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Hip implants are artificial devices that form the essential parts of the hip joint during a hip replacement surgery. The hip implants vary by size, shape, and material. Implants are made of biocompatible materials that are accepted by the body without producing any rejection response. Implants can be made of metal alloys, ceramics, or plastics, and can be joined to the bone. The metals used include stainless steel, titanium, and cobalt chrome, whereas the plastic used is polyethylene.  Various components of a hip implant may be used for a hip replacement surgery.  The components used may depend on the extent of damage to the hip joint, and the preference of the orthopedic surgeon performing the procedure.

Components of a Hip Implant

Hip joint is a ball-and-socket joint. The ball or the spherical head of the thighbone (femur) moves inside a cup shaped socket (acetabulum) of the pelvis.

The components of a hip implant replicate the natural shape and structure of the ball-and-socket joint. The components used may depend on the size of the body and vary from patient to patient.  A total hip replacement implant has three parts:

• Stem: The stem fits into the femur
• Ball: The ball replaces the spherical head of the femur
• Cup: The cup replaces the worn out hip socket

Types of Hip Implants

Based on the patient’s activity level, any of the following types of hip implants may be used in a hip replacement surgery.

Metal-on-polyethylene implant: The ball is replaced with a metal ball and the socket is replaced with polyethylene or has a polyethylene lining.

Ceramic-on-polyethylene implant: The ball is replaced with a ceramic ball and the socket is replaced with polyethylene or has a polyethylene lining.

Ceramic-on-ceramic implant: The ball is replaced with a ceramic ball, and the socket has a ceramic lining. They wear less than metal-on-metal implants, and are most durable among the available hip implants.

Types of Implant Fixation

Depending on the age and activity level of the patient undergoing hip replacement surgery, an orthopedic surgeon may recommend any of the available three types of implant fixation.

Cemented Fixation: The femoral and acetabular components are held together with special bone cement. The bone cement is made from a special polymer called polymethylmethacrylate (PMMA). Patients can often immediately be full weight bearing and walk after a cemented fixation. Cemented fixation is an option for less active patients. However, too much stress on cemented fixation can lead to fatigue fractures.

Cementless Fixation: Cementless implants are coated with a porous material.  They attach to the new bone that grows to the surface of the implant via bone ingrowth. The implant may be fixed using screws or pegs until bone ingrowth. Patients need to limit weight bearing and use crutches or walker following cemented fixation to allow the bone to attach itself to the implant. Cementless fixation is an option for more active patients with good bone quality.

The hip joint is a ball and socket joint, where the head of the thigh bone (femur) articulates with the cavity (acetabulum) of the pelvic bone.

Sickle cell disease, a group of disorders that affect the hemoglobin or oxygen carrying component of blood, causes avascular necrosis or the death of bone tissue in the hip due to lack of blood supply.

Avascular necrosis commonly affects the head of the femur. Necrosis leads to tiny cracks on the bone which finally causes the head of the femur to collapse. The condition causes pain due to increased pressure in the blood vessels of the bone marrow at the region of the necrosis.

Early stages of avascular necrosis can be treated by core decompression surgery, which reduces pressure, promotes blood flow and encourages healing of the bone.

Indications

Core decompression is indicated in the early stages of avascular necrosis, when the surface of the head is still smooth and round.  It is done to prevent total hip replacement surgery, which is indicated for severe cases of avascular necrosis and involves the replacement of the hip joint with an artificial device or prosthesis.

Surgical Procedure

Core decompression is done under spinal or general anesthesia. The patient is placed on their back in supine position. Live X-ray imaging or fluoroscopy is used to guide your surgeon during the procedure.

A small incision is made on your hip and a guide wire is passed from the incision through the neck of the femoral bone to the necrotic area in the femoral head. A hole is then drilled along the wire. The necrotic bone is then removed. This reduces the pressure immediately and creates space for the new blood vessels to grow and nourish the existing bone.

The cavity that is left behind in the bone is sometimes filled with bone graft taken either from another part of your body or a cadaver.  Sometimes synthetic bone graft material is used. The incision is then closed with sutures. Another variation of the same surgery involves drilling very small diameter holes from a single point. The surgical wound in this case is very small and may require only a single suture.

Post-Operative Care

After the operation, crutches are to be used for 6 to 12 weeks to prevent weight bearing at the hip joint until the femur bone heals completely. You will be able to resume your regular activities 3 months after the surgery.

Advantages

The advantages of core decompression include the following:

• Prevents complications of collapse of the femoral head
• Preserves bone of the femur
• Delays the need for total hip replacement where the diseased femur head is replaced with an artificial prosthesis.

Risks and Complications

As with all surgeries, core decompression may be associated with certain complications such as:

• Fracture along the core track
• Perforations in the femoral head
• Deep vein thrombosis

The hip joint is one of the body's largest weight-bearing joints and is the point where the thigh bone (femur) and the pelvis (acetabulum) unite. It is a ball and socket joint in which the head of the femur is the ball and the pelvic acetabulum forms the socket. The joint surface is covered by a smooth articular cartilage that cushions and enables smooth movements of the joint.

Hip hemiarthroplasty is a surgical technique employed to treat hip fractures. In this procedure, only one half (ball section) of the hip joint is substituted by a metal prosthesis.

Surgical procedure

The procedure is performed under general anesthesia. An incision is made along the outer aspect of the affected hip. The surgeon gains access to the hip joint and the head of the femur is removed using surgical instruments and prepared to accept the prosthesis. The stem of the metal prosthesis is placed inside the femoral bone. The surgeon now connects the metal ball that forms the femoral head. The stem prosthesis can be press-fit in patients with a strong, healthy bone or cemented in cases of weak, osteoporotic bone. The method of implantation depends on the patient’s age and condition of the bone. At the end of the procedure, the incisions are closed with sutures and a dressing is applied.

After surgery

The post-procedural instructions to be followed hip hemiarthroplasty include:

• You will be prescribed medications to reduce pain and inflammation.
• Crossing your legs should be avoided.
• Avoid lifting of heavy objects.
• Avoid standing for long hours.
• Your surgeon may recommend physical therapy to strengthen the joint and the muscles and to help restore mobility to the hip joint.

Complications of hip hemiarthroplasty include infection, dislocation, deep vein thrombosis, loosening of the prosthesis, and failure to relieve pain. Discuss with your surgeon if you have concerns regarding hip hemiarthroplasty surgery.

Periacetabular osteotomy is a surgical procedure to treat a congenital hip condition called hip dysplasia. Hip dysplasia is either present from birth or develops in the first few months of life. Patients suffering from this condition have a shallow socket (acetabulum) of the hip joint. This causes misalignment of the head of the thigh bone (femur) in the acetabulum and leads to premature wear and tear of the joint over the years.

The proper functioning of the hip joint can be restored by a surgical procedure called periacetabular osteotomy. This involves cutting the acetabulum from the pelvic bone and repositioning it with screws to allow for a better fit of the femoral head.  The procedure reduces pain, restores function and prevents further deterioration of the hip joint, thereby increasing the life of the hip joint and postponing total hip replacement.

The surgery can be done for children over 10 years of age and adults, preferably less than 40 years of age.

Hip Anatomy

The hip joint is a ball and a socket joint where the ball shaped head of the femur articulates with the acetabulum of the pelvic bone. The edge of the socket is lined with cartilage to form a rim around it called the labrum. The labrum deepens the socket providing more stability to the joint.

The articulating surfaces of both the head of the femur and the acetabulum are covered with cartilage. Cartilage is a tough but flexible tissue that allows two bones to move over each other smoothly without friction.

Hip dysplasia is a congenital hip condition where patients have either a shallow acetabulum (Acetabular dysplasia) or an abnormality in the shape of the upper portion of the femur. This causes symptoms of limping, waddling or walking on their toe. Hip Dysplasia progressively leads to premature degeneration of the cartilage of the hip joint and may cause a rim fracture or labral tear. Patients usually start experiencing pain in the groin region at 20 to 30 years of age.

Diagnosis

Based on the patient’s medical history, symptoms and physical examination, the doctor may suspect hip dysplasia. The diagnosis is confirmed by an X-ray of the hip joint. An MRI scan may be ordered to check the condition of the labrum.

 Initial treatment is aimed at managing the symptoms of pain and inflammation. Hip Dysplasia can only be treated surgically by either periacetabular osteotomy or total hip replacement. If left untreated it leads to progressive arthritis with increasing pain and progressive loss of hip function.

Surgical Procedure

Periacetabular osteotomy is a technically challenging surgery and is therefore done under Fluoroscopy to provide the surgeon with continuous live X-ray guidance.

The procedure is performed under general anesthesia with the patient lying on their back. An incision is made over the hip joint. The acetabulum is cut completely from the rest of pelvis using a surgical saw. The fragment of the bone containing the acetabulum is then rotated to a new position so that it covers the head of the femur more naturally. It is then fixed in the new position by inserting screws into the bone. The incision is closed with sutures and surgical staples.

Sometimes cutting and repositioning of the femoral head may be needed but is not known until during the operation. If necessary, this procedure, called a femoral osteotomy, will be done at the same time but does require another separate incision.

Risk and Complications

 Periacetabular osteotomy is a relatively safe surgery however complications can occur and may include lack of healing of the involved bones, wound infection, deep vein thrombosis (blood clots in large veins mainly of leg), nerve damage and pulmonary embolus (blood clots traveling to the lungs).

Post-Operative Care

After surgery, pain and anticoagulant medications are given. Crutches are to be used for the first 6 weeks to prevent full weight bearing on the operated hip until it has healed. X-rays are taken 2 to 3 days after the surgery to confirm the new position of the acetabulum. Physical therapy is started as soon as possible after the surgery to strengthen the hip muscles and improve hip function. Full recovery after the surgery takes around 4 months.

Periacetabular Osteotomy surgery has several advantages for young patients with dysplastic hip over a total hip replacement (THR) surgery. 

• Patients who undergo total hip replacement have hip restrictions to follow to avoid dislocation of the artificial joint. Patients who undergo periacetabular osteotomy have no such restrictions and can have an active life as much as their joint allows without fear of dislocation.
• A natural joint is better than an artificial joint as the natural bone is preserved. Artificial implants can wear out with use and time. Moreover, there is a small risk of release of metal ions from the artificial hip joint which could pose a risk to the fetus of women in child bearing age.
• Periacetabular osteotomy is done in young patients who would likely outlive the life of an artificial implant. As a result, revision surgery would be needed at a later date which has a higher complication rate. However, THR can be done after periacetabular osteotomy if the need arises.
• Full sensations of the hip joint are retained in periacetabular osteotomy as compared to THR where the natural bone is lost to artificial material implant.

The hip joint is also known as a ball and socket joint, where the ball (femoral head) of the thigh bone fits into the socket (acetabulum) of the pelvis bone.

Damage to the hip bones can be treated by hip resurfacing, which is a surgical procedure in which the damaged parts of the femoral head are trimmed, and the socket is removed and replaced with metal caps.

Hip resurfacing is an alternative to total hip replacement surgery where both the ball and socket of the hip joint are completely removed and replaced with plastic, metal, or ceramic prosthetics.

Indications and contraindications

Your Surgeon may recommend hip resurfacing surgery if you suffer from severe hip arthritis affecting your quality of life and the symptoms have not been relieved with conservative treatment options such as medications, injections, and physical therapy.  

In addition, younger, larger-framed patients with strong and healthy bone are more suitable candidates for hip resurfacing surgery.

Hip resurfacing surgery is not recommended in patients with known metal hypersensitivities, osteoporosis, diabetes, impaired kidney function, and large areas of dead bone (avascular necrosis).

Surgical procedure

Hip resurfacing surgery is performed with the patient under spinal or general anesthesia.

Your surgeon makes an incision over your thigh to locate the hip joint. The femoral head is displaced from its socket, trimmed of the damage using special instruments, and fitted with a metal cap. The damaged bone and cartilage lining the socket is removed and a metal cup is fixed. Finally, the femoral head is repositioned into the socket, and the incision is closed.

Advantages of hip resurfacing

The advantages of hip resurfacing over total hip replacement include:

• Easier to revise: Components used in both procedures usually wear out, loosen or fail after a period of 10 to 20 years, requiring revision surgery. As hip resurfacing involves less removal of bone, the revision surgery should be easier to perform.
• Lower risk of hip dislocation: As the ball size in hip resurfacing is larger and closer to the normal size, the risk of hip dislocation is less.
• More natural walking pattern
• Greater range of hip motion

Disadvantages of hip resurfacing

The disadvantages of hip resurfacing are:

• Femoral neck fracture: There is a likelihood of femoral neck fracture with hip resurfacing, which eventually necessitates a total hip replacement.
• Metal ion risk: Tiny metal particles or ions produced by the movement of the metal ball against the metal socket may produce an allergic reaction causing pain and swelling.
• Difficulty: Hip resurfacing is a more difficult procedure to perform, requiring a larger incision.

Risks and complications

As with any surgery, complications are rare but can occur. Hip resurfacing patients may have complications including:

• Formation of blood clots in the leg veins
• Injury to nerves or blood vessels
• Infection
• Femoral neck fracture
• Dislocation