Differential Forces Within the Proximal
Patellar Tendon as an Explanation
for the Characteristic Lesion of Patellar
Tendinopathy
An In Vivo Descriptive Experimental Study
Edwin Mark Dillon,*† MD, Pieter J. Erasmus,† MD, Jacobus H. Müller,‡ Cornie Scheffer,‡
and Richard V. P. de Villiers,§ MD
†
From the Knee Clinic Stellenbosch, Department of Orthopaedic Surgery,
‡
Stellenbosch University, Stellenbosch, South Africa, Biomedical Engineering
Research Group, Department of Mechanical and Mechatronic Engineering,
§
Stellenbosch University, Stellenbosch, South Africa, and Drs. Van Wageningen
and Partners, Stellenbosch Medi-Clinic, Stellenbosch, South Africa
Background: Patellar tendinopathy is a common condition affecting the posterior region of the proximal patellar tendon, but the
reason for this typical location remains unclear.
Hypothesis: The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding
anterior region.
Study Design: Descriptive laboratory study.
Method: An optic fiber technique was used to detect forces in both the anterior and the posterior regions of the proximal patellar tendon in 7 healthy persons. The optic fiber force sensor works on the principle of the amplitude modulation of transmitted
light when the optic fiber is geometrically altered owing to the forces acting on it. Longitudinal strain in the tendon or ligament
produces a negative transverse strain, thus causing a force that effectively squeezes the optic fiber. Measurements were
recorded during the following exercises: closed kinetic chain quadriceps contraction (eccentric and concentric), open kinetic
chain quadriceps contraction (eccentric and concentric), a step exercise, and a jump exercise.
Results: During all the exercises, the peak differential signal output in the posterior location of the proximal patellar tendon was
greater than in the corresponding anterior location. The greatest differential signal output was found in the jump and squat exercises.
Conclusion: The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. This finding supports the tensile-overload theory of patellar tendinopathy.
Clinical Relevance: Jump activities and deep squat exercises expose the patellar tendon to very large tendinous forces.
Keywords: patella; tendinopathy; tendon forces; optic fiber
Patellar tendinopathy (PT) was first described by Blazina
et al in 1973 as jumper’s knee.5 It is a common condition
affecting the proximal attachment of the patellar tendon to
the inferior pole of the patella and is one of the most common forms of chronic tendinopathy.13 It has, in fact, been
shown to have an incidence of up to 20% in an athletic population.28 The lesion in PT is located within the posterior
*Address correspondence to Edwin Mark Dillon, MD, Knee Clinic
Stellenbosch, Department of Orthopaedic Surgery, Stellenbosch University,
Stellenbosch 7600, South Africa (e-mail: [email protected]).
No potential conflict of interest declared.
The American Journal of Sports Medicine, Vol. X, No. X
DOI: 10.1177/0363546508319311
© 2008 American Orthopaedic Society for Sports Medicine
1
2
Dillon et al
fibers of the proximal patellar tendon. Investigations into
the region-specific mechanical properties of the patellar
tendon are, however, limited14; hence, the reason for this
typical location remains unclear.
There appears to be little or no support for an inflammatory origin for PT,13 as the absence or scarcity of inflammatory cells has been consistently reported.16 It has also been
shown that the levels of the inflammatory mediator PGE2
are not raised in PT.1 Johnson et al15 proposed the impingement of the inferior pole of the patella against the patellar
tendon as the cause of PT, but recent studies have disputed
this.26,27 It is accepted, however, that the long inferior pole
of the patella sometimes seen in patients with PT represents a traction osteophyte secondary to the high tensile
loads in this area.23 A recent hypothesis suggests that a
compressive force exists at the proximal-posterior aspect of
the patellar tendon, which enacts an “adaptive process”
leading to the formation of an altered tissue.13
It has also been proposed that the cause of PT is chronic
stress overload,15,19 resulting in microscopic tears of the
tendon and degeneration. Kannus18 speculated that tendinosis may begin with horizontal collagen tearing due to
mechanical overload.
Lian et al21 found that volleyball players with PT performed better in a standardized series of jump and power
tests than did a matched control group. Ground reaction
force, knee flexion angle during landing, and external tibialtorsional movement are all increased in elite volleyball
players with PT.25 Furthermore, it is believed that eccentric force production is a primary cause of tendon
microruptures.14 These findings suggest that the knee
kinematics and forces during jumping contribute to PT.
Subjects who developed PT have also been shown to be less
flexible in the quadriceps and hamstring muscles.28 It has
furthermore been shown that the incidence of PT increases
as the intensity or frequency of training increases.10
As the characteristic lesion in PT typically occurs in the
deep posterior part of the proximal patellar tendon, it has
been hypothesized that loading exposes this part of the
tendon to greater strain.4 This hypothesis was tested in 2
recent cadaveric studies, with conflicting results.2,4 Several
authors have stated that further study is necessary, specifically with regard to the differential forces applied to the
proximal patellar tendon.2,4,13
This study is the first to evaluate differential strains in
the proximal patellar tendon in vivo. It tests the hypothesis that the posterior fibers of the proximal patellar tendon
are subjected to greater tendinous forces than are the
corresponding anterior fibers.
MATERIALS AND METHODS
Optic Fiber Technique
An optic fiber was used as a sensor to record the difference in
forces in the proximal patellar tendon. This technique, which
was first introduced by Komi et al,20 has previously been
used in in vivo studies on the Achilles tendon3,12 and the
patellar tendon.11 The technique entails the optic fiber being
The American Journal of Sports Medicine
inserted through the entire cross section of the tendon and
the ends being attached to a transmitter-receiver unit for
light intensity monitoring. The optic fiber force sensor works
on the principle of the amplitude modulation of transmitted
light when the optic fiber is geometrically altered owing to
the forces acting on it. Longitudinal strain in the tendon or
ligament produces a negative transverse strain, thus causing
a force that effectively squeezes the optic fiber.24
The sensing unit is composed of a transceiver unit (2
HFBR 1414T transmitters and 2 HFBR 2416T receivers,
Agilent Technologies, Santa Clara, Calif) and 2 pieces of optic
fiber (300 mm in length with an outside diameter of 0.5 mm,
Raytela PGR-FB 500, Toray Industries, Tokyo, Japan). The
optic fibers are connected to the transceiver unit by means of
ST connectors. A regulated voltage supply (of 5 V) drives a
small LED, which transmits the light signal through the connected pieces of optic fiber. At the receiver end, a photodiode
gives a voltage output that is proportional to the sensed light
intensity. In this study, 2 optic fibers were inserted through
the proximal patellar tendon, 1 anteriorly and 1 posteriorly.
Calibrating an in vivo–implanted optic fiber sensor to
yield absolute forces is challenging. It was deemed unnecessary for the purposes of this study to calibrate the sensors to record forces in newtons because we were
investigating only the differences in loading at 2 positions
on the tendon. Standard off-the-shelf electronic components were used in building the 2 sensor channels, which
caused a difference in the sensitivity of the channels of
approximately 3%, as revealed by in vitro testing of the
sensor in a hydraulic press.22
Because transducer output is also influenced by optic
fiber bending, it was necessary to investigate the relative
movement between skin and tendon. Finni et al11 measured
the forces during maximum voluntary contraction on 4 volunteers, using the exact same type of fiber optic sensor.
They concluded that the effect of skin movement introduced
a less than 2% error on the sensor output. Erdemir et al9
did a cadaveric simulation of walking during which the
optic fiber measurements of Achilles tendon forces were
compared with known applied loads on the tendon. They
found that skin movement induced high errors (24%-81% of
peak forces) and that measurement errors decreased considerably after the skin was removed (10%-33% peak
forces). In our study, we manually moved the skin 1 cm
proximally and 1 cm distally relative to the underlying tendon with the optic fibers in situ. The manually induced
errors resulted in a maximum error of 3.7% that could be
found during the squat exercise, 3.4% for the step exercise,
and 2% for jumping. Our results and conclusions regarding
the effect of skin movement are hence in keeping with those
of Finni et al.11 It should also be kept in mind that the effect
of skin movement will be almost the same for both channels
because of their close proximity, and hence the skin movement does not influence our conclusions.
Goniometer
A custom-designed goniometer was used to measure the knee
flexion angles during the exercises. A variable-resistance
Vol. X, No. X, XXXX
Characteristic Lesion of Patellar Tendinopathy
3
the sampled data were specified on the user interface.
After the data collection of each trial, all sampled data
were exported to Matlab (version 7.0.1.24704 [R14],
MathWorks, Natick, Mass) for processing.
Data Processing
Figure 1. Custom-designed knee flexion brace.
potentiometer (a 10-k Ω precision potentiometer) was
mounted over the hinge joint of an orthopaedic knee brace
in such a way that the potentiometer’s resistance (the
goniometer output) varied linearly as the brace’s arms
swiveled around the hinge. A frame was mounted on the
brace to support the 2 transmitters and receivers (see
Figure 1).
Experimental Setup
Both the optic fiber force sensor and the goniometer have
analog outputs. The analog signals were converted into
digital format for data processing with a PC. The conversion was performed with a USB-based I/O module (PMD1608FS, Measurement Computing, Norton, Mass) with
eight 16-bit single-ended analog channels, each with its
own A/D converter. The anterior and posterior output
channels of the sensing unit, a switch used to record the
landing in the jump exercise, and the goniometer were coupled to this device through BNC cables.
Data acquisition was performed by a customized
LabVIEW application (LabVIEW 7.1), which consisted of
universal libraries that were supplied with the USB-based
I/O module. The number of channels that needed to be
scanned on the module, the number of required data
points, the sampling frequency, and the voltage ranges for
Both channels are zeroed before implantation, but after
implantation there is a DC offset between the channels.
This offset is attributed to 2 factors: the termination of
the optic fiber ends and the pretension in the tendon (it
is not possible to guarantee an absolute zero loading condition during implantation). Both factors introduced a
subject-specific offset in sensor readings at zero load conditions.
Regarding the quality of the termination of the fiber
optic ends, 10 fibers of the same length were cleaved and
the readings taken at zero loading (ie, not implanted). The
mean steady-state output for the anterior channel was
3.56 V (SD, 0.197 V; n, 10). The mean steady-state output
at zero load for the posterior channel was 3.54 V (SD, 0.194
V; n, 10). This resulted in a mean offset of 2 mV between
the 2 channels, which can introduce a 0.1% error in the
full-scale output for all the exercises, which is negligible.22
The mean recorded steady-state DC offset between the
channels after implantation was 20 mV, and we believe
this is owing to the effect of a pretension in the tendon (ie,
there already exists greater tension in the posterior
region).22 However, even if this hypothesis is wrong, the
reading of 20 mV can introduce a maximum error of 1% to
2% on the full-scale output for all the exercises and hence
does not influence our conclusions presented in this article.
Because of these factors, we reported the sensor outputs
as a deviation in the sensor readings from a reference
value, and we termed this the differential output. The differential output was determined by the subtraction of the
“raw” sensor signals from the reference value. The reference chosen was the smallest sensor output of the anterior
channel at a physiologically minimal load condition (lower
sensor outputs indicated lower forces and vice-versa). The
same reference was then used on both channels, hence
establishing the zero baseline according to which we
reported the data. Through the comparison of the magnitudes of the differential outputs of the posteriorly and
anteriorly placed optic fibers, it was possible to state
whether the forces were larger in the one compared with
the other despite the inability to quantify the results in
absolute force values.
A last processing step was to calculate a 10-point running average of the data to filter out sampling noise.
In Vivo Study
Participants. Seven healthy males with no previous
history of PT, knee injury, or knee pain requiring consultation with a doctor volunteered for this study (Table 1). They
were informed of all the risks of the study, they gave their
written consent for participation, and they were free to
stop the experiment at will. The study was approved by the
ethics committee of the authors’ university.
4
Dillon et al
The American Journal of Sports Medicine
TABLE 1
Anthropometric Data of the Study Participants
Participant
1
2
3
4
5
6
7
Mean ± SD
Age, y
31
32
22
23
24
26
27
26.4 ± 3.9
Sex
Height, cm
Male
Male
Male
Male
Male
Male
Male
186
189
185
183
172
167
180
180.3 ± 8
Experiment protocol. An ultrasound examination of the
patellar tendon was performed before the insertion of the
optic fibers. The transverse width of the proximal patellar
tendon, the anteroposterior diameter of the proximal
patellar tendon, the length of the patellar tendon, and the
presence or absence of a hypoechoic lesion in the proximal
patellar tendon were recorded.
With the patient supine and the knee flexed to 90°, 2 sterile 20-gauge spinal needles were inserted into the proximal
patellar tendon under ultrasound guidance with the use of
an aseptic technique and after the infiltration of local anesthetic. The first 20-gauge spinal needle was passed through
the tendon from lateral to medial, 1 to 2 mm anterior to the
posterior border of the tendon. The second needle was placed
1 to 2 mm posterior to the anterior border of the tendon and
parallel to the first needle. The obturator of each spinal needle was removed, and a 0.5-mm optic fiber was fed through
each needle. The needles were removed, leaving the optic
fibers in the tendon (Figure 2). Transmitter-receiver units
were attached to the free ends of both optic fibers.
The positions of the needles were verified with the ultrasound (Figure 3) to be within 5 to 10 mm of the apex of the
patella within the anterior half of the patellar tendon
(anterior fiber) and within the posterior half of the patellar tendon (posterior fiber).
The hinged knee brace fitted with the goniometer was
fitted to the leg. The transmitter and receiver units of the
2 optic fibers were attached to the light source and the
receiver mounted on the brace.
Measurements were recorded during the following
activities:
1. Closed kinetic chain eccentric and concentric
quadriceps contraction during a 1-leg squat:
• Concentric quadriceps contraction from 110° of
knee flexion to full extension.
• Eccentric quadriceps contraction from full
extension to 110° of knee flexion.
2. Open kinetic chain concentric quadriceps contraction with a 10-kg weight attached to the foot:
• Concentric: active knee extension from 90° of
flexion to full extension.
• Eccentric: controlled knee flexion from full
extension to 90° of knee flexion.
Weight, kg
Body Mass Index, kg/m2
88
85
91
77
79
67
78
80.7 ± 8.1
25.4
23.8
26.6
23
26.7
24
24.1
24.8 ± 1.5
Q Angle, deg
14
12
11
12
12
12
14
12.4 ± 1.1
3. Step up and step down:
• With the right leg as the leading leg.
4. Jump from a height of 30 cm:
• Landing on the right leg with hip and knee
flexed to the participant’s discretion.
On completion of the experiment, the optic fibers were
removed, and an adhesive dressing was applied.
Statistical Methods
The means of the anterior and posterior peak differential
signal deflections were compared using repeated-measures
analysis of variance. Normal probability plots were
inspected to check for deviations from normality and possible outliers, and none were found that could have significantly influenced the results. Means and SDs were
calculated for descriptive purposes. A minimum significance
level of P ≤ .05 was set for all statistical tests.
RESULTS
The sonographic characteristics of the proximal patellar
tendon are represented in Table 2.
The positions of the 2 fibers were well within the anterior and posterior halves of the proximal patellar tendon as
confirmed by sonographic measurement (Table 3).
A hypoechoic lesion in the posterior aspect of the proximal
patellar tendon was noted in 1 of the 7 participants (participant 5). We did not notice any obvious difference between
this person’s results and those of the other 6. In addition,
normal probability plots were inspected for possible outliers,
and none were found. The results from each exercise are
described below.
Closed Kinetic Chain Eccentric and Concentric
Quadriceps Contraction During a 1-Leg Squat
The peak differential outputs were greater in the posterior
fiber than in the anterior fiber in all 7 participants (Table 4).
This finding was statistically significant (P = .0097 for
the concentric contraction and P = .0087 for the eccentric
Vol. X, No. X, XXXX
Characteristic Lesion of Patellar Tendinopathy
5
Figure 3. Ultrasound: sagittal view showing the position of
the 2 optic fibers in the patellar tendon.
Open Kinetic Chain Concentric
Quadriceps Contraction
The posterior fibers had a greater peak differential output
than did the anterior fiber in all 7 participants (Table 5). This
trend was shown to be statistically significant (P = .0078
for the concentric contraction and P = .0075 for the eccentric contraction). The peak differential output during the
concentric contraction occurred at a mean of 3.4° ± 4.8° of
knee flexion in the anterior fibers and at 4.8° ± 9.5° of knee
flexion in the posterior fibers. During the eccentric phase
of the exercise, this occurred at 10.1° ± 12.4° of knee flexion
in the anterior fibers and at 10.3° ± 15.7° of knee flexion in
the posterior fibers.
Figure 2. Position of the optic fibers in the patellar tendon.
Step Exercise
contraction). The peak differential output during the concentric contraction occurred at a mean of 102.4° ± 8.8° of
knee flexion in the anterior fibers and at 94.8° ± 18.9° of
knee flexion in the posterior fibers (Figure 4). During the
eccentric phase of the exercise, this occurred at 97.9° ± 14.2°
of knee flexion in the anterior fibers and at 94.9° ± 16° of
knee flexion in the posterior fibers.
Once again, the posterior fiber showed a larger peak differential output than did the anterior fiber in all 7 participants
(Tables 6 and 7). This trend was once again statistically
significant (P = .0020 for the step-up and P = .0023 for the
step-down). There was a greater peak differential output
on the step-down than on step-up in both the anterior (P =
.0250) and the posterior (P = .0280) fibers.
TABLE 2
Sonographic Findings of the Patellar Tendona
Participant
Transverse Width, mm
AP Diameter, mm
Length, mm
Hypoechoic Lesion
1
2
3
4
5
6
7
Mean ± SD
28.4
22.4
30.5
30.4
27.1
29.6
22.5
27.3 ± 3.5
4.6
4.8
4.0
4.9
3.5
4.8
4.9
4.5 ± 0.5
55.4
47.7
60.0
41.6
52.2
41.9
48.7
49.6 ± 6.8
No
No
No
No
Yes
No
No
a. AP, the anteroposterior measurement taken from 5 to 10 mm from the patella apex; length, measured from the patella apex to the insertion of posterior fibers on the tibial tuberosity.
6
Dillon et al
The American Journal of Sports Medicine
TABLE 3
Positions of Fibers in the Proximal Patellar Tendona
1
2
3
4
5
6
7
Mean ± SD
AP Diameter, mm
Anterior, mm
4.6
4.8
4.0
4.9
3.5
4.8
4.9
4.5 ± 0.5
1.7
1.3
1.4
1.9
1.0
1.2
1.7
1.5 ± 0.3
Posterior, mm
1.5
1.2
1.3
1.6
1.2
1.3
1.4
1.4 ± 0.2
anterior
posterior
0.50 V
1.8 V
landing
1.8
differential signal output, [volts]
Participant
2
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
a. Anterior, the distance of the anterior fiber from the anterior
margin of the tendon; posterior, the distance of the posterior fiber
from the posterior margin of the tendon.
0
0
1
2
3
4
5
6
time, [seconds]
Figure 4. Example of a squat exercise (participant 5).
Jump From a Height of 30 cm
The results of this exercise were the most striking (Figure 5).
All 7 participants had a greater differential signal output in
the posterior optic fiber than in the anterior optic fiber
(Table 8). This finding was statistically significant (P = .0019).
The knee flexion angles at peak differential output ranged
from 32.2° to 96.7° (mean, 54.5°) for the anterior fibers and
from 32.2° to 57.8° (mean, 45°) for the posterior fibers.
TABLE 4
Peak Differential Output With a Closed Kinetic Chain Quadriceps Contraction During a 1-Legged Squata
Concentric Contraction
Participant
Anterior, V
Posterior, V
1
2
3
4
5
6
7
Mean ± SD
0.35
0.19
0.40
0.47
0.98
0.25
0.23
0.41 ± 0.27
0.86
0.56
1.72
1.68
1.37
0.73
0.37
1.0 ± 0.55
Eccentric Contraction
Ratio = Posterior/Anterior
2.5
3.0
4.4
3.6
1.4
2.9
1.7
2.8 ± 1.0
Anterior, V
0.32
0.22
0.39
0.50
0.98
0.25
0.27
0.42 ± 0.27
Posterior, V
0.9
0.6
1.7
1.5
1.3
0.7
0.4
1.0 ± 0.49
Ratio = Posterior/Anterior
2.8
2.7
4.5
3.0
1.4
2.9
1.5
2.7 ± 1.0
a. Concentric contraction, the posterior maximum forces are larger than the anterior maximum forces (P < .05). Eccentric contraction, the
posterior maximum forces are larger than the anterior maximum forces (P < .05).
TABLE 5
Peak Differential Output With an Open Kinetic Chain Concentric Quadriceps Contractiona
Concentric Contraction
Participant
Anterior, V
Posterior, V
1
2
3
4
5
6
7
Mean ± SD
0.22
0.13
0.23
0.18
0.30
0.17
0.21
0.21 ± 0.054
0.58
0.51
1.7
1.0
1.2
0.70
0.35
0.86 ± 0.47
Eccentric Contraction
Ratio = Posterior/Anterior
2.6
3.8
7.3
5.6
3.8
4.1
1.9
4.2 ± 1.8
Anterior, V
Posterior, V
0.20
0.13
0.23
0.21
0.30
0.19
0.22
0.21 ± 0.051
0.58
0.51
1.7
1.1
1.2
0.71
0.36
0.88 ± 0.47
Ratio = Posterior/Anterior
2.8
3.9
7.2
5.1
3.9
3.8
1.8
4.1 ± 1.7
a. Concentric contraction, the posterior maximum forces larger than the anterior maximum forces (P < .05). Eccentric contraction, the
posterior maximum forces are larger than the anterior maximum forces (P < .05).
Vol. X, No. X, XXXX
Characteristic Lesion of Patellar Tendinopathy
TABLE 6
Peak Differential Output With a Step-up Exercisea
Participant
Anterior, V
Posterior, V
1
2
3
4
5
6
7
Mean ± SD
0.090
0.11
0.48
0.14
0.27
0.18
0.25
0.22 ± 0.13
0.59
0.45
1.53
0.75
0.93
0.72
0.41
0.77 ± 0.38
TABLE 7
Peak Differential Output With a Step-down Exercisea
Ratio = Posterior/
Anterior
6.2
3.9
3.2
5.5
3.5
4.1
2.4
4.1 ± 1.3
a. The posterior maximum forces are larger than are the anterior
maximum forces (P < .05).
differential signal output, [volts]
Anterior, V
Posterior, V
1
2
3
4
5
6
7
Mean ± SD
0.14
0.14
0.46
0.18
0.32
0.2
0.25
0.24 ± 0.12
0.61
0.49
1.59
0.75
1.09
0.76
0.44
0.82 ± 0.40
Ratio = Posterior/
Anterior
4.42
3.6
3.43
4.05
3.42
3.82
2.55
3.6 ± 0.59
studies6,17 and provides further evidence that ultrasonographic appearance and clinical symptoms do not correlate
precisely. It has, however, previously been shown that in
elite junior basketball players, the presence of an asymptomatic ultrasonographic hypoechoic area was associated with
a 4 times greater risk of developing PT.7
The 2 previous in vivo studies using this technique in
the Achilles tendon12 and the patellar tendon11 noted that
the insertion of a needle through the tendon was virtually
pain free. The experience of our first 2 participants did not,
however, bear this out, hence our attempt to achieve better
analgesia. The resultant excellent anesthetic effect of the
The transverse width of the proximal patellar tendon in our
study of 27.3 ± 3.5 mm correlated well with the findings of
an MRI study by El-Khourny et al8 (31.3 ± 9.3 mm). The AP
diameter of 4.6 ± 0.6 mm in our study was midway between
the 3.7 ± 1.2 mm of El-Khourny et al and the 5.5-mm AP
diameter of an earlier MRI study by Johnson et al.15
The 1 participant with a hypoechoic lesion in the proximal
posterior aspect of the patellar tendon was involved in recreational sports activity only. This finding of an asymptomatic
hypoechoic lesion has been shown before in previous
1
Participant
a. The posterior maximum forces are larger than are the anterior
maximum forces (P < .05).
DISCUSSION
1.5
concentric: anterior
concentric: posterior
1.1 V
1.4 V
0.5
0
0
20
40
60
80
100
120
80
100
120
differential signal output, [volts]
angle, [degrees]
1.5
1
eccentric: anterior
eccentric: posterior
1.1 V
1.3 V
0.5
0
0
20
7
40
60
angle, [degrees]
Figure 5. Differential output during a jump exercise (participant 3).
8
Dillon et al
The American Journal of Sports Medicine
TABLE 8
Peak Differential Signal Output With a Jump Exercisea
Participant
Anterior, V
Posterior, V
1
2
3
4
5
6
7
Mean ± SD
0.19
0.17
0.53
0.19
0.57
0.4
0.53
0.37 ± 0.18
0.78
0.57
1.79
0.83
1.27
0.82
0.86
0.99 ± 0.41
Ratio = Posterior/
Anterior
3.99
3.41
3.41
4.45
2.21
2.12
1.63
3.0 ± 1.1
a. The posterior maximum forces are larger than are the anterior
maximum forces (P < .05).
infiltration of local anesthetic adjacent to the deep margin
of the proximal tendon was of particular interest, and it
can be postulated that this region of the tendon receives
rich innervation from the underlying fat pad.
This was the first study to evaluate the differential
tendinous forces in the proximal patellar tendon in vivo.
Our results indicated that an optic fiber located in the posterior region of the patellar tendon was subjected to
greater tensile forces than was an optic fiber located in the
corresponding anterior region. This finding was statistically significant for all the exercises performed in this
study, which included an open and closed kinetic chain
exercise, a step exercise, and a jump exercise. The greatest
signal deflections were reported for the jump exercise.
Two previous cadaveric studies assessing differential
tensile strain in the proximal patellar tendon showed conflicting results.2,4 This could be attributed to methodological differences.
Almekinders et al2 measured strain patterns in 8 patellar tendons with the use of a buckle sensor. When a 45-N
load was applied to the quadriceps muscle with the knee in
full extension, a strain increase in the central proximal posterior location resulted. The difference between the anterior
and the posterior sides, however, was not statistically significant. As the knee was brought into flexion, the tensile
strain increased on the anterior side but decreased on the
posterior side in the central proximal location of the tendon. Relatively low loads and velocities were used, and the
authors mentioned that higher loads and velocities could
transfer more tensile load to the posterior tendon surface.
Basso et al4 measured tensile strain in the anterior and
posterior fibers of 10 cadaveric knees with a technique of documenting changes in suture length. Quadriceps loading in
this study caused significantly greater strains in the posterior fascicles between 60° and 90° of knee flexion. A significantly higher load of 1 kN was used in this study compared
to the 45-N load in the previously mentioned study.
Both these cadaveric studies were performed on a much
older study population, although PT occurs in a young, athletically active population. The techniques of strain measurement used required significant soft tissue dissection
and the removal of retinacular structures. Our findings,
however, are in agreement with the second study, which
found that quadriceps loading in knee flexion caused significantly greater strains in the posterior fascicles.4
The peak signal outputs toward the end of extension
during the open kinetic chain quadriceps contraction suggest that the last ±10° of knee extension should be avoided
during a rehabilitation program when resisted knee extension exercises are performed. When squat exercises are
performed, deep knee flexion should also be avoided, as
peak deflections during these exercises occurred from 94°.
A safe recommendation would be to avoid squat exercises
of greater than 80° of knee flexion.
This study had some limitations. One of these was that
the optic fiber sensors were not calibrated, and we were
therefore not able to quantify the forces encountered. We
accept this as a limitation but would argue that the aim
of the study was to assess the differential forces in the 2
locations, and we are satisfied that our methods met this
aim. Another limitation was that the small numbers precluded meaningful deductions regarding the differences
between eccentric and concentric exercises and knee
flexion angles at peak differential output during some of
the exercises. There was, however, a trend toward
greater differential signal output during eccentric exercises than during concentric exercises, although this was
not statistically significant in any of the relevant exercises. This was in keeping with previous findings regarding the link between eccentric force production and the
overload theory.25
This was the first in vivo study to evaluate differential
forces between the anterior and the posterior fibers of the
proximal patellar tendon. The results clearly showed
higher forces in the posterior fibers than in the anterior
fibers. This supports the repeated tensile-overload theory
of PT. It also showed that deep-squat exercises and jumping result in very high tensile forces in the patellar tendon.
ACKNOWLEDGMENT
This research was partly funded by research grants from
Stellenbosch University and the South African Orthopaedic
Association.
REFERENCES
1. Alfredson H, Forsgren S, Thorsen K, Lorentzon R. In vivo microdialysis and immunohistochemical analysis of tendon tissue demonstrated
high amounts of free glutamate and glutamate NMDAR1 receptors
but no signs of inflammation in jumper’s knee. J Orthop Res.
2001;19:881-886.
2. Almekinders LC, Vellema JH, Weinhold PS. Strain patterns in the
patellar tendons and the implications for patellar tendinopathy. Knee
Surg Sports Traumatol Arthrosc. 2002;10:2-5.
3. Arndt AN, Komi PV, Brüggemann GP, Lukkariniemi J. Individual muscle contributions to the in vivo Achilles’ tendon force. Clin Biomech
(Bristol, Avon). 1998;13:532-541.
4. Basso O, Amis AA, Race A, Johnson DP. Patellar tendon fiber strains:
their differential responses to quadriceps tension. Clin Orthop Relat
Res. 2002;400:246-253.
5. Blazina ME, Kerlan RK, Jobe FW, Carter VS, Carlson GJ. Jumper’s
knee. Orthop Clin North Am. 1973;4:665-678.
Vol. X, No. X, XXXX
6. Cook JL, Kahn KM, Harcourt PR, et al. Patellar tendon ultrasonography in asymptomatic active athletes reveals hypoechoic regions: a
study of 320 tendons. Clin J Sport Med. 1998;8:73-77.
7. Cook JL, Kahn KM, Kiss ZS, Purdam CR, Griffiths L. Prospective
imaging study of patellar tendinopathy in elite junior basketball players. J Ultrasound Med. 2000;19:473-479.
8. El-Khourny GY, Wira RL, Berbaum KS, Pope TL Jr, Monu JU. MR
imaging of patellar tendinitis. Radiology. 1992;184:849-854.
9. Erdemir A, Hamel AJ, Piazza SJ, Sharkey NA. Fiberoptic measurement of tendon forces is influenced by skin movement artifact.
J Biomech. 2003;36:449-455.
10. Ferretti A. Epidemiology of jumper’s knee. Sports Med. 1986;3:
289-295.
11. Finni T, Komi PV, Lepola V. In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement
jump. Eur J Appl Physiol. 2000;83:416-426.
12. Finni T, Komi PV, Lukkariniemi J. Achilles’ tendon loading during walking: application of a novel optic fiber technique. Eur J Appl Physiol
Occup Physiol. 1998;77:289-291.
13. Hamilton B, Purdam C. Patellar tendinosis as an adaptive process: a
new hypothesis. Br J Sports Med. 2004;38:758-761.
14. Haraldsson BT, Aagaard P, Krogsgaard M, Alkjaer M, Kjaer M,
Magnusson SP. Region-specific mechanical properties of the human
patellar tendon. J Appl Physiol. 2005;98:1006-1012.
15. Johnson DP, Wakely CJ, Watt I. Magnetic resonance imaging of patellar tendonitis. J Bone Joint Surg Br. 1996;78:452-457.
16. Kahn KM, Cook JL, Bonar F, Harcourt P, Astrom M. Histopathology of
common tendinopathies: update and implications for clinical management. Sports Med. 1999;27(6):393-408.
17. Kahn KM, Visentini PJ, Kiss ZS, et al. Correlation of ultrasound and
magnetic resonance imaging with clinical outcome after open patellar tenotomy: prospective and retrospective studies. Clin J Sport
Med. 1999;9:129-137.
18. Kannus P. Etiology and pathophysiology of chronic tendon disorders
in sport. Scand J Med Sci Sports. 1997;7:78-85.
Characteristic Lesion of Patellar Tendinopathy
9
19. King J. Patellar dislocation and lesions of the patella tendon. Br J
Sports Med. 2000;34:467-470.
20. Komi PV, Belli A, Huttunen V, Bonnefoy R, Geysantt A, Lacour JR.
Optic fiber as a transducer of tendomuscular forces. Eur J Appl
Physiol Occup Physiol. 1996;72:278-280.
21. Lian ∅, Engebretsen L, ∅vreb∅ RV, Bahr R. Characteristics of the leg
extensors in male volleyball players with jumper’s knee. Am J Sports
Med. 1996;24:380-385.
22. Müller JH. Design, Building and Testing of a Fiber Optic Force
Transducer and Its Application in an In Vitro and In Vivo Study
Investigating the Biomechanical Properties of the Patella Tendon
[MScEng thesis]. Stellenbosch, South Africa: Department of Mechanical
and Mechatronic Engineering, Stellenbosch University; 2007.
23. Popp JE, Yu JS, Kaeding CC. Recalcitrant patellar tendinitis: magnetic resonance imaging, histologic evaluation and surgical treatment. Am J Sports Med. 1997;25:218-222.
24. Ravary B, Pourcelot P, Bortolussi C, Konieczka S, Crevier-Denoix N.
Strain and force sensors in human and veterinary tendon and ligament biomechanical studies. Clin Biomech (Bristol, Avon). 2004;19:
433-447.
25. Richards DP, Ajemian SV, Wiley JP, Zernicke RF. Knee joint dynamics
predict patellar tendinitis in elite volleyball players. Am J Sports Med.
1996;24:676-683.
26. Schmid MR, Hodler J, Cathrein P, Duewell S, Jacob HA, Romero J. Is
impingement the cause of jumper’s knee? Dynamic and static MR
imaging of patellar tendonitis in an open configuration system. Am J
Sports Med. 2002;30:388-395.
27. Tyler TF, Hershman EB, Nicholas SJ, Berg JH, McHugh MP. Evidence
of abnormal anteroposterior patellar tilt in patients with patellar tendinitis with use of a new radiographic measurement. Am J Sports
Med. 2002;30:396-401.
28. Witvrou E, Bellemans J, Lysens R, Danneels L, Cambier D. Intrinsic
risk factors for the development of patellar tendinitis in an athletic
population: a two-year prospective study. Am J Sports Med.
2001;29:190-195.