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Intrinsic Risk Factors for the Development
of Patellar Tendinitis in an Athletic
Population
A Two-Year Prospective Study*
Erik Witvrouw,†‡§ PT, PhD, Johan Bellemans,‡ MD, PhD, Roeland Lysens,‡ MD, PhD,
Lieven Danneels,‡ PT, and Dirk Cambier,† PT, PhD
From the †Department of Physical Therapy, Faculty of Medicine, University of Gent,
Gent, Belgium, and ‡Faculty of Physical Therapy & Rehabilitation Sciences,
Catholic University of Leuven, Leuven, Belgium
ABSTRACT
Retrospective studies have suggested various factors
that might cause a disposition to develop patellar ten-
dinitis, but no prospective data exist to determine any
relationships. The purpose of this study was to deter-
mine the intrinsic risk factors for the development of
patellar tendinitis in an athletic population. Before the
study, 138 male and female students of physical edu-
cation were evaluated for anthropometric variables, leg
alignment characteristics, and muscle tightness and
strength parameters. During the 2-year study, 19 of the
138 students developed patellar tendinitis. In all cases
the diagnosis was confirmed by the presence of a
hypoechogenic nodular lesion in the proximal region of
the patellar tendon. Univariate and stepwise discrimi-
nant function analyses were performed comparing the
various measurements. These analyses revealed that
the only significant determining factor was muscular
flexibility, with the patellar tendinitis patients being less
flexible in the quadriceps and hamstring muscles (
P
⬍
0.05). The results of this study demonstrate that lower
flexibility of the quadriceps and hamstring muscles
may contribute to the development of patellar tendinitis
in an athletic population. Therefore, the prevention of
this condition in athletes should be focused on screen-
ing for and treating poor quadriceps and hamstring
muscle flexibility.
Musculoskeletal overuse injuries are common, and the
number of injuries related to physical activity and sport is
increasing.
8,25, 56
Tendon injuries are common in sports
because much stress and force are focused to the tendon
part of the muscle-tendon unit during physical activity,
increasing the risk of injury to tendon tissue.
20,25, 51
The term “jumper’s knee,” or patellar tendinitis, is used
to describe an overuse injury that involves pain at either
the attachment of the quadriceps tendon to the superior
pole of the patella, at the distal insertion of the extensor
mechanism at the tibial tuberosity, or (most commonly) at
the proximal attachment of the patellar tendon at the
inferior pole of the patella.
2,13, 45
The onset of this pain is
often insidious. Pain may be related to microtearing of the
collagen and elastin fibers, inflammation, edema, mucoid
degeneration, fibroid necroses, erosion of the inferior
pole of the patella, or rupture of the patellar ten-
don.
1,3, 14,17, 49,58
Histologically, the lesions of jumper’s
knee have been localized to the bone-tendon junction.
There is no particular age predilection, although patellar
tendinitis is most common in skeletally mature patients.
The cause of jumper’s knee is, as of most injuries, con-
sidered to be multifactoral,
6
including both intrinsic and
extrinsic factors.
7
The term “extrinsic risk factor” refers to
the amount of training, the intensity of training, the train-
ing surfaces, the environmental conditions, the footwear
and equipment, and training errors.
7,42
The importance of
these in the development of patellar tendinitis is well
known. Studies have shown that as the intensity or fre-
quency of training increases, the incidence of patellar
tendinitis rises.
52,53
The fact that not every athlete who is exposed to intense
and repetitive training develops jumper’s knee suggests
that there may be intrinsic risk factors for this condition
as well. Several authors hypothesize that malalignment,
*Presented at the 26th annual meeting of the AOSSM, Sun Valley, Idaho,
June 2000.
§ Address correspondence and reprint requests to Erik Witvrouw, PT, PhD,
Gent University Hospital, Faculty of Medicine, Department of Rehabilitation
Sciences and Physiotherapy, De Pintelaan 185, 9000 Gent, Belgium.
No author or related institution has received any financial benefit from
research in this study.
0363-5465/101/2929-0190$02.00/0
THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 29, No. 2
© 2001 American Orthopaedic Society for Sports Medicine
190
limb-length discrepancy, muscular imbalance or insuffi-
ciency, or a combination predispose an athlete to patellar
tendinitis.
7,32, 64
Two retrospective studies have been re-
ported in which these intrinsic factors were examined.
2,33
However, no studies have been undertaken to scientifi-
cally determine intrinsic risk factors for patellar tendini-
tis on a prospective basis. The purpose of this study was,
therefore, to analyze, with a prospective study design,
which of the presumed risk factors play a significant role
in the development of patellar tendinitis.
MATERIALS AND METHODS
A 2-year prospective study was undertaken of 480 students
who entered an undergraduate program in physical educa-
tion. All students followed the same sports program, which
consisted of 12 to 14 hours of various sports a week. This
program was composed of swimming (2 hrs/week), track and
field (2 hrs/week), gymnastics (2 hrs/week), soccer (1 or 2
hrs/week), basketball (1 or 2 hrs/week), volleyball (1 hr/
week), jazz dance (1 hr/week), handball (1 hr/week), and judo
(1 hr/week). During this 2-year period 198 subjects stopped
the program for various reasons, not because of knee prob-
lems, and were excluded from the study. Of the remaining
282 students, 132 were excluded because they had pain on
palpation at the lower pole of the patella at the outset of the
study or had a history of knee complaints or surgery. Since it
was important in this study that all students have the same
hours of exposure to sports activities, 12 students were ex-
cluded from statistical analysis because they missed regu-
larly scheduled activities for more than 2 months. Finally,
138 students with no history of knee complaints or surgery
(99 boys and 39 girls) took part in the study. Their mean age
was 18.8 years (range, 17 to 21). All participants knew the
goals of the study and gave informed consent.
Before the program commenced, all students underwent
screening that included a survey of their participation in
external, or extracurricular, sports activities and of any
prior complaints of knee problems, trauma, or prior knee
surgery. All students’ knees were examined for signs of
ligamentous instability, meniscal abnormalities, effusion,
or tenderness.
Before the start of the study, all students were also
evaluated for anthropometric variables, leg alignment
characteristics, muscular tightness, and muscle strength.
Anthropometric variables consisted of height and weight
measurements. Leg alignment characteristics were ob-
tained by clinically measuring the leg-length discrepancy,
Q-angle, and the medial tibial intercondylar distance. The
leg-length discrepancy was evaluated by gauging the dis-
tance between the anterior superior iliac spine and the
medial malleolus of both legs from the supine posi-
tion.
28,41
This measurement is described by Martin et
al.
46
as a standardized and valid method for determining
segmental leg length. The Q-angle was measured using a
goniometer and anatomically placed lines with the subject
in a supine position with relaxed quadriceps muscles.
23
The resulting measurement has been shown to be a reli-
able and valid clinical assessment of the quadriceps fem-
oris muscle’s resultant force.
5,60
A clinical evaluation of
the medial tibial intercondylar distance was performed
with the student in a supine position with relaxed quad-
riceps muscles. The distance between the medial condyles
of the tibiae was measured to the nearest centimeter.
Isokinetic strength of the quadriceps and hamstring
muscles was evaluated on the Cybex 350 dynamometer
(Cybex International, Inc., Medway, Massachusetts).
Concentric knee extensor and flexor peak torque was mea-
sured at three speeds: 60, 180, and 240 deg/sec. Each
subject was positioned according to the Cybex testing
manual (Cybex I: Isolated Joint Testing and Exercise.
Ronkonkoma, NY, Cybex, 1983).
The tightness of the hamstring and quadriceps femoris
muscles was measured goniometrically. Hamstring mus-
cle tightness was tested with the subject in a supine po-
sition. The examiner lifted one of the straightened limbs
and measured the angle at the hip.
22
The subject was
placed in a prone position to test the quadriceps femoris
muscle. The knee was maximally flexed, while the foot on
the noninvolved side was placed on the floor in a 90° hip
flexion position, and the angle of the knee was meas-
ured.
22
The flexibility measurements were taken bilater-
ally by the same person (EW). Previous research indicates
that these measurements appear to be reliable if taken by
the same examiner.
4,35, 59
The same doctor and orthopaedic surgeon reviewed the
students every 3 months and questioned them about pain
in the knee. To be considered a patellar tendinitis patient,
the subject had to have all of the following diagnostic
criteria: 1) characteristic history of pain in the quadriceps
or patellar tendons or their patellar or tibial insertions, 2)
tenderness to palpation corresponding to the painful area,
and 3) the presence of a hypoechogenic nodular lesion in
the patellar tendon.
47,55
Ultrasound has been held by many to be the best
method for diagnosing patellar tendinitis.
9,48, 49
Khan et
al.
29
have shown that jumper’s knee is characterized by
consistent changes at ultrasound imaging. These findings
are in accordance with the results of other research-
ers,
18,44
who state, on the basis of their findings, that
ultrasound is a noninvasive, timesaving, and economical
diagnostic tool for evaluating patellar tendinitis. How-
ever, it must be mentioned that the specificity and sensi-
tivity of ultrasound is found to be good in severe cases of
patellar tendinitis but low in the evaluation of patients
who have only mild symptoms.
40
Statistical Procedures
Data were analyzed using the SAS statistical software
package Version 6.12 (SAS Institute, Inc., Cary, North
Carolina).
To study the relationship between a binary response
variable (patellar tendinitis) and one or more explanatory
variables (evaluated parameters), the logistic regression
model has become, in many fields, the standard method of
analysis.
24
Therefore, to identify the intrinsic risk factors
in this study, a multivariate analysis of all measured
parameters was performed with the use of stepwise logis-
tic regression.
Vol. 29, No. 2, 2001 Intrinsic Risk Factors for Patellar Tendinitis 191
In addition, means and standard deviations were calcu-
lated for all characteristics. Univariate analyses (indepen-
dent t-tests) were performed to compare the continuous
variables between the students who did and did not have
patellar tendinitis when there was a normal distribution.
A nonparametric Wilcoxon test was performed when the
distribution did not meet the criterion of normality (quad-
riceps strength at 240 deg/sec; hamstring strength at 180
deg/sec; and medial tibial intercondylar distance). A chi-
square or Fisher’s exact test was used to compare the two
groups of students for the discrete variables (sex and
external sports activity). Significance was accepted at the
0.05 level. Since there was an unequal sex distribution
and because some of the measured parameters are signif-
icantly influenced by sex, we transformed the data so that
they became sex-independent. At first, we calculated for
each variable the mean for the girls and that for the boys.
When statistical analysis showed that the difference be-
tween the sexes was significant, we subtracted or added
up this difference from the data of the girls. Through this
procedure the data became sex-independent. Without this
transformation of the data the power of this study would
have been inadequate.
RESULTS
During the 2-year period of the study patellar tendinitis
was diagnosed in 19 (13.8%) of the 138 students. The
location of pain on palpation was well correlated to the
location of the hypoechogenic nodular lesion observed in
all of these 19 subjects. Power analysis revealed that the
statistical power for the univariate and multivariate anal-
ysis exceeded 80%. Of the 99 boys, 11 developed patellar
tendinitis, as did 8 of the 39 girls. Statistical analysis
showed no significant sex difference in the incidence of
patellar tendinitis in this study (P⫽0.36). None of the
patients had bilateral complaints, and 10 of the 19 stu-
dents had patellar tendinitis in their dominant leg.
No significant difference was found in the amount of
external sports activity between the students who did and
did not have patellar tendinitis. Students with patellar
tendinitis participated in sports outside their education
program for 2.6 hours a week, as compared with 2.3 hours
a week for those who did not develop patellar tendinitis
(control subjects) (P⫽0.62). Eighty-nine percent of the
TABLE 1
Number of Subjects Participating in Competitive Sports for the
Students Who Developed Patellar Tendinitis and for Those
Who Did Not (Control)
Sport
Group
Patellar tendinitis
(N⫽17)
Control
(N⫽98)
N(%) N(%)
Soccer 5 (29) 23 (24)
Volleyball 4 (24) 19 (20)
Track and field 2 (12) 10 (10)
Basketball 1 (6) 9 (9)
Tennis 1 (6) 6 (6)
Judo 1 (6) 4 (4)
Handball 1 (6) 3 (3)
Jazz dance 1 (6) 3 (3)
Gymnastics 1 (6) 4 (4)
Triathlon 5 (5)
Table tennis 3 (3)
Surfing 1 (1)
Squash 1 (1)
Sailing 1 (1)
Badminton 1 (1)
Karate 1 (1)
Field Hockey 1 (1)
Rugby 1 (1)
Water polo 1 (1)
TABLE 2
Univariate Analysis of Variables for the Students Who Developed Patellar Tendinitis and Those Who Did Not (Control)
Variable
Group
PValue
Patellar tendinitis
(N⫽19)
Control
(N⫽119)
Mean SD Mean SD
Height (cm) 177 9.2 175 8.4 0.43
Weight (kg) 64.6 8.1 66.6 7.9 0.72
Age (years) 18.8 1.2 18.9 1.6 0.81
Q-angle (deg) 7.7 4.8 8.7 4.3 0.40
Leg-length difference (cm) 0.5 0.3 0.7 0.4 0.59
Medial tibial intercondylar distance (cm) 1.7 1.22 2.4 1.94 0.49
Flexibility (deg)
Hamstring 83.7 20.9 100.8 16.4 0.04
a
Quadriceps 86.0 12.4 132.6 14.9 0.009
a
Quadriceps strength (N䡠m/kg)
60 deg/sec 285.9 45.5 285.1 42.2 0.94
180 deg/sec 221.6 29.2 217.7 36.5 0.71
240 deg/sec 195.7 29.0 194.7 33.2 0.92
Hamstring strength (N䡠m/kg)
60 deg/sec 174.9 27.5 180.8 28.2 0.48
180 deg/sec 147.6 28.1 145.3 26.8 0.78
240 deg/sec 132.7 28.7 130.5 25.2 0.77
a
Significant difference.
192 Witvrouw et al. American Journal of Sports Medicine
students with patellar tendinitis participated in competi-
tive sports, as compared with 82% of the control subjects.
Because of the wide variety of sports activities and the
small number of patellar tendinitis patients in this study,
it was impossible to make a statistical comparison regard-
ing the possible differences in the type of sports activity
between both groups. However, Table 1 lists the specific
sports activities engaged in by all of the students. This
table shows very clearly that participation in external
sports did not differ between the symptomatic and the
nonsymptomatic groups.
We were unable to identify any statistically significant
differences in muscle tightness between the groups of soccer,
volleyball, track and field, and basketball players (P⬎0.05).
Because of the small number of participants in the other
sports, we were unable to statistically investigate the asso-
ciation between these different sports and muscle tightness.
Among the variables that were assessed before the be-
ginning of the study, the only significant difference de-
tected between the groups was in their quadriceps and
hamstring muscle flexibility (Table 2). The patellar tendi-
nitis group showed a statistically significant lower flexi-
bility for those two muscle groups compared with the
control subjects (quadriceps muscles, P⫽0.009; ham-
string muscles, P⫽0.04). With stepwise logistic regres-
sion analysis, of all of the variables entered into the model,
only the flexibility of quadriceps muscles (P⫽0.004) and
the hamstring muscles (P⫽0.03) showed a significant
association with patellar tendinitis.
DISCUSSION
Patellar tendinitis is one of the most common knee disor-
ders in an athletic population.
26,34, 36,50
The incidence of
this condition varies among different sports and is found
to be dependent on training frequency and intensity.
13
In
our study, jumper’s knee affected 13.8% of the students.
This finding is consistent with earlier investigations that
report up to a 20% incidence of patellar tendinitis in an
athletic population.
25,26, 35
Many authors cite both extrinsic and intrinsic parame-
ters as being etiologic factors for patellar tendini-
tis.
12,41, 42,62
The association between clinical overloading
of the tendon (extrinsic parameters) and the development
of jumper’s knee is well known.
15,16, 37,39, 58,62
Ferretti et
al.
16
found jumper’s knee to be more common in volleyball
players who play more than four times a week. Despite this,
it remains unknown why some athletes develop patellar
tendinitis whereas others do well despite an equally high
training volume. This observation suggests a role of intrinsic
parameters in the development of jumper’s knee, but the
contribution of these intrinsic factors remains enigmatic.
We found no significant difference between the students
who developed patellar tendinitis during the 2-year period
and the students who did not in terms of the context, the
type, and the amount of their external sports activities.
This finding indicates that both groups were comparable
in external load and suggests that the incidence of patellar
tendinitis in the students in this study is mainly a result
of intrinsic risk factors.
No association was found between the students’ muscle
tightness and their participation in different competitive
sports. This finding is not in agreement with the results of
previous studies that identified significant differences in
muscle tightness between gymnasts, swimmers, tennis
players, and soccer players.
31,43
However, because of the
small number of students participating in different sports
in this study the power of the statistical analysis was too
low to allow conclusions. Despite this, we believe that the
influence of the type of external sports participation (2.6
hrs/week) on muscle tightness was negligible compared
with the students’ common sports program, which con-
sisted of 12 to 14 hours per week of different sports.
In this study the location of painful palpation was well
correlated with the presence of a hypoechogenic nodular
lesion, in agreement with findings from other studies in
subjects with severe symptoms of jumper’s knee.
29,40, 54,61
However, because we performed ultrasonography only in
the subjects with patellar tendinitis symptoms, we are
unaware of the presence of any hypoechogenic nodular
lesions in the asymptomatic group.
In a review of the literature, we found that malalign-
ment, patella alta, abnormal patellar laxity, muscular
tightness, and muscular imbalance are suggested as possible
intrinsic risk factors for jumper’s knee. Although it is fre-
quently stated, there is no convincing evidence to support
the assumption that jumper’s knee may be associated with
these variables, because of a lack of prospective studies.
21
Kujala et al.
34
prospectively studied factors predispos-
ing athletes to exertion-related injuries involving the
knee, but not specifically to patellar tendinitis. They found
that taller height, increased patellar passive medial-lat-
eral range of motion, increased knee laxity, inequality of
limb lengths, and a high riding patella due to a long
patella (patella alta) had a statistically significant associ-
ation with injuries of the knee related to overactivity. The
latter finding of patella alta as an intrinsic risk factor of
exertion-related injuries involving in the knee was also
found by Kannus et al.
27
In contrast, none of the evaluated malalignment param-
eters measured in our study (limb-length discrepancy,
Q-angle, medial intercondylar distance) were significantly
different between the two groups. Although these param-
eters were measured clinically, they were found to be
reliable and valid.
5,60
Our results suggest that malalign-
ment variables are not discriminators for patellar tendi-
nitis in an athletic population. The fact that Kujala et al.
34
studied knee exertion injuries, whereas in our study the
results of patients with patellar tendinitis were used, ex-
plains, in our opinion, these differences in results.
The main finding of this study is, not completely unex-
pected, that decreased flexibility of the quadriceps and
hamstring muscles are predisposing factors for the devel-
opment of patellar tendinitis. Our results are consistent
with the suggestion of many experts in sports medicine
who believe that muscular flexibility plays a role in inju-
ries, be they strains, sprains, or overuse injuries.
21,30, 32
However, the small number of patients with patellar ten-
dinitis in our study limits the interpretation and results.
In a review of the literature, Gleim and McHugh
21
con-
Vol. 29, No. 2, 2001 Intrinsic Risk Factors for Patellar Tendinitis 193
cluded that prospective studies that have demonstrated
significant changes in flexibility concurrent with signifi-
cant reductions in injury do not seem to exist. However,
several cohort studies have examined the relationship of
flexibility and overuse injuries.
6,11, 63
Ekstrand et al.
11
found in their study of soccer players that tightness was
associated with tendinitis. This result, and the results of
other retrospective studies supports the concept that de-
creased flexibility will lead to an increase in tendon strain
with joint movements, and therefore predisposes athletes
to tendon overload.
19
Nevertheless, causal relationships
are extremely difficult to discern from the available liter-
ature, so no conclusive statements can be made about the
relationship of flexibility and jumper’s knee.
The observed quantitative association between de-
creased muscular flexibility and jumper’s knee that we
observed in our study has, in our opinion, some important
clinical consequences. This finding suggests that great
attention should be given to the regular assessment of
muscular flexibility during screening of athletes, and sub-
sequent prescriptive measures should be taken to correct
noted lack of flexibility. Expertise in such assessment and
the prescription of proper stretching techniques is essential
in tackling this problem. On the basis of our results, we
believe it is crucial to emphasize the use of proper stretching
in both treatment and prevention of patellar tendinitis. It is
recommended that such procedures be carried out frequently
by appropriately trained personnel and that these persons
guide and teach athletes proper self-stretching. Further re-
search is required to examine, on a prospective randomized
basis, the effect of a preventive stretching program on the
incidence of patellar tendinitis.
It is very tempting to interpret our results as indicating
that flexibility of the quadriceps and hamstring muscles is
the most important causative factor for this condition, but
that temptation must be resisted because of the relatively
small number of subjects in this study, especially in the
stepwise discriminant analysis (N⫽19). Therefore, it may
be wise to consider these data as trends that require
further verification.
The widely held belief that muscle strength is related to
tendinitis cannot be supported by the results of this study,
since no association whatsoever between muscular
strength and patellar tendinitis was observed. However,
the isokinetic concentric measurement used in this study
to evaluate muscle strength does not well represent func-
tional explosive performance. It could be argued that mea-
suring explosive or eccentric strength would have given us
different results. However, the presumed causal relation-
ship of muscle strength to patellar tendinitis might be a
false extrapolation from retrospective studies in which
quadriceps muscle atrophy is often observed in patients
with patellar tendinis.
2,18, 23
The results of this study combined with the results
reported in the literature suggest that decreased quadri-
ceps and hamstring muscle strength is associated with
patellar tendinitis, probably not as a significant etiologic
factor but rather as an altered parameter as a conse-
quence of this condition. Nevertheless, until further study
can substantiate these findings, we agree that players
should have sufficient muscular strength and recommend
a very cautious and progressive strength-training pro-
gram, especially in athletes participating in sports with a
high prevalence of patellar tendinitis.
In this study we found no significant differences be-
tween groups with respect to weight or height, and, there-
fore, we do not consider them as predisposing factors for
patellar tendinitis in an athletic population. This finding
is supported by that of Martens et al.,
45
who found no
correlation between height and the presence of patellar
tendinitis. However, Blazina et al.
2
stated that patients
suffering from jumper’s knee were usually tall athletes.
Lian et al.
39
reported, in contrast to our findings, a signif-
icantly higher body weight in volleyball players with pa-
tellar tendinitis compared with a matched control group of
healthy players. The results from their retrospective
study cannot answer the question of whether the in-
creased weight of their subjects was a consequence of the
patellar tendinitis, or vice versa, and therefore the differ-
ence in study design may account for the difference in
results. In addition, by combining the results of the retro-
spective study of Lian et al.
39
with those of our prospective
study, it could be conjectured that an increase in weight
can be expected in players with jumper’s knee.
Richards et al.
57
found in their study of volleyball play-
ers that subjects with substantial tibial external torsion
moments at takeoff and deep knee flexion angles at that
moment were more likely to develop jumper’s knee. How-
ever, as these authors state themselves, these data were
gathered from retrospective studies, and therefore have
an indeterminate cause-and-effect relation. Nevertheless,
their results suggest that biomechanical parameters may
be related to the prevalence of jumper’s knee. Because in
our study no biomechanical parameters were evaluated,
these results cannot be verified. Another limitation of our
study is that we tested a restricted number of variables.
Anthropometric variables, malalignment characteris-
tics, and muscular strength were not identified as predis-
posing factors for patellar tendinitis. However, the small
number of patients with jumper’s knee in our study con-
fines the interpretation and results and warrants further
research before conclusions about the relationship be-
tween jumper’s knee and extrinsic or intrinsic risk factors
can be drawn. Studies on larger and more homogeneous
populations (for example, volleyball players, basketball
players) and investigating other potential variables (body
fat content, presence of “pes planus,” the use of orthotics,
the types of shoes worn, the impact of different playing
surfaces) are necessary. In addition, further research
needs to better quantify the appropriate intensity of train-
ing, especially related to jumping and landing activities.
Future researchers will need to ensure a more rigorous
definition and data collection related to athlete exposure,
and the battery of possible injury predictors should be
expanded to include injury history. Future researchers
might attempt to determine whether a dose-response re-
lationship exists between flexibility of the quadriceps and
hamstring muscles and the severity of patellar tendinitis.
The existence of such a relationship would add credence to
194 Witvrouw et al. American Journal of Sports Medicine
our hypothesis that poor flexibility of these muscles in-
creases the risk for patellar tendinitis.
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