|
|
SURFACE HIP REPLACEMENT (Double cup or double shell) To understand the anatomical terms in this chapter please look at the chapter Hip joint Details of anatomy The difference between the surface and the total hip replacement. Show picture: Difference between surface and total hip replacement A : Left side: Surface hip replacement: surgeon removes only the surface of the damaged femoral head and replaces it with a (relatively) thin shell of metal. Right side: Total hip replacement: the surgeon removes totally (=amputates) the femoral head and a large part of the collum femoris (neck of the thigh bone) and replaces it with a metallic ball. The amputated head and neck are shown in the insert. The metallic ball used for replacement of the amputated femoral head is part of a big and clumsy femoral (thighbone) component. The larger part of the component is anchored in the upper part of the thighbone’s marrow hole. In this picture the femoral component is superposed over the thigh bone, look how big the whole femoral component is. The larger part of the component is anchored in the upper part of the thighbone’s marrow hole. Thus, for fixation of the femoral component the surgeon must open and remove the contents of the upper marrow bone cavity B: Left side: The surface replacement shells are in place. It is obvious that surface hip replacement does not touch the upper part of the thigh bone. The hip joint is restored, the stability and mobility of the new surface replacement hip is disturbed minimally because the dimensions of the artificial ball and cup components are close to the size of the original hip joint. Right side: The total hip replacement changed profoundly the upper part of the thigh bone. The sizes of the ball components are smaller than the sizes of the original hip joint heads. Thus, the stability of the total hip joint is diminished.. Too much of the upper part of thighbone is away; in case of failure of the total hip device there may be problem to find enough intact skeleton tissues. Quit the pictures
The total hip operation causes big operation trauma when the surgeon opens the marrow hole (see the Picture Opening the marrow hole – OPTH_Operation3.jpg) with large reamers. The contents of the marrow hole, mostly marrow fat, are at this moment forced out into the patients circulation where they may cause emboli and confusion of the mental state of the patient. Total hip patients usually need more blood transfusion, and their return to function is slower than in patients with surface replaced hips.. Quit the pictures Now, the damage of the femoral head that needs replacement may be very different in individual patients. Show the picture: Damages of the femoral head decide who is candidate Left side picture: Suitable candidate A - In some patients the damage is limited really only to the surface of the femoral head. In this patient the femoral head is still round and there are no defects in the skeleton of the femoral head. B - After removal of the diseased surface there remains still enough healthy bone stock (white colour of the bone tissue because the surgeon flushed off oozing blood. The suction tube protrudes from the middle of the femoral head). The surgeon also made several holes in the skeleto to improve the fixation of bone cement. Operation pictures adapted from Amstutz 2006). This patient is suitable candidate for surface hip replacement.
Right side picture: Unsuitable candidate In other patients, on the other hand, the femoral head is distorted so much by disease that there is not enough bone tissue left to support the metallic shell. Upper picture: In this patient the femoral head is not round but flattened out and moreover there is big bone cyst that weakens furthermore the the skeleton. Lower Picture: After chamfering the there are only rests of the femoral head skeleton. These rests cannot support the metallic shell. For this patient is thus the total hip device the given alternative.
Quit the pictures The operation of surface replacement The operation of surface replacement is en exact technical procedure. The replacement of the hip joint socket with the acetabular shell does not differ from the procedure done in total hip replacement (Show Picture Replacement of the hip socket. The precise placement of the cup over the chamfered femoral head is decisive for the lasting result. Show Picture: The surface hip replacement operation
A - The skeleton of the femoral neck is made from tiny beams of spongy bone. These beams are strong only when all of them act together. When preparing the femoral head for the femoral shell the surgeon must be very careful not to disturb the fine architecture of the beams. In the right position the rim of the shell is flush with the femoral neck walls and parallel with the long axis of the neck. In this position the neck does not engage the tinny beams of sponge bone. In a wrong position the rim of the metallic shell encroaches the skeleton of the femoral neck and weakens it. Such weakened neck fractures easily.
B - Ready surface hip replacement operation. Note on this picture that the femoral ´shell’s rim is flush with the outer contours of the femoral neck and does not encroach on them. The direction pin of the femoral shell is parallel with the long axis of the femoral neck. The chamfered femoral head is fitted with a spherical metal shell and the hip socket is fitted with a thin spherical metal shell too. Both spherical shells (cups) are congruent and together they form a pair of metal bearings. Quit the pictures Note also that surface hip replacement differs from the total hip replacement only on the thighbone side. On the hip socket side there is no difference between these two replacement methods if the surgeon uses the metallic cup for replacement of the acetabulum (hip socket) in total hip replacement.
Why surface hip replacement? 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 and socket shells enhance the stability of the surface replacement arthroplasty and prevent dislocation. Preservation of bone stock is especially attractive for young, active patients who may outlive their first hip arthroplasty operation and will have a new arthroplasty operation during their lifetime. 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 blood supply of the femoral head and the surface replacement. The intricate blood supply of the femoral head may be a problem for surface replacement surgery of the hip. The surgeon must ream, chisel off, and chamfer the bone surface of the femoral head to make it flush with the hole in the shell. The nourishing vessels reach the femoral head across the surface of the neck where they are vulnerable to surgeon's chisels and chamfers. Some surgeons believe that damage of these nourishing vessels during surgery may cause death of the spongy tissue in the femoral head. The dead bone tissue will break under already moderate load and the the whole neck will eventually break. See also the chapter Blood supply of femoral head for details. The demands on the modern surface replacement device: Both shells must be thin to avoid too large resection of the femoral head's and acetabulum's (socket’s) bone stocks. 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 at present are the modern Cobalt Chrome alloys. Both shells must be highly congruent but must also have enough spell space between them ("just right" spell is necessary): Large spell produces too much wear, too little spell increases fraction tremendously. Show picture: Modern surface hip replacement device: Note how thin the shells of modern surface replacement devices are. In some devices -Conserve Plus models for example - the walls are3.5 millimeters thick. Thinner walls spare space; the outer diameter of the shell can be made larger. This is advantageous; the more the size of the replacement shell nears the size of the normal head the more is the replaced hip stable. On the other hand, there is a limit for the thinness of the walls: too thin walls will deform under body weight load and eventually loosen. The shells for the femoral head have pins for precise placement of the femoral shell. Cement is used for fixation of the femoral shell. The acetabular shells are fixed without use of bone cement. They are thus porous- and apatite- coated on their surfaces. At present (2007) the majority of all total joint manufacturers produce their own surface replacement devices. Quit pictures Who is a candidate for this type of the hip arthroplasty operation? The typical successful patient is a young male patient > 82 kg weight!, with good skeleton quality and normal kidney function! The term “ Young” means <65 years for men and for women <55 years of age. Risk factors: there are some factors that increase risk of the failure of surface hip replacement:
Patients who absolutely should not have surface hip replacement are patients with marked osteoporosis in their skeletons and patients with known long term kidney disease.
The operation and postoperative treatment specifics: The length of the operation carried out by an experienced surgeon is not longer than the 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 are 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 after surface hip replacement. The mobilization of the patients after surgery varies: Some surgeons allow weight bearing “as tolerated” from the first postoperative day for the majority of patients. Other surgeons are more restrictive with weigh bearing, starting with partial weight bearing first. Patients with defects in their femoral necks that needed special treatment during operation may have restricted weight bearing for 6 or more weeks. 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. The pain relief after the surface hip replacement is as good as that after the conventional total hip replacement in published studies. The short term results (3 -5 years) showed 95 to 99% of satisfied patients. See also the chapter: Recent reports on surface hip replacements
The complications Complications that occur after surface replacement operation are the same as the complications after the total hip replacement, except for one complication that occurs only in surface hip replacement surgery – fracture of the femoral (thighbone) neck. Is the occurrence of other complications following surface hip replacement higher than the occurrence of these complications after total hip replacement? There are only short term comparisons of the results of surface with total hip replacement operations. Such comparisons found no significant difference in the frequency of postoperative complications between both types of replacement operations.
Neck Fracture 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 2.0% 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.
Show picture Neck fracture
A - The upper schematic picture shows the fracture that starts as a hair-fine line through the skeleton of the femoral neck (white arrow). Note also that the neck’s skeleton consists of fine beams of spongy bone and that the femoral shell is placed in wrong position. Its lower rim encroaches on the skeleton (red arrow) and weakens it This hair-fine fracture causes pain but it may be difficult to see the fine fracture line on x-rays. Every surface hip replaced patient with suspicion of such incipient facture should be put on the non weight bearing regime and the fracture will often heal. B - The second (schematic) picture shows a fracture that widens and breaks the neck totally. Such fracture is easily seen on x-ray pictures (the insert). Unfortunately, this fracture displaced both fracture ends too much, it will never heal. The surface replacement failed. The patient must have a new operation. The surgeon removes the broken femoral neck with its shell and replaces it with a total hip device. The acetabular shell replaced at surface replacement operation stays in place and serves well with the new total hip component. Quit pictures
Two consequences follow from these pictures: First: The surgeon learns the right placement of the cup component successively as he operates on more patients. In most reports the fractures of the neck occur in the surgeon’s first 50 or so surface replacement operations. Thus – seek a surgeon who already has this learning period behind. Second: Surface hip replaced patients with sudden occurrence of pain in the hip, especially after a preceding trauma, must have a careful x-ray examination of their surface replaced hip. When the surgeon suspects the hair fine neck fracture, these patients should be followed closely until the subsequent x-ray control (after 4 -6 weeks) either confirms the fracture or shows an intact skeleton of the neck. Show x-ray pictures of the development of the neck facture after surface hip replacement: On this pictures you may see the development and treatment of a collum fracture on three x-ray pictures of the same patient (adapted from Shimmin 2008): A- A hair-fine fracture line starts at the white arrow and propagates down. The upper collum part is also encroached. B- complete fracture some time later. C- the patient was operated on: the cup component was retained, the broken part of collum and shaft component was removed and replaced with a conventional total hip component. This bild sequence demonstrates that the treatment of collum fracture after surface replacement may be relatively simple, easy, and durable. _________________________________________ High blood levels of trace metals: Cobalt and chromium are the principal components of the modern metallic alloys used for manufacture of surface and total joints. Yet, high blood and urine levels of these two metals are causing trouble to the surgeons and their patients who are bearers of metal on metal surface and total hip joints. The blood metals are dissolved metallic salts of cobalt and chromium, their right name is ions. The origin of the salts are the small wear particles of the metals produced continually on the bearing surfaces of the replacement devices. The total volume of the metallic particles is relatively small but the particles are very small and their numbers are enormous. The body's liquids react with the metals in the small particles and produce soluble metal salts. The production of metallic wear particles is very high in the first one-two years after surgery, then the production of wear particles diminishes but does not cease entirely. Several studies demonstrated that prolonged exposure to thigh levels of these metals produced damages among others to blood cells. See Picture: Damaged chromosomes.
Other studies, however, demonstrated that the body posses mechanisms that monitor and repair these damages. Cobalt is constituent of the vitamin B12, chromium is necessary for uptake of glucose. These two metals are essential for life, although in minimal quantities. For more information see also the chapter Metal on metal total hips Several studies were published about the levels of these metals in patients with metal on metal artificial hip joints, but the published blood levels are impossible to compare because individual authors used different techniques, some of them more easy and less accurate than other. Yet, some conclusions from all these studies could be drawn. Practically all surface hip replaced patients have elevated blood and urine levels of metals chrome and cobalt. Some studies demonstrated that blood metal levels are higher in surface replacement than in total hip replacement patients and also higher in patients who had both hip replaced. The levels of blood levels of both metals vary between individual patients depending on the length of time since surgery and on the patient’s activity. As yet, there is no proof that the elevated blood metal levels go down to normal values longer time ( >2 years) after surgery. The Birmingham surgeon DJW McMinn (England) carried a comprehensive study of blood levels and urine excretion of cobalt and chromium at his Centre in Birmingham ( Daniel 2007). This study may serve as a benchmark. The scientists in Birmingham followed the blood levels and the daily output of cobalt and chrome in urine in 26 patients for 4 years. Show Diagram of blood levels of cobalt and chromium The upper diagram shows the blood levels of cobalt and chromium. Before the surface replacement operation the patents had normal blood levels of both metals (zero line). The curves show how much were the blood levels elevated above normal values. The metal blood levels began to rise directly after surgery. One year after surgery the cobalt blood level /blue line) was 6.5 times higher and the chromium (red line) was 8 times higher than the levels before the operation. Both curves peak about one year after surgery and then successively drop down during the four postoperative years; bud the curves did not come near normal levels. 4 years after surgery the blood level of cobalt was still 6 times higher and that of chromium 4 times higher than normal levels. Fortunately, the kidney excrete both metals (or rather their ions) so that in patients with healthy kidneys the metals do not accumulate in the body (very probably). The scientists are not entirely sure because the metallic wear particles and even some of their salts may concentrate on certain parts of the body. The lower diagram shows how much increases the urine excretion of both metals in patients during the four postoperative years. The curves show how much increased the excretion of both metals after surgery. The diagram shows that the excretion of cobalt peaked already six months after surgery when it was forty! times higher then normal. Then the excretion sinks slowly but is still 13 times higher than normal 4 years after surgery - witness of continuing production of cobalt metal ions . Maximal Chromium’s excretion was “only” about ten times the normal half year after surgery, and then it drops slowly. But it is still seven times higher than normal four years after surgery. Obviously, the patients who will be transplanted metal on metal artificial hips must have healthy kidney. In patients with chronic renal diseases, who cannot excrete the steady overproduction of metallic ions, were observed toxic blood levels of these metals. Metal blood levels in pregnant women and their developing children (fetus): Surface hip replacement is an operation that is offered to young patients, some female patients are still in childbearing age. One must suppose that pregnant female patients will have equally high metal blood levels as the other patients. It is important to know whether the high metal blood levels would pass placenta and increase the blood metal levels of the growing foetus. Study of this issue is difficult. In the past there were published some individual measurements that concluded that no cobalt and no chromium pass placenta. Some scientists even considered such findings as "assuring". Which was of cause foolish; the growing child needs both cobalt and chromium for healthy development. Placenta in normal healthy women must pass these metals. These findings were the result of bad methods used to measure the blood metal levels. Again, the study done in Birmingham's McMinn Centre is a very comprehensive summary of our knowledge in this area (Ziaee 2007). Among 2700 patients who had surface replacement done in Birmingham’s McMinn’s Centre, there were 10 otherwise healthy women with surface hip replacements who were pregnant. The metal blood levels of these women and the metal blood levels in the umbilical cord of their babies were studied. Show graph Metal blood levels of mothers and their babies The diagram shows that the pregnant women had elevated metal blood levels, cobalt four times and chromium seven times. The placenta, however, did not let the mother's high blood level of metals pas into the blood of the baby. Whereas the mothers had elevated blood levels of cobalt four times and of chromium seven times, their babies had only moderate elevation of these levels (cobalt two times and chromium less than two times). There are, however, two important points to be considered here: One, the mean interval between hip resurfacing and parturition in these patients was almost five years (56 months). The maternal metal blood levels were thus relatively "low". Were these women to give birth to their children during the first two years after surgery, the metal levels in babies’ blood would be probably considerably higher. Thus, wait with pregnancy at least two years after surface hip replacement. The other one: As yet, we do not know what cobalt and chrome blood levels are well tolerated by developing fetuses. Is twofold increase of metal blood levels still well tolerated? In every case, the children born in this study had no signs of damage. See also manufacturer's sites (most useful for information on wear and friction of surface hips) : Corin Medical : www.coringroup.com Midland Medical technology : www.midmedtec.co.uk Zimmer / Centerpulse : www.metasul.com Wright Medical Technology: www.wmt.com References: Amstutz H et al.: Metal on metal hybrid surface arthroplasty. J Bone Joint Surg-Am 2006; 88-A, Supplement 1, Part 2: 234 - 49 Daniel J et al: Blood and urine metal ion levels…J Bone Joint Surg-Br 2007; 89-B; 169 - 173 Jacobs et al. Clin Orthop 1996;329 Suppl: S256-63 Shimmin A et al : J Bone Joint Surg-Am, 2008; 90-A: 637 - 54 Merrit K. Clin Orthop 1996;329 Suppl: S233 -43 Ziaee H: Transplacental transfer… J Bone Joint Surg-Br 2007; 89-B; 301 - 305
NEXT to Recent reports on surface replacement Before you take any action, please read the DISCLAIMER Revised January 2008
|
