Acutely, static widening of the medial joint space on plain radiographs can indicate a medial-sided injury or structure incarceration, e.g., medial capsule or MCL (≥5 mm). The “irreducible” knee dislocation can present this way following posterolateral joint subluxation or vastus medialis entrapment [46, 47]. Valgus stress radiographs can confirm suspicions of medial-sided injury [14, 17]. LaPrade et al. quantified side-to-side differences of 1.7 mm and 3.8 mm at 0° and 20°, respectively, in isolated MCL tears and 6.5 mm and 9.8 mm at 0° and 20°, respectively, in combined MCL and posteromedial corner disruption [40]. Otherwise, an examination under anesthetic can be used to detect rotatory injuries not previously detected by preoperative imaging or examination [42].

Chronically, radiographic changes can provide clues to the pattern of underlying injury. A Pellegrini-Stieda lesion, an ossified posttraumatic avulsion lesion of the MCL from the medial epicondyle of the femur [48], a deep femoral notch sign, peaked tibial spines, or cupula lesions can indicate long-standing MCL and ACL injuries.

MRI without contrast remains the gold standard where the diagnosis of medial-sided knee injuries can be performed with an accuracy of 87 % [49]. Its greatest advantages are in suspected complete MCL tears, suspected ACL tears, persistent clinical instability, and identifying the location of tear where surgery is required [50]. Individual medial-sided structures and the exact location of the injury can be visualized (see Fig. 34.1) [51]. MRI arthrograms enhance the identification of PMC injuries. Kimori et al. found arthrography to be more useful than arthroscopy and clinical examination in detecting tears, but interpretation can be difficult [51–53].

Nakamura et al. (contributing author) developed a new classification for MCL injuries based on the appearance of the superficial medial collateral ligament (sMCL) on MRI: femoral insertion site injury (type I), tibial insertion site injury (type 2), or injury throughout the length of the MCL (type 3) [53]. All five of their type 3 injuries required MCL reconstruction and there were no type 2 injuries. No differences were observed in IKDC sagittal laxity or valgus stability in all injuries.

Ligament discontinuity, subcutaneous edema, internal (ligament) change of signal intensity, and contrecoup bipolar bone bruises have all been associated with MCL tears [54–56]. When the pivot-shift mechanism does not dissipate all the deforming forces of certain high-energy injuries, varus, the internal rotation impaction on the anteriorly subluxated proximal tibia, is thought to lead to central medial femoral condyle and posterior tibial plateau contusion [57, 58].

The recently described “wave sign” indicates a distal tibial avulsion injury (Fig. 34.2a–c) [36]. This is thought to occur because the distal end of the ligament is not tethered to other soft tissue structures locally and takes on a serpiginous appearance when it retracts proximally. Taketomi et al. described three types: an avulsion injury where the distal end of the torn ligament remains under pes anserinus, the so-called “Stener” lesion of the knee where the distal end of the ligament sits outside pes anserinus [59], and MCL incarceration within the joint. They make the argument that potentially all of these types of MCL tears require surgical intervention.

LaPrade et al. have extensively described (1) bony landmarks, (2) ligaments, and (3) tendons (adductor magnus, medial head of the gastrocnemius, semimembranosus, and the pes anserinus) of the medial side of the knee [21]. The MCL complex is made of the sMCL, the deep medial collateral ligament (dMCL), and POL (part of PMC) [23]. The other constituent components of the PMC are the semimembranosus tendon (and its multiple reflections), the oblique popliteal ligament, posterior horn of the medial meniscus, and medial joint capsule [60]. The sMCL has one femoral and two tibial attachments (proximal and distal). The femoral attachment is located 3.2 mm proximal and 4.8 mm posterior to the medial epicondyle [21]. Many reconstruction techniques incorrectly identify the medial epicondyle as the attachment site of the MCL [15, 26, 61–64]. The tibial insertion is broader, attaching primarily to soft tissues proximally and to bone distally, 60 mm from the joint line [65]. The dMCL is a vertical thickening of the medial joint capsule and consists of the MF (attaching 15.1 mm posterior and distal to the medial epicondyle) and MT ligaments (3.2 mm from medial tibial plateau) [65].

The posterior oblique ligament (POL) arises from behind the medial femoral epicondyle, 7.7 mm distal and 6.4 mm posterior to the adductor tubercle [65]. It fans out from its origin with three fascial arms: superficial, central, and capsular [21, 61]. The central arm is the largest, inserting near the margin of the tibial articular surface, the capsular arm reinforces the PM joint capsule, and the superficial arm blends with semimembranosus.

The MCL complex is a primary restraint to direct valgus stress. It also secondarily contributes to external rotation and anteroposterior stability [23]. The sMCL provides the majority of this stability in all degrees of flexion; the dMCL only providing secondary stability. The distal division of the sMCL is a primary stabilizer for external rotation and the POL, the primary stabilizer for internal rotation, highlighting its importance in counteracting AMRI [41].

The PMC provides one third of the restraint to valgus stress in full extension, slackening off in flexion [66]. It has a secondary role in the prevention of posterior translation of the tibia. However, in the context of combined injuries, it has a more important role in the resistance to external rotation. When damage to the PMC is combined with an MCL tear, external rotation is increased by 30° [42]. Failing to address the rotational component of this injury is what is thought to lead to residual laxity and functional compromise and the main source of controversy surrounding repair or reconstruction techniques.

Pes anserinus tendons and semimembranosus have a role in tightening medial structures in external rotation and flexion. In the context of damage to medial structures, utilization of these tendons to reconstruct MCL or POL may compromise the results of surgery inadvertently. Avoiding the harvest of hamstring autograft may be preferable, instead of favoring other graft options in these cases.

ACL reconstruction and nonsurgical treatment of grade I and II MCL injuries have outcomes similar to that of isolated ACL injury reconstructions [67, 68]. Based on this, many authors propose protection of the MCL with a knee brace and delaying ACL reconstruction surgery [1, 69, 70]. Usually a period of 6–8 weeks is required for MCL injuries to heal.

The abovementioned approach can be utilized with grade III MCL injuries even among professional athletes with successful results [7]. However, the persistent valgus and/or AMRI of certain MCL tears can compromise ACL reconstructions if the medial side is not addressed [8, 11–13]. Both of these situations of compromised stability can prevent athletes from returning to pivoting sports [1, 2, 17].