Most anatomy textbooks describe the supraspinatus, infraspinatus, and teres minor as running almost parallel to each other and as inserting separately into discrete and flattened impressions on the greater tuberosity, with the supraspinatus inserting into the highest impression, the infraspinatus inserting into the middle impression, and the teres minor inserting into the lowest impression1,2. The authors of most studies in which humeral insertions have been assessed have reported footprints similar to those described in the anatomy textbooks3-5. In the present study, however, the supraspinatus was found to be localized to the anteromedial area of the highest impression on the greater tuberosity, while the infraspinatus occupied the anterolateral area of the highest impression and all of the middle impression. We believe that differences in the dissection methods brought about these discrepancies. Clark and Harryman reported that fibers from the supraspinatus and infraspinatus fuse together and that it is difficult to separate them7. Dugas et al. selected a sharp dissection along with a discrete muscular interval between the supraspinatus and infraspinatus4. Minagawa et al. removed the overlying thick and parallel fiber bundles to expose the rotator cuff; however, they did not state how they had separated these muscles6. In the present study, we removed the overlying coracohumeral ligament and the loose connective tissues, which enabled us to detect a distinct border between the supraspinatus and infraspinatus and to separate them by precisely tracing the anterior margin of the superior tendinous portion of the infraspinatus. The overlying coracohumeral ligament comprised loose connective tissue as described by Edelson et al.19; therefore, it was easy to distinguish and remove the overlying coracohumeral ligament from the underlying supraspinatus and infraspinatus tendons.
In the additional dissection, the removal of the muscular portion from the supraspinatus and infraspinatus demonstrated that both muscles had a long, thick tendinous portion anteriorly and a short, thin tendinous portion posteriorly. These findings ensured the accuracy of our dissection method with regard to separating the infraspinatus from the supraspinatus along with the superior tendinous portion of the infraspinatus.
The footprint of the supraspinatus was found to have a triangular shape that tapered away from the joint capsule. The footprint of the infraspinatus was trapezoidal in shape and became wider laterally. The average maximum medial-to-lateral length of the footprint of the supraspinatus was 6.9 mm, and that of the infraspinatus was 10.2 mm. Both were smaller than previously reported3,4. We believe that this finding is primarily due to the fact that, in the previous reports, the footprint contained the insertion area of the joint capsule. The total medial-to-lateral length of the footprint plus the capsular attachment in our study was almost the same as that reported by Dugas et al.4. The maximum anteroposterior width of the footprint of the supraspinatus in our study averaged 12.6 mm, which was shorter than previously reported3,4,6. On the other hand, the maximum anteroposterior width of the footprint of the infraspinatus averaged 32.7 mm, which was longer than previously reported3,4,6.
In summary, the supraspinatus insertion at the level of the articular side was found to be almost as broad as previously reported3,4,6. Only laterally, where its insertion was anteriorward, did it become very narrow. Additionally, the infraspinatus insertion was located posteriorward at the level of the anatomical neck, just as previously described in anatomy textbooks1,2. The difference that we found was that the infraspinatus then curved anteriorly as it extended laterally (Fig. 6).
The frequency of rotator cuff tears and the classification of the involved tendons have been described in several anatomical8-11 and clinical studies12-14. Jerosch et al. reported, in their cadaver study, that the supraspinatus was involved in all fifty-six specimens with rotator cuff tears, whereas the infraspinatus was involved in only eleven shoulders (20%)8. Several clinical studies have demonstrated the frequency of supraspinatus involvement in rotator cuff tears to range from 86% to 100% and the frequency of infraspinatus involvement to range from 14% to 48%12-14. However, in light of the finding in the present study that the infraspinatus had a substantially wider footprint than had been previously believed, the infraspinatus tendon may be involved in a much higher proportion of rotator cuff tears.
Muscle atrophy of the infraspinatus is sometimes observed in patients with an apparently isolated supraspinatus tear; this atrophy has been investigated in several studies15-17. In a cadaver study, Albritton et al. observed that retraction of the supraspinatus muscle following rotator cuff tears increased tension on the suprascapular nerve15. In clinical studies, Vad et al. reported that two of seven patients with an abnormal electromyographic finding following a full-thickness rotator cuff tear were diagnosed as having a suprascapular neuropathy16 and Warner reported that seven of twenty-six patients with a massive rotator cuff tear and fatty replacement of the muscles showed dysfunction of the suprascapular nerve17. The authors of these two clinical studies proposed that suprascapular nerve damage could result in the infraspinatus atrophy in association with some rotator cuff tears, although most of their patients with muscle atrophy did not show dysfunction of the suprascapular nerve16,17. Our anatomical findings suggest that there may be a higher frequency of involvement of the infraspinatus in rotator cuff tears, which can explain the development of infraspinatus atrophy without suprascapular nerve damage.
The supraspinatus has traditionally been considered to be an important abductor among the rotator cuff muscles20,21. However, several researchers have reported that the infraspinatus contributes as much to abduction as does the supraspinatus22-25. Although the analysis of the footprints in our cadaver study did not allow us to draw conclusions regarding the function of the rotator cuff, our findings that the infraspinatus occupied about half of the highest impression on the greater tuberosity, which has been believed to be the footprint of the supraspinatus, support the concept that the infraspinatus may contribute more to shoulder abduction than previously believed.
Recently, on the basis of both biomechanical26,27 and clinical28,29 studies, the concept of “footprint reconstruction” with use of a double row of suture anchors has been suggested to be superior to single-row fixation in rotator cuff repair. On the basis of the anatomical findings of our study, it appears that rotator cuff reconstruction to the anatomical footprint requires repair not only of the supraspinatus but also of the infraspinatus to the wider footprint on the highest impression on the greater tuberosity. Although it is difficult to distinguish the infraspinatus from the supraspinatus during surgery, the long, thick tendinous portions of both the supraspinatus and the infraspinatus may help to identify each muscle. Additionally, repair of these thick tendinous portions to the original insertions may be important for the full restoration of shoulder function.
The anatomical findings in the present study suggest that surgeons need to have increased awareness of pathological conditions of the infraspinatus tendon, especially when delamination is observed30,31. It may be important for surgeons to incorporate these new anatomical findings in order to properly restore the geography of the footprint of the torn anterior rotator cuff.
NOTE: The authors thank Akimoto Nimura, MD, for measurements of the footprint; Joji Moriishi, MD, PhD, for proffering advice regarding our studies; and Kumiko Yamaguchi, MD, PhD, for technical assistance.
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. 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. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Unit of Clinical Anatomy, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
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