Femoroacetabular impingement secondary to the cam effect is thought to be associated with an insufficient anterior concavity in the sagittal/axial plane of the femoral head-neck junction. Using three-dimensional computed tomography the anterior and posterior concavity of the femoral head-neck junction was assessed in 36 painful non-dysplastic hips (30 patients). The mean age of the symptomatic hips was 40.7 with 13 females and 17 males. Eighteen out of the 36 hips had a pistol grip deformity. Magnetic resonance gadolinium arthrography was performed to assess for labral and cartilage lesions. Alpha and beta angles measuring the anterior and posterior femoral head-neck junction concavities were also determined in 20 asymptomatic hips (12 patients; mean age 37, 5 females and 7 males) using three-dimensional computed tomography. The mean alpha angle for the symptomatic and the control group were: 66.4 versus 43.8 (p=0.001), and for the beta angle 40.2 versus 43.8 (p=0.011), respectively. All but one of the symptomatic hips had a labral tear with 61% of these hips having associated cartilage damage. Three-dimensional computed tomography represents an accurate tool to quantify the femoral head-neck concavity providing a non-invasive assessment of hips at risk of femoroacetabular impingement.

Osteoid osteoma is a small, benign but painful lesion with specific clinical and imaging characteristics. Computed tomography is the imaging modality of choice for visualization of the nidus and for treatment planning. Complete surgical excision of the nidus is curative, providing symptomatic relief, and is the traditionally preferred treatment. However, surgery has disadvantages, including the difficulty of locating the lesion intraoperatively, the need for prolonged hospitalization, and the possibility of postoperative complications ranging from an unsatisfactory cosmetic result to a fracture. Percutaneous radiofrequency (RF) ablation, which involves the use of thermal coagulation to induce necrosis in the lesion, is a minimally invasive alternative to surgical treatment of osteoid osteoma. With reported success rates approaching 90%, RF ablation should be considered among the primary options available for treating this condition.

Introduction In our institutional experience, determination of the alpha (α) angle at MR arthrography as an indicator of the likelihood of cam-type femoroacetabular impingement (FAI) is fraught with inconsistency. The aims of this study were to quantify the degree of variability in and calculate the diagnostic accuracy of the α angle in suggesting a diagnosis of cam impingement, to determine the accuracy of a positive clinical impingement test, and to suggest alternative MR arthrographic measures of femoral head-neck overgrowth and determine their diagnostic utilities. Materials and methods We carried out a retrospective analysis of MR arthrographic studies performed during a 4-year period, combined with chart analysis, which allowed identification of 78 patients in whom surgical correlation was also available. The status of a preoperative clinical impingement test was also noted. Patients were designated as having cam-type FAI (Group A, n= 39) if intra-operative femoral head-neck junction bony osteochondroplasty/arthoscopic femoral debridement was performed. Group B (n=39) acted as controls. Three radiologists independently and blindly performed a series of measurements (α angle and two newly proposed measurements) in each patient on two separate occasions. An α angle of greater than 55°was considered indicative of the presence of cam-type FAI.Results Performance values for α angle measurement were poor for each observer. There was considerable (up to 30% of the mean value) intra-observer variability between the first and second α angle measurements for each subject. Binary logistic regression analysis confirmed that the α angle is of no value in predicting the presence or absence of cam-FAI. A statistically significant difference existed between Groups A and B with regard to the newly proposed anterior femoral distance (AFD; p=0.004). Using an AFD value of 3.60 mm or greater as being indicative of the presence of cam-FAI yields a 0.67 performance measure (95% confidence interval 0.55-0.79). The second proposed parameter (femoral neck ratio) was of no value in suggesting the presence or absence of this condition. The sensitivity, specificity, and positive and negative predictive values of the clinical impingement test were 76.9%, 87.2%, 85.7% and 79.1% respectively. Conclusions Femoral α angle measurement is associated with considerable variability. This index performed poorly in our patient population and was statistically of no value in suggesting the presence or absence of cam-FAI. One of our proposed measures, the AFD, outperformed the α angle, though to an insufficient degree to suggest its routine incorporation into clinical practice. Our experience suggests that the clinical impingement test remains the most reliable predictor of the presence of this condition.

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