Injury-related causes of acute knee pain

This detailed overview covers the anatomy, function, and examination of the knee and describes the phases of treatment for acute knee pain secondary to injury.

Jacqueline S. Dascola, MMSc, PA-C; Kelly Ward, PA-C

Jacqueline Dascola is a recent graduate of the Emory University Physician Assistant Program. She works at the Atlanta ID Group, Atlanta, Ga. Kelly Ward practices at the Athens Orthopedic Clinic, Athens, Ga. The authors have indicated no relationships to disclose relating to the content of this article.

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Knee pain affects 20% of people in the United States and is the fifth most common health complaint, accounting for 12.5 million doctor visits in 2000.^1,2 Acute knee pain leads to more than one million emergency department and 1.9 million outpatient primary care visits annually.¹ The most common causes of acute knee pain in the adult primary care setting are osteoarthritis (OA) exacerbations (34%), meniscal injuries (9%), collateral (7%) and cruciate (4%) ligament injuries, gout (2%), and fractures (1.2%).³ Although musculoskeletal conditions are common, many clinicians receive limited instruction in musculoskeletal medicine during training and therefore may provide suboptimal treatment.⁴

Distinguishing acute injuries from chronic causes of knee pain may be difficult—especially in the clinical setting, because patients frequently present with acute exacerbations of chronic problems. Acute knee pain is defined as pain that begins less than 1 week before the patient seeks medical attention.¹

Anatomy

The largest articulating joint in the body, the knee is comprised of the medial and lateral femoral condyles, medial and lateral tibial plateaus, and the patella. Soft tissue structures include the anterior cruciate (ACL), posterior cruciate (PCL), medial collateral (MCL), and lateral collateral (LCL) ligaments; the medial and lateral menisci; the joint capsule; and the tendons associated with the knee musculature.

The knee ligaments provide stability. The ACL and PCL prevent anterior and posterior displacement of the tibia on the femur, respectively, attaching to the intra-articular portions of the femur and tibia. The MCL, originating below the adduction tubercle of the femur and attaching to the upper medial tibia, limits abduction and assists in controlling knee rotation. The LCL controls adduction, attaching to the lateral epicondyle of the femur and head of the fibula. The menisci are semilunar crescent-shaped structures on the tibial plateaus. They increase joint stability, facilitate nutrition, and provide joint lubrication and shock absorption⁴ (see Figure 1).

Common knee injuries

Meniscal The medial meniscus is three times more likely to tear than is the lateral.⁵ The meniscus is usually injured by a noncontact rotational force on a partly or completely flexed knee, an injury commonly seen in tennis players. Patients may report hearing their knees “pop” but are able to continue with their activities, noting an effusion more than 12 hours after injury. Patients commonly experience stiffness, painful locking, or clicking and sometimes describe the knee as “giving way.” They may report pain and difficulty with squatting and/or climbing and descending stairs.¹

MCL This is the most commonly injured knee ligament. The damage usually results from a valgus blow but may also occur with external rotation while the foot is planted. Patients experience localized swelling and tenderness over the injured area within 12 hours of the injury. Football, soccer, hockey, and rugby players typically damage the MCL by a direct sideways blow. Skiers and wrestlers may also hurt the MCLs by twisting the knee.⁶

ACL ACL injuries have a presentation that is more than 70% accurate for diagnosis.⁷ The ACL is the second most commonly injured knee ligament, and damage is usually caused by a noncontact pivoting/twisting movement with the foot planted, a sudden deceleration, or hyperextension.^5,8 The patient will notice an audible “pop” at the time of injury, experience extreme pain, and be immediately disabled. The history includes swelling a few hours after the injury and the patient’s sense that the knee will give way. This injury is seen in skiers, gymnasts, and football, basketball, and soccer players.

PCL and LCL The PCL is three times stronger than the ACL,⁷ so an anterior blow to the tibia with the knee flexed has to occur in order for it to be damaged. Patients may report pain at the back of the knee that worsens when they kneel and a mild effusion within a few hours after injury. Football, basketball, soccer, and rugby players suffer from this type of injury. The mechanism of LCL injury is a varus blow or rotational force on a planted foot or extended knee. In addition to a mild effusion within a few hours after injury, there is tenderness along the lateral joint line. The LCL is the ligament least likely to be injured because the blow to the medial aspect of the knee would be to an area that is usually shielded by the opposite leg.

Physical examination

The knee examination—when performed by a trained clinician—is an accurate way to diagnose meniscal and ligamentous tears. If findings are negative, the likelihood of a ligamentous or meniscal tear is less than 1.5%, allowing such injuries to be followed clinically. Follow-up is essential, however, since the physical examination is limited by the acuteness of the injury and a few lesions may be missed.¹ Since certain tests cannot be performed while the patient is in acute pain, most experts suggest waiting and evaluating a swollen or painful knee after 2 to 3 days. This approach is safe as long as fractures, infections, or other emergent conditions have been ruled out.⁵ There are three components to the knee examination: inspection, range of motion (ROM) evaluation, and palpation.

Inspection Always begin the examination with inspection. Observe the gait for general knee function, compare both knees, and look for and assess any effusions by noticing any general fullness to the knee anteriorly or a loss of peripatellar dimples.⁸ It is important to assess the hips and ankles because problems in adjacent joints can exacerbate knee injuries or cause referred pain to the knee.

ROM Observe active extension by having the patient sit at the edge of the examination table and extend the knee against gravity. Observe full, active flexion by seeing if the patient can touch the buttocks with the heel while in the supine position.⁵

Palpation The third component in the examination is palpation to assess for normal knee anatomy and tenderness. With the patient sitting on the edge of the table and knees in flexion, palpate the unaffected knee first. Review bony landmarks by placing your thumbs in the dimples on either side of the patella, move thumbs up to palpate the femoral condyles and epicondyles, and then move thumbs down to palpate the tibial plateaus. Palpate the joint line and, lastly, palpate the patella and tibial tuberosity.  

Specific knee tests

If the knee examination leads you to suspect an injury, particular tests can help you determine certain internal knee derangements.

Meniscal Joint line tenderness is the most sensitive test for meniscal injuries at 76%, but with a specificity of 29%.¹ Palpate medially and laterally along the knee joint; pain on palpation is a positive sign. Use the McMurray maneuver to test for meniscal tears through passive flexion and extension of the knee (see Figure 2).^1,4 The McMurray maneuver has a specificity of 97% and a sensitivity of 52%.¹ Other tests for meniscal tears are the Apley’s compression test and medial lateral grind test.

ACL The best evaluated methods of detecting a ligamentous tear are the Lachman maneuver (see Figure 3), the pivot test, and the anterior drawer test.⁹ The Lachman test is the most sensitive at 87% and specific at 93%.¹ The pivot test has a 97% specificity.¹ Both the Lachman maneuver and pivot test have better sensitivities and specificities than the anterior drawer test.¹

PCL Use the posterior sag and posterior drawer tests if you suspect PCL injury (see Figure 4).⁴ In the posterior drawer test, have the patient lie supine with the injured knee flexed at 90°. This is similar to the anterior drawer test for an ACL tear except that you would apply an anterior force pushing back on the tibia. The result is positive if there is posterior translation of the tibia.⁴

MCL and LCL In an MCL injury, there will be tenderness along the medial aspect of the knee, especially at the point of disruption.⁷ The integrity of the ligament is tested by the abduction stress (valgus) test (see Figure 5).¹⁰ For an LCL injury, use the adduction (varus) stress test (see Figure 6).¹⁰ Table 1 provides a summary of the presentations and physical examination findings for common knee injuries.

Imaging

To evaluate the knee for injury, use plain radiographs and MRI. Plain films are of limited value in assessing meniscal or ligamentous tears but are important in ruling out fractures or avulsions. Obtain anterior-posterior and lateral views if you suspect fractures and sunrise views to look for condylar fractures and evaluate the posterior patella after an acute patellar dislocation. Plain films may also show degenerative changes, calcifications of the menisci, or loose bodies. The tunnel view provides better visualization of the weight-bearing surface of the femoral condyles.

MRI is better for demonstrating ligamentous and meniscal lesions. Although not necessary for all patients with acute internal derangement, MRI should be used if the diagnosis is in question. For example, if the history and examination are consistent with an isolated MCL or meniscal injury and the patient responds to conservative therapy, MRI is not necessary. It may be indicated, however, if an initial period of therapy produces no improvement. MRI is approximately 90% accurate in identifying surgically confirmed ACL and meniscal injuries, although it is not more accurate than examination by an experienced clinician.^11-15  

Management

The management of acute knee injuries can be divided into four overlapping phases.¹⁶

Phase 1 Management in the primary care setting often begins with conservative measures. Thus, RICE—rest, ice, compression, and elevation—is usually first-line therapy in controlling knee swelling.^5,17

Rest is indicated when patients have signs of instability or pain with weight bearing. You can immobilize the knee adequately with a 6-inch elastic wrap and give the patient crutches with instruction on their use. Straight-leg immobilizers are also commonly used, but they offer no structural support and are not indicated in acute knee injuries.¹⁶ To control swelling, the patient should apply plastic bags of ice (or bags of frozen vegetables) directly on the knee for 20 minutes at a time at least twice a day. During the first day or two, ice should be applied every 1 to 2 hours while the patient is awake. Heat should not be used during the first phase of treatment. A six-inch elastic wrap or a tubular stockinette not only immobilizes the knee, but also provides compression. The wrap should be applied from the distal to proximal ends and extend from the midcalf to about 10 cm above the patella. The patient should wear the wrap during waking hours, removing it only to ice the knee. The knee should be elevated above the level of the heart as much as possible throughout the day, promoting venous return and reducing swelling. Analgesia with medications such as acetaminophen and NSAIDs should be used during the first 5 to 7 days to decrease pain and swelling and to prevent the cycle of pain leading to disuse leading to atrophy.¹⁶ Patients can take more potent pain medications if necessary.

For all acute knee injuries, phase 1 management continues with quadriceps muscle strengthening after fracture has been ruled out. The patient can maintain quadriceps strength by performing quad-sets, which are pain-free isometric quadriceps contractions. These can be straight-leg raises while supine, knee extensions while seated, or quadriceps muscle contractions with the heel contacting the ground and the knee in full extension. The patient should initiate these exercises immediately, starting even on the day of injury so long as the exercise is not painful. Prescribe 10 sets of 60 to 100 repetitions daily until the patient can use the knee normally. A physical therapist may be needed to facilitate quadriceps strengthening and to teach the patient how to perform the contractions.

Phase 2 When pain and swelling are well controlled, the patient can progress to the second phase. In this phase, the goals are to continue improving strength, flexibility, and endurance while protecting the injured structure. There is some overlap with phase 1, however, because the patient has already begun performing the quad sets. Phase 2 involves relative rest, meaning that the patient can perform any movement or activity so long as there is no pain or swelling during the activity or within 24 hours before or after it. Athletes and others who resume exercise too soon despite pain or swelling increase the risk of developing a chronic injury.¹⁶ The patient should begin active and passive ROM knee exercises, applying moist heat before exercise to improve ROM and ice after it to help control swelling and inflammation. As ROM improves, strengthening exercises against gravity can begin, always starting with low resistance. It is wise to refer the patient to a physical therapist for proper exercise training.

Phase 3 The goals of the third phase are to achieve near-normal strength, flexibility, endurance, and neuromuscular control while continuing to protect the healing structure. In this phase, resistance exercises can involve heavier weights and fewer repetitions to increase strength and power. Neuromuscular control can be rehabilitated with balance and resistance band exercises and/or jumping on a small trampoline. Bicycling is also beneficial in this phase.

Phase 4 The last phase involves functional rehabilitation for athletes. The first step is a walk-jog program, starting with walking and progressing to jogging for 20 minutes without pain or swelling for at least 1 week. After that the patient progresses to straight-away sprints and jog turns. Athletes can perform without restriction if they remain asymptomatic while performing 10-yard figure-8 paths at full sprint.

Follow-up After this program, ROM should return in 1 week. Consult an orthopedist if a patient has continued pain and swelling after 1 week.⁵ If a fracture, infection, or other emergent condition is suspected, immediately refer the patient to a specialist. Surgery may be indicated for any acute injuries involving more than one structure. Isolated injuries of the ACL, PCL, MCL, and LCL may not require surgery; the initial treatment of choice is conservative therapy.¹⁶ However, if the patient does not progress to baseline, consider surgery in follow-up, including knee arthroscopy and repair of ACL, PCL, and LCL tears. For meniscal tears, surgeons perform complete or partial menisectomies or allograft tissue replacements.  

Prognosis

Meniscal tears After conservative or surgical treatment, patients with torn menisci frequently have a good prognosis, and most can resume normal athletic activities after repair. Many patients feel satisfied with results even 10 to 11 years after surgery.¹⁸ However, some may eventually develop arthritis in the injured knee, most likely as the result of a partial or complete menisectomy that removed some of the cushioning effects of the meniscus.

Ligament tears The prognosis of ligament tears is also excellent, with 90% of ACL injuries and 80% of PCL tears healing fully after proper conservative and/or surgical treatment and physical therapy. Almost all MCL and LCL injuries also have excellent prognoses.⁶ As with meniscal repairs, patients may experience long-term complications of OA in the injured knee at any time, but most likely after 10 to 25 years. 

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