Biomechanics of the Hip Joint and Causes of Dislocation

PART 1:

The biomechanics of the hip joint is quite complex due to the pelvic motion, alongside the range of movements it produces. In science, biomechanics is the study of forces acting on a living body. Due to its anatomical figure and secure attachments of muscles and ligaments, the hip is quite mobile. The orientation and neck of the femur play an essential role in the mobility of the hip (ARUN PAL SINGH ). The neck angulates to the femur in the sagittal and coronal plane. The neck-shaft during birth is approximately 140 degrees and lowers to about 120-135 degrees in adults. The orientation of acetabulum is also of importance in hip stability and the transferring of forces. It is directed forwards about 15-20 degrees and downwards by 45 degrees. The mechanical axis of the lower limb passes between the centre of the hip joint and the centre of the ankle joint. Anatomical axis line is between the tip of the greater trochanter to the centre of the knee joint. The angle between the two axes is about 7 degrees (Byrne, Mulhall & Baker 2010). Biomechanics of the hip joint is essential in the diagnosis and treatment of pathological-related conditions. Areas which have benefited from the understanding of hip biomechanics include the evaluation of joint function and the evolution of programs for the treatment of joint problems. Also, any procedures for reconstructive surgeries and the design of the total hip prostheses (ARUN PAL SINGH ).

In evaluating the effects of modifications in joint anatomy or different treatment modalities of the hip-joint reaction force, the balance of forces and moments about the hip joint can be beneficial. Figure 1 portrays the hip joint is the first-class lever; the fulcrum is placed between the effort and load. The fulcrum here is the hip, the amount is bodyweight, and the abductor tension is the effort (ARUN PAL SINGH ). To sustain a stabilised hip, the torques that are fabricated by the bodyweight oppose by the pull of the abductor’s muscles. Hence the forces acting across the hip joint is bodyweight, abductor muscles force and joint reaction force.  The hip is generated to balance the moment arms of the body weight and abductor tension to maintain a level pelvis (Byrne, Mulhall & Baker 2010). The constant loading of the hip joint has frequently been imprecise with a simple, two-dimensional analysis performed in the frontal plane. When the weight of the body is on both legs, the centre of gravity pivots between the two hips. Thus, the force exerted equally distributes on both hips. Each hip supports about one-third of the bodyweight in the two-leg stance. Figure 1 displays the process of these loading conditions, the weight of the body minus the weight of both legs is supported similarly on the femoral heads. Thus, the resultant vectors are vertical (ARUN PAL SINGH ).

Figure 1: This diagram displays the body weight as the load applied to a lever arm extending from the body’s centre of gravity to the centre of the femoral head.

In a single leg stance, the efficient centre of gravity moves distally and away from the substantiating leg. Through, the single-leg position, the force is three times the body weight. The limb on which the weight is supported, it would have to sustain the upper body weight and contralateral limb (Byrne, Mulhall & Baker 2010). As the non-substantiating leg quantifies as part of body mass acting upon the weight-bearing hip. This downward force exerts as a rotating motion around the centre of the femoral head. It is the moment created by body weight, and its moment arm (distance from the femur to the centre of gravity). The muscles that endure this movement counterbalanced by the combination of the abductor’s muscles. This group of muscles encompass the upper fibres of the gluteus maximus, the tensor fascia lata, the gluteus medius and minimus, the piriformis and obturator internus. The abductor muscles force creates a moment around the centre of the femoral head; however, this moment arm is considerably shorter than the capable lever arm of body weight. Thus, the merged force of the abductors must be a multiple of body weight

The magnitude of the effects critically depends on the lever arm ratio, which is the ratio between the bodyweight moment arm and the abductor muscle moment arm (ARUN PAL SINGH ).

The average levels for the single-leg stance are three times body weight. Thus, anything that increases the lever arm ratio also increases the abductor muscle force required for gait — also, the force on the head of the femur (ARUN PAL SINGH ). Normal gait, on heel-strike the hip moves into flexion at 30 degrees and at toe-off (when the foot is off the ground) of extension at about 10 degrees. The range of the abduction and adduction of the hip is approximately 11 degrees, and for the internal-external rotation, the rage is roughly 8 degrees. As there are different phases during the gait cycle, various forces act on the femoral head. Approximately, two-thirds of the hip force is constructed by the abductors (ARUN PAL SINGH ). The orientation of the resultant force on the joint is vital to the function of the entire hip. It is essential to consider the forces relative to the axes based on the long axis of the femur. The coronal plane is where the forces acting makes an angle of about 15-27 degrees to the long shaft of the femur during the stance phase of gait. This results in axial compression, varus and mediolateral forces. The sagittal plane of the anteroposterior forces on the femoral head is consequently in torsion (Byrne, Mulhall & Baker 2010). Trendelenburg sign is done to check the abductor function. The patient is required to stand on a single limb. As mentioned before, the bodyweight centre of gravity shifts to the lifted limb and lowers the pelvis. Abductor muscles on the side of the patient are standing on exerts the force to lift the pelvis on the contralateral side. A positive Trendelenburg sign is when there is a failure in dropping the pelvis on a single leg stance, thus indicates abductor malfunction (Byrne, Mulhall & Baker 2010).  

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Some factors affecting hip biomechanics is shortening of the abductor lever arm and weight gain. Shortening of the abductor lever arm would increase abductor workouts. The abductor lever arm may reduce through arthritis, femoral anteversion, external rotational deformities and developmental dysplasia of the hip. Thus, if the muscles aren’t able to generate the necessary force to counterbalance body weight, it can result in gait or pelvic tilt. Weight gain would increase the bodyweight, which leads to an increase in forces acting on the joint (Byrne, Mulhall & Baker 2010).Conventionally, the tissues and bones of the hip joint function without causation of pain. However, multiple diseases and injuries can harm the tissues so that the deformations associated with loading are painful.  To reduce the joint reaction force is to accommodate and manage painful hip disorders. This can be achieved by lowering the body weight and its moment arm or helping the abductor force and its moment arm (ARUN PAL SINGH). Any form of an increase in body weight will harm the total forces applied to the joint. The efficient loading of the joint can be lowered notably by bringing the centre of gravity closer to the centre of the femoral head (decreasing the moment arm) (ARUN PAL SINGH). This can be accomplished by limping; however, the lateral movements required take a considerable amount of energy and is a much less efficient means of ambulation. Another strategic way to reduce joint reaction force involves using a cane or walking stick in the opposite hand. The moment assembled from both the cane and abductor’s muscles together produces a moment equal and opposite to that of the provided by the competent body weight (ARUN PAL SINGH).

PART 2:
Discuss the possible physical causes of hip joint dislocation based on its structure

This hip is made up of numerous structures that assist in supporting the movements of the lower limbs. The location of the hip joint is lateral and anterior to the gluteal region, inferior to the iliac crest, and overlying the greater trochanter of the femur (Dawson-Amoah et al. 2018). The hip joint is the largest joint of the body and is a ball and socket joint. The joint is sealed inside a tight capsule made of membrane synovial capsule, which contains a lubricating fluid that aids the motion of the joint.  The ball is anchored firmly into the socket with tough connective tissue called ligaments. The muscles of the legs overlay these ligaments (Dawson-Amoah et al. 2018). This type of joint allows for the movements of flexion and extension, abduction and adduction, and external rotation. Thus, it will enable a wide range of motions while still supporting the weight of the body (Martin & Gómez-Hoyos 2019). The hip joint is scientifically known as the acetabulofemoral joint, as it is a joint between the femur and acetabulum of the pelvis. The main function is to support the weight of the body in both static (e.g. standing) and dynamic (e.g. running) postures. It also has roles in retaining balance and for maintaining the pelvic inclination angle (Martin & Gómez-Hoyos 2019).

Hip dislocation is a disarrangement of the joint between the femur and pelvis. Predominantly it is when the ball-shaped head of the femur comes out of the cup-shaped acetabulum of the pelvis (Martin & Gómez-Hoyos 2019). Some symptoms mainly include pain and an inability to move the hip. It can lead to complications such as avascular necrosis of the hip, injury to the sciatic nerve or even arthritis (Shukla 2016). Dislocations are typically due to severe trauma. Some traumas include a motor vehicle collision, fall from a specific height, sports participation, and it being heredity can also lead to dislocate the hip joint. Hip joint dislocations can also occur following a hip replacement or from a developmental abnormality known as hip dysplasia (Shukla 2016). The main reason hip dislocations are so unusual is that the ball is held deeply within the hip socket.

Additionally, the bone anatomy of the hip creates a stable joint. The body has strong ligaments, many muscles and tendons that contribute to the stability of the hip joint. Thus, for a hip dislocation to occur, significant force must be applied to the joint (Shukla 2016).

A common cause of a hip joint dislocation is displayed in a motor vehicle collision. This traumatic incidence often happens when a vehicle collides in from of them. Most people will usually extend their legs at the front of the car as they slam on the brakes. To try and avoid the accident, they lock out their knees. Thus, forces from the collision transmitted from the front of the vehicle up the femur of the patient. Then into the head of the femur, which joins the hip at the acetabulum. If the force is high enough. Hence, the head of the femur can be dislocated out of the socket of the hip, leading to hip dislocation. Bone fractures often accompany this to some of the surrounding structures. If this injury isn’t recognised within its earlier stages, there can be severe complications which could require surgery (Hospital 1986). Another form of physical cause is susceptibility to falls, as falling increases your chances of a dislocated joint. Even if you use your arms to brace for impact or if you land forcefully on the hip (Hospital 1986). An additionally acquired dislocation is hip dislocation that occurs within the first three months following total hip replacement surgery. This happens when the patient reaches the extremes of the prosthetic range of motion. The femoral neck levers in the acetabular cup, thus allowing the femoral head to escape from the acetabulum (Dawson-Amoah et al. 2018). Further physical causes include sports participation, as many dislocations occur during high impact or contact sports, such as basketball, dancing and football (Knapik & Salata 2019). The nature of an acquired dislocation to be heredity, where some people are born with ligaments that looser and quite prone to injury or accidents than those of other people (Dawson-Amoah et al. 2018).

Hip dislocation is known to be a potentially devastating injury that can lead to either short-term or long-term problems with the hip joint. Patients perpetuate hip dislocation through plausible causes. Some common examples include motor vehicle accident, susceptibility to falls, post-surgery concerning areas associated with the hip joint, sports participation.  Further, it being heredity, as they require local anaesthesia for the hip joint to be repositioned back in place. After the hip dislocation, it is vital to ensure the joint is stable, and there aren’t any injuries surrounding the bone. Patients who have sustained injuries are at high risk for developing further complications such as osteonecrosis and arthritis of the hip joint. Eventually, hip replacement may become necessary if there was long term damage that resided to the hip joint (Dawson-Amoah et al. 2018).

PART 3:

Reflection

Within my group, I was assigned to the role of researching the biomechanics of the hip joint. I feel as though my response to the sub-question was an application of how the bones, muscles and joints of the hip work together, whether it is through movement or certain diseases. This sub-question I focus on analyses the hip’s structure; thus, this further understanding allowed my other group members to understand and respond to the main question. This is because hip joint dislocation involves a certain degree of knowledge to the biomechanics of the hip joint. It analyses the movements and forces of the hip structure, which results in further understanding of how hip dislocation occurs. I feel as though I researched as much as I could have. I have provided my group members with a detailed response that also answers the analysis of the main question. The research I completed was extensive, as I utilised a range of sources and read in detail to understand each point I made in my response to the sub-question.

In terms of communication, I was always able to respond to my group members questions and helped them out the best way possible. This also applied in the discussions we had about the assignment on our group chat and in-person after class. I felt as though I was a team player, as I tried to answer my sub-question in a way that would help all of my group members with their main question. This is particularly evident in the detailed mentioning of my understanding and interpretation of the biomechanics of the hip joint. Thus, relating this understanding to how specific physical causes can lead to hip joint dislocation. My other group members also displayed this aspect of knowledge through answering their sub-question, however not as detailed when compared to my analysis. The reason I was comprehensive in my response to the sub-question was that I wanted to make it easier to understand.  Also, to conceptualise my understanding of the biomechanics of the hip joint. I also wanted my teammates to have more fundamental knowledge than myself for them to answer the main question.

Therefore, I believe that I was successful in my role as the person in charge of researching the biomechanics of the hip joint. However, I could have improved slightly more on communication with my other team members. This can be carried out with discussing our approach on how we should answer this question, the format and the structure of our responses to the main problem. This is because I realised after completing the main issue that it would have been easier for users to have a structured plan of the required response to the main question. To focus on two to three of the physical causes, and expand them with information regarding the structure of the hip joint that applied to hip dislocation. Despite this, I fell as though my response to the main question reflects a high-quality answer. I feel as though my team members would agree I did what was required for me in answering the sub-question. I was supportive and provided them with thorough information that was the analysis of their response to the main question. Overall, the main thing I would have done differently is to have this assignment completed earlier. In saying that, I feel as though I finished my role quite well.

References

• ARUN PAL SINGH Hip Biomechanics and It’s Clinical Uses, <https://boneandspine.com/hip-biomechanics/>.
• Byrne, D.P., Mulhall, K.J. & Baker, J.F. 2010, ‘Anatomy & Biomechanics of the Hip’, The Open Sports Medicine Journal, vol. 4, no. 1, pp. 51-7.
• Dawson-Amoah, K., Raszewski, J., Duplantier, N. & Waddell, B.S. 2018, ‘Dislocation of the Hip: A Review of Types, Causes, and Treatment’, The Ochsner Journal, vol. 18, no. 3, pp. 242-52.
• Houcke, J., Khanduja, V., Pattyn, C. & Audenaert, E. 2017, ‘The history of biomechanics in total hip arthroplasty’, Indian Journal of Orthopaedics, vol. 51, no. 4, pp. 359-67.
• Martin, H.D. & Gómez-Hoyos, J., 2019, Posterior Hip Disorders, Springer International Publishing, Cham, <https://doi.org/10.1007/978-3-319-78040-5>.