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GLENOID FRACTURE AND SHOULDER INSTABILITY
EIJI ITOI, MD
Department of Orthopedic Surgery, Akita University School of Medicine Akita, Japan
Introduction
Fracture or erosion of the anteroinferior rim of the glenoid is sometimes observed in shoulders with recurrent anterior dislocation. Fracture of the anterior glenoid rim with a large fragment is known to cause anterior shoulder instability. There is a consensus that a large fragment needs to be fixed and a large defect needs to be bone grafted, but there is a lack of consensus how large a defect should be to necessitate a bone grafting. Some proposed a guideline that a bony defect larger than 1/3 of the glenoid surface may require bone graft. However, there remain two questions to be answered. 1) How can we say a defect is 1/3 of the glenoid surface? 2) Is this size of bony defect critical to the stability of a Bankart-repaired shoulder?
Bony defect that causes instability?
The first study was performed 1) to create various sizes of bony defect of the glenoid quantitatively and 2) to determine the effect of bony defect on the anteroinferior stability of the shoulder after Bankart repair. The glenoids from 16 dried scapulae were photographed, and the images were scanned into a computer. The average shape of the glenoid was determined on the basis of the scans, and this information was used to design custom templates for the purpose of creating various sizes of osseous defects. Ten fresh-frozen cadaveric shoulders then were obtained from individuals who had been an average of 79 years old at the time of death, and all muscles were removed to expose the joint capsule. With use of a custom multiaxis electromechanical testing machine with a six-degrees-of-freedom load-cell, the humeral head was translated 10 mm in the anteroinferior direction with the arm in abduction and external rotation as well as in abduction and internal rotation. With a 50-N axial force constantly applied to the humerus in order to keep the humeral head centered in the glenoid fossa, the peak force that was needed to translate the humeral head a normalized distance was determined under 11 sequential conditions: (1) with the capsule intact, (2) after the creation of a simulated Bankart lesion, (3) after the capsule was repaired, (4) after the creation of an anteroinferior osseous defect with a width that was 9% of the glenoid length (average width, 2.8 mm), (5) after the capsule was repaired, (6) after the creation of an osseous defect with a width that was 21% of the glenoid length (average width, 6.8 mm), (7) after the capsule was repaired, (8) after the creation of an osseous defect with a width that was 34% of the glenoid length (average width, 10.8 mm), (9) after the capsule was repaired, (10) after the creation of an osseous defect with a width that was 46% of the glenoid length (average width, 14.8 mm), and (11) after the capsule was repaired. With the arm in abduction and external rotation, the stability of the shoulder after Bankart repair did not change significantly regardless of the size of the osseous defect (p = 0.106). With the arm in abduction and internal rotation, the stability decreased significantly as the size of the osseous defect increased (p<0.0001): the translation force in shoulders in which the width of the osseous defect was at least 21% of the glenoid length (average width, 6.8 mm) was significantly smaller than the force in shoulders without an osseous defect. The range of external rotation in shoulders in which the width of the osseous defect was at least 21% of the glenoid length was significantly less than that in shoulders without a defect (p<0.0001) because of the pretensioning of the capsule caused by closing the gap between the detached capsule and the glenoid rim. The average loss of external rotation was 25˚/cm of defect. In conclusion, an osseous defect with a width that is at least 21% of the glenoid length may cause instability and limit the range of motion of the shoulder after Bankart repair. The results of this cadaver study suggest that in cases of glenoid defect greater than 21% of the glenoid length, any measures to restore the arc of glenoid concavity may be beneficial both in terms of stability and motion.
How to assess the bony defect?
The next question that we have is how to assess this critical size of bony defect. We hypothesized that we would be able to estimate the critical size of glenoid defects using radiography or CT. Thus, we performed a controlled laboratory study using 12 cadaveric scapulae. We created osseous defects of 0%, 9%, 21%, 34%, and 46% of the glenoid length stepwise. With each size of a defect, 1) plain radiographs simulating the axillary and West Point views and 2) CT images were obtained. The maximum width of the remnant glenoid was measured under each condition and expressed as a percentage to the width of the intact glenoid. On West Point view, a 21% defect appeared 18.6% of the intact glenoid. Using CT images, the 21% defect resulted in loss of 50% of the glenoid width on a single CT slice across the lower 1/4 of the glenoid. We conclude that we can estimate the critical size of the glenoid defect using the West Point view or CT at the inferior 1/4 of the glenoid. In a case with an osseous glenoid defect, these images give decisive information as to whether or not the defect necessitates bone grafting to achieve stability after Bankart repair.
References:
1. Itoi E, Lee SB, Berglund LJ, Berge LL, An KN: The effect of a glenoid defect on anteroinferior stability of the shoulder after Bankart repair: A cadaveric study. J Bone Joint Surg 2000; 82-A: 35-46
2. Itoi E, Lee SB, Amrami KK, Wenger DE, An KN: Quantitative assessment of classic anteroinferior bony Bnakart lesions using radiography and computed tomography. Am J Sports Med, In Press.
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