Rotator cuff tears are a common cause of shoulder pain treated by Christopher C. Schmidt, MD, a board-certified orthopedic surgeon who practices in Pittsburgh, Pennsylvania. Widely regarded as one of the best shoulder surgeons in the United States, Dr. Schmidt also performs extensive clinical and biomechanical research to continually improve both pre-and intra-operative approaches to rotator cuff tear repair. The findings of his studies are incorporated into best-practice guidelines utilized by other surgeons across the nation.
Recently, Dr. Schmidt led an important biomechanical study focused on the rotator cable, a thick bundle of fibers perpendicular to the supraspinatus and infraspinatus tendons. During shoulder abduction, the rotator cable functions similarly to a “suspension bridge,” transmitting force to the humeral head while possibly strain-shielding the rotator crescent. The goal of this research initiative was to further the understanding of the rotator cable and its role in strain-shielding the crescent area.
Does the Rotator Cable Effectively Strain-Shield the Crescent Area?
Through extensive testing, Dr. Schmidt and his team found that the magnitude and direction of the principal strain on the rotator crescent were relatively consistent regardless of the condition of the rotator cable. These findings were inconsistent with the theory that the rotator cable transmits the load while strain-shielding the crescent area. As a result, Dr. Schmidt and his team concluded that the rotator cable and the rotator crescent are important structures in initiating shoulder abduction, and therefore rotator cuff tears in these areas are biomechanically significant and should be treated accordingly.
The abstract of the study is reproduced below:
Rotator Crescent Area Is Important To Force Transmission
R Blake1, M Smolinski1, C Spicer1, A Davidson2, MC Miller1, P Smolinski1,2, T Zink2, D Papadopoulos2, C Schmidt1,2
1 Department of Mechanical Engineering and Material Science, University of Pittsburgh, PA
2 Department of Orthopedic Surgery, University of Pittsburgh, Pittsburgh, PA
INTRODUCTION: The rotator cable (RCa) is a thickening band of fibers within the rotator cuff originating at the coracohumeral ligament (CHL) insertion and terminating between the infraspinatus (IS) and teres minor (TM). The structure is responsible for transmitting load to the humeral head and is thought to strain shield the lateral rotator cuff (crescent area CA). Surface strains were measured on the rotator cuff under physiological loading to determine the role of the RCa.
METHODS: Eighteen fresh-frozen cadaveric specimens (average age 67 + 17) without rotator cuff pathology were employed for the study and dissected to the level of the rotator cuff complex with approval from the institutional review board. All shoulder specimens were placed into a custom shoulder simulator. The muscles of the rotator cuff were individually loaded with physiological force vectors to imitate abduction [2,3]. The upper and lower subscapularis (SubS), supraspinatus (SS), IS, and TM were attached to servo driven actuators on the simulator. For the testing sequence, all shoulders were placed in neutral forearm rotation and tested at both 0° and 30° of scapular plane abduction. A digital image correlation system [accuracy + 10µm] (Vic-3D Correlated Solutions Inc, Irmo, SC) was used to measure the magnitude and direction of surface principal strain. After testing the native intact cable condition, the specimens had both the anterior and posterior cable insertions transected (full release) and the specimen was retested. Strain measurements were taken in the RCa and at a position one-half cable widths distance to the lateral border of the RCa along a line that bisected the SS. The values at each position were averaged over a 2 mm radius circle region. Statistical comparisons were made using paired t-tests with p < 0.05.
RESULTS: The maximum principal strains and directions in the CA and RCa as a function of cuff condition and flexion angle are given in Table 1 and shown in Figure 2. There was no significant difference in strain direction or magnitude with cable state.
DISCUSSION: Based upon the hypothesis that the rotator cable transmits the load and shields the crescent area, higher strain patterns should develop in the CA after transection of the RCa, and strain direction would deviate to compensate for the RCa’s inability to transmit load around the CA. However, our results show that the magnitude and direction of the principal strain is relatively consistent regardless of cable condition.
CLINICAL RELEVANCE: The crescent area is a load bearing structure and tears in this area are mechanically significant, which should be addressed clinically.
REFERENCES: 1. Burkhart, et al., Arthroscopy 1993 2. Kedgley, et al., J Biomech. 2007 3. Omi, et al., J Anatomy 2010
Figure 1: A) Specimen in testing system and B) contour plot of the maximum principal strain in the cuff.
Figure 2: Major principal strain direction for native and full release states at 0° abduction.
Table 1: Cable and crescent major principal strain directions and magnitudes for native and full release states.
If you would like to learn more about the latest techniques in arthroscopic rotator cuff tear repair, call (877) 471-0935 to request an appointment with Dr. Schmidt at one of his three office locations in the greater Pittsburgh, PA, area.
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