NONTRAUMATIC KNEE INJURIES
Cindy Y. Lin and Kelvin T. Chew
While sex differences in traumatic knee injuries are more commonly described in the literature, a subset of nontraumatic knee injuries also have demonstrated sex differences that are important to note in the athletic population. Patellofemoral pain syndrome (PFPS), iliotibial band syndrome (ITBS) and pes anserine tendinitis bursitis (PATB) are common nontraumatic knee injuries typically related to overuse. Gender differences have been described in their incidence and risk factors (see the table in the Conclusion section). These differences should be considered in the evaluation and treatment of common nontraumatic knee injuries in a sports medicine population.
Patellofemoral Pain Syndrome
PFPS is anterior knee pain characterized by retropatellar or peripatellar discomfort (1). Symptoms are typically provoked when the patellofemoral joint is loaded, such as in stair climbing, running, cycling, squatting, or rising from a seated position.
Several studies report a higher incidence of PFPS in female than in male athletes, while other studies report a similar incidence in both sexes, such as in military populations (2–8). In a retrospective study of over 4,000 runners, PFPS accounted for more injuries in women than in men (2). PFPS comprised 19.6% of all injuries in females compared to 7.4% of males presenting to an academic sports medicine center (3). In a retrospective review, Taunton et al. reported a PFPS prevalence approximately 1.5 times greater in females than in males (4). A prospective study of 810 adolescent basketball players also found that patellofemoral dysfunction was significantly more common in females than in males (7.3% versus 1.2%; P < .05) (5).
PFPS prevalence and incidence has also been examined in military populations where the literature is mixed. In a prospective study of United States Naval Academy enrollees, there was no statistically significant sex difference in PFPS rates on enrollment; however, females were 2.23 times more likely to develop PFPS (95% confidence interval [CI]: 1.19–4.20) compared with males over 2.5 years of follow-up (6). In contrast, a study of 97,000 Israeli defense force recruits found that males had a 4.56% prevalence of anterior knee pain compared to 2.39% in females (7). Almeida et al. reported that female military recruits reported more musculoskeletal injuries than males (relative risk [RR]: 1.72, 95% CI: 1.29–2.30) (8). Furthermore, PFPS was the most commonly reported injury affecting 10% of female recruits (8). However, when reported and unreported musculoskeletal injuries were combined, total injury rates were similar between the groups, indicating possible sex differences in symptom reporting (8).
PFPS pathogenesis is multifactorial in origin and relates to several intrinsic and extrinsic risk factors (1). Intrinsic risk factors include anatomic, biomechanical, and hormonal factors and abnormal neuromuscular control, while extrinsic risk factors include training and environmental conditions.
Sex differences in PFPS incidence have been evaluated with respect to proximal and distal kinetic chain factors that contribute to dynamic maltracking of the patellofemoral joint. Abnormal or lateral patellar tracking results in decreased patellofemoral contact area and causes the same amount of patellofemoral force to be applied over a smaller area (see Figure 8.1).
Historically, the wider pelvis in women has been associated with a larger quadriceps femoris muscle angle (Q angle), which is defined as the acute angle formed by the vector for the combined pull of the quadriceps femoris muscle and the patellar tendon (9). The resultant abnormal patellar tracking is believed to play a role in PFPS development. Horton and Hall reported a mean Q angle for women of 15.8 ± 45° and for men of 11.2 ± 30° (10). Several studies have cited a Q angle greater than 20° as abnormal (11,12). The larger Q angle of women has been associated with greater dynamic lateral patellar shift during quadriceps contraction (10), thereby making them more prone to developing PFPS.
However, recent studies suggest that a greater Q angle as well as other anatomic factors may not be as significant of risk factors as previously thought for PFPS (13–15). Contrary to conventional belief, a study by Park and Stefanyshyn found a negative correlation between Q angle and the magnitude of peak knee abduction moment (P = .005) (15). Further substantiating this discovery was an MRI study that found that increased Q angle correlates with medial rather than lateral patellar displacement and tilt (13). In a study of women with PFPS compared to controls, no significant difference was identified between the patients’ symptomatic versus nonsymptomatic knee, nor between the patients’ and controls’ Q angle and leg-heel alignment measurements (16). Another prospective study by Barber Foss et al. found no relationship between relative body composition and relative body mass to height ratio and the development of PFPS in middle school-age female basketball players (17).
Muscle imbalance in terms of strength and flexibility differences have also been described inconsistently as a risk factor for PFPS in females (1). The greater flexibility and generalized ligamentous laxity of women, as well as the higher incidence of benign hypermobility syndrome, is a risk factor for patellar hypermobility (18–21). Patellar hypermobility may lead to PFPS due to the increased tendency for patellar maltracking.
While local factors that influence patellofemoral biomechanics such as patellar hypermobility, lateral patellar tilt, and vastus medialis strength have been well described in the literature (22), recent studies have highlighted the importance of proximal kinetic chain factors.
Hip strength deficits are another identified risk factor for PFPS in women. When hip isometric strength was measured using a handheld dynamometer in females with and without PFPS, the hip adductor/abductor isometric strength ratio in the PFPS group was 23% higher than the control group (P = .01), suggesting a tendency towards increased dynamic knee valgus and patellofemoral loading in this group (23). Lower hip abduction strength, lower knee extension peak torque, and less hip external rotation strength have also been reported in females with PFPS compared to controls (24). Females with PFPS had less gluteus medius muscle activation (P = .017) during the single-leg squat compared to controls (25). Prins and van der Wurff also found that females with PFPS had decreased hip abduction, external rotation, and extension strength compared to controls (26). However, a study by Thijs et al. found no significant difference in baseline isometric strength of the hip flexor, extensor, abductor, adductor, and external and internal rotator muscles between healthy female novice runners who did and did not develop PFPS (27). Hip strength deficits in men with PFPS have not been identified consistently (28). While some studies may disagree, the current body of evidence suggests that evaluating and addressing proximal kinetic chain deficits in hip muscle strength is important in the rehabilitation of PFPS in males and females.
Sex differences in lower limb dynamic alignment, strength, and central motor planning may contribute to altered neuromuscular control patterns during activities such as single-leg squat, running, and jump landings. Reduced lower limb frontal plane neuromuscular control has been described in female athletes (29). Females with PFPS had more hip internal rotation (P < .05) during the single-leg squat compared to controls (30). In adolescent girls, elevated (greater than 15 Nm) knee abduction load during jump landing was associated with a greater likelihood of developing PFPS (31). A prospective study of male military recruits found that those who developed PFPS had significant delay of vastus medialis obliquus electromyographic activity onset compared with controls (P = .023), even before basic military training (32). This finding suggests that altered quadriceps muscle activation timing may be a risk factor for PFPS in men (32).
Running gait mechanic differences have been evaluated as a possible risk factor for PFPS. Females with PFPS demonstrated greater hip internal rotation (P = .04) compared to controls when ground reaction forces were greatest during running, suggesting that altered transverse plane hip kinematics may be involved in the etiology or exacerbation of PFPS (33). When running mechanics were compared, males with PFPS ran and squatted with greater peak knee adduction and demonstrated a greater peak knee external adduction moment compared with healthy male controls. In addition, males with PFPS ran and squatted with less peak hip adduction and greater peak knee adduction compared with females with PFPS (34).
The role of menstrual phase and hormonal factors in the development of PFPS is inconclusive, although preliminary studies suggest a possible link (35,36). Menstrual cycle fluctuations in female sex hormones may influence ligament mechanical properties and increased knee laxity has been related to ACL injuries (35). Tenan et al. found that vastus medialis and vastus medialis oblique initial firing rates differ across the menstrual cycle during an isometric ramp knee extension exercise (36). However, whether these hormonal factors relate to PFPS needs further investigation.
Potential extrinsic risk factors that might contribute to sex differences in PFPS include training volume, intensity, type, and environment. As PFPS pain is typically associated with increased activities or athletic training, some studies suggest that the cause of PFPS is overloading and overuse of the patellofemoral joint, rather than anatomic or biomechanical malalignment (16). Abrupt increases in running mileage have been identified as a risk factor for PFPS (37). Early specialization in a single sport also increases the relative risk of PFPS incidence by 1.5 fold (95% CI: 1.0–2.2; P = .038) in adolescent females (38). These studies highlight the importance of identifying training factors during history taking in both sexes when PFPS is suspected.
PFPS is a clinical diagnosis based on history and physical exam. However, no single physical exam maneuver reliably confirms PFPS, and the reliability of individual tests for PFPS is generally low (39). Sex differences in the physical exam for PFPS have not been well described in the literature. The history taking should include evaluation of common intrinsic and extrinsic risk factors described previously. The physical exam of any patient with suspected PFPS should include evaluation of anatomic factors as well as static and dynamic alignment of the lower limb kinetic chain. On standing examination, it is helpful to evaluate for abnormal foot position such as pes planus. Navicular drop, a measure of foot pronation, has been identified as a risk factor for running-related injuries in novice female but not in novice male runners (40).
Dynamic patellar tracking and stability is best evaluated by a functional test such as single-leg partial squat (see Figure 8.2), step up or step down, single-leg sit to stand from a chair, or a sports-specific movement that is associated with symptom onset. Performing a functional, weight-bearing activity test allows for a visual assessment of patellar kinematics over a range of knee flexion angles (41). A running gait evaluation via a video gait analysis may be helpful to identify factors related to running mechanics. Abnormal biomechanics at the hip, knee, foot, or ankle joint should be identified and addressed in the rehabilitation of PFPS as these factors can contribute to dynamic knee valgus. If generalized ligamentous laxity is suspected, the Beighton score evaluation is useful. One should also assess for tightness of the iliotibial band and quadriceps and hip abductor strength (39).
Preparticipation screening may be useful for identifying athletes at risk for PFPS and other common musculoskeletal injuries. In a preparticipation screening study of middle and high school female basketball players, 26.6% had anterior knee pain (42). Risk factors, prior injuries, muscle imbalance, or functional deficits identified during the screening can be used to guide a prehabilitation injury prevention program (43). Waryasz and McDermott suggest that prehabilitation programs may help prevent PFPS (44). The role of prehabilitation programs in preventing anterior cruciate ligament (ACL) injuries in females is described in Chapter 9 on traumatic injuries of the knee; this framework may be a useful guide for PFPS prevention as dynamic knee valgus, which is a risk factor for ACL injuries, also increases patellofemoral loading.
As PFPS is a clinical diagnosis based on history and physical examination, diagnostic imaging is typically not indicated. Sex differences in the indications for diagnostic imaging in PFPS have not been described. Radiographs may help identify excessive patellar tilt, patellar alta, or trochlear dysplasia, as these factors may predispose patellar instability (45).
Treatment and Rehabilitation
The treatment of PFPS should be individualized based on modifiable risk factors identified during the history and physical examination. Exercise therapy is generally beneficial in reducing pain and improving functional outcomes in PFPS (46,47). Sex differences in PFPS risk factors suggest a potential benefit of sex-specific interventions for individuals with PFPS.
Hip abductor strengthening and neuromuscular control training should be included in the rehabilitation of females with PFPS (31). Knee exercises supplemented by hip posterolateral muscle-strengthening exercises were more effective than knee exercises alone in improving long-term function and reducing pain in sedentary women with PFPS (48). Another study of women with PFPS also found that a functional stabilization program including hip muscle strengthening and lower-limb and trunk movement control exercises was more beneficial in improving pain, physical function, and muscle strength compared to a program of quadriceps-strengthening exercises alone (49).
Pain can limit the ability of a person with PFPS to participate actively in rehabilitation. An exercise progression involving 4 weeks of initial hip strengthening has been found to result in earlier pain reduction than exercises focused on the quadriceps alone in females with PFPS (50). Adolescent females with PFPS have lower mechanical pressure pain thresholds, characterized by localized and distal hyperalgesia, than those with no musculoskeletal pain (51). These findings may have implications for treating PFPS, as both peripheral and central mechanisms may influence pain sensitization in females.
Iliotibial Band Syndrome
ITBS is well described in the active population including runners, cyclists, and military recruits (52–54). ITBS is characterized by lateral knee pain associated with repetitive motion activities. Proposed etiologies include friction of the iliotibial band against the lateral femoral condyle, compression of the fat and connective tissue deep to the iliotibial band, and inflammation of the iliotibial band bursa (53).
The prevalence and incidence of ITBS have been described in military recruits and runners. A study of running injuries presenting to a sports medicine clinic found that ITBS was the second most common injury following PFPS for both females and males. ITBS was diagnosed in 62% of the females and 38% of the males (4). Among U.S. Marine Corps male and female recruits, there was no significant difference in reported rates of ITBS, at 5.8% and 4.0%, respectively (8).
Biomechanical differences related to altered hip frontal plane running mechanics have been described between females with and without ITBS. Greater knee internal rotation and increased hip adduction angles may play a role in ITBS pathogenesis (55). Phinyomark et al. found that females with ITBS demonstrated greater hip external rotation during the stance phase of running as compared with males with ITBS (56). Females with ITBS also have a significantly greater peak rearfoot inversion moment, peak knee internal rotation angle, and peak hip adduction angle compared to controls while running (57). However, causality is not clear as Foch and Milner found that of 34 female runners with and without ITBS, both groups had similar peak trunk lateral flexion, peak contralateral pelvic drop, peak hip adduction, and peak external knee adduction moment during running (58).
Male runners with ITBS had significantly less ITB flexibility on Ober test measurement, weaker hip external rotators, greater hip internal rotation, and greater knee adduction than controls on handheld dynamometer testing (59).
The literature is inconclusive as to whether specific patterns of muscle strength deficits exist in runners with ITBS (55). Some studies have suggested a role of gluteus medius and minimus muscle weakness or inhibition during running, resulting in decreased ability to stabilize the pelvis during the single leg support phase (52). A case series of male and female long-distance runners with ITBS found that those with ITBS had weaker hip abduction strength in the affected leg compared with their unaffected leg than healthy long-distance runners. The runners’ successful return to their preinjury training program paralleled improvement in hip abductor strength (60). Further sex-specific prospective studies are needed to evaluate the anatomic, biomechanical, strength, and neuromuscular control factors contributing to ITBS.
Sex differences have not been described in extrinsic risk factors in the development of ITBS. It is important in both sexes to evaluate for inciting factors such as changes in training volume, intensity, equipment, and environment. In cyclists, ITBS can be aggravated by improper seat height and training errors (61). In runners, a repetitive knee flexion angle between 20° and 30° can increase iliotibial band friction against the underlying lateral femoral epicondyle (62,63). Thus, jogging or running at slower speeds or running downhill is more likely to aggravate symptoms than sprinting or faster running (63).
Sex differences have not been described in the physical examination and diagnostic imaging indications for ITBS. The diagnosis of ITBS is made from a thorough history and clinical examination, with an infrequent need for imaging studies.
Treatment and Rehabilitation
Given the lack of consensus regarding sex-specific biomechanical risk factors for ITBS, there are also no clear guidelines regarding the sex-specific rehabilitation of ITBS. Noehren et al. suggested that men with ITBS may benefit from intervention strategies that target neuromuscular control of the hip and knee (59). Phinyomark et al. found that female ITBS runners exhibited significant differences in transverse plane hip kinematic gait patterns while male ITBS runners exhibited significant differences in transverse plane ankle kinematic gait patterns as compared with controls (56). These results suggest that female runners may benefit from focusing on proximal muscle strengthening, whereas male runners should focus on distal muscle strengthening to prevent ITBS (56). Individualized assessment of contributing risk factors and tailored treatment are advised for both sexes in ITBS.
Pes Anserine Tendinopathy Bursitis Syndrome
The pes anserine bursa is located at the anteromedial aspect of the proximal tibia at the tendinous insertion of the sartorius, gracilis, and semi-tendinosus muscles. This bursa can become inflamed due to overuse or from direct trauma. Tendinitis or tendinopathy can occur at the pes anserine site as well.
PATB has been more commonly reported in women than in men, although the exact difference in prevalence rates has not been well-defined (64–67). Risk factors that have been proposed, although controversial, include diabetes mellitus, obesity, and knee osteoarthritis (64,68). The presence of valgus knee deformity alone or in conjunction with ligamentous instability has been cited as a risk factor (64).
There are no published studies specifically describing sex differences in the diagnosis, imaging, treatment, and rehabilitation of PATB syndrome. Risk factors that may be helpful to consider during diagnosis and treatment planning include proximal kinetic chain gluteal weakness and lumbopelvic and core instability. Imaging is typically not necessary in the diagnosis of PATB syndrome. Diagnostic ultrasound, if readily available, may be a useful supplement to the physical exam.