Common Soft Tissue Injuries About the Knee in American Football

Introduction

Since the first game was played in 1869, American football has become one of the most popular contact sports in the United States [56]. Participation carries a risk of injury from the non-impairing to the career-ending, with potential neurological, psychological, and musculoskeletal sequelae, and it is the team sport with the most injuries at each level of involvement (high school, collegiate, and professional) [41,75]. Knee injuries are frequently seen at all levels of participation and can lead to significant dysfunction and time away from competition. Some of these injuries should be treated surgically to achieve good outcomes and return the athlete to function at high levels of competition. The purpose of this review is to synthesize the literature on surgical and non-surgical management of soft tissue knee injuries in American football, including extra-articular injuries such as muscle strain, contusion, and intra-articular injuries of the ligaments and menisci.

Epidemiology of Knee Injuries in Football

Lower extremity injuries, particularly those of the knee, are prevalent, with medial collateral ligament (MCL) and anterior cruciate ligament (ACL) tears among the most frequently reported football injuries [33]. This trend is seen across participation levels; in high school football players, several studies have found that the most common injuries occur in the knee. In fact, 15.2% of total injuries in this population are knee injuries, with ligament sprain occurring most frequently in this category [71,75]. In comparison, head/face injuries, of which 96.1% were concussions, accounted for 11.5% of total injuries, while shoulder injuries accounted for 12.4% of total injuries [75]. The overall rate of injury per 1000 athlete exposures (AE/1000) is lower in high school athletes (4.36) than it is in National Collegiate Athletic Association (NCAA) athletes (8.61), but as with the high school athletes, knees, and shoulders are the most common sites of injury in college participants, making up 16.4% and 13.2% of total injuries, respectively [75]. A more recent study of collegiate American football players in the United Kingdom conducted from 2014 to 2015 found the knee (10.2%), ankle (8.4%), hand (6.9%), and head (6.6%) as the most common sites of injury, with ligamentous injuries predominating [11]. Another epidemiologic study of NCAA football players from 2004 to 2009 similarly found that knee, ankle, shoulder, and head/face injuries were most common, at 19.9%, 17.3%, 13.7%, and 8.8% of total injuries, respectively [38]. While there is some variance in the percentage and incidence across these studies, the knee is frequently reported as the most commonly injured anatomic region in various levels of football.

A study of injuries during the first 4 weeks of the 2019 to 2020 preseason and the regular season of the National Football League (NFL) found that injuries to the knee, upper leg/thigh, ankle, and concussion were the most common injuries at .38, .32, .28, and .23 AE/1000, respectively [10]. Weeks 1 to 4 of the 2018 to 2019 preseason and regular season are consistent with this trend, as the knee, upper leg/thigh, and ankle were the most common injuries during this period. However, during the 2016 to 2017 period, the groin/hip was the second most common injury following the knee, with the ankle again as the third most common injury [10]. A study of NFL training camp injuries from 1998 to 2007 found contusion, knee sprains, ankle sprains, and hamstring strains the most common injuries in games [27]. During practice, knee sprain becomes the most common injury, followed by hamstring strain, ankle sprain, and contusion [27]. As with collegiate and high school players, the knee is the most commonly injured region in professional athletes.

Muscle Injury and Contusion

In football, muscle contusions are common at all levels of play. They arise when a blunt external force is violently exerted on a part of the body, causing muscle trauma. If the force is strong enough, core muscle fibers and connective tissue are aggressively compressed, potentially leading to diffused, or circumscribed hematoma within the muscle [57]. This can cause localized inflammation, stiffness, pain, and diminished range of motion in the affected muscle. Particularly in football, contusions tend to occur in the quadriceps, primarily in the vastus intermedius and rectus femoris. While thigh padding aids in the prevention of these injuries, the risk of injury persists. Direct or repeated blows to quadriceps from a shoulder, knee, or helmet can cause contusions in varying degrees of severity [41]. In most cases, contusions minimally affect the athlete and resolve without complication, but some cases are more severe and require the athlete to be taken out of play and can keep them out for prolonged periods.

In addition to pain, stiffness, and swelling, athletes may report bruising triggered by tissue and blood vessels damaged by the traumatic blow. Blood may collect under the skin at the site of the injury and form a hematoma. Quadriceps contusions in football are diagnosed largely without advanced imaging [41], although magnetic resonance imaging (MRI), ultrasound, and computed tomography (CT) can aid in determining injury severity, a history and physical examination is typically used to make the diagnosis. Treatment for contusions in football vary based on severity. Most cases are treated conservatively, usually involving nonsteroidal anti-inflammatory drugs (NSAIDs) and the rest, ice, compression, and elevation (RICE) protocol. For quadriceps contusions, initial immobilization in 120° flexion for 24 hours has been shown to decrease time to return to full athletic activity [8]. In some cases, more invasive measures such as a hematoma being drained intraoperatively may be required [69].

Hamstring strain injuries (HSIs) are non-contact injuries that occur frequently in football. A common muscle injury in the NFL, HSIs affect both offensive and defensive players. The injury pattern is diverse, ranging from a simple strain or mild muscle damage without structural damage to complete muscle tearing with structural impairment or muscle avulsion. There are numerous known risk factors, such as inadequate hamstring flexibility or warm-up, reduced hamstring strength and core stability, low fitness level, and anatomical abnormalities. However, the most significant risk factor is previous hamstring injury, which has been found to elevate the re-injury rate 2 to 6 times [1,30,45,67,81]. The hamstring muscle group spans both the hip and knee joints, thus enabling rapid muscle acceleration and elongation. The injury typically occurs with an eccentric muscle contraction and simultaneous hip flexion and ipsilateral knee extension, as may happen at the end of the swing phase while sprinting, when the hamstrings reach maximal length and undergo eccentric contraction before heel strike. The hamstring muscle group consists of 3 different muscles: the long and short heads of the biceps femoris, semimembranosus, and the semitendinosus. The 3 muscles originate at the posterior thigh from the ischial tuberosity to form a common tendon, except for the short head of the biceps femoris, which originates medial to the linea aspera in the posterior distal femur. The common tendon becomes 3 separate tendons 5 to 10 cm distal to the ischium, which act on the thigh according to their distal insertion site. The hamstrings group crosses the hip and knee joints (except the short head of the biceps femoris), thus affecting both joints by extending and stabilizing the posterior thigh and flexing the knee. The long head of the biceps femoris inserts into the fibular head and the lateral tibia. It provides posterior pelvis stability and posterolateral knee stability while extending the hip and flexing the knee. The semimembranosus inserts at the posteromedial corner of the knee and extends, abducts, and medially rotates the thigh, while stabilizing, flexing, and medially rotating the leg at the knee [1,30,45,67,81].

Hamstring tear may occur with a sudden pain in the posterior thigh while exercising (occasionally with an audible “pop”), hamstring tightness, avoidance of sitting and knee or hip flexion (ie, stiff-leggedness), ecchymosis at the tearing area, and neurological complaints of pins and needles sensation resembling sciatica, due to acute compression of a hematoma or chronic scarring in the sciatic route. Hamstring injuries can be classified by their clinical presentation (mild, moderate, severe), the integrity of the muscle fibers (overstretching, partial or incomplete tearing, complete rupture), the amount of muscle involved (1 or more), timing (acute or chronic), and avulsion with or without retraction [1,30,45,67]. As with contusions, HSIs do not require diagnostic imaging for a diagnosis, but advanced imaging can help shed light on the severity of the strain. Conservative treatment, NSAIDs, and the RICE method are the gold standard and preferred method of treatment for most HSIs. Although controversial, it has been shown in select patient populations that the use of biologics, platelet-rich plasma (PRP) in particular, may lead to more rapid return to full activity with no change in reinjury rate. Thus, some authors recommend the use of PRP for acute hamstring injuries in professional football players [15]. There is yet no conclusive data on indications for surgical treatment if conservative treatments fail. This research is still evolving. However, in the rare case of acute proximal hamstring rupture, surgical intervention is recommended for the best outcome [6,12]. Additionally, distal avulsions of the semitendinosus with retraction are reported to have poor outcomes with nonoperative treatment, and thus may warrant surgical intervention, either with resection or repair [23,37]. Other accepted surgical indications include involvement of all 3 tendons and/or of a high-demand athlete, displaced bony avulsion, gap formation with retraction of 2 cm or more, displaced bony avulsion, and chronic pain associated with functional impairment [45,67,81].

Meniscal Injuries

Meniscal pathology is the most prevalent among all intra-articular knee injuries, with an incidence of 60 to 70/1000 000 people per year [22,44,78]. Sports-related injuries comprise roughly 1/3 of all meniscal injuries, with a high prevalence in sports such as American football, which usually involves non-contact mechanisms including repeated pivoting movement, deceleration, cutting, and landing from a jump [16,68,72]. Bradley et al [13] evaluated 332 elite football players and found that 54% (179 athletes) had a history of a knee injury, out of which 25.5% (51 athletes) had a history of meniscal pathology.

The meniscus’s unique circumferential wedge shape compensates for the mismatch between the articular surfaces of the relative flat tibial plateau and the curved femoral condyles—thus stabilizing the knee joint and allowing transmission and endurance of compression, tension, and shear forces [72,78,79]. Additionally, the meniscus absorbs and transfers forces acting on the knee, as well as joint lubrication and nutrition of the articular cartilages [68,72,78,79]. The average meniscus covers roughly 60% to 70% of the knee contact area, with the medial meniscus being smaller, less mobile, and more c-shaped compared to the lateral one [78,82]. Lastly, the meniscus has a tenuous blood supply mainly to its periphery, with diminishing vascularization toward the center. This unique structure leaves the middle 2/3 largely avascular, increasing risk of injury with reduced healing capacity. The amount of vascularization defines 3 distinct meniscal zones: central avascular white–white zone, intermediate red–white zone, and peripheral red–red zone. Of note, the lateral meniscus is somewhat less vascularized (10%–30%) than the medial meniscus [7,24,78].

Meniscal injuries can lead to dysfunction in sports and sequelae such as accelerated degenerative change. Due to its more constrained structure and reduced mobility, the medial meniscus is more prone to isolated injury than the lateral meniscus, with a 1:3 ratio, respectively [50,73]. However, lateral meniscal tears are more common in younger patients (ranging from 51% to 72%), especially in combination with acute ACL injury [44,50,73,78]. In a cadaveric study by Musahl and colleagues, the authors did a stepwise resection of the ACL, followed by the medial meniscus and the lateral meniscus, while measuring the amount of anterior tibial translations. They found that the lateral meniscus was a more important restraint to anterior tibial translation during combined valgus and rotatory loads applied during a pivoting maneuver [58]. This injury pattern mimics sport-related injuries that involve repeated pivoting movement, deceleration, cutting, and landing from a jump [16,68,72].

The most essential objective of meniscal surgery is to preserve function. In their cadaveric study involving 12 fresh-frozen human cadaveric knees, Lee et al [43] found that resection of the meniscus (ie, medial meniscectomy) resulted in significantly decreased contact areas and increased mean and peak contact stresses compared with the intact state. Additionally, as more peripheral meniscus was removed, greater decreases in contact areas and in contact stresses were observed. Likewise, Paletta et al [64] found that complete total lateral meniscectomy resulted in a 235% to 335% increase in peak local contact pressure. Hence, any impairment in the meniscus function may lead to accelerated degenerative change, cartilage damage, and osteoarthritis [43,44,58,64,78].

According to the National Football League Players Association (NFLPA) the average NFL career lasts 3.3 years [62]. Hence, athletes are motivated to return to play (RTP) as quickly as possible to maximize their career potential. Moreover, by most accounts, coaches, and teammates exert significant pressure [9,19,78]. Therefore, treating meniscal pathologies in athletes can be challenging, with a focus on short-term outcomes such as RTP rather than long-term knee preservation. Meniscal treatment may vary from partial meniscectomy with removal of the non-functional tissue and the goal of preserving vital meniscus tissue as much as possible. Meniscal repair requires a substantially longer recovery and postoperative rehabilitation time but is favored for preservation of meniscal tissue and prevention of downstream degenerative changes (Fig. 1) [9,19,44,78]. In a systemic review by Lee et al [44] reporting on the time and rate of RTP after meniscal surgery, most players returned to their pre-injury activity level 7 to 9 weeks after partial meniscectomy. Kim et al [39] reported that partial medial meniscectomy required significantly more time for RTP than lateral meniscectomy, with a higher incidence of adverse outcomes, while Nawabi et al [61] reached the opposite results. Interestingly, Aune et al [9] found no difference in patients undergoing concomitant procedures and their RTP, nor did any specific procedure affect the odds of RTP. Finally, elite and competitive-level athletes exhibit significantly shorter time to RTP than recreational athletes [39,44], possibly due to strict, multidisciplinary rehabilitation protocols, which are more accessible to them, as well as their high motivation.

Fig. 1.

Arthroscopic images of meniscal repair. Left: demonstration of vertical longitudinal tear. Right: completed all-inside arthroscopic meniscus repair with apposition of the torn tissue.

Lee et al [44] also reported 81% to 88.9% of RTP in athletes who underwent isolated meniscal repair, with an average time of 5.6 months after surgery. Although concurrent anterior cruciate ligament reconstruction (ACLR) extended the RTP time, it had no effect on the percentage of RTP [3,44,80]. Meniscal allograft transplantation is scarcely performed, especially in elite athletes in a contact pivoting sport [2,44,53,84]. Marcacci et al [53] reported on 12 professional soccer players who had undergone meniscal transplantation after subtotal meniscectomy (6 medial and 6 lateral). Of these, 11 patients (92%) returned to play soccer within a mean of 10.5 ± 2.6 months, and 9 patients (75%) were still playing as professionals after 36 months of follow-up. Similarly, Alentorn-Geli et al [2] reported on 14 patients who were followed for 36 months; 12 patients (85.7%) returned to play soccer and demonstrated significant improvements in all patient-reported outcomes.

In summary, meniscal treatment in the professional athlete population is challenging as different procedures can significantly impact RTP and, as a result, affect career length. While the highest RTP rate and the shortest recovery time are seen after partial meniscectomy, this treatment option may be deleterious to long-term knee outcomes.

Ligamentous Injury

Knee stability depends primarily on bony anatomy and ligamentous integrity. Of the 4 main knee ligaments, the cruciates are intra-articular and control anteroposterior and rotatory stability while contributing to varus and valgus stability in full extension. The medial and lateral collateral ligaments are extra-articular and responsible for varus and valgus stability throughout knee motion. In addition, because of the presence of synovial fluid for the cruciate ligaments, the ultrastructure of the collateral ligaments differs from that of the cruciates [47]. As a result, healing potential differs. Isolated acute collateral ligament injuries are more likely to heal spontaneously, with basic science and clinical results to support this theory [47]. In the setting of the cruciate ligaments, it is known that native enthesis tissue is not regenerated following injury. Rather, intra-articular healing is unpredictable and by way of fibrovascular scar tissue [29]. Thus, to best reproduce native anatomy and function, surgical reconstruction of the ACL is often performed. The most common ligamentous knee injuries in football are to the MCL and ACL [33].

It is known that ACL deficiency leads to functional instability. While athletes participating in linear sports such as cycling or rowing may be able to perform with ACL deficiency, functional stability is required for cutting and pivoting or contact sports such as football, basketball, and soccer [59]. Restoration of stability allows cutting and pivoting athletes to RTP; numerous studies have shown improvement in anteroposterior and rotatory stability following ACLR [55,59,85]. Furthermore, ACL deficiency leads to consequences such as meniscus pathology and need for subsequent surgery due to increased load on secondary knee stabilizers [20,32].

A study of NFL Combine participants from 2009 to 2015 showed that 13.2% of prospects had a history of MCL injury [48]. In studies of various levels of play, injuries to the MCL have been found to be the most common knee injury [25,48]. Medial collateral ligament injuries are diagnosed based on history and physical examination, and in severe cases or suspected concomitant injury, advanced imaging is performed. The mechanism is either contact or non-contact, with a valgus moment to the knee, and medial sided pain. Physical examination is remarkable for tenderness along the course of the MCL from its origin at the medial femoral epicondyle to the medial proximal tibia. In addition, valgus stress examination at 30° flexion may reveal increased opening compared to the contralateral side or lack of endpoint. Important injury characteristics include location (proximal, midsubstance, or distal), and grade of injury on advanced imaging (edema, partial tear, complete tear, chronic scarring).

Mechanistically, ACL ruptures in NFL players often occur through non-contact mechanism, but players with elevated body mass index (in particular offensive linemen) have a greater likelihood of ACL injury through direct contact [17]. Diagnosis begins with history and physical examination. The anterior drawer, Lachman, and pivot shift tests are used to evaluate the structural integrity of the ACL to anteroposterior and rotatory stress. There is often a hemarthrosis, and it is important to examine for meniscal or other ligamentous injury that can occur concomitantly. Imaging can confirm the diagnosis, with a Segond fracture seen on radiographs, and direct visualization of the torn ACL and pathognomonic bone bruising pattern on MRI. In addition, injury to the cartilage, menisci, and other ligaments can be visualized on MRI, and malalignment can be identified with long leg alignment films.

Treatment for MCL injury is usually nonsurgical. For the patient who fails nonoperative treatment and has persistent laxity, high-grade injury, or concomitant injury, surgical management may be indicated, in the form of MCL reconstruction. Biologics are generally not used for MCL injuries. While there are rare case reports of PRP use for MCL injury in the literature, there are no large studies [5], and laboratory studies in animal models have shown no benefit in MCL healing with the use of PRP; dosage, composition, and timing of PRP are important factors that are not yet fully understood [4,42].

Treatment of ACL injuries can be conservative or surgical. Conservative management is typically reserved for older, less active individuals who do not wish to participate in cutting or pivoting sports. Thus, in football players, the treatment of choice is operative management with surgical reconstruction (Fig. 2) [14,59]. Primary ACL repair has gained some attention in the literature, with the advent of newer techniques and biologic augmentation; however, reconstruction remains the gold standard treatment with superior outcomes [63]. When planning surgical reconstruction, graft selection is an important consideration. Graft types can be categorized as allograft or autograft. Allograft is typically used for patients over the age of 40, as higher failure rates have been reported in younger active patients [34]. There is limited data on individual types of allografts, but it is known that irradiated allografts have increased failure rate [65]. The main types of autografts are bone-tendon-bone (BTB), quadriceps, and hamstring. Bone-tendon-bone is generally considered the gold standard, with a long track record of good functional outcomes and low failure rate in cutting and pivoting sports [66]. Advantages of BTB include presence of bone plugs at either end, which allow for robust fixation and bone-to-bone healing, as well as the presence of native enthesis tissue at either end; however, there is donor site morbidity in the form of anterior knee pain, kneeling pain, and theoretical risk of patella fracture or tendon rupture [46]. Hamstring autograft is typically in the form of a quadrupled graft using the gracilis and semitendiniosus. Hamstring autograft is commonly used, but some studies have reported increased early failure rate [51,70]. Recently, quadriceps autograft has gained popularity. The central portion of the quadriceps can be harvested as a soft tissue-only graft, or with the presence of a patellar bone plug, and produces a robust high-diameter intraarticular graft [46]. Quadriceps autograft has demonstrated good outcomes in laxity and failure rate [76], but long-term high-quality studies are lacking.

Fig. 2.

Arthroscopic ACL reconstruction. Left: arthroscopic image of ruptured ACL stump. Right: completed arthroscopic ACL reconstruction using quadriceps tendon autograft, with suture brace augmentation.

While graft selection has been abundantly studied, there are other important surgical and postoperative considerations for ACLR. Anatomic tunnel position is paramount, as tunnel malposition is cited as a leading cause of ACL graft failure [60,77]. Treatment of concomitant meniscal pathology is crucial, as the menisci are secondary stabilizers of the knee, with the medial meniscus serving as a brake-stop to anteroposterior translation, and the lateral meniscus providing stability to the pivot shift [31,40]. Finally, there has been increasing evidence for addition of concomitant stability procedures, in particular lateral extra-articular stabilization, in cases of high-grade laxity or revision surgery [28]. Post-operative rehabilitation consists of an initial period of light activity focusing on regaining full range of motion and quadriceps strengthening. Return to sport is through a graduated progression, from normalized gait to straight line activity, cutting and pivoting exercises, then graduated sport-specific protocol starting with individual drills and progressing to full contact practice and game activity. Formal return to sport is dependent upon objective performance on strength, performance, and functional tests, as well as psychological readiness [35]. Typically, football players can return to same level of play by 10 to 12 months [21,74].

Return to play following ACL injury varies by position. National Football League quarterbacks’ rate of RTP is 92%, with no significant change in in-game performance after return [26]. Return to play is lower in other positions: 64.3% in NFL linemen [21], 64.5% in running backs, and 60.1% in wide receivers [52]. Running backs and wide receivers have decreased statistical performance following ACLR, while in quarterbacks statistical performance after RTP remains consistent with pre-injury [18]. Similar trends of decreased performance in receivers and running backs following RTP from ACLR have been reported in collegiate football [83]. When compared with athletes in other sports, professional football players fare the worst after ACLR, with the lowest survival rate, shortest postoperative career length, and decreased statistical productivity [49]. Nonetheless, players with prior ACLR have similar objective performance metrics at the NFL Combine compared to those without prior ACLR [36]. Return to play in high school and collegiate football players is similar to that in the NFL, with 63% and 69% return in high school and collegiate athletes, respectively [54].