From the topics covered in my previous posts, it’s clear that I enjoy working and learning about hips. But this hasn’t always been the case. I used to find hips very overwhelming. There are so many anatomical structures that could be generating pain, not to mention referral from the SI joint, low back and red flags! Hips were my kryptonite.  

I remember at one point after feeling like I did a terrible hip assessment going back to my school notes to study up and do better the next time. Turns out, my assessment wasn’t terrible at all, my problem was my thought process. I was going through all the right motions, but my intent and analysis was lacking. I remember feeling overwhelmed trying to remember what structures cause anterior groin pain, what causes pain with this movement, why do they say they feel pain there but not here, what special tests do I need to do, are there even special tests for the hip???? (Yes, I now know that there are special tests for the hip). I soon realized I wasn’t going to be successful at treating hips if I kept approaching my assessment this way. 

I started digging into research and reading more about hips, and I came across this approach of compartmentalizing the hip in layers. It’s a systematic approach to assessing hips and it has honestly changed the game for me. By compartmentalizing the hip anatomy, it’s easier to create a clear picture of possible differential diagnoses.    

The Layered Approach 

This way of thinking about the hip will help you paint a better picture of what structures are involved during your hip assessment. Your actual assessment of the hip may not change but having a more systematic thought process will hopefully help with your analysis and in guiding your treatment. 

Layer 1: Osteochondral Layer  

The first layer is the deepest – it is the osteochondral layer. This layer consists of the bony and cartilaginous structures including the femur, acetabulum and pelvis. Anatomical variations can predispose the cartilage to different loading forces, which can predispose people to degeneration of the cartilage and eventually OA. Such variations can include acetabular and femoral anteversion/retroversion, coxa vara/valga and acetabular overcoverage/undercoverage. Hip morphologies such as cam and pincer deformities are also included in the osteochondral layer. These bony changes can lead to dynamic impingement of structures in layers 2 and 3 when reaching end ranges of motion, as well as dynamic instabilities if excessive range of motion is required for sport. Additionally, when the required range of motion exceeds the available range of motion and motion is forced, increased stress will be placed on the SI joint, pubis symphysis and lumbar spine potential leading a compensatory injury.  

Layer 2: Inert layer  

Layer 2 consists of the labrum, joint capsule and ligaments including the iliofemoral, pubofemoral, ischiofemoral, ligamentum teres, and zona orbicularis. Often underlying mechanical stress from layer 1 will lead to reactive hip pain from layer 2 caused by increased strain on the tissue and/or impingement of the tissues. In other words, morphology of layer 1 will directly affect layer 2. These pathologies can include labrum injuries, ligamentum teres tears, ligamentous laxity and adhesive capsulitis. Side to side range of motion comparison as well as a Beighton score can give you an idea of hip laxity or stiffness and general laxity. Impingement tests will also provide information on the interaction between layer 1 and layer 2. 

Layer 3: Contractile layer 

Layer 3 is the contractile layer and consists of all the muscles surrounding the hip, including the lumbosacral muscles and the pelvic floor. Just like abnormal mechanics of layer 1 can increase stress on layer 2, the same can occur to layer 3. Altered stress caused by bony morphology of the osteochondral layer can increase strain on the muscles around the hip and lead to compensatory injuries. On top of this, there is evidence to suggest that layer 1 pathology can lead to poor muscle function, meaning a decrease in muscle strength and timing of activation (also part of layer 4). Other pathologies can also occur such as muscle strains and tendinopathies. Because of the large number of muscles surrounding the hip, they are usually grouped together based on location and function (ie hip flexors, hip extensors, etc.). As I have previously wrote about here, this is more practical when testing muscle strength and length considering the challenge of isolating just one muscle.  

Layer 4: Neurokinetic layer 

Layer 4 consists of the nerves from the thoracolumbosacral plexus that innervate the joint, tissues and muscles. This layer is responsible for proprioception, pain and partially responsible for muscle function. Pathologies within this layer can include nerve compressions, neuromuscular dysfunction and radicular referral from the lumbar spine. With the high prevalence of labrum tears in athletes and potential ligamentous changes, joint proprioception can be altered. This could lead to poor control of the femur on the acetabulum as well as acetabulum on the femur. Pain has been shown to cause neurogenic inhibition of muscles surrounding the painful area. This will change the muscle activation patterns around the hip potentially leading to imbalances. Additionally, poor muscle firing patterns can also lead to poor control of the hip. Finally, protective guarding occurs when the body is trying to protect a structure. This will commonly occur in people with pathological structures, such as the irritated labrum or joint laxity. This leads to increased tone of muscles which are perceived as tight but are not necessarily short.  

Layer 5: Kinetic Chain  

The final layer is the kinetic chain. This includes everything above and below the hip. The influence of the rest of the body can affect what happens at the hip and vice versa. As previously mentioned, early impingement of hip structures due to bony morphology in a sport where ROM requirements exceed the available ROM will lead to compensations above the hip, at the SI joint, lumbar spine and pubis symphysis.  Another example is dynamic knee valgus. This can be caused by poor hip control or by over pronation of the foot, which will also create a dynamic impingement situation at the hip. So even though the symptoms may be occurring at the hip and there may be pathologies at the hip, it’s important to look up and down the kinetic chain to identify potential causes of hip pain or potential compensatory changes caused by hip pathology. 

What does a layered differential diagnosis look like? 

Here is an example of what a differential diagnosis using the layered approach might look like in an athlete with anterior hip pain.  

In this case, the athlete would have developed a cam deformity and labrum tear from playing their sport. Over time, this will have led to increased arthrokinematic joint motion and capsular laxity. The labrum tear leads to poor proprioceptive feedback and control of the femoral head. It then gets irritated from repeated strain from increased translation of the femoral and/or from early impingement from the cam deformity resulting in anterior hip pain. On top of a cam deformity, the athlete has an anterior pelvic tilt which also leads to early impingement of the labrum. Finally, the iliopsoas reflexively tightens to prevent the femoral head from sliding anteriorly and trying to protect the anterior labrum from further damage, resulting in a tight hip flexor. 

Based on this differential diagnosis, we can create a treatment plan to attack the problematic layers of the hip and provide a complete and comprehensive rehab program to optimize recovery and return to sport. 

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References

Nho, S. J., Leunig, M., Larson, C. M., Bedi, A., & Kelly, B. T. (Eds.). (2015). Hip arthroscopy and hip joint preservation surgery. Springer New York.