Orthopaedic Surgery - Knee Anatomy and Examination

Basics

The knee is a complex synovial joint composed of bone, articular cartilage, menisci, ligaments, tendons, muscles, nerves, and vascular structures.

A systematic understanding of this anatomy is essential for interpreting pain, instability, swelling, mechanical symptoms, and loss of motion.

Bony Anatomy

The principal bones forming the knee are the:

Femur, tibia, and patella.

Femur

The medial and lateral femoral condyles articulate with the tibial plateau to form the tibiofemoral joint.

Anteriorly, the femoral trochlea articulates with the patella.

Tibia

The medial and lateral tibial plateaus articulate with the corresponding femoral condyles.

The tibia transmits the majority of axial load through the knee.

Patella

The patella articulates with the femoral trochlea and is embedded within the extensor mechanism.

It acts as a fulcrum that increases the mechanical advantage of the quadriceps, thereby improving knee-extension strength.

Articular Cartilage

Hyaline cartilage covers the articulating surfaces of the:

Femoral condyles, tibial plateaus, trochlea, and posterior patellar surface.

Loss or injury of this cartilage may produce pain, crepitus, swelling, and degenerative arthritis.

Menisci

The medial and lateral menisci are fibrocartilaginous structures located between the femoral condyles and tibial plateaus.

They contribute to:

Load distribution, shock absorption, joint congruity, stability, lubrication, and protection of the articular cartilage.

Medial Meniscus

The medial meniscus is relatively firmly attached to the joint capsule and deep medial collateral ligament, making it less mobile than the lateral meniscus.

Lateral Meniscus

The lateral meniscus is more mobile and covers a larger proportion of the lateral tibial plateau.

Extensor Mechanism

The extensor mechanism consists primarily of:

Quadriceps muscle and tendon, patella, patellar tendon, and tibial tuberosity.

Quadriceps Tendon

The quadriceps tendon inserts on the superior pole of the patella.

Patellar Tendon

The patellar tendon extends from the inferior pole of the patella to the tibial tuberosity.

Together with the quadriceps tendon and patella, it transmits quadriceps force to extend the knee.

Anterior Cruciate Ligament

The ACL originates from the posteromedial surface of the lateral femoral condyle and inserts on the anterior intercondylar region of the tibia.

Its major function is to resist:

Anterior translation of the tibia relative to the femur.

It also contributes to rotational stability.

Posterior Cruciate Ligament

The PCL originates from the anterolateral surface of the medial femoral condyle and inserts on the posterior intercondylar region of the tibia.

Its primary role is to resist:

Posterior translation of the tibia relative to the femur, particularly during knee flexion.

Medial Collateral Ligament

The MCL originates near the medial femoral epicondyle and inserts along the medial proximal tibia.

Its deep fibers have connections with the medial capsule and medial meniscus.

The MCL is the principal restraint to valgus stress.

Lateral Collateral Ligament and Posterolateral Corner

The LCL originates from the lateral femoral epicondyle and inserts on the fibular head.

Together with the posterolateral corner structures, it helps resist:

Varus stress, external rotation of the tibia, and posterolateral instability.

The posterolateral corner is a complex of several stabilizing structures rather than the LCL alone.

Muscles

Knee Extensors

The quadriceps muscle group consists of:

Rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis.

These muscles extend the knee.

Knee Flexors

Major flexors include the:

Biceps femoris, semimembranosus, semitendinosus, and gracilis.

Other muscles also contribute to knee flexion and rotation.

Nerves

Femoral Nerve

The femoral nerve supplies the quadriceps and is therefore critical to active knee extension.

Sciatic Nerve

The sciatic nerve travels posteriorly in the thigh and divides into the tibial and common peroneal nerves near the knee.

Common Peroneal Nerve

The common peroneal nerve passes laterally around the fibular neck, where it is relatively superficial and vulnerable to injury.

Diagnosis and Examination

A complete knee examination should follow a consistent sequence and should generally include:

Inspection, gait assessment, range of motion, palpation, evaluation for effusion, ligament testing, meniscal testing, patellofemoral assessment, and a complete neurovascular examination.

Initial Assessment

The unaffected knee should be examined for comparison.

The clinician should also consider referred pain from the:

Hip, lumbar spine, or surrounding soft tissues.

Neurovascular Examination

Motor function, sensation, peripheral pulses, and capillary refill should be assessed when appropriate.

Range of Motion

Normal knee motion is approximately:

0–140°, although values vary with age and body habitus.

Many individuals have a small degree of physiologic hyperextension.

Flexion Contracture

A flexion contracture is present when the knee cannot reach full extension either actively or passively.

Extension Lag

An extension lag exists when:

Full passive extension is possible, but the patient cannot actively achieve it.

This may indicate weakness or disruption of the extensor mechanism.

Symmetry

Knee motion should be compared with the contralateral side.

Inspection

Observe for:

Erythema, swelling, effusion, abrasions, scars, deformity, muscle wasting, and abnormal patellar position.

Muscle Atrophy

Quadriceps wasting may accompany chronic knee pain, instability, arthritis, or prolonged immobilization.

Alignment

Standing alignment should be assessed in the coronal and rotational planes.

Varus and Valgus

Physiologic lower-extremity alignment usually includes several degrees of valgus.

Historical averages are approximately:

5° of valgus in males and 7° in females, although individual variation is substantial.

Rotational Alignment

The direction of the patellae and feet during standing and walking can provide clues about rotational alignment of the lower extremities.

Gait Examination

The patient should be observed walking whenever possible.

Antalgic Gait

An antalgic gait is characterized by:

Reduced stance time on the painful side and shortened stride length.

Palpation

With the knee flexed to approximately 90°, systematically palpate:

Quadriceps tendon, patella, patellar tendon, medial and lateral joint lines, MCL, LCL, tibial tuberosity, iliotibial band, femoral epicondyles, and other areas of reported tenderness.

The location of tenderness should be correlated with underlying anatomy.

Knee Effusion

An intra-articular effusion may result from:

Hemarthrosis, inflammation, infection, arthritis, meniscal injury, ligament injury, or intra-articular fracture.

Hemarthrosis

Acute hemarthrosis may occur with:

ACL rupture, patellar dislocation, intra-articular fracture, or major meniscal or ligament injury.

Warmth

Increased warmth may indicate active inflammation or infection.

Patellar Ballottement / Blot Test

The patella is gently compressed toward the femoral trochlea.

With a substantial effusion, the patella may feel as though it floats or rebounds as fluid redistributes within the joint.

Prepatellar Bursitis

Prepatellar bursitis presents as:

Localized swelling directly anterior to the patella, often accompanied by anterior knee pain and tenderness.

The swelling is extra-articular.

Popliteal or Baker Cyst

A Baker cyst appears as swelling or fullness in the popliteal fossa.

It often reflects underlying intra-articular pathology such as:

Arthritis, meniscal disease, or chronic synovitis.

Iliotibial Band Syndrome

Iliotibial band syndrome commonly produces:

Lateral knee pain over the lateral femoral epicondyle, particularly in runners and cyclists.

It is usually an overuse condition.

Ober Test

The Ober test assesses iliotibial band tightness.

Technique

The patient lies on the unaffected side.

The pelvis is stabilized while the affected hip is abducted and extended.

The examiner then allows the leg to move toward adduction.

Positive Test

If the leg remains abducted rather than dropping toward the table, iliotibial band tightness is suggested.

Lateral knee pain may also be reproduced.

Extensor Mechanism Injury

Patellar fracture, quadriceps tendon rupture, and patellar tendon rupture may disrupt active knee extension.

Findings

Potential findings include:

Palpable tendon defect, extension lag, abnormal patellar position, and inability to perform an active straight-leg raise or actively extend the knee.

Patellofemoral Pain

Patellofemoral disorders commonly produce anterior knee pain.

Symptoms are often aggravated by:

Running, stair climbing, squatting, prolonged sitting, or rising from a seated position.

Q Angle

The Q angle is formed by the intersection of:

A line from the anterior superior iliac spine to the center of the patella, and a line from the patella to the tibial tuberosity.

Historically, an angle greater than approximately 15° has been associated with patellofemoral symptoms, although Q angle alone is a limited predictor of pathology.

Patellar Glide Test

With the knee extended or slightly flexed, the patella is translated medially and laterally.

Patellar mobility may be described in quadrants.

Excessive Translation

Translation greater than approximately two quadrants may suggest patellar hypermobility or instability, especially when asymmetric.

Patellar Apprehension Test

With the knee relaxed, the patella is translated laterally.

A positive test occurs when the patient develops marked apprehension or attempts to stop the maneuver.

This suggests lateral patellar instability.

Patellar Grind Test

The patella is compressed against the trochlea while the patient contracts the quadriceps.

Pain or crepitus may be produced in patients with patellofemoral pathology.

Because this maneuver can be uncomfortable even in normal knees, it should be interpreted cautiously.

Ligamentous Instability

The mechanism of injury often suggests which ligament is injured.

ACL Mechanism

ACL rupture commonly follows a:

Noncontact pivoting, cutting, or twisting injury with the foot planted, sometimes accompanied by an audible or palpable “pop.”

An acute hemarthrosis frequently develops.

PCL Mechanism

PCL injury classically follows a posteriorly directed force on the proximal tibia with the knee flexed, as in a dashboard injury.

MCL Mechanism

MCL injury typically results from a valgus force applied to the lateral side of the knee.

Symptoms of Instability

Patients may describe the knee as:

Giving way, buckling, or shifting, particularly during cutting or pivoting activities.

Varus and Valgus Stress Tests

These tests evaluate the collateral ligaments.

Technique

Apply valgus or varus stress with the knee:

In full extension and at approximately 30° of flexion.

Testing at 30° better isolates the collateral ligaments, whereas instability in full extension suggests injury to additional capsular or cruciate structures.

Grading

Traditional grading based on joint opening is:

Grade 1: less than 5 mm.

Grade 2: approximately 5–10 mm.

Grade 3: greater than 10 mm.

Comparison with the opposite knee is important.

Lachman Test

The Lachman test is one of the most sensitive clinical tests for ACL insufficiency.

Technique

With the knee relaxed at approximately 20–30° of flexion, stabilize the distal femur with one hand and translate the proximal tibia anteriorly with the other.

Positive Test

A positive test demonstrates:

Increased anterior translation and/or a soft or absent endpoint compared with the opposite knee.

Historically, more than approximately 5 mm of translation or more than 3 mm of side-to-side difference has been considered abnormal.

Grading

Grade 1: less than 5 mm.

Grade 2: 5–10 mm.

Grade 3: greater than 10 mm.

The endpoint may be described as:

A = firm endpoint

B = soft endpoint

Anterior Drawer Test

The anterior drawer test assesses ACL integrity.

Technique

With the knee flexed to 90° and the foot stabilized, grasp the proximal tibia and pull it anteriorly.

Excessive anterior translation suggests ACL insufficiency.

Posterior Drawer Test

With the same position, the tibia is pushed posteriorly.

Excessive posterior translation indicates PCL insufficiency.

Drawer Grading

Traditional grading is:

Grade 1: less than 5 mm.

Grade 2: 5–10 mm.

Grade 3: greater than 10 mm.

Dial Test

The dial test assesses posterolateral rotational instability.

Technique

The patient is often positioned prone.

Externally rotate both feet and compare the thigh-foot angles at approximately:

30° and 90° of knee flexion.

Positive Test

An increase in external rotation greater than approximately 10° compared with the opposite side is considered abnormal.

Interpretation

Increased external rotation primarily at 30° suggests an isolated posterolateral corner injury.

Increased rotation at both 30° and 90° suggests combined posterolateral corner and PCL injury.

Meniscal Pathology

Meniscal tears commonly cause:

Joint-line pain, swelling, catching, clicking, locking, or episodic mechanical symptoms.

A displaced tear such as a bucket-handle tear may produce a knee locked in flexion.

Joint-Line Tenderness

Tenderness is best assessed with the knee flexed to approximately 90°.

Focal tenderness along the affected joint line supports a meniscal lesion.

McMurray Test

The McMurray test attempts to reproduce symptoms from a meniscal tear during knee rotation and extension.

Medial Meniscus

To test the medial meniscus:

Flex the knee fully, externally rotate the tibia, and apply a valgus stress while gradually extending the knee.

Palpate the medial joint line.

Lateral Meniscus

To test the lateral meniscus:

Flex the knee, internally rotate the tibia, and apply a varus stress while extending the knee.

Palpate the lateral joint line.

Positive Test

A reproducible:

Painful click, clunk, or mechanical sensation at the joint line

suggests meniscal pathology.

Apley Compression Test

The patient lies prone with the knee flexed to approximately 90°.

Axial compression is applied through the heel while the tibia is internally and externally rotated.

Interpretation

Pain at the:

Medial joint line suggests possible medial meniscal injury.

Pain at the:

Lateral joint line suggests possible lateral meniscal injury.

The test should be interpreted together with the history and other examination findings.

Knee Arthritis

Patients with knee arthritis commonly report:

Activity-related pain, stiffness, swelling, loss of motion, and reduced walking tolerance.

Physical Findings

Examination may demonstrate:

Crepitus, palpable osteophytes, effusion, flexion contracture, reduced range of motion, deformity, and functional limitation.

Pseudolaxity

Advanced compartmental cartilage loss may create apparent varus or valgus laxity.

This is called pseudolaxity because the apparent opening results from joint-space loss and deformity rather than true ligament rupture.

Imaging in Arthritis

Radiographs may demonstrate:

Joint-space narrowing, osteophytes, subchondral sclerosis, cystic change, and deformity.

Weight-bearing radiographs are especially useful for evaluating compartmental joint-space loss.

General Examination Strategy

A complete knee assessment should integrate:

History, mechanism of injury, gait, alignment, range of motion, effusion, tenderness, patellofemoral examination, ligament testing, meniscal testing, and neurovascular status.

No single special test should be interpreted in isolation.

The most reliable diagnosis comes from correlating the history, examination findings, and appropriate imaging.