IntroductionThe anterior bundle of the ulnar collateral ligament (UCL) of the elbow is the primary static stabilizer against valgus stress between 20-120 degrees of elbow range of motion. Attenuation or rupture of the UCL in overhead throwing athletes results in valgus laxity and altered posteromedial elbow compartment biomechanics. It is not yet known whether the UCL functions as a whole or whether some portion of the intact UCL has a primary role in maintaining valgus stability. We investigated the biomechanical effect of proximal and distal transection of the UCL at the ulnar insertion with loading in two elbow positions representing the arc of the throwing motion.MethodsI-scan pressure sensors were inserted into the posteromedial elbow joint of 13 male cadaveric elbows (average age 54, range 26-61 years). Contact area and pressure were measured with valgus torque with the elbow in 90° and 30° of flexion, simulating late cocking/early acceleration and deceleration phases of the throwing motion, respectively. The intact specimen was tested, and then the anterior band of the UCL was transected from the sublime tubercle proximal to distal in increments of 50% and 100% of the ligament width and the testing was repeated. This methodology was then repeated in 13 additional specimens (average age 49, range 26-66 years), except that the anterior band was transected from distal to proximal in the same increments.ResultsWhen the proximal 50% of the anterior band was transected, contact area decreased significantly compared with the intact state at 90° (91.3±23.6 mm2 vs 112.2±26.0 mm2) and 30° (69.3±14.8 mm2 vs 83.1±21.6 mm2)(p<0.001) of elbow flexion. Center of pressure (COP) also moved significantly proximally compared with the intact state at 90° (3.7±2.5 mm vs 5.4±2.3 mm) and 30° (5.9±2.8 mm vs 7.4±1.9 mm)(p<0.001) of elbow flexion. Change in valgus laxity was not significant. With 100% proximal-to-distal transection, significant change compared with the intact state was found in contact area and movement of COP at 90° and 30° of elbow flexion (<0.001). Valgus laxity increased significantly compared with the intact state at 90° (<0.001) and 30° (p=0.01) of elbow flexion. When the distal 50% of the anterior band was transected, no significant change was observed compared with the intact state in contact area, movement of center of pressure, or valgus laxity. With 100% UCL transection distal to proximal, a significant decrease in contact area, proximal movement of center of pressure, and increase in valgus laxity were observed compared with intact state at both 90° and 30° of elbow flexion (<0.001).ConclusionThe significant change in contact area and proximal movement of center or pressure with 50% proximal transection of the UCL and the lack of significant change with 50% distal transection suggest that the proximal half of the UCL has a primary role in maintaining posteromedial elbow stability. The findings may suggest that surgical reconstruction should at least aim to reestablish stability in the proximal 50% of the UCL ulnar footprint. IntroductionThe anterior bundle of the ulnar collateral ligament (UCL) of the elbow is the primary static stabilizer against valgus stress between 20-120 degrees of elbow range of motion. Attenuation or rupture of the UCL in overhead throwing athletes results in valgus laxity and altered posteromedial elbow compartment biomechanics. It is not yet known whether the UCL functions as a whole or whether some portion of the intact UCL has a primary role in maintaining valgus stability. We investigated the biomechanical effect of proximal and distal transection of the UCL at the ulnar insertion with loading in two elbow positions representing the arc of the throwing motion. The anterior bundle of the ulnar collateral ligament (UCL) of the elbow is the primary static stabilizer against valgus stress between 20-120 degrees of elbow range of motion. Attenuation or rupture of the UCL in overhead throwing athletes results in valgus laxity and altered posteromedial elbow compartment biomechanics. It is not yet known whether the UCL functions as a whole or whether some portion of the intact UCL has a primary role in maintaining valgus stability. We investigated the biomechanical effect of proximal and distal transection of the UCL at the ulnar insertion with loading in two elbow positions representing the arc of the throwing motion. MethodsI-scan pressure sensors were inserted into the posteromedial elbow joint of 13 male cadaveric elbows (average age 54, range 26-61 years). Contact area and pressure were measured with valgus torque with the elbow in 90° and 30° of flexion, simulating late cocking/early acceleration and deceleration phases of the throwing motion, respectively. The intact specimen was tested, and then the anterior band of the UCL was transected from the sublime tubercle proximal to distal in increments of 50% and 100% of the ligament width and the testing was repeated. This methodology was then repeated in 13 additional specimens (average age 49, range 26-66 years), except that the anterior band was transected from distal to proximal in the same increments. I-scan pressure sensors were inserted into the posteromedial elbow joint of 13 male cadaveric elbows (average age 54, range 26-61 years). Contact area and pressure were measured with valgus torque with the elbow in 90° and 30° of flexion, simulating late cocking/early acceleration and deceleration phases of the throwing motion, respectively. The intact specimen was tested, and then the anterior band of the UCL was transected from the sublime tubercle proximal to distal in increments of 50% and 100% of the ligament width and the testing was repeated. This methodology was then repeated in 13 additional specimens (average age 49, range 26-66 years), except that the anterior band was transected from distal to proximal in the same increments. ResultsWhen the proximal 50% of the anterior band was transected, contact area decreased significantly compared with the intact state at 90° (91.3±23.6 mm2 vs 112.2±26.0 mm2) and 30° (69.3±14.8 mm2 vs 83.1±21.6 mm2)(p<0.001) of elbow flexion. Center of pressure (COP) also moved significantly proximally compared with the intact state at 90° (3.7±2.5 mm vs 5.4±2.3 mm) and 30° (5.9±2.8 mm vs 7.4±1.9 mm)(p<0.001) of elbow flexion. Change in valgus laxity was not significant. With 100% proximal-to-distal transection, significant change compared with the intact state was found in contact area and movement of COP at 90° and 30° of elbow flexion (<0.001). Valgus laxity increased significantly compared with the intact state at 90° (<0.001) and 30° (p=0.01) of elbow flexion. When the distal 50% of the anterior band was transected, no significant change was observed compared with the intact state in contact area, movement of center of pressure, or valgus laxity. With 100% UCL transection distal to proximal, a significant decrease in contact area, proximal movement of center of pressure, and increase in valgus laxity were observed compared with intact state at both 90° and 30° of elbow flexion (<0.001). When the proximal 50% of the anterior band was transected, contact area decreased significantly compared with the intact state at 90° (91.3±23.6 mm2 vs 112.2±26.0 mm2) and 30° (69.3±14.8 mm2 vs 83.1±21.6 mm2)(p<0.001) of elbow flexion. Center of pressure (COP) also moved significantly proximally compared with the intact state at 90° (3.7±2.5 mm vs 5.4±2.3 mm) and 30° (5.9±2.8 mm vs 7.4±1.9 mm)(p<0.001) of elbow flexion. Change in valgus laxity was not significant. With 100% proximal-to-distal transection, significant change compared with the intact state was found in contact area and movement of COP at 90° and 30° of elbow flexion (<0.001). Valgus laxity increased significantly compared with the intact state at 90° (<0.001) and 30° (p=0.01) of elbow flexion. When the distal 50% of the anterior band was transected, no significant change was observed compared with the intact state in contact area, movement of center of pressure, or valgus laxity. With 100% UCL transection distal to proximal, a significant decrease in contact area, proximal movement of center of pressure, and increase in valgus laxity were observed compared with intact state at both 90° and 30° of elbow flexion (<0.001). ConclusionThe significant change in contact area and proximal movement of center or pressure with 50% proximal transection of the UCL and the lack of significant change with 50% distal transection suggest that the proximal half of the UCL has a primary role in maintaining posteromedial elbow stability. The findings may suggest that surgical reconstruction should at least aim to reestablish stability in the proximal 50% of the UCL ulnar footprint. The significant change in contact area and proximal movement of center or pressure with 50% proximal transection of the UCL and the lack of significant change with 50% distal transection suggest that the proximal half of the UCL has a primary role in maintaining posteromedial elbow stability. The findings may suggest that surgical reconstruction should at least aim to reestablish stability in the proximal 50% of the UCL ulnar footprint.