Hip Joint Anatomy and Pathology 

Anatomy of the hip joint 

The hip joint consists of a ball (femoral head) and socket (acetabulum). The socket of the joint is lined by an important structure called the labrum. The labrum acts as a “seal” around the socket and produces a suction affect which helps in load distribution and support of the hip joint as well as stability.

Surrounding the hip joint is another layer of tissue called the capsule. The capsule also provides stability to the joint as well as dictating how much movement the hip joint can achieve. For example, a tight or inflamed hip joint capsule may present as hip joint pain and stiffness. The labrum and hip capsule are closely related.

Muscles of the hip joint 

The hip joint is surrounded by a vast network of muscle groups. They can be broadly categorised into four groups (please note this article does not cover wider groin anatomy). 

• Hip Flexors. The psoas and iliacus muscles (or iliopsoas) are the predominate hip flexors. The lie directly on the front of the hip joint. They are often very reactive to underlying hip joint or labral irritation and can become tight and tense. They commonly “snap” over the hip joint or rim of the pelvis and can produce a clunk.

• Hip Adductors. This group of muscles attach around the pubic area and although do not directly overlie the hip joint, they can also become very tight/tense and weak when the hip joint is inflamed. Patients usually report pain in the inner groin or thigh area.

• Hip Abductors. These muscles counterbalance the hip adductors and are also referred to as the gluteal muscles. They too can often become reactive in response to hip joint pain and inflammation. Patients with hip abductor pain can present with outer gluteal and thigh pain, with it sometimes difficult to lie on the affected side at night. 

• Deep gluteal muscles. There are a multitude of small muscles that surround the hip joint and are responsible for fine control movements such as rotation. The Piriformis muscle is the best known of them. Patients with “Piriformis Syndrome” present with deep buttock pain, with the hip joint itself often being the primary culprit. Another important structure, the sciatic nerve, passes through the deep gluteal muscles and can become irritated producing Sciatica symptoms.

In reality, the hip joint and hip musculature have an intimate relationship. It can sometimes be difficult for a clinician to work out where the primary driver for pain or pathology is originating from. However, for people who continue to experience problems in the hip and groin area despite good rehabilitation and exercise, hip joint pathology should always be considered.

Movements of the hip joint 

The muscle groups described above can contract together (synergistically) or against each other (antagonistically) to produce the movement in hip joint. The hip joint allows a wide range of movement in six key planes as shown in the diagram below. 

The hip joint can also move in a combination of these directions, called circumduction. The range of movement that the hip joint is able to achieve depends on; 

1. The anatomy of the ball and socket. We are all born with different shape and size joints. The main variations that effect the way the hip joint moves include:

• The angle of the acetabulum (socket) in the pelvis (e.g., more forward or backward facing) 
• The amount of coverage provided by the socket (e.g., over or under coverage).
• The shape of the ball (femoral head and neck) 

Hip joint anatomy will affect the amount of available range and may bias certain planes of movement making them easier or harder to perform (e.g., external vs internal rotation).

2. The tightness/tension of the hip capsule. Inflammation of the hip capsule can cause significant restriction of movement, even if the bony anatomy allows. This can sometimes occur after labral irritation or hip joint sprains. Capsule tightness can also be caused by sedentary lifestyle and inactivity.

3. The flexibility of the surround hip muscles. Tightness in the hip flexors and deep gluteal muscles can also cause a restriction in range of motion. Conversely, people who are hypermobility have lots of range of movement due to very flexible muscle groups (e.g., in dancers and gymnasts).

Common hip problems and symptoms 

The hip joint is the largest joint in the body and when inflamed can cause significant pain and disability. Hip joint issues present similarly with subtle variations depending on the pathology. Patient usually describe pain in the groin, buttock and even lower back. This can travel down the thigh and into the knee. Movement is restricted as are activities of daily living such as getting dressed and sleeping. There can sometimes be nerve involvement is the deep gluteal muscles become tight and tense. 

Once the hip joint has flared up it can take a good while for things to settle down, needing careful thought and attention. Below are the common pathologies seen in the hip joint.

1. Hip Impingement 

The most common way for the hip joint to become irritated is by impingement. Certain directions of movement in isolation or in combination (particularly flexion, adduction and internal rotation, FADIR) can trap the upper part of the ball against the socket. Common examples of functional movements of when this hip impingement may occur is in a deep squat, getting in and out of a car or bending over whilst seated to tie shoelaces. Pain is often felt in the centre of the groin but can radiate out to the buttock or outside of the thigh. 

As described above, common anatomical variations of the hip joint can predispose to hip impingement. There may be over coverage from the socket (pincer), or a bumpy, large femoral head (CAM), or even both. It is important to remember that these are not abnormalities per se, but just different variations in hip anatomy. There is evidence to now suggest that children and adolescents who play a lot of sport are more likely to develop CAM and pincer variations. There are also strong genetic and ethnic correlations that make certain patient groups more vulnerable to hip impingement.

2. Labral irritation and tears 

As a consequence of hip impingement, the labrum of the hip socket can become irritated and inflamed. Sometimes there can be a physical defect such as a tear in the labrum itself. Sports people that are more predisposed to labral pathology are those who are required to push the hip into a more impinged position e.g., footballers, dancers, Olympic weight lifters. Labral tears can also happen acutely without any previous impingement signs or symptom e.g., catching the labrum whilst running. 

The MRI scan above shows a healthy labral on the left (white arrow) and a labrum with a split in on the right (white arrow). 

Labral tears are very common indeed. They are often found incidentally on scans having not caused any previous issues or pain. In patients with hip pain, they are often seen in the non-problematic side where they have been asymptomatic. Labral tears can however be a good indicator to highlight movement patterns, lifestyle choices or biomechanics that lead to hip impingement.

Labral tears don’t often need any intervention unless they are causing significant pain, inflammation, or are affecting the function or stability of the hip joint. To determine this requires a detailed clinical assessment, examination and investigation by a specialist. 

3. Hip degenerative change (arthritis) 

As a primary load bearing joint, the hip is subject to degenerative change (wear and tear) over time. The rate at which a hip joint degenerates is dependent on the amount of overuse and trauma (major or micro) over a lifetime. Genetic and lifestyle factors such as diet and exercise also play a significant role.

It is important to note that hip function does not always correlate with the degree of degenerative change. Even mild degenerative change can cause the hip to become very stiff and restricted if muscle and capsule restriction is not identified and resolved. Conversely, a severely arthritic hip can still have good function and mobility with man people continue to run and lead fit and active lives. With any degree of arthritis, it is important to keep the hip joint capsule and muscles strong and without tension or stiffness.

Diagram above shows a left hip joint with arthritis and x-ray findings.

4. Hip Dysplasia 

Hip Dysplasia (sometimes called developmental dysplasia of the hip, DDH) refers to under coverage of the femoral head (ball) by the acetabulum (socket). Most people with hip dysplasia are born with the condition. Doctors will check a new-born baby for signs of hip dysplasia shortly after birth and during well-baby visits. If hip dysplasia is diagnosed in early infancy, a soft brace can usually correct the problem. Milder cases of hip dysplasia might not start causing symptoms until teenage or young adult years, particularly in sporty people who play high impact sports.

Significant hip dysplasia can affect the cartilage lining the joint, and it can also injure the labrum that rims the socket portion of the hip joint. This can present similarly to hip impingement symptoms as described above. 

For more severe cases, there is evidence that hip dysplasia can increase the risk of hip arthritis throughout life. Therefore, in young adulthood, surgery may be needed to move the hip bones into betters positions to optimise and smooth joint movement. This would be conducted with the aim to preserve the joint for as long as possible (hip preservation surgery).

Left image. Normal hip x-ray.

Right image. Hip dysplasia with under coverage of the femoral head by the socket highlighted by the red arrows.

5. Stress Fractures of the hip 

Stress fractures on the hip are common, particularly in people who are energy deficient, overtraining and under fuelling. Most occur in the femoral neck, and do not affect the actual hip joint directly. This means most stress fractures of the hip heal completely without any long-term detriment to the joint itself. Please read the LBSM guide on bone injury to find out more about bone stress injury.

Management of hip issues 

Regardless of the type of hip pathology, rehabilitation principles are largely the same. These can be summarised below.

1. Hip muscle strength and flexibility. Keeping the hip muscles strong through exercise is key. This includes all muscle groups described above. A good hip strength and mobility programme should have routines to work on the power muscles (e.g., hip flexors and abductors), but also exercises that work on fine balance and more precise hip movements (e.g., deep gluteals and adductors). 
2. Hip range and capsule release. With the knowledge that every hip has a different range available to it due to its unique anatomy, efforts should be made to ensure the hip is unrestricted and moving smoothly through that available range. This can be done through mobility exercise (e.g., stretching, yoga, dynamic stretching) and soft tissue release under experienced therapy hands (e.g., hip mobilisation and muscle release). 
3. Avoiding aggravating movements. It is important to identify the key triggers for hip joint pain and stiffness and removing them from day to day life. This could mean reducing the amount of time at the desk (e.g., standing desk, movement alarm), or substituting a high impact exercise (e.g., running, HITT) for a lower impact one (e.g., cycling, swimming). This is a very individual for each patient and needs some thought. Often subtle changes such as improving squat technique or car seat height and make a big difference.
4. Diet. Along with rehabilitation, improving diet has a significant role in reducing joint inflammation. The key principles are to reduce foods that cause inflammation, and increase those that have anti-inflammatory properties. Please read the LBSM guide on Diet and Supplements to find out more.
5. Anti-inflammatories. If hip joint and capsule inflammation has become too severe, medication can help settle this down. Anti-inflammatories taken orally or injected directly into the joint and settle the symptoms and allow a period of rehabilitation to work on the above points 1-4). In an ideal world, we wouldn’t rely on regular anti-inflammatories but use them to aid rehabilitation as part of a wider plan.
6. Regenerative treatments. It is important to state that there are no proven medications or treatments that regenerate joints (i.e., reverse wear and tear changes). However, certain novel therapies may help slow down degenerative change and improve capsule inflammation. These may include Platelet Rich Plasma, Stem Cells, Hyaluronic acid and Lipogems. Evidence however for all these treatments is very limited so they should not be relied on as a one stop treatment per se but be part of a wider rehabilitation programme.
7. Surgery. In fit and active people who are able to commit to a rehabilitation programme, surgery is usually not required for any hip pathology. Certain cases, however, may warrant surgical opinion if patient are not making progress. 
   • Severe hip arthritis. For those patients with severe degenerative change, which is not responding to rehabilitation or anti-inflammatories, a hip replacement can be great procedure in terms of improvement quality of life and function.
   • Acute labral tears. In certain cases, repair a labral tear via hip arthroscopy can be of benefit to improve the stability of the hip joint. Making this decision requires expert opinion as the hip arthroscopy procedure comes with its own risks.
   • Severe dysplastic hips. As described above, a severely dysplastic or under covered hip may benefit from bone realignment to protect it from future wear and tear. Detailed scans and calculations of angles are required before making this decision. 

Managing hip problems can be complex. However, if we are able to keep the hip joint strong and mobile, whilst avoiding aggravating activities, any hip joint pathology can be managed successfully. If you need any further advice around you hip joint, please do not hesitate to contact the LBSM team.