The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 92, Issue 11

Scientific Articles | September 01, 2010

Stabilizing Mechanism in Bone-Grafting of a Large Glenoid Defect

Nobuyuki Yamamoto, MD¹; Takayuki Muraki, PhD¹; John W. Sperling, MD¹; Scott P. Steinmann, MD¹; Robert H. Cofield, MD¹; Eiji Itoi, MD²; Kai-Nan An, PhD¹

¹ Biomechanics Laboratory, Division of Orthopedic Research (N.Y., T.M., and K.-N.A.) and Department of Orthopedic Surgery (J.W.S., S.P.S., and R.H.C.), Mayo Clinic, 200 First Street S.W., Rochester, MN 55905. E-mail address for K.-N. An: an.kainan@mayo.edu
² Department of Orthopaedic Surgery, Tohoku University School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan

View Disclosures and Other Information

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the Uehara Memorial Foundation. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.

Investigation performed at the Mayo Clinic, Rochester, Minnesota

J Bone Joint Surg Am, 2010 Sep 01;92(11):2059-2066. doi: 10.2106/JBJS.I.00261

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Abstract

Background:

Conventional wisdom suggests that the glenoid defect after a shoulder dislocation is anteroinferior. However, recent studies have found that the defect is located anteriorly. The purposes of this study were (1) to clarify the critical size of the anterior defect and (2) to demonstrate the stabilizing mechanism of bone-grafting.

Methods:

Thirteen cadaver shoulders were investigated. With use of a custom testing machine with a 50-N compression force, the peak translational force that was needed to move the humeral head and lateral humeral displacement were measured. The force was used to evaluate the joint stability. An osseous defect was created stepwise in 2-mm increments of the defect width. The bone graft was harvested from the coracoid process. The defect size was expressed as the estimated defect size divided by the measured glenoid length. Testing was performed with (1) the glenoid intact, (2) a simulated Bankart lesion, (3) the Bankart lesion repaired, (4) a 2-mm defect, (5) the Bankart lesion repaired, (6) the defect bone-grafted, (7) a 4-mm defect, (8) the Bankart lesion repaired, (9) the defect bone-grafted, (10) a 6-mm defect, (11) the Bankart lesion repaired, (12) the defect bone-grafted, (13) an 8-mm defect, (14) the Bankart lesion repaired, and (15) the defect bone-grafted.

Results:

Force and displacement decreased as the size of the osseous defect increased. The mean force after the formation of a defect of =6 mm (19% of the glenoid length) with the Bankart lesion repaired (22 ± 7 N) was significantly decreased compared with the baseline force (52 ± 11 N). Both the mean force (and standard deviation) and displacement returned to the levels of the intact condition (68 ± 3 N and 2.6 ± 0.4 mm, respectively) after bone-grafting (72 ± 12 N and 2.7 ± 0.3 mm, respectively).

Conclusions:

An osseous defect with a width that is =19% of the glenoid length remains unstable even after Bankart lesion repair. The stabilizing mechanism of bone-grafting was the restoration of the glenoid concavity.

Clinical Relevance:

Reconstruction of the glenoid concavity may be necessary in shoulders with an anterior glenoid defect that is =19% of the glenoid length.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

It is known that glenoid defects are a common injury associated with recurrent anterior dislocation of the shoulder. In the treatment of traumatic recurrent anterior shoulder instability, patients with a large glenoid defect are at risk for recurrent instability after arthroscopic Bankart repair^1-8. In the literature, conventional wisdom suggests that the osseous defect after a shoulder dislocation is anteroinferior^9,10. However, recent studies^4,11-14 have found that the defect was located anteriorly rather than anteroinferiorly. Both Griffith et al.¹¹ and, most recently, Saito et al.¹² described the osseous defect of the glenoid as being located anteriorly at approximately the three o'clock position on a right shoulder.

Bone-grafting has been performed in shoulders with a large glenoid defect, with excellent clinical outcomes^15-18. However, there have been few biomechanical studies clarifying the stabilizing mechanism of the bone graft. Montgomery et al.¹⁹ demonstrated that the effectiveness of a bone graft in restoring stability was related both to its height and the extent to which it was contoured as long as the graft was not prominent. They created a glenoid defect anteroinferiorly and studied only the glenoid without the ligaments, labrum, and capsule. The purposes of the present study were (1) to clarify the critical size of the anterior glenoid defect and (2) to demonstrate the stabilizing mechanism of bone-grafting.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Preparation of Specimens

Thirteen fresh-frozen cadaver shoulders (six from male donors and seven from female donors) were used in this study. The average age of the subjects at the time of death was seventy-seven years (range, fifty-five to ninety-eight years). The specimens demonstrated no evidence of moderate or severe glenohumeral arthritis or joint contracture. A glenohumeral joint contracture was thought to be present when at least one of the following conditions was observed: <60° of elevation relative to the scapula, <45° of external rotation, and <20° of internal rotation. Also, we excluded specimens with moderate to severe glenohumeral arthritis from our study when the radiographic image demonstrated at least one of the following findings: extensive deformity of the glenoid or humeral head, a large osteophyte, or loss of joint space. The shoulders were thawed overnight at room temperature prior to testing, and the skin, subcutaneous tissue, and all soft tissues superficial to the rotator cuff muscles were removed. The rotator cuff muscles were elevated from the scapula, and the tendinous portions were bluntly separated from the capsule. The medial margin of the scapula was osteotomized along a line parallel to the glenoid surface. The glenoid was potted in an aluminum container attached to a custom multiaxis electromechanical testing machine (Avalon Technologies, Rochester, Minnesota) with use of resin so that its face was oriented parallel to the floor. To make the glenoid articular surface be parallel to the floor, two Kirschner wires (1.2 mm in diameter) were passed through the glenohumeral joint space (just above the labrum), one parallel to a line connecting the anterior and posterior aspects of the glenoid rim and the other parallel to a line connecting the superior and inferior aspects of the glenoid rim. When the wires were parallel to the floor in both directions, the glenoid articular surface was believed to be parallel to the floor. The specimen was kept moist with a spray of saline solution applied every ten to fifteen minutes during the test, which was performed at room temperature (24°C).

Testing Apparatus

The testing apparatus (Fig. 1) consisted of a six-degrees-of-freedom load-cell (model 45E15A-E24ES-A; JR3, Woodland, California) mounted on a motorized x-y stage (DCI Design Components, Franklin, Massachusetts), onto which the aluminum box with the potted scapula was fixed. According to the manufacturer, the capacity and resolution of the load-cell were 1000 N and 1.3 N, respectively. This custom testing apparatus has been validated in previous studies^9,20. In the present study, the x and y axes were defined in the horizontal plane, and the z direction was defined as the vertical direction. The x-y table allowed for motor-driven computer-controlled translations in the horizontal plane. The relationship of the stepper motor and lead screw was 0.025 mm/step; however, the accuracy of the linear potentiometer used to record linear motion was 0.2 mm. This value was determined by considering the resolution of the analog-digital card (12 bit), the voltage supplied to the potentiometer, and the linearity of the potentiometer (0.075%). The vertical slide provided unconstrained movement in the z direction. A linear-position transducer (TR-50; Novotechnik, Stuttgart, Germany) was attached to the sliding device to measure the lateral movement of the humeral head along the z axis. According to the manufacturer, repeatability of this linear-position sensor was ±0.002 mm and independent linearity was ±0.15%. A 50-N compressive force was constantly applied to the humerus by a low-friction pneumatic cylinder to keep the humeral head centered in the glenoid fossa. Bovine serum was used as a lubricant between the glenoid and humeral head articular cartilage.

Fig. 1

The custom mechanical testing device consisted of a six-degrees-of-freedom load-cell mounted on a motorized X-Y table.

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Stability Test

First, the reference position was determined by measuring the lateral humeral displacement during an 8.0-mm translation in the superior-inferior and anterior-posterior directions as previously reported^9,20. In both directions, the least-lateral position of the humeral head on the glenoid surface was defined as the reference neutral position. Starting from the neutral position on the glenoid, the humeral head was translated in the anterior direction for 10 mm at a rate of 2.0 mm/sec. The peak translational force that was needed to move the humeral head and the lateral displacement of the center of the humeral head were recorded. Two trials were performed in each condition, and the mean value was used for data analysis. Although all specimens were tested with use of this 10-mm-displacement protocol²¹, the analysis of force was based on a normalized displacement, which was proportional to the size of the glenoid. A displacement length of 10 mm was used for the glenoid with the largest length (38 mm), and the displacement lengths for all other glenoids were scaled accordingly; e.g., if the glenoid length of one specimen was 19 mm, the displacement length for this specimen was determined to be 5 mm. Then, the peak force that was necessary to translate the humeral head the normalized distance was determined for each test condition from the force-displacement curve.

Test Conditions

The arm position that was used was 60° of abduction relative to the scapula (90° of abduction relative to the trunk) and neutral rotation. We chose this position because it simulated the midrange position of the glenohumeral joint where compression of the humeral head into the glenoid concavity is an important mechanism of stability^22,23. Testing was performed under fifteen conditions with (1) the glenoid intact, (2) a simulated Bankart lesion, (3) the Bankart lesion repaired, (4) a 2-mm osseous defect, (5) the Bankart lesion repaired, (6) the defect bone-grafted, (7) a 4-mm osseous defect, (8) the Bankart lesion repaired, (9) the defect bone-grafted, (10) a 6-mm osseous defect, (11) the Bankart lesion repaired, (12) the defect bone-grafted, (13) an 8-mm osseous defect, (14) the Bankart lesion repaired, and (15) the defect bone-grafted. A Bankart lesion was created by elevating the capsulolabral attachment subperiosteally from the glenoid from two o'clock to eight o'clock in the right shoulder. In addition, the labrum was disrupted by using a sharp blade at three o'clock in the right shoulder as previously reported⁹.

Simulation of the Glenoid Defect

Before a glenoid defect was created, the glenoid width and length were measured. Glenoid defects were then created stepwise in 2-mm increments of the defect width by cutting off the anterior rim of the glenoid at the three o'clock position (Fig. 2) as previously reported²³. The osteotomy lines were determined as follows. First, an outer fitting circle that fit the superoinferior diameter of the glenoid was constructed for each specimen. After that, the center of the outer fitting circle was determined. The y axis was drawn passing through the superior and inferior contact points of the outer fitting circle. Next, the x axis was drawn perpendicular to the y axis and passing through the center of the outer fitting circle. Osteotomy lines at 2 mm, 4 mm, 6 mm, and 8 mm in width were drawn perpendicular to the x axis. The osteotomy line at "0 mm" was defined as a line parallel to the y axis and tangential to the anterior rim of the glenoid. We defined the defect for which the peak translational force was significantly smaller than the baseline force (without a defect after creation of the Bankart lesion) as the critical size of a glenoid defect.

Fig. 2

Photograph of a glenoid showing the simulated osteotomy lines (blue lines) in 2-mm increments, which were drawn parallel to the y axis. The x and y axes pass through the center (C) of the circumcircle (dotted line), which was fitted to the superoinferior diameter of the glenoid.

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Reconstruction

The coracoid process was chosen as the bone graft in the present study because it has been commonly used^24-29, and it was conveniently available from each cadaver shoulder compared with harvesting the iliac crest. With a small angulated saw, the 2.0-cm distal tip of the coracoid process was osteotomized. The coracoid graft was transferred to the glenoid neck, on top of the capsule. The bone block was secured to the defect with the use of two AO 4.5-mm malleolar screws that were placed through the graft and into the native glenoid subchondral bone. The graft was placed so that it functioned as an extension of the glenoid articular arc at the anterior-inferior margin of the glenoid, thereby adjusting the inclination of the graft. If needed, the graft was contoured in three dimensions with a power burr to fit the shape of the glenoid and to recreate the anterior-posterior curve of the glenoid articular surface^15,25. The type of Bankart repair that was performed in the present study involved placing transosseous sutures at the glenoid rim. The repair was performed with the arm in adduction and 30° of external rotation relative to the scapular plane, which was equivalent to adduction and neutral rotation relative to the trunk.

Three Factors Related to Osseous Stability

Three factors believed to be related to osseous stability—the depth, curved articular surface, and arc length of the glenoid—were evaluated (Fig. 3). The depth of the glenoid was defined as the distance from the rim to the bottom of the articular surface of the glenoid in the mediolateral direction; the curved articular surface, as the curvature from the rim to the bottom of the articular surface of the glenoid; and the arc length, as the distance from the rim to the bottom of the articular surface of the glenoid in the anteroposterior direction. These factors were evaluated from the humeral displacement curves.

Fig. 3

Diagram demonstrating the glenoid with a large osseous defect. When there is a large glenoid defect, the glenoid loses three factors related to osseous stability: depth, curved articular surface, and arc length of the glenoid.

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Data Analysis

One-way repeated-measures analysis of variance was used to compare the peak translational force with the various glenoid defects with and without Bankart lesion repair. When a significant effect was observed, it was further analyzed separately with use of the Dunnett test. The conditions without a defect after the creation of the Bankart lesion and after the repair of the Bankart lesion were used as the baseline conditions for the groups without and with Bankart repair, respectively. A paired t test was used to determine if a significant difference was present between the forces with the glenoid intact and after creating a Bankart lesion. It was also used to determine if a significant difference was present between the forces after creating a defect and after bone-grafting and if a significant difference was present between the humeral displacements with the glenoid intact and after creating a Bankart lesion. The level of significance was set at p < 0.05. With thirteen cadaver shoulders, there was 80% power to detect an effect size of >0.85, where the effect size is the mean of the change divided by the standard deviation of the change.

Source of Funding

The equipment and devices were purchased by the Mayo Foundation. The Uehara Memorial Foundation supported a portion of the research project.

Results

Introduction | Materials and Methods | Results | Discussion | References

Stability

The average force that was needed to move the humeral head with the glenoid intact (68 ± 3 N) decreased significantly after the creation of the Bankart lesion (37 ± 9 N) (p = 0.001) (Fig. 4). The force decreased as the size of the osseous defect increased (p < 0.001), with the force in shoulders without a Bankart lesion repair that were measured after creation of a defect of =4 mm (16 ± 5 N) being significantly smaller than the baseline force (without a defect after creation of the Bankart lesion) (p = 0.0015). Similarly, the force in shoulders with a Bankart lesion repair significantly decreased after creation of a defect of =6 mm (22 ± 7 N) (p = 0.036) compared with the baseline force (Bankart repair, 52 ± 11 N). For these conditions, the mean glenoid length and width were 31.9 ± 2.6 mm and 24.0 ± 2.7 mm, respectively; thus, a 6-mm defect was equivalent to 19% ± 1% of the glenoid length or 25% ± 2% of the glenoid width. After bone-grafting of the defect, the mean force (72 ± 12 N) had significantly increased to the level of the intact glenoid (68 ± 3 N) regardless of the size of the osseous defect (p < 0.001).

Fig. 4

Histogram demonstrating stability. The peak translational force decreased significantly in both the shoulders without Bankart lesion repair (yellow bars) and those with Bankart lesion repair (red bars) compared with the peak translational force in the intact condition. In shoulders with a repair, the force decreased significantly when the width of the osseous defect was =6 mm. The force values are expressed as the mean and the standard deviation. #Compared with the intact glenoid, the difference was significant (p < 0.05). *Compared with shoulders with a Bankart lesion without a defect, the difference was significant (p < 0.05). **Compared with shoulders with a Bankart lesion repair without a defect, the difference was significant (p < 0.05). †Compared with shoulders with a defect after Bankart lesion repair, the difference was significant (p < 0.05).

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Lateral Humeral Displacement

The lateral humeral displacement decreased as the size of the osseous defect increased (p < 0.0001) (Figs. 5 and 6). The displacement that was measured after creating a defect that was =2 mm (1.5 ± 0.5 mm) with and without a Bankart lesion repair was significantly smaller than that with the glenoid intact (2.6 ± 0.4 mm) (p < 0.001). The displacement of the intact glenoid (100%) was decreased to 0.4 ± 0.4 mm (15%) after creation of a 6-mm defect. However, it increased to 2.7 ± 0.3 mm (104%) after bone-grafting. After bone-grafting, the displacement significantly increased to the level of the intact glenoid regardless of the size of the osseous defect (p < 0.001).

Fig. 5

Histogram demonstrating the lateral humeral displacement. The lateral humeral displacement decreased significantly in both the shoulders without Bankart lesion repair (yellow bars) and those with Bankart lesion repair (red bars) compared with the displacement in the intact condition. In both the shoulders with and those without a Bankart lesion repair, the displacement decreased significantly after creation of a glenoid defect that was =2 mm. The displacement values are expressed as the mean and the standard deviation. *Compared with shoulders with a Bankart lesion without a defect, the difference was significant (p < 0.05). **Compared with shoulders with a Bankart lesion repair without a defect, the difference was significant (p < 0.05). †Compared with shoulders with a defect after Bankart lesion repair, the difference was significant (p < 0.05).

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Fig. 6

Lateral humeral displacement curves for three conditions in one specimen. The lateral humeral displacement, which decreased after the creation of a 6-mm glenoid defect, returned to almost the intact-glenoid level after bone-grafting. From these curves, it can be seen that the curved articular surface and arc length of the glenoid also decreased when a glenoid defect was created, and that they returned to the intact-condition level after bone-grafting.

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The humeral displacement curves also showed that after creation of a glenoid defect, the glenoid depth was decreased and the curved articular surface and arc length were shortened. In contrast, after bone-grafting, the glenoid depth was increased and the curved articular surface and arc length were widened (Fig. 7).

Fig. 7

Diagram demonstrating the glenoid after bone-grafting. Bone graft can restore the glenoid concavity by increasing the glenoid depth, extending the curved articular surface, and widening the arc length.

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Discussion

Introduction | Materials and Methods | Results | Discussion | References

The purpose of this study was to clarify the critical size of the anterior glenoid defect in anterior shoulder dislocation and to demonstrate the stabilizing mechanism of bone-grafting. From the data of the peak translational force, it was demonstrated that a 6-mm defect, which was equivalent to 25% of the glenoid width or 19% of the glenoid length, was the critical size of a glenoid defect. If the size of the defect is calculated as a percentage of the area of the entire glenoid surface, a 6-mm defect involves approximately one-fourth (26%) of the glenoid surface. However, expressing the size of the defect in terms of glenoid length may be more useful because the anterior rim of the glenoid is deficient in shoulders with anterior instability, and thus the glenoid width cannot be directly measured. On the other hand, the glenoid length is almost always measurable because the most inferior portion of the glenoid is seldom involved in the defect^7,11,12.

This 19% length is comparable with the 20% length that was reported to be the critical size of an anterior glenoid defect in the biomechanical study by Yamamoto et al.²², which used glenohumeral joints without soft-tissue attachments. This indicates that the stability of the shoulder in the midrange of motion depends solely on the osseous contour of the glenoid, not on the soft-tissue status. In other studies in which the soft tissues were preserved, it was found that, in the midrange of motion, where the capsuloligamentous structures were lax and did not contribute to the stability, the stability depended mainly on the osseous structures^30-32. We can conclude that a large osseous defect, which would cause postoperative anterior instability, is a defect that is =19% of the glenoid length. If there is a glenoid defect of this size, the findings suggest that the glenoid concavity should be reconstructed with a bone graft.

The lateral humeral displacement represents the total depth of the glenoid socket as previously reported^30,33. The displacement with the glenoid intact was decreased to approximately 15% (an 85% reduction) after creation of a 6-mm defect. It was increased to 104% (a 4% increase relative to the native glenoid and an 89% increase relative to the glenoid with a 6-mm defect) by bone-grafting. Our data indicate that the glenoid lost depth when a defect was created but recovered it after bone-grafting. Looking at the lateral humeral displacement curves (Fig. 6), we can see that the curved articular surface and the arc length of the glenoid were shortened after the creation of a defect, and they returned to the intact-condition level after bone-grafting (Fig. 7). Thus, in our study, the transplant did not function as a bone block (the bone-block effect²⁶), but it anatomically reconstructed the glenoid concavity, creating a so-called glenoidplasty effect²⁴.

The peak translational force results show that the reconstruction of the glenoid defect by bone graft restored stability comparable with the intact condition. Interestingly, even if there was a large glenoid defect, such as an 8-mm defect, the shoulder attained stability comparable with the intact condition after transfer of the coracoid process. Some surgeons have suggested that for severe bone loss, an iliac crest graft, not the coracoid process, should be employed^8,24,34. However, our findings support the view that the glenoid concavity can be reconstructed by a coracoid process graft, resulting in a stable shoulder.

The present study had several limitations. First, we created an osseous defect by cutting off the anterior glenoid rim parallel to the longitudinal axis of the glenoid. In this manner, we were able to create osseous defects quantitatively. However, the margin of an actual glenoid defect may not be parallel to the longitudinal axis of the glenoid. Therefore, the simulated osseous defect in the present study may be different from the osseous defect in patients. Second, the bone graft was contoured in three dimensions with a power burr to fit the glenoid and to recreate the anterior-posterior curve of the glenoid articular surface. However, some surgeons^24,27-29 have recommended that the graft be placed flush to the articular surface of the glenoid (not to fit the curvature of the articular surface) to prevent postoperative osteoarthritis. We chose the former procedure because it has been used commonly^15,26,30. Third, we used the same size bone graft for the various sizes of defects. Clinically, when the defect is relatively small, we need only a small piece of bone graft. On the other hand, when the defect is large, the graft also needs to be large. However, if the harvested graft is too small, it cannot be made larger. Therefore, 2.0 cm of the coracoid process was osteotomized in the present study so that it fit the glenoid curvature with a large defect. Thus, our bone graft technique may be slightly different from the clinical scenario. Fourth, the soft-tissue condition of the cadaver specimens used in the present study may have different structural qualities compared with those of shoulders in young individuals in whom anterior shoulder dislocation usually occurs. Fifth, the rate of displacement throughout the cycle chosen in the present study was 2.0 mm/sec. However, anterior dislocation of the shoulder typically occurs at much faster displacement rates.

In conclusion, a 6-mm anterior glenoid defect, which was equivalent to 19% of the glenoid length, may be sufficient to cause instability even after repair of a Bankart lesion. If there is a glenoid defect of this size or greater, our findings suggest that the deficient glenoid cavity should be reconstructed.

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Introduction | Materials and Methods | Results | Discussion | References

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Topics

bone transplantation ; humerus ; reconstructive surgical procedures ; shoulder region ; bankart lesion

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Philipp Moroder MD, Mark Tauber MD, Herbert Resch MD

Posted on December 13, 2012

When glenoid bone loss becomes significant

Department of Traumatology and Sports Injuries, Paracelsus Medical University, Salzburg, Austria

In literature there is a lack of consensus on the extent glenoid bone loss has to exceed before being deemed significant and thus indicating bone grafting instead of capsulo-labral repair.(1) The study “Stabilizing Mechanism in Bone-Grafting of a Large Glenoid Defect“ by Yamamoto et al.(2) represents one of the main biomechanical reference studies in the above-mentioned discussion. The role of glenoid bone loss in recurrent anterior shoulder instability has been recognized and gained rising scientific interest over the last few years as documented by the increasing number of clinical, radiological and biomechanical studies focusing on this topical issue.

Due to the importance of this biomechanical study for the debate about the critical glenoid bone loss extent we would like to make two remarks on the otherwise excellent research by Yamamoto et al. The first remark is a mathematical correction. According to the published data, Yamamoto deems the significant bone-defect size to correspond to 19% of the glenoid length or 25% of the glenoid width. Both values are drawn from division of the critical 6mm defect created in the biomechanical experiment, after which Bankart repair provided significantly reduced stability, by the corresponding anatomical structure, be it glenoid length or width. Yamamoto adds that according to the 25% width a defect area of 26% of the glenoid surface should be regarded as significant. This statement is mathematically incorrect since the outer segments of a circle always contain less area than the inner ones as can be seen when cutting a tomato into equally thick slices. Therefore, the critical glenoid defect area according to Yamamoto’s data should be 19.6% instead of 26%. This correction becomes important since recently the trend of measuring glenoid bone loss has shifted toward area measurements rather than linear measurements.(3)

The second remark is an interpretational one. The authors of this biomechanical study created bone-loss situations starting with 0mm defect width and increasing by 2mm steps. With increasing defect extent a decrease of the peak translational force (PTF) despite Bankart repair was noted indicating decreasing shoulder stability. The intact shoulders had a PTF of 68N, shoulders with 0mm defect + Bankart repair 52N, 2mm + Bankart repair 43N, 4mm + Bankart repair 35N, 6mm + Bankart repair 22N, and 8mm + Bankart repair 13N. The authors noted a statistically significant reduction of the PTF in the case of a 6mm defect in comparison to the 0mm defect situation despite Bankart repair in both cases. Biomechanically this comparison is correct because it factors out stability loss due to the repaired but not intact labrum. In reality, however, the goal of surgical shoulder stabilization is not to make a shoulder as stable as a previously repaired one. Obviously, patients expect to re-obtain their normal shoulder stability, especially those with shoulder-demanding work or athletic activities. In that case the comparison of the shoulders with increasing glenoid bone loss should be made with the intact shoulder (PTF of 68N) and not with the one with Bankart lesion and undergone repair (PTF of 52N). If doing so, a 4mm bone loss situation with Bankart repair, which only achieved half the PTF (35N) of an intact shoulder, might render a statistically significant difference yielding critical defect sizes of 13% of the glenoid length, 17% of the glenoid width, and 11% of the glenoid area according to Yamamoto’s data. While probably both interpretations are correct and only offer different views of the same data the possible implications for clinical decision-making are of relevant importance. The cut-off between significant and non-significant glenoid bone loss appears to be found at lower values than determined by Yamamoto et al. However, the clinical threshold value always remains subject to the patient’s individual expectations and need for proper stability.

References
1. Bushnell BD, Creighton RA, Herring MM. Bony instability of the shoulder. Arthroscopy. 2008 Sep;24(9):1061-73.
2. Yamamoto N, Muraki T, Sperling JW, Steinmann SP, Cofield RH, Itoi E, et al. Stabilizing mechanism in bone-grafting of a large glenoid defect. J Bone Joint Surg Am. 2010 Sep;92(11):2059-66.
3. Bois AJ, Fening SD, Polster J, Jones MH, Miniaci A. Quantifying glenoid bone loss in anterior shoulder instability: reliability and accuracy of 2-dimensional and 3-dimensional computed tomography measurement techniques. Am J Sports Med. 2012 Nov;40(11):2569-77.

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Nobuyuki Yamamoto, Takayuki Muraki, John W. Sperling, Scott P. Steinmann, Robert H. Cofield, Eiji Itoi, Kai-Nan An; Stabilizing Mechanism in Bone-Grafting of a Large Glenoid Defect. The Journal of Bone & Joint Surgery. 2010 Sep;92(11):2059-2066.

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