopaedic Surgery, University of Washington Medical Center, Seattle
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Abstract
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One of the primary stabilizing mechanisms of the glenohumeral joint is concavity-compression, the maintenance of the humeral head in the concave glenoid fossa by the compressive force generated by the surrounding muscles. This mechanism is active in all glenohumeral positions but it is particularly important in the functional mid-range, in which the capsule and ligaments are slack. The effectiveness of concavity-compression in the stabilization of a joint can be characterized in terms of the ratio between the maximum dislocating force that can be stabilized in a given direction and the load compressing the head into the glenoid (the stability ratio). Glenoid concavity can be described by the lateral humeral displacement during translation across the glenoid. The purpose of the present investigation was to characterize the concavity and stability ratios of normal cadaveric glenoids, to measure the effect of an anteroinferior chondral-labral defect on these parameters, and to measure the effectiveness of a simulated operative reconstruction on the restoration of glenoid concavity and the stability ratio.
The chondral-labral defect created in this study reduced the height of the glenoid by approximately 80 per cent and the stability ratio by approximately 65 per cent for translation in the direction of the defect. Reconstruction of the anteroinferior aspect of the glenoid concavity with use of an autogenous biceps-tendon graft restored normal values for these variables.
CLINICAL RELEVANCE: Loss of glenoid concavity may be an important factor in glenohumeral instability, and reconstruction of this concavity may effectively restore stability.
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Introduction
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Recent investigations have highlighted the importance of glenoid concavity, and specifically the contribution of the labrum to that concavity, in the maintenance of glenohumeral stability. Howell and Galinat9 measured an approximately 50 per cent increase in the depth of the glenoid with an intact labrum, which demonstrates the fossa-deepening effect of the labrum. Flatow et al. demonstrated the effectiveness of the labrum and thickened peripheral cartilage in extending the area of surface contact of the glenoid with the humeral head. Pappas et al. and Baker et al. noted clinical instability that was presumably secondary to lesions of the glenoid labrum. Lippitt et al. studied the contribution of the labrum to glenohumeral stability. With use of the stability ratio, a factor defined by Fukuda et al. as the transverse force necessary to cause dislocation divided by the compressive load that provides stability multiplied by 100 per cent, Lippitt et al. found that excision of the labrum decreased the stability ratio by an average of 20 per cent. Lippitt et al. also characterized the lateral displacement of the humeral head as it translated from the centered position in the glenoid and used this measurement to describe the glenoid concavity.
Stability can be defined as the individual's ability to maintain the humeral head precisely centered within the glenoid fossa16,18. This definition is supported by the work of Poppen and Walker22 as well as that of Howell et al.10, who demonstrated that the humeral head in normal shoulders remains nearly perfectly centered within the glenoid until the extremes of motion. Harryman et al. found that the capsule remains loose during passive glenohumeral motion until the end-range of motion is reached, at which point the humeral head translates away from the tight capsule. Since the capsule is lax during all but the extremes of motion, the mechanism stabilizing the humeral head in the mid-range is independent of capsular constraint. Therefore, it is unlikely that capsular repairs and reconstructions restore normal mid-range stability without substantial reduction of the range of motion or alteration of capsular compliance.
Compromise of the glenoid concavity has been noted with both traumatic and atraumatic glenohumeral instability1,3,4,15,17,20,21,24,27. Defects of the glenoid concavity may be related to loss of bone, cartilage, or labrum, or to a combination of these factors. Avulsion of the anterior portion of the labrum has been seen in as many as 97 per cent of patients (twenty-eight of twenty-nine27) who have traumatic anterior instability. Although the etiology of atraumatic instability is not clear and capsular laxity is often proposed as the fundamental deficit1,4,20, flattening of the surface of the glenoid and labral tearing may play a role. Neer and Foster as well as Altchek et al. noted labral abnormality of varying degrees in most of their patients who had atraumatic instability.
Excellent results have been reported with the use of Bankart-type reconstructions for the treatment of recurrent traumatic anterior instability3,15,17,24,27. For patients with atraumatic instability for whom rehabilitation has been unsuccessful, capsular shift has been associated with good results1,4,5,13,20. In our practice, however, we are seeing an increasing number of patients for whom multiple operative attempts to treat instability have failed. Whether secondary to traumatic or atraumatic instability, marked deficiency of the anterior portion of the glenoid labrum and chondral surface is typical in these patients.
The purpose of the present study was to characterize a group of normal cadaveric glenoids in terms of concavity and stability ratios in various directions, to measure the effect of a standardized anteroinferior chondral-labral lesion, and to measure the effectiveness of a simulated operative reconstruction of this lesion.
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Materials and Methods
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We studied five fresh-frozen cadaveric glenohumeral joints. The average age of the subjects at the time of death was seventy-six years (range, sixty-nine to eighty-nine years). Any joint that was noted to have crepitance on movement through the range of motion or that lacked 90 degrees of humeroscapular elevation was discarded. Although there was substantial variability with regard to the shape of the glenoid and the size and quality of the labrum, every shoulder that did not meet the criteria for rejection was included in the test.
The specimens were prepared by sectioning the scapula from the thorax and dividing the humerus at the middle of the shaft. All muscles and tendons were resected. The glenohumeral capsule was excised just lateral to the glenoid labrum. The vertebral border of each scapula was potted in plaster of Paris in a five-centimeter-deep container. The container was fixed to a six-degrees-of-freedom force-transducer (Astek Model FS160A-600; Barry Wright, Watertown, Massachusetts). The surface of the glenoid was oriented parallel to the floor. By convention, the direction proceeding from the center of the glenoid to the origin of the long head of the biceps was defined as superior (0 degrees) and the anterior perpendicular to this plane was defined as anterior (90 degrees).
The humerus was abducted and externally rotated to place the center of its articular surface into the center of the glenoid. This position, approximately 45 degrees of abduction, 35 degrees of external rotation, and neutral flexion, was used for all of the experiments. A wooden dowel, one-half inch (1.27 centimeters) in diameter, was placed from anterior to posterior through the center of the humeral head with the humerus in the position of maximum joint congruity just described. The sensors of a six-degrees-of-freedom spatial digitizer (Polhemus; Navigational Sciences, Colchester, Vermont) were attached to the humeral shaft and the scapula (Fig. 1). This system has been shown to be accurate to less than one millimeter of translation and 1 degree of rotation8. Data were recorded by computer (Orthokine Software; University of Washington, Seattle, Washington).
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Photograph of the potted scapula attached to a six-degrees-of-freedom force-transducer, with the face of the glenoid oriented parallel to the floor. The sensors of a six-degrees-of-freedom spatial digitizer were attached to the proximal aspect of the humerus. A load was applied to the humeral head through a wooden dowel that was one-half inch (1.27 centimeters) in diameter.
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To locate the center of the humeral head, the head was pressed into the glenoid and its position was recorded during twelve maneuvers as the shoulder was flexed, extended, abducted, and rotated. The center was defined as the point on the head that moved the least during these maneuvers.
The humeral head was centered in the glenoid fossa with a fifty-newton compressive load applied to the dowel and normal to the face of the glenoid. This force was maintained for thirty minutes. Data points were recorded every fifteen seconds to determine whether appreciable chondral deformation occurred during this time-interval.
The humeral head was then centered with a five-newton compressive load applied with the dowel. While the five-newton compressive load was maintained, the head was translated in eight directions (superior, anterosuperior, anterior, anteroinferior, inferior, posteroinferior, posterior, and posterosuperior). Two trials were performed in each direction. The lateral displacement of the center of the head during each of these translations was measured (Fig. 2).
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Illustration demonstrating the measurement of glenoid concavity and the plot of the path of the center of the humeral head as it translates across the glenoid.
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Next, a fifty-newton compressive load was applied to the humeral head with the dowel. We chose fifty newtons as the compressive load for two reasons. Inman et al. as well as Poppen and Walker23 calculated that, with active in vivo abduction, the force normal to the glenoid was always greater than fifty newtons. In addition, Lippitt et al. demonstrated that, with an applied compressive load of fifty newtons, dislocation did not cause gross damage to the tissue. While the compressive load was maintained, a force applied parallel to the face of the glenoid was increased progressively until the humeral head dislocated. This sequence was repeated for each of the eight directions around the glenoid. As noted by Lippitt et al., dislocation occurred suddenly with minimum previous translation. The translatory force at dislocation, therefore, was easily determined. Two trials were performed in each direction. The glenoid was moistened with normal saline solution between each two trials. The translating force divided by fifty newtons multiplied by 100 per cent gave the stability ratio for each direction.
Next, a chondral-labral defect was created in the anteroinferior quadrant of the glenoid, from 90 degrees (anterior) to 180 degrees (inferior) (Fig. 3-A, 3-B, and 3-C). The lesion was created with use of a number-15 blade to remove the labrum and cartilage down to the level of subchondral bone and medially to the center of the glenoid. Lateral humeral translations and stability ratios were again measured as described previously.
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Photograph (Fig. 3-A) and illustrations (Figs. 3-B and 3-C) of the chondral-labral defect, created from anterior (90 degrees) to inferior (180 degrees), to the depth of the subchondral bone and medially to the center of the glenoid.
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Photograph (Fig. 3-A) and illustrations (Figs. 3-B and 3-C) of the chondral-labral defect, created from anterior (90 degrees) to inferior (180 degrees), to the depth of the subchondral bone and medially to the center of the glenoid.
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Photograph (Fig. 3-A) and illustrations (Figs. 3-B and 3-C) of the chondral-labral defect, created from anterior (90 degrees) to inferior (180 degrees), to the depth of the subchondral bone and medially to the center of the glenoid.
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There are many possible approaches to reconstruction of the glenoid concavity. We selected attachment of an autogenous tendon graft onto the rim of the glenoid. We used the long head of the biceps tendon, not because it represented a realistic operative choice but because it was conveniently available from each humeroscapular preparation. The biceps was divided at its insertion in the glenoid and at its musculotendinous junction. The tendon was trimmed from its distal end until the length was twice that of the chondral-labral defect. Drill-holes, 1.8 millimeters in diameter, were then placed through the lip of the glenoid in the area of the lesion approximately five millimeters apart and four millimeters from the rim of the glenoid, as has been recommended for reattachment of an avulsed glenoid labrum18,27. The biceps graft was folded onto itself to double its width, and it was attached to the glenoid with simple interrupted sutures of number-2 Tevdek (Deknatel, Fall River, Massachusetts) through the drill-holes, with care taken to be sure that the knots were placed on the periphery (Fig. 4-A, 4-B, and 4-C). The thickness and quality of the biceps varied greatly among the specimens, but each specimen was reconstructed with its own biceps. No attempt was made to standardize the thickness of the tendon. After this reconstruction, lateral humeral displacements and stability ratios were measured as described.
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Photograph (Fig. 4-A) and illustrations (Figs. 4-B and 4-C) of the simulated operative reconstruction of the glenoid concavity with use of an autogenous biceps-tendon graft.
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Photograph (Fig. 4-A) and illustrations (Figs. 4-B and 4-C) of the simulated operative reconstruction of the glenoid concavity with use of an autogenous biceps-tendon graft.
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Photograph (Fig. 4-A) and illustrations (Figs. 4-B and 4-C) of the simulated operative reconstruction of the glenoid concavity with use of an autogenous biceps-tendon graft.
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Statistics
One-way analysis of variance was used to compare the lateral humeral displacement and the stability ratio for each test condition (intact, incised, and reconstructed) for each of the eight different directions around the glenoid. Significance was determined with the Scheffé multiple comparisons test as p < 0.05.
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Results
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Glenoid compression: No change in the zero position of the humeral head was noted in any shoulder after thirty minutes of compression with a fifty-newton load, indicating the absence of appreciable chondral deformation.
Lateral humeral displacement: Representative pathways of the humeral head from one shoulder for the intact and incised conditions showed that incision of the anteroinferior portion of the glenoid labrum and cartilage decreased the lateral humeral displacement in this direction (Fig. 5). Reconstruction of this defect restored the normal height and shape of the curve (Fig. 5). For all five shoulders, no significant differences in the center pathway of the humeral head in the anteroinferior (135-degree) direction were seen between the intact and the reconstructed conditions (Fig. 6). In each of the three directions influenced by the chondral-labral defect, lateral humeral displacement started to change significantly at five millimeters of translation (p < 0.0001). Furthermore, the reconstruction effectively restored the lateral humeral displacement to or beyond the values for the intact condition (Table I).
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Lateral humeral displacement curves for one shoulder, for translation in the anteroinferior (135-degree) and posterosuperior (315-degree) directions for the three conditions.
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Lateral humeral displacement curves for the three conditions in all five shoulders in the anteroinferior (135-degree) direction. Each point represents the mean, and the I-bars represent one standard deviation.
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TABLE I
LATERAL DISPLACEMENTS OF THE HUMERAL HEAD AT 2.5, 5.0, AND 12.5 MILLIMETERS OF TRANSLATION FROM THE CENTER OF THE GLENOID
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Stability ratio: The stability ratio was related to the direction of the testing and to the condition of the labrum (Fig. 7). In the anterior (90-degree), anteroinferior (135-degree), and inferior (180-degree) directions, the chondral-labral defect significantly (p < 0.0001) reduced the stability ratio compared with that in the intact condition. Reconstruction of the defect restored the stability ratio in all three directions. In the anteroinferior direction, the stability ratio for the reconstructed condition was significantly greater (p < 0.03) than for the intact condition.
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Illustration showing the mean stability ratios (and standard deviations) in the eight directions of testing. By convention, superior was defined as 0 degrees and anterior, as 90 degrees. For each direction, the stability ratios are listed in order for the intact, incised, and reconstructed conditions. *P < 0.0001 compared with the incised condition. P < 0.03 compared with the intact condition.
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Maximum lateral humeral displacement versus stability ratio: In the anterior, anteroinferior, and inferior directions, the maximum lateral humeral displacement was compared with the stability ratio for each condition (Fig. 8). As noted by Lippitt et al., there was a nearly linear relationship between the two factors regardless of the condition (Pearson correlation coefficient, 0.88). If the humeral head and glenoid are assumed to be rigid, frictionless, conforming spheres, the stability ratio can be approximated with the equation: two times the maximum height of the lip of the glenoid divided by the width of the glenoid, where the width is equal to the magnitude of horizontal translation at the point of maximum lateral displacement. This equation was used to generate the calculated relationship between maximum lateral displacement and the stability ratio (Fig. 8).
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The stability ratio as a function of the maximum lateral humeral displacement for all three conditions in the anterior (90-degree), anteroinferior (135-degree), and inferior (180-degree) directions. The graph shows the stability ratios observed experimentally compared with those calculated with the equation: two times the maximum height of the lip of the glenoid divided by the width of the glenoid, where the width is equal to the magnitude of horizontal translation at the point of maximum lateral displacement.
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Statistical variation: Lateral humeral displacement curves for the translation in the anteroinferior (135-degree) and posterosuperior (315-degree) directions for all three conditions were demonstrated for one shoulder (Fig. 5). Each pathway of the humeral head was constructed from two trials. Examination of the points of the curve showed that there was little variation between the two trials with respect to the lateral displacement of the humeral head. The magnitude of the standard deviations for the mean values for lateral humeral displacement (Fig. 6) was primarily due to differences between the specimens. Similarly, the standard deviations for the mean stability ratios (Fig. 7) presumably represent variations in the anatomy of the shoulders. The mean difference (and standard deviation) in the stability ratios between the two trials for each specimen were 2.7 ± 3.7 per cent, 1.1 ± 0.9 per cent, and 2.2 ± 1.7 per cent for the intact, incised, and reconstructed conditions, respectively. Values for the second trial were not consistently lower than those for the first trial.
Translation to dislocation: As the translating force was applied progressively, minimum translation occurred before dislocation. Once the threshold translating force was exceeded, dislocation occurred suddenly (Fig. 9) (Table II).
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Graph of the translatory force plotted against the translation until dislocation for one shoulder. The direction of application of the translatory force was anterior, and the compressive load was fifty newtons.
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Discussion
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The anteroinferior glenohumeral ligament has been thought to be an important stabilizer of the shoulder against traumatic anterior instability15,17,19,24,26-29. For this reason, repair of an avulsion of this ligament from the glenoid is the primary operative treatment for traumatic anterior instability. Atraumatic multidirectional instability is often associated with a patulous capsule. Therefore, capsular shift is the mainstay of operative treatment for patients for whom rehabilitation has been unsuccessful1,4,5,13,20. However, the capsule tightens only at the extremes of glenohumeral motion8. Hence, the capsule can be viewed as the final checkrein against dislocation from over-rotation. Since the humeral head in a normal shoulder remains precisely centered within the glenoid during the mid-range of motion10,22, other mechanisms of stability must be operative.
Compression of the humeral head into the glenoid concavity is an important mechanism of glenohumeral stability, especially in the mid-range, in which most functions are carried out and the capsuloligamentous structures are lax14,18. Lesions that compromise the glenoid concavity are thought to jeopardize glenohumeral stability3,9,14,18. The relationship between glenoid concavity and stabilization of the shoulder has not been explored in detail, to our knowledge. Furthermore, the potential for restoring lost glenoid concavity has received little attention previously.
The objective of the present investigation was to explore the effect of glenoid concavity on humeral stabilization in cadaveric shoulders. We created a chondral-labral lesion at the anteroinferior aspect of the glenoid in an attempt to recreate a defect similar to that which we have observed in patients who had numerous anterior subluxations or dislocations. This lesion significantly decreased both the lateral humeral displacement and the stability ratio for translations in the direction of the defect. In the important anteroinferior direction, the height of the glenoid was decreased by approximately 80 per cent and the stability ratio was decreased by approximately 65 per cent.
Traumatic and atraumatic instability can be associated with chondral and labral abnormality1-4,15,17,20,21,24,27. Although satisfactory results may be obtained after capsular repair or reconstruction, our clinical experience indicates that a subset of patients still have continued instability despite numerous attempts at such procedures. Defects of the glenoid labrum, cartilage, or osseous rim, with a resultant decrease in the glenoid concavity, may be responsible for this recalcitrant instability.
Simulated operative reconstruction of the lost anteroinferior glenoid concavity with use of an autogenous tendon graft restored both the normal lateral humeral displacements and the stability ratios to or beyond the values for the intact shoulder. Also, the over-all shape of the pathway of the humeral head was corrected by the reconstruction.
In the anteroinferior direction, the reconstruction significantly increased the stability ratio beyond that of the intact glenoid. Since the maximum lateral humeral displacement and the stability ratio are related linearly, larger grafts or a bone-cartilage-labrum composite could conceivably increase the stability ratio even more. In essence, the stability ratio is a measure of the efficiency of concavity-compression in providing stability of the shoulder. For a shoulder with a larger glenoid concavity (and hence a greater stability ratio), less compressive load would be required.
We have shown that the efficiency of concavity-compression can be estimated mathematically (Fig. 8). Soslowsky et al. demonstrated that the humeral and glenoid radii of curvature closely approximate conforming spheres. Therefore, the difference noted between the calculated and experimental stability ratios is probably due to the lack of perfect rigidity of the articular surfaces and glenoid labrum as well as to frictional effects.
During stability testing, minimum translation takes place before the dislocating force is reached and, as noted by Lippitt et al., once this threshold force is attained, dislocation occurs suddenly. In the intact specimens, the predislocation translation averaged 1.6, 2.2, and 2.1 millimeters in the anterior, anteroinferior, and inferior directions, respectively. There were no significant differences in the predislocation translation among the intact, incised, and reconstructed specimens in any direction of dislocation, although there was more variability among the incised specimens. Regardless of how flat the translation pathway of the humeral head became after the creation of a chondral-labral lesion, some concavity was always present. As stability has been defined as the ability to keep the humeral head perfectly centered in the glenoid fossa16,18, the minimum translation before the dislocation force is reached points to concavity-compression as one of the primary mechanisms responsible for the maintenance of stability.
The cadaveric shoulders used in this study were all from elderly individuals. Conceivably, in the younger age-group of patients who typically have glenohumeral instability, thicker chondral surfaces and a more robust labrum might lead to a deeper glenoid and greater stability by the concavity-compression mechanism than that demonstrated here. As a result, compromise of this mechanism in a younger population may be even more substantial than we have shown.
The goals of this study were purely mechanical. Specifically, we questioned whether the lateral humeral displacement curve and stability ratio could be restored with a labral reconstruction. No attempt was made to investigate the biological characteristics of the reconstruction. While a variety of operative methods for the enhancement of the concavity of the glenoid could be explored, we used only an autogenous tendon graft sutured to the lip of the glenoid. Interestingly, Kohn et al. used a tendinous graft attached through tunnels in bone to reconstruct the medial meniscus in a sheep model. After one year, they found that not only had the grafts healed but they also had taken on a meniscoid appearance. In the present study, the long head of the biceps tendon was used for the reconstruction solely because it was available from the humeroscapular specimens. We do not advocate its use clinically because of the important function of the biceps tendon in the normal shoulder. Additional research is necessary to determine which graft material best satisfies the mechanical and biological requirements of the glenohumeral joint.
In conclusion, the concavity-compression mechanism is an important glenohumeral stabilizer. Creation of an anteroinferior chondral-labral defect similar to that seen in patients who had multiple episodes of anterior instability caused a decrease in the lateral humeral displacement and the stability ratio. Simulated operative reconstruction of this defect re-established the normal glenoid concavity with consequent restoration of normal stability by concavity-compression. Dislocation under compressive load occurs as an abrupt and sudden event with minimum initial translation before dislocation. In patients in whom multiple capsular procedures have failed or who have instability predominantly in the mid-range of motion, reconstruction of the glenoid concavity may be preferable to capsular tightening. Additional research into the clinical aspects of this type of reconstruction is needed before the procedure can be recommended.
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Footnotes
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*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. No funds were received in support of this study.
Department of Orthopaedic Surgery, Albert Einstein Medical Center, 5501 Old York Road, Willocrest 4, Philadelphia, Pennsylvania 19141-3098.
Department of Orthopaedic Surgery RK-10, University of Washington Medical Center, 1959 N.E. Pacific Street, Seattle, Washington 98195.
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Abstract
Introduction