Anterior Cruciate Ligament Rupture Case File

Anterior Cruciate Ligament Rupture Case File

Eugene C. Toy, MD, Lawrence M. Ross, MD, PhD, Han Zhang, MD, Cristo Papasakelariou, MD, FACOG

CASE 7

A 25-year-old woman is on her first ski trip to Colorado. She has advanced from the bunny (gentle) slopes and, during the last run of the day, falls and twists her right leg. She cannot stand on her right leg because of pain and is brought down the hill in a snowmobile. On examination, the right knee is swollen and tender. With the patient sitting on the stretcher with her knee flexed, the lower leg seems to have several centimeters of excess anterior mobility.

⯈ What is the most likely diagnosis?

⯈ What is the mechanism of the injury?

ANSWER TO CASE 7:

Anterior Cruciate Ligament Rupture

Summary: A 25-year-old woman twists her right lower limb in a ski accident. She has right knee swelling and tenderness, and excessive anterior mobility with the knee flexed.

• Most likely diagnosis: Anterior cruciate ligament (ACL) tear

• Mechanism of injury: Excessive rotational force strains or ruptures the ligament

CLINICAL CORRELATION

Injuries to the knee are very common because it bears weight, combines mobility in flexion and extension, and allows some rotation. The stability of the knee depends entirely on its ligaments and muscles. Sports injuries to the knee are most commonly caused by high-speed and rotational forces applied to the leg through the knee joint. In addition, certain ligaments are anatomically related to the menisci, on which the distal femur articulates. This 25-year-old woman was involved in a ski injury, a common setting for ACL injury. The twisting force to the lower limb when a ski becomes lodged in snow and the body continues to rotate can produce significant trauma to the knee. The ACL passes from the posterior aspect of the distal femur to the intercondylar region of the anterior aspect of the proximal tibia; it limits anterior movement of the tibia in relation to the femur. Thus, on examination, this patient exhibits the “anterior drawer sign,” or excessive anterior mobility of the tibia with the knee flexed. This injury will usually require surgical repair.

APPROACH TO:

The Knee Joint

OBJECTIVES

1. Be able to describe the anatomy of the knee joint, including the bones, ligaments, possible movements, and muscles responsible for these movements

2. Be able to describe the mechanism of injury to the four main ligaments of the knee

DEFINITIONS

Knee: Hinge joint between the femur and proximal tibia

Patella: A triangular bone approximately 5 cm in diameter situated in the front of the knee at the insertion of the quadriceps tendons

Meniscus: Crescent-shaped intraarticular cartilage

DISCUSSION

The knee joint is a synovial hinge joint formed by the distal femur, the proximal tibia, and the patella. It is a relatively stable joint; its movements consist primarily of flexion and extension, with some gliding, rolling, and locking rotation. The distal femur forms two large knuckle-like lateral and medial condyles, which articulate with lateral and medial tibial condyles. The superior surfaces of the tibial condyles are flattened to form the tibial plateau. An intercondylar eminence fits between the femoral condyles, and the proximal fibula articulates with the lateral tibial condyle but is not a part of the knee joint. The patella articulates with the femur anteriorly. The flat tibial condylar surfaces are modified to accommodate the femoral condyles by the C-shaped lateral and medial menisci. These fibrocartilaginous structures are wedge-shaped in cross section; they are thick peripherally but thin internally, are firmly attached to the tibial condyles, and serve as shock absorbers. The lateral meniscus is the smaller of the two, and is somewhat circular, whereas the medial meniscus is C-shaped. The femoral and remaining portions of the tibial condyles are covered with articular cartilage (Figure 7-1).

The knee joint is surrounded by a capsule, lined with synovial membrane, and reinforced by several ligamentous thickenings. Anteriorly, the patella is embedded within the tendon of the quadriceps femoris muscle group. Inferior to the patella, the tendon becomes the patellar ligament, which inserts into the tibial tuberosity. Laterally, the capsule is thickened to form the fibular (lateral) collateral ligament from the lateral femoral epicondyle to the fibular head. The fibular collateral ligament remains separated from the lateral meniscus by the tendon of the popliteus muscle. It prevents increase of the lateral angle or adduction of the leg at the knee. The tibial (medial) collateral ligament extends from the medial femoral epicondyle to the medial tibial condyle. The deep aspect of this ligament is firmly attached to the margin of the medial meniscus. It prevents increase of the medial angle or abduction of the leg at the knee. Posteriorly, the capsule is reinforced by oblique and arcuate popliteal ligaments. The knee is unique because of the presence of two intraarticular ligaments: the ACL and the posterior cruciate ligament (PCL). The cruciate ligaments are covered by synovial membrane and thus are external to the synovial cavity and are named for their attachment to the tibia. The ACL extends from the anterior tibial plateau near the intercondylar eminence to the posteromedial aspect of the lateral femoral condyle. It limits anterior displacement of the tibial in relation to the femur and limits hyperextension. The PCL extends from the posterior aspect of the tibial plateau to the anterolateral aspect of the medial femoral condyle. In its course, it crosses the ACL on its medial side and is larger and stronger than the ACL. The PCL limits posterior displacement of the tibia on the femur and limits hyperflexion. A dozen or so bursae are associated with the knee joint, and four of these communicate with the synovial cavity of the joint: suprapatellar, popliteus, anserine, and gastrocnemius. Thus, inflammation of any of these bursae (bursitis) will likely result in swelling of the entire knee joint. The knee joint is richly supplied by several genicular and recurrent arteries from the femoral, popliteal, and anterior tibial arteries.

Additional strength and stability to the knee joint are provided by the muscles that cross and produce movement at the joint. The action and innervation of these muscles are listed in Table 7-1.

Figure 7-1. The knee joint. Ligaments of the knee in full extension (a). The superior aspect of the knee showing the menisci (b). (Reproduced, with permission, from Lindner HH. Clinical Anatomy. East Norwalk, CT: Appleton & Lange, 1989:615.)

An abnormal force applied to the lateral aspect of the knee with the foot planted stretches the tibial (medial) collateral ligament, causing a sprain or, if sufficiently forceful, rupture of this ligament. The exposed lateral knee makes this injury more frequent. Because the medial meniscus is firmly attached to the deep surface of the tibial collateral ligament, it also is frequently damaged. Forces applied to the medial aspect of the knee can damage the fibular (lateral) collateral ligament in a similar manner. However, because the lateral meniscus is not attached to the ligament, 

Table 7-1 • MUSCLES ACTING ON THE KNEE JOINT
	Muscles	Innervation
Extension	Quadriceps group: rectus femoris, vastus lateralis, interomedialis, and medialis	Femoral nerve
Flexion	Hamstring group: semitendinosus, semimembranosus, and biceps femoris	Tibial portion of sciatic nerve
Lateral rotation	Biceps femoris	Tibial portion of sciatic nerve
Medial rotation	Popliteus, semitendinosus, and semimembranosus	Tibial nerve

it typically is not damaged. Excessive force to the anterior aspect of the tibia will move it posteriorly, thus stretching or tearing the PCL. The ACL is most often damaged when forces or activities produce hyperextension of the knee.

COMPREHENSION QUESTIONS

7.1 Your patient has sustained an external force to the knee. Which of the following ligaments has prevented abduction of the leg at the knee?

A. Oblique popliteal

B. Anterior cruciate

C. Posterior cruciate

D. Lateral collateral

E. Medial collateral

7.2 In this same patient, which of the following ligaments prevented posterior displacement of the tibia on the femur?

A. Oblique popliteal

B. Anterior cruciate

C. Posterior cruciate

D. Lateral collateral

E. Medial collateral

7.3 You have examined a patient and find that there is weakness in the ability to flex the knee. This indicates a problem with which of the following nerves?

A. Femoral nerve

B. Tibial nerve

C. Common fibular nerve

D. Deep fibular nerve

E. Superficial fibular nerve

ANSWERS

7.1 E. Abduction of the leg at the knee is limited by the medial or tibial collateral ligament.

7.2 C. Posterior displacement of the tibia on the femur is limited by the PCL.

7.3 B. The muscles that flex the knee are innervated by the tibial portion of the sciatic nerve.

ANATOMY PEARLS

⯈ The anterior cruciate ligament is so named because it is attached to the anterior aspect of the tibia and prevents anterior displacement of the tibia on the femur.

⯈ The tibial collateral ligament is attached to the medial meniscus; thus, both are often injured by an abnormal force to the lateral knee.

⯈ The ACL is injured most often by hyperextension of the knee.

References

Gilroy RM, MacPherson BR, Ross LM. Atlas of Anatomy, 2nd ed. New York, NY: Thieme Medical Publishers; 2012:406−414. 

Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 7th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:634−645, 662−663. 

Netter FH. Atlas of Human Anatomy, 6th ed. Philadelphia, PA: Saunders; 2014: plates 494−498.

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