We investigated the instantaneous rotation axis of the elbow joint in normal subjects during extension-flexion motion with wrist in neutral position by 4D CT. The major finding of the current study was that the mean variation between the instantaneous rotation axis and the X axis in the coronal plane and horizontal plane is 12.3° and 45.5°, respectively. The intersections in the medial aspect of the humerus were mostly located in the superior and posterior quadrant and showed the trend from anterior-inferior to posterior-superior with the increment of the elbow flexion. The intersections in the lateral aspect of the humerus were mostly located in the middle half of the anterior quadrant and showed the trend from posterior-inferior to anterior-superior with the increment of the elbow flexion. The isometric point in the humerus for collateral ligament reconstruction was changing during the elbow extension-flexion motion.
The instantaneous rotation axis was shown to be inconsistent at the coronal plane. On the contrary, there was a variation of instantaneous rotation axis at the horizontal plane which corresponded to the increment of elbow flexion angle. When the elbow was put no more than 30°, the instantaneous rotation axis was close to the center of the capitulum and trochlea. Box-loop ligament reconstruction of the elbow was raised up to treat ligament tears both in MCL and LUCL [4, 17]. The attachment chosen in the medial aspect of the humerus was the origin point of the MCL, which was located in the anterior-inferior epicondyle. The attachment chosen in the lateral aspect of the humerus was the center of the capitulum. But the previous study by Finkbone et al. did not specify the flexion angle of the elbow joint when doing all collateral ligament reconstruction . The distal humeral tunnel was created near the X axis which we defined in our study. Our study found the instantaneous rotation axis was shown to be close to the X axis when elbow flexion angle was less than 30°. For this reason, we recommended that during box-loop ligament reconstruction, distal humeral tunnel should be created while maintaining elbow in less than 30° position.
We also found that the intraindividual variation in the rotation axis ranging from 9.9 ° to 15.9° in the coronal plane and from 34.2° to 64.6° in the horizontal plane which was greater compared with the previous study by Ericson et al. . Ericson et al. reported that the rotation axis was located close to a line joining the center of the trochlea and capitellum and intraindividual variation of the axis ranged from 2.1° to 14.3° in the coronal plane and 1.6° to 9.8° in the horizontal plane. The inconsistent result between Ericson et al. and the current study may be resulted from the different experiment setting which used plain x-ray and was taken at full extension and at 30°, 60°, 90° and 120° of flexion (static imaging) instead of the 4D CT scan with 30 positions used in the current study which represented the dynamic analysis. Since Duck et al. reported the impact of active/passive motion and forearm pronation/supination to the screw displacement axis , we speculated that another reason for the different result between our study and Ericson’s study was wrist position. The experiment in the current study was performed at neutral wrist position which represented an anatomic position in contrast with the supinated hand position performed in Ericson’s study.
The optimal method for MCL reconstruction has not yet been defined. Despite many techniques proposed, most of the technique emphasized on the importance of an appropriate graft attachment sites with the aim to achieve isometric ligament reconstruction as to restore normal elbow kinematics [5, 22], which was on the base of the theory that the rotation axis of the ulnohumeral joint passed through the centers of the capitellum and trochlea. Traditionally, the sublime tubercle served as the ulnar footprint of the reconstructed neo-ligament [13, 30]. However, quantitative analysis showed that the anatomic attachment of the MCL ulnar footprint was located more distal, namely the median ulnar ridge . Also, there were different viewpoints about the location of the axis of the ulnohumeral joint [14, 15, 19]. For reasons above, the anatomical reconstruction may not serve as an isometric reconstruction because of the inconsistency of information regarding anatomic footprint of the ulnar site and different recognition of the axis of the ulnohumeral joint. Additionally, the MCL distal humeral footprint was widely accepted at the anteroinferior region of the distal humerus [2, 13, 30], which was regarded as the isometric point during the elbow motion. However, our study found that the axis of the ulnohumeral joint was not near the line connecting the centers of the medial and lateral aspect of the humerus and the isometric point in the humerus during the extension-flexion mode of the elbow joint was shifted from anterior-inferior to posterior-superior and mostly located at the posterior-superior quadrant of the medial epicondyle enface.
To the current knowledge, it is unclear to what position does the elbow joint need to be maintained during collateral ligament reconstruction. Patel et al. reviewed the outcomes and complications for the MCL reconstruction in 0° to 30° and 45° to 70° of the elbow joint, he came to the conclusion that the elbow flexion may not influence the return to the same or higher level of competition but appeared to influence the need for a revision after MCL reconstruction . 0° to 30° flexion degree would result in a high revision rate. In our study, with the elbow moving from maximum extension to maximum flexion with elbow in neutral position, the intersection between the axis of the elbow and medial aspect of the humerus started near the anatomical attachment of the MCL in humerus and showed the trend from anterior-inferior to posterior-superior quadrant of the medial aspect of the humerus. For this reason, 0° to 30° flexion degree was recommended for the MCL reconstruction, which was also recommended by Cohen et al. because the reconstructions fixated at 30° more closely resembled the biomechanical characteristics of the intact elbow than did reconstructions fixated at 90° . Current surgical techniques regarding the LUCL reconstruction were Morrey’s original tunnel technique and the contemporary docking technique . The elbow was placed in 30° to 40° of flexion and forced pronation in docking technique and was placed in 30° of flexion and forced fully pronation in Morrey’s technique [21, 25]. The humeral tunnel was determined as the isometric point in the surgery, which was close to the anatomical footprint of the LUCL in the humerus. In our study, 30° of flexion was closer to the anatomical footprint of the LUCL compared with 30° to 40° of flexion since the isometric point in the lateral side was started near the anatomical attachment of the LUCL and showed the trend from posterior-inferior to anterior.
There has been an inconsistency report regarding isometric LUCL reconstruction [3, 18, 24]. Moritomo et al. found the most isometric point of the LUCL was located at the 2 mm proximal to the center of the capitellum in vivo MRI study . Goren et al. reported that most isometric point on the humerus was located between the 3:00 and 4:30 o’clock positions on the lateral epicondyle in cadaveric biomechanical study . Alaia et al. reported that the humeral center of rotation was the most isometric point for the humeral reconstruction site . We postulated that the inconsistency of the reports was due to the different experiment setting. The current study found that there was no fixed isometric point during elbow motion and the points were started near the anatomical attachment of the LUCL and showed the trend from posterior-inferior to anterior-superior quadrant of the lateral aspect of the humerus. The isometric area for the LUCL reconstruction was located at the middle half of the anterior part of the lateral aspect of the humerus which was supported by Goren et al. .
Angle between the rotation axis and transcondylar line is 1.8°(SD6.3°) in maximum extension and 47.3°(SD13.9°) in maximum flexion. With the flexion angle grows, the intersections shifted from anterior-inferior to posterior-superior in the medial side and from posterior-inferior to anterior-superior in the lateral side. In other words, the axis showed the trend from posterior-superior in the medial side to anterior-superior in the lateral side in sagittal plane with the flexion angle grows.
There are several limitations to this study. Firstly, the flexion mode during CT scanning may not represent flexion in normal daily activity because wrist was naturally at the supination position when elbow is flexed. Secondly, the registration of the greater sigmoid notch of the ulna was performed manually. Thirdly, the flexion angle was not as large as the normal maximum flexion angle owing to the posture of the participant. Fourthly, the length of the reconstructed ligament and the translation along the axis were not evaluated.