Medial Ulnar Collateral Ligament Injury
Dr. KS Dhillon
Introduction
The medial ulnar collateral ligament (MUCL), also referred to as the medial collateral ligament (MCL), ulnar collateral ligament (UCL), and anterior bundle (AB) is the primary restraint to valgus instability of the elbow (1-5). The MUCL is one of three ligaments that comprise the “medial ulnar collateral ligament complex” of the elbow. The posterior bundle (PB) and transverse ligament (TL) are the other two (Fig 1).
Fig 1.
The MUCL is the primary stabilizer of the elbow during valgus stress, followed by the radial head and dynamic stabilizers of the elbow such as the flexor-pronator muscle mass (6-10). The MUCL is composed of two separate bands (posterior and anterior) that provide reciprocal function with the posterior band tight in flexion, and the anterior band tight in extension.
The MUCL is commonly injured in overhead-throwing athletes when a valgus moment is placed on the elbow during the early acceleration and late cocking phases (11-15). Incompetence or rupture of the ligament leads to valgus instability of the elbow which has varying clinical presentations. Patients may complain of instability, most however will report pain, reduced accuracy, and decreased velocity. Clinically significant pathology usually requires surgery. Appropriate graft positioning at both the humeral origin and the ulnar insertion is necessary when doing ligament reconstruction. A thorough understanding of the native anatomy of the MUCL facilitates the surgeon’s ability to restore stability and function effectively.
Morrey and An (10) in 1985 were the first to publish the quantitative analysis of the medial ulnar collateral ligament complex. They dissected 10 fresh frozen cadavers and described the dimensions of the AB and PB at various degrees of elbow flexion. Since then, multiple studies have assessed the anatomy and biomechanics of the AB (2,3,6,16,17). In the past general consensus held that the AB inserts solely and directly onto the sublime tubercle. Recent studies however have shown that the AB insertion is in fact broader, tapered, and significantly larger in terms of surface area.
Anatomy
The MUCL (AB) is the primary restraint to valgus instability of the elbow (1-5). The PB is a soft tissue stabilizer of the elbow with greatest contributions during flexion (17). It is generally believed that the TL does not provide a significant contribution to elbow stability (10). A recent study however has revealed a direct insertion of the TL onto the AB that may potentially play a role in elbow stability (18). The AB, in particular, is the primary stabilizer of the elbow in valgus stress, with the radial head and dynamic stabilizers of the medial elbow also contributing (6-10). It originates on the anterioinferior surface of the medial epicondyle and goes on to insert on the sublime tubercle of the ulna (Fig 1).
Origin
Surface area and footprint center: The origin of the AB is posterior to the elbow’s axis of rotation. It is on the anterior, inferior, and lateral aspect of the medial epicondyle (10). The surface area of the humeral origin has been widely variable. Dugas et al (19) carried out a study in 13 fresh frozen cadavers utilizing a three-dimensional (3-D) electromagnetic tracking and digitalizing device and showed that the AB origin was round with a mean surface area of 45.5 mm2 (range of 25.9-59.4 mm2). A recent study by Camp et al (18) found a mean origin surface area of 32.3 mm2. In contrast to these two studies, Frangiamore et al (20) also studied 10 fresh frozen cadaver specimens and found the measurement to be notably smaller at 17.0 mm2 (range of 14.9-19.1 mm2). It is possible that due to differences in measurement techniques, these studies demonstrated variable results.
Frangiamore et al (20) and Dugas et al (19) described the center of the ligament origin in different terms. This is of clinical importance when determining the location of humeral tunnel placement during MUCL reconstruction surgery. Dugas et al (19) described the center of the origin in an area on a flat surface anterior and inferior to the medial epicondyle. The mean distance they measured was 13.4 mm from the center of the medial epicondyle to the center of the origin. Camp et al (18) found a similar mean distance of 11.7 mm. Frangiamore et al (20) described this measurement in terms of two separate measurements. They reported the center of the origin to be located, on average, 8.5 mm distal (inferior) and 7.8 mm lateral (anterior) to the medial epicondyle.
Insertion
Surface area and footprint center: The ulnar footprint of the AB inserts solely onto the sublime tubercle. It serves as the anatomical landmark for surgical repairs and reconstructions. In one early report, the mean AB insertional surface area was 66.4 mm2 (20).
Recently some authors have described the AB insertion as a longer, distally tapered area that follows the ulnar ridge. Dugas et al (19) reported that the surface area of this broader insertion has a mean surface area of 127.8 mm2. They found that the length of the ulnar footprint measured an average of 29.2 mm. Other authors have found similar lengths of tapered insertion with means of 30.2 mm and 29.2 mm (21,22). Camp et al (18) found a tapered insertion with a mean surface area of 187.6 mm2 and an ulnar footprint length averaging 29.7 mm.
Since the footprint center of a broader tapered insertion may not occur in the location previously assumed at the apex of the sublime tubercle, the optimal position of the ulnar tunnel in reconstructive surgery may still need to be elucidated. The clinical relevance of the broader tapered ulnar insertion described in studies by Camp et al (18) and Dugas et al (19) requires further investigation.
A study of 10 cadaveric specimens by Camp et al (18) has shown the mean proximal insertional width to be 9.4 mm which is greater than previously reported (18). This discrepancy in widths leaves room for further investigation.
Overall ligament dimensions
Length: The AB is the longest ligament of the medial elbow. It spans the inferior aspect of the medial epicondyle to the sublime tubercle and extends distally along the ulnar ridge. The reported average length of AB is between 21.1 mm and 31.4 mm (2,9,10,17,20,23). These lengths were measured from the origin's center to the sublime tubercle's center, based on a direct insertion onto the sublime tubercle without a distal extension. In contrast, more recent reports measured the length from the center of the humeral origin to the most distal point of tapered insertion. They reported mean lengths of 53.9 mm (21) and 51.7 mm (22). The difference in length measured between a non-tapered sublime insertion and a tapered insertion calls for further study to evaluate native AB anatomy. The appropriate length of the ligament component has important implications for ligament reconstruction.
It is important to remember that the AB is not an isometric soft tissue stabilizer. It changes in length throughout flexion. The length of AB changes by 18%, between 2.8 mm and 4.8 mm as the elbow moves from extension to flexion (6,10,24). The dynamic length of the AB must also be kept in mind during reconstruction procedures.
Width: The width of the AB varies. It increases distally to its greatest width at the sublime tubercle before tapering to a point as it inserts distally along the ulnar ridge. Generally, there has been limited variability in widths of the AB, ranging from 4.0 to 7.6 mm (2,5,10,17,25,26).
Surface area: The reported mean surface area of the AB is between 108 mm2 to 135 mm2 (2,10) in studies that did not consider the full distal footprint. Given that Dugas et al (19) have shown the tapered ulnar footprint alone to have a mean surface area of 127.8 mm2, the overall surface area of the ligament will undoubtedly be significantly greater than previously assumed. A more recent study published by Camp et al (18) has shown the mean surface area of the entire AB to be 324.2 mm2.
Biomechanics
Valgus instability
When a valgus stress is applied the MUCL provides a vital contribution to the stability of the elbow. Morrey and An in a fresh frozen cadaveric model showed that with an intact radial head, the MUCL contributes 31% and 54% to valgus restraint at 0° and 90° of flexion, respectively (1). In this study, varying valgus loads from 0 to 3 nm were applied at 0° and 90° of flexion in 4 fresh frozen cadavers. There was 3° of valgus laxity in full extension and 2° of laxity in flexion with the maximum force applied. There are several studies that have performed similar biomechanical testing at various degrees of flexion and various amounts of force (2,3,5,6,10,25,27). The amount of valgus laxity varies from 2° to 8° when the MUCL is intact.
Callaway et al (3) showed valgus laxity of 3.6° under a 2nm load compared to an unloaded elbow at 30° and 90° of flexion. Safran et al (5) studied 12 cadaveric specimens with a 2Nm load applied at 30 degrees of elbow flexion, and showed a mean alignment of 10.7° of valgus with a neutral forearm rotation. This study did not determine the inherent valgus alignment in an unloaded elbow. Hence it is not possible to compare these studies. With an intact MUCL and a 2 nm load applied, the amount of valgus laxity is generally greatest at 30° of flexion (5).
The effect of transecting the AB on elbow stability has been studied by several authors. In cadaveric models when the AB is disrupted, the amount of valgus instability increases until the point at which secondary osseous stabilizers such as the radial head provide stability. Callaway et al (3) demonstrated the greatest instability at 90° of flexion in the presence of AB deficiency. The study reported a gain of 1.6, 2.8, 3.2, and 3.0 degrees of valgus motion at 30, 60, 90, and 120 degrees of flexion, respectively compared to the intact state. Mullen et al (28) showed that at 90 degrees of flexion, AB transection increases valgus instability by 150%. Floris et al (25) and Søjbjerg et al (29) showed that the greatest instability occurs at 70 degrees of flexion, with recorded valgus angles of 11.8˚ and 14.2˚. Safran et al (5) produced a maximal gain of 6.3˚of laxity under 2 nm of valgus load with a transected AB at elbow flexion of 50˚. Finally, Morrey et al (4) showed a gain of laxity over baseline ranging from 3.3˚ to 4.8˚ in cadaver specimens at 20 degrees of elbow flexion. An intact AB is vital in maintaining valgus stability of the elbow throughout the flexion range.
Rotational instability
Forearm internal rotation is constrained by the soft tissue stabilizer of the medial elbow. While there is inherent internal rotation of the forearm during flexion, the degree of rotation is limited between 2.8˚-6˚ in an uninjured elbow (4,6,25). Transection of the AB allows internal rotation of the forearm to increase to 18.5˚ at 60˚ of joint flexion (25). A correct understanding of these biomechanics is important in repair and reconstruction since rotatory moment is part of the mechanism of injury.
Tissue strength
The AB is the most frequently injured ligament in overhead throwing athletes, although it has been shown to have the most inherent strength and stiffness (6,17). This shows that significant loads are placed on the medial side of the elbow during the late cocking and early acceleration phase (30,31). In a cadaveric model when each soft tissue stabilizer was evaluated under stress, the AB was found to be the strongest with an average load to failure of 260.9 N (17). During overhead throwing, the elbow experiences 64nm of mean valgus torque and 290N of tensile force on the medial side which is greater than the threshold for failure of 260.9N (30,31). A maximal mean valgus load of 90Nm has been reported (32-34). In a recent study of 81 professional baseball pitchers (MLB and MiLB), over 82000 throws showed a mean valgus torque of 60 nm with individual participant means ranging from 41 nm to 94 nm (35). Hence, it is evident why the AB fails in this subset of athletes based on the load to failure being below the force imparted on the elbow.
Conclusion
The AB of the medial ulnar collateral ligament complex plays a crucial role in elbow stability, especially as a valgus and rotational constraint. The AB originates on the humerus. It inserts onto the sublime tubercle of the ulna. Based on recent studies, the ulnar footprint has a broader insertion that is more tapered and elongated than previously thought. The data regarding the centers of the ulnar and humeral footprints guides proper tunnel placement during reconstructive procedures.
Although the ligament is quite strong, the amount of force placed across the elbow in elite overhead-throwing athletes routinely exceeds the ligament's average load to failure. Hence it is not surprising why MUCL injuries are so common amongst baseball players and pitchers.
Understanding the native anatomy and biomechanics of the MUCL is clinically important to help understand the pathoanatomy and also to guide surgical techniques when treating MUCL injuries.
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