Soft Tissue Knee Injury 

Soft tissue injuries of the knee are some of the most common and clinically challenging musculoskeletal disorders in patients presenting to the ED. Annually, more than 1 million emergency department (ED) visits and 1.9 million primary care outpatient visits are for acute knee pain. Therefore, establishing clear-cut diagnostic and therapeutic objectives for these injuries is important.

Knee pain and related symptoms may derive from damage to one or more of the soft tissue structures that stabilize and cushion the knee joint (including the ligaments, muscles, tendons, and menisci), from infection to the knee joint or surrounding structures, or from trauma to the bones forming the joint. In addition to the etiology of the patient's presenting symptom, determine the acuity of the pathologic process as an acute traumatic or infectious event or exacerbation of a chronic overuse or degenerative syndrome.

Accurate and timely diagnosis increases the likelihood of fully restoring normal and pain-free use of the affected knee. For most patients, the severity of the etiology and the injury or pathologic process, acute or chronic, can be determined from a targeted history, focused physical examination, and thoughtful workup including diagnostic imaging (eg, plain radiography).

To understand the various injury patterns associated with trauma to the knee, understanding the anatomy is important. The anatomy of the knee is shown in the image below.

Knee symptoms arise from an alteration or disruption of the normal anatomic structures that impede normal knee function. In mechanical terms, the knee performs like a rolling cam rather than as a simple hinged (ginglymus) joint. As the knee proceeds from flexion to extension, a complex screw-type of motion takes place, with the femoral condyles locking into the tibial plateau as the femur rotates internally. Full knee extension increases the tautness of the major bracing ligaments, transforming the knee into a mechanically rigid structure. Flexion loosens the knee joint by unlocking and disengaging the bracing structures, including retraction of the menisci, thereby enhancing ligamentous laxity and increasing the range of motion (ROM) of the joint.

Two separate but interdependent joints forming the knee are the tibiofemoral articulation and the patellofemoral coupling. Weight-bearing forces, as much as 5 times an individual's body weight, are transmitted through the opposing condyles of the femur and the tibia. Two shock-absorbing cartilaginous menisci interpose between the femur and the tibia, forming the largest synovial joint in the body. The medial meniscus is smaller and more fixed than the lateral meniscus; these features predispose it to injury. A fibrous capsule lined by a synovial membrane also surrounds and bolsters the knee joint but does not contribute to the inherent stability of the joint.

Fitness of the knee joint largely depends on the fortifying ligaments and muscles binding together the femur, tibia, and patella. Two sets of knee ligaments are frequently affected. The first set, lying outside of the knee joint proper, are the extracapsular collateral ligaments. These ligaments consist of the medial collateral ligament (MCL), which opposes extreme abductive and/or valgus forces, and its counterpart, the lateral collateral ligament (LCL), which limits excessive adductive and/or varus pressures. The second set, crisscrossing in the knee joint, are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), which individually brace against excessive translation in the anteroposterior (AP) plane. The ACL serves as the primary knee stabilizer, preventing forward displacement of the tibia on the femur.

Primarily formed by the quadriceps muscles, the extensor apparatus envelops and stabilizes the patella. At its distal aspect, the quadriceps muscle consolidates into the patellar ligament, ultimately inserting onto the tibial tubercle. Several bursae envelop the knee, including the prepatellar, superficial and deep infrapatellar, and pes anserine bursae, which permit friction-free movement between the various structures. Acute trauma or repetitive occupational stress may incite inflammation; infection and metabolic disorders (eg, gout) are less common etiologies. Inflammation of the bursa then leads to localized tenderness, erythema, and increased warmth. Extensive bursae in this area alleviate potentially damaging frictional forces between the susceptible structures. Fixed in the back of the knee joint, in the popliteal fossa, are vital neurovascular structures, including the popliteal artery.

Definitions and grades of sprains and strains

Sprains to the knee are characterized by the stretching or tearing of noncontractile structures, such as the investing ligaments or of the joint capsule itself, whereas a strain refers to stretching or severing along the course of muscles or tendons. Both collateral ligament and cruciate ligament sprains, as well as muscular strains, are relatively common. Ligamentous (sprain) and muscular (strain) injuries may be classified according to the degree of impairment.

• Grade I sprain - Stretching but no tearing of the ligament, local tenderness, minimal edema, no gross instability with stress testing, firm end point
• Grade II sprain - Partial tears of the ligaments, moderate local tenderness, mild instability with stress testing (but firm end point), moderately incapacitating
• Grade III sprain - Complete tear, discomfort with manipulation but less than expected for degree of injury, variable amount of edema (ranging from negligible to grossly conspicuous), clear instability with stress testing (expressing a mushy end point), severe disability

Specific injuries

• ACL injury: Rupture of the ACL is among the most serious of the common knee injuries and results from a variety of mechanisms. Most patients with ACL damage complain of immediate and profound pain, exacerbated with motion, and inability to ambulate. Disruption of the ACL may occur alone or with other knee injuries, especially a meniscal injury or tear of the MCL.
• PCL injury: Patients typically report falling on a flexed knee or sustaining a direct blow to the anterior aspect of the knee (eg, when the knee strikes the dashboard in a motor vehicle accident). PCL harm signifies a major injury and rarely occurs as an isolated injury.

Frequency

United States

Knee pain is a complaint in as many as 20% of the general adult population, accounting for almost 3 million outpatient and emergency department visits per year. Trauma to the knee is the second most common occupational accident. The MCL is the most frequently injured ligament in the knee. ACL damage causes the highest incidence of pathologic joint instability.

In National Collegiate Athletic Association (NCAA) football, one major knee injury occurs per team every year. Sports-related activity accounts for approximately 60% of knee injuries producing ligamentous laxity.

International

As in the United States, the incidence of knee injuries is increased in countries where sporting activities, such as skiing, soccer, and basketball, are popular.

Mortality/Morbidity

Patients receiving inappropriate or ill-timed care of knee dislocations may have undue morbidity (eg, amputation) due to vascular complications in the distal leg. Oversight of the magnitude of soft tissue injuries of the knee may result in a failure to expeditiously consider compartment syndrome and its resultant complications, including loss of a limb. Misdiagnosis or mismanagement of damage to supporting structures of the knee may lead to chronic knee instability with subsequent development of degenerative joint disease and/or loss of knee function, including but not limited to an inability to bear weight or ambulate.

Sex

Disorders of the patella and lateral meniscus are generally more common in girls and women than in boys and men. Recent studies also suggest that females are more prone to ACL injuries, which is believed to be due to the fact that the female ACL is both structurally weaker and has a relatively smaller cross sectional diameter. Chondromalacia patellae or patellar malalignment syndrome (ie, premature erosion and degeneration of patellar cartilage) predominates in young women.

Larsen-Johansson disease of the patella, also known as inferior pole patellar chondropathy, is 9 times more prevalent in boys and men than in girls and women, especially in boys aged 10-14 years.

Age

Ligamentous and meniscal injuries are most likely in young to middle-aged adults, whereas children and adolescents are most susceptible to osseous damage. Most patients with a meniscal tear are aged 20-30 years, but a second peak occurs in patients older than 60 years. Meniscal injuries are rare in children younger than 10 years with morphologically normal menisci.

In general, knee dislocations arise from high-energy trauma, such as motor vehicle accidents.^[1] The epidemiology of injury follows that of car accidents; therefore, patients tend to be young men.

Overall, 18.1% of US men and 23.5% of US women aged 60 years and older reported knee pain on most days for 6 weeks prior to their medical examination. Additionally, elderly patients may sustain fractures after minimal trauma that typically produces only soft tissue injuries in younger patients.

The region of the extensor mechanism susceptible to disruption is correlated with the patient's age. The older the patient, the more proximal the area of rupture. Disruption of the quadriceps tendon most often occurs in elderly patients, whereas more distal severance of the patellar tendon and avulsion of the tibial tubercle occurs in younger patients.