≫ Understand the functional anatomy and physiology of the knee.
≫ Learn about the serious limb-threatening injuries that can occur as a result of trauma to the knee.
≫ Learn practical tips to help you successfully evaluate and manage serious knee injuries in the prehospital setting.
Foot drop: Gait abnormality in which the forefoot is unable to be lifted upward due to weakness, irritation or damage to the peroneal nerve.
Knee dislocation: Complete displacement of the tibia in relationship to the femur in either the anterior or posterior direction. Associated with serious injury to the cartilage, ligaments, blood vessels and nerves of the knee and lower leg.
Patella dislocation: Displacement of the patella (kneecap) out of its normal midline position in the patellofemoral groove. The patella typically dislocates laterally.
Popliteal artery: Continuation of the femoral artery that passes directly behind the knee.
It’s 11:00 p.m. when you’re dispatched to the southbound lanes of Interstate 5 at mile marker 171 for a confirmed multi-vehicle crash. You’re the second unit on scen eand upon arrival are directed by the incident commander to a front seat passenger inside her vehicle. She’s 32 years old and was restrained when her car rear-ended a truck that suddenly stopped in the left lane of the freeway. There’s extensive front-end intrusion but your patient is awake, alert and denies losing consciousness or hitting her head. Her chief complaint is left knee pain.
As the rescue company completes extrication and your partner begins the rapid trauma assessment, you can’t help but notice your patient’s left knee is swollen and her lower leg looks slightly dusky. You’re eight minutes from a busy community hospital and 30 minutes from a trauma center.
There are more than 1.3 million annual visits to EDs for knee trauma in the United States, with many of these patients initially being treated by EMS.1 The knee’s anatomic and functional complexity mean trauma can result in diverse injury patterns, including fractures, dislocations, sprains/strains, ligamentous and cartilaginous injuries, as well as potentially devastating neurovascular compromise.
Knee trauma can be a high- or low-energy mechanism of injury. High-energy knee injuries are largely caused by motor vehicle crashes (MVCs), falls from a great height and pedestrians struck by motor vehicles. They warrant immediate recognition, emergent evaluation and timely transport. Low-energy mechanisms include routine sports-related injuries, ground level falls and repetitive overuse trauma that can also cause significant long-term complications and decreased functional ability.
All knee trauma warrants a thorough prehospital assessment as missed or delayed recognition of serious injury can result in potentially limb-threatening consequences.
ANATOMY & PHYSIOLOGY
The knee is the largest joint in the human body.2 It’s a complex synovial hinge joint with many components vulnerable to injury. The four major components are bones, cartilage, ligaments and tendons. (See Figure 1, below.)
Figure 1: Anatomy of the knee
The three major bones that make up the knee joint are the femur, tibia and patella. The femur and the tibia form the articular component of the joint, and between the mare the medial and lateral menisci, which are wedge-shaped pieces of cartilage that distribute weight and act as shock absorbers.
Interfacing bone surfaces are covered with articular cartilage that allows gliding across one another as the knee flexes and extends.
Ligaments are bands of tissue that connect bones to one another, while tendons are bands of tissue that connect muscle to bone. The knee contains two categories of ligaments: collateral and cruciate. The collateral ligaments are located medially and laterally, bracing the knee against extreme sideways motion. The cruciate ligaments, both anterior and posterior, cross one another to form an X shape deep inside the joint. This provides rotational support and prevents the tibia from sliding forward or backward on the femur.
The patella protects the front of the knee and provides an attachment for both the quadriceps and patellar tendon. The quadricep tendon attaches the strong muscles of the anterior thigh to the patella and allows for extension of the lower leg at the knee. The patellar tendon anchors the patella to the anterior proximal tibia and also contributes to this motion.
The popliteal artery and peroneal nerve are two key structures that EMS providers must consider with every knee injury. Significant trauma to the knee or mismanagement and poorly applied splints can result in serious, often permanent, injury.1–4
The popliteal artery is the major artery that supplies blood to the lower leg. It’s formed by the femoral artery as it travels down the leg toward the knee. Its pulse can often be palpated on exam and is found on the posterior side of the knee, in the popliteal fossa. With its close and fixed proximity to the knee bones, the popliteal artery can be compressed or disrupted by knee dislocations and severe fractures. Assessing for the presence of a dorsalis pedis in the foot and posterior tibial pulse in the ankle are critically important when evaluatinginjuries.5
Originating from the sciatic nerve in the posterior thigh, the peroneal nerve supplies sensation to the front and lateral aspect of the lower leg and to the top of the foot, as well as motor function to the lower leg muscles that dorsi flex the ankle and toes upward. The peroneal nerve branches course superficially around the head of the fibula, just lateral and below the knee, and can also be injured in knee dislocations and fractures and by poorly applied splints. Injury to the peroneal nerve can result in a “foot drop,” or the inability to raise the forefoot during walking.
Fractures (patella, distal femur, proximal tibia): The most commonly fractured bone in the knee is the patella,2 which usually occurs from direct trauma to the front of the knee, such as with a fall or MVC. If the patella fractures into multiple pieces, the patient may be unable to actively straighten the knee. (See Figure 2, below.) Fractures to the distal femur and the proximal tibia can also occur and can be devastating to the overall function of the knee. In younger patients these fractures are typically from high-energy mechanisms, but in elderly patients with less dense bones, proximal tibia fractures can occur with lower energy mechanisms such as ground-level falls. Significantly displaced fracture fragments of the distal femur and proximal tibia can cause limb-threatening neurovascular compromise to the lower leg.
Figure 2: Open fracture of the patella
Ligamentous injuries: The anterior cruciate ligament (ACL) is the most commonly injured major ligament in the knee1 and is frequently injured in snow skiing accidents and in contact sports such as football.1,6 The ACL can also be injured in a non-contact fashion, classically when the patient is decelerating, pivoting, or changing direction with weight applied to thatleg.7 (See Figure 3, below.) About 67–80% of patients will report feeling or hearing a “pop” in their knee at the time of injury and will report significant joint instability.7 ACL injuries are often associated with immediate and significant swelling due to tearing of the synovium and small blood vessels. This swelling typically occurs within three hours but can take up to a day to develop.7 Injuries can be devastating, resulting in significant functional impairment. In addition, about half of ACL injuries are associated with damage to other structures in the knee such as a meniscus, cartilage or other ligaments.2
Figure 3: Ligamentous injuries of the knee (right knee, front view)
The posterior cruciate ligament (PCL) is less commonly injured due to its inherent strength and location within the knee. It’s typically torn after a direct high-energy impact to the anterior tibia of a bent knee such as by the dashboard during an MVC. Similar to ACL injuries, PCL injuries can cause significant joint instability and are often associated with damage to other structures in the knee.
The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) help to stabilize the medial and lateral aspects of the knee and are typically injured by forces applied to the opposite side of the knee. This commonly occurs in lower-energy mechanisms such as a collision playing sports. The MCL is more commonly injured than the LCL and often associated with an ACL injury. These can be classified as sprains, representing partial injuries to the ligament, or as complete tears.
Meniscus tears: Meniscus injuries often occur as a result of a twisting motion put onto a flexed or weight-bearing knee. Meniscus injuries are often caused by a low-energy mechanism but can coexist with more serious injuries to other structures of the knee.1 Patients with meniscus injuries often note increased pain with weight bearing and describe a “popping” or “locking” sensation in the joint. Locking of the knee (i.e., fixed in flexion) immediately after injury is due to a mechanical block from the displaced cartilage.1
Tendon injuries: The quadriceps and patellar tendon can also be torn. Tears are more common in middle-aged patients during running or jumping sports such as basketball ortennis.2 Direct trauma to the knee such as a significant fall can also cause traumatic tendon rupture. Although the patient may lose the ability to actively extend the lower leg, remember that patients with quadriceps or patellar tendon ruptures often can still walk by leaning forward and allowing gravity to extend theknee.1 Early diagnosis of both quadriceps and patellar tendon rupture is important as urgent surgical repair is frequently necessary to preserve the extensor mechanisms of the knee.
Dislocations: A joint dislocation occurs when an injury forces the surfaces of two bones out of normal contact with each other, preventing the joint from moving through its normal range of motion. Dislocations of the patella and knee are often confused by both lay people and healthcare providers alike, but represent different pathologies with different treatment modalities.
Patellar dislocations are more common than true knee dislocations and often occur with a low-energy twisting mechanism while the foot is planted.1 Patellar dislocations mainly affect young active people, specifically young women, with a peak age between10 and 20 years old.5,8 They’re typically caused by the patella becoming displaced laterally out of the groove where it normally lies and can be very obvious on physical exam because of the grossly deformed appearance of the knee. Many patellar dislocations will self-reduce when the muscles of the thigh relax and the leg is straightened, such as for splinting, but some may require sedation and manual reduction in the ED. Although dislocation of the patella may spontaneously reduce, providers should remember that 12% of these dislocations will have a major coexisting knee injury.1,9 Lastly,patellar dislocations can be associated with ruptures of the medial patellofemoral ligament(MPLF), which can make future and recurrent dislocations more common.8
True knee dislocations are less common than patellar dislocations but are associated with much greater morbidity. Given that the knee is normally a very stable joint, a high-energy mechanism is typically required to dislocate the joint.5 Knee dislocations are termed anterior or posterior based on the direction the tibia is displaced in relation to the femur, but 50–60% are anterior.10
By its very definition, a knee dislocation is associated with serious injury to other structures in the knee such as the ACL, PCL and MCL ligaments, as well as the joint capsule, nerves, arteries and cartilage.3,11 Knee dislocations can be dangerous because the blood vessels (namely the popliteal artery) that run behind the knee can be compressed or torn during a dislocation event, thus compromising distal circulation to the lower leg.3,11 Popliteal artery injury has been reported in20–40% of knee dislocations, and between 20–25% of patients who suffer a serious popliteal artery injury require a lower leg amputation.4,5,12,13 In addition, injury to the peroneal nerve occurs in 25–35% of knee dislocations, thus proper immobilization and neurovascular checks before and after splinting are criticallyimportant.5,11,12
The key to prehospital management of a true knee dislocation is maintaining a high level of suspicion for the injury. Up to 50%of patients who suffered a knee dislocation will have spontaneous reduction in the field prior to ED evaluation, thus making diagnosis more difficult.10 The patient may not always give the classic history of the knee “popping out of place” and then suddenly returning to normal. Typically, if spontaneous reduction of a knee dislocation has occurred, the knee will still be significantly swollen, painful and structurally unstable on exam. However, if the patient has had complete disruption of the joint capsule, the hematoma may spread into the thigh or calf, resulting in the knee appearing almost normal in size.1
Lastly, it’s entirely possible to have a popliteal artery injury and still have a warm foot with palpable dorsalis pedis and posterior tibialpulses.12 It’s imperative providers remember that palpable pulses don’t rule out significant arterial injury.
TREATMENT & TRANSPORT
High-energy mechanisms that result in devastating knee injuries also frequently cause other potentially life-threatening injuries to the head, chest, abdomen, etc. Thus, personnel may become task saturated in caring for these time-sensitive, critical trauma patients. Providers should follow their department protocols and standard Prehospital Trauma Life Support principles in management of these multisystem trauma patients.
Initial assessment by EMS can prove critically important in successfully managing knee trauma. First, it’s imperative to establish the mechanism of injury, taking note of the position of the leg at the time of injury and the patient’s ability to walk post-injury. Understanding the injury pattern can raise suspicion for hidden neurovascular injury, resulting in very specific triage and management decisions for both EMS and in the ED.
A thorough examination of the knee should then follow. This includes a visual inspection of the entire leg, assessing peripheral pulses and determining if there are any sensory or motor deficits distal to the injury. An important area to assess for sensory loss is on the top of the foot, between the first and second toe. Numbness in this area following knee trauma is highly suggestive of a peroneal nerve injury.
Splinting/immobilization: At the cornerstone of prehospital management for knee trauma is splint application and joint immobilization.4,14 The goal of splinting isn’t only to support potentially unstable fractures, but to also decrease the patient’s pain and reduce the chance of further neurovascular or soft tissue injury from uncontrolled bone motion.14,15 (See “The Lost Art of Splinting: How to properly immobilize extremities & manage pain,” by Jennifer Cuske, RN, EMT-P, at jems.com/art-of-splinting.)
Prehospital providers are classically taught to splint musculoskeletal injuries in the position found unless there’s compromised distal circulation. However, in recent years there’s been increased recognition of the importance of realigning extremities into near anatomic position as early as possible to control pain and protect the site from additional vascular injury.11,14-16 The decision to reduce or realign a fracture or dislocation in the field is controversial and thus truly situation dependent.14,19 When splinting the knee, immobilize the limb in the position found or that of maximum comfort.18,19 However, providers should discuss with their medical direction specific indications and contraindications to attempting repositioning or realignment of serious knee deformities based on specific patient care environments (e.g., wilderness vs. urban setting), available local resources, level of training, and geographic proximity to the trauma center.17,18
Regardless of whether a knee injury is splinted in the position found or after a gentle attempt at realignment, care should be taken not to splint the leg fully extended as this may compress the neurovascular bundle against the posterior tibia.18,19 Splinting the knee with approximately 10 degrees of flexion is thought to be ideal.15 There are a variety of options for splinting the knee, ranging from preformed cardboard and vacuum splints to the sophisticated Reel Splint Immobilizer. Frequently utilized by the U.S. military, the Reel Splint Immobilizer with its unique multi-hinge system allows you to easily adjust the length and angle of the splint to fit almost any knee deformity.3
Regardless of the splint utilized, it’s most important to ensure it adequately prevents movement of the joint, is appropriately sized both circumferentially and lengthwise above and below the knee, is well padded, and allows continued assessment of the injured extremity during transport. Lastly, it’s critically important to complete and document a neurovascular exam both before and after any manipulation or splinting of the injured knee.14
Pain control: Knee injuries can be extremely painful—especially those sustained from a high-energy mechanism. It’s been well documented that prehospital providers often don’t provide adequate pain relief for patients with lower extremity injuries.20-22
This is especially true of pediatric patients, who are much less likely to receive appropriate prehospital analgesia than adults with similar extremity injuries.20 In addition to splinting and elevating and icing the injured knee, providers should strongly consider early administration of IV, intranasal or intramuscular pain medication per department protocol. This classically includes narcotic analgesia such as morphine or fentanyl; however, more recently the role of low-dose ketamine in prehospital pain management is being explored and gaining favor.14,23
Transport considerations: Vascular injuries from knee trauma may require time-sensitive intervention by orthopedic and vascular surgeons in an attempt to restore distal leg blood flow and salvage the patient’s limb. Thus, patients who have any evidence of vascular injury, knee dislocation or neurological deficit, or those patients with significant mechanisms that raise suspicion fora spontaneously reduced knee dislocation, should be transported directly to a traumacenter.12,13 Upon arrival at the trauma center, it’s important EMS providers clearly communicate their initial neurovascular exam of the injured extremity and clinical suspicion for a potentially devastating knee injury.
Your rapid trauma exam reveals a Glasgow coma scale of 15, patent airway, clear and equal lung sounds, a non-tender abdomen and vital signs significant only for tachycardia to 116.As you complete your secondary survey, you again note the left leg appears slightly dusky below the knee. There’s no obvious deformity of the knee bones but there’s swelling to the posterior knee and the patient refuses to bend her knee secondary to pain.
The goal of splinting isn’t only to support potentially unstable fractures, but to also decrease the patient’s pain and reduce the chance of further neurovascular or soft tissue injury from uncontrolled bone motion. Photo courtesy Colerain Township Department of Fire and EMS
You have difficulty finding a dorsalis pedis pulse but are able to palpate a weak posterior tibial pulse. The patient also has decreased sensation to the top of her foot. You astutely recognize the patient may have suffered a knee dislocation with spontaneous reduction and recall that this injury can result in significant damage to the popliteal artery and peroneal nerve. You discuss your findings with your partner and decide to transport your patient the extra distance to the regional trauma center where you know there’s 24-hour orthopedic and vascular surgery capability. You apply a vacuum splint to appropriately immobilize the leg in a position of comfort and begin transport.
En route, you establish an 18-gauge IV and administer two weight-based doses of IV fentanyl per protocol for her significant knee pain. You cover the patient and the affected limb with warm, dry blankets to promote circulation.
On arrival to the trauma center, you note that the patient’s posterior tibial pulse seems stronger and you can now palpate a dorsalis pedis pulse. The color of the lower left leg has also improved. Upon transferring care to the ED you discuss your initial exam findings and clinical suspicion with the attending emergency physician. Later in the evening you return to the trauma center to drop off a different patient and the physician tells you your patient with the knee injury indeed had a popliteal artery injury on CT angiogram, likely from a spontaneously reduced, posterior knee dislocation. The physician compliments you on your astute prehospital examination skills and intuition, as diagnosing this limb-threatening injury could have been easily delayed without the exam findings you noted in the field.
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3. Heightman AJ. Articulating knee injuries: Placing proper emphasis on the recognition & stabilization of severely dislocated knees. JEMS.2004;29(7):46–55.
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21. McEachin CC, McDermott JT, Swor R. Few EMS patients with lower extremity fractures receive prehospital analgesia. Prehosp Emerg Care.2002;6(4):406–410.
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