Understanding the medial ulnar collateral ligament of the elbow: Review of native ligament anatomy and function.

The medial ulnar collateral ligament complex of the elbow, which is comprised of the anterior bundle [AB, more formally referred to as the medial ulnar collateral ligament (MUCL)], posterior (PB), and transverse ligament, is commonly injured in overhead throwing athletes. Attenuation or rupture of the ligament results in valgus instability with variable clinical presentations. The AB or MUCL is the strongest component of the ligamentous complex and the primary restraint to valgus stress. It is also composed of two separate bands (anterior and posterior) that provide reciprocal function with the anterior band tight in extension, and the posterior band tight in flexion. In individuals who fail comprehensive non-operative treatment, surgical repair or reconstruction of the MUCL is commonly required to restore elbow function and stability. A comprehensive understanding of the anatomy and biomechanical properties of the MUCL is imperative to optimize reconstructive efforts, and to enhance clinical and radiographic outcomes. Our understanding of the native anatomy and biomechanics of the MUCL has evolved over time. The precise locations of the origin and insertion footprint centers guide surgeons in proper graft placement with relation to bony anatomic landmarks. In recent studies, the ulnar insertion of the MUCL is described as larger than previously thought, with the center of the footprint at varying distances relative to the ulnar ridge, joint line, or sublime tubercle. The purpose of this review is to consolidate and summarize the existing literature regarding the native anatomy, biomechanical, and clinical significance of the entire medial ulnar collateral ligament complex, including the MUCL (AB), PB, and transverse ligament.

Core tip: The anterior bundle of the medial ulnar collateral ligament complex plays a crucial role in elbow stability, specifically as a valgus and rotational constraint. Based on recent studies and our own cadaveric dissections, the ulnar footprint has a broader insertion that is more tapered and elongated than previous considered. A comprehensive understanding of the anatomy and biomechanical properties of the medial ulnar collateral ligament is imperative to optimize reconstructive efforts, and to enhance clinical and radiographic outcomes.

INTRODUCTION

The medial ulnar collateral ligament [MUCL, also referred to as the ulnar collateral ligament (UCL), medial collateral ligament (MCL), 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 with the posterior bundle (PB) and transverse ligament (TL) being the other two (Figure 1). The MUCL, in particular, has been shown to be 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 (anterior and posterior) that provide reciprocal function with the anterior band tight in extension, and the posterior band tight in flexion.

Figure 1

Medial ulnar collateral ligament complex of the elbow with outlined ligaments generated by co-registering the three dimensional digitized anatomy and computed tomography scan of a cadaveric elbow. Note the tapered and distally elongated insertion of the medial ulnar collateral ligament on the sublime tubercle and ulnar ridge. MUCL: Medial ulnar collateral ligament.

The MUCL is commonly injured in overhead throwing athletes when a valgus moment is placed on the elbow during the late cocking and early acceleration phases[11-15]. Incompetence or rupture of the ligament leads to valgus instability which has varying clinical presentations. Patients may complain of instability, however, most will report pain, reduced accuracy, and decreased velocity. Clinically significant pathology often requires surgical intervention. Ligament reconstruction relies on appropriate graft positioning at both the humeral origin and the ulnar insertion. A thorough understanding the native anatomy of the MUCL facilitates the surgeon’s ability to effectively restore stability and function.

In 1985, Morrey and An[10] published the first quantitative analysis of the medial ulnar collateral ligament complex. Based on 10 fresh frozen cadavers, they described the dimensions of the AB and PB at various degrees of elbow flexion. Since that time, multiple studies have assessed the anatomy and biomechanics of the AB[2,3,6,16,17]. Historically, general consensus held that the AB inserts solely and directly onto the sublime tubercle. However, recent studies have shown that the AB insertion is in fact broader, tapered, and significantly larger in terms of surface area than previously thought.

The purpose of this review is to consolidate and summarize the existing literature regarding the anatomy, biomechanical function, and clinical significance of the native (non-reconstruction) MUCL. It is our hope that this work may serve as a framework for better understanding valgus instability in the elbow and refining surgical techniques in order to optimize post-operative outcomes.

ANATOMY

As mentioned, 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 contributions greatest during flexion[17]. It is generally thought that the TL does not provide a significant contribution to elbow stability[10]; however, recent study 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, has been shown to be 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 inserts onto the sublime tubercle of the ulna (Figure 1).

Origin

Surface area and footprint center: The origin of the AB is posterior to the elbow’s axis of rotation, on the anterior, inferior, and lateral aspect of the medial epicondyle[10]. The surface area of the humeral origin has been widely variable in the literature (Table 1). Dugas et al[19] showed that the AB origin was round with a mean surface area of 45.5 mm2 (range of 25.9-59.4 mm2) in 13 fresh frozen cadavers utilizing a three-dimensional (3-D) electromagnetic tracking and digitalizing device. Similar to the findings of Dugas et al[19], 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] found this measurement to be notably smaller at 17.0 mm2 (range of 14.9-19.1 mm2) through analysis of 10 fresh frozen cadaver specimens. Potentially due to differences in measurement techniques, these studies demonstrated variable results. Additionally, Frangiamore et al[20] and Dugas et al[19] described the center of the ligament origin in different terms, which 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 as an area of tissue 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, with Camp et al[18] finding a similar mean distance of 11.7 mm. Rather than describing this as a linear distance with no specific angle, Frangiamore et al[20] described this measurement in terms of two separate measurements, reporting 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. The use of two measurements relative to a single point in the latter study may assist with better reproducibility.

Table 1

Summary of anatomic studies describing the length, width, and surface area of the anterior bundle

Ref.	Specimen	AB length (mm)	AB width (mm)	Origin surface area (mm2)	Insertion surface area (mm2)	AB surface area (mm2)
Alcid et al[2] 2004	-	27	4.5	-	-	121.5
Beckett et al[23] 2000	39	26.7	-	-	-	-
Camp et al[35] 2017	10	-		32.3	187.6	324.2
Dugas et al[19] 2007	13	-	6.8	45.5	127.8	-
Eygendaal et al[9] 2002	5	26	5	-	-	-
Farrow et al[22] 2014	10	53.9	-	-	-	-
Farrow et al[21] 2011	12	51.7	-	-	-	-
Floris et al[25] 1998	18	-	5.8	-	-	-
Morrey et al[10] 1985	10	27.1	4.7	-	-	-
Regan et al[17]	8	21.1	7.6	-	-	-
Safran et al[5] 2005	12	-	7.2	-	-	-
Timmerman et al[26] 1994	10	-	6	-	-	-
Frangiamore et al[20] 2018	10	21.5	-	17	66.4	-

AB: Anterior bundle.

Insertion

Surface area and footprint center: Historically, the ulnar footprint of the AB has generally been described as inserting solely onto the sublime tubercle, serving as the anatomical landmark for surgical repairs and reconstructions. In one such early report, the mean AB insertional surface area was 66.4 mm2[20].

Recently, authors have described the AB insertion as a longer, distally tapered area that follows the ulnar ridge. The surface area of this broader insertion has was reported by Dugas et al[19] to have a mean surface area of 127.8 mm2. In this study, length of the ulnar footprint measured an average of 29.2 mm. Others have found similar lengths when appreciating a tapered insertion with means of 30.2 mm and 29.2 mm[21,22]. Further study by Camp et al[18] identified a tapered insertion with a mean surface area of 187.6 mm2 and an ulnar footprint length averaging 29.7 mm.

Because the footprint center of a broader tapered insertion may not occur in the location previously assumed (apex of the sublime tubercle), the optimal position of the ulnar tunnel in reconstructive efforts may still need to be elucidated. Clinical relevance of the broader tapered ulnar insertion described in recent studies by Dugas et al[19] and Camp et al[18] is in need of further investigation.

One recent study of 10 cadaveric specimens has shown the mean proximal insertional width to be 9.4 mm which is greater than previously noted[18]. This discrepancy in widths leaves room for further investigation to ensure that the native anatomy of the AB is fully documented and understood (Figure 2).

Figure 2

Cadaveric specimen outlining all ligaments of the medial ulnar collateral ligament complex including the medial ulnar collateral ligament/anterior bundle, posterior bundle, and transverse ligament. MUCL: Medial ulnar collateral ligament.

Overall ligament dimensions

Length: The AB is the longest ligament of the medial elbow, spanning the inferior aspect of the medial epicondyle to the sublime tubercle and extending distally along the ulnar ridge. The average length of AB has been reported between 21.1 mm and 31.4 mm in several older studies[2,9,10,17,20,23]. These lengths were measured from the center of the origin to center of sublime tubercle, based on a direct insertion onto the sublime tubercle without a distal extension. In contrast, more recent reports measured from the center of the humeral origin to the most distal point of tapered insertion 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 and the clinical relevance of different measurements. In particular, appropriate length of the ligament component has important implications for ligament reconstruction.

It is important to note that the AB is not an isometric soft tissue stabilizer but instead changes in length throughout flexion. Studies have shown the length of the 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 is an aspect of native biomechanics that must also be considered during reconstruction procedures.

Width: The width of the AB varies, increasing 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 the reported widths of the AB, ranging from 4.0 to 7.6 mm[2,5,10,17,25,26].

Surface area: The mean surface area of the AB has been reported between 108 mm2 to 135 mm2[2,10] in studies that did not take the full distal footprint into consideration. Given that Dugas et al[19] has 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 in historical reports. In contrast to these historical reports, and in support of the Dugas et al[19] findings, a more recent study published by Camp et al[18] identified the mean surface area of the entire AB to be 324.2 mm2 (Figure 3).

Figure 3

Cadaveric specimen showing origins and insertions of all ligaments of the medial ulnar collateral ligament complex including: Anterior bundle, posterior bundle and transverse ligament. AB: Anterior bundle; PB: Posterior bundle; TL: Transverse ligament.

BIOMECHANICS

Valgus instability

The MUCL provides a vital contribution to the stability of the elbow when a valgus stress is applied. With an intact radial head, a fresh frozen cadaveric model demonstrated that the MUCL contributes 31% and 54% to valgus restraint at 0° and 90° of flexion, respectively[1]. In this study, 4 fresh frozen cadavers had a varying valgus load from 0 to 3 nm applied at 0° and 90° of flexion. With the maximum force applied, there was 3° of valgus laxity in full extension and 2° of laxity in flexion. Several studies have performed similar biomechanical testing at various degrees of flexion and various amount of force[2,3,5,6,10,25,27]. The amount of valgus laxity varies from 2° to 8° with an intact MUCL (Table 2).

Table 2

Maximum physiologic valgus demonstrated in various studies with noted elbow position and load applied during testing

Authors	Specimens	Maximum valgus (degrees)	Elbow flexion (degrees)	Load (nm)
Callaway et al[3] 1997	28	3.6	30, 60, 90, 120	2
Floris et al[25] 1998	18	6	20	0.75
Morrey et al[4] 1991	6	5	20	Gravity
Safran et al[5] 2005	12	11.1	70	2

At 30° and 90° of flexion, Callaway et al[3] showed valgus laxity of 3.6° under a 2nm load compared to an unloaded elbow. Safran et al[5] analyzed 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 position. This study did not determine the inherent valgus alignment in an unloaded elbow, which affects the ability to compare these studies. Thus, with an intact MUCL and a 2 nm load applied, the amount of valgus laxity is generally greatest at 30° of flexion[5].

Other authors have evaluated the effect of transecting the AB on elbow stability (Table 3). When the AB is disrupted in cadaveric models, the amount of valgus instability increases until the point at which secondary osseous stabilizers such as the radial head impart stability. In the setting of AB deficiency, Callaway et al[3] demonstrated the greatest instability at 90° of flexion. 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 when compared to the intact state. Additionally, Mullen et al[28] showed that at 90 degrees of flexion, the transected AB increases valgus instability by 150%. Floris et al[25] and Søjbjerg et al[29] showed the greatest instability occurred at 70 degrees of flexion, with recorded valgus angles of 14.2˚and 11.8˚. Safran et al[5] produced a maximal gain of 6.3˚of laxity under 2 nm of valgus load with a transected AB at 50˚ of elbow flexion. 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 flexion under gravity. In summary, an intact AB is vital in maintaining valgus stability of the elbow throughout the entire range of flexion.

Table 3

Maximum valgus reported when the anterior bundle is transected in various studies with noted elbow position and load applied during testing

Ref.	Specimens	Maximum valgus gain (degrees)	Maximum absolute valgus	Elbow flexion (degrees)	Load (nm)
Ahmad et al[8] 2004	7	7.37	-	30	2
Callaway et al[3] 1997	28	3.2	-	90	2
Floris et al[25] 1998	18	5.7	-	30	0.75
Morrey et al[4] 1991	6	3.9	-	20	gravity
Safran et al[5] 2005	12	6.3	-	50	2
Sojbjerg et al[29] 1987	12	-	11.8	70	1.5

Some studies reported gain in valgus from physiologic state, while others noted absolute valgus.

Rotational instability

Internal rotation of the forearm relative to the humerus is constrained by the soft tissues stabilizer of the medial elbow. While there is inherent internal rotation during flexion, the degree of rotation is limited between 2.8˚-6˚ in an uninjured elbow[4,6,25]. Transection of the AB permits internal rotation of the forearm to increase to 18.5˚ at 60˚ of joint flexion (Table 4)[25]. Correct understanding of these biomechanics is important in repair and reconstruction, as a rotatory moment is part of the mechanism of injury.

Table 4

Summary of the studies evaluating the effect of the anterior bundle on relative internal rotation of the forearm in the native and pathologic state

Ref.	Specimen	Intact AB internal rotation (degrees)	Cut AB internal rotation (degrees)	Gain of internal rotation with cut AB
Bryce et al[6] 2008	-	4	-	-
Floris et al[25] 1998	18	6	18.5	12.5
Morrey et al[4] 1991	6	2.8	7.8	5

AB: Anterior bundle.

Tissue strength

Despite being the most frequently injured ligament in the overhead throwing athlete, the AB or MUCL has been shown to have the most inherent strength and stiffness[6,17]. This fact emphasizes the significant loads placed on the medial side of the elbow during the late cocking and early acceleration phase[30,31]. In a cadaveric model with each soft tissue stabilizer evaluated under stress, the AB was the strongest with an average load to failure of 260.9 N[17]. During overhead throwing, the elbow experiences 64 nm of mean valgus torque and 290 N of tensile force on the medial side (which is greater than the threshold for failure of 260.9 N)[30,31]. Furthermore, a maximal mean valgus load of 90Nm has been reported[32-34]. 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]. Thus, it is clearly 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, specifically as a valgus and rotational constraint. The AB originates on the humerus and inserts onto the sublime tubercle of the ulna. Based on recent studies and our own cadaveric dissections, the ulnar footprint has a broader insertion that is more tapered and elongated than previous considered. The data regarding the centers of the ulnar and humeral footprints provides guidance for proper tunnel placement during reconstructive efforts. The width and stiffness of the AB is described and can be used to guide graft selection during recons ruction.

Lastly, although the ligament is quite strong, the amount of force placed across the elbow in elite overhead throwing athletes is routinely exceeds the ligaments average load to failure. Accordingly, it is not surprising why MUCL injuries are so common amongst baseball players and pitchers.

Understanding native anatomy and biomechanics of the AB/MUCL is clinically important to help understand the pathoanatomy and guide surgical techniques when treating MUCL injuries.