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NEWER TRENDS - SURFACE HIP REPLACEMENT

The principle of the operation

The total hip replacement surgery entails amputation of the whole femoral head and invasion of the marrow cavity in the thighbone. Yet, in many patients the damage is limited only to the hip joint surfaces. Obviously, in these patients it would suffice to remove only the damaged surfaces and leave the rest of the skeleton untouched.

This is the idea of the hip surface replacement operation. (Such surface replacement is the procedure carried out regularly on the knee joint surfaces although there it is still called "total knee replacement").

The principle of surface hip replacement

The Principle

The principle: much less bone tissue is removed at surface replacement operation from thighbone end.

Left picture: During total hip replacement the surgeon removes the whole femoral head, part of the femoral neck, and opens the marrow cavity of the thighbone.

Right picture: During surface hip replacement the surgeon removes only the damaged hip joint surfaces from the femoral head to prepare place for the metal shell. Femoral neck and contents of bone marrow cavity are left in place.

Note that hip socket (acetabulum) is reamed equally much in both operations (not shown in the picture)

Note also that in some patients with avascular necrosis of the femoral head, the surgeon fits only the femoral head surface with a metallic shell. The acetabulum is left untouched because it is not changed by the disease.

This is hemiresurfacing arthroplasty ("half resurfacing") of the hip. See for more details in Alternative hip operations

The Surface hip replacement operation

The principle of surface hip operation

In the hip surface replacement operation the surgeon removes only the diseased or damaged surfaces of the head of the femur and the hip socket (acetabulum).

The femoral head is fitted with a spherical metal shell and the hip socket is lined with a thin spherical metal shell too. Both spherical shells (cups) are congruent and together they form a pair of metal bearings.

Note that the femoral shell has a central pin that helps the surgeon to choose the right position of the shell on the reshaped (chamfered) femoral head. The reshaped femoral head has a rather cylindrical form.

The hip surface replacement concept remains attractive because the femoral head and neck are preserved, the femoral (thigh) bone is loaded in a more natural way, and the large femoral shell enhances the stability of the surface replacement arthroplasty and prevents dislocation.

Preservation of bone stock is especially attractive for young, active patients who are likely to outlive their first hip arthroplasty operation and will have a new arthroplasty operation during their lifetime.

The demands on the modern surface replacement device:

Both shells must be thin to avoid an undue resection of the femoral head or acetabulum bone stock.

Both shells must be also made from a material that does not produce excessive quantities of wear debris. Consequently, the only passable material for the shells are the modern Cobalt Chrome alloys.

Both shells must be highly congruent but must also have a spell space between them ("just right" spell is necessary): Large spell produces too much wear, too little spell increases fraction tremendously.

The placement of the shell on the femoral head in right position is very important. Too much tilting of the shell will notch the neck of the thigh bone and may even cause a femoral neck fracture. The femoral shell should thus have a pin or another arrangement to alleviate the right placement of the shell.

In (2002) there were four manufacturers of hip surface replacement models on the market, At present (2005) the majority of all total joint manufacturers produce their own surface replacement device

All hip surface replacement models are made from Cobalt Chrome alloys. The majority of these devices is press- fitted on the raw bone surfaces of the femoral head and on the hip socket surface without use of cement, Some models use bone cement for fixation of the femoral shell.

The majority of models use a central pin placed in the femoral shell to secure the right position of the femoral shell on the shaved off femoral head.

Modern surface replacement device.

Birmingham hip, Finsbury.

Modern surface replacement device.

The modern surface hip replacement device. The shell for the femoral head has a pin for precise placement of the femoral shell. Cement is used for fixation of the femoral shell. The acetabular shell is porous- and apatite- coated on its surface. No cement is used for fixation of the acetabular shell

Who is a candidate for this type of the hip arthroplasty operation?

Risk index

Young, active patients, usually under 65 years of age who will meet also the other requirements for total hip replacement, i.e. severe limitation and stiffness in the hip joint.

It is important that the patient does not have any risk factors that may increase the risk of failure of the surface hip replacement operation. Such risk factors are:

Previous operation on the hip joint, especially an operation that left the neck of the thighbone deformed.

Very active, heavy patients

Patients with bone cysts (voids) in the femoral heads and necks

Patients with very small and /or severely deformed hip joints

The operation and postoperative treatment

The length of the operation carried out by an experienced surgeon on a not too much deformed hip joint is not longer than the "usual" total hip replacement operation. The need for blood transfusion is lower and the postoperative complications, such as deep vein thrombosis, are encountered less often than after total hip replacement according to the available reports.

The postoperative treatment and mobilization of the patients is similar to the treatment and mobilization of patients operated on with conventional total hip replacement. Many patients will have a short course of NSAID treatment to prevent development of postoperative ossifications because in some reports this complication occurred relatively often.

The mobilization of the patients varies: the patients with cemented femoral shells (McMinn) were allowed full weight bearing from the third postoperative day,

the patients with cementless shells have been on partial weight bearing for 6 - 12 weeks. This also varies.

The results :

The available studies show that the majority of patients were relieved from their pain, gained better motion in their hips, and improved considerably their walking ability. It seems also that the pain relief after the surface hip replacement is as good as that after the conventional total hip replacement.

Late complications are the same as the complications after the total hip replacement. There is as yet no report comparing the rate of complications after the surface hip replacement with those observed after the conventional total hip replacement. The short term results (3 -5 years) showed 95 to 99% of satisfied patients.

Complications:

Neck Fracture

x-ray picture of the femoral neck fracture one year after surface replacement. Note: an old model (1983) of the surface replacement device

Complications

There is one specific complication of surface hip replacement not observed in total hip replacements: the fracture of the remaining femoral neck. In the published reports about the modern surface hip replacement this complication occurred in about 0.8 to 1.0% of all patients. Usually it occurs within one year after surgery; the first signs are pain. In some patients it occurs after a previous trauma.

It is important that patients with sudden occurrence of pain in the hip, especially after a preceding trauma, will have a careful x-ray examination of their surface replaced hip. With suspicion on this complication the patient should be put on non-weight-bearing regime and followed up closely.

Heterotopic Ossifications

Are equally frequent as after total hip replacement. Predisposed are same categories of patients as the patients at risk after after total hip replacement. See the chapter Other complications of total hip surgery

High blood levels of trace metals:

The surface replacement shells have larger surfaces than conventional metal on metal total hip prostheses and, at least theoretically, they will produce more metallic particles and other corrosion products than conventional metal on metal total hip prostheses. Consequently one may suppose higher levels of these metals in the blood and urine of patients with surface hip replacements.

One report showed that patients with hip surface replacements had very high values of Chromium and Cobalt in their blood and urine.

Trace Metal	Elevation compared with normal people
Chromium in blood	28-fold
Chromium in urine	146-fold
Cobalt in blood	12-fold

These values were also 3 - 4 fold higher then the already high values of these metals in patients with metal on metal total hip prostheses. (Jacobs 1996). Patients with bilateral surface replacements have had still higher serum & urine levels of these metals.

These elevated values of trace metals were found in patients who have had their surface replacement surgery less than two years ago. It is known that the wear of metal on metal surfaces is very high during the initial period. It may be that these very high serum levels of the trace metals only reflect the initially high wear rates of metal on metal bearing couples.

Although the use of metal on metal bearing couples in the surface hip replacement eliminates the production of polyethylene particles, there is concern about the high blood concentrations of the metals that are part of the alloys used for fabrication of surface shells.

The importance of these very high serum & urine levels of the trace metals for the development of cancer is not known. (Merrit 1996).

Patients with impairment of kidney function (urea blood levels and creatinin clearance levels too high) should not have modern surface hip replacement with metal-on-metal shells.

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