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Abstract

Background: Limited information is available with regard to the dynamic stability of the elbow joint, ie, the role of muscle function. Biomechanical analysis of common activities, such as overhead throwing, suggests that muscles must play a role in the elbow to aid in counteracting valgus load. We hypothesize that the dynamic action of the flexor pronator mass will decrease the strain on the medial collateral ligament and provide a protective varus correction at the elbow. Method and Materials: Eight fresh frozen cadaveric elbows were tested utilizing a custom made testing device. Muscles of the flexor pronator mass (the flexor carpi ulnaris, flexor carpi radialis, flexor digitorum superficialis, and the pronator teres), the extensor supinator mass and the triceps, biceps, and brachialis were isolated and wrapped in a mesh and wire construct. The insertion of each muscle was left undisturbed while eyelets were placed at the origins of each muscle. The wire and mesh construct was passed through each eyelet and to a pulley system attached to the testing apparatus. The elbow was locked in neutral pro/supination, neutral wrist flexion, and 30 degrees of flexion at the metacarpophalangeal joint. The humerus was potted into the testing device, while the forearm was allowed 6 degrees of freedom. The arm was placed in a position of throwing and tested at 45 and 90 degrees of flexion. A differential variable reluctance transducer (DVRT) microstrain gauge was placed on the posterior aspect of the anterior band of the anterior bundle of the medial collateral ligament. The arm was repeatedly cycled to remove crimp from the ligament and insure that the strain gauge had no interference from any overlying tissue. In no instance was more than 5% of the muscle mass removed. Fasttrack transmitters were attached to the radius and the humerus to record kinematic data. Each individual muscle was loaded to 2% of maximum muscle force, while the tested muscle was loaded at sequentially greater loads – 20 N, 40 N, and 60 N. Strain and kinematic data was recorded simultaneously and plotted as a function of force. A least square linear analysis was used to fit the data and interpreted as strain/rotation over 10 N of load. Results: The flexor pronator mass created a varus moment and decreased the strain on the ligament. At both 45 and 90 degrees the flexor carpi ulnaris provided the greatest decrease in strain, 3 and 4 times greater than the flexor digitorum superficialis, respectively (p < .05). In both the ligament intact and cut conditions, the flexor carpis ulnaris provided the greatest varus correction at 90 degrees (p < .05) while at 45 degrees the flexor carpi ulnaris, the flexor digitorum superficialis and the flexor carpi radialis all provided statistically equivalent amounts of correction. The flexor carpi ulnaris was the only external rotator of the construct (p < .05) and the pronator teres the only internal rotator (p < .05) at both 45 and 90 degrees. Conclusions: In vitro, dynamic activation of the flexor pronator mass provides a significant varus moment at the elbow joint and a significant decrease in strain of the medial collateral ligament. Dynamic stabilizers may help explain why an incompetent medial collateral ligament is often functionally well tolerated by many patients. Proper muscle training may be protective of the medial collateral ligament, and preservation of the muscle mass may protect the reconstructed ligament and aid rehabilitation. If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use).