PM R 7 (2015) 584-592
Femoral Neck Stress Fractures and Imaging Features of Femoroacetabular Impingement Michael Goldin, MD, Christian N. Anderson, MD, Michael Fredericson, MD, Marc R. Safran, MD, Kathryn J. Stevens, MD
Abstract Background: Prior literature has suggested an association between the radiographic signs of femoroacetabular impingement (FAI) and femoral neck stress fractures (FNSF) or femoral neck stress reactions (FNSR). At the time of the writing of this article, no study has described the association of FAI and FNSF/FNSR along with the need for surgical intervention and outcomes. Objective: To determine the prevalence of radiographic features of FAI in patients diagnosed with FNSF. Design: Retrospective case series. Setting: Tertiary care, institutional setting. Patients: A medical records search program (Stanford Translational Research Integrated Database Environment, Stanford University, California) was used to retrospectively search for patients 18-40 years old with a history of FNSF or FNSR. The records were obtained from the period July 25, 2003, to September 23, 2011. Methods: For assessment of risk factors, plain radiographs and magnetic resonance imaging studies were reviewed for features of cam or pincer FAI. Medical records were reviewed to determine whether patients required operative intervention. Main Outcome Measures: Incidence of abnormal alpha (a) angle, abnormal anterior offset ratio, abnormal femoral head-neck junction, coxa profunda, positive crossover sign, and abnormal lateral center-to-edge angle. Results: Twenty-one female and 3 male participants (mean age 27 years, range 19-39 years) were identified with magnetic resonance imaging evidence of femoral neck stress injury. Cam morphology was seen in 10 patients (42%). Pincer morphology could be assessed in 18 patients, with coxa profunda in 14 (78%) and acetabular retroversion in 6 (14%). Features of combined pincer and cam impingement were observed in 4 patients (17%). Seven patients (29%) had operative intervention, with 3 (12%) requiring internal fixation of their femoral neck fractures, and all had radiographic evidence of fracture union after surgery. Four patients (17%) had persistent symptoms after healing of their FNSF with conservative treatment and eventually required surgery for FAI, 3 had no pain at final follow-up 1 year post-surgery, and one patient was lost to follow-up. Conclusion: The results of the current study suggest that patients in the general population with femoral neck stress injuries have a higher incidence of bony abnormalities associated with pincer impingement, including coxa profunda and acetabular retroversion, although it is unclear whether pincer FAI is a true risk factor in the development of FNSF.
Introduction Stress fractures can occur anywhere along the femur, but most commonly occur along the femoral neck or shaft . Femoral neck stress fractures (FNSF) can be classified into 3 categories, namely, tension-sided, compression-sided, or displaced. Classification systems of FNSF have been used to describe the degree of bony injury observed with plain radiographs  or magnetic resonance imaging (MRI)  MRI is the imaging modality of choice because it is highly sensitive and specific for diagnosing stress reactions of the bone, and it avoids
the ionizing radiation associated with computed tomography (CT) and scintigraphy [4-7]. Incomplete and compression-sided FNSF are typically treated nonoperatively . Operative fixation is usually reserved for tension-sided and complete fractures or for cases that fail nonoperative management . A number of factors are involved in the development of FNSF, including training intensity, diet, footwear, age, gender, lower limb alignment, and bone mass . An increase in training intensity is the most common risk factor involved  and can produce repetitive stresses that outpace the remodeling capacity of bone . The
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abnormal bony anatomy associated with femoroacetabular impingement (FAI) has also been implicated as a factor involved in the development of FNSF [11-13]. FAI is a common cause of hip pain and pathology that results from abnormal contact between the proximal femur and the acetabular rim during terminal range of motion . This pathomechanical process eventually results in characteristic damage to the labrum and acetabular cartilage, depending on the location of the osseous abnormality  The 2 most common osseous abnormalities that lead to FAI are a loss of the normal femoral head-neck offset, resulting in cam impingement, and acetabular over coverage, resulting in pincer impingement . In 2010, a retrospective study demonstrated an increased incidence of radiographic features of FAI in military recruits with FNSF compared to those without stress injuries . A more recent study has also shown an association between FAI and femoral neck stress fractures in military personnel ; however, the association between FAI and FNSF has not been determined in the general population. The purpose of this study was to determine whether there is an increased incidence of FAI in patients in the general population who presented with stress fractures or stress reactions of the femoral neck. Method Institutional review board approval was obtained for data collection. A medical records search program (Stanford Translational Research Integrated Database Environment, Stanford University, California) was used to retrospectively search for patients 18-40 years old with a history of FNSF or femoral neck stress reaction (FNSR). The records were obtained from the period July 25, 2003, to September 23, 2011. After reviewing the records that the database obtained, we found 24 patients who had radiographic evidence and/or MRI evidence of a FNSF or FNSR. Radiographic images were reviewed by both a fellowship trained musculoskeletal radiologist (K.J.S.) and an experienced hip arthroscopist (M.R.S.). Anteriorposterior (AP) pelvis and cross-table lateral radiographs of the affected hip were assessed for the presence of radiographic features of FAI. Anterior-posterior radiographs with excessive tilt or rotation were excluded from the final analysis of acetabular morphology. Pelvic tilt was determined to be acceptable if the distance between the sacrococcygeal junction and pubic symphysis was 8-50 mm in males and 15-72 mm in females . Pelvic rotation was acceptable if the distance from the center sacral line to the center of the pubic symphysis was 50 , and an AOR of 55 , an AOR of 35 , Abnormal profunda acetabular 1 > 40 femoral retroversion head-neck junction in 6
4 >50 8 AOR >0.18
Anterosuperior No Posterosuperior chondrolabral separation 11 Grade 4, Labral tear in 14 5 grade 3, 8 grade 2
None None None None
M. Goldin et al. / PM R 7 (2015) 584-592
Adequacy Side of Femoral of Film (No Rotation Coxa Neck Stress Age (y) Reaction/fx of Pelvis*) Profunda
None None None
Fibrocystic change in 2
Pt ¼ patient; fx ¼ fracture; NL ¼ normal; N/A ¼ not available. * Adequacy of film: sacrococcygeal junction to pubic symphysis 0.8-5 cm in males and 1.5-72 cm in females. 587
Femoral Neck Stress Fractures and Imaging Features
Figure 3. (A) Anterior-posterior radiograph of the pelvis in a 37-year-old man with right groin pain after running a marathon demonstrates a small bone bump on the lateral aspect of the femoral head-neck junction (arrowhead), and subtle sclerosis of the femoral neck medially (arrow). (B) Coronal proton densityeweighted image also demonstrates a small bone bump on the femoral head-neck junction laterally (arrowhead), and an early stress fracture is seen medially (arrow). (C) Axial T2-weighted image with fat saturation demonstrates a focal labral tear anteriorly with a small paralabral cyst (arrow).
Figure 4. (A) Anterior-posterior radiograph of the pelvis in a 22-year-old woman with right hip pain, demonstrating coxa profunda, where the medial wall of actabulum (dotted line) projects medial to the ilioischial line (solid line). (B) Coronal fat saturated T2-weighted image of the right hip demonstrates a grade 3 stress reaction (arrow) of the femoral neck medially. (C) Axial fat saturated T2-weighted image demonstrates chondrolabral separation anteriorly (arrow).
M. Goldin et al. / PM R 7 (2015) 584-592
Figure 5. (A) Anterior-posterior radiograph of the pelvis in a 20-year-old female runner with left hip pain. A crossover sign of the acetabular margins is seen bilaterally, where the posterior wall of the acetabulum (solid line) crosses over the anterior wall (dotted arrow). There is also a posterior wall sign bilaterally, where the center of the femoral head (circular markers) lies lateral to the posterior wall of the acetabulum. Subtle periosteal reaction is seen along the left femoral neck medially (arrow). (B) Coronal fat saturated T2-weighted images demonstrate a stress fracture of the left femoral neck (arrow).
fractures (grade 4 stress reactions) were seen in 11 (46%) (Figures 3 and 5): of these, 2 (8%) were complete, 2 (8%) extended through 50% of the femoral neck, and 6 (25%) extended across 50 in the asymptomatic population has been determined to be 14%, which does not differ substantially from that in our cohort . The lateral center-edge angle, described by Wiberg  has been used to determine the presence of abnormalities in acetabular morphology associated with pincer impingement. A higher LCEA correlates with more acetabular over-coverage ; however, the normal reference range reported in the literature is variable [27,28,30]. In a cadaveric study, Armbuster et al  determined the mean LCEA was 39 (range 24 59 ) for cadavers >40 years old, compared to 38 (range 23 -56 ) for those 40 years. Fowkes et al  determined the mean CEA in an asymptomatic population to be 36.3 (range 22.5 -50.1 ). In contrast, Werner et al determined the mean CEA in pelvic radiographs obtained from trauma patients was 33.6 with a 95% confidence interval of 33.2 -34 . In the current study, 28% of patients had a CEA of >35 and 6% had a CEA of >40 . Although these values are higher than the normal values determined by Werner et al , they are within the normal reference range according to both Fowkes et al  and Armbuster et al . Other radiographic findings used to detect the presence of pincer FAI include acetabular retroversion and coxa profunda . The prevalence of acetabular
retroversion has been found to be 6% in an asymptomatic population . One study on elite soccer players found that acetabular retroversion was present in 27% of males and 10% of females . In the current study on patients with FNSF, the overall prevalence of acetabular retroversion was 33% (31.25% of females and 50% of males), which is substantially higher than that observed in both the general population and elite athletes. In a cross-sectional study of the general population, the prevalence of coxa profunda was found to be 15.2% in men and 19.4% in women . The incidence of coxa profunda in the athletic population has also been described. It was found to be 34% in asymptomatic male football players . The prevalence of coxa profunda in our study was 78% (75% females and 100% males), again substantially higher than that observed in the general population, although the number of patients in our study was small. The increased prevalence of pincer-type FAI in patients with FNSF suggests a potential correlation with pincer morphology and the development of a stress injury. However, more recently coxa profunda has been disputed as a useful parameter for determining pincer FAI, and suggests that coxa profunda may be considered a normal finding in females . Pincer impingement can also be associated with labral tears, which were found in 58% of our cohort. This incidence was similar to that described in an asymptomatic group of hockey players, of whom 56% had labral tears . Previous research has also found a correlation between FAI and the development of FNSF [11,13]. Kuhn et al  determined that 57% of patients diagnosed with femoral neck stress fractures had a positive crossover sign, compared to 31% in a control group. Carey et al  found a crossover sign in 51% cases, a center-edge angle of >40 in 47%, and an a angle of >50 in 55%. Similar to the results of these studies, we also found a higher prevalence of pincer morphology with femoral neck stress injuries; however, in contrast to Carey et al, we did not find an increased prevalence of cam morphology in our cohort. Furthermore, these previous studies were performed in military personnel, which may not be applicable to the general population, and did not include subsequent patient management outcomes. In our study, 29% of individuals with FNSF eventually required surgical intervention; 12% had internal fixation of the FNSF; and 17% remained symptomatic after their femoral neck fracture had healed and required subsequent arthroscopic intervention for FAI. A prior description of pincer impingement has suggested that the pincer deformity creates a fulcrum that causes chronic leverage of the femoral head in the acetabulum . This repetitive pathologic motion results in chondral damage where the femoral head contacts the posterior acetabulum . FAI has also been shown to increase stress across the femoral neck, leading to
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non-union of traumatic femoral neck fractures treated with internal fixation . In a computer-simulated biomechanical model, Voo et al  assessed stress patterns on the femoral neck with activity. Simulation of a vertical force through the hip comparable to that of jumping or running shows significant stress at both the inferior root and superior aspect of the femoral neck  The authors hypothesized that a purely vertical force simulates the absence of hip abductor strength, which may occur with fatigue of these muscle groups. Hip abductor weakness or fatigue, coupled with the fulcrum effect produced by pincer deformity, may increase the mechanical stress at the femoral neck beyond the remodeling capacity of bone, resulting in stress injury (Figure 6). The current study has some limitations. This study had a relatively small number of patients; however, the low incidence of femoral neck stress fractures makes it challenging to have a large cohort of patients. Even so, of the individuals included in the cohort, only 78% had radiographs that were adequate for assessment of the coxa profunda and acetabular retroversion, which may have introduced selection bias to the study. There was no control group, which is a drawback of a retrospective cohort study. Asymptomatic age-matched athletes would not be having radiographs and MRI scans, and therefore it would be difficult to envisage getting a suitable control group of asymptomatic age-matched athletes without completely redesigning the study and changing it to a prospective study. In addition, the literature does describe the incidence of coxa profunda and acetabular retroversion in the athletic population, and therefore we believed it reasonable to compare our rates to those already described. In addition, although the results of the current study suggest an association between pincer impingement and the development of FNSR, we were unable to control for other risk factors involved in the development of FNSFs, such as training intensity, diet, footwear, and gender . Consequently, it is unclear whether the results of this study were confounded by these unknown variables. Even so, the previous research by Kuhn et al  and Carey et al  also demonstrated a similar association between pincertype morphology and FNSFs in military personnel. Therefore it is probable that this association occurs in athletic individuals in the general population as well. Conclusion The results of the current study suggest that patients in the general population with femoral neck stress injuries have a higher incidence of bony abnormalities associated with pincer impingement, such as coxa profunda and acetabular retroversion. However, it is unclear whether pincer FAI is a true risk factor in the development of an FNSF. Consequently, further biomechanical research and higher-level prospective
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Disclosure M.G. Department of Physical Medicine and Rehabilitation, Washington Township Medical Foundation, Fremont, CA Disclosure: nothing to disclose C.N.A. Department of Orthopaedic Surgery, Stanford Medical Center, Stanford, CA Disclosure: nothing to disclose M.F. Department of Orthopaedic Surgery, Stanford Medical Center, Stanford, CA Disclosures outside this publication: senior editor, AAPM&R (money to author); consultancy, Cool Systems, Inc, Genesolve (money to author); grants/grants pending, Ipsen, Inc (money to institution); royalties, OPTP (money to author) M.R.S. Department of Sports Medicine, Stanford Medical Center, Stanford, CA Disclosures outside this publication: board membership, AOSSM Board, ISAKOS Board, ISHA Board (no money); consultancy, ConMed Linvatec, Biomimedica,
Cool Systems (money to author); grants/grants pending, ISAKOS, Smith and Nephew, Ferring Pharmaceuticals (money to institution); patents, Distraction for Hip Arthroscopy, FAI Brace (money to author); royalties, Stryker, DJO, Arthrocare, Lippincott, Mosby (money to author); stock/stock options, Cool Systems, Biomimedica, Eleven Blade Solutions (money to author) K.J.S. Department of Radiology, Stanford Medical Center, 300 Pasteur Drive, Grant Building S062A, Stanford, CA 94305. Address correspondence to: K.J.S.; e-mail: [email protected]
Disclosure: nothing to disclose Submitted for publication May 7, 2014; accepted December 25, 2014.