Surg Radiol Anat (1997) 19: 265-268 © Springer-Verlag France 1997

Radiologic anatomy Three dimensional reconstruction of the fibrous frame of the rotator cuff* L. Le Gars1, O. Gagey2, N. Saidani1, N. Gagey2 and J. Bittoun3 1 Institute of Anatomy, Paris V University, France 2 Orthopaedic Research Laboratory, Paris-South University, Orthopaedic Department, University Hospital of Bicêtre, France 3 CIERM (MRI center), Paris-South University, France Received July 12, 1996 / Accepted in final form February 11, 1997 Key words: Shoulder Scapulohumeral joint 3D-reconstruction Anatomy * Work done in the Orthopaedic Research Laboratory Paris-South University, and in the Anatomy Institute of Paris, Paris V University, Prof. J.P. Lassau Correspondence to: O. Gagey, University Hospital of Bicêtre, 78, avenue du Général Leclerc, F-94270 Le Kremlin-Bicêtre, France

Abstract The authors present the methodology and the results of a trial of 3D-reconstruction of the fibrous frame of the rotator cuff taken from high resolution MRI. The acuracy of the method used is discussed. A precise reconstruction of the soft tissues is possible and provides an interesting model to study the mechanical role of the rotator cuff muscles. An example of biomechanical application of the 3D images is given.

Many anatomical and IRM studies have demonstrated the presence of a strong fibrous frame within the muscles of the rotator cuff [1-8]. Such reinforcements of the muscle's structure suggest an adaptation to strong forces. Thus the study of the fibrous frame is of great interest to improve our knowledge about the biomechanics of the rotator cuff and the stability of the humeral head. Most of the studies concerning the morphology of the shoulder take in account either the humeral head of the humerus or the scapula alone. To be acurate and useful for biomechanical research a morphological study must involve both bony structures and soft tissues of the joint. The aim of this work was to study the possibility and the value of 3D-reconstruction of the fibrous frame of the rotator cuff and of the scapulohumeral joint. Material and methods MRI pictures demonstrate fibrous prolongation of the tendons of the cuff within the muscle bellies (Figs. 2, 3). High resolution shoulder MRI provided by the data bank of the CIERM (GE Sigma 1.5) has been used. Volunteers with no shoulder pathology were placed in prone position, with the upper limb in neutral rotation. The surface coil 5 inch GP was centered upon the rotator interval. The following main parameters were used: thickness of the slice 3 mm, distance between the slices 1 mm, field of view 10 cm, matrix 256/256, spin echo T2-weighted sequence (TE = 20, TR = 2000). A Camra Allegro workstation (ISG technologies) was used for the reconstructions. Data were directly transfered from the MRI device to the workstation. Sagittal views (Fig. 1) were used for the reconstructions because they were perpendicular to the main axis of the muscles and thus gave the best informations.

Fig. 1 Sagittal view of a shoulder. 1: supraspinatus m., 2: fibrous structure of the supraspinatus m., 3: biceps brachii tendon, 4: subscapularis m., 5: infraspinatus m., 6: teres minor m., 7: capsule Volumes of interest were drawn on each slide using the seeding method wich is the most currently used method in 3D reconstruction. Four non-embalmed cadavers were dissected. The muscles of the rotator were entirely removed from the shoulder. The muscular fibers were carefully removed leaving only the fibrous structures of the muscles. We compared the anatomical fibrous structures obtained after dissection to the reconstructed images. After the reconstruction was completed, the fibrous structures of the muscles were allowed to draw the main axis of the forces exerted by the muscle. The position of the axis was determined in degrees related to the horizontal plan. Each reconstruction was made twice at different moments in order to verify the reliability of the reconstrution. Each determination of the main axis was verified by the same way. Results The quality of the pictures provided by the MRI (Figs. 1, 2, 3) permits the isolation of every fibrous component within the muscles of the rotator cuff.

Fig. 2 Coronal view of the shoulder. 1: fibrous frame of the subscapularis m.

Fig. 3 Coronal view of the shoulder. 1: tendon of the supraspinatus m., 2: glenoid fossa, 3: belly of supraspinatus muscle, 4: deltoid m. 3D-reconstruction evidenced (Figs. 4, 5, 6) the same results that the anatomic or radiologic litterature did, above all in the cases of the supraspinatus and the subscapularis muscles. We found the deep fibrous reinforcement within the supraspinatus m. and three to five fibrous bands within the subscapularis m. In two cases the fibrous structure of the teres minor m. was not reconstructed because of a too small field of view in MRI. The "rotator interval" was plainly evidenced between the supraspinatus and subscapularis mm. (Fig. 4).

Fig. 4 3D-reconstruction of the fibrous frame (AP view). 1: subscapularis m., 2: supraspinatus tendon and fibrous structure

Fig. 5 3D-reconstruction of the fibrous frame (superior view). 1: subscapularis m., 2: supraspinatus tendon and fibrous structure, 3: infraspinatus m., 4: teres minor m.

Fig. 6 Exemple of calculation of the direction of the tendons (AP view). For the supraspinatus m. the direction is given by the superior edge of the reconstructed tendon, for the subscapularis m. we divided the tendon in two superior and intermediate parts The distal end of the tendon ended abruptly at the level of their bony insertion because of the difficulty to differentiate the bony and the tendinous MRI signals at time of reconstruction. That is one of the limits of the three dimentional reconstruction. The intraoperator control of the reconstruction evidenced an excellent morphologic correlation in every case. The results of determination of the orientation of the main axis are presented in Table 1a. The teres minor was oriented downward (from 12 to 30°) as is the infraspinatus m. (from 0 to 28°) and the suscapularis m. (6 to 32°). The direction of the fibrous axis of the supraspinatus m. is more variable. In 6/12 cases it was oriented upward, in 6/12 cases it was oriented downward.

Table 1a. Orientation of the vectors related to the horizontal plan Plus sign means the force is oriented upward, minus sign means the force is oriented downward. S-SP: Supraspinatus m., S-SC: subscapularis m., I-SP: infraspinatus m., TM: teres minor m., sup: superior, mid: middle The corrected values (Table 1b) (see discussion) are as follow: the teres minor was oriented downward (from 9 to 39°) as is the infraspinatus m. (from 0 to 26°) except in two cases where the tendon was oriented upward. The suscapularis m. was oriented downward (9 to 36 °) except in one case. The direction of the fibrous axis of the supraspinatus m. was more variable. In 9/12 cases it was oriented upward (from 3 to 23°), in 3/12 cases it was oriented downward (5°). The mesurements were totally reproductible.

Table 1b. Inclination of the tendons after substraction of the value of the inclination of the glenoid fossa Discussion MRI exploration of the shoulder gives images of structures of about 1 mm thickness (for example the coracoacromial lig.). In these conditions, careful 3D-reconstruction allows reproduction of good virtual images of the anatomy even of the soft tissues. The higher the number of slices, the better the quality of the reconstruction. The number of slices depends on the thickness of the object and on the thickness of the slices. Comparison between anatomy and reconstructed images evidenced an excellent morphological correlation. 3D images are even more interesting than anatomical study since they showed the exact spatial morphology of the fibrous frame of the rotator cuff, whereas dissection of the fibrous frame provides only flat objects. The main limit we experienced in our work was due to the MRI device. In MRI, the structure of the tendons is often heterogenous even in absence of tendinopathy. The deep fibrous structure is difficult to analyse because of its numerous thin prolongations: the thinner the fibrous structure, the lower the contrast between the fibrous tissue and the surrounding muscle's fibers, especially in the aeras where the size of the fibrous tissue is close to this one of the pixel (400 microns). Using a surface coil in order to obtain the best resolution, the field of view was limited so that the proximal part of the fibrous strctures was not visible. This limit was particularly strong in the case of the infraspinatus m. whose fibrous structure begins very proximally. The insertion of the muscles on the humeral head are large. As origin of the vector we have chosen the middle of the tendon.The determination of the vector representing the force of the muscle remains unprecise because of the limitation of the field of view. Nevertheless the fibrous structures give good approximation of the direction of the main forces. In the case of the supraspinatus m., the direction of the vector is given by this one of the main anterior fibrous structure. The direction of the muscles depends on the position of the joint at time of the MRI. We have studied the scapulohumeral joint in only one position, all volonteers were installed in the same position: the arm at rest along the chest. The position of the main axis of the humerus was the only to be constantly checked at time of MRI. The absence of rigid link between the bone components of the shoulder makes impossibe the choice of a constant position of the joint for all subjects tested. The small field of view used because the good quality of the images provided did not allow the study of the humeral shaft. For this reason we do not know the exact position of the joint. Nevertheless it is possible to make a correction of the values calculated for each tendon provinding an hypothesis about the glenoid fossa: we assume that the position of the glenoid fossa depends only on this one of the joint. In these conditions the values of the orientation of the glenoid fossa may be substracted from this one of the tendon. With such a correction we have found the supraspinatus tendon being oriented upward in 9 out of 12 cases (from 3 to 21°) and oriented downward in 3 cases (4°). The field of view used in this work apprears to be too small in order to determine the exact position of the joint at time of MRI. The new softwares allowing high resolution with a matrix of 512/512 will solve this problem. In most of the cases the rotator muscles pull the head downward. The results concerning the supraspinatus m. are more surprising: our results suggest that, in 9/12 of the cases, the supraspinatus m. pulls the head upwards: that is controversial regarding its currently admitted role. Anatomical studies on biomechanics or functional anatomy of the joint are often a destructive one. To use 3D reconstruction makes possible to work on a "permanent model" and to perform more accurate studies. 3D reconstructions give us interesting information about the role of the muscles in the position studied, (in the present work at the very beginning of the movement). Further studies would be required in different positions of the joint in order to study the geometric variations of the muscles around the head. In addition reconstruction of the whole muscles should allow to improve the determination of the direction of the forces and make possible, knowing the volume of the muscles, to advance some hypotheses on the amount of the forces. Conclusion 3D-reconstructions of the soft tissues of the shoulder jont are possible with an excellent resolution. Providing a larger field of view, (which is now possible without loss of quality of the images), a 3D model of the joint may be obtained. Our work emphasizes the possiblity to apply the "reconstructed anatomy" to the biomechanical evaluation of the joints providing the use of a larger field of view in order to know the position of the joint. This opens an additional new way to study the functional anatomy of the musculoskeletal system. This method is of great interest because many biomechanical works are based on dramatically simplified anatomical models.

References

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