Download The incidence and risk factors in the development of medial tibial

Ben Yates,*t MSc, FCPod(S), and Shaun White,+ BApplSci
From the tPodiatry Department, University College Northampton, Northampton, United
Kingdom, and the tDepartment of Podiatr.x La Trobe Universit.x Melbourne, Australia
Purpose: To identify the incidence of medial tibial stress syndrome (MTSS) in a group of naval recruits undergoing a 10-week
basic training period and to determine potential risk factors.
Method: One hundred and twenty-four recruits (84 men and 40 women) were followed prospectively during basic training.
Anthropometric and lower limb biomechanical data were recorded at the start of the program along with injury history and previous sporting activity for the 3 months prior to enlisting. Recruits were monitored during training for development of medial tibial strees syndrome and were asked to complete an exit interview at the end of the program.
Results: Forty recruits (22 men and 18 women) developed medial tibial stress syndrome, giving an incidence of 35%. A significant relationship existed between gender and medial tibial stress syndrome (P = .012), with femare recruits more likely to develop medial tibial stress syndrome than male recruits (53% vs 28%). A risk estimate revealed a relative risk of 2.03. The biomechanical results indicated a more pronated foot type (P = .002) in the medial tibial stress syndrome group when compared to the
control group. A risk estimate established that recruits with a more pronated foot type had a relative risk of 1.70.
Conclusion: Identifying a pronated foot type prior to training may help reduce the incidence of medial tibial stress syndrome by
early intervention to control abnormal pronation. Findings of a higher incidence of medial tibial stress syndrome among female
recruits require further investigation.
Keywords: shin splints; foot pronation; injury rates
Medial tibial stress syndrome (MTSS) is one of many
overuse lower leg injuries found under the umbrella term
of exercise-induced leg pain or shin splints. Stress fractures, chronic compartment syndrome, and MTSS are the
3 most common forms of exercise-inducedleg pain, with
MTSS having the highest prevalence. MTSS was first
defined "as a symptom complex seen in athletes who complain of exercise induced pain along the posteriormedial
border of the tibia."36(p209)
It is a very commoninjury experienced by runners and military personnel. It accounts for
between 13.2% and 17.3% of all running injuries14.18and
up to 22% of injuries seen in aerobic dancers.51Among
male and 241 female naval recruits through an II-week
(men) and 12-week (women) training period, finding an
overall incidenceof 6.417c
for MTSS. In the only prospective
civilian study, an incidence of 13%was identified among
125 high schoolrunners. I I
Although various studies have attempted to find the
exact pathophysiology for this common condition. it still
remains unresolved. Until recently, inflammation of the
periosteum due to excessivetraction was considered the
most likely etiology of MTSS.I,23,36,40.4S
Detmerl6 opposed
this theory, proposing periostalgia as the likely cause of
MTSS after he found no evidenceof inflammatory changes
and consistently found adiposetissue interposed between
the periosteum and the bone surface. Johnell et al24first
proposedthe bone stress reaction theory after taking biopsies and finding osseousmetabolic changes in 37 limbs
\vith MTSS and no evidenceof inflammatory changes.
Recent studies have supported this view that ),ITSS is
not an inflammatory processof the periosteum but instead
a bone stress reaction that becomespainful.4,s.20
'Vhen a
person begins an exercise program, the bone undergoes
metabolic changes.These changesin the tibia are characterized by initial bone porosity due to osteoclastic chan-
studies of military personnel, Almeida et al2 followed 176
'Address correspondence to Ben Yates, Podiatry Department, University
College Northampton,
Boughton Green Road, Northampton,
NN2
7AL, UK.
No author or related institution has received financial benefit from
research in this study.
The American Journal of Sports Medicine. Vol. 32, No.3
001: 10.1177/0095399703258776
@ 2004 The American Orthopaedic Society for Sports Medicine
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Medial Tibial StressSyndromeAmongNaval Recruits 773
f
neling on the compressedconcaveposteriormedial border.
This is followed by the laying down of new bone to resist
these compressiveforces and strengthen the bone.3.10
The
result is that the tibia becomes stronger than the preexercise state on the posteriormedial border. However, in
casesof long-standing MTSS, it has been shown that the
affected part of the tibia is 15% more porous than in control subjects and 23% less than in athletic control sub.Jects.30
A thorough clinical history and physical examination
can accurately diagnoseMTSS. MTSS commonly presents
as diffuse, palpable pain, localized to the posteriomedial
t1b.1a1 border.48.1620374055 It can occur anywhere a1ong the
.
posteriomedial border but most commonlyaffects the middle to distal thirds.5.10.11.16.32.36.53.55
The pain is usually
describedas a dull ache following exercise,which may last
for several hours or days. In severecases,pain may persist
during normal activities of daily living.
Additional investigations such as plain radiographs,
bone scans,and magnetic resonanceimaging can be useful
to clinically diagnoseMTSS as well as rule out other forms
of exercise-inducedleg pain suchas stressfracture. However,
all 3 types of investigation have beenassociatedwith both
false-positive and false-negative results.3.6-8.39.47.57
It has
therefore been suggested that if the clinical picture indicates MTSS and there is no diagnostic dilemma, further
investigations are unwarranted.52
Various authors have proposeda wide variety of etiological factors for MTSS, including training on hard surfaces
or uneven terrain, improper training techniques, increasing training intensity too quickly, changes in footwear,
muscle imbalances or inflexibility, and biomechanical
abnormalities.29.31.35.49.50.55
Although not linked directly to
MTSS, a high body mass index (BMI) and a previous history of injury have been linked to the development of
lower-limb overuse injuries.25.26.34
Abnormal subtalar joint pronation has been associated
with MTSS in a number of static and dynamic studt d.
11 t
'.
.31
50 53 Th
1es.
..ese
s u 1eswere a re rospecbve In nature,
consisted of small sample groups, and often failed to correctly define the condition as MTSS, using instead the
broader definition of shin splints. The only prospective
study that determined foot pronation as a risk factor for
MTSS is that of Bennett et al,ll \vho followed high school
cross-country runners through a training period. They
measured the degree of foot pronation by recording the
amount the navicular bone lowered betweentwo standing
positions, the neutral calcaneal stance position and the
relaxed calcaneal stance position. This distance is termed
the navicular drop, and the procedure measures the
amount the medial longitudinal arch (MLA) lowers in the
sagittal plane. However, Bennett et al performed this test
only after the runner developedMTSS, so in truth it was a
retrospective measurementand may have been an effect of
developing MTSS rather than the cause of the condition.
The purpose of the present study \vas therefore to
prospectively measure and monitor naval recruits during
basic training to determine the incidence and potential
anthropometric, foot posture, exercise history, and previ-
ous lower-limb injury risk factors for the development of
MTSS.
METHOD
The study was granted ethics approval from the
Australian DefenceMedical Ethics Committee and the La
Trobe University Faculty of Health Sciences Human
Ethics Committee. All subjectswere recruited from HMAS
Cerberusin Western Port, Victoria, Australia. All recruits
at the commencementof training were offered the opportunity to voluntarily participate in the study. Subjects
were assured that all information obtained would be confidential; they would remain anonymous,and a failure to
participate would have no detrimental effect on their
naval careers or the managementof their MTSS if such a
condition was detected.
Prospective data were collected at the start of basic
training and consisted of the measuring of biomechanical
parameters and the administering of a questionnaire. The
exercisesundertaken by the recruits during basic training
took up an averageof 16.2hours per week; they involved a
mixture of marching, doubling (running in step), circuit
training, and cross-countryrunning. During the training
period, recruits were monitored for the development of
MTSS. Following the completion of basic training, all
recruits underwent a confidential exit interview to determine the incidence of lower limb injuries incurred during
the training.
For the purpose of this study, MTSS was defined as
"pain along the posteriomedial border of the tibia that
occurs due to exercise excluding pain from ischaemic origin or signs of stress fracture." The diagnosis of MTSS was
based on the following criteria.
.Pain History. The pain was induced by exercise and
lasted for a few hours or days after exercise. Pain
was located on the posteriomedial border of the
tibia. There was no history of paraesthesia or other
symptoms indicative of other causesof exerciseinduced leg pain.
.Location. The recruits identified pain along the posteriomedial border of the tibia. The site had to be
spread over a minimum of 5 cm. Focalareas of only
2 to 3 cm are typical of stress fracture.4,s
.Palpation. Palpation of the posteriomedial border of
the tibia produced discomfort that was diffuse in
nature and confined to the posteriomedial border of
the tibia. In the areas of discomfort, the bone surface may feel uneven.
Those subjects who developedMTSS during the study
were placed in the symptomatic MTSS group after meeting the following strict criteria:
.positive MTSS diagnosis,
.absence of symptoms indicative of other causesof
exercise-inducedleg pain, and
.absence of MTSS at the commencementof the
study.
774
Yatesand White
The American
Journal of Sports Medicine '..f
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~
Any subjects with symptoms suggestiveof other causesof
exercise-induced leg pain were excluded from the study.
Imaging investigations were not undertaken to confirm or
exclude MTSS for the reasons previously stated. Any subject with a history of lower limb surgery or fracture likely
to alter their normal lower limb alignment was excluded
from the study.
Recruits were questioned about anthropometric data
such as age, sex, height, and weight. A previous history of
lower limb injuries, including exercise-inducedleg pain,
was recorded. A photograph dividing the anterior part of
the lower leg into 5 distinct areas helped identify the site
of any previous exercise-induced leg pain (Figure 1). For
all previous or current injuries, the duration of pain,
treatments implemented, and activity inciting the pain
were recorded. All recruits were then questioned about
their training schedule or sporting activities in the previous 3 months. The type of exercise,weekly frequency,duration, and type of exercising surface were recorded.
Following the questionnaire, all subjects underwent a biomechanical assessmentincluding measurementsof ankle
joint dorsiflexionand foot posture. The person undertaking
these measurements was blinded to the results of the
questionnaire.
The foot posture index (FPU is an observational test
that determines whether a foot is in a pronated, supinated, or neutral position based on 8 parameters.41-43
These
parameters can also distinguish between frontal, sagittal,
or transverse plane positional differences. The FPI has
been shown to have good intratester and intertester reliability, ranging from 0.73 to 0.87 and 0.66 to 0.78, respectively.41.43
This reliability coefficient is much higher than
most clinical foot biomechanical measurements.
The FPI is based on the observation of 7 visual parameters, and the 8th parameter is determined by palpating the
position of the head of the talus. All measurements are
recorded with the patients standing at their natural angle
of base of gait. The participants were required to stand on
a platform and were instructed to walk up and down on
the spot. After 10 seconds,they were asked to stop, place
their arms by their side, face for\vard, and distribute their
weight evenly through both legs and feet.
Different observations are seen for both foot pronation
and supination. The 8 parameters required to assessoverall foot posture for each foot were the following:
Figure 1. Location of symptoms of exercise-induced leg
pain. 1, proximal posteromedial tibial border; 2, middle posteromedial tibial border; 3, distal posteromedial tibial border;
4, anterior compartment; 5, anterior tibial border.
5.
6.
1. Prominence in the region of the talonavicular
joint. Bulging in this area is associated with a
pronated foot. Indentation is observed in the
supinated foot.
2. Calcaneal frontal plane position. It is everted in
the pronated foot and inverted in the supinated
foot.
3. Helbing's sign. As the calcaneus everts in the
pronated foot, the tendo achilles can "bow," with
the inferior part directed to the lateral side. In a
supinated foot, the tendo achilles can bow,with the
inferior part directed to the medial side.
4. Supra- and infra-lateral malleolar curvature. In a
pronated foot, the curve below the malleolus is
7.
8.
more acute than the curve above. In a supinated
foot, the curve above the malleolus is more acute
than the curve below.
Congruenceof the lateral border of the foot. In a
pronated foot, the lateral border develops a concaveprofile as the forefootabducts on the rear foot.
In a supinated foot,the lateral borderdevelopsa convex profile as the forefootadductson the rear foot.
Talar head palpation. In a pronated foot, the head
of the talus is more prominent on the medial side.
and in a supinated foot, it is more prominent on
the lateral side.
Congruenceof the MLA. The MLA is lo\vered in a
pronated foot, particularly the proximal part of the
arch. In a supinated foot, the proximal part of the
arch is high.
Abduction/adduction of the forefoot on the rear
foot. In a pronated foot, the forefoot abducts.
resulting in more of the toes being visible on the
lateral side (the too-many-toessign). In a supinated foot, the forefoot adducts relative to the rear
foot,resulting in the toes being more visible on the
medial side.
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Some of these parameters can be seen in Figure 2. The
parameters for the FPI were measured for both feet. Each
parameter is graded from -2 to +2, whereby +2 is assigned
for marked positive signs of pronation, +1 for moderate
signs of pronation, 0 for neutral, -1 for moderate signs of
supination, and -2 for marked signs of supination.
Therefore, the final score ranges between -16 and +16. A
normal foot has an aggregate scoreof +1 to +5. A pronated
foot posture ranges from +6 to +11, with a value of +12 to
+ 16 indicating a highly pronated foot. A supinated foot
posture ranges from 0 to -4, whereas a significantly negative aggregate of -5 to -16 indicates a highly supinated
foot posture.43The mean values for groups are categorized
based on the whole number and are not rounded up or
down.
A decreasein ankle joint dorsiflexion has also beenassociated with excessive subtalar joint pronation and has
been identified as a risk factor for MTSS.21,50,55
Testing
inkle joint dorsiflexion with the knee both fully extended
and flexed at 90° determines any difference in flexibility
between the gastrosoleus complex and the soleus muscle
in isolation. Ankle joint dorsiflexion can be measured clinically using a universal goniometer. Most studies of the
reliability of goniometric measurement for passive or
active ankle joint dorsiflexion have demonstrated high
intratester reliability (0.82-0.90) with comparatively low
intertester reliability (0.20-0.50).17,46,56
Intratester reliability for ankle joint dorsiflexion, subtalar joint measurement, and the FPI were completed during a pilot study
and were 0.83, 0.70, and 0.91, respectively.
The nonweightbearing parameter involved the measurement of the degreeof ankle joint dorsiflexion with the knee
fully extended and then flexed at 90°. Ankle joint dorsiflexion was measured using a universal goniometer
(Zimmer Ltd., United Kingdom) that had numerical divisions marked on the goniometer in 1° increments. To
ensure greater accuracy,the subtalar joint was held in the
neutral position, preventing abnormal dorsiflexion that
occurs through subtalar and oblique midtarsal joint pronation.54Subtalar joint neutral is found by moving the joint
through its range of motion to a point where the joint is
neither supinated nor pronated. This point is where the
medial and lateral aspects of the talus are congruous on
both sides.15
At the completion of the 10-weekbasic training course,
all recruits who participated in the study were required to
undergo a brief anonymous and confidential exit interview. Recruits were questioned on the developmentof any
lower limb injuries during the training period. For any
injury, the site, duration, treatment, and activity that produced the discomfort were all recorded. For those with a
history of exercise-induced shin pain, the criteria established for the diagnosis of MTSS were applied at this
moment. The subject was questioned about the pain history and location of the pain using the photograph in Figure
1. If the pain was located along the posteriomedial tibial
border, the site was palpated to see if the pain could be
reproduced. If all the componentsof the established criteria for MTSS were met, the subject was placed in the
Medial Tibial StressSyndromeAmongNaval Recruits 775
Figure 2. Posterior view of a pronated foot as defined by the
foot posture index. 1, talar navicular prominence; 2, calacaneal frontal plane position; 3, Helbing's sign; 4, inferior and
superior lateral malleolar curves; 5, congruence of the lateral
border.
symptomatic MTSS group. All subjects not meeting these
strict criteria were placed in the non-MTSS group.
Statistical analysis included t tests to determine any
statistical significance between variables and bet\veen
thosewho developedMTSS and those who did not. For any
factor found to be significant, the clinical degreeofsignificancewas determined using relative risks and 95% confidence intervals using SPSS software (SPSS Science,
Chicago, Ill). Relative risk is the ratio of the t\\"o risks.
for example, the risk of developing MTSS with a previous history of the condition compared to risk of developing MTSS with no previous history.
RESULTS
One hundred and twenty-four naval recruits (84 men and
40 women) participated in the study. The mean age was
21.06 years with a range of 17 to 35 years (SD = 4.26).
Prior to naval training, 116 (950/()recruits had undertaken
some form of physical exercise.Eight recruits did not per-
4.91
776 Yatesand White
The American Journal of Sports Medicine
(
TABLE 1
Mean, Standard Deviations, and t Tests for the Total Weekly Hours of
Exercise by the Medial Tibial Stress Syndrome (MTSS) and Non-MTSS Groups
Non-MTSS Group (n = 72) MTSS Group (n = 40)
Mean
Total weekly hours exercising
Total weekly weightbearing exercisehours
Total weekly weightbearing exercisehours excluding walking
form any weekly physical activity. The mean time spent
exercising weekly for all recruits was 6.81 hours (SD =
6.06 hours).
During the training period, the recruits were required to
complete 16.2 hours of intensive physical conditioning per
week. Week 1 and 2 were composed of relatively light
training compared to the following weeks. The physical
training consistedof marching, doubling (running in step),
circuit training, and cross-country running. Australian
Defenceforce running shoeswere worn for circuit training
and running. In all other weightbearing activities, standard-issue heavy-duty military boots were worn.
At the completion of training, 112 of the initial 124
recruits remained in the study, thus representing a
dropout rate of 10% within the study. Of the 12 recruits
unavailable, 5 were discharged, 3 back classed,2 hospitalized, and 2 unavailable due to other commitments. Of the
112 recruits followed prospectively through the training
period, 78 (70%)were men and 34 (30%)were women.
Forty naval recruits (22 men and 18 women) developed
MTSS during the training period, an incidence of 35%.
Female recruits had a high incidence of MTSS, with 18 of
them (52.9%) developing the condition compared to 22
male recruits (28.2%). Using a chi-square analysis, the
relationship between group membership and gender was
found to be P = .012,indicating a significant relationship
between the developmentofMTSS and gender.A risk estimate for gender and the development of MTSS showed a
relative risk of 2.03. Therefore, female recruits had twice
the risk of developing MTSS than male recruits.
The mean age of the MTSS group was 20.95 years (SD =
3.92) with an agerange of 17to 33 years.The control group
had a similar mean age of 21.40 (SD = 4.58) with an age
range of 17 to 35 years. The BMI of the MTSS group was
23.95 with a range of 18.12 to 30.35 (SD = 2.50). The control group had a mean BMI of 23.90 and a range of 17.91
to 32.37. There was no significant difference betweenthe 2
groups (P = .917).
Prior to the training regime, 11 (28%.)of the MTSS subjects did not perform any weightbearing training compared to 7 (10%)of control group subjects. If walking and
nonweightbearing training (eg, swimming) are excluded,
17 (43%)of the MTSS and 10 (14%)of the control group did
not undertake any exercise. However, as can be seen in
Table 1, these differenceswere not statistically significant.~
SD
6.12
Mean
SD
7.454.00 5.45
4.11
4.82
3.68
5.53
2.41
3.25
7.12
t test
t(110) = 0.66, P =.95
t(110) = 1.06, P = .29
t(110) = -1.33, P = .22
The location of symptoms in recruits who experienced
exercise-induced shin pain can be seen in Figure 3. All
recruits diagnosed with MTSS complained of pain along
the distal, middle, or proximal posteriormedial border of
the tibia. The pain occurred mostly in the middle third of
the posteriomedial tibia, with 61% of all symptoms occurring in the middle third.
Twenty-eight recruits (70%) with MTSS did not seek
medical assistance for their injury. Eight (67%) of those
who sought medical assistancewere restricted to lighter
activities (ie, no running or marching). During training,
recruits with MTSS missed or "trained lightly on 64 days,
comparedto a total of 46 days for all other injuries. Forty
of these forty-six days were seen in the non-MTSS group
and six in the MTSS group.
Of the 26 recruits that had a history of MTSS prior to
enlisting, 11 (42%) developed the condition during the
training period. Therefore,28% of the MTSS group had a
previous history of MTSS.This produced a relative risk of
1.52,so recruits with a previous history of MTSS were statistically more likely to develop the condition than were
their colleagueswith no history.
By the beginning of the 4th week of training, 27 recruits
(68%)of the MTSS group had developedthe condition. All
recruits diagnosed with MTSS were symptomatic at the
end of the training program.
All recruits were questioned about the specific training
exercisesthat causedthe pain associatedwith MTSS. The
results can be seen in Figure 4. Thirty-four of the forty
recruits identified some form of training as the main cause
of the symptoms, with running accounting for 63% of
recruits' symptoms.
MTSS was bilateral in 35 cases and unilateral in 5.
resulting in 75 limbs being examined. The means and
standard deviations for ankle joint dorsiflexion with the
knee extended and flexed for both the MTSS and control
groups can be \iewed in Table 2. These results indicate
that although the MTSS group had slightly less ankle joint
dorsiflexion with both the knee extended and flexed, the
results ,vere not statistically or clinically significant (P = .11),
Table 3 reveals the means and standard deviations for
the FPI of the l\ITSS and non-MTSS groups. There was a
statistically significantly greater FPI total for the MTSS
group compared to the non-MTSS group (P = .002). This
indicates that l\ITSS subjects have a significantly more~
j:fI:' Vol.32, No.3, 2004
Medial Tibial StressSyndromeAmongNaval Recruits 777
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TABLE 2
Means and Standard Deviations
for Ankle Joint Dorsiflexion
t/J 70
E 60
:J 50
'0 40
111 30
.D
E 20
:J 10
Z
0
.D
..
Knee position
Group
Mean (degrees)
SD
10.5
11.7
16.86
18.5
4.9
4.5
5.7
5.8
MTSSa
ControlMTSS
Proximal Medial
Third
Middle Medial
Third
Distal Medial
Third
Figure 3. Location of symptoms in limbs diagnosed with
media! tibial stress syndrome.
12.
Control
a MTSS, medial tibial stress syndrome.
TABLE 3
Means and Standard Deviations
for the FootPosture Index
-Running
I-Marching
Flexed
Flexed
Group
Mean
SD
Control
7.45
5.47
3.17
3.15
[J Doubling
8
0 Running/Marching
j ~ Running/Doubling
j 0 Marching/Doublin9
I
2
I-Basic training
0
Figure 4. Specific training exercises that were associated
with causing medial tibial stress syndrome.
pronated foot type. A risk estimate was performed, and a
relative risk of 1.70was identified. Recruits with a pronated foot type were almost twice as likely to developMTSS
than were those with a normal or supinated foot posture.
Figure 5 showsthe FPI categoriesof the MTSS and nonMTSS groups. The MTSS group occupieda higher relative
percentageof the pronated FPI category.Of the MTSS subjects, 22.5% (9 of 40) were placed in the normal (+1 to +5)
category,compared to 51.4% (37 of 72) of non-MTSS subjects. Twenty-nine MTSS subjects (72.5%) occupied the
pronated category,comparedto thirty-two (44.4%)of nonMTSS subjects.Only 2 subjects fell into the supinated category,both of which were non-MTSSsubjects.
DISCUSSION
The incidence ofMTSS has varied among civilian and military populations. In 2 military studies, the incidence has
varied from 4.0% to 6.4%.2.5In contrast, the only civilian
population study reported a much higher incidence of
13%.11Compared to these reports, the present study
reported a very high incidence ofMTSS. A number offactors exist within the study that could explain this higher
incidence rate. In the current study, both male and female
recruits performed an average of 16.2 hours of physical
activity weekly. In comparison, naval recruits in the
Almeida et al2 study undertook 12.7 hours of physical
activity weekly.This representeda 33% increase in weekly
ExtendedExtended
MTSS
~
Subjects
Figure 5. The percentage of medial tibial stress syndrome
(MTSS) and non-MTSS subjects by foot posture index category (no subjects were highly supinated).
training for recruits participating in the current study.The
weekly level of exercise in the Andrish et al5 study is
unknown.
Recruits' awareness of the condition could have also
affected the reported incidence. Prior to the commencement of the study, all recruits were briefed and given information regarding the symptoms ofMTSS. They were notified that reporting the condition to the researcherswould
remain confidential and not detrimental to their career.As
a consequence,recruits \vere comfortable in coming forward with any type of shin pain to the researchers. It is
known that military trainees often hide injuries from fellow recruits or officers. In the Almeida et al2 study, 35% of
all injuries were unreported. This may be reflected in the
current study as only 30l7cof recruits who developedMTSS
sought medical treatment. This hiding of injuries highlights one of the problems associatedwith using military
populations in injury epidemiologicalstudies.
rhe
778
Yatesand White
It is also possible that the symptoms associated with
MTSS were relatively mild in somerecruits and therefore
did not require medical treatment. Recruits with minor or
vague symptoms were not included in the symptomatic
group, and all recruits diagnosed with MTSS had complained of symptoms for a minimum of 7 days. It is difficult to gauge the severity of MTSS, particularly when
motivation to continue exercising is high. Some studies
have tried to grade the condition using bone scintigraphy7.8.13.57
or MRI.6.2oHowever, the results have been
mixed and are often conflicting. 52The use of a visual analog pain scale could have helped gaugeseverity, and this is
being incorporated in a further study. Confirmation of
the diagnosis by imaging techniques would have
strengthened the study, and it is recognized that this is
a potential limitation.
Two thirds of the MTSS group developedthe condition
by the 4th week of training. It is known that the boneremodeling sequence at the start of an exercise program
commences approximately 5 days after stimulation and
that the bone is relatively weakened for the first 8
weeks.9.19
This is one of the main concepts of the bone
stress reaction theory in the pathophysiologyofMTSS and
stress fractures. The early developmentof symptoms, most
commonly in the middle third of the tibia, in relatively
unconditioned recruits as seen in this study would tend to
support the bone stress reaction theory as the cause of
MTSS.
Until recently, few well-controlled studies reporting
whether gender affected the development of MTSS have
existed. Yates55 reviewed all MTSS studies, finding a
cumulative number qfmen and women in previous studies
that were almost equal (89 men and 80 women). In a large
study reporting gender differences in musculoskeletal
injury rates, Almeida et al2 followed 176 men and 241
women through boot camp army training. During this
period, 12 men (6.9%) and 14 women (5.9%) developed
MTSS. More recently, Bennett et alII followed 125 high
schoollong-distance runners through an 8-week training
period. They found that women were at an increased risk
of developing MTSS, with 13 of 68 women (19%)and 2 of
57 men (3%)developingMTSS.
The increased risk to women was also seen in the present study, with a risk estimate for gender and MTSS
demonstrating a relative risk of 2.03. Therefore, female
recruits had twice the risk of developingMTSS than males
in the study. Foot posture, BMI, previous injury history, or
prior exercise levels cannot account for the differences in
incidence rates betweengenders as they were comparable
betweenthe sexes.
Higher female injury rates among military populations
have been well documented.2i.28.38.44
It has been shown
that injury rates are higher when women train alongside
men and are expectedto achieve the same exit fitness leveIs.'1222Th1S
'
may be becausewomen are generally smaller
in stature. Anecdotal evidence in the current study may
support this, as many female recruits reported that MTSS
was precipitated when they had to "keep up" with male
recruits while marching or doubling (jogging in step). This
AmericanJournal of SportsMedicine
may have resulted in overstrenuous gait changes with
abnormally long stride lengths, thus increasing the risk of
developing MTSS. This theory is supported by Jones et
al,25who found that the shortest 25% of women had a
greater risk of injury than the taller 75% (relative risk =
1.7, P = .02). Men and women trained separately in the
Almeida et al2study, which may have resulted in the nonsignificant result when comparing gender. Changes have
now been made to the naval-training program with male
and female recruits training separately. The incidence of
MTSS and other lower limb injuries with the new training
program is now the subject of a further study.
There has been some confusion in the literature as to
the exact location of the pain associated with MTSS.
Various authors have describedMTSS pain as occurring in
the proximal, middle, or distal thirds of the posteriomedial tibial border or the anterior border. This study attempted to identify the location of pain associated \vith MTSS
clinically, without the use of imaging techniques.This \vas
achieved by dividing the tibia into the anterior muscle
compartment, anterior tibial crest, and the posteriomedial
distal, middle, and proximal thirds of the tibia. The results
from the current study supported those by Batt et aI,s who
found that the distribution of posteriomedial pain in
MTSS subjects occurred mo.,Stcommonly in the middle
third, followed by the distal third and far less frequently in
the proximal third. Therefore,our results support the commonly held belief that MTSS can occur within all posteriomedial tibial thirds but most frequently occurs along the
posteriomedial distal t\VOthirds of the tibia.
Although variability existed, 91% of all symptoms of
MTSS from the current study occurred to the posteriomedial distal and middle tibial thirds. Beck and OsterniglO
found that although large individual differences existed
for inferior attachments of the soleusand flexor digitorum
longus muscles,the mean inferior attachments \vere 48%
and 35%, respectively, along the posteriomedial tibia.
Therefore,based on our clinical findings, the current study
supports the view held by Batt et al8 that MTSS is more
likely to be related to a bone stress injury rather than a
primary traction periosteal injury becausethe soleus,tibialis posterior, and flexor digitorum longus muscles originate too far proximally to be linked to distal s)mptoms.
Milgrom et al33 found that maximum tibial bending
occurred at the narro\vest diaphyseal \vidth, the weakest
portion of the bone. The middle third of the tibia corresponds to the narro\vest diaphyseal width, where the
symptoms of MTSS most commonly occurred. These facts
would support the bone stress reaction theory. It is kno\\-n
that bending forces playa significant role in the development of stress fractures.33Several authors have hypothesized that MTSS and stress fractures are related along a
continuum of bone micro-damageand reparative processes, whereby MTSS is a relatively mild expression and
T.b.
.S
stress firact ure IS
a severeextreme." 1357 1 la I stress fractures most commonly occur in the middle or distal thirds
on the posteriormedial border.
One of the main aims of the study \vas to determine if
there were biomechanical differences between the 2
Vol. 32, No.3, 2004
'.,.,
!
Ii groups.The MTSS subjects scored higher on the FPI com, ;, pared to the non-MTSS subjects. The mean score for the
::
Medial Tibial StressSyndromeAmongNaval Recruits 779
":c
'-:1
i
.1
I
:3
:]
MTSS group placed them in the pronated category (+6 to
+11), whereas the mean score for the non-MTSS group
positioned them in the normal category (+1 to +5). The
results were statistically significant (P = .002).The results
of the asymptomatic group are comparableto the results of
the largest study using the FPI, where the meanvalue was
4.9.43The FPI results of the current study also appearclinically significant as the mean score between groups differed by 2 reference points. For a pronated foot type, the
relative risk of developing MTSS was 1..70.
The results from the present study support previous retrospectivestudies that have identified a pronated foot type
or excessive subtalar joint pronation as being a cause of
MTSS.11,Sl,50,5S,55
Limitations exist within this study
becausethe FPI does not assessdynamic foot function and
the pronated foot posture may not have been reproduced
Juring walking, marching, or running gaits. However, it
has many advantages,including its reliability, its validity,
the short time it takes to complete the test, and its not
requiring of expensive gait analysis equipment.
Subtalar joint pronation is an integral component in
absorbing ground reaction forces during gait. An excessive
amount or change in timing or velocity of pronation has
beenproposedto require the smaller intrinsic foot muscles
and larger extrinsic anti-pronatory muscles to fire for
longer while contracting eccentrically.5O.55
As a result, muscle fatigue occurs earlier, which subsequently increases
the amount of force absorbed more proximally on the
tenoperiosteum and bone. The cause of the increased
pronated foot position seen in the MTSS group was not
due to a lack of ankle joint dorsiflexion.
Excessive eccentric muscle contraction has been linked
to the development of overuse injuries, including MTSS.
Richie et al45studied the difference betweeneccentric and
concentric muscle activity in 10 runners who ran on 3 different surfaces of varying hardness. He found that subjects who exercised on hard, noncompliant surfaces such
as concrete produced the greatest eccentric medial shin
muscle activity and muscle pre activation to improve
intrinsic shock absorption, subsequently leading to
fatigue. Naval training consists of high-impact exercises
such as running and marching on hard, unyielding surfaces such as asphalt. Therefore, considerable levels of
intrinsic shock absorption are required, accentuating
eccentric muscle activity, and may contribute to the development of the condition. Therefore, it is likely that an
increased amount of force is absorbedmore proximally on
the tenoperiosteum and bone and this may contribute in
the pathogenesis of MTSS. High-risk groups such as
women or those with a pronated foot type may require
shoe inserts or orthoses to reduce intrinsic shock absorption, thus reducing the force absorbed by the tenoperiosteum and bone. Possible changes to the running shoe or
navy boot to incorporate more shock-absorbingmaterials
or anti-pronation structure could be beneficial. Also,training on softer surfaces suchas grass will reduce the amount
of intrinsic shock absorption required.
A pre-fitness-training program that incorporates the
essential naval-training activities of walking, marching,
and running should be designed that gradually increases
in intensity. This program may better prepare recruits for
the demandsof physical training, thus decreasingthe incidenceof MTSS.
CONCLUSIONS
MTSS is a commoninjury amongmilitary recruits undergoing basic training and frequently results in lost training
days. This research has highlighted both gender and a
pronated foot type as 2 significant risk factors for MTSS.
Controlling excessivefoot pronation and enabling female
and male recruits to train separately should be undertaken to attempt to reduce the incidence ofMTSS among military recruits.
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