TYPES OF KNEE INJURIES & HOW THEY OCCUR
By Matthew Donohoe, MA, ATC; Helen Aslanian, BS; and Kenneth Solomon, PhD, PE, Post PhD
³This article originally appeared in The Forensic Examiner (Vol 14, No. 1). It is being reprinted with permission of
the American College of Forensic Examiners. www.acfei.com, (800) 423-9737²
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The hamstring group is made up of the biceps femoris, semimembranosus, and the semitendinosus. These muscles work together to produce knee flexion and hip extension. The muscles of the hamstring group produce hip extension because they also cross the hip joint and have a proximal attachment on the ischial tuberosity of the pelvis. Only the short head of the biceps femoris does not produce hip extension due to its proximal attachment on the posterior femur.¹⁶

The unclassified muscle group contains the sartorius, gracilis, popliteus, gastrocnemius, and plantar muscles.¹⁵ These muscles have less influence on the knee than the other two muscle groups. They primarily act on another joint, such as the ankle or the hip, and only assist with flexion, extension, and internal and external rotation of the tibia. The popliteus is the one exception to that statement; its primary function is to initiate internal rotation of the knee and unlock the knee at the onset of knee flexion.⁶

There are 13 bursae in the knee joint. A bursa is a sac containing a viscid fluid that helps reduce friction between moving parts. Bursae are usually found over bony prominences and beneath tendons.⁵ The knee bursae are grouped in three general areas: the anterior bursae, medial bursae, and lateral bursae. The anterior bursae consist of the deep infrapatellar bursa, subcutaneous prepatellar bursa, subcutaneous infrapatellar bursa, and suprapatellar bursa. The medial bursae consist of the bursa between the medial head of the gastrocnemius and the fibrous capsule, the bursa between tendons of the semimembranosus and semitendinosus, bursae deep to the tibial collateral ligament, the bursa superficial to the tibial collateral ligament, and the semimembranosus bursa. The lateral bursae consist of the bursa between the fibular collateral ligament and the tendon of the biceps femoris, the bursa between the fibular collateral ligament and the tendon of popliteus, the bursa between the lateral head of the gastrocnemius and joint capsule, and the bursa between the tendon of the popliteus and the lateral femoral condyle.¹⁴

Posterior Cruciate Ligament Rupture
Posterior cruciate ruptures are created when a posterior force is applied to the head of the tibia while the knee is in flexion.¹⁷ Ruptures of the posterior cruciate ligament may also occur during both hyperflexion and hyperextension. Hyperflexion without coupled posterior translation of the tibia often results in an avulsion of the posterior cruciate ligament from the femur.⁷ When ruptures of the posterior cruciate ligament occur as a result of hyperextension, first the anterior cruciate ligament is ruptured, followed by injury to the posterior capsule. Then, at 30 degrees of hyperextension, injury to the posterior cruciate ligament occurs; finally, injury to the poplitieal artery occurs at 50 degrees of hyperextension.⁷

Injury to the posterior cruciate ligament most frequently occurs as a result of motor vehicle accidents and has been referred to as “the dashboard injury.” During a frontal collision, the occupants of the front seat travel forward and strike their knees on the dashboard. This forces the head of the tibia to move posteriorly and causes the posterior cruciate ligament to rupture.⁷ Another common means of rupturing the posterior cruciate ligament is falling onto a flexed knee, which also drives the tibia posteriorly, rupturing the posterior cruciate ligament (Figure 6).²

Medial Collateral Ligament Rupture
Ruptures of the medial collateral ligament are caused by a direct valgus force applied to the knee, as well as by an external rotation of the knee joint (Figure 7).^2,4 A medial collateral ligament injury is usually more severe than a lateral collateral ligament injury because the medial collateral ligament is part of the joint capsule and is attached to the medial meniscus, while the lateral collateral ligament is not.^2,9

Lateral Collateral Ligament Rupture
Ruptures of the lateral collateral ligament are uncommon; however, when they do occur, they are caused by a varus force applied to the knee joint (often coupled with internal rotation of the tibia) (Figure 8) or during complete dislocation of the knee.^2,17 Injury to the lateral collateral ligament is rarely an isolated injury and often is concomitant with stretching or rupturing of the lateral popliteal nerve.¹⁷

Knee Osteoarthritis
Osteoarthritis of the knee is a disease that is characterized by degeneration of the articular cartilage of the joint. Osteoarthritis is also referred to as osteoarthrosis and degenerative joint disease. The degeneration of articular cartilage leads to a loss of shock absorption, which in turn leads to trabecular micro-fractures. Subsequently, the subchondral bone begins to degenerate and osteophytes form at the joint margin.^19,18 The cause of osteoarthritis is not well known; however, risk factors have been established and include age, gender, previous trauma to the joint, and possibly obesity. The prevalence of knee osteoarthritis increases with age because as cartilage ages, it undergoes changes that decrease its ability to withstand compression, putting more stress on the subchondral bone.^8,11 Females seem to be more likely to get osteoarthritis than males, but males tend to get osteoarthritis at a younger age.^1,12 It has been suggested that males get osteoarthritis at a younger age due to the influence of trauma or occupation.¹² Any damage to the joint cartilage, no matter how minimal, can lead to osteoarthritis. This is because cartilage has limited healing abilities due to poor vascularity. Once injured, the degeneration process of the joint cartilage begins and is difficult to halt or reverse.^19,11 The increased body weight of obese individuals increases the stress placed on the cartilage in the joint and can become a contributing biomechanical factor for knee osteoarthritis.¹² Logically, increased loading stress will diminish the capacity for protection of the subchondral bone and injury to these structures is more likely, thus increasing the risk of osteoarthritis.

Injury

Mechanism of Injury

How the Injury May Occur