Knee pain is a common complaint in the primary-care environment. The pain may be acute, following an injury or overuse, or it may be chronic and evolve slowly over a period of time. Pain may occur in one or both knees. To provide a thorough assessment, there are a few important questions to ask, observations to make, and simple physical tests to perform. This process will help determine the type and severity of injury and guide you to the best treatment. While many conditions will require referral to an orthopedist for treatment, most can be diagnosed by the primary-care clinician.
Basic knee anatomy
There are two joints in the knee—the tibiofemoral and the patellofemoral. The knee also consists of:
- Three bones: the femur, tibia, and patella. The femur and tibia line up evenly with each other, and the patella rests in a groove on the femur. A fourth bone, the fibula, is not involved in articulation but anchors some knee ligaments.
- Two types of cartilage: articular and meniscal. The bone-to-bone contact surface of all three bones is covered by articular cartilage. Several millimeters thick, avascular, and roughly the texture of Formica, this cartilage follows the contours of the bone to which it adheres and provides strength and durability to the articulating portions of bones.
The space between the tibia and the femur is occupied by the meniscus. Four to five millimeters thick (but thinner in the middle), avascular, and similar in texture to hard rubber, this cartilaginous structure comprises two C-shaped disks (one medial, one lateral) and absorbs some of the forces knee bones must withstand. - Four major ligaments: two cruciate ligaments and two collateral ligaments. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are found deep within the tibiofemoral joint. These two ligaments keep the femur from sliding forward or backward on the tibia. The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) are on the medial and lateral aspects of the tibiofemoral joint, respectively, and keep the femur from sliding to one side of the tibia. The ACL, PCL, and MCL have one attachment on the femur and one on the tibia. The LCL has one attachment on the tibia and one on the nonarticulating fibula. The knee has several smaller stabilizing ligaments as well.
- The patellar tendon: This tendon connects the quadriceps muscle with the tibia and runs from the patella to the tibial tubercle.
- Bursae: Several small bursae are located around the patellar tendon in the front portion of the knee. These small sacs facilitate patellar motion.
Understanding the basic anatomy of the knee aids in understanding how certain types of activities or processes will affect the various structures. A good patient history can also provide many clues about which structures are involved. Ask about the circumstances of injury (if applicable) and any prior injuries. Have the patient use one finger to indicate where pain is most severe. Also ask about duration of pain and any associated symptoms (e.g., popping or clicking noises, swelling, or joint instability) to help narrow your diagnosis.
Examining the knee
A brief but thorough knee exam and appropriate radiologic tests will help further isolate and quantify any injuries. Always perform tests on each leg separately and compare the results. Have the patient sit on the exam table with his or her lower legs hanging free. Compare both knees, and look for any asymmetry (i.e., areas of swelling, skin changes, or thigh-muscle atrophy). To determine if knee motion is affected, place your left hand over the knee joint, covering the patella, and grasp the ankle with your right hand. Slowly move the tibia up and down several times, bending and straightening the knee as far as comfort will allow. While doing this, note any unusual noises or limitations of movement. Without pathology, the knee should swing easily and smoothly and elicit no pain or noise. Normal range of motion is at least 130° of flexion and 0° of extension (shin straight in line with the thigh).1 In some individuals, the lower leg may appear to hyperextend in relation to the thigh; up to 10° of hyperextension is normal.2
To quickly assess the condition of the collateral ligaments, use your right hand to hold the leg out until it is almost straight. Grasp the underside of the knee with the left hand, and gently apply a valgus stress followed by a varus stress. For a valgus stress, the right hand pulls the lower leg away from the midline while the left hand pushes the knee joint toward the midline. For a varus stress, the hands remain in the same position, but the grip and forces are reversed (right hand pushes while left hand pulls). A few degrees of motion are normal as the collateral ligaments are slightly stretched. Note any excessive laxity or pain.
Since hip joint pathology commonly results in referred pain to the knee, a quick check of the hip joint is important. With the knee bent to 90°, place the left hand over the knee and press down lightly to stabilize the knee joint. Grasp the ankle with your right hand, and rock the tibia from side to side. This pendulum motion, which may cause some pain in the knee, is actually a test of the hip joint as it rocks the head of the femur in the hip socket. A normal hip will allow the tibia to swing at least 30° in each direction.3 If the leg will not rock more than about 20° in either direction or the rocking causes visible wincing, consider hip pathology in addition to any suspected knee problems.
A complete knee exam includes a test of the patellar reflex to look for any pathology of spinal nerves L2, L3, and L4. This is especially important if the patient has an altered gait. The patellar reflex is one of the deep-tendon reflexes and will help differentiate an upper motor neuron lesion from a lower motor neuron lesion.1 Strike the patellar tendon sharply with a reflex hammer just below and slightly lateral to the bottom of the patella. The knee should be hanging free or crossed over the opposite knee and relaxed. Note whether the reflex is brisk, mild, or absent. While no solid conclusion can be reached from this single neurologic test, it provides an important contribution to the full picture.
Palpation is a key component of every knee exam. If possible, sit on a stool facing the patient. With the knee flexed 90°, move your thumb across the joint line, feeling for the tibial plateau (a hard transverse ridge just below the patella) and the softer indentation just above it (where the meniscus lies). Note any painful areas. Next, palpate in a circular motion with two fingers over the MCL and LCL, which extend more than an inch above and below the joint line. Note any areas of point tenderness or diffuse tenderness. If the patient is complaining of anterior or posterior knee pain, palpate there as well, identifying the precise area of maximal pain. Then, with the leg flat on the exam table, palpate around the edges of the patella, looking for any signs of discomfort or apprehension. Also place one hand on either side of the patella and press down with the fingers of one hand followed by the other. If you feel a fluid shift, the test is positive for an effusion. An effusion occurs with inflammation and usually points to underlying injury inside the joint capsule.
If the patient has described an acute injury accompanied by a popping noise, perform the following two tests to look for an ACL tear or a meniscal tear.
The anterior/posterior drawer test detects injury to the ACL or PCL. Have the patient recline on the table with the injured leg bent to 90°. Rest your body against the patient’s foot to anchor it to the table, and grasp the injured leg with both hands just below the joint line. Firmly pull the tibia forward (toward yourself) in relation to the femur. Normal motion is <5 mm. Test for posterior motion by pushing the tibia backward (toward the patient) in relation to the femur. Any movement >5 mm usually indicates a tear of the ACL or PCL. 
The Apley compression test (Figure 1) helps detect a meniscal tear. Have the patient lie face down on the exam table. Gently grasp the injured leg and bend it to 90°. While maintaining downward pressure on the heel (pressing the patient’s knee against the table), slowly rotate the foot (hence the tibia) first internally then externally on the femur. Pain felt medially suggests a medial meniscus tear. Pain felt laterally suggests a lateral meniscus tear.
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