Femoral cause
Young athletic male
Causes:
Femoral neck anteversion
Reluctant posterior tilt
Previous fracture neck
Femoral waist deficiency (Pistol grip deformity)
Insufficient femoral head-neck offset
Subtle displacement of femoral epiphysis
SUFE
Post surgical/ post op
Mechanism:
Peripherally increasing femoral head radius causes damage to acetabular rim during flexion with internal rotation
Abnormal anterior femoral head-neck junction causes premature contact between femur and acetabular rim in flexion, adduction and internal rotation. This leads to abrasion and avulsion of anterosuperior acetabular cartilage from labrum and subchondral bone, which in turn leads to detachment (tear) of anterosuperior acetabular labrum.
Pathology:
Initial cartilage loss, which in turn leads to labral loss
Cartilage loss:
Most are acetabular; Anterosuperior is most common region (Anterior 55%, Superior 22%, Posterior 23%)
May have cartilage flap tear (difficult to see on MR)
Alpha angle:
Line parallel to the femoral neck. A best-fit circle is drawn outlining the femoral head. Alpha angle is the angle between the line and the point where the femoral head protrudes anterior to the circle. Angle greater that 55° are likely to be associated with impingement (asymptomatic controls mean angle 42°)
Moderately reliable measurement
Herniation pit:
Anterosuperior aspect of the neck
??Pathognomonic of cam impingement
Os acetabuli/ Avulsion fracture:
May represent fatigue fracture
Plain film:
Abnormal head
Short neck
Herniation synovial pits
Os acetabuli
'Bump' (pistol grip deformity) at anterolateral femoral head-neck junction: specific but not sensitive
CT:
Radial reformations
MR:
Sagttal plane is best plane
Radial reformations
Cartilage is most difficult to seen even on MRA. Traction while scanning helps
Management:
Femoral neck excision osteoplasty
