Expected Prevalence From the Differential Diagnosis of Anterior Knee Pain in Adolescent Female Athletes During Preparticipation Screening

An estimated 30 million school-aged youngsters participate in sports in the United States each year.¹ This number represents a 21% increase over the past decade.² Of these, 34% of middle school–aged and 38% of high school–aged participants will be injured and seek medical treatment at an annual cost of $1.8 billion.^1,3 In young athletes, the knee is the most common joint injured.^1,4 Up to 54% of athletes experience some form of knee pain each year,^5,6 and girls are more likely than boys to sustain a knee injury.^4,7,8 Participation in the sport of basketball results in increased incidence of knee injury relative to other sports.^4,8 Adolescent girls suffer anterior knee pain 2 to 10 times more frequently than similarly aged males.⁹ The symptoms of anterior knee pain may cause up to 74% of school-aged athletes to limit their sport participation or even cease participation altogether.^10–12 In the primary care setting, one-third of all visits for musculoskeletal problems are related to knee pain, which not only affects sport participation but may also impair activities of daily living.^5,6,13–15 Restricting participation in physical activity can lead to a reduction in the health benefits, both physical and psychosocial, gained from physical activity.^16,17 There is currently a paucity of literature that defines the relative prevalence of specific patellofemoral disorders associated with anterior knee pain in adolescent females.

The purpose of our study was to determine the prevalence of specific patellofemoral disorders from the differential diagnosis of anterior knee pain in adolescent female athletes during preparticipation screening. The differential diagnosis was used to classify the blanket term anterior knee pain into discrete, identifiable conditions. All athletes were screened before the onset of their competitive sport season. Based on a prior report⁹ in similar populations, which indicated that anterior knee pain has the greatest incidence of new onset during the middle school years, we hypothesized that anterior knee pain would persist into the high school years, leading to an increased prevalence in high school–aged versus middle school–aged female athletes in the current sample.

METHODS

RESULTS

Over the course of 3 basketball seasons, 688 knee evaluations were performed. Of these 688 examinations, 183 (26.6%) were positive for anterior knee pain. Prevalence between school levels differed, with 34.4% (67 cases) in high school–aged athletes (n = 195) versus 23.5% (116 cases) in middle school–aged athletes (n = 493, P < .05; Table 1).

Table 1.  Prevalence of Anterior Knee Pain by School Level, n (%)

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Of 1376 knees evaluated, PFD was the most common diagnosis, with an overall prevalence of 7.3% (n = 100). This was followed by Sinding-Larsen-Johansson disease or patellar tendinopathy (n = 71, 5.0%), Osgood-Schlatter disease (n = 31, 2.3%), and plica (n = 29, 2.1%). The remaining diagnoses (trauma, fat pad irritation or inflammation, iliotibial band syndrome, and pes anserine bursitis) had a combined prevalence of 1.9% (n = 10; Table 2). The only diagnosis shown to be statistically different between levels was Sinding-Larsen-Johansson disease or patellar tendinopathy, with 9.7% (38 cases) at the high school level and 3.1% (31 cases) at the middle school level (P < .005). The Figure shows the differential diagnosis of anterior knee pain within the 1376 knees evaluated.

Table 2.  Prevalence of Anterior Knee Pain by Diagnosis

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No difference was evident between school levels on the AKPS (high school mean = 84.16 ± 9.2, middle school mean = 83.11 ± 9.9) or IKDC scores (high school mean =80.14 ± 15.5, middle school mean = 81.64 ± 14.77) in participants with anterior knee pain. No difference was observed between groups by IKDC score and differential diagnosis (Table 3).

Table 3.  International Knee Documentation Committee Form Score^a and Differential Diagnosis

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DISCUSSION

Thorough evaluation and determination of the origin of pain may also aid in understanding the underlying contributing causes. Differential diagnosis can be determined by location of pain (Table 4).²² The characteristics of the pain itself (eg, onset, duration, severity, and quality) must also be ascertained.^22,23 Physical examination by the clinician may be better suited to diagnosing anterior knee pain conditions than radiographic evaluation.²⁴

Table 4.  Differential Diagnosis of Anterior Knee Pain by Location

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Our finding of the overall 26.6% prevalence of anterior knee pain corroborates prior reports^1,15,25 in the literature that up to 25% of all athletes experience anterior knee pain. Based on a previous study⁹ in similar populations, which indicated that anterior knee pain has the greater incidence of new onset during middle school relative to high school, we hypothesized that anterior knee pain would persist and lead to increased prevalence in high school–aged versus middle school–aged female athletes in the current sample. Our results showed a difference in the prevalence of anterior knee pain between these levels at P < .05: 34.4% of female high school athletes had a positive anterior knee pain evaluation versus 23.5% at the middle school level. Our investigation did not determine incidence, nor did it take into account the severity and duration of anterior knee pain symptoms, but it does provide a clear understanding of anterior knee pain prevalence in this target population. This differentiation of prevalence rates between high school–aged and middle school–aged athletes could have profound effects on the timing of intervention strategies aimed at preventing the development of anterior knee pain. It may be appropriate to expand targeted intervention programs to include middle school–aged athletes as well as those in high school. Further study is warranted to understand the causes and duration of anterior knee pain in younger athletes over time.

It is important to note that, based upon the prior reports of incidence and the present findings, the prevalence of anterior knee pain increases with school level and does not run the self-limited, benign course previously theorized. If new cases develop at the middle school level but the prevalence is higher at the high school level, this would indicate that anterior knee pain persists in the athletic female population and is not easily resolved.

The most prevalent form of anterior knee pain was PFP at 7.3%. Our findings corroborate a prior report¹⁵ that 10% of visits to physicians were for patellofemoral pain. The proposed causes of PFP are multifactorial, ranging from dynamic malalignment to muscular imbalance to biomechanical factors, including large amounts of loading of the patellofemoral joint.^15,24,26 Increased joint loads across the patellofemoral joint have been estimated at 7.6 times body weight during squatting and up to 20 times body weight with jumping activities.²⁶ The onset is often insidious, and the location of pain is vague. Pain may occur under or around the patella or toward the medial or lateral facets.¹ Pain is often increased with prolonged sitting, squatting, jumping, running, or climbing stairs.^1,12,27

Similar to Osgood-Schlatter disease, Sinding-Larsen-Johansson disease is a traction apophysitis at the inferior pole of the patella, often presenting in adolescents aged 10–12 years.²⁸ Clinical examination reveals pain, swelling, and point tenderness at the inferior pole of the patella. Patellar tendinopathy, sometimes referred to as jumper's knee, is an injury to or degeneration of the patellar tendon. We grouped Sinding-Larsen-Johannson disease and patellar tendinopathy in this study because the diagnosis was based upon physical examination and radiographs are necessary for a definitive diagnosis of the former over the latter. Stair climbing, jumping, kneeling, and running are activities that commonly exacerbate these conditions.²⁹ In the adolescent female population studied, Sinding-Larsen-Johannson disease and patellar tendinopathy was the second most common contributor to anterior knee pain with a prevalence of 5.0%. Of all of the anterior knee pain diagnoses, this was the only one to show a statistically significant difference between levels. Among the knees examined, 3.1% were diagnosed with Sinding-Larsen-Johannson disease or patellar tendinopathy in middle school athletes; the prevalence increased to 9.7% in high school athletes.

During the screening, Osgood-Schlatter disease was identified in 2.3% of knees. Classified as a traction apophysitis of the tibial tubercle, the condition is more commonly found in adolescent boys, but it also develops in girls between 11 and 13 years of age, coinciding with the adolescent growth spurt.^28,29 Osgood-Schlatter disease is often bilateral.^28,30 Pain is typically intermittent in nature and exacerbated with activities such as jumping, kneeling, and squatting, but it generally does not impair activities of daily function.^28,29 As the condition progresses, additional ossification develops around the tibial tubercle. As many as 90% of all patients with Osgood-Schlatter disease have complete resolution of symptoms without additional intervention.²⁸

Inflammation of the synovial plica may result in pain, catching, locking, and clicking of the knee.^29,30 A medial plica is more frequently symptomatic than a lateral plica. The initial inflammation in adolescents generally results from trauma to the anterior-medial aspect of the knee, either from a fall or blow.^28,29 This direct trauma results in thickening and hemorrhage of the plica, leading to increased pain. Pain is the chief complaint; however, a loud clicking or snapping can also be present, which may be confused with a subluxing patella, especially in adolescent females.³¹ The pain is described as dull, achy, and located medial to the patella.³² Our results showed a prevalence of 2.1%, which was lower than the incidence of 3.8% to 5.5% reported by Szajderman et al.³²

The least common findings consisted of trauma, fat pad irritation or inflammation, iliotibial band syndrome, and pes anserine bursitis and accounted for a combined prevalence of 1.9%. The infrapatellar fat pad lies between the patella and patellar tendon anteriorly and the femoral condyles posteriorly.^29,33 Symptomatically, irritation of the fat pad (often called the Hoffa syndrome) produces a dull, aching pain that is exacerbated by squatting, kneeling, climbing stairs, or prolonged sitting.^27,29,34,35 We found a prevalence of fat pad irritation of 0.4%.

Anterior knee pain may not be the benign condition previously described. In 2 longitudinal studies,^36,37 females with anterior knee pain initially diagnosed as adolescents continued to have symptoms on follow-up 15 years later; 25% reported continued symptoms, 45% felt that pain affected daily life, and 36% reported that anterior knee pain actually restricted daily activities. The current findings reaffirm that it is imperative to not only accurately diagnose anterior knee pain and treat the condition properly but ultimately to ascertain the underlying mechanism and morphology. Future researchers should determine the underlying pathomechanics, especially in middle school–aged girls. In addition, further investigation into potential sex differences for the differential diagnoses may aid in developing sex-specific, targeted interventions to prevent anterior knee pain over the course of a competitive season in young athletes.

There are potential limitations to the current study. The first potential limitation is that the screening examinations were performed by different physicians each year. However, we standardized both the questionnaires and examinations and the physician training for identifying PFP to limit the potential for interrater diagnostic differences. In addition, we reported on the expected prevalence for PFP in young athletic populations. Relying on 3 qualified physicians over the 3-year timeframe may actually increase the generalizability of the current results to multiple sports medicine settings. Third, we only screened basketball players, so we cannot compare multiple sports, and analyses were conducted on per-athlete and per-knee levels. This analysis does not differentiate between those athletes with unilateral or bilateral anterior knee pain. The final limitation deals with information about the anterior knee pain condition itself, including treatment that was undertaken, severity, outcome measures, and the lack of calculated incidence rates during the season. All cases of anterior knee pain were treated equally in the determination of prevalence and the classification into a differential diagnosis. We acknowledge that anterior knee pain often occurs on a continuum, and treatment is related to severity. Some athletes may not mention anterior knee pain unless specifically asked, whereas for others, it is quite disabling. Although more study is needed to determine both who should be treated and what interventions should be used, the reported prevalence should trigger greater attention to screening and study of the natural history of the condition in this population.

CONCLUSIONS

Anterior knee pain was observed in 26.6% of the adolescent female athletes screened over 3 years. Patellofemoral dysfunction was the most common cause of anterior knee pain in these participants, followed by Sinding-Larsen-Johansson disease and patellar tendinopathy, Osgood-Schlatter disease, and plica. Fewer than 2% had trauma, fat pad impingement or inflammation, iliotibial band syndrome, or pes anserine bursitis. The accurate differential diagnosis of the underlying cause of anterior knee pain symptom will aid in providing prompt and efficient treatment and perhaps in developing potential interventions for young female athletes. This differentiation could have a profound effect on the timing of intervention strategies aimed at preventing the development of anterior knee pain. Our findings indicate that for the introduction of preventive interventions, targeting middle school–aged girls is most appropriate.