In hip arthritis, the hip joint is a synovial joint by which the leg articulates with the trunk. It consists of a spherical head, at the top of the femur (or thigh bone) which is contained within, and articulates with, the cup-shaped acetabulum in the pelvis.
As a result of a number of conditions the cartilage of the hip can be destroyed, giving rise to arthritis of the joint. The pain of an arthritic hip is generally felt in front of the hip or in the groin, although it sometimes radiates into the back or down the leg to the knee. Wherever the pain, the affected person finds is that it progressively increases from a dull ache to a sharp discomfort as time goes by. The pain is aggravated by movement and the person finds that his (or her) activities become increasingly limited because of it. The other problem which people encounter is that the hip becomes very stiff. Walking becomes difficult and it often becomes awkward to undertake even the simplest of activities which require bending the hip, such as putting on ones shoes, climbing stairs, or getting into a car.
Trying the measures outlined in the previous section are often helpful but, in some cases, it becomes apparent that the only course open to the individual is to have the affected hip replaced by an artificial one.
During the operation the hip joint is exposed after incision through the skin and muscles. The joint is opened and the hip is dislocated before preparing the acetabulum and femur to take their respective prosthetic components. The damaged cartilage of the acetabulum is removed, using a set of hemi-spherical reamers, and a cup shaped artificial acetabulum is inserted. The head of the femur is removed and the upper part of the bone is reamed or fashioned to take the artificial femoral component which is then fitted. The hip is then reduced once more and the wound closed.
The artificial hip joint is designed to move and act in the same way as the normal joint. In doing so it must carry the load of the body across it and to do this it has to be bonded to the bone. This is done in one of two ways. The artificial hip may be ‘cemented’ i.e. the prosthesis is bonded to the bone by way of orthopaedic cement, or it may be uncemented. The uncemented prostheses are specially designed to encourage the bone to grow into them and bond with the material of the prosthesis. This is a living function and it is usually necessary to protect the hip for a period until this process is established.
Longivity of the prosthesis
The hip prosthesis, like the normal hip, is subject to ‘wear and tear’. Being an artificial construction it is not able to repair itself like the normal tissues of the body and is likely to ‘wear out’ and loosen after a period and need to be replaced.. This applies particularly in the younger patient who, being more active, is likely to subject his (or her) artificial hip to more stress than the elderly patient and require revision of the prosthesis at an earlier date.
One of the main factors which has been shown to contribute to the loosening of an artificial hip are wear particles which arise from the plastic substance of the cup inserted into the acetabulum. In order to minimise these minute plastic wear particles the construction of the acetabular component has been changed. At present three different methods of are in use. The first method is to harden the plastic which is used for the acetabular component, making it more resistant to wear. The second is to use some other substance for the articulating surfaces which rub together as the hip moves. In this respect ceramic components and metal components are routinely used. These substances can now be machined and polished to give a very smooth surface which is extremely resistant to mechanical wear. It has been shown that use of one of these methods in the construction of the prosthesis significantly prolongs the working life of the artificial hip. It must be noted, however, that the release of metal ions from some ‘metal-on-metal’ articulations can result in a reaction in the bone about the prosthesis which leads to the collapse of the joint.
Construction of the artificial hip
In order to transmit the forces of walking and climbing through the artificial hip it is necessary to ensure that the prosthetic components bond securely with the surrounding bone. This is done in one of two way. The first is to use a ‘cement’ substance which locks into the bone and around the prosthesis to hold it securely. The second is to encourage the surrounding bone to ‘grow’ into the surface of the prosthesis and bond with it. This second method is achieved by using an irregular surface to the prosthesis or by coating the surface of the prosthesis with bone like calcium hydroxyapatite onto which the bone fuses.
The ‘traditional’ prosthesis on the femoral side of the hip sits in the medullary cavity in the middle of the bone after removal of its articulating head. Hip ‘resurfacing’ is a different concept which attempts to preserve as much of the hip substance as possible. This being so the upper part of the femur is retained but fashioned so that it can be covered by an artificial head. A prosthetic cup is inserted into the acetabulum as normal. Seemingly the advantages of this procedure is to preserve as much of the natural bone as possible and, by having a large head, increasing the stability of the joint. In addition the articulation is ‘metal on metal’ which, it was thought, reduces the amount of articular wear.
While this prosthesis has given good results, particularly in the younger patient, it must be remembered that it is still an artificial joint and subject to the same problems as other artificial joints, particularly dislocation, fracture of bone and, ultimately, some wear.
It has nevertheless also been found, as noted above, that destructive, inflammatory granulomas develop in relation to some of these prostheses. This has been attributed to a relatively large number of metal ions being released by the wear process in a sensitive individual. As a result this particular type of prosthesis has ‘gone out of favour’ in some areas until better types of articulation are found, such as a ‘ceramic on ceramic’.
After the surgery
The pain due to the operation usually settles within a day or two and early mobilisation is usually encouraged. One of the problems with hip replacements is that the hip may be slow to regain its normal range of motion and early movement and exercise is to be encouraged. In the early days, however, the hip is potentially unstable, particularly in the flexed position and there are, therefore, some actions which should be undertaken with caution:
- In the early stages it is necessary to walk with the support of a pair of crutches or a walking frame, particularly if an uncemented prosthesis has been inserted. As the hip gains strength the crutches are exchanged for a cane or walking stick which is finally discarded when full walking ability has returned.
- Sitting in a low chair is to be discouraged. If the toilet seat is too low it may be necessary to obtain a commode. Similarly, a cushion may be necessary when sitting in a car.
- The operated leg should be supported while in bed and a pillow placed between the legs when lying on the side.
It takes about six weeks for the tissues to heal completely and, after this period, it is safe to undertake most activities.
The joint which is inserted into the hip is artificial and subject to its own particular problems:
- It cannot repair itself, as normal body tissues can, and will ultimately wear out. Various surgical techniques and materials are used in an attempt to diminish this wear and to prolong the ‘life’ of the prosthesis.
- Wear of the components can ultimately lead to an instability of the implants necessitating revision of the prosthesis. This can be due to the wear products from the implants causing a ‘reactive granuloma’ which damages the surrounding tissues.
- Infection after the operation is an ever present threat to the prosthesis and must be guarded against. Following such a procedure care should be taken with any infections as there is a small risk that bacteria from such a lesion may find their way to the prosthesis and cause it to be infected. Similarly, any person with an artificial joint should consult their doctor or surgeon regarding antibiotic cover prior to any urological surgical procedure or to a tooth extraction as these can cause bacteria to circulate in the blood for a while.
Under certain circumstances the prosthesis can become unstable and can dislocate.
- The arthritis of the hip sometimes causes the affected leg to be shorter. Surgery often corrects this inequality but there are instances where this is not possible and the differences in length of the legs persists.
- A relatively unusual complication is the development of ‘ectopic’ bone about the hip joint following the insertion of a total hip replacement. While the presence of bone fragments has been implicated in its causation, it is more likely a tissue response to the operation in susceptible individuals.
The operation may not take away all the pain. A specific problem in this respect is that very occasionally pain is experienced in the outer aspect of the thigh. This is usually associated with the prostheses inserted without ‘cement’ and usually settles after a while.
All these problems notwithstanding, arthritis of the hip is a relatively common condition and the operation of ‘total hip replacement’ has revolutionised countless thousands of peoples lives – allowing them to regain the lost movement of the hip and move about again with little, if any, pain.
Complications of hip surgery
You must note that surgical procedures on the spine are not without their problems. While every effort is made to undertake the procedure carefully and safely, things sometimes (inadvertently) do go wrong. Although the risks are very low some of the following mishaps rarely can occur during the operation, some of which may require further surgery:
- Continuing pain
- Stiffness of the hip
- Dislocation of the hip
- Deep vein thrombosis (blood clots)
- Fat embolism to lungs
- Leg length discrepancy
- Injury to an artery or vein leading to uncontrolled bleeding
- Injury to a nerve with permanent weakness and/or numbness in an area of the leg.
- Malplacement or shifting of the prosthesis
- Subsidence of the femoral component
- Fracture of the bone
- Extra bone formation
- Allergic reaction to prosthesis
- Reaction to anaesthetic agents