[Three-dimensional geometry of the proximal humerus and rotator cuff attachment and its utilization in shoulder arthroplasty].

PURPOSE OF THE STUDY The aim of the study was to provide a method of measurement and data collection, based on morphologic analysis of the proximal humerus, that would facilitate precise placement of the humeral stem and would be easy to apply in clinical medicine in patients requiring shoulder arthroplasty. MATERIAL Three groups of materials were used. Materials for the first and second groups were provided by the Department of Anatomy, First Faculty of Medicine, Charles University. The first group included 10 specimens (five left and five right humeral bones) obtained at routine anatomical dissection. The second group contained 110 so-called dry preparations from the collections of the Department of Anatomy and the third group comprised nuclear magnetic resonance (NMR) scans of 20 patients. To show spatial relationships between the proximal and the distal humerus, another coil was applied to the epicondylar region in addition to the one placed over the proximal humerus. METHODS The first group material was used to study proximal humerus morphology and to determine reference points for the other two groups. The points were constructed to make seven planes perpendicular to the axis of the proximal humeral metaphysis. Based on the reference points, parameters of the proximal humerus were assessed in the defined planes also in the other two groups. We measured angles between the reference points and the transepicondylar line or the humeral head axis. The vertex of each angle was always placed in the point of intersection of the metaphyseal axis and the given transverse plane. Reference points of the greater tubercle were marked on the medial margin continuous with the intertubercular groove, on the lateral margin of the lesser tubercle and in the "deepest" place of the intertubercular groove. We also measured humeral head retroversion and the position of maximal bony mass of both the greater and the lesser tubercle (this parameter can be used with advantage for optimal insertion of screws in proximal humerus reconstruction). RESULTS The angle between the medial margin of the greater tubercle and the humeral head axis was on average 164.8 degrees on the left side and 163.2 degrees on the right side; the angle between the great tubercle margin and the transepicondylar line was 137.0 degrees on the left humerus and 137.7 degrees on the right humerus. The lateral margin of the lesser tubercle and the humeral head axis formed on average an angle of 124.4 degrees and of 122.6 degrees on the right and the left side, respectively. The intertubercular groove/ humeral head axis relationship was 143.4 degrees and 144.8 degrees for the left and the right humerus, respectively, and the intertubercular groove/transepicondylar line angle was 115.6 degrees for the left and 119.5 degrees for the right humerus. The humeral head axis and the transepicondylar line made an angle of 27.8 degrees for the left and 25.3 degrees for the right humerus. These values corresponded to the angle of the humeral head retroversion. The reference point of maximal bony mass of the greater tubercle and the humeral head axis made an angle of 181.1 degrees and of 180.2 degrees for the left and the right humerus, respectively; between this point and the lesser tubercle was an angle of 120.2 degrees for the left and 126.9 degrees for the right humerus. DISCUSSION One of the most important parameters in restoring shoulder function by alloplasty is humeral head retroversion. If this is not correct, ventral or, less frequently, dorsal instability of the shoulder may result. The correct setting of retroversion is guided by the transepicondylar line or several specific landmarks on the greater and the lesser tubercle of the humerus. Another important factor is the correct reconstruction of anatomic position of the greater and the lesser tubercle in relation to the insertion of rotator cuff muscles into the humeral head. Relationships of diaphyseal, metaphyseal and humeral head axes have been reported in the relevant literature dealing with proximal femoral morphology. None of the reports, however, has dealt with tubercular angles and position of the maximal body mass, which is a decisive factor for insertion of screws fixing the prosthetic stem. CONCLUSIONS The values provided here can be used for a more precise construction of implants for shoulder replacement. The method of three-dimensional presentation of the proximal humerus may aid in a more exact implantation procedure during shoulder arthroplasty. An optimal position of the implant can also be based on parameters obtained from the healthy contralateral shoulder joint.