Although anterior knee pain can occur in anyone, particularly athletes, women who are not athletic clearly are more prone to this problem than men who are not athletic.11 The purpose of the current study is to better understand the nature of anterior knee pain in women and to understand why women are more commonly afflicted with this problem than are men. Psychologic, anatomic, hormonal, postural, and sociologic factors will be considered.
The Nature of Anterior Knee Pain
Anterior knee pain is commonly caused by an abnormal alignment of the knee extensor mechanism. Problems in this area are readily accentuated by physical activity, causing stress on an already distressed patellofemoral joint that is not functioning correctly. Ultimately, imbalance in the knee extensor mechanism can lead to articular cartilage softening, chondromalacia, and eventual loss of articular cartilage attributable to excessive pressure on the lateral patella facet. The distal central aspect of the patella may undergo degeneration related to abnormal shear stress and deficient contact early in knee flexion. Pain can evolve from articular cartilage change and resulting subchondral bone irritation, from synovitis and inflammatory response within the knee, and from retinacular stress causing small nerve injury, particularly in the lateral retinaculum.8
Another common cause of anterior knee pain is direct trauma. Most typically, anterior knee trauma occurs in accidents involving an anterior fall onto the front of the knee with a blow to the patellofemoral joint. Trauma of this nature also can occur in a dashboard injury or with any other direct trauma to the anterior knee. Twisting injuries also can cause excessive loading of the patellofemoral joint and cartilage injury. Most bothersome, however, are the injuries involving direct forceful trauma to the anterior knee. Although cartilage injury at the time of such impact is a clear cause of pain in some patients, retinacular injury around the front of the knee also can be a source of patellofemoral pain that sometimes is overlooked.
The other major cause of anterior knee pain is overuse. Virtually, any athlete or person involved in repetitive activity can experience anterior knee pain related to repetitive stress involving the knee extensor mechanism. Similarly, repeated stress to the lateral knee involving the iliotibial band can cause pain that may mimic patellofemoral pain in some patients. Even repetitive stress around the hip or quadriceps muscle can cause referred pain to the anterior knee. In most patients, particularly athletes, this possible cause of pain may be implied by the nature of the individual's activity.
The most obvious reason why women have more anterior knee pain than men is the difference in lower extremity orientation and alignment. Compared with the pelvis of a male, the pelvis of a female is larger relative to the individual's overall structure. Genetically and developmentally, this is a natural evolution related to childbirth. From the point of view of knee structure and function, however, the broader pelvis moves the hip joints farther lateral relative to the midline and therefore produces an increased valgus angle from the hip to the knee and then to the ground. Bringing lower extremities toward the midline minimizes the energy expenditure necessary for normal walking.7 In addition to a broader pelvis, females have a higher prevalence of increased femoral anteversion, which in turn affects the biomechanics of the patellofemoral joint.18 This increase in femoral anteversion and a wider pelvis also are associated with an increased quadriceps angle (Q angle).12 Because the lower extremity muscles (most notably the quadriceps muscles) follow the orientation of the femur, the knee extensor mechanism in the female generally has a greater valgus orientation than in the male. This valgus load onto the patella creates a normal increase of pressure on the lateral facet of the patella, ultimately leading to the concavity of the lateral facet developmentally during formation of the embryo and thereafter. This trend toward increased lateral thrust on the patella is a significant factor in the difference between men and women regarding anterior knee pain. Although the increased valgus thrust on the patella is not necessarily a problem, it increases the tendency of excessive lateral pressure on the patella, which then can lead to cartilage softening and retinacular stress around the patella and, in some people, anterior knee pain. Therefore it can be seen why women are predisposed to anterior knee pain based on structure alone.
With excessive femoral anteversion, there is also a compensatory increase in external tibial torsion and accommodative pronation of the foot to receive a plantar grade position. This triad is termed the "miserable malalignment".13 Associated features with miserable malalignment syndrome can include a valgus knee, hypermobile patella, and a hypoplastic vastus medialis obliquus muscle. With a pronated foot, there is an accompanying increased rotation of the tibia during the stance phase of running and walking. Some authors think that this increased rotation of the tibia leads to increased patella motion and subsequent patella overuse.13 More recent attention has focused on the cause of miserable malalignment farther up the extremity at the pelvifemoral joint. Miserable malalignment syndrome also can include an anteriorly rotated pelvis, which is associated with a compensatory internal rotation of the femur.4,19 Internal femoral rotation produces dynamic valgus knee alignment and alters the motor or muscle activity of the limb by increasing the demand on the vastus lateralis and iliotibial band. These muscle groups, now aligned more anteriorly on the malrotated limb, act as the primary extensors of the knee. This pelvifemoral dysfunction also can be associated with additional compensatory alterations on the kinetic chain with an increase in lumbar lordosis, an increase in thoracic kyphosis, cervical extension, and a forward thrust of the head. There can be an alteration in the orchestration of neuromuscular control and postural muscle activity, leading to a loss of coordinated muscle activity between the lumbopelvic region and the hip. The individual then is unable to maintain pelvic stability. Although no population-based studies on this miserable malalignment exist, certainly a popular opinion shared by orthopaedists and therapists is that it is more prevalent in females.
Muscular and Strengthening Differences
Bennett et al2 reported the differences in vastus medialis obliquus orientation in females which may make the patella more prone to instability. Diminished support for the patella by the medial dynamic stabilizer and an overall increased valgus thrust places the patella in the female at particular risk for subluxation and excessive strain on the patella stabilizers (retinaculum).
Quadriceps strengthening activities have long been regarded as the cornerstone of treatment for patients with patellofemoral pain and dysfunction. Muscular differences between men and women and how they impact the development of fitness are just beginning to be investigated and recognized. Gender-specific differences in body composition and size have been observed readily and have been attributed to differences in sex hormones on muscular and skeletal development.1 Although studies differ in conclusions, gender-related differences in strength training have been reported.17 Whether gender-related differences in strength training are related to patellofemoral joint pain and injuries is speculative, but certainly the critical role of strength training in treatment of patients with patellofemoral joint disorders is uncontested. Quadriceps strengthening activities have been used, with more recent emphasis on closed-chained activities that emphasize eccentric contractions. More proximal in the lower extremity, muscular maintenance of pelvic stability is key.
The gluteus maximus and hamstrings play an important role in posteriorly rotating the pelvis and controlling limb activity, particularly rotation.10 The hamstrings also may play a role in maintaining limb stability (patella stability) by controlling internal and external rotation of the tibia.19 Abductor strength helps to maintain pelvic stability in single leg stance and control rotational alignment of the limb. These components are critical to analyze in treating patellofemoral pain, particularly in women because of the notable vulnerability of the lower extremity as outlined.
Kowal14 showed that in Army recruits, inadequate conditioning and relatively greater body weight and fat percent and limited leg strength, will predispose women to a greater risk of injury. In a similar study, Cox and Lenz6 also reported that women were more prone to injury relative to lower initial fitness at the time of entering the United States Naval Academy. Cox stated however, that the "number of stress-related visits to sick-call" diminished with time and as the women midshipmen "became accustomed to minor injuries and health problems and learned to ignore them." Conditioning and tolerance of pain, then, have been implicated as factors influencing the increased incidence of anterior knee pain in women.
Other sociologic issues were raised by Bergenudd et al.3 In a study of 574 residents in Malmo, Sweden, the authors found that average income was less in those individuals with knee pain. In another sociologic and demographic study of 2102 people in Bristol, England, McAlindon and coworkers16 established that knee pain is more common in women (27% incidence) and that women were more likely to report a disability, particularly with increasing age. In another study, Cooper et al5 found that repeated knee-bending predisposes the individual to anterior knee pain. One may speculate that secretarial work and clerical work in which women typically have been employed more frequently than men, may introduce other predisposing factors in the incidence of anterior knee pain.
Other characteristics that may predispose women to anterior knee include postural factors, such as sitting in the hip-adducted position. Sitting with legs adducted produces an increased valgus stress at the knee. Because women already have a tendency toward lateral stress on the patella based on pelvic width, sitting with legs adducted additionally accentuates the valgus moment at the knee and may be another contributing factor in causing excessive lateral pressure at the patella and resulting anterior knee pain.
Wearing high heels moves the body center of gravity forward which requires sitting back slightly, thereby necessitating slight knee flexion to maintain balance. This slight knee flexion while wearing high heels, and the adducted knee gait and relatively increased pelvic width, additionally accentuates load on the lateral patella facet during normal gait.
Women experience hormonal fluctuations throughout life that inevitably affect body structure including bone and ligaments. The effects of estrogen, in particular on bone, have been well established.9 Liu et al15 recently established that local estrogen increases can diminish collagen synthesis, thereby reducing connective tissue structure and strength. Although there has been some implication that this may make women more prone to cruciate ligament injury, there has been no specific evidence of an effect on the knee extensor mechanism. In fact, reduction of connective tissue synthesis or increased flexibility around the anterior knee and thereby less patellofemoral compression might even ameliorate patellofemoral pressure. This is an area about which very little is known.
Regarding anterior knee pain, it is most appropriate to recognize male and female differences and how these differences affect pain around the knee. There is little doubt that women differ from men structurally, sociologically, and hormonally. In general, the differences are not subtle. Because the knee is uniquely prone to any changes that affect posture, gait, or lower extremity structure, it is not surprising that women have a higher incidence of anterior knee pain related to the factors outlined in the current study. Although there is little that can be done about the structural differences between men and women, other than conditioning to help increase patella support, it certainly is feasible to modify some sociologic factors. Physical conditioning, in particular, seems to help diminish musculoskeletal disability among women. When anterior knee pain becomes evident, diminishing the use of high heels, adducted lower extremity posture, and aggravating occupational activities should help in the treatment of patellofemoral pain in women. Focus on the kinetic chain in women is particularly important, such that alterations and malfunctions related to structural differences in women might be controlled effectively.
The authors thank Orthopedic Associates Donna Wasowicz and Donna Moriarty for their help in producing this manuscript.
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