Total Strain Fields of the Antero-Inferior Shoulder Capsule Under Subluxation: A Stereoradiogrammetric Study

[+] Author and Article Information
David M. Malicky

Department of Mechanical Engineering, Valparaiso University, Valparaiso, IN 46383

Louis J. Soslowsky

McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA 19104

John E. Kuhn, Cameron M. Mouro

Orthopaedic Research Laboratories, University of Michigan, Ann Arbor, MI 45454

Michael J. Bey

McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA 19104; Orthopaedic Research Laboratories, University of Michigan, Ann Arbor, MI 45454

Jonathan A. Raz

Division of Biostatistics, University of Pennsylvania, Philadelphia, PA 19104

Changying A. Liu

Clinical Reporting System, Parke-Davis, City, ST 90909

J Biomech Eng 123(5), 425-431 (Apr 17, 2001) (7 pages) doi:10.1115/1.1394197 History: Received October 21, 1999; Revised April 17, 2001
Copyright © 2001 by ASME
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Diagram of the shoulder showing muscles, ligaments, and the AIC (Antero-Inferior Capsule). This is a lateral view of a right shoulder socket as seen from the humeral head. The AIC (dashed lines) spans a region of joint capsule from glenoid to humerus, from the 2 o’clock position to the 6 o’clock position. Muscles: SP=Supraspinatus, SB=Subscapularis, IF=Infraspinatus, TM=Teres Minor, BI=Long head of Biceps. Ligaments: I=Inferior Glenohumeral Ligament, AB=Anterior Band of the IGHL. Also shown: G=Glenoid, Ac=Acromion, Co=Coracoid, CH=Coraco-Humeral Ligament.
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A supero-lateral view of the humerus and scapula, mounted in the experimental fixture. Also shown are the calibration frame, x-ray film cartridge, subluxation arm, and universal joint. The direction of subluxation is towards the bottom of the picture. Radiographic source is above frame of picture.
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An example of an anterior-inferior radiographic view (inverted) used in SRG reconstruction. The scapula is off the frame of the radiograph, to the left. The direction of subluxation is out of the page. Denoted are the calibration frame, calibration markers, and 60 object markers on the AIC.
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Infero-medial view of the three dimensionally reconstructed Antero-Inferior Capsule (AIC) in nominal strain and strained states. The bottom row of elements corresponds to the 6 o’clock position on the capsule. The direction of subluxation is antero-inferior, as shown in the axes of the figure.
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Schematic of experimental methodology, showing progression of experiment from specimen to SRG to strain calculation. Nominal strain state (X ) includes pressure (P), distraction (D), and rotation (R) as variables. Strained state (x ) includes muscle forces (Fm) and subluxation (S). Direct linear transform (DLT) algorithm reconstructs three-dimensional positions of markers. Strain (ε) is calculated from X and x states.
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Maximum principal strains of the AIC in three specimens, from glenoid to humerus, at 16 mm subluxation (orientation specified in Fig. 6(a)). Dashed lines denote borders of the anterior band of the IGHL when distinct within the AIC. First and last columns represent the AIC insertion zones. Four shoulders displayed high strains typically on the glenoid side (e.g., Fig. 6(a) and 6(b)) while the other four shoulders displayed a more even distribution of strain (e.g., Fig. 6(c)). The most typical high strain region occurred on the glenoid side near the inferior border of the AIC. Other subluxations generally appeared to be scaled from these plots.
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Vectors showing total principal strain directions and magnitudes at 16 mm subluxation in a typical specimen. Maximum principal strain vectors were relatively consistent for all specimens, typically directed from the inferior glenoid border to the superior humeral border. The direction of the maximum principal strain was misaligned with the AB-IGHL by 38±36 deg.




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