DISLOCATION OF THE TOTAL HIP
CONTENTS:
Dislocation of the hip joint
Facts for your decision
Dislocation of the polyethylene liner
DISLOCATION OF THE TOTAL HIP JOINT
"The point must be made that an occasional post-operative dislocation ...is no disgrace. Patients can sometimes be quite irresponsible and unreasonable during this period...It is only in recurrent subluxation or dislocation that the surgeon might have to hold himself responsible.." (???)
Sir John Charnley 1979, one of the Founding Fathers of the total hip surgery.
3What is it?
The total hip dislocation is a painful complication in which the femoral ball component comes out (dislocates) of its place in the cup component and moves outside the total hip.
The x-ray picture reveals that the cup component of the right total hip is void. The ball component left the cup and reposes now above it resting on the pelvic bone. The left hip joint is normal on this x-ray picture. Note that the centre of the dislocated right ball component lies above the centre of the left normal hip joint. The upper position of the dislocated ball is due to forceful and painful contracture of thigh muscles that pushed the dislocated ball into this position.
At examination the doctor found the right leg stretched and rotated outwards and shorter.
Picture: X-ray picture of a dislocated total hip. (Click on the icon for a full size picture)
The mechanism that causes the majority of dislocations is impingement of the neck of the femoral component against the rim of the cup. See picture:
Picture
Dislocation of the total hip joint backwards
(Click on the icon for a full size picture)
Upper picture: Profile picture of the total hip. Patient has moved his/her leg up to 90 degrees of flexion. The neck of the femoral component comes close to the rim of the cup.
Middle picture. If the patient continues to move (flex) the leg above 90 degrees of flexion the neck of the femoral component hits the rim of the cup = the neck impinges on the rim of the cup (Red circle). The thighbone acts as a prolongated lever of the femoral component and pushes the neck against the rim of the cup; this very forceful lever mechanism eventually pushes the ball out of the cup.
Lower picture. The ligaments and the capsule on the backside of the hip are weak and do not restrain the movement of the ball backwards. Once outside the cup, the ball travels backwards through the weak backside capsule and out of the total joint. The ball component dislocates backwards.
Why is it a problem?
- Total hip dislocation is a very painful condition
- Total hip dislocation distress the patients who are scared to continue their every day activities and don’t dare to continue with gymnastics for fear of a dislocation
- For reposition of the dislocated ball component the patient must have anesthesia combined with muscle relaxing medicines; the anesthesia has inherent risks for the patient.
- Repeated dislocations need revision operation ; these operation imply often severe surgical trauma for the often old and frail patients.
Stability of the total hip joint.
Depends on the following factors:
Restoration of balance in soft tissues around the total hip
(See also the chapter Function of the total hip.)
Removal of the diseased hip joint leaves the soft tissues slack. The surgeon should choose such total hip joint model that will restore appropriate tension in muscles and soft tissues around the new total hip. This need experience. If the muscles around the new total hip are out of balance the risk of dislocation increases.
The scar in the severed and sutured tissues is a weak spot. Dislocation succeeds better when the dislocating head pushes against the weak spot.
Successively the scar tissue matures and a new capsule forms around the new total hip that further stabilizes the new total hip joint. At 6 weeks after the surgery the soft tissues are reasonably healed, but further thickening and strengthening of the scars continues for months.
The position of the components:
Components placed in wrong position make the total hip joint unstable. See Picture
Picture:
Wrong position of the cup makes the total hip unstable
Upper picture: The opening of the cup should face forwards, the opening angle should be about 45 degrees with horizontal plane. If the cup has this position the ball is still well contained in the cup even if the patient bends up the thigh to 90 degrees flexion.
Lower picture: The cup faces too much downwards. When the patient bends the thigh close to 90 degrees the femoral component (its neck) impinges on the rim of the cup and the ball begins to leave the cup. Such total hip is unstable.
The size of the ball component:
Total hip joints have had always the femoral ball smaller than the removed femoral head. Small femoral ball makes the total joint less stable.
Statistics demonstrate that total hip models with large femoral balls (diameter 32 mm or larger) have less dislocations than total hips with small balls ( 22 or 26 mm in diameter).
Large ball makes the total hip joint more stable.
Upper picture: Total hip with large ball
The ball is well contained within a large cup even at flexion above 90 degrees. Large force is necessary to move the large ball out of the cup. Note that the neck is thin, the thinner the neck the less is the risk of impingement when the patient bends the thigh.
Note that the walls of the cup are thin to accommodate the large ball. It must be so because the limiting factor for the size of the cup is the size of the natural hip socket. The surgeon cannot enlarge it much. So that if the femoral ball should be large, the thickness of the walls of the cups must be kept at minimum. For metallic cups the limit is about 5 millimeters. (Thinner walls will deform under load). For this purpose the modern total hips with really large balls are metal-on-metal systems. Ceramic or polyethylene cups must have thicker walls.
Lower picture: Total hip with small ball.
The neck impinges against the cup already when the patient is bending the thigh less than 90 degrees. Already a small force can push the small ball out of the cup.
Note that the walls of the cup are thick to accommodate the small ball. The thick neck impinges against the thick walls. Again, a disadvantage of small ball component.
How often does it happen?
Determining the true rates of postoperative dislocation is difficult because the statistis are often incomplete.
Most reports from clinics with high-volume operations suggest that this complication occurs in 0.3 to 3% of patients]. A recent review of Medicare patients in the United States described varying rates according to surgeon operation volume: patients operated on by surgeons who performed less than 6 THR annually experienced 4.2% dislocations, patients operated on by surgeons with > 50 THR annually experienced only 1.5% dislocations.
Very long follow-up evaluation is necessary to determine the true frequency of dislocations because late dislocations, occurring after > 10 years after the surgery has become a more common problem related to polyethylene wear and implant impingement. The cumulative frequency of dislocations in patients followed up for > 20 years was 7 % (von Koch 2002).
Studies showed that the majority of total hip dislocations appeared during the first two months after the surgery; only 20 % of all dislocations occurred later than 2 years after the surgery. Long time (>20 years) after surgery, however, the frequency of dislocations rises again; when the age of the patients rises over 80 years the frequency of dislocation rises again. The mobility and balance control of the old patients decreases and the frequency of dislocation increases. The cumulative frequency of dislocations in older patients followed up for > 20 years was 7 % (von Koch 2002).
Revision operations for repeated total hip dislocations are, after aseptic loosening and deep infection, the third most frequent cause of revision operations.
Factors increasing risks of dislocation.
Usually, not one but several risk factors "collaborate" to eventually dislocate the total hip joint.
Patient, surgical, and implant factors play an important role in the etiology of dislocation after THA.
Important patient risk factors include advancing age, female gender, prior surgery or fracture through the hip joint, neuromuscular disorders that damage the muscles around the total hip, dementia, and alcohol abuse.
Important surgical risk factors leading to dislocation include
wrong positioning of the total hip components,
failure to restore leg length and / or proper tension of the tissues around the total hip
failure to preserve the strength in the abductor muscles (the strong muscles that move the leg sideways and keep the femoral ball in the cup)
.using the posterior surgical approach (
approach to the hip joint from the back. Statistics of surgeons who carefully repaired their cuts through tendons and joint capsule after the surgery demonstrated, however, low rates of dislocation after the posterior approach. Obviously, the risk factor is not the posterior approach itself, but omission to repair the divided tendons and joint capsule at the end of the operation.
Implant
risk factorsinclude total hip models with small femoral balls (22 mm), and femoral balls with thick femoral neck component.
Diagnosis
Signs of total hip dislocation:
Usually the patient feels very painful "popping" in the total hip joint. Often this popping occurs after a sudden vigorous movement or accident.
The patient is keeping the whole leg stiff and firmly pushed to the midline and the other leg (if the ball is dislocated backwards), or rotated outwards and pushed from midline (if the ball is rotated frontwards). He/she resists every attempt to move the leg because every such attempt is very painful.
If the patient has had many dislocations in the past, the pain may be only moderate, but the ability to move the leg is still severely restricted.
If it happens to you and you suspect a dislocated total hip:
call your doctor immediately
call the ambulance
do not eat or drink anything. You will very probably need anesthesia to get the total hip back in place.
Try to remember and tell your doctor the following:
Did you have an accident immediately preceding dislocation?
Did you faint away immediately before the dislocation?
Did you use drugs that may make you prone to the accident?
Was there intake of alcoholic beverages before the dislocation?
X-ray pictures
X-ray pictures make the diagnosis. Often these pictures are taken on the emergency room and may be of "varying" quality.
The pictures show that the femoral ball lies outside the cup component.
The radiologist should take more than one picture of the total hip with suspected dislocations to pinpoint the position of the dislocated ball.
(When the radiologist takes only one x-ray picture the ball component may be projected on just this one picture by chance in the middle of the cup although it in reality lies dislocated behind the cup component; such picture may give falsely although in reality it lies dislocated behind the cup component negative diagnosis of dislocation.)
Treatment of the first dislocation
Most of the time, the dislocated femoral ball may be pulled back in place without operation. This is called reposition. Good anesthesia and muscle relaxation are always necessary to accomplish the reposition with so little damage to soft tissues as possible. Good muscle relaxation is extremely important in patients with weak bones (osteoporosis) because forceful pulling of the leg against painfully contracted muscles may cause a fracture through the weak thighbone. The majority of repositions through simply pulling on the leg succeed well; in rare cases the ball may get firmly stuck in tissues around the hip and cannot be moved by pulling applied on the leg. In these cases the surgeon may be forced to open the dislocated total hip and carry out the reposition through an open operation.
After reposition the surgeon takes x-ray control pictures to verify that the ball is in place again and to verify that the total hip joint was not damaged by dislocation and succeeding reposition.
When x-ray control verified that the reposition succeeded the patient’s leg is placed in a brace in a "secure position". The methods for bracing the leg and the length of bracing vary – from days to weeks, from complete bed rest to walking with a special brace.
In the secure position the ball component of the total hip prosthesis reposes securely in the socket while the damaged soft tissues are healing. Healing takes some weeks and there may be problems with the brace – the brace may cause back pain, local pressure, etc.
After reposition of the dislocated total hip, the surgeon should control the nerve function and circulation in the leg, because dislocation may damage the nerves or the vessels crossing the dislocated hip joint.
The repeated dislocation
is always a serious matter
The first dislocation that occurred during the first three postoperative weeks and was treated accordingly has a low risk of recurrence: about 20 to 30 % during the next years.
After another (second) dislocation the risk that the total hip will continue dislocate increases substantially; according to some statistics about 50% of patients who had two dislocations will continue to dislocate their total hip repeatedly; this risk is especially high if the total hip operation was done through posterior approach or if the total hip is a model with a small femoral ball.
If the x-ray pictures do not demonstrate any deviation that may explain the dislocation, the surgeon may still try treatment with the stabilizing brace, but he should explain to the patient that the stabilization of the total hip with this treatment cannot be guaranteed.
If the patient has had a third dislocation, the hip is very probably unstable and will not stabilize without surgery. 80% of patients who have had three dislocations continue to have further dislocations within three years. Further attempt of conservative treatment would probably be a waste of time.
After the third dislocation the surgeon should thus discuss with the patient the possibility of operative treatment.
Examinations after second and further dislocations:
Comprehensive x-ray study of the total hip to assess any deviations of the total hip components from the "normal" position.
Possibly examination of the total hip in narcosis combined with simultaneous fluoroscopic (x-ray) examination. This examination may disclose weak soft tissues and/or deviations in the position of the components not apparent on ordinary x-rays.
It is important to realize that the surgeon should know what defect caused the dislocation before he/she proceeds to the surgery to repair it.
If this evaluation demonstrates any clearly identifiable cause of the repeated dislocations that the surgeon can repair, it is advisable not to wait too long with the revision operation. Every new dislocation increases the damage of soft tissues and the repair will be more difficult if one waits too long.
OPERATIONS TO IMPROVE THE STABILITY
of the recurrently dislocating total hip joint
Remember that usually there is not only one but rather numerous factors that together make the total joint unstable. Accordingly, there are also many operations designed to cure the unstable total hip.
Revision of faulty positioned components
Any faulty position of the total hip device or any damage of it that is suspected to cause repeating dislocations should be repaired in a revision operation. Removing the worn, old, damaged components may be a difficult surgery and the surgeon should discuss the risks of such operation with the patient.
Some other operations:
Picture
Operations to relieve slackness of soft tissues around a total hip
(Click on the icon for a full size picture)
In the total hip on this picture the most apparent cause of the repeated dislocations is the slackness of the muscles around the total hip. The goal is restoration of proper strength in the soft tissues around the total hip, especially abductor muscles (the strong muscles that move the leg sideways and press the femoral ball in the cup).
The surgeon has many choices to achieve this goal:
A:
The surgeon moves the attachment of the abductor muscles together with a piece of bone downwards until the muscles are taut again and fixates the piece of bone there with a screw. After this operation the patient is placed in a special brace until the attachments heal in a new place (4 – 6 weeks). This operation increases the strength of the abductor muscles. This operation is done in total hips with non - modular femoral components. A simple surgery but a long aftertreatment.
B:
When the femoral component is of the modular type, the surgeon may exchange the femoral head module; the exchange will lengthen the neck of the device and stretch the weak muscles.
There is, however, a significant risk that a longer neck component will make the whole leg longer after the surgery. Some total hip systems have femoral components with several lengths of the femoral offset. (See in chapter Function of the total hip). It is preferable, if possible, to use femoral components with longer offsets instead of components longer necks. Statistics demonstrated that the latter are more effective to cure repeated dislocations.
C:
The surgeon may remove all components of the total hip and replace them with a special total hip device with a very large ball ( about 36 - 40 mm in diameter). These total hip models are sometimes called Tripolar.
Because the wear increases with increasing joint surface area these cups cannot be made from conventional polyethylene which is much wear sensitive. The available models are thus metal-on-metal total hip models. The new crosslinked polyethylene is considered to be a kind of polyethylene that could be used as liner for these tripolar cups.
Revision operations of dislocating total hips with these large ball prostheses are a big surgery with considerable surgical trauma. Known results demonstrate that revision operations with the Tripolar total hips render the hip stable, but these revision operations have a high rate of other complications (Beaule 2002).
Constrained cups
Another choice of the operative treatment is the use of special "constrained " acetabular socket components. These cup components, which envelop and retain the femoral ball, should replace the lacking joint capsule. The constraining component is sometimes poetically called "lip" and the whole such cup component "lip cup".
Some models of constrained cups need removal of the old cup - a rather big surgery with considerable surgical trauma. Other models allow only exchange of polyethylene liners. The surgeon removes the old conventional liner and replaces is with a constraining liner. Some studies showed very good stabilizing effect of these cups, other studies showed that these patients experienced higher loosening rates of the constrained total hip models.
Picture:
Schematic picture of total hip joint with constrained cup
(click on the icon for a full size picture)
Left side: Unconstrained conventional cup, made of polyethylene.
Top picture: The cup covers about one half of the ball component
Lower picture: when patient bends the thigh, the ball component has insufficient containment in the cup - it is on the way out of the joint
Right side: Constrained polyethylene cup.
Top picture: The cup covers more than one half of the ball component; the constraining ring provides increased coverage of the backside of the ball component.
Lower picture: when patient bends the thigh, the ball component is still well contained within the cup.
Bipolar hip prosthesis
The surgeon may remove all components of the total hip and replace them with a single bipolar prosthesis. This prosthesis has a large mobile cup component (48 to 56 mm in diameter) coupled firmly with the femoral ball component. The outer surface of the cup is made from a highly polished metal. The metallic backup articulates directly with the raw skeleton of the pelvic socket on the outside. Such bipolar hip joint is stable but often painful. The increased stability is due to the large diameter of the new bal component.
Because only femoral head (i.e. only half of the hip joint) is replaced , the term used for this kind of replacement device has been hemiarthroplasty ("half" arthroplasty
Picture: Bipolar hip prosthesis.
Top: The polyethylene cup moves against the ball component. The polyethylene cup is contained in a metallic cup. The outer surface of the metallic outer surface is highly polished and articulates directly with the raw bone of the socket. The motion against the raw bone of the socket may be painful.
Bottom: On cross section it is apparent that the cup consists of a large polyethylene cup and a metallic back up. Ball component and both components move freely against each other. A closing ring keeps the cup firmly coupled with the ball.
Facts for your decision
How long to wait?
80% of patients who have had three dislocations continue to have further dislocations within three years. Further attempt of conservative treatment after the third dislocation would probably be a waste of time.
After the third dislocation the surgeon should thus discuss with the patient the possibility of operative treatment. The surgeon should, however, extensively inform the patients about the risks of revision operations for recurrent dislocation. The expectations of a revision surgery for recurrent dislocations should be realistic. Unfortunately, many long-term results of these revision operations are still less than optimal.
Results of individual revision operations
for total hip dislocations (Bourne 2004):
Success = no recurrence of dislocation
: success in 69% to 96% of cases (the longest follow up, 6 years, has the lowest success rate).Modular components exchange (longer femoral neck)
Trochanter advancement success in 81 –90% of cases.
Bipolar hemiarthroplasty success in 81%; the functional result was, however, dismaying. Many patients have had stable, yet painful hip bipolar hip joints
Jumbo femoral balls (also called tripolar arthroplasty) success in 81 –100%, depending on the model. The diameter of femoral balls is usually 34 to 36 mm (the normal femoral head has a diameter 44 - 56 mm).Follow-up was however very short and the other complications were relatively many.
Constrained acetabular components: The outcomes of using these constrained acetabular liners seems to be implant dependent: Osteonics constrained liner: 4% dislocation and 2% aseptic loosening; S-ROM constrained liner 9% - 29%) and 4% aseptic loosening rates.
Disadvantages are restricted range of motion resulting in impingement and socket failure. The thin polyethylene liner probably will restrict the use of these devices to elderly low-demand patients.
Which revision operation to choose
Revision operations for recurrent dislocation of total hip are among the most difficult in the total hip surgery.
No obvious reason for dislocation = bad results of surgery
Experience learned the surgeons that when the surgeon has not found an obvious cause of dislocation, such as bad position of components, the results of revision operation have been often disappointing. "Quick fix" results in bad result.
9When faced with recurrent dislocation of a THA, a decision scheme is helpful.
One such scheme is as follows:
If the total hip components are in acceptable position and of modular type (see Function of the total hip prosthesis), the surgeon can take advantage of the modularity, and recommend exchange of the modular femoral head using a component with longer femoral neck. If possible the surgeon may exchange a polyethylene insert and use a larger, lipped liner articulating against a larger femoral ball component. These are relatively simple operations with little surgical trauma.
If the x-ray pictures show that the cup or femoral components are in bad position, especially if the cup faces downwards or backwards, then the best strategy is to remove them and replace them with new components placed right. This is big surgery with big surgical trauma.
If the dislocating total hip model is not of the modular type, trochanteric advancement can often be a useful operation.
At this time, it seems wise to reserve the use of constrained acetabular components to elderly low-demand patients
Questions to ask your surgeon
How long should I have restriction on motions in your hip? Who will tell me about them?Questions to ask after the first dislocations:
Who did the reposition of the dislocated total hip, emergency room doctor or your surgeon?
What show the x-ray pictures after the reposition?
Do these x-ray picture show the signs of damage of the skeleton around the total hip or signs of damage of the total hip joint itself?
What is the position of the components of the total hip on the x-ray pictures? Are the components placed in the right position?
If they are not, would you recommend a revision operation just now to place the components right? (If the deviation of the components from the ideal position is small and it is the first dislocation, the majority of surgeons will recommend waiting).
Do I need special physical therapy and balance training?
What is the risk, in your opinion, that I will have another dislocation?
What should I do to prevent the recurrence?
Questions to ask your surgeon after second and further dislocation of your total hip:
What is in your opinion the cause of repeated dislocations of my total hip?
Will you order any special examination – e.g. examination of my total hip in narcosis?
Do you believe that I will need revision operation to heal my repeated dislocation?
Precisely what will the surgery be?
What happens if I will not have a revision operation?
How long should I wait with the operation?
What will be the treatment after the revision operation?
Which restrictions and for how long will I have them after the revision operation?
How many patients like me did you treat and what was their success rate?
Prevention:
Proper positioning of the legs and well-developed musculature are the key factors to prevent the dislocations. You were taught the preventive measures before and after your total hip surgery. Follow them carefully.
Statistics demonstrated that good balance is very important to prevent dislocation. Good balance must be trained after the total hip surgery. Ask your physical therapists to teach you balance training.
Total hips with large femoral balls?
Should you have a total hip prosthesis with a very large femoral ball to be sure that your total hip will be stable?
Here are your options:
Recently manufacturers introduced total hip joints with femoral balls equally large as the removed femoral head (46 to 54 mm). These models are metal-on-metal models, with a thin cup made from cobalt-chrome alloy.
(Polyethylene or ceramic cups cannot be used. Polyethylene layer must be thick and must be covered be metallic envelope – back up. The same apply to ceramic material too. Together, the walls of such cups are too thick and leave not place for femoral ball of really large diameter.)
These new large ball total hips are mechanically more stable; moreover the metal bearing surfaces are kept together by suction force. (See the chapter Function of the total hip). These two characteristics should make these new total hips, at least in theory, very resistant against dislocation.
References:As yet, I have not seen any patient statistics.
There is also at least one theoretical disadvantage of the metal-on-metal total hips with large balls. These total hips have increased range of motion and increased diameter of articulation surfaces. This combination will produce, in spite of theoretically better lubrication, more metallic wear particles and higher blood levels of metals (cobalt, chrome) in the blood of patients.
How important are increased blood levels of metals? As yes there is no answer, but investigations of patients operated on in the 1970’s with metal-on-metal total hips did not demonstrate increased general rate of cancer in these patients. (See also the chapter Total hip and cancer)
Bourne J, Mehin R.: J Arthroplasty 2004, Suppl 1, 111-4
Charnley J Low Friction Arthroplasty of the Hip, Springer -Verlag,1979, p 319
Von Koch M et al.: J Bone Joint Surg-Am 2002; 84-A: 1949-53
(Revised June 2006)
Dislocation of the polyethylene liner
"Dislodgment of the polyethylene liner is an increasingly common complication following total hip arthroplasty"(Della Valle, 2001)
What is it
In cementless hips the cup component consist of metallic shell and a polyethylene liner. (See Cemented and cementless total hips). The polyethylene liner may dislodge from its metallic encasement / shell. Both modular and non-modular cup models experience this complication. Reports also demonstrate that the locking mechanism, that should keep the polyethylene liner inside its shell, may fail quite so often in certain total hip models.
What are the symptoms
In most patients the symptoms start spontaneously. The patients experience sudden or increasing hip pain, clicking and instability in the replaced hip joint, painful limp, and shortening of the limb. In some patients the symptoms start with an accident / fall.
How often does it occurs
Nobody really knows, because (curiously enough) it is not recorded as a total hip failure in the large, nationwide registers. Statistics from the USA’s Food and Drug Administration Medical Device Report of failed total hip replacement prostheses reveal that failure of acetabular components was reported twice as often as the failure of the femoral components.
Fracture of the polyethylene liner was reported in 38 % of cases and dislodgment in 34 % of cases. The statistics also indicate that the rate of this complication is increasing. The authors of the report believe than only a small part of all complications have been reported.
Treatment:
Operation of the dislodged liner is necessary, the surgery should be done as soon as possible.
At the operation the surgeon has several options, depending on the state of the metallic shell, the locking mechanism for the liner (state of the tines), and the state of the skeleton around the metallic shell.
If the (modular) metallic shell with its locking mechanism is intact the surgeon may remove only the dislocated liner, which usually shows several damage, and put in a new polyethylene liner. (There is, however, no guarantee that the new liner will hold better than the old one)
If the (modular) metallic shell has screw holes and is well fixated to the bone, the surgeon may cement a new liner into the retained metallic shell. This will eliminate the problem of deficient locking mechanism; but it may create other problems, because the cement "eats up" the space for a new polyethylene liner
The surgeon may remove both the dislocated liner and the metallic shell and
implant a new, "better", non-modular cup component,
or cement in an all polyethylene component (see The conventional total hip).
The results of revision operation of the dislodged liner
there is as yet no statistic large enough to allow general conclusion, but the individual reports report success rates of 80 - 90%.
References:
Della Valle et al . J Bone Joint Surg - Am, 83-A, 2001, 553-9
Parvizi J et al.: Revision total hip arthroplasty for instability: J Bone Joint Surg - Am, 2008; 90-A: 1134 - 42
(Under revision July , 2008)
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