Knee joint biomechanics

The knee joint acts as a hinge, allowing flexion and extension in the sagittal plane of motion (Fig. 29-1). However, flexion and extension occur about a constantly changing center of rotation, called polycentric rotation.⁴² This allows for a process called femoral rollback. As knee flexion starts in full extension, the femoral condyle begins to roll without sliding, and with further knee flexion sliding becomes more predominant. The knee joint also undergoes axial rotation during flexion and extension. Axial rotation occurs because of a differential radius of curvature between the medial and lateral femoral condyles, the convex shape of the medial tibial plateau, the concave shape of the lateral tibial plateau, and the ability of the MCL to stretch more rapidly than the lateral collateral ligament. As a result, when the knee joint continues to flex throughout the entire range of motion, there is inward rotation of the tibia, whereas the reciprocal is true with knee extension.

Fig. 29-1 A, With knee flexion there is pure sliding of the femur on the tibia. If the knee was restricted to pure sliding, then posterior femoral metaphyseal impinge would prohibit increased knee flexion angles. B, With knee flexion there is pure rolling of the femur on the tibia. If the knee was restricted to pure rolling, then both tibial and femoral contact points would change as the femur rolls on the tibia. The knee would dislocate with increased knee flexion. C, Actual knee motion involves both rolling and sliding.

From Scott WN: The knee, St. Louis, 1994, Mosby.

Brace design

Knee braces are designed to transfer load while allowing for normal knee motion (Fig. 29-2). This depends on the amount of leverage the knee brace can provide. Longer braces produce a greater amount of leverage; therefore, athletes typically select the longest brace that provides the best fit to the extremity.¹⁸ The optimal position to apply the leverage depends on the goal of the knee brace. Prophylactic braces usually are designed with unilateral, single-hinge systems. Some newer designs have bilateral uprights and polycentric hinges. They are used to prevent MCL strain by applying leverage laterally on the femur and tibia to inhibit excessive valgus. Functional knee braces typically are designed with bilateral, polycentric hinge systems. These braces act to prevent recurrent instability in patients with ACL tears. They act by applying leverage to resist abnormal forward translation of the tibia on the femur.³⁵

Fig. 29-2 Knee braces are designed to apply leverage to the extremity by preventing excessive varus/valgus forces and abnormal tibial translation.

From Goldberg B, Hsu JD: Atlas of orthoses and assistive devices, ed 3, St. Louis, 1997, Mosby.

Hinge design

In order for the knee brace to function properly, the knee hinge must recreate polycentric rotation and femoral rollback (Fig. 29-3). Simple unicentric hinges, as used in prophylactic braces, do not allow normal knee joint kinematics. Newer polycentric hinges, as used in functional braces with dual hinges, have been developed that more closely approximate normal knee kinematics.²⁴ Ultimately, optimal brace performance depends on placing the hinge(s) in correct relation to the femoral condyles.⁷⁴

Fig. 29-3 Kinematic hinge designed to replicate femoral rollback with knee flexion.

From Goldberg B, Hsu JD: Atlas of orthoses and assistive devices, ed 3, St. Louis, 1997, Mosby.

There is a great deal of debate on which factor is the most important. Based on mathematical models, some authors have found that the mechanical characteristics of the brace, the structural integrity of the brace, and the interaction of the brace with the limb during loading are the most important.⁵⁷ Others authors have suggested that hinge position is the most important.⁹⁴

Clinical relevance

Knee injuries compose a significant portion of the injuries seen in athletic training rooms, emergency departments, and physician offices. These include injuries to the ACL, PCL, and MCL.

The actual ACL injury rate to the knee remains quite low, approximately 80,000 per year annually in the United States.²⁹ The highest incidence appears to occur among 15- to 25-year-old athletes who participate in pivoting sports. Some studies have suggested that the rate of knee injuries, specifically ACL tears, may be increasing⁸⁸; however, other authors have concluded that the injury rate remains unchanged.¹ Gender differences in the rate of knee injuries do exist. In fact, data suggest that collegiate female athletes participating in the same activity are two to eight times more likely to sustain an ACL injury than are males.^1,6 Some authors have suggested that delays in the treatment of these injuries may be associated with an increased risk of medial meniscus tears.⁶⁶

The amount of force acting on the normal ACL depends upon the activity the knee is experiencing. Forces range between 400 N during normal daily activities to much higher forces during strenuous activity.^70,71 Although the ultimate strength of the ACL is age dependent, the average ultimate strength is 2,100 N.⁹⁷ Some researchers have attempted to use knee braces to limit the force experienced by the ACL in order to prevent injury. However, if the force exerted on the ACL is greater than its ultimate strength, an ACL rupture will still occur.

Approximately 30% of ACL injuries are thought to be due to direct contact between another player or object; the other 70% are believed to be the result of noncontact mechanisms, including pivoting and decelerating activity, “out of control play,” and awkward landings.⁴⁰ Risk factors for noncontact injuries, including environmental, anatomic, hormonal, and biomechanical factors, have been suggested.

Because the ACL is composed of fibrocartilage, it does not have an intrinsic ability to heal itself.⁴⁵ ACL-deficient athletes often note a “giving way” or instability after attempts to return to sport because the ACL is the primary restraint to anterior tibial displacement. As a result of the functional instability, the secondary restraints of the knee are often injured.²³ Current recommendations for return to play in the ACL-deficient athlete mostly include surgical ACL reconstruction because of the repercussions of conservative management of the unstable knee. However, some have used knee braces to facilitate functional return to sport in an ACL-deficient knee and as an adjunct to surgical intervention in the postoperative rehabilitative period.

PCL tears represent approximately 3% to 37% of all knee ligament injuries, depending on the mechanism of injury.^22,67 A PCL injury is sustained as the result of contact (posterior force directed against a flexed knee) and noncontact (hyperextension) mechanisms.⁴⁴ No study has reported a trend toward an increasing incidence or gender difference in PCL injury. The PCL is the primary restraint to posterior tibial displacement and secondary restraint to external rotation.¹⁹ With PCL rupture there is posterior subluxation of the tibia on the femur. This has been shown to result in increased contact pressures in the patellofemoral joint and medial femoral condyle.^60,87 Some older short-term studies have noted that patients with PCL injuries who are treated nonoperatively do well.^28,34,92,93 Other long-term studies have shown that the clinical and radiological incidence of degenerative joint disease increases in proportion to length of time after PCL injury to the knee.^22,26,52,75 Interestingly, the PCL has the intrinsic ability to heal itself because it is made of fibroblastlike cells.^83,84 This may allow for full functional recovery with proper nonoperative management. In some cases this may include the use of rehabilitative braces.⁴⁴ However, one must be aware that the PCL may heal in a lengthened position.³²

The incidence of MCL injury is difficult to determine because of the wide spectrum of injury severity. Additionally, the ACL, PCL, or LCL may be injured concomitantly. Most injuries occur as a result of significant valgus stress applied to the lateral aspect of the knee, as the MCL is the primary medial restraint to valgus loading.⁹⁵ Then, with increasing stress, the ACL and PCL provide medial restraint.^36,77 The MCL also provides resistance to abnormal external tibial rotation.⁴¹

Because the MCL has an intrinsic ability to heal itself, treatment of isolated MCL tears for the past 25 years has been a conservative, nonsurgical approach. Knee braces are used frequently during rehabilitation to facilitate early range of motion while protecting the healing ligament. Some also have recommended a bilateral upright knee orthosis when returning to play.

Historical perspective

Historically, casting was used to treat knee injuries and fractures about the knee.⁵⁹ Unfortunately, some patients had residual knee stiffness and functional instability of the knee joint.⁵³ Knee orthoses originally were designed to treat congenital and acquired deformities around the knee, including genu varum and quadriceps paralysis in polio patients. More recently, Sarmiento⁸¹ used knee orthoses in a rehabilitative setting to treat fractures about the knee while preventing loss of range of motion. In the 1960s and 1970s, physicians began using orthoses to treat athletes with functional knee instability from ACL injuries.²⁵ In the late 1970s, Anderson et al.⁵ modified a rehabilitative brace, the Anderson Knee Stabler, to prevent recurrent injury to the medial collateral ligament. Physicians, coaches, and players then started to use these braces to prevent knee injuries.

In the mid-1980s, some of the negative aspects of brace wear began to emerge, and assumptions were made that knee braces were bad. More recent epidemiological studies have not shown any negative consequences of brace wear.

Currents issues regarding knee braces are multifaceted, including whether or not to wear a brace in a prophylactic, functional, or rehabilitative capacity and how long to wear these braces, whether the performance inhibition is worth the perceived decreased risk of injury, and whether an off-the- shelf or custom design is better.

Prophylactic braces

Prophylactic knee braces have been used mostly by football and lacrosse players to prevent MCL and ACL injury. The prophylactic knee braces attempt to limit the strain on the MCL and ACL by redirecting a lateral impact force away from the joint line to points more distal on the tibia or femur (Fig. 29-4).³³ There is a great deal of controversy surrounding the use of a prophylactic knee brace in sport.

Fig. 29-4 McDavid lateral hinge prophylactic knee brace.

From Insall J, Scott W: Surgery of the knee, ed 3, Philadelphia, 2001, Churchill Livingstone.