A hip dislocation happens when the femoral head parts company with the acetabular cup (the pelvis side). The dislocation causes surrounding tissue to tear and these won't repair as readily as they did after surgery.
Once a joint has dislocated there is a significantly higher chance of it doing so again within three months.
But for every patient there is a life long risk of a dislocation and caring for your new joint should become second nature.
Why Do Hips Dislocate Post-Surgery?
A normal size femoral head is about 45 - 55 mm (about 2") but your replacement one is only about half that. The most frequently used heads are 22, 26 or 28mm although larger ones are sometimes used.
So why not use a larger head? The bigger the femoral head the more torque (stress) is placed on the cup as you move. This results in loosening and therefore wearing out faster than it would with a smaller sized cup. However, the smaller the femoral head is the greater the possibility of dislocation.
So a big head leads to faster wear but a smaller head puts you at greater risk of a dislocation
The surgeon will chose a head that is suitable for you and your life style.
Other Factors Associated with Hip Dislocation
The depth of the femoral head within the cup - the deeper the better
The diameter of the femoral neck
The distance between the femoral head and the start of the stem (the bit embedded in your femur)
Incorrect placement of the cup during surgery
How Many Hip Replacements Dislocate?
It is thought to occur in between 1- 4 % of all primary total hip replacement patients.
Dislocated hips are more common in patients who have undergone revision surgery (i.e. have had their hip surgery re-done) where the rate may be as high as 16%.
Treatment for hip dislocation
The process of putting ball back is called reduction.
It is often possible to restore the joint manually using the Havard Technique.
If you're squeamish don't watch the video.
In some cases surgery will be required particularly if any of the hip's components need to be realigned.
Before surgery, one of your care team will go through a list of hip precautions with you.
The length of the list and the how long you will need to stick with them will depend on the surgical approach used (where the first incision is made), how strong you are pre-surgery, how well you recover and your surgeon's personal preferences.
The first 6 - 12 weeks are the most crucial.
The list will probably include
Not bending your body more than 90 degrees
Not twisting your leg inward (particularly if weight bearing)
Use of pillows when sleeping - I particularly recommend using a Slumber Support cushion
Not crossing your legs
Use of a stick or other walking aid
How to get out of a chair
How to get in and out of a car
How to put on socks and shoes
Use of public transport
Sex - safe positions
How to pick things up off the ground
A hip dislocation is classed as an orthopaedic emergency
One of the ways that surgeon's are rated are on the percentage of hip replacements that end up back in theatre within a certain time frame.
So it is possible that his advice may not just reflect his genuine concern for your welfare but also some concern about his statistics. One physio has suggested to me that the advice you may get will err on the side of caution. But although the degree of risk you are ready to take is totally your decision please bear in mind that a hip dislocation is very painful and that once it happens it is much more likely to happen again. If you're going to take risks watch the video first.
Special Types of Acetabular Cups
Some cups are designed with a lip, which reduces the likelihood of dislocation.
For patients at particular risk of dislocation a constrained cup can be used but this increases the torque on the cup and therefore the risk of it loosening.