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Simultaneous bilateral posteromedial tibial epiphysis stress fractures in a healthy young man: A case report

We present a compelling case of simultaneous, bilateral tibial stress fractures occurring in a unique epiphyseal and posterior location, with unclear aetiology. An overweight, Caucasian male in his late 20s developed synchronous bilateral medial knee pain following an intense 10-day training regimen. His radiographies were normal, but MRI revealed almost identical bilateral stress fracture lines in the posteromedial tibial epiphyses. Bone mineral densitometry and a full metabolic and hormonal panel were performed to further investigate potential underlying metabolic bone disease. He was found to have normal bone mineral densitometry and low Vitamin D serum values. Symptomatology greatly improved with activity modification. There were no further complaints and complications at 12 months’ follow-up. Diagnosis can be challenging and the treating physician should be acquainted with the basic science of stress fractures and main discriminating clinical, biochemical and radiological characteristics from insufficiency fractures, to avoid pitfalls in treatment decision.


Introduction
Pain in the medial tibial condyle may be a challenging disorder for physicians, particularly if it presents bilaterally and simultaneously. Stress fractures in this particular location are easily misdiagnosed, especially in the absence of risk factors. In the general population including the elderly, microtrabecular insufficiency fractures, also known as "spontaneous osteonecrosis", evolving osteoarthritis with or without meniscal degenerative tear, traumatic meniscal tear and pes anserinus bursitis are included in the differentials 1 . In athletes, meniscal injury, medial collateral ligament sprain, synovial plica entrapment, patellofemoral impingement, chondral pathology, medial tibial crest friction syndrome, and stress fractures are included in the differentials 2 . Stress (fatigue) fractures present more often in athletes or military recruits following sudden increased intensity of the training regimen. They are most commonly unilateral and located in the posteromedial tibia diaphysis followed by the distal third 3,4 . The anterior cortex is a rare location of a high risk fracture of this tension side, and even more rare is the tibial plateau 5,6 . We present herein a unique case of simultaneous bilateral posteromedial tibial epiphysis stress fracture in a healthy individual without underlying causative factors.
his first medical consultation, without any history of specific traumatic incident. Prior to pain presentation, the patient had just completed a ten-day training period in order to meet specific criteria to join the police academy. The training consisted of long walks and running sessions on hard tarmac. Before the training period, his lifestyle was rather sedentary. The patient provided a detailed account of his daily training sessions because he was using a specific application on his smart phone (Table 1). He had no previous knee injuries or locking-like symptomatology. The patient denies altering his daily diet habits before, during or after training.
On physical examination, no limb malalignment was present. His knees had no apparent effusion/swelling and there were no skin abnormalities. Temperature of both knees was normal. There was significant tenderness over the medial tibial condyle in a wide area including the medial joint line, the pes anserinus and the medial collateral ligament insertion, consistent with the reporting pain site in both knees. Both passive and active range of motion were full and unrestricted and elicited only mild discomfort. Special tests assessing cruciate ligament instability were within normal range. Valgus force test elicited pain, without gross instability. The McMurray test for medial meniscus was positive for both knees, although the examining physician felt that it was nonspecific for the patient's symptomatology. Anteroposterior and lateral plain radiographs of both knees were ordered and were reported back as normal ( Figure 1). Simple painkillers were prescribed and non-weight bearing ambulation with a wheelchair was strongly advised. An MRI was requested before his next outpatient appointment.
The MRI revealed almost identical findings for both knees in keeping with medial-posterior epiphyseal stress fractures of the medial tibial condyle (Figures 2 and 3). Posterior slope of the medial tibia plateau, as measured from the sagittal MR images, were found to be 4.14 o for the right knee and 5.4 o for the left knee 7 . A complete hormonal and metabolic workout was consequently requested, as well as bone mineral densitometry (BMD). PTH was 58.7 pg/mL, serum calcium was 9.6 mg/dL, FT4 was 1,2 ng/dL and TSH was 2,1 mIU/L. The only parameter below normal values was 25(OH) Vitamin D (17 ng/mL, potential deficiency). His BMD, measured at the neck of his non-dominant (left) hip and lumbar spine by dual energy x-ray absorptiometry, were 1,01 g/cm 2 (T-score:0.5 and Z-score:0.1) and 1,166 g/ cm 2 (T-score:-0.4 and Z-score:-0.3) respectively, which was within normal range for his age.
The patient was classified as low-risk for an imminent complete fracture. Vitamin D supplementation and typical anticoagulation medication was prescribed and he was directed to continue with no weight bearing (with the use of a wheelchair) for 6 weeks until the next outpatient visit. In his 6 weeks' follow-up consultation, the patient's symptoms had greatly improved. His VAS pain was 0/10 at rest and 5/10 when full weight bearing. He was advised to continue for 6 weeks with protected weight bearing with the use of a walking frame (20% of body weight). The patient continued to improve, his 25(OH) levels normalised and in his next six weeks' appointment (three months since pain onset), symptomatology was completely resolved and the patient was able to walk pain-free and without support. A 12-month follow-up consultation took place over the phone, the patient continued to be symptom-free, at both rest and exercise. The patient gave full written informed consent for publication of his history and imaging details.

Discussion
Stress fracture is a common entity encountered across all patient demographics. Stress fractures do not occur from a single traumatic event and they are classified as either fatigue when there is increased loading on normal bone or insufficiency when they result from normal forces applied on weak bone. Fatigue fractures in particular, are produced by submaximal loading characterized by increased frequency, duration, or intensity over a relatively short period of time 8 . The pathophysiology is quite simple: when repetitive loading is continued without appropriate rest, the continued loading on a more porous bone may result in a positive feedback cycle of increased strain, microdamage accumulation, and bone resorption due to remodelling, until microcracks coalesce and stress fracture occurs 9 . This more porous bone with decreased elastic modulus, results from repair of microdamage accumulation via coupled remodelling, a paradoxical process that unfortunately takes time. Bone remodelling introduces an acute increase in porosity but may also prevent stress fractures by replacing damaged bone, given time. This process creates two possible candidates susceptible to the possibility of lower extremity stress fracture: a professional or semi-professional athlete suffering from overuse injury after altering his training intensity abruptly or the "weekend warrior" type of athlete, a category that our patient falls in, who only intermittently engages in strenuous activity without regular training 10 . In a recent study, examining the relationship between obesity and risk of stress fracture, risk increased in subjects with high percentage of body fat as calculated by the equation %BF=-7.53+1.43xBMI+0.13xAge-14.73xSex. According to this study, our patient's body fat percentage is at the highest decile and therefore has a 15% greater risk of stress fracture compared to men in the mean body fat decile 11 . Other studies, although some of them are statistically underpowered, have failed to show a relationship between obesity and stress fractures [12][13][14][15] .
History taking is of utmost importance when stress fractured is suspected. Training regimen, footwear, training surface, and type of sport are modifiable extrinsic causative factors of stress fractures 10 . Pain pattern can also help guide diagnostic process towards or away from neoplastic causes 16 . History alone is not merely enough. The common site for stress fractures is the middle and distal thirds of the tibia. The proximal metaphysis is rarely affected and the proximity to the knee joint can make the diagnosis difficult to differentiate from a lesion within the joint or systematic disease. Furthermore, the treating physician needs to have a thorough knowledge of bone metabolism and disease, to avoid diagnostic and therapeutic pitfalls. Plain x-rays may show the fracture in 50% of cases, in more than 3 weeks after the onset of symptoms. MRI provides the most comprehensive evaluation of stress injuries and is capable of excluding other disorders which might show a similar location of the symptoms 17 . The fracture is demonstrated on MRI with a band-like low signal intensity irregular line corresponding to  the fracture, which extends up to the cortex and is surrounded by bone marrow edema 18,19 . Thus, in our patient, the irregular low signal intensity fracture line with surrounding edema on fat suppressed images, had the typical appearance.
Apart from imaging, full and targeted laboratory work is required, such as thyroid and parathyroid function, vitamin D, calcium and phosphorus, in order to exclude and differentiate from insufficiency fractures. Vitamin D levels and BMD do not reliably correlate. PTH appears to predict BMD in a more predictable way, especially in young individuals. Recently published research demonstrates that loss of BMD is observed in vitamin D-deficient adults (both young and old) as long as secondary hyperparathyroidism is present 20 . The serum levels of 25(OH)D may not correlate with BMD, but definite causative relationship between low serum 25(OH)D levels at diagnosis and lower extremity stress fractures has been established among a large young athletic population (military personnel) 21,22 . The minimum 25(OH)D serum level that can guarantee adequate skeletal health in a patient with increased functional demands remains to be determined 23 .
Non traumatic fractures in the proximal tibia involving the medial metaphysis, correspond to insufficiency fractures 24 . They tend to occur in the elderly, mainly postmenopausal women, located inferior to the epiphyseal scar 25 . In one study including patients with exercise-induced knee pain, bilateral location was seen in 25% of the cohort and the most common anatomic location was the medial tibial plateau (31%) 26 . However, the "tibial plateau" was not defined and indeed in one figure, it is the medial tibial metaphysis showing the lesion. Bilateral location of proximal tibial stress fractures have been described in case reports 25,[27][28][29] and treating physicians have been misled from the clinical presentation of bilateral proximal tibia pain, performing even unnecessary surgery, before establishing diagnosis 25,28 . However, in all of them, the location was in the metaphysis and in only one report the location of the fracture was posterior 29 . In one Japanese study on the tibial plateau morphology and stress fractures, posterior and medial slope was measured from MRI a small sample of 14 patients with medial proximal tibial fractures 30 . In the aforementioned study, the "tibial plateau" again was not accurately defined and the fractures were located mostly posteriorly or posteromedially (area of greater weight-bearing stress) rather than anteromedially in the metaphysis, not in the epiphysis. The posterior slope in posterior (11.4 o ) or posteromedial (7.9 o ) type of fractures was significantly larger than that in anteromedial type (5 o ). In general, posteromedial and anteromedial posterior slope of medial tibial plateau fractures appeared to be lower than normal reported values (7.8-14.7 o ). Values range significantly between ethnicities, and different techniques have been used for measurement 7,[31][32][33][34] . There were no differences between fracture types in relation to the medial slope. Our patient also had a small posterior slope of the medial tibia plateau compared to normal published values.
In runners, tibia stress fractures take up 3.35% of total injuries, and they mostly occur in the middle and distal third of the tibia diaphysis, posteromedially or anteriorly [35][36][37] .
Proximal tibial stress fractures, metaphyseal or epiphyseal are considered a rarity and their incidence remains unknown. Our patient suffered from a stress fracture of both medial tibial epiphyses, an unexpected site, following running. Several biomechanic and simulation studies, have described medial tibial plateau contact stresses during walking, climbing stairs, or running [38][39][40][41] . The posterior portion of the medial plateau has been found to endure higher peak contact pressures than the anterior portion in the stance phase of gait at 14% of the gait cycle, which correspond to 15° knee flexion with axial loads of 2280N 38 . Wang et al also reported a contact stress pattern of the medial tibial plateau that consisted of a single peak with an average magnitude of 1.30 MPa during the stance phase of gait, which corresponded to the timing of the first axial force peak during normal gait (also at 14% of the gait cycle) 39 . Akpinar et al found that, during level walking and downhill running, the medial tibial plateau had a greater cartilage contact area across all percentages of the gait cycle (walking: 0-15%, running: 0-10%) versus the lateral compartment and a significantly smaller cartilage contact area during running versus walking from 8% to 10% of the gait cycle. Furthermore, along the tibial anteriorposterior axis, the medial compartment cartilage contact paths were significantly more posterior and longer at each percentage of the gait cycle (0-10%) during downhill running versus level walking 40 .
What makes our case unique is the epiphyseal and posterior location of the stress fracture which has not been previously described, to the best of our knowledge. In our patient, MRI shows the typical findings in favor of a fatigue fracture 18 . Possible causative factors include a combined effect of the posterior tibial slope, potential Vitamin D deficiency and the increased BMI, perhaps with a poor technique of running. MRI is not required for follow-up, as the clinical improvement alone allows return to normal activities 42 .
In conclusion, even in non-elite athletes, a careful history together with the increased awareness of exercise induced injuries, contribute to early and accurate diagnosis. MRI contributes to fracture detection and treatment planning.