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First published on September 16, 2005, doi:10.1177/0363546505278699
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The American Journal of Sports Medicine 34:78-83 (2006)
© 2006 American Orthopaedic Society for Sports Medicine

Bone Stress Injuries Causing Exercise-Induced Knee Pain

Maria H. Niva, MD*, Martti J. Kiuru, MD, PhD, MSc*,{dagger},{ddagger}, Riina Haataja, MSc§,|| and Harri K. Pihlajamäki, MD, PhD*,¶

From the * Research Institute of Military Medicine, Central Military Hospital, Helsinki, Finland, the {dagger} Department of Radiology, Helsinki University Central Hospital, Helsinki, Finland, the § Tampere School of Public Health, University of Tampere, Tampere, Finland, the ll Research Unit, Tampere University Hospital, Tampere, Finland, and the Department of Surgery, Central Military Hospital, Helsinki, Finland

{ddagger} Address correspondence to Martti J. Kiuru, MD, PhD, MSc, Topeliuksenkatu 5, PO Box 266, Helsinki, Finland 00029 HUS (e-mail: martti.kiuru{at}hus.fi).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: No comprehensive studies of bone stress injuries in the knee based on magnetic resonance imaging findings have been published.

Purpose: Assess the incidence, location, nature, and patterns of bone stress injuries in the knee in military conscripts with exercise-induced knee pain.

Study Design: Case series; Level of evidence, 4.

Methods: During a period of 70 months, 1330 patients with exercise-induced knee pain underwent magnetic resonance imaging of the knee. A total of 1577 knees were imaged; the images with bone stress injury findings were retrospectively reevaluated with respect to location and type of injury. The person-based incidence of bone stress injuries in the knee was calculated, based on the number of conscripts within the hospital’s catchment area.

Results: Of the 1330 patients, 88 (7%) met the inclusion criteria, and 141 bone stress injuries were found in the 110 knees imaged. The incidence of bone stress injuries was 103 per 100 000 person-years. Of the patients, 25% had bilateral bone stress injuries; 28% had 2 solitary bone stress injuries in the same knee simultaneously, all situated in the femoral condyle and tibial plateau. The most common anatomical location for a bone stress injury was the medial tibial plateau (31%), which was also the most typical location for a more advanced injury. After the commencement of military service, a bone stress injury in the medial tibial plateau caused knee pain earlier than did a bone stress injury elsewhere in the knee (P = .014).

Conclusion: The incidence of bone stress injuries in the knee with exercise-induced knee pain is relatively high in conscripts. Multiple and bilateral injuries can occur. For accurate diagnosis and to ensure appropriate treatment, magnetic resonance imaging is recommended as a routine imaging method when a physical activity can be regularly associated with the onset of symptoms.

Key Words: stress fracture • incidence • injury • knee • magnetic resonance imaging (MRI)


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Bone stress injuries resulting from overuse are frequent not only in athletes and in military trainees but also in healthy people who have recently started a new or intensive physical activity.10 However, reported cases of bone stress injuries in the knee in young adults are rare; only case reports have been previously published (Table 1Go). The diagnosis of bone stress injury is based on a history of increased physical activity and on imaging findings.1 The clinical diagnosis of bone stress injury is difficult, and plain radiographs can produce false-negative findings.5,11,21,32,43,51 In bone scans, false-positive cases are common, especially in the knee area, where osteonecrosis, osteochondritis dissecans, or ligamentous injury can result in abnormal uptake.8 One of the first choice diagnostic modalities for stress-related bone changes 1,2,6,12,21,25,46 and for the internal derangement of the knee joint is MRI.28,38 The purpose of the present study was to assess the incidence, location, nature, and patterns of bone stress injuries in physically active young adults based on MRI.


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  		TABLE 1 Reported Cases of Knee Stress Fractures (SF) in Previous Studiesa
 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
This retrospective study took place in the Central Military Hospital in Helsinki, Finland. It involved 1577 knees of 1330 consecutive military conscripts (34 women, 1296 men; age range, 17–29 years; mean, 20 years), who during a period of 70 months, beginning in 1997, underwent MRI because of stress-related knee pain. The conscripts were referred to the Central Military Hospital for an orthopaedic surgeon’s consultation because exercise-induced knee pain had disturbed their military training. The inclusion criteria for the present study were exercise-induced knee pain during the military service, a physical examination by an orthopaedic surgeon in the Central Military Hospital, a negative finding on a plain radiograph taken in the primary military health care unit, and a finding on an MRI of a bone stress injury. The exclusion criteria were infection and a known recent trauma involving the knee. The original medical records and MRIs of patients meeting these inclusion and exclusion criteria were reviewed and retrieved. The study was approved by the Medical Ethics Committee of the institution.

Physical examination by the orthopaedic surgeon involved careful history taking and palpation. The range of movement (extension-flexion) and the ligamentous stability (anterior-posterior and side stability) of both knee joints were examined. Skin changes were recorded. The ability to walk and to jump on 1 foot was examined, and any other aberrant observations were recorded. The symptomatic knees, based on patient history and on physical examination, underwent MRI.

The MRI scans were performed with a 1.0 T scanner (Sigma Horizon, GE Medical Systems, Milwaukee, Wis). A knee coil with a field of view of 10 to 16 cm was used. Slice thickness was 3 to 4 mm, with a 0.5-mm or 1.0-mm intersection gap. Sagittal proton density spin-echo sequence images with fat suppression (repetition time/echo time = 3400 ms/17 ms, with 2 signals averaged and a 256 x 256 [516] matrix) or sagittal T1-weighted spin-echo sequence images (repetition time/echo time = 680 ms/11 ms, with 2 signals averaged and a 256 x 256 [512] matrix) were obtained. T2-weighted fast spin-echo sequences with fat suppression were obtained in the axial images (repetition time/echo time = 2560 ms/85 ms, with 2 signals averaged and a 256 x 256 [512] matrix) and in the coronal images (repetition time/echo time = 4000–4600 ms/72–90 ms, with 2 signals averaged and a 256 x 256 [512] matrix).

Two musculoskeletal radiologists reevaluated the MRIs retrospectively and independently, without knowledge of the clinical data. In case of disagreement of the interpretation, a third musculoskeletal radiologist interpreted the MRI. The MRIs of the knees were analyzed for bone stress injury location and bone stress injury type. Bone stress injuries were graded as follows 22: grade I, endosteal marrow edema; grade II, periosteal edema and endosteal marrow edema; grade III, muscle edema, periosteal edema, and endosteal marrow edema; grade IV, fracture line; and grade V, callus in cortical bone. Internal derangement of the knee was recorded.

All Finnish men become liable for a 6-, 9-, or 12-month-long military service at the age of 18. Within the service area of the hospital, the total exposure time for the population at risk during the study period was 85 318 person-years. This number was calculated by registering the dates of entry and transfer or discharge of every conscript in the catchment area of the hospital. The catchment area was clearly defined. The Mann-Whitney U test was used to test the differences in the continuous skewed data between groups. Differences in cross tables were determined using the Fisher exact test. The limit for significance was set equal to .05. Data analysis was performed using SPSS/Win (version 11.0, SPSS Inc, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Based on MRI, 88 (1 woman, 87 men; age range, 19–29 years; mean, 21 years) of the 1330 patients had 141 bone stress injuries in the knee. Of the 113 knees in these 88 patients that underwent MRI, bone stress injuries were seen in 110 knees, and 3 knees appeared normal. Of the 1577 knees that underwent MRI, at least 1 bone stress injury occurred in 7% (110 of 1577) of the cases; internal derangement—meniscal lesions, ligamentous injuries, patellar chondromalacia, osteochondritis dissecans, and plicae—occurred in 8.6% (135 of 1577) of the cases; and in 84.3% (1329 of 1577) of the knees, there were no aberrant findings. The incidence of conscripts having sustained a bone stress injury in the knee was 103 per 100 000 person-years. Of the 88 patients, 53% had 1 bone stress injury, and 47% had 2 or more solitary bone stress injuries either in one or both knees. Altogether, 55% (77 of 141) of bone stress injuries occurred in the right knee and 45% (64 of 141) in the left knee.

After the commencement of military training, the median time for an orthopaedic consultation was 42 days (range, 11–207 days; n = 43 knees) in patients with bone stress injury in the medial tibial plateau and 59 days (range, 7–280 days; n = 70 knees) in patients with bone stress injury in another location (P = .014). In all 88 patients, the mean duration of knee pain before MRI was 55.6 days (median, 48.5 days; range, 13–180 days; n = 92 knees with pain data available). The median time to diagnosis of the bone stress injury by MRI after entering military service was 72 days (range, 21–319 days; n = 110 knees). There were no individual clinical findings found to be specifically related to bone stress injuries in the knee.

Both knees were imaged in 25 patients (28%) because of bilateral knee pain. Twenty-two patients (25%) had bilateral bone stress injuries, giving them the incidence of 26 per 100 000 person-years. Of these 22 patients, 13 (59%) had bilateral bone stress injuries in varying anatomical locations, 8 (36%) had bone stress injuries in the bilateral medial tibial plateau, and 1 (5%) had bilateral supra-condylar bone stress injuries. Two solitary bone stress injuries in the same knee simultaneously were found in 25 patients (28%; 29 knees, 4 bilateral), and all these cases involved the tibial and femoral condyles. The most common combinations of anatomical locations involving the same knee were medial tibial plateau with medial femoral condyle (11 patients, 13 knees) (Figure 1Go), entire tibial plateau with entire femoral condyle (7 patients, 8 knees), and entire tibial plateau with medial femoral condyle (5 patients, 6 knees). The most common anatomical location for bone stress injury was the medial tibial plateau (43 of 141; 31%), which was also the most typical location for a fracture line (67%) (Table 2Go, Figure 2Go). Of all 141 bone stress injuries, 83 (59%) were in the tibial plateau, 7 (5%) were in the proximal tibial shaft, 40 (28%) were in the femoral condyles, and 6 (4%) were in the femoral supra-condylar area (Figure 3Go). None of the fracture lines of the condylar bone stress injuries extended into the articular surface. There were no fibular bone stress injuries. Of the 141 bone stress injuries, bone marrow edema was exhibited in 65% of the cases. Patellar bone stress injuries were low grades (Figure 4Go). In the 25 patients with 2 bone stress injuries in the same knee simultaneously, no relationship between the anatomical locations and the grades was observed. Based on the MRIs of these 88 patients, 12 patients had a simultaneous intra-articular abnormality in the same knee as the bone stress injury: 6 patients had a meniscal tear, 2 patients had patellar chondromalacia, and 4 patients had a medial synovial plica.



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  		Figure 1. A 20-year-old male conscript suffering from left knee pain for 41 days. A, coronal T2-weighted fat saturated MRI reveals a high signal intensity indicating endosteal bone marrow edema both in the medial femoral condyle and in the medial tibial plateau (arrows). A tiny intraosseous fracture line is visible (arrowhead). B, axial T2-weighted fat saturated MRI demonstrates not only endosteal edema (arrow) but also periosteal edema (arrowhead) in the medial tibial plateau.

 

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  		TABLE 2 Anatomical Locations and Magnetic Resonance Imaging (MRI) Grades (I–V) of Bone Stress Injuries
 


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  		Figure 2. A 22-year-old male conscript suffering from right knee pain for 14 days. A, coronal T2-weighted fat saturated MRI demonstrates a fracture line (arrowheads) with associated bone marrow edema of the medial tibial plateau. B, sagittal T1-weighted image indicates a nearly horizontal, posteriorly wide, slightly irregular line of reduced signal intensity (arrowheads) in the proximal medial tibia around the epiphysial line.

 


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  		Figure 3. A 20-year-old male conscript suffering from right knee pain for 14 days. Coronal T2-weighted fat saturated MRI indicating a transverse supracondylar femoral fracture line (arrows) with associated bone marrow edema and soft tissue edema.

 


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  		Figure 4. A 19-year-old male conscript suffering from left knee pain for 21 days. A and B, sagittal and coronal T2-weighted fat saturated MRI reveal a high signal intensity indicating endosteal bone marrow edema of patella (arrows). C, axial proton density image demonstrates not only endosteal edema and periosteal edema but also soft tissue edema (grade III) (arrow).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In the present study, as compared with the previous literature (Table 1Go), a surprisingly high proportion, almost a tenth of conscripts, had a bone stress injury in the knee as a cause of exercise-induced knee pain. The incidence of bone stress injury to the knee among Finnish military conscripts is 103 per 100 000 person-years. This incidence is representative of the general conscript population during military service. Previous studies concerning knee bone stress injuries have been mainly case reports, and consequently, the real incidence is unknown. The reports on the general incidence of all lower extremity bone stress injuries have varied from 2% to 31%.3,4,19,31,33,42

In the present study, the exercise-induced knee pain and bone stress injuries appeared mainly during the basic training period, that is, the first 3 months of military service. This finding is in agreement with the findings of previous studies on bone stress injuries involving lower extremities.13,15,31,33,35,36,42 The abrupt increase in the intensity or duration of training, to which the trainees are often not accustomed in civilian life, is usually reported.11,13,35 In this study, after the commencement of military training, the conscripts with bone stress injuries in the medial tibial plateau sought medical advice about 2 weeks earlier than did patients with bone stress injuries in other anatomical locations. In a study by Engber et al,11 the delay between the inception of knee pain and the time when medical advice was sought was even shorter, a mean of 9 days (range, 3–50 days).

In this study, a quarter of all patients had a bilateral knee bone stress injury. The proportion of the bilateral bone stress injuries involving the medial tibial plateau, 47%, was lower than in earlier reports,11,13 in which the proportions were 74% and 70%, respectively. Depending on the determination of the bone stress injury, the proportion of bilateral bone stress injuries may have increased if the pain-negative knees had also undergone MRI and bone stress injuries had been calculated. However, an intense isotope accumulation in the medial tibial plateau in asymptomatic patients has also been considered to indicate an early physiological abnormality, a stress-related bone remodeling, and, therefore, a false-positive.11 The finding in the present study that about one third of the patients had 2 bone stress injuries in the same knee simultaneously seems to represent a very unusual combination of bone stress injuries in previously healthy individuals because we found no similar references in the English literature.

The weightbearing stress in the proximal tibia is thought to be greatest in the medial and posterior parts of the bone.40 In the shaft, the thick cortex handles most of the stress, whereas in the ends of the long bones, the cancellous trabeculae handle much of the weightbearing stress.43 In this study, the medial tibial plateau was the most common anatomical location involved with bone stress injury, accounting for about one third of all locations. In more than half of the cases, the location was exhibited by a fracture line surrounded by bone marrow edema. An explanation for this anatomical location may be found in the varying individual biomechanical factors23 and in the differences in exercise, terrain, and genetic factors as well. It is possible that with a larger female population, the number and distribution of bone stress injuries might have been different. Variation in the results of the previous studies (female recruits showed a greater risk and incidence as well as a different distribution of stress fracture than did male recruits) probably reflects not only the differences in the composition of each case series but also the differences in the biomechanical features and endocrine factors. 3,7,9,27,41,48

Military recruits with a running background had fewer stress fractures,15 whereas conscripts with multiple bone stress fractures had not participated in sports before their military service.30 In a recent report by Hohmann et al,17 MRIs of the hips and knees in long-distance runners displayed no bone marrow edema or periosteal stress reactions before and after a marathon race. It was thought that the runners’ long-lasting training history compensated for the extreme demands.

The study of choice for evaluating the trabecular bone of the condyles is MRI.7 It is usually adequate for making a distinction between a spontaneous osteonecrosis and an insufficiency stress fracture in the medial femoral condyle and in the medial tibial plateau.44,52 On an MRI, a highly sensitive early sign of a stress-related bone stress injury is the endosteal marrow edema, whereas the only specific MRI finding for a bone stress injury is a fracture line.25 In this study, about two thirds of all bone stress injuries, regardless of the anatomical location, appeared as grade I. However, a bone marrow edema is a nonspecific finding that can also be evident in bone bruises, infections, malignancies, and in asymptomatic physically active persons.2,20,39,44,46,50 In the present study, the differentiation between bone stress injuries and bone bruises was made on the basis of a negative trauma history.

The diagnosis of internal derangement of the knee has commonly been used as a name for the condition of the painful knee joint. In the literature review, we were not able to find any reference to an association between bone stress injury and internal derangement of the knee joint in young adults to support the findings of the present study. It remains unclear whether the meniscal tears observed in this study as causing medial knee pain might have had an effect in the development of bone stress injuries or whether those tears were such as found by MRI in approximately one third of the patients without knee complaints.8,24 In the present study, however, neither subchondral marrow edema nor sclerosis was seen adjacent to the ruptured menisci or chondral changes, as is usually the case with transient posttraumatic abnormalities.34

The typical diagnostic features in young adults with bone stress injuries are unaccustomed or unusual activity preceding localized pain with insidious onset, worsening of pain with progressive activity, and relief of pain by rest.7,11,15,19 The proximity of a bone stress injury to the knee joint can confuse the clinical diagnosis with a lesion within a joint 11,51; medial knee pain can raise a suspicion of a rupture of the medial meniscus.31,51 In conclusion, a typical patient history without recent trauma, unclear findings in the physical examination, and negative plain radiograph results should emphasize MRI as the diagnostic method for bone stress injuries before an arthroscopic procedure is performed.


   FOOTNOTES
 
No potential conflict of interest declared.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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