Hip Replacements

SUMMARY


  • arthritis in the hip is very common

  • hip arthritis causes pain, stiffness and difficulty walking

  • the pain can be felt deep in the hip, in the groin, in the thigh or even in the knee

  • early arthritis can be helped by anti-inflammatories, physiotherapy or injections into the joint

  • severe arthritis requires hip replacement surgery

  • there are many different options for different types of hip replacement and these should be discussed with your surgeon

  • about 95% of hip replacement patients report excellent results, with loss of pain and significant improvement in function

  • about 95% of hip replacements are still working fine after 10 years, with about 80% still working well after 20 years


The procedure of total hip replacement (THR) has been practiced for many years now, with excellent reported results. Modern hip replacements should be expected to last many many years, with in excess of 95% of the hips that we put in lasting more than 10 years.

THR involves removing the arthritic bone around a hip and replacing the joint with a new artificial bearing. The hip is a ball and socket joint, with the ball being at the top of the thigh bone (femur) and the socket (acetabulum) being in the side of the pelvis.

 

 The various steps in performing a total hip replacement:-  A, The femoral head is removed. B, the acetabulum is reamed. C, the femur is reamed. D, a plastic cup is cemented into the socket of the hip, a metal stem is inserted down the centre of the femur and a metal ball is placed onto the top of the metal stem. E, the hip is 'reduced' - the new head is relocated into the new socket.

CLICK HERE TO SEE AN ANIMATION OF HOW A HIP REPLACEMENT IS PERFORMED

With a traditional hip replacement, the ball of bone (the femoral head) is cut away, and a metal stem is fixed inside the bone of the top of the femur. The metal is fixed into the bone with a special bone 'cement', made of an artificial substance called polymethylmethacrylate, which sets solid. A metal ball is then placed onto the top of the metal stem. The bone of the socket in the pelvis is then scraped out (reamed), and a polyethylene cup is cemented into the pelvis. The metal ball is then located into the new plastic cup, giving a good strong artificial ball and socket joint.

 

 

 The Stryker Exeter total hip replacement. A polished double-taper femoral stem with a metal head that sits in a plastic cup. Both the stem and the cup are cemented onto the bone.

A post-op X-ray showing an Exeter total hip replacement cemented into the femur and the pelvis, with a metal ball in a plastic socket.

The results of hip replacements are generally excellent, with in excess of 95% of hip replacements still in place and working well after 10 years.

However, the operation itself should not be underestimated. It requires either a general or a spinal anaesthetic, and the surgery itself can last 1 hours. Patients can, not infrequently, require blood transfusions after the surgery, due to the blood loss that inevitably occurs during the procedure. Patients are generally kept in hospital for something in the region of 5 - 10 days postoperatively, depending on their general fitness.

Different Types of Hip Replacement

Most traditional THRs use a special type of bone cement to fix the prostheses into the bone of the femur and the pelvis. However, there are many THRs available now that do not rely on cement, and which instead are press-fit into the bone. They are normally textured and/or coated with chemicals (hydroxyapetite) that encourage the bone to grow onto the surface of the prosthesis.

Some companies have developed hip replacements with harder bearing surfaces, such as ceramics or metal-on-metal. These each have their potential merits and potential problems. In the past there have been some concerns with the use of ceramic balls and sockets, where there were cases where the ceramics were too brittle and therefore cracked and broke. Modern ceramics, however, are more reliable and the wear rate with a ceramic-on-ceramic hip can be as much as 1000 times less than is seen in a traditional metal-on-plastic hip. 

 

 The Stryker ABG uncemented hip replacement. A ceramic socket sits inside the metal acetabular shell, and a ceramic head sits at the top of the uncemented stem. The acetabular shell and the femoral stem are coated in a layer of hydroxyapetite, so that the bone grows onto the surface of the metal.

Other companies have focused on the development of hip replacements using metal balls in metal sockets. In these hips, the opposing metal surfaces actually polish each other smooth, and after an initial 'bedding in' period, the wear rate can be very low.

 

Metal-on-metal hip replacement - a metal head sits in a metal actetabular shell. 


However, one of the potential disadvantages of a metal-on-metal THR is that they produce very high numbers of very tiny metal particles. These metal particles are too small to activate the local bone cells to resorb bone, which is a very positive thing. However, studies have shown that the metal particles to accumulate in the local soft tissues and in lymph nodes throughout the body, and there are some concerns regarding what the potential long-term consequences of this might be, such as the formation of tumours. However, to-date, no studies have demonstrated any such association.

One particular concern that does exist with metal-on-metal THRs is that the metal always contains small traces of impurities, such as Nickel. As much as 10% of the population may have some degree of Nickel sensitivity. There have been cases reported where metal-on-metal THRs have loosened prematurely due to what appears to have been an 'allergic' reaction to the metal particles. Therefore, especially in any individual with a history of reacting to metals (such as cheap jewellery, which has a high nickel content) it is probably advisable to perform patch-testing on the skin prior to surgery, or to even avoid metal-on-metal bearings altogether.

Risks & Complications

As with any major surgery, there are risks and complications. With Total Hip Replacement, the main risks are infection , deep vein thrombosis , and dislocation of the joint . The replacement joint may wear out and require another replacement.

Infection

Hip replacements are major surgery, and as is the case with any major procedure, there are various potential complications that can occur. Probably the biggest worry that is at the forefront of most people's minds is the risk of infection. Bacteria exist all around us; in our skin, in the air and even sometimes in our blood. Bacteria can sometimes stick to the metal surface of a hip replacement, where they can multiply. They can then form a sticky membrane around themselves that makes it difficult for antibiotics to get to them. Superficial infections near the surface of a wound are more common and can normally be treated effectively with a simple course of antibiotics. However, deep infections affecting the joint itself are far harder to treat, and sometime it becomes necessary to remove the entire artificial hip joint, leaving a shortened leg with a stiff hip.

For the above reasons, the risk of infection is taken very seriously in THR, especially with the prevalence of new antibiotic resistant strains of bacteria, such as MRSA. Patients are therefore given 1 dose of prophylactic antibiotics before the surgery, followed by 2 doses postoperatively. Furthermore, the procedure is carried out under full aseptic conditions, in a theatre with ultra-filtered air that is carefully controlled using a laminar airflow environment.

The risk of infection is considerably lower in the private sector than it is in the NHS. However, it should be stressed that this does not necessarily reflect better surgical or nursing care, or cleaner hospitals, and it may potentially simply be a reflection of the more complex cases and sicker patients that generally tend to be treated within NHS hospitals, with the inherent greater risk of infection that this carries.

Deep vein thrombosis

One of the other main potential complications that is associated with joint replacement is the formation of deep vein thromboses (DVTs). DVTs can develop in the veins of any limb that remains static for too long. The risk of thrombosis also increases due to the stress hormones that are released after any kind of major surgery. Small DVTs, below the level of the knee, are frequent and can often cause no actual symptoms. However, larger DVTs, especially those that develop above the level of the knee can actually cause painful swelling in the leg, and they can break off and pass into the circulation and potentially lodge in the lungs (a pulmonary embolus) - which can on rare occasions be fatal.

Again, therefore, this potential complication is also taken very seriously and to try and reduce the risk patients are given low-molecular-weight heparin injections once a day after the surgery, which slightly thins the blood, reducing the potential for clots to form. In addition, patients are mobilized as early as possible postoperatively, and are encouraged to repeatedly exercise and contract the calf muscles in order to pump blood and keep it moving through the veins of the leg.

Dislocation and Leg Length Discrepancy

Artificial joints are never as stable as the natural hip joint. Dislocation can occur with a THR post-operatively, with the literature suggesting that this may occur in as much as 2% of patients. Most dislocated hip replacements can be put back into socket fairly easily, either under sedation or under a brief general anaesthetic. A majority of these dislocations then stabilize after a period of rest. However, on rare occasions where THRs become recurrently unstable and dislocate repeatedly, further surgery may become necessary in order to, for example, change the socket to a more stable prosthesis.

When arthritis is severe, the bone around a hip joint can wear aware significantly, resulting in a slightly shortened leg. When surgeons replace a hip joint, the aim is always to restore leg length where possible, and to match the length of both legs equally. However, the main priority in THR surgery is to obtain a stable joint, and therefore balancing of the joint length and the soft tissue (muscle/tendon) tension is paramount. It is not uncommon for there to be minor discrepancies, therefore, in leg length post-operatively. However, this is rarely any greater than about 1cm in magnitude, and can normally be corrected for very easily with a heel raise shoe insert on the appropriate side.

Wearing out and loosening of the THR

Natural hip joints are made of living tissue that is constantly in a cycle of repairing and rebuilding itself. Hip replacements, however, use metals, plastics and ceramics - all of which can wear out with time.

In a standard hip replacement, there is a metal ball sitting in a plastic (polyethylene) socket. The ball can wear into the plastic, creating a debris of tiny plastic particles. These particles can affect the local bone cells, causing them to resorb bone tissue, thus making the hip replacement become loose. A loose hip replacement often causes significant pain, and if the loosening is sufficiently severe, then the THR may need to be removed, and a new hip inserted (a revision hip replacement).

Revision Hip Replacements

Revision THRs are technically more difficult procedures, with a greater risk of blood loss, and with perhaps twice the potential complications of a standard primary THR. Furthermore, the longevity of a revision THR is less than it is for a primary THR, meaning that the revision THR may not actually last as long as the first one did. 


Article written by
Mr Ian McDermott
Consultant Hip Surgeon and Knee Surgeon, London & Northwood
Last updated 13/01/10


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