Hip dislocations are relatively uncommon during athletic events.  Injuries to small joints (eg, finger, wrist, ankle, knee) are much more common. However, serious morbidity can be associated with hip dislocations, making careful and expedient diagnosis and treatment important for the sports medicine physician.
Large-force trauma (eg, motor vehicle accidents, pedestrians struck by automobiles) are the most common causes of hip dislocations. [1, 2, 3, 4, 5] This type of injury is also associated with high-energy impact athletic events (eg, American football, rugby, water skiing, alpine skiing/snowboarding, gymnastics, running, basketball, race car driving, equestrian sports). [5, 6, 7, 8, 9, 10, 11] Diagnosing and correctly treating these injuries to avoid long-term sequelae of avascular necrosis and osteoarthritis is imperative.
Note the contrasting images below.
Hip dislocations are either anterior or posterior, with posterior hip dislocations comprising the majority of traumatic dislocations.
Several classification systems are used to describe posterior hip dislocations.
The Thompson-Epstein classification is based on radiographic findings.
- Type 1 – With or without minor fracture
- Type 2 – With large, single fracture of posterior acetabular rim
- Type 3 – With comminution of rim of acetabulum, with or without major fragments
- Type 4 – With fracture of the acetabular floor
- Type 5 – With fracture of the femoral head
The Steward and Milford classification is based on functional hip stability.
- Type 1 – No fracture or insignificant fracture
- Type 2 – Associated with a single or comminuted posterior wall fragment, but the hip remains stable through a functional range of motion
- Type 3 – Associated with gross instability of the hip joint secondary to loss of structural support
- Type 4 – Associated with femoral head fracture
- Some case series have found that most posterior hip dislocations are type 1.
- The Thompson-Epstein classification is based on radiographic findings.
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Up to 70% of all hip dislocations are due to motor vehicle accidents.
Hip dislocations in younger individuals are relatively rare, with only 5% of cases occurring in patients younger than 14 years. Most injuries are in boys and are related to low-energy sports injuries or falls. 
Very little documentation concerning the occurrence of hip dislocations during sporting events exists. American football and rugby are the sports in which hip dislocations have been most widely reported.  An estimated 3% of all football injuries involve hip fracture or dislocation. Rugby, followed by alpine skiing and snowboarding, is the sport with the second highest number of hip dislocations. 
One study found rates of hip dislocation with or without fracture of the hip joint significantly higher in snowboarders than skiers over a 10-year period (5 times higher in snowboarders than in skiers),  and one case each of hip dislocation has been documented in the literature in competitive gymnastics and professional basketball. [1, 5] Case reports also exist of hip dislocations and fractures in racecar drivers and equestrians. 
The hip joint is based on the articulation of the femoral head and the acetabulum of the pelvis, and it is a synovial ball-and-socket type joint. The femur is held in the acetabulum by 5 separate ligaments as follows:
The iliofemoral ligament attaches to the anterior inferior iliac spine of the pelvis and the intertrochanteric line of the femur.
The pubofemoral ligament originates at the superior ramus of the pubis, also attaching to the intertrochanteric line of the femur.
The ischiofemoral ligament connects the ischium to the greater trochanter of the femur.
The transverse acetabular ligament consists of the labrum covering the acetabular notch.
The femoral head ligament joins the femoral head with the transverse ligament and acetabular notch.
The relative strength of these ligaments joined together, along with the angulation of the proximal femur in relation to the acetabulum, make dislocation of the hip joint difficult. The large sciatic nerve lies just inferoposterior to the hip joint, whereas the femoral nerve lies just anterior to the hip. The proximal shaft of the femur and the femoral neck has a plentiful blood supply from the medial circumflex femoral artery and its branches. The femoral head, on the other hand, has an extremely tenuous blood supply from a small branch of the obturator artery that passes with the femoral ligament.
Two general categories of hip dislocations exist, anterior and posterior. Posterior dislocations compose 70-80% of all hip dislocations and 90% of all sports-related hip dislocations. Alpine skiing is an exception, with one study showing higher rates of anterior dislocations in skiers.  In order to cause a posterior dislocation, a large force is required to strike the flexed knee with the hip flexed, adducted, and internally rotated. This injury occurs more commonly during contact and collision sports (eg, American football, rugby) when a running player is tackled from behind and falls onto a flexed knee and hip. As the opposing player falls onto the tackled player's back, his added weight drives the torso and pelvis toward the ground, and the femoral head is thus driven out the socket posteriorly.
Anterior dislocations occur when an athlete's hip is flexed, with the leg abducted and externally rotated. The thigh and leg act as a lever, with the fulcrum being the posterior edge of acetabular socket, popping the femoral head out of the socket anteriorly. These injuries are more common in sports (eg, basketball, gymnastics) in which players are running at high speeds, jumping, and landing awkwardly on the inner or medial aspect of the knee. This force drives the femoral head out of the acetabulum anteriorly, tearing ligaments, and often fracturing the femoral head and/or acetabulum. The increased rates of dislocation in alpine skiing are likely due to the large rotational forces, abduction, and external rotation applied to the hip by the ski equipment during a fall.
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