The hip joint is made up of the femoral head and the acetabulum. It is a very constrained ball and socket joint where the ball fits almost perfectly in the socket. Surrounding the socket is a fibrocartilage called the labrum. Some hip joints develop in life without good coverage of the cup and they are said to be dysplastic. Dysplasia can occur as one develops and the socket does not deepen but stays shallow. People with dysplasia can have earlier onset of arthritis as a result of the loss of coverage and increased contact forces that occur.
Muscles and ligaments attach the pelvis to the femur. The muscles of the hip can move the hip in all directions forward back and rotate it internally toward the midline or externally. The gluteus muscles are responsible for lifting the leg back and to the side or abducting the leg. Here is a view from the back.
Notice the Gluteus Medius and how, if it contracts, it is responsible for keeping the pelvis level when walking or running. If the maximus contracts it will pull the femur posteriorly or extend the hip joint. There are small rotators on the front and the back and then there is the iliopsoas or the filet mignon of people. It is the biggest hip flexor and can cause bursitis as it attaches to the femur. Tendonitis will also present as groin pain and pain with motion from overuse. The iliopsoas actually starts on the lumbar spine vertebrae and moves forward towards the hip joint. You can see why you need to extend the hip and back to get a great iliopsoas stretch.
The joint capsule is made of ligaments that hold the joint tightly. In order to operate on the hip and perform arthroscopy, the joint has to be entered and filled with fluid. We can then get a better picture for what is going on with our digital camera that is placed into the joint.
Labral or cartilage tears can occur through repetitive stress, traumatic events, or as a result of pathologic anatomy that can cause pain and pinching of the cartilage. Here you can see a tattered labrum that can pinch in the joint causing pain in the groin.
Repairing vs. trimming the labrum depends on certain factors like age and the condition of the labrum. A bone spur on the femoral head is called a CAM lesion and can be trimmed to prevent tearing of the labrum; a pincer lesion is on the cup side and can be trimmed also.
Things that we need to appreciate in the CrossFit world. When squatting, rotate your feet outwards the hips then rotate and allow more motion in deep flexion and prevent impingement. New clicking in the groin where something feels like it is mechanically moving in the joint is a sign of a labral tear. People can feel groin pain or pain along the side of the hip in what is called a C sign where you grab the outer side of the hip in the front and the back. Other conditions that can occur in the joint are loose bodies or arthritis. One can also develop chondral injuries or injury to the shiny smooth substance that cushions the ball or socket.
X-ray of arthritis shows the bone on bone from loss of cushion.
Here is a nice site for basic stretches that can keep the muscle around the hip and the capsule loose and flexible.