Dislocation risk and 90 degree rule

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By Josephine Fox, BoneSmart Nurse Emeritus
Updated: September 19, 2022 by Jamie


If you are worried about your newly replaced hip dislocating, you have lots of company. Most people are concerned about this happening, especially in the early weeks following surgery while they are still getting used to their hip. Keep reading to find out why this is not a big concern for most patients today.

Years ago, a majority of surgeons imposed what has come to be known as the "90 degree rule." You were told never to bend your hip beyond 90 degrees. You may still hear about this rule and maybe even some additional restrictions on movement if you've had posterior approach surgery. But with the introduction of the anterior and lateral approaches along with improvements in implant design and surgical techniques, today's patients are usually told they have no restrictions other than to be cautious and stop any movement that causes discomfort or pain.

No matter what approach you have for surgery, the dislocation factor is a RISK, not an inevitability - so don't get too anxious or afraid that a dislocation is right around the corner if you move a certain way. Also don't be scared if you are told any restrictions are for life. Again, that type of warning was more common in the past. Today's hips should allow people to enjoy almost any activity they want to do once recovery is complete. Here are the actual statistics about the post-op risk of dislocation:

Anterior .7%
Direct Lateral .43
Posterior 1.01%

When a hip is replaced, for some time the muscles will be weak and have very little tone. This can cause the stability of the joint to be somewhat compromised and presents a slight risk dislocations. Your surgeon may have some specific guidelines for the early weeks and months of your recovery and you should follow them. But here are some things you can do to lower the risk of dislocation and ensure that your hip is as comfortable as possible while you are rebuilding your strength and muscle tone.

1. Sleep with a pillow between your legs - in abduction. If you remain in the hospital, they may give you a special triangular wedge sometimes called an abduction pillow or a Charnley wedge (image on the left, below). This makes you keep your legs wide apart, lessening the risk of dislocation while your muscles are at their weakest. Upon discharge, you may be advised to use one or two pillows between your legs for a specified period of time. You can also purchase special contour pillows on sites such as Amazon that allow you to comfortably maintain this separation while sleeping (image on the right, below).

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2. Observe the 90 degree restriction. The 90 degree restriction simply means not bringing your knee close to your body in order to reduce the risk of dislocation. You can bend UP to that point, but just not beyond it. This guidance applies more to patients with the posterior approach than an anterior or lateral incision (THR approaches or incisions). With today's improved implants and surgical techniques, you may not see a lot of surgeons who ask their patients to observe this restriction any longer. Instead, they may simply say to not move your leg beyond any mild discomfort and avoid sitting in low chairs for the first months following surgery.

This is 90 degrees of flexion at the hip

aflagsforworship.co.uk_jo_pic_images_flextotht.jpg



Here are some images to further explain the "degree of bend" for hips:

flex to 0 a-horz.jpg



And here is a chart explain some of the precautions you may want to observe for at least a while:

aflagsforworship.co.uk_jo_pic_images_flexto90b.jpg


There is also this tip to pick things up from the floor - it's called the "Golfer's Reach." Make sure you have one hand on something stable like a kitchen counter. Put your operated leg back as far as you can so it is in line with the straight of your back. Then bend your good leg and reach down with your free hand to pick up the item you're retrieving. As you can see from the images below, the operated leg remains in a straight line with the body, keeping stress off the new joint while it heals.

golfers reach 2-horz.jpg

This is a very easy maneuver, but be careful during the first week or so of recovery and always hold on to something firm like a counter or an armchair until you are confident with it.

Please note this movement may not be appropriate for patients with an anterior approach as it can stress the front of the joint which is the most vulnerable area as you are recovering.

3. No twisting. Twisting motions can really stress any new hip joint and should be avoided until the movement can be made with absolutely no discomfort at all. When you want to turn around, instead of spinning on the ball of your foot, take a number of small steps to turn in order to minimize dislocation risk. When you want to turn over in bed, try to keep your feet apart and avoid crossing one leg over the other as you slowly make the turning motion.

4. Don't cross your legs. This is always a good idea after any surgery, even a nose job! Having an anesthetic temporarily increases your risk of blood clots. Lying with one foot on the other ankle can restrict blood flow in the both legs so it's best avoided. And following a hip replacement, this seemingly simple movement can put the new hip at risk of dislocation.

aflagsforworship.co.uk_jo_pic_images_crossedleg.jpg


5. Reaching to get something from a bedside table. This involves half turning the body, so the type of approach you had for your hip replacement is important.

Posterior approach risk for dislocation: The risk is present if the table is on the same side as your operated leg. If you turn to grab something on the table, you will roll onto your surgical hip and cause it to internally rotate. This increases the risk for dislocation. Rolling onto your surgical hip might be very uncomfortable anyway and cause you to stop this motion. But the better choice is to make sure that you always keep a side table on the side opposite of your new hip for the first month or so of recovery.

Anterior or lateral approach risk for dislocation: Here the risk is when the table is on the opposite side to the operated hip and half turning will cause you to externally rotate your hip and risk dislocation.

6. Sliding scale of dislocation risk. The biggest worry THR patients have is dislocation. But the risk is simply that - a risk. And that risk, while worse in the first few days, lessens as the weeks of healing go by. This chart visually shows how things improve with time during your recovery.

aflagsforworship.co.uk_jo_pic_images_dislocatio.jpg



Why is a hip at more risk of dislocating after a replacement than before?

As shown in the illustration below, the hip capsule is comprised of a fibrous bag which encompasses the joint, keeps the joint fluid in place, and contributes to the stability of the joint. There are many small but strong muscles surrounding the bag that keep the joint from dislocating with normal movements.

1663643149987.png
During surgery when a new joint is put in place, part of the joint capsule is cut out and removed. During recovery as you heal, fibrous scar tissue grows around the joint and takes the place of the excised capsule. This is known as 'psuedo' (or false) capsule and it's what restores the integrity of the joint.

After a couple of months, your muscles will have regained most of their strength and tone, the new 'pseudo' capsule will have formed, and your hip joint will be as secure as a natural one. This healing process allows you to be able to do pretty much do anything you want to after recovery. There should be no lifelong restrictions for anyone having a hip replaced these days.
 

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